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24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24
RLE Progress Report 144
Optics and Quantum Electronics
Academic Staff
Prof. James G. Fujimoto, Prof. Hermann A. Haus, Prof. Erich P. Ippen, Professor Franz X.
Kärtner
Research Staff, Visiting Scientists and Affiliates
Dr. Kazi S. Abedin, Dr. Stephane Bourquin, Dr. Mark Brezinski, Dr. Katherine Hall, Christian
Koos, Dr. Chris Kroeger, Dr. Christina Manolatou, Dr. Chan H. Park, Dr. Lelia A. Paunescu, Dr.
Poh-Boon Phua, Dr. Thomas R. Schibli, Karl Schneider, Dr. Ping Xue
Graduate Students
Aaron Aguirre, Juhi Chandalia, Ravi Ghanta, Juliet Gopinath, Felix Grawert, Matthew Grein, Paul
Hertz, Pei-lin Hsiung, Leaf Jiang, Mohammed Jalal Khan, Tony Ko, Andrew Kowalevicz, O. Onur
Kuzucu, J.P. Laine, Nirlep Patel, Milos Popovic, Rohit Prasankumar, Peter Rakich, Daniel Ripin,
Bryan Robinson, Shelby Savage, Hanfei Shen, Jason Sickler, Laura Tiefenbruck, Aurea Tucay,
Michael Watts, Samuel Wong
Undergraduate Students
Karen Robinson
Technical and Support Staff
Mary Aldridge, Donna Gale, Cindy Kopf
Research Areas and Projects
Ultrashort Pulse and Laser Generation Technology
Few-Cycle Pulse Generation and Dispersion Compensating Laser Optics
• Double-Chirped Mirror Design
• Record Pulse Generation of 5 fs Pulses with Octave Bandwidth
• Few-Cycle Laser Pulses from Cr:YAG and Cr:Forsterite Lasers at 1.3 µm and 1.5 µm
• Ultra-broadband Prismless Ti:sapphire lasers
Ultra-low-threshold, Low Cost, Femtosecond Laser Technology
Continuous Wave and Q-switched Mode-locked Microchip Lasers
Broadband Oxidized Saturable Bragg Reflector
Ultrafast Cr:YAG Laser
Non-epitaxially Grown Semiconductor-Doped Silica Films for Laser Modelocking
Active Harmonically Modelocked Fiber Lasers
High-Repetition-Rate Fiber Ring Laser Passively Modelocked With a Saturable Absorber Mirror
Self-Stabilized Harmonic Passively Modelocked Stretched-Pulse Erbium Fiber Ring Laser
Noise in Harmonically Modelocked Lasers
Timing Jitter Reduction in Modelocked Semiconductor Lasers with Photon Seeding
Quantum-Limited Noise Performance of a Semiconductor Modelocked Laser
Experimental Demonstration of a Timing Jitter Eater
Novel Low-Coherence Light Sources for Optical Imaging Applications
• Spectral Broadening in Tapered Fiber using a Femtosecond Nd:Glass Laser
• Continuum Generation in the Visible Wavelength Region Using High Nonlinearity AirSilica Microstructure Optical Fibers
• Broadband Fluorescence Sources Using Ti:Al2O3 Crystals
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Optical Phase Control and Stabilization Techniques
Control and Stabilization of the Absolute Optical Phase Evolution
Few-Cycle Nonlinear Optics and Absolute Optical Phase Effects in Carrier-Wave Rabi Flopping
Nonlinear Fabry-Perots for Synchronization of Independent Laser Oscillators
Control of the Absolute Optical Phase in Picosecond Lasers
Control of Q-switching Instabilities in Mode-locked Lasers by Active Feedback
Ultrafast Phenomena and Quantum Electronics
Resonance Raman Studies on 0.4nm Single Wall Carbon Nanotubes
Enhanced Light Extraction and Lasing in Two Dimensional Photonic Crystals
Highly Nondegenerate Four-Wave Mixing in Tapered Microstructure Fiber
Femtosecond Pump-Probe Spectroscopy Using a Two-Dimensional Smart Pixel Detector Array
Photonics and Devices
Resonant Channel Add/Drop Filters
Fiber-Chip Coupling
Polarization Mode Dispersion
Suppression of Radiation from Quarter-Wave Shifted Bragg Resonators
Micron-size Bending Radii in Silica-based Waveguides
Micromachined Photonic Devices using Nonlinear Materials Processing
Publications
Conference Papers
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Ultrashort Pulse and Laser Generation Technology
Few-Cycle Pulse Generation and Dispersion Compensating Laser Optics
Sponsors
U.S. Air Force – Office of Scientific Research
Grant F49620-01-1-0084
MIT Lincoln Laboratory
Grant ACC 334
MIT Presidential Fellowship
Project Staff
Felix Grawert, Onur Kuzucu, Dr. Uwe Morgner Dr. Thomas R. Schibli, Professor Franz X.
Kärtner, Professor James G. Fujimoto, Professor Hermann A. Haus, Professor Erich P. Ippen,
Richard Ell
The generation of ultrashort laser pulses continues to be a very active field of research. This
technology has found applications in the areas of biomedical optics, high speed communications,
and the investigation of ultrafast nonlinear processes in semiconductor materials and devices.
Generally, these laser sources aim to be cost effective, robust, and technologically simple. Kerrlens modelocking (KLM), which utilizes the electronic Kerr effect to create an artificial fast
saturable absorber, has been the most successful technique for the generation of ultrashort
pulses. We have developed a theoretical model which provides a foundation for understanding
and optimizing short-pulse KLM lasers. Our program investigates several areas of ultrafast laser
technology, with the objective of developing new technologies that can be applied across a range
of laser materials and systems.
Double-Chirped Mirror Design
Solid state lasers can have gain over extremely broad bandwidths of several hundred
nanometers, enabling both the generation of few cycle pulse durations or longer pulse durations
with broad tunability. Intracavity dispersion is the limiting factor in laser performance for sub-10 fs
pulses due to their broad bandwidth. Intracavity prisms have been used for dispersion
compensation. However, prisms have parasitic higher order dispersion, which limits pulse
duration and also makes laser tuning difficult. Self-phase modulation (SPM) is a temporal
nonlinear effect also originating in the Kerr nonlinearity at high intensities that generates new
frequencies and spectrally broadens the pulse. When dispersion is carefully compensated, SPM
can aid in generation of pulses with spectra that extend beyond the gain bandwidth.
Double chirped mirrors (DCMs) have recently emerged as a powerful technology which permits
intracavity dispersion management [1-6]. DCMs enable both broadband operation and intracavity
dispersion compensation without prisms. Figure 1 shows the differences between standard
Bragg mirrors, simple chirped mirrors, and double-chirped mirrors. In a simple chirped mirror the
Bragg-wavelength of the grating increases with increasing penetration depth of light into the
mirror. Light is reflected at the index discontinuity between the surrounding air and the first layer
in the mirror and during chirping of the Bragg-wavelength. These reflections interfere with the
strong reflection from the back of the mirror which leads to Gires-Tournois like interferences
resulting in strong dispersion oscillations. In a double-chirped mirror we avoid these spurious
reflections by a consistent impedance matching from the ambient air all the way to the classical
turning point of the wave in the chirped mirror. This is achieved by matching from air to the first
low index layer of the actual DCM by a broadband AR-coating. Additional spurious reflections
inside the chirped mirror are avoided by slowly switching on the grating, which can be done by
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increasing the thickness of the high-index layer adiabatically from an almost vanishing value to
the corresponding quarter wave thickness. The proper design of DCMs enables the intracavity
dispersion to be almost perfectly compensated, removing all higher order dispersion terms over
the laser bandwidth which limit the generation of short pulses. We have recently demonstrated a
5 fs Ti:Al2O3 laser where we compensated in average dispersion up to sixth order over one
octave of bandwidth [7].
Bragg-Mirror:
a)
Chirped Mirror:
SiO 2
Substrate
λB
- Layers
4
TiO2 / SiO2
SiO 2
Substrate
Air
Bragg-Wavelength λ chirped
B
b)
Negative
λ1
Dispersion:
λ2 λ > λ
2
1
Double-Chirped Mirror: Bragg-Wavelength and Coupling Chirped
d = λB /4
h
SiO 2
Substrate
ARCoating
Air
“Impedance” - Matching
c)
Figure 1: Schematic drawing of layer sequence for various dielectric mirrors: a) standard quarter
wave Bragg mirror, b) simple chirped mirror, c) double chirped mirror (DCM).
We have recently developed an analytic theory for the design of dispersion compensating mirrors
and mirror pairs [4, 5, 8, 9]. Dispersion compensating mirrors, and especially double-chirped
mirrors developed in our group, provide high reflectivity and well controlled dispersion over 400
nm in the case of single mirrors or even over one octave using specially matched mirror pairs.
These components are a prerequisite for miniaturized ultrabroadband femtosecond lasers
necessary for biomedical optical imaging, ultrafast instrumentation, and other applications.
In our previous work pulses shorter than 5.4 fs at a center wavelength of 800 nm, corresponding
to a bandwidth greater than 350 nm, were generated directly by a Kerr-lens mode-locked
Ti:sapphire laser at a repetition rate of 90 MHz and an average output power of 200 mW. In this
laser, the pulse duration was limited by the bandwidth over which the DCMs can balance the
dispersion inside the cavity. The gain bandwidth of Ti:Al2O3 extends from 600 nm to almost 1200
nm and would enable the generation of even shorter pulses, in the single-cycle regime. However,
increasing the bandwidth of the DCMs results in stronger oscillations of the group delay, which
limits the pulse duration. This effect is a consequence of fundamental properties of DCM
operation. This problem can be solved by developing complementary sets of mirrors. The phase
of the group delay oscillations can be controlled since it depends on the index and layer thickness
of the dielectric layers. Two complementary sets of ultra-broadband DCMs can be designed with
excess group delay oscillations that are exactly out of phase. Using these two mirror sets, the
excess oscillations can be made to cancel.
This novel approach enables dispersion
compensation over a much greater spectral range than possible using a single mirror set and
enables the generation of unprecedented bandwidths and pulse durations. Figure 2 shows the
calculated and measured reflectivity and group delay dispersion of the mirror pairs. The mirrors
are designed such that the dispersion of all intracavity components is exactly compensated up to
sixth-order.
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600
80
400
0
40
0
600
800
1000 1200
WAVELENGTH, NM
400
80
200
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0
40
-200
20
-400
0
-600
1400
2
-400
2
-200
20
600
GDD, FS
200
60
100
REFLECTIVITY (%)
100
GDD, FS
REFLECTIVITY (%)
24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24
RLE Progress Report 144
600
800
1000 1200
WAVELENGTH, NM
-600
1400
Figure 2: Calculated and measured reflectivity and dispersion of ultrabroad bandwidth DCM pairs
specially designed to cancel parasitic dispersion oscillations.
Record Pulse Generation of 5 fs with Octave Bandwidth
Using this novel design, we recently demonstrated the generation of pulses with durations of only
5 fs and spectral bandwidths over one octave directly from a Ti:Al2O3 laser [7]. This is the
shortest pulse ever generated directly from a laser. Figure 3 shows a schematic of our laser
system. This laser has a standard z-cavity design with the addition of a second intracavity fold.
A second focus is generated by M4 and M5 in which a 2.4 mm thick plate of BK7 is positioned.
This provides enhanced self phase modulation and increases the laser bandwidth.
Figure 3: Schematic of Ti:sapphire laser that generates octave bandwidths and 5 fs pulse
durations. A second focus was used to increase bandwidth via self phase modulation.
The optical power spectrum at the laser output is displayed in Figure 4a) on a linear and
logarithmic scale. On a log scale the spectrum extends from 600 to 1350 nm above the noise
floor. The FWHM of the corresponding pulse assuming a flat phase would be 4.3 fs. The
structure in the spectrum is correlated with oscillations in the measured intracavity GDD
suggesting that improved DCM design would improve the laser performance. The two peaks at
700 nm and 1050 nm are caused by the increasing output coupling mirror transmission. Despite
the large oscillations in the GDD caused by fabrication tolerances, the spectrum is relatively
smooth, which can be explained by the enhanced SPM due to the second intracavity focus and
the strong KLM action, which continuously cleans up the pulse shape. The interferometric
autocorrelation measurement is displayed in Figure 4b). A phase retrieval algorithm [10] was
used to reconstruct the actual pulse shape from the autocorrelation. The intensity envelope of
the reconstructed pulse indicates a FWHM of 5 fs. The phase measurement suggests that
reductions in duration to 4.5 fs should be possible.
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SPECTRUM, a.u.
IAC
6
4
2
0
a)
0
1.0
8
10
-1
0.8
10
0.6
10
-2
-3
10
0.4
-4
10
0.2
-5
10
0.0
-20
-10
0
10
20
TIME DELAY, FS
30
b)
600
800
1000
1200
WAVELENGTH, NM
Figure 4: a) Interferometric autocorrelation of the 5 fs pulse b) corresponding spectrum on a
linear and logarithmic scale. These are the shortest pulses ever generated directly from a laser.
Few-cycle laser pulses from Cr:YAG and Cr:Forsterite lasers at 1.3 µm and 1.5 µm
Dispersion management techniques using DCMs can be applied to a wide range of solid state
laser materials. The spectral range at 1.3 µm and 1.5 µm is of particular interest because it falls
nd
rd
into the 2 and 3 telecommunication window. Our group recently demonstrated all-solid-state,
Kerr-lens mode-locked Cr:forsterite and Cr:YAG lasers producing 14 fs pulses with 250 nm
bandwidth at 1.3 µm and 19 fs pulses with 240 nm bandwidth at 1.5 µm, respectively [11,12]. In
this wavelength range dispersion compensation is particularly challenging because higher order
dispersion becomes the dominant factor near the zero dispersion wavelength. Higher order
dispersion is hard to compensate over an extended wavelength range even with DCMs. The total
spectral coverage on a logarithmic scale of the three few-cycle lasers, Ti:sapphire, Cr:forsterite
and Cr:YAG, developed so far is shown in Figure 5.
Figure 5: Total spectral coverage of the few-cycle laser systems Ti:sapphire, Cr:forsterite and
Cr:YAG.
The spectral coverage of these three novel laser sources will allow us to perform unique
spectroscopic investigations and excitations of semiconductors and semiconductor devices in the
next years. Some of them are further discussed below.
Ultra-broadband prismless Ti:sapphire lasers
To date, extremely stable, all mirror-dispersion controlled Ti:sapphire lasers emitting 8 fs pulses
with spectra extending over 105 nm full width at half-maximum (FWHM) have been demonstrated
[13]. As shown above, additional use of prism pairs for dispersion compensation results in an
increased flexibility in the mirror design and, therefore, in pulses as short as 5 fs with octave
spanning spectra directly from the laser. However, it has been found, that the fluctuations in the
intracavity beam-pointing angle translates into undesired dispersion fluctuations in lasers with
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prisms, which is detrimental in experiments concerned with carrier-envelope phase stabilization.
Here, an all mirror-dispersion controlled Ti:sapphire laser, which generates sub-8fs pulses and
most notably, emits over a spectral width as large as 250 nm (FWHM) on a linear scale and even
generates significant spectrum over 400 nm on a logarithmic scale. This ultrabroadband laser is
suitable for carrier-envelope phase stabilization based on interference of second and third
harmonic light [14] and may have many other applications, such as in high-resolution optical
coherence tomography [15]. Due to the reduced intracavity-losses, an optical to optical efficiency
of more than 10% in the mode-locked state is achieved allowing for pump-powers below 3 W with
a total output power of typically 300 mW.
Telescope
DCM - 10cm ROC
Ti:Al2O3
Pump Laser
2.4% Output Coupler
DCM
DCM
Figure 1. Schematic diagram of the astigmatically compensated, prism-less Ti:sapphire laser. All
mirrors except the output coupler mirror are double-chirped mirrors (DCMs), which compensate
the second and third order dispersion of the laser crystal and the air, which resides inside the
laser cavity.
b)
1.000
0.999
0.998
1.0
0
-20
2
GDD [fs ]
Reflectivity
a)
0.5
-40
-60
-80
0.0
700
wavelength (nm)
750
800
850
900
wavelength (nm)
950
1000
Fig. 2: a) Reflectivity of the DCMs. b) Desired (dotted line), designed (solid line) and measured
(dashed line) group delay dispersion of the DCMs.
Figure 1 shows a schematic diagram of the prism-less Ti:sapphire laser. All cavity mirrors except
the output coupler are double-chirped mirrors (DCMs). The DCM-design is such that there is high
transmission of the pump light at 532 nm and high reflectivity greater than 99.9% from 700-1000
nm. On average, the DCMs generate within six bounces per roundtrip the necessary dispersion to
compensate the positive second and third order dispersion of the laser crystal and the 4.5 meters
of air which are present in this 75 MHz laser cavity, see Fig. 2. The laser crystal has a path length
of 2.3 mm and absorbs approximately 70% of the pump light emitted from a frequency doubled,
diode pumped Nd:YVO4 laser.
Figure 3 shows a typical spectrum and the corresponding interferometric autocorrelation of the
pulses emitted by the laser. A phase-retrival algorithm [10] results in 7.8fs pulses. (The Fourier
transform limit of the spectrum shown in Fig. 3 assuming constant spectral phase is 6.9 fs).
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0
0.8
-20
0.6
-40
0.4
-60
0.2
0.0
600
800
1000
1 200
-80
1400
Wavelength [nm]
8
M eas ured
Fitted (7.8fs )
6
IAC
1.0
Norm. Spectral Power (log)
Norm. Spectral Power
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RLE Progress Report 144
4
2
0
-40
-20
0
20
40
Time [fs]
Figure 3. Typical spectrum and interferometric autocorrelation generated by the laser shown in
Figure 1.
Since the resonator dispersion is independent of the alignment of the cavity, stable mode-locking
is observed over a day without dropout. Also no fine-tuning of the dispersion is needed. The
operation of such a laser is significantly easier than a laser with additional prism pairs fordispersion compensation.
References:
1. R. Szipöcs, K. Ferencz, C. Spielmann and F. Krausz, "Chirped multilayer coatings for
broadband dispersion control in femtosecond lasers," Opt. Lett. 19(3): 201-3 (1994).
2. R. Szipöcs, A. Stingl, C. Spielmann and F. Krausz, "Chirped dielectric mirrors for dispersion
control in femtosecond laser systems," paper presented at the in Generation, Amplification,
and Measurement of Ultrashort Laser Pulses II, Proc. SPIE, San Jose, California. Feb. 6-7,
1995.
3. R. Szipöcs and A. Kohazi-Kis, "Theory and design of chirped dielectric laser mirrors," Appl.
Phys. B 65(2): 115-136 (1997).
4. F.X. Kärtner, N. Matuschek, T. Schibli, U. Keller, H.A. Haus, C. Heine, R. Morf, V. Scheuer,
M. Tilsch and T. Tschudi, "Design and fabrication of double-chirped mirrors," Opt. Lett.
22(11): 831-33 (1997).
5. N. Matuschek, F.X. Kärtner and U. Keller, "Theory of Double-Chirped Mirrors," IEEE J. of
Select. Topics Quantum Electron. 4(2): 197 (1998)
6. U. Morgner, F.X. Kaertner, S.H. Cho, Y. Chen, H.A. Haus, J.G. Fujimoto, E.P. Ippen, V.
Scheuer, G. Angelow and T. Tschudi, "Sub-two-cycle pulses from a Kerr-lens mode-locked
Ti:sapphire laser," Opt. Lett. 24(6): 411-13, (1999).
7. R. Ell, U. Morgner, F.X. Kärtner, J.G. Fujimoto, E.P. Ippen, V. Scheuer, G. Angelow and T.
Tschudi, "Generation of 5 fs pulses and octave-spanning spectra directly from a Ti:sapphire
laser," Opt. Lett. 26(6): 373-5 (2001).
8. N. Matuschek, F.X. Kärtner and U. Keller, "Analytic design of double-chirped mirrors with
custom tailored dispersion characteristics," IEEE J. Quantum Electron. 35(2): 129-37 (1999)
9. .F.X. Kärtner, U. Morgner, T.R. Schibli, E.P. Ippen, J.G. Fujimoto, V. Scheuer, G. Angelow
and T. Tschudi, "Ultrabroadband double-chirped mirror pairs covering for single cycle pulses,"
submitted to J. Opt. Soc. Am. B.
10. A. Baltuska, A. Pugzlys, M. Pshenichnickov, D. Wiersma, B. Hoenders and H. Ferwerda,
“How to retrieve amplitude and phase from an autocorrelation and spectrum,“ Proceedings of
Ultrafast Optics 1999, 221, Th10, Ascona, Switzerland, (1999).
11. C. Chudoba, J.G. Fujimoto, E.P. Ippen, H.A. Haus, U. Morgner, F.X. Kärtner, V. Scheuer, G.
Angelow and T. Tschudi, "All-solid-state Cr:forsterite laser generating 14 fs pulses at 1.3 um,"
Opt. Lett. 26(5): 292-94 (2001).
12. D.J. Ripin, C. Chudoba, J.T. Gopinath, J.G. Fujimoto, E.P. Ippen, U. Morgner, F.X. Kärtner,
4+
V. Scheuer, G. Angelow and T. Tschudi, "Generation of 20 fs pulses by a prismless Cr :YAG
laser," Opt. Lett. 27(1), 61-3 (2001).
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13. A. Stingl, M. Lenzner, Ch. Spielmann, F. Krausz and R. Szipöcs, "Sub-10-fs mirrordispersion-controlled Ti:sapphire laser," Opt. Lett. 20(3): 602-4 (1995).
14. U. Morgner, R. Ell, G. Metzler, T.R. Schibli, F.X. Kärtner, J.G. Fujimoto, H.A. Haus and E.P.
Ippen, ''Nonlinear optics with phase-controlled pulses in the sub-two-cycle regime,'' Phys.
Rev. Lett. 86(24) :5462-5 (2001).
15. W. Drexler, U. Morgner, F.X. Kärtner, C. Pitris, Y. Chen, S.A. Boppart, X.D. Li, E.P. Ippen
and J.G. Fujimoto, "In vivo ultrahigh resolution optical coherence tomography," Opt. Lett.
24(17): 1224 (1999).
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Ultra-low-threshold, Low Cost, Femtosecond Laser Technology
Sponsors
National Science Foundation
Grant ECS-019452
Air Force Office of Scientific Research
Grant F49620-98-01-0084
Air Force Office of Scientific Research (MFEL)
Grant F49620-01-1-0186
Project Staff
Andrew M. Kowalevicz, Rohit P. Prasankumar, Dr. Thomas Schibli, Dr. Ingmar Hartl, Dr. Uwe
Morgner, Professor Franz X. Kärtner, Professor Erich P. Ippen, and Professor James G. Fujimoto
Kerr lens modelocked (KLM) Ti:Al2O3 lasers can generate extremely short pulse durations with
broad bandwidths and have widespread applications in ultrafast studies as well as in biomedical
imaging [1]. A standard Kerr lens modelocked laser operating with a 5W pump can produce
output powers of 500 mW with 5 nJ of pulse energy. Unfortunately, the high cost of today’s
femtosecond lasers severely limits their widespread use. The cost of femtosecond Ti:Al2O3
lasers is strongly dependent on the pump power requirements. Diode pumped solid-state lasers
capable of generating 5 W can be prohibitively expensive, while lasers generating several
hundred mW are considerably more affordable.
Figure 1. Schematic of the ultra-low-threshold Ti:Al2O3 laser. Arm lengths are 163 cm and 130 cm
for prismatic and OC arms respectively. Intracavity dispersion compensation and tuning is
provided by double chirped mirrors (DCM) and a pair of fused silica prisms separated by 31 cm.
Here we report the development of an ultra-low-threshold modelocked Ti:Al2O3 laser. Since the
pulse energy of a laser is given by its average power divided by its repetition rate, increasing the
cavity length produces an increase in pulse energy [2, 3]. This increased pulse energy enables
high performance KLM operation at low powers while also decreasing the spot size of the laser
mode to reduce pump thresholds. .
Previous investigations have achieved low threshold operation in Ti:Al2O3 with a KLM threshold of
500 mW pump power and sustained modelocked operation below 400 mW, generating pulses of
18 fs with bandwidths of 66 nm [4]. We report what is, to our knowledge, the lowest threshold
achieved to date for a Kerr lens modelocked Ti:Al2O3 laser. Modelocking can be started with
<260 mW of incident pump power, and once initiated, maintained with as little as 170 mW pump.
Pumping at 260 mW, pulse durations of 11 fs with bandwidths of ~125 nm are generated with 13
mW output power at 50 MHz repetition rate.
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Figure 1 shows a schematic of the ultra-low-threshold KLM Ti:Al2O3 laser. The pump source is a
frequency doubled, diode pumped Nd:Vanadate laser. The cavity is a standard folded X
–1
configuration with a 2 mm thick Ti:Al2O3 laser crystal having an α = 5.0 cm . The focusing
mirrors are 7.5 cm radius of curvature and transmit >92% of the pump beam at 532 nm. The
output coupler has a transmission of ~2% between 700 nm and 1000 nm and is low dispersion.
All of the mirrors except the output coupler are doubled chirped mirrors (DCM). Intracavity
dispersion compensation and tuning is provided by a pair of fused silica Brewster prisms
separated by 31 cm.
Figure 2. (a) The output spectrum and (b) interferometric autocorrelation of the pulses.
In order to achieve high performance KLM operation at low thresholds the laser cavity is
extended and has a 50 MHz repetition rate. In addition to increasing the energy per pulse by a
factor of 2 over the standard 100 MHz configuration, the increased arm lengths also reduces the
laser mode size in the crystal to about 8 µm radius. Increasing the pulse energy by cavity length
scaling enables stronger KLM and shorter pulses at low pump powers. The tight focusing of the
laser mode also reduces the laser threshold.
In order to make more efficient use of all available pump power, we use a double pass pump
configuration where the unabsorbed pump transmitted through the laser crystal is collimated and
retroreflected. Because our crystal absorbs only ~63% of the light on the first pass and the laser
operates close to threshold, this second pass of the pump gives a factor of 2 increase in CW
output power.
The use of both DCMs and prisms for intracavity dispersion compensation enables the dispersion
operating point of the laser to be tuned while maintaining a low amount of third order dispersion.
KLM can be started by inducing high intensity fluctuations in the laser by either translating the
end mirror or one of the prisms. The KLM threshold is sensitive to the dispersion operating point.
To start modelocking at low pump thresholds, the dispersion must first be set to a more negative
value than for optimum pulse durations. After modelocking is initiated, the dispersion operating
point can be tuned toward zero to increase the bandwidth and achieve minimum pulse duration.
Typically we observe an output spectrum with 85 nm bandwidth at the dispersion operating point
for starting KLM and bandwidths >125 nm when dispersion is tuned for optimum pulse duration.
At higher pump powers, the lasers can be started at a dispersion operating point closer to the
optimum pulse duration.
The pulse duration was measured with a collinear interferometric autocorrelator. The excess
dispersion from the output coupler and external optical elements was compensated using two
reflections from DCMs prior to the autocorrelator. Second harmonic was generated by focusing
2
with a mirror into a thin KDP crystal. Fitting the interferometric autocorrelation with a sech pulse
yields a pulse duration of 11 fs. The time-bandwidth product of 0.614 is above the transform limit
of 0.315, indicating that there is some residual pulse chirp.
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In conclusion, we have demonstrated ultra-low-threshold operation of a modelocked Ti:Al2O3
laser. Output bandwidths of 80 nm to greater than 125 nm FWHM can be generated with pulse
durations as short as 11 fs with KLM thresholds of <260 mW. By reducing the pump power
requirements a factor of 10x lower than conventional KLM lasers, ultra-low-threshold lasers
should enable a significant reduction in the cost of femtosecond technology.
References
1.
2.
3.
4.
D. E. Spence, P. N. Kean, and W. Sibbett, “60-fsec pulse generation from a self-modelocked Ti:Sapphire laser,” Optics Lett., 16: 42-44, 1991.
S. H. Cho, B. E. Bouma, E. P. Ippen, and J. G. Fujimoto, “Low-repetition-rate high-peak
power Kerr-lens mode-locked Ti:Al2O3 laser using a multiple-pass cavity,” Optics Letters,
24: 417-419, 1999.
A. R. Libertun, R. Shelton, H. C. Kapteyn, and M. M. Murnane, “A 36 nJ-15.5 MHz
extended-cavity Ti:sapphire oscillator,” presented at Conference on Lasers and ElectroOptics, Baltimore, MD, 1999.
K. Read, F. Blonigen, N. Riccelli, M. Murnane, and H. Kapteyn, “Low-threshold operation
of an ultrashort-pulse mode-locked Ti:sapphire laser,” Optics Letters, 21: 489-491, 1996.
24-12
24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24
RLE Progress Report 144
Continuous Wave and Q-switched Modelocked Microchip Lasers
Sponsors
MIT Lincoln Laboratory
Grant ACC 333
National Science Foundation
MIT Presidential Fellowship
Project Staff
Felix Grawert, Dr. Thomas R. Schibli and Professor Franz Kärtner
University of Karlsruhe: Jochen Hetzler, Dr. Uwe Morgner, Professor Martin Wegener
University of Stuttgart: R. Butendeich, J. Schwarz, Dr. H. Schweizer, Dr. Ferdinand Scholz
Compact, reliable and cheap femtosecond laser sources with fundamental repetition rates of 10
GHz and more are needed for high-speed optical data transmission in the 1.5 µm wavelength
region for optical analog to digital conversion and optical imaging techniques. A promising
4+
approach towards these goals are Cr :YAG microchip lasers (for microchip lasers, see e.g. [1]).
4+
Cr :YAG satisfies both the requirements for femtosecond pulse generation as well as for
applicability in fiber-optic communication systems. First experimental results towards mode
4+
locking of Cr :YAG microchip lasers are reported. Continuous wave (cw) operation of the
microchip laser with up to 300 mW of output power has been achieved. Kerr-lens mode-locked
4+
(KLM) operation of a 8.2 mm long Cr :YAG microchip laser using a Q-switched Nd:YVO4 laser
for pumping as well as Q-switched mode-locked operation with saturable Bragg-reflectors (SBR)
and cw pumping at a fundamental repetition rate of 10 GHz with 200 fs pulses has been
demonstrated.
Dichroic
mirror
Pump beam
Heatsink
Output
coupler
Saturable
Absorber
Mirror
(SAM)
Nd:YVO 4 /
Laser diode
Telescope
Cr 4+:YAG-Crystal
Laser Output
Fig. 1: Set-up of the modelocked femtosecond microchip laser.
Intensity [a.u.]
Spectral energy density [dB]
0
-10
1.0
0.5
0.0
-0.4
-20
0.0
0.4
Time [ps]
-30
-40
-50
1420
1440
1460
1480
1500
Wavelength [nm]
Fig. 2: Mode-locked spectra emitted by the system sketched in Fig.1. The inset shows the
intensity-autocorrelation trace of the pulses emitted by the micro-chip laser when operated in the
Kerr-lens mode-locked regime.
24-13
24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24
RLE Progress Report 144
The principle structure of the SBRs can be found in [2]. The experimental setup of the microchip
laser is sketched in Fig. 1. A diode-pumped Nd:YVO4 laser which can be operated either in
4+
continuous-wave (cw) or in Q-switched mode is used for pumping the Cr :YAG material. The
microchip laser consists of the active material sandwiched between two flat mirrors, one of them
being used as the output-coupler mirror (OC). The other one is either a high-reflector (HR) for
KLM operation or a saturable Bragg-reflector (SBR) as a mode-locker. Figure 2 shows a typical
spectrum of the laser when Kerr-lens mode-locked using a Q-switched pump. The inset shows
the intensity-autocorrelation of the pulse train of 200 fs pulses. A severe problem in building
lasers with a high fundamental repetition-rate is the Q-switching instability, since the achievable
intracavity pulse energy is too low to saturate the absorber strongly. In the past, we have
investigated several passive schemes to overcome this instability, such as two-photon absorption
[3] and soliton formation [4], respectively. To evaluate the effectiveness of these techniques in
the micro-chip setup we performed numerical simulations of the temporal dynamics of the
system. The results of such a simulation are shown in Fig. 3.
Fig. 3: Critical pulse energy Wcrit plotted as a function of the intracavity net-dispersion per
roundtrip for different thicknesses of an intracavity indium-phosphide layer which is used as a
two-photon absorber.
The minimum pulse energy Wcrit needed for continuos-wave mode-locking of the system, shown
in Fig. 1, is plotted as a function of the intracavity net-dispersion per round-trip for different
thicknesses of an intracavity indium-phosphide (InP) layer which serves as a two-photon
absorber. It can be clearly seen that appropriate dispersion compensation in conjunction with a
two-photon absorber can lower Wcrit by one order of magnitude, which should allow cw-modelocked operation of the microchip laser in the near future.
References:
1. J. J. Zayhowski and A. Mooradian, ''Single-frequency microchip Nd lasers,'' Opt. Lett. 14(1):
24-6 (1989)
2. E. R. Thoen, E.M. Knootz, D.J. Jones, D. Barbier, F.X. Kärtner, E.P. Ippen and L.A.
Kolodziejski, ''Erbium-Ytterbium Waveguide Laser Mode-locked with a semiconductor
saturable absorber mirror,'' IEEE Photon. Technol. Lett. 12(2): 149-151 (2000).
3. E.R. Thoen, E.M. Knootz, M. Joschko, P. Langlois, T.R. Schibli, F.X. Kärtner, E. P. Ippen and
L. A. Kolodziejski, “Two-photon absorption in semiconductor saturable absorber mirrors,”
Appl. Phys. Lett. 74(26): 3927-9 (1999)
4. S. Namiki, E.P. Ippen, H. A. Haus, and C. X. Yu, ''Energy rate equations for mode-locked
lasers,'' J. Opt. Soc. Am. B. 14(8): 2099 (1997)
24-14
24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24
RLE Progress Report 144
Broadband Oxidized Saturable Bragg Reflector
Sponsors
U.S. Air Force – Office of Scientific Research
Grant F49620-01-1-0084
MRSEC Program of the National Science Foundation
Award DMR 98-08941
Project Staff
Dr. Daniel Ripin, Juliet Gopinath, Hanfei Shen, Alexei Erchak, Dr. Gale Petrich, Professor Franz
Kärtner, Professor Leslie Kolodziejski, Professor Erich P. Ippen
4+
Kerr lens modelocked Cr :YAG lasers are used to generate femtosecond laser pulses in the
wavelength range from 1300 nm to 1600 nm. Pulses as short as 20 fs with a spectral bandwidth
of 190 nm fwhm have been produced from a laser using double-chirped mirrors (DCMs) [1] for
group delay dispersion (GDD) compensation. In general, without precise alignment, Kerr lens
modelocking (KLM) will not be self-starting. External perturbations are then used to initiate
modelocking by creating transient power spikes.
Saturable absorber mirrors based on semiconductor quantum wells have been used in a varietyof
4+
solid-state lasers to initiate modelocking. For Cr :YAG, saturable absorber mirrors have
beendemonstrated consisting of InGaAs quantum wells grown upon a highly reflecting mirrors [2,
3]. In most cases the mirrors were GaAs/AlAs Bragg stacks. These mirrors typically have a
bandwidth of ~100 nm and can therefore limit the minimum pulsewidth by spectral filtering.
Overcoming this difficulty, Zhang et. al. generated 44 fs pulses from a laser started by a
InGaAs/InAlAs quantum well saturable absorber bonded onto a broadband enhanced gold mirror
[4]. It is likely that the pulsewidth in this laser was limited by higher-order dispersion.
In this work, a novel high-index-contrast mirror-based saturable Bragg reflector (SBR) was used
4+
to generate 35 fs pulses with a fwhm bandwidth of 68 nm Kerr lens modelocked Cr :YAG laser.
The laser DCMs to compensate the laser crystal GDD. The SBR consists of a broadband
oxidized 7-period GaAs/AlxOy Bragg mirror substrate supporting an InGaAs/InP quantum well
absorber.
The refractive index and square of the electric field standing wave pattern in the high-dielectric
contrast SBR are shown as a function of position in the structure in Figure 1. The SBR consists
of a 7-period GaAs/AlxOy Bragg stack a 10 nm InGaAs quantum well in a λ/2-thick InP layer.
Each layer thickness was chosen for a center wavelength of λ = 1440 nm. GaAs and AlxOy layers
have indices of refraction of 3.39 and 1.61 at 1.5 µm respectively, creating a high-index-contrast
mirror that has a calculated reflectivity of 99.9% within the wavelength range of 1220 to 1740 nm
using a dielectric stack of only 7-periods.
The SBRs are fabricated with III-V semiconductor-based material growth techniques. First, a
GaAs/AlAs multilayer stack is grown by gas source molecular beam epitaxy (GSMBE). The AlAs
layers are converted to AlxOy through a wet-oxidation process. It is estimated that the resulting
AlxOy layers extended as far as 300 µm into the structure. Side-view scanning electron
micrograph (SEM) images of an unoxidized and oxidized SBR structure are shown in Figure 2(a)
2
and 2(b). Using pump-probe spectroscopy, the saturation fluence is on the order of ~ 10 µJ/cm
and the maximum saturable loss is 0.3%.
4+
The broadband SBR was introduced into a Cr :YAG laser cavity to start modelocking. The laser
cavity was then optimized for Kerr-lens modelocking. A plot of the KLM pulse spectrum is shown
with linear and logarithmic scales in Figure 3. The pulse spectrum is centered at 1490 nm, and
has a full-width half maximum of 68 nm. Spectral components are detected from 1200 to > 1700
nm. An interferometric autocorrelation trace, used to determine the pulsewidth, is shown in
24-15
24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24
RLE Progress Report 144
Figure 4. The autocorrelation fits well to a 32 fs sech-shaped pulse. A sech-shaped pulse fit may
underestimate the true pulsewidth for non-sech-shaped pulses. Using the measured spectrum, a
bandwidth limited pulsewidth of 35 fs is calculated.
InGaAs
4
2
GaAs
3
2
AlxOy
InP
2
|E(z)|
Index of Refraction
4
1
Air
0
1
2
0
3
Position (µm)
Fig. 1. Index of refraction and electric field energy amplitude of the designed Bragg reflector
(SBR) mirror consisting of a GaAs/AlxOy high-index contrast mirror and an InGaAs/InP quantum
well.
(a)
(b)
Fig. 2. Scanning electron micrograph (SEM) images of an (a) unoxidized and (b) oxidized SBR
structure. After oxidation, the AlAs layers are converted to polycrystaline AlxOy, which appears to
be granular.
24-16
24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24
RLE Progress Report 144
-30
0.8
-35
-40
0.6
-45
0.4
-50
-55
0.2
-60
0.0
-65
1200
1300
1400
1500
1600
Intensity (dB)
Intensity (Arb. Units)
-25
1.0
-70
1700
Wavelength (nm)
4+
Fig. 3. Pulse spectrum from a self-started Cr :YAG laser plotted on a linear (black) and
logarithmic (gray) scale.
Autocorrelation
8
6
4
2
0
-100
-50
0
50
Time Delay (fs)
4+
Fig. 4. Interferometric autocorrelation of a saturable absorber Cr :YAG laser.
24-17
100
24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24
RLE Progress Report 144
References:
[1] D. J. Ripin, C. Chudoba, J. T. Gopinath, J. G. Fujimoto, E. P. Ippen, U. Morgner, F. X. Kärtner,
4+
V. Scheuer, G. Angelow, and T. Tschudi, "Generation of 20-fs pulses by a prismless Cr :YAG
laser," Opt. Lett. 27, 61-63 (2002).
[2] B. C. Collings, J. B. Stark, S. Tsuda, W. H. Knox, J. E. Cunningham, W. Y. Jan, R. Pathak,
4+
and K. Bergman, "Saturable Bragg reflector self-starting passive mode locking of a Cr :YAG
laser pumped with a diode-pumped Nd:YVO4 laser," Opt. Lett. 21, 1171-1173 (1996).
[3] S. Spälter, M. Böhm, M. Burk, B. Mikulla, R. Fluck, I. D. Jung, G. Zhang, U. Keller, A.
Sizmann, and G. Leuchs, "Self-starting soliton-modelocked femtosecond Cr(4+):YAG laser using
an antiresonant Fabry-Pérot saturable absorber," App. Phys. B 65, 335-338 (1997).
[4] Zhigang Zhang, Tadashi Nakagawa, Kenji Torizuka, Takeyoshi Sugaya, and Katsuyuki
4+
Kobayashi, "Self-starting mode-locked Cr :YAG laser with a low-loss broadband semiconductor
saturable-absorber mirror," Opt. Lett. 24,1768-1770 (1999).
24-18
24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24
RLE Progress Report 144
Ultrafast Cr4+:YAG Laser
Sponsors
U.S. Air Force – Office of Scientific Research
Grant F49620-01-1-0084
MRSEC Program of the National Science Foundation
Award DMR 98-08941
Project Staff
Dr. Daniel Ripin, Juliet Gopinath, Hanfei Shen, Professor Franz Kärtner, Professor Erich P. Ippen
Short pulsewidth optical sources are ideal for time-resolved studies of ultrafast phenomena and
devices, as optical clocks with precise timing at the cavity repetition rate, and for ultra-high speed
optical communications. Their broad coherent bandwidth can be exploited for spectroscopy, to
generate synchronized multi-wavelength optical sources, or for metrological optical frequency
standards. Specific laser crystals are chosen for applications based on their material properties.
4+
Cr :YAG is a promising laser crystal with broad emission from 1300 to 1600 nm. This gain
spectrum make this laser crystal ideal for optical telecommunications applications. In general,
intracavity dispersion due to the laser crystal, prisms, and mirrors limits the bandwidth and
4+
pulsewidth of the emitted light pulses. The shortest pulses previously reported in the Cr :YAG
laser system are 43 fs pulses [1], which were generated in a laser cavity using two intracavity
4+
fused silica prisms to compensate the group delay dispersion (GDD) from the Cr :YAG crystal. It
is thought that third-order dispersion (TOD) limits the pulsewidth in this system [2].
4+
We have demonstrated the generation of 20 fs pulses from a Cr :YAG laser using doublechirped mirrors for dispersion compensation [3]. Double-chirped mirrors allow shorter pulse
widths to be generated by reducing higher-order dispersion in the laser cavity. A schematic of the
4+
laser cavity is shown in Figure 1. A 2 cm Cr :YAG crystal was placed inside a Z-fold cavity
designed to maximize Kerr-lens modelocking while simultaneously compensating for astigmatism.
The laser is optically pumped by a Nd:YVO4 laser emitting cw light at 1064 nm. Six round-trip
bounces off double-chirped mirrors (DCMs) are used to simultaneously compensate GDD and
4+
higher-order dispersion. A plot of the calculated Cr :YAG and net cavity dispersions are shown
in Figure 2. By purging the laser cavity with N2 gas to eliminate water absorption, it was possible
to generate optical pulses with a center wavelength of 1450 nm, and a bandwidth of 190 nm from
1310 to 1500 nm fwhm.
Kerr-lens modelocking was used to generate ultrashort pulses. A minimum pulsewidth of 19.5 fs
was determined by fitting an interferometric autocorrelation (shown in Figure 3) using a pulse
phase retrieval algorithm. The average power of the 110 MHz pulse train was 200 to 400 mW, for
9 W of absorbed pump. An example of the modelocked pulse spectrum is shown on both linear
and log scale in Figure 4. The spectrum has a full width at half maximum of 190 nm, extending
from 1310 to 1500 nm.
24-19
24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24
RLE Progress Report 144
Nd:YVO4
Laser - 1064nm
f = 10 cm
M3
M4
M1
M2
OC
2 cm Cr4+:YAG
4+
4+
Fig. 1. Schematic of the Cr :YAG laser Z-fold cavity. A 2 cm Cr :YAG crystal was surrounded
by two 10 cm radius-of-curvature folding mirrors (M1 and M2). One arm contains an output
coupler (OC), while the second arm contains two flat mirrors (M3 and M4). Pulses experience six
reflections off double-chirped mirrors (M1-M3) each roundtrip through the cavity for dispersion
compensation. The laser is optically pumped by a Nd:YVO4 laser emitting cw light at 1064 nm.
1500
4+
Cr :YAG
2
GDD (fs )
1000
500
0
4+
Cr :YAG and 6 DCM Bounces
-500
1300
1400
1500
1600
Wavelength (nm)
4+
Fig. 2. Plot of Cr :YAG and net cavity dispersion. The net cavity dispersion includes the
4+
dispersion from Cr :YAG and reflections from 6 double-chirped mirrors (DCMs).
24-20
24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24
RLE Progress Report 144
Autocorrelation
8
6
IAC
19.5 fs Fit
4
2
0
-75
-50
-25
0
25
50
75
Time Delay (fs)
4+
Fig. 3. Autocorrelation of ultrafast Cr :YAG laser pulses. The autocorrelation matches a fit
corresponding with a 19.5 fs pulse.
Intensity (Arb. Units)
1.0
-20
0.8
-30
0.6
-40
0.4
-50
0.2
-60
0.0
Intensity (dB)
-10
1.2
-70
1100 1200 1300 1400 1500 1600 1700
Wavelength (nm)
4+
Fig. 4. Spectrum from a Cr :YAG laser plotted on a linear and logarithmic scale. The pulse has
a spectral bandwidth of 190 nm spanning from 1310 to 1500 nm fwhm.
References:
[1] Y. P. Tong, P. M. W. French, J. R. Taylor, J. O. Fujimoto, "All-solid-state femtosecond
sources in the near infrared"; Opt. Comm., 136, 235-238 (1997).
24-21
24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24
RLE Progress Report 144
[2] Tatsuya Tomaru and Hrvoje Petek, "Effect of third-order dispersion on the phases of
4+
solitonlike Cr :YAG-laser pulses characterized by the second-harmonic generation frequencyresolved optical gating method"; J. Opt. Soc. Am. B, 18, 388-393 (2001).
[3] D. J. Ripin, C. Chudoba, J. T. Gopinath, J. G. Fujimoto, E. P. Ippen, U. Morgner, F. X. Kärtner,
4+
V. Scheuer, G. Angelow, and T. Tschudi, "Generation of 20-fs pulses by a prismless Cr :YAG
laser," Opt. Lett. 27, 61-63 (2002).
24-22
24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24
RLE Progress Report 144
Non-epitaxially grown semiconductor-doped silica films for laser modelocking
Sponsors
Air Force Office of Scientific Research (MFEL)
Grant F49620-01-1-0186
Air Force Office of Scientific Research
Grant F49620-98-01-0084
National Science Foundation
Grant ECS-019452
Project Staff
Rohit P. Prasankumar, Aurea Tucay, Dr. Thomas Schibli, Dr. Christian Chudoba, Dr. Ingmar
Hartl, Professor Michael Ruane and Paul Mak (Boston U), Dr. James N. Walpole and Leo J.
Misaggia, (MIT Lincoln Laboratory), Professor Franz X. Kärtner, Professor Erich P. Ippen, and
Professor James G. Fujimoto
The development of more reliable, compact, and inexpensive modelocked lasers is a major thrust
of current research. Semiconductor saturable absorbers are a common technology used to
generate self-starting, stable femtosecond pulses in solid state lasers. These devices are
typically fabricated by molecular beam epitaxy (MBE) and have been used for both saturable
absorber modelocking and initiation of Kerr lens modelocking (KLM) in many solid state laser
systems [1, 2]. However, they suffer from some disadvantages, such as lattice matching
constraints that limit the choice of semiconductor materials as well as reliance on a complicated,
expensive fabrication system.
In previous work, we developed non-epitaxially grown saturable absorber devices and applied
them to self-starting KLM in a Ti:Al2O3 laser [3]. The devices consist of InAs nanocrystallites
doped into SiO2 films in a 10%InAs/90%SiO2 ratio and deposited on sapphire substrates using a
non-magnetron radio frequency (RF) sputtering system. RF sputtering is an inexpensive, simple
device fabrication technique that offers flexibility in the choice of semiconductor dopant and
substrate materials. We found that rapid thermal annealing (RTA) from 500-750 °C was an
effective method of controlling the absorption saturation dynamics of our saturable absorbers.
The structural and optical properties were comprehensively characterized [4] and the devices
were used to initiate KLM in a Ti:Al2O3 laser. Self-starting 25 fs pulses were obtained with a
bandwidth of 53 nm and tuning range of 80 nm. The saturation fluence of these devices was
2
measured to be 25 mJ/cm , which is too high to enable saturable absorber modelocking without
KLM and also limits the minimum achievable pulsewidth.
Pump probe
10% InAs / 90% SiO2
-3
3x10
925 nm
-∆α/α
2
800 nm
750 nm
1
0
0
5
10
time (ps)
15
20
Figure 1. Tunable pump probe measurements between 750 and 900 nm revealing a decrease in
saturation fluence with wavelength. The magnitude of the signal is inversely proportional to the
saturation fluence.
24-23
24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24
RLE Progress Report 144
Recently, our focus in this work has been to lower the saturation fluence of these devices to
obtain shorter pulses and enable saturable absorber modelocking without KLM. We developed a
novel pump-probe system using a broadband 5.5 fs Ti:Al2O3 laser [5] to obtain 17 fs time
resolution and independent pump and probe wavelength tunability over a range of 700 to 1000
nm. Using this system, we characterized the nonlinear optical properties of our non-epitaxially
grown semiconductor-doped silica film saturable absorbers and discovered trends that aid in
device optimization [6]. The devices used in this study were fabricated with a magnetron RF
sputtering system, while varying growth parameters such as InAs/SiO2 ratio and substrate
temperature. Degenerate pump probe experiments at wavelengths between 750 and 925 nm
(Figure 1) and linear transmission measurements (Figure 2) indicate that operation closer to the
band edge and fabrication of devices with larger quantum dots results in lower saturation
fluences.
100
10% InAs / 90% SiO2
80
60
%T
40
40% InAs / 60% SiO2
20
0
500
1000
1500
2000
wavelength (nm)
2500
3000
Figure 2. Linear transmission measurements on semiconductor-doped silica films with different
InAs/SiO2 ratios, showing an increase in quantum dot size with increasing InAs/SiO2 ratio.
Using these guidelines, InAs-doped silica film saturable absorbers with an InAs/SiO2 ratio of
40% / 60 % were fabricated and their saturation fluence at 1.25 µm was measured from pump
2
probe experiments (Figure 9) to be 3.35 mJ/cm , which is a significant improvement over the
previous work in Ti:sapphire.
-3
3x10
40% InAs / 60% SiO2
2.5
1260 nm
2
-∆α/α
1.5
1
.5
800 nm
0
-0.5
0.0
0.5
1.0
time (ps)
1.5
2.0
Figure 3. Comparison of pump-probe measurements at 1260 and 800 nm on annealed 40%
InAs/60% SiO2 films.
We investigated the application of these devices to self-starting modelocking in a Cr:forsterite
laser operating at 1.25 µm. This laser produces 14 fs pulses in a standard 4 mirror configuration
[7], which are currently the shortest pulses ever generated at this wavelength. The cavity was
then modified to include an additional fold for focusing onto the saturable absorber in a
transmissive geometry. Using films with a 40%/60% InAs/SiO2 ratio that were designed to have
24-24
24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24
RLE Progress Report 144
1% absorption at 1.25 µm, self-starting KLM was obtained, with a bandwidth of 91 nm and
pulsewidth of 30 fs measured by interferometric autocorrelation (Figure 4). We measured the
modelocking buildup time in this system and found it to be approximately 2.5 ms, about 20 times
faster than in Ti:Sapphire; this can also be linked to the lower saturation fluence at this
wavelength. We were also able to obtain saturable absorber modelocking without KLM, although
a long background pulse coexisted with the shorter (~300 fs) modelocked pulse. We expect that
optimization of the dispersion and the use of devices with lower saturation fluence will overcome
this problem.
1
Spectral Intensity (norm.)
IAC Measurement (norm.)
8
7
6
5
4
3
2
1
0
-100
-50
0
50
100
Time Delay (fs)
FWHM 91 nm
0
1.1
1.2
1.3
1.4
1.5
1.6
Wavelength (µm)
Figure 4. Self-starting KLM in Cr:forsterite using 40%InAs/60% SiO2 semiconductor-doped silica
films.
References
1.
2.
3.
4.
5.
6.
7.
U. Keller, K. J. Weingarten, F. X. Kaertner, D. Kopf, B. Braun, I. D. Jung, R. Fluck, C.
Honninger, N. Matuschek, and J. A. D. Au, “Semiconductor saturable absorber mirrors
(SESAM's) for femtosecond to nanosecond pulse generation in solid-state lasers,” IEEE
Journal of Selected Topics in Quantum Electronics, 2: 435-453, 1996.
S. Tsuda, W. H. Knox, S. T. Cundiff, W. Y. Jan, and J. E. Cuningham, “Mode-locked
ultrafast solid-state lasers with saturable Bragg reflectors,” IEEE Journal of Selected
Topics in Quantum Electronics, 2: 454-464, 1996.
I. P. Bilinsky, B. E. Bouma, and J. G. Fujimoto, “Self-starting KLM Ti:Al2O3 laser using
semiconductor-doped glass structures,” Technical Digest. Summaries of Papers
Presented at the Conference on Lasers and Electro Optics. Conference Edition, 6: 333-4,
1998.
I. P. Bilinsky, R. P. Prasankumar, and J. G. Fujimoto, “Self-starting mode locking and
Kerr-lens mode locking of a Ti:Al2O3 laser by use of semiconductor-doped glass
structures,” Journal of the Optical Society of America B Optical Physics, 16: 546-9, 1999.
U. Morgner, F. X. Kartner, S. H. Cho, Y. Chen, H. A. Haus, J. G. Fujimoto, E. P. Ippen, V.
Scheuer, G. Angelow, and T. Tschudi, “Sub-two-cycle pulses from a Kerr-lens modelocked Ti:sapphire laser,” Optics Letters, 24: 411-413, 1999.
R. P. Prasankumar, I. Hartl, J. T. Gopinath, E. P. Ippen, J. G. Fujimoto, P. Mak, and M. F.
Ruane, “Ultrafast dynamics of non-epitaxially grown semiconductor-doped silica film
saturable absorbers,” presented at Quantum Electronics and Laser Science Conference,
Baltimore, MD, 2001.
C. Chudoba, J. G. Fujimoto, E. P. Ippen, H. A. Haus, U. Morgner, F. X. Kärtner, V.
Scheuer, G. Angelow, and T. Tschudi, “All-solid-state Cr:forsterite laser generating 14 fs
pulses at 1.3 um,” Optics Letters, 26: 292 - 294, 2001.
24-25
24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24
RLE Progress Report 144
Active Harmonically Modelocked Fiber Lasers
Sponsors
U.S. Air Force – Office of Scientific Research
Grant F49620-01-1-0084
DARPA - Defense Advanced Research Projects Agency
Grant F49620-96-01266
Project Staff
Matthew E. Grein, Leaf A. Jiang, Professor Erich Ippen, Professor Hermann A. Haus
Actively modelocked fiber lasers can generate streams of transform-limited picosecond pulses
locked to an external frequency reference at GHz repetition rates with low amplitude and timing
jitter. Such a source can potentially be used for optical sampling in precision, high-speed analogto-digital converters and as optical transmitters in a high speed time-division-multiplexed
transmission system. Much of the low-noise performance of fiber lasers—compared with
semiconductor lasers—arises due to the much larger intracavity pulse energy and larger signalto-noise ratio. The goal of this work has been to study the timing jitter in actively modelocked
fiber lasers. Pursuant to that goal, we have developed a theory for the timing jitter, identified the
characteristic retiming constants that govern the timing jitter for the case of amplitude (AM) and
phase (PM) modulation, developed a timing-jitter measurement scheme using a balanced
microwave homodyne detection scheme with high dynamic range, and built an actively
modelocked fiber laser that produces picosecond pulses at 10 GHz whose timing jitter is quantum
limited.
The active modelocking of fiber lasers is achieved with two types of modelockers: amplitude (AM)
and phase (PM) modulation, each yielding similar performance with respect to pulse shaping and
stability. We have found, however, the modulation strongly affects the noise characteristics in
qualitatively different ways[1-2]. The timing jitter is expressed as excitations (driven by noise) that
are damped by characteristic time constants that describe the laser’s dynamic response to
noise[3]. The time constants are related to the laser parameters, such as modulation depth,
filtering, and group-velocity dispersion (GVD), and are qualitatively different for the cases of AM
and PM. For the case of AM, the pulse timing is directly restored by the modulator, leading to an
exponential-type of timing recovery. In contrast, for the case of PM, pulse timing is not directly
restored. Mistimed pulses first experience a shift in frequency from the phase modulator. These
frequency shifts then convert to a timing shift through group-velocity dispersion. Upon successive
round trips, the accumulated frequency shifts are damped out by filtering. The characteristic
timing recovery resembles that of a damped, harmonic oscillator that can be underdamped,
overdamped, or critically damped depending on the relative strengths of the imposed frequency
shift by the modulator and GVD, and the filtering strength.
The fiber laser setup—shown in Fig. 1--is arranged in a sigma-type configuration in which the
linear portion is composed of non-polarization-maintaining elements. The amplifying medium is
an Er:Yb double-clad fiber side-pumped with a multimode 980 nm laser diode. The sigma laser
works as follows: a pulse exiting the polarizing beam splitter (PBS) from the ring depolarizes due
to environmentally-induced birefringence in the linear segment. A faraday rotator at the end of
the linear segment ensures that the backward-propagating pulse travels along the orthogonal
polarization axis with respect to the forward-traveling pulse. In this way, the polarization effects in
the forward and backward propagating directions are averaged out so that the pulse arrives at the
polarization beam splitter again with a linear polarization.
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24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24
RLE Progress Report 144
.
Figure 1: Sigma laser configuration. OSC is an external microwave frequency reference, G
microwave amplifier, F optical bandpass filter, HWP and QWP half- and quarter-wave plates,
PBS polarizing beam splitter, DSF and DCF dispersion-shifted and dispersion-compensating
fiber, EYDFA erbium-ytterbium co-doped fiber amplifier, FR faraday rotator, AS aspheric lens, MR
dielectric mirror.
The laser produces transform-limited, hyperbolic-secant pulses at 1.5 µm with repetition rates
upwards of 10 GHz with pulsewidths from 900 fs to 2 ps, depending on the optical filtering and
pump power. The suppression of supermodes in the RF spectrum is typically greater than 70
dB, indicative of excellent laser stability. A typical autocorrelation trace and microwave RF plot
are shown in Fig. 2. The laser is locked to the external microwave frequency reference by
Figure 2. Background-free autocorrelation trace showing a fit to an hyperbolic secant with a
pulsewidth of 1.55 ps, and RF spectrum of the directly-detected photocurrent, showing greater
than 70 dB of supermode suppression.
stabilizing the cavity length using a phase-locked loop (PLL) consisting of a microwave phase
detector, control electronics, and a fiber-wound piezoelectric transducer. The measurement of the
laser timing jitter is achieved using a residual phase-noise technique that is typically used to
compare the relative phase noise between two microwave frequency sources. The detected
laser signal (10 GHz) is compared with that of the external microwave frequency reference using
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24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24
RLE Progress Report 144
Figure 3: Timing jitter measurement scheme. C is a 50/50 polarization-independent directional,
PD 16 GHz photodiode, F microwave bandpass filter centered at 9.00 GHz, A microwave
amplifier, M double-balanced microwave mixer, PS microwave phase shifter, LO microwave
frequency reference.
homodyne phase detection, shown in Fig. 3. The laser pulses are first split in a directional
coupler. The pulses impinge on a photodetector (16 GHz bandwidth), and the harmonics of the
laser repetition rate are filtered from the photogenerated current, leaving only the photogenerated
current at the laser repetition rate. This signal is amplified and compared to the microwave
frequency reference using a double-balanced mixer. Because the microwave frequency
reference and laser repetition rate are locked using a PLL, the output of the mixer is a voltage
proportional to the phase difference between the microwave frequency reference and laser
repetition rate. This phase-error noise voltage is displayed using an fast-Fourier transform (FFT)
analyzer. By using two separate channels and electronically cross-correlating them using the
FFT dual signal analyzer, we were able to reduce the measurement noise floor by an additional
15 dB[4]. The spectrum of the phase noise can be related to the spectrum of the timing jitter.
The resulting phase-noise spectrum L(f) is shown in Fig. 4 for the case where the
Figure 4: Timing jitter spectrum for the case of mostly AM. Upper solid curve, data; lower solid
curve, measurement noise floor; dotted curve, theory.
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24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24
RLE Progress Report 144
modulation type was set for mostly AM. The phase noise of the LO (Poseidon Shoe-Box
Oscillator) was measured in a separate measurement (-112 dBc/Hz @ 100 Hz, -141 dBc/Hz @ 1
kHz, -161 dBc/Hz @ 10 kHz, -172 dBc/Hz @ 100 kHz) and is lower than the laser noise. The
many sharp spurs from 10 Hz to 10 kHz and 70 kHz and 90 kHz are due to environmental
electrical interference. The peak near 20 kHz is the laser relaxation oscillation and contributes
mostly amplitude noise[8]. The knee near 2.5 kHz is predicted from the theory (also shown in Fig.
4), after which the spectrum falls off by approximately 30 dB per decade. The supermodes at
harmonics of 483 kHz have the same spectrum as the baseband mode. The noise floor in the
data that is above the measurement noise floor near –150 dBc is due to the overlapping
supermode tails, as shown in comparison with the theoretical curve that includes the overlapping
supermode tails.
Fig. 5 shows the case where the laser is modulated with an increased amount of PM. The laser
retiming is underdamped, resulting in a characteristic oscillatory peak near 4 kHz and a rolloff of
approximately 40 dB per decade for f >4 kHz, as expected from theory. The root-mean-square
timing jitter is given by
σ=
1
2πf m
2∫ L( f )df
where fm is the 9.0 GHz pulse repetition rate frequency. The jitter integrated from 10 Hz to 241.5
kHz is 9.66 fs for Fig. 4 and 80.0 fs for Fig. 5.
From the theoretical model, we have determined that for AM, the quantum-limited timing jitter is
proportional to the square of GVD and inversely with the modulation slope. The minimum
quantum-limited timing jitter is achieved using a vanishing GVD and strong optical filtering to
reduce the Gordon-Haus contributions to the timing jitter. For PM, the minimum jitter requires an
optimum dispersion because large dispersion increases the timing recovery from noise that
comes from timing fluctuations, but dispersion also increases the noise that comes from
frequency shifts via the Gordon-Haus effect. It turns out that the quantum-limited timing jitter
grows linearly with GVD for large GVD and inversely proportional to GVD for small GVD. In both
cases, the timing jitter is proportional to the internal ratio of signal photons to ASE photons.
Figure 5: Timing jitter spectrum for the case of mostly PM. Upper solid curve, data; lower solid
curve, measurement noise floor; dotted curve, theory.
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RLE Progress Report 144
References
1. M. E. Grein, L. A. Jiang, Y. Chen, H A. Haus, and E. P. Ippen, “Timing restoration
dynamics in an actively mode-locked fiber ring laser”, Opt. Lett. 24(23): 1687-1689
(1999).
2. M. E. Grein, L. A. Jiang, Y. Chen, H A. Haus, and E. P. Ippen, “A study of the of the
dynamics governing timing restoration in the actively modelocked soliton laser”, in
Conference on Lasers and Electro-Optics, OSA Technical Digest (Optical Society of
America, Washington DC, 1999), p. 100.
3. H. A. Haus and A. Mecozzi, “Noise of modelocked lasers”, IEEE J. Quantum Electron.
29(3): 983-996 (1993).
4. W. F. Walls, “Cross-correlation phase noise measurements,” In IEEE Frequency Control
Symposium, (Institute of Electrical and Electronics Engineers, New York, 1992), pp. 257260.
Publications
M. E. Grein, L. A. Jiang, H. A. Haus, E. P. Ippen, C. McNeilage, J. H. Searls, and R. S.
Windeler, “Observation of quantum-limited timing jitter in an active, harmonically
modelocked fiber laser”, submitted to Opt. Lett.
M. E. Grein, H. A. Haus, Y. Chen, and E. P. Ippen, “Timing jitter in actively modelocked
fiber lasers”, to be submitted to IEEE J. Quantum. Electron.
Conference Papers
M. E. Grein, H. A. Haus, E. P. Ippen, and Y. Chen, “The quantum limit of timing jitter in
actively mode-locked soliton fiber lasers”, in OSA Trends in Optics and Photonics (TOPS)
Vol. 56, Conference on Lasers and Electro-Optics (CLEO 2001), pp. 243-244.
M. E. Grein, L. A. Jiang, H. A. Haus, and E. P. Ippen, “Timing jitter in modelocked lasers,”
invited talk presented at the IEEE Lasers and Electro Optics Society Annual Meeting,
November 12-14, San Diego CA, USA, paper MWP.
J. J. Hargreaves, P. W. Juodawlkis, J. J. Plant, J. P. Donnelly, J. C. Twichell, F. Rana, M.
E. Grein, R. J. Ram, E. P. Ippen, “Timing jitter in modelocked lasers,” invited talk
presented at the IEEE Lasers and Electro Optics Society Annual Meeting, November 1214, San Diego CA, USA, paper MWQ.
Reports
H. A. Haus and M. E. Grein, “Quantum limit on timing jitter of actively mode-locked lasers,”
internal Optics and Quantum Electronics Memo No. 92 (1999).
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RLE Progress Report 144
High-Repetition-Rate Fiber Ring Laser Passively Modelocked with a
Saturable Absorber Mirror
Sponsors
U.S. Air Force – Office of Scientific Research
Grant F49620-01-1-0084
Japanese Science and Technology Agency
Project Staff
Dr. K. S. Abedin, J. T. Gopinath, M. E. Grein, Professor L. A. Kolodziejski, and Professor E. P.
Ippen
Recently, there has been significant interest in increasing the repetition rate of passively modelocked erbium fiber lasers for applications such as high-speed optical communication and
precision optical sampling. Mode-locked pulses at fundamental cavity repetition rates of 300 MHz
have been generated from erbium/ytterbium fiber lasers with short cavity lengths [1]. In addition,
harmonic operation of such lasers provided pulses with GHz repetition rates [1]. Recently, Er/Yb
codoped phosphate glass waveguides, which exhibit higher pump absorption at ~980 nm and
higher gain near 1.53 µm, have also been used in high repetition rate lasers [2].
Passively mode-locked fiber laser cavities contain, in addition to the gain media (Er-doped fiber),
a section of undoped single mode fiber (SMF), with anomalous dispersion at 1.5 µm. This SMF,
which is typically 2~4 times longer than the doped fiber, is intended to compensate the positive
dispersion of the doped fiber and to initiate nonlinear pulse shaping. If the undoped fiber can be
replaced with a doped anomalous dispersion fiber, it would be possible to increase the total gain
and saturation energy of the laser. We have demonstrated a compact ring laser, consisting of
erbium-doped fiber sections with both normal and anomalous dispersion, that is mode-locked with
the aid of semiconductor saturable absorber mirror (SESAM). The self-starting laser produces a
mode-locked pulse train with a fundamental cavity repetition rate of 140 MHz and average output
power as high as 5.3 mW.
Pump
Output
EDF1
(+0.075 ps2/m)
11 ~ 130 cm
λ/4
λ/4 λ/4
Dichroic
Mirror
PBS
SESAM
Isolator
λ/4
λ/2
SPLICE
Splice
EDF2
(-0.011 ps2/m)
0 ~ 96 cm
Figure 1: Schematic diagram of the erbium fiber ring laser and the structure of the SESAM.
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The experimental setup is shown in Fig. 1. The gain section consists of a section of high-gain
2
erbium-doped fiber having normal dispersion (EDF1, β 2 : +75 ± 10 ps /km), and a section of
2
erbium doped fiber with anomalous dispersion (EDF2, β 2 : –10.8 ps /km). The SESAM (Ref. 3)
was incorporated in the ring cavity by using a polarization beam splitter and a quarter-wave plate.
-30
Intensity (dB)
Intensity (a.u.)
1
0.8
-60
0.6
-90
0
100
200
300
RF Frequency (MHz)
0.4
0.2
0
-10
2a
-5
0
Delay (ps)
5
10
-5
0.08
140.6 MHz
Chirp Parameter
-6
Gain
-7
0.04
108.5 MHz
-8
g
d
a
|σ |, σ , σ
Dispersion
98.3 MHz
0.02
0
Chirp Parameter, C
Saturable Absorber
0.06
-9
0
2b
0.02
0.04
0.06
0.08
Averaged Dispersion (ps*ps/km)
-10
0.1
Figure 2: (a) Autocorrelation trace showing the compressed output pulse. Inset shows the RF
spectrum. (b) Plot showing the contributions of GVD, gain dispersion and saturable absorber
action to pulse broadening /shortening at different average dispersion.
gain dispersion;
dispersion: σ d .
σa :
σg :
shortening due to
shortening due to saturable absorber; broadening due to cavity
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24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24
RLE Progress Report 144
The pulse width, spectral width, and time-bandwidth product were 5.9 ps, 4.6 nm, and 3.5,
respectively. The chirped high-energy output pulses from the laser could be compressed to 450
fs by using a 400 m long large effective-area fiber (LEAF). The autocorrelation trace of the
compressed pulse output is shown in Fig. 2(a), and in the inset RF spectrum is plotted.
According to the theory that describes passively mode-locked lasers, the pulse envelope for such
. For such pulses, the balance
systems can be represented as a(t ) = Ao sech( t / τ )
between the pulse broadening and shortening can be described by [4]
(1+ jC )
g
Ωτ
2 2
g o
(2 − C ) − 3C2τβ ′′
2
cav
2
o
= σa
(1)
Here, g is the saturated gain per pass,
the total GVD of the cavity, and
and second terms, defined as
σg
σa
Ω g is the gain linewidth, C is the chirp parameter, β cav is
"
is the modulation index due to saturable absorber. The first
and
σ d , represent the pulse broadening due to gain dispersion
and GVD of fiber, while the term on the right side represents the shortening due to the SESAM.
Since it was difficult in our cavity to measure the contribution of the absorber, we estimated it by
solving Eq. 1. Figure 2b plots the contribution of GVD, gain dispersion, absorber and the chirp
parameter for different values of average cavity dispersion. Since the laser was operated with C
>> 2 , the gain dispersion plays the role of pulse shortening rather than the broadening typically
observed in a laser with small anomalous cavity dispersion. The combined effect of pulse
shortening due to the saturable absorber and gain dispersion was counterbalanced by the
broadening due to GVD.
References
1
2
3
4
B.C. Collings, K. Bergman, S.T. Cundiff, S. Tsuda, J.N. Kutz, J.E. Cunningham, W.Y. Jan, M.
Koch, and W.H. Knox, “Short cavity erbium/Ytterbium fiber lasers mode-locked with a
saturable Bragg reflector,” IEEE J. Sel. Topics Quantum Electron. 3: 1065-1075 (1997).
E. R. Thoen, E.M. Koontz, D.J. Jones, D. Barbier, F.X. Kartner, E.P. Ippen, L.A. Kolodziejski,
“Erbium-Ytterbium waveguide laser mode-locked with a semiconductor saturable absorber
mirror,” Photon. Technol. Lett. 12: 149-151 ( 2000).
E.R. Thoen, E.M. Koontz, M. Joschko, P. Langlois, T.R. Schibli, F.X. Kartner, E.P. Ippen and
L.A. Kolodziejski, “Two-photon absorption in semiconductor saturable absorber mirrors,”
Appl. Phys. Lett. 74, 3927-3929 (1999).
J.N. Kutz, B.C. Collings, K. Bergman, S. Tsuda, S.T. Cundiff, W.H. Knox, P. Holmes, and M.
Weinstein, “Mode-locking pulse dynamics in a fiber laser with a saturable Bragg reflector,”
Opt. Soc. Am. B. 14: 2681-2690 (1997).
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RLE Progress Report 144
Self-Stabilized Harmonic Passively Modelocked Stretched-Pulse Erbium
Fiber Ring Laser
Sponsors:
U.S. Air Force – Office of Scientific Research
Grant F49620-01-1-0084
Japanese Science and Technology Agency
Project staff:
Dr. K. S. Abedin, J. T. Gopinath, L. A. Jiang, M. E. Grein, Professor H. A. Haus, and Professor E.
P. Ippen
Multiple pulses generated in a soliton fiber laser cavity due to energy quantization effects are
often randomly spaced. Grudinin et al. has reported [1] a means of self-stabilizing a harmonic
passively mode-locked soliton fiber laser through the electrostrictive interaction between the
successive pulses. In the anomalous dispersion regime, the electrostrictive interaction between
the pulses in the cavity can lead to repulsive forces, thus producing a periodic harmonic pulse
train. In this work, we demonstrate self-stabilized harmonic operation of a stretched pulse fiber
laser [2]. Highly periodic harmonic pulses with a repetition rate as large as 220 MHz and with an
average power of 80 mW have been achieved. These harmonic pulses organize themselves by
repelling each other in a manner similar to that observed in soliton fiber lasers. The pulses have
picosecond jitter, supermode noise suppression of > 75 dB, and could be compressed to ~125 fs
by external chirp compensation.
1.8m Erbium-Doped
Fiber
4.4 m SMF-28
Fiber
PMT
WDM
Coupler
Birefringent
PBS
Plate
λ/4 λ/4
MOPA
Collimator
Isolator λ/4 λ/2
SMF-28
Fiber
Lock-In
Amplifier
Corner
Cube
Nonlinear
Crystal
Chopper
Corner
Cube
Corner
Cube
Fig. 1. Experimental setup.
The schematic diagram of the experimental setup is shown in Fig. 1. Highly doped Er fiber (1.77
m), is used as a gain medium. The large normal dispersion of this gain fiber is partially
compensated with 4.35 m of standard single mode fiber, producing a total cavity dispersion of
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24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24
RLE Progress Report 144
2
0.036 ps . The total length was 6.55 m and the fundamental cavity repetition rate was 31.52 MHz.
By adjusting the waveplates, Gaussian-like pulses with widths varying between 1.7-1.9 ps and
average output powers as high as ~96 mW were produced. For particular settings of the
waveplates, it was possible to produce multiple uniformly-spaced pulses. As the waveplates
were adjusted initially, the high energy pulse splits into a group of two to seven pulses with
random spacing. Next, the pulses uniformly distribute themselves over the round-trip period in a
time scale of 15 sec to about a minute. Stable operation, with lower order cavity harmonics
suppressed by >70 dB (repetition rates of 157.6 MHz, N = 5) and >50 dB (repetition rate of 220.6
MHz, N = 7) can be achieved, as shown in Fig. 2. These states continued for many hours without
further adjustment. For a pump power of 550 mW, output powers as much as 82 mW, as shown
in Fig. 2(c), were obtained (7th harmonic operation) at a repetition rate of 221 MHz. Chirped
pulses, with widths of ~1.7 ps and spectral width of 50 nm, were obtained from the laser. The
pulses were compressed with 2.5-3 m of standard single mode fiber. The minimum pulse width
obtained was 125 fs, and the time-bandwidth product 0.77.
-20
N=5
100
-60
-80
Output Power (mW)
80
-100
(a)
-120
-20
N=7
Intensity (dBm)
-40
-60
6
Output Power (mW)
5
60
4
3
40
2
20
1
-80
0
-120
0
0
-100
(b)
7
Harmonic Number (N)
0
50
100
150
RF Frequency (MHz)
200
250
(c)
Harmonic Number (N)
Intensity (dBm)
-40
100
200
300
400
500
600
Pump Power (mW)
Fig. 2 Laser output. (a) RF spectra of for laser operating at the 5th and 7th harmonic of the
fundamental cavity repetition rate. (b) output power and harmonic number as a function of
launched pump power.
To measure the noise of the harmonically mode-locked laser, we performed crosscorrelations
between successive pulses in the train, delaying one arm by the pulse period (shown in Fig. 1).
The envelope of the crosscorrelation trace was irregular, and had a typical width of about ~2 ps.
From the scan speed of ~120 fs/s and an average separation of 50 fs between successive peaks,
we conclude that the distance between successive pulses in harmonic operation varies on the
order of 0.5 s. This slow change suggests that it takes a relatively "long" time for the laser to
settle after perturbation, an indication that the repulsive force leading to self-stabilization is rather
weak.
This weak force can result either from dynamic saturation of the gain caused by the high energy
pulses [3] or the electrostrictive interaction as seen in soliton lasers. The transfer function of
electrostrictive response of fiber contains a number of peaks or resonants as a result of reflection
from the cladding-coating boundary of the fiber. This transfer function also exhibits anti-resonants
in which the real component changes its sign [4]. A periodic pulse train whose repetition rate
approaches these anti-resonant points would experience electrostrictive phase modulation, with
curvature opposite to what it assumes for resonant frequencies. In a stretched pulse fiber laser
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24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24
RLE Progress Report 144
with net normal dispersion, self-stabilization is expected to occur when the repetition rate
becomes close to one of the anti-resonant frequencies. The frequency shift resulting from the
electrostrictive effect and the group velocity dispersion will help to stabilize the pulse train.
Investigations to further study this mechanism are now in progress.
References
5
6
7
8
A.B. Grudinin, D.J. Richardson, D.N. Payne, "Passive harmonic modelocking in soliton fiber
lasers", J. Opt. Soc. Am. B 14: 144-153 (1997).
K. Tamura, E.P. Ippen, H.A. Haus and L.E. Nelson, "77-fs pulse generation from a stretchedpulse mode-locked all-fiber ring laser", Opt. Lett. 18: 1080-1082 (1993).
J.N. Kutz, B.C. Collings, K. Bergman and W.H. Knox, "Stabilized pulse spacing in soliton
lasers due to gain depletion and recovery, IEEE J. Quantum Electron. 34: 1749-1757 (1998).
A. Fellegara, S. Wabnitz, "Electrostrictive cross-phase modulation of periodic pulse trains in
optical fibers", Opt. Lett. 23: 1357-1359 (1998).
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RLE Progress Report 144
Noise in Harmonically Modelocked Lasers
Sponsor
U.S. Air Force – Office of Scientific Research
Grant F49620-01-1-0084
DARPA - Defense Advanced Research Projects Agency
Grant F49620-96-01266
Project Staff
Leaf A. Jiang, Mathew E. Grein, Farhan Rana, John M. Fini, Professor Rajeev J. Ram, Professor
Erich P. Ippen, Professor Herman A. Haus
The noise of modelocked lasers is often measured by converting the optical signal to a
microwave signal and then comparing its phase noise to a quiet local oscillator. The
interpretation of the resulting noise spectrum, which is a combination of both amplitude and timing
noise, follows the work of von der Linde [1]. This model was developed for fundamentally
modelocked lasers and does not directly apply to the complicated correlations that exist in
harmonically modelocked lasers. By using von der Linde’s noise model, researchers have
underestimated the noise of their harmonically modelocked lasers [2-4] and hence mistakenly
report extremely low timing jitters.
A model for amplitude and timing noise in harmonically modelocked lasers was developed for the
case that the pulses in the cavity are uncorrelated. We show that the power spectral density of
the first cavity axial mode is dominated by amplitude noise, which allows us to distinguish
amplitude and timing fluctuations with a smaller electronic detection bandwidth. Our model
predicts that increasing the cavity length while keeping the same gain and loss per round-trip
changes the frequency content of the timing jitter but does not change the integrated value. In
addition, we clarify how to interpret the spectrum from residual phase noise measurements and
RF analyzer measurements.
Models for the noise in gain-switched lasers [5,6] and fundamentally modelocked lasers [1,7] are
well known, but there is currently no treatment for patterning effects, which are inherent in
harmonically modelocked lasers. These two cases cover two extremes: in the case of gainswitched lasers, each pulse independently builds up from ASE and therefore the pulse-to-pulse
timing jitter is mainly independent and uncorrelated. In the case of fundamentally modelocked
lasers, the same pulse recirculates through the cavity and hence the pulse-to-pulse jitter is highly
correlated. In a harmonically modelocked laser, there is a mix of these two effects. Just like a
gain-switched laser, all M pulses in the cavity at any time instant are independent of each other
since they all arise separately from spontaneous emission. On the other hand, similar to a
fundamentally modelocked laser, the pulses recirculate in the laser cavity and hence the output is
correlated, e.g. pulse 1 is correlated with pulse M+1, but pulse 1 and 2 are not strongly
correlated. The assumption that all M pulses in the cavity are independent is a good
approximation as long as the gain dynamics do not cause pulse-to-pulse correlations, i.e. the gain
recovers between pulses (semiconductor laser) or is not significantly affected by a single pulse
(Erbium laser). The case in which neighboring pulses are highly correlated has also been
investigated [8].
References
[1] D. von der Linde, “Characterization of noise in continuously operating mode-locked lasers,”
Appl. Phys. B. 39: 201-217 (1986).
[2] W. Ng, R. Stephens, D. Persechini and K. V. Reddy, “Ultra-low jitter modelocking of Er-fibre
laser at 10 GHz and its application in photonic sampling for analogue-to-digital conversion,”
Electron. Lett. 37(2): 113-115 (2001).
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24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24
RLE Progress Report 144
[3] T. R. Clark, T. F. Carruthers, P. J. Matthews, and I. N. Duling III, “Phase noise measurements
of ultrastable 10 GHz harmonically modelocked fibre laser, “ Electron. Lett 35(9):720-721 (1999).
[4] C. M. DePriest, P. J. Delfyett, J. H. Abeles, and A. Braun, “Low noise external-cavity
semiconductor diode ring laser actively modelocked at 10 GHz,” Proceedings of the Conference
on Ultrafast Electronics and Optoelectronics, Lake Tahoe, Nevada, Optical Society of America,
January, 2001.
[5] D. A. Leep and D. A. Holm, “Spectral measurement of timing jitter in gain-switched
semiconductor lasers,” Appl. Phys. Lett. 60(20):2451-2453 (1992).
[6] M. Jinno, “Correlated and uncorrelated timing jitter in gain-switched laser diodes,” IEEE
Photon. Tech. Lett. 5(10):1140-1143 (1993).
[7] U. Keller, K. D. Li, M. Rodwell, and D. M. Bloom, “Noise characterization of femtosecond fiber
raman soliton lasers,” IEEE. J. Quantum Electron. 25(3): 280-288 (1989).
[8] F. Rana, H. L. T. Lee, M. E. Grein, L. A. Jiang, R. J. Ram, E. P. Ippen, and H. A. Haus,
“Characterization of the noise and correlations in harmonically mode-locked lasers,” submitted to
J. Opt. Soc. Am. B.
Publications
L. A. Jiang, M. E. Grein, J. M. Fini, E. P. Ippen and H. A. Haus, “Noise of harmonically
modelocked lasers,” presented at the Gordon Research Conference on Nonlinear Optics and
Lasers, Colby-Sawyer College, New London, New Hampshire, July 29 - August 3, 2001.
F. Rana, H. L. T. Lee, M. E. Grein, L. A. Jiang, R. J. Ram, E. P. Ippen, and H. A. Haus,
“Characterization of the noise and correlations in harmonically mode-locked lasers,” submitted to
J. Opt. Soc. Am. B.
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RLE Progress Report 144
Timing Jitter Reduction in Modelocked Semiconductor Lasers with Photon
Seeding
Sponsor
DARPA - Defense Advanced Research Projects Agency
Grant F49620-96-01266
Project Staff
Leaf A. Jiang, Dr. Kazi S. Abedin, Mathew E. Grein, Professor Erich P. Ippen, Professor Herman
A. Haus
Reflecting a small fraction of the output light of a laser back into the cavity is called photon
seeding and has been shown to reduce timing jitter in modelocked semiconductor lasers [1,2] and
gain-switched semiconductor lasers [3]. Reduction of timing jitter is crucial for high-speed optical
sampling systems [4] in which it is important to have both short pulses and low-timing jitter. These
two requirements on optical sampling streams are difficult to achieve simultaneously [5]. In this
work, we demonstrate improved noise performance though coherent photon seeding (CPS)
without significant pulse broadening.
Improved timing jitter performance from CPS has been attributed to the suppression of noise
initially located just ahead of the pulse. This noise migrates into the pulse and causes severe
perturbations of the pulse profile due to the random phase and amplitude of the spontaneous
emission [6]. Numerical simulations have shown that the pulses suffer large scale perturbations
originating from the weak spontaneous emission noise background and that the power of the
seeding pulse should be large enough to swamp the noise background, but small enough so as
to not affect the pulse forming dynamics of the laser [7].
Fig. 1. Experimental setup for photon seeding.
The experimental setup for photon seeding with a modelocked semiconductor laser is shown in
Fig. 1. The output pulses of the laser are 2.5 ps wide with a carrier wavelength of 1.5 µm. Less
than one-thousandth of the photons are reflected back into the laser cavity by tapping 50% of the
light with a beam-splitter, attenuating the signal and then reflecting the photons back into the
laser cavity with gold mirror mounted on a micrometer stage. The number, delay, and
polarization of the returning photons were optimized to obtain the best noise performance. To
monitor the performance of the pulses as well as the quality of the laser pulses, the optical
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24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24
RLE Progress Report 144
intensity autocorrelation, optical spectrum, and RF spectrum of the optical pulses were measured
simultaneously. Fig. 2 shows the RF spectrum of the detected laser output as a function of delay
between the main and seed pulses. The RF noise skirts decrease as the optimal delay is
approached. The inset plots show the corresponding intensity autocorrelations and optical
spectra. These plots are almost invariant with respect to delay. This indicates that the pulse
shape is not significantly changed with photon seeding and that the noise performance of the
laser can be improved without degrading the output pulses.
2
-50
-70
-4 -2 0 2 4
Delay (ps)
1552
0
-30
1548
-60
4
-10
1544
Power (dBm)
-50
Power (dBm)
-40
6
1540
SHG Signal (a.u.)
-30
Wavelength (nm)
-70
-80
-90
-100
Noise Floor
RBW: 10 kHz
-4.0M
-2.0M
0.0
2.0M
4.0M
Frequency Offset (Hz)
Fig.2. The RF spectra, intensity autocorrelations (left inset), and optical spectra (right inset) of
the modelocked laser. The RF spectra are centered at the second harmonic (18 GHz). The RF
carrier power at the second harmonic was measured separately using a wider resolution
bandwidth and was -15.50 to -17.00 dBm depending on the delay. The gray line is the noise floor
of the RF measurement.
The timing jitter of a hybridly modelocked semiconductor laser was reduced from 0.75 to 0.22 ps
using CPS. There was no penalty to the pulse width of the laser indicating that CPS may be
useful in obtaining short pulses and low noise simultaneously.
References
[1] H. Kawaguchi and K. S. Abedin, “Coherent photon seeding of actively mode locked laser
diodes,” Appl. Phys. Lett. 62(18): 2164-2166 (1993).
[2] P. Langlois, D. Gay, N. McCarthy, and M. Piché, “Noise reduction in a mode-locked
semiconductor laser by coherent photon seeding,” Opt. Lett. 23(2): 114-116 (1998).
[3] M. Schell, W. Utz, D. Huhse, J. Kässner, and D. Bimberg, “Low jitter single-mode pulse
generation by a self-seeded, gain-switched Fabry-Perot semiconductor laser,“ Appl. Phys. Lett.
65(24): 3045-3047 (1994).
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24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24
RLE Progress Report 144
[4] P. W. Juodawlkis, J. C. Twichell, G. E. Betts, J. J. Hargreaves, R. D. Younger, J. L.
Wasserman, F. J. O’Donnell, K. G. Ray, and R. C. Williamson, “Optically sampled analog-todigital converters,” IEEE Trans. Microwave Theory Tech. 49(10): 1840-1853 (2001).
[5] L. A. Jiang, M. E. Grein, E. P. Ippen, C. McNeilage, J. Searls, and H. Yokoyama, “Quantumlimited noise performance of a mode-locked laser diode,“ Opt. Lett 27(1):49-51 (2002).
[6] P. Beaud, J. Q. Bi, W. Hodel, and H. P. Weber, “Experimental observation of the selfstabilization of a synchronously pumped dye laser,” Opt. Commun. 80(1): 31-36 (1990).
[7] G. H. C. New, ”Self-stabilization of synchronously mode-locked lasers,” Opt. Lett. 15(22):
1306-1308 (1990).
Publications
L. A. Jiang, K. S. Abedin, M. E. Grein, and E. P. Ippen, “Timing jitter reduction in modelocked
semiconductor lasers with photon seeding,” Appl. Phys. Lett., forthcoming.
24-41
24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24
RLE Progress Report 144
Quantum-Limited Noise Performance of a Semiconductor Modelocked
Laser
Sponsor
U.S. Air Force – Office of Scientific Research
Grant F49620-01-1-0084
DARPA - Defense Advanced Research Projects Agency
Grant F49620-96-01266
Project Staff
Leaf A. Jiang, Mathew E. Grein, Professor Erich P. Ippen, Professor Herman A. Haus, Cameron
*
*
**
*
Poseidon Scientific Instruments, Unit 1, 95
McNeilage , Jesse Searls , Hiroyuki Yokoyama
Queen Victoria Street, Freemantle Western Australia 6160 **NEC Opto-Electronics Basic
Research Lab, NEC Corporation 34 Miyukigaoka, Tsukuba Ibaraki 305-8501 Japan
Quantum-limited noise performance of a semiconductor modelocked laser was demonstrated.
The integrated timing jitter of the 6.7 ps output pulses was only 47 fs (10 Hz to 10 MHz) and 86 fs
4
(10 Hz to 4.5 GHz), which is the best-reported figure of merit (pulsewidth x timing jitter) for
semiconductor modelocked lasers. Record performance was achieved by using a narrow
intracavity optical filter and an ultra-low noise sapphire crystal microwave oscillator at 9 GHz.
High-speed optical sampling systems [1] and optical time-division multiplexed transmission
(OTDM) systems [2,3] have stringent timing jitter requirements. Timing jitter less than 500 fs is
required for OTDM transmission at 160 Gb/s (timing jitter should be less than 10% of bit period).
An optical source with less than 50 fs of timing jitter (10 Hz to 5 GHz) would be needed for a
sampler with 8-bit quantization at a sampling rate of 10 GS/s.
-80
Vector Signal Analyzer Data
L (f) dBc/Hz
-90
Spectrum Analyzer Data
-100
-110
-120
-130
-140
-150
Vector Signal
Analyzer
Noise Floor
-160
10
100
1k
Spectrum Analyzer Noise Floor
10k 100k
1M
10M 100M 1G
Offset Frequency (Hz)
Fig.1. Single-sideband phase noise of the hybridly modelocked laser diode and corresponding
noise floor.
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24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24
RLE Progress Report 144
The timing jitter of the modelocked laser was measured with a residual phase noise technique [4]
and the resulting single-sided phase noise is shown in Fig. 1. Spurs due to 60 Hz and harmonics
thereof are apparent. Vibration spurs at harmonics of 20 Hz and 55 Hz due to AC fans and other
ambient vibrations are also visible. At higher offsets, radio stations pickups appear in the 10-100
MHz decade. A spur due to wireless telephones is visible in the 100 MHz to 1 GHz decade. In
addition, contributions of noise due to the fast gain dynamics in the semiconductor laser at the
relaxation oscillation frequency are also evident in this decade [5]. In the 100 kHz to 1 MHz
decade, the spurs in the laser noise are due to the switching power supply in the current source.
For offsets less than 1 kHz, the laser noise increases by approximately 13 dB/decade. A direct
measurement of the noise of the voltage supply to the saturable absorber and current source
reveals an increase by 13 dB/decade for offsets less than 1 kHz, which indicates that the lowfrequency noise is due to the current source driver and voltage supply. For offsets greater than 5
kHz, the noise reduces to that produced by the fundamental amplified spontaneous emission
(ASE) quantum noise. Since the noise less than 5 kHz contributes a small fraction of the
integrated timing jitter, the predominant source of the total value is ASE. Fig. 4 shows the
integrated timing jitter in each decade. The timing jitter from 10 Hz to 4.5 GHz is 86 fs, or 154 fs if
all spurs are included.
With an integrated timing jitter of 86 fs, it is important to have a quiet oscillator drive these lasers
since the oscillator can easily be the dominant noise source. The SBO used in these
measurements had 5.6 fs of timing jitter (10 Hz to 10 MHz), which allows quantum-limited noise
performance of the MLLD. Quantum-limited noise performance means that the noise is
dominated by spontaneous emission noise and not the microwave oscillator noise.
References
[1] P. W. Juodawlkis, J. C. Twichell, G. E. Betts, J. J. Hargreaves, R. D. Younger, J. L.
Wasserman, F. J. O’Donnell, K. G. Ray, and R. C. Williamson, “Optically sampled analog-todigital converters,” IEEE Trans. Microwave Theory Tech. 49(10): 1840-1853 (2001).
[2] H. Yokoyama, “Highly stabilized mode-locked semiconductor diode lasers,''
Review of Laser Engineering 27: 750--755 (1999).
[3] U. Feiste, R. Ludwig, C. Schubert, J. Berger, C. Schmidt, H.-G. Weber, B. Schmauss, A.
Munk, B. Buchold, D. Briggmann, F. Kueppers, and F. Rumpf, “160 Gbit/s transmission
over 116 km field-installed fibre using 160 Gbit/s OTDM and 40 Gbit/s ETDM,'' Electron. Lett.
37(7): 443--445 (2001).
[4] D. J. Derickson, A. Mar, and J. E. Bowers, “Residual and absolute timing jitter in actively
mode-locked semiconductor lasers,'' Electron. Lett. 26(24):2026-2028 (1990).
[5] L. A. Jiang, M. E. Grein, H. A. Haus, and E. P. Ippen, “Noise of mode-locked semiconductor
lasers," IEEE J. Select. Topics Quantum Electron. 7(2): 159-167 (2001).
Publication
L. A. Jiang, M. E. Grein, E. P. Ippen, C. McNeilage, J. Searls, and H. Yokoyama, “Quantumlimited noise performance of a mode-locked laser diode,“ Opt. Lett 27(1):49-51 (2002).
24-43
24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24
RLE Progress Report 144
Experimental Demonstration of a Timing Jitter Eater
Sponsor
DARPA - Defense Advanced Research Projects Agency
Grant F49620-96-01266
Project Staff
Leaf A. Jiang, Mathew E. Grein, Professor Erich P. Ippen, Professor Herman A. Haus
The timing jitter of a pulse train can be reduced with dispersive fiber and a phase modulator. We
experimentally demonstrate the reduction of the timing jitter of a 10 GHz modelocked
semiconductor laser by 60%.
Timing jitter can severely limit system performance in fiber-optic communications [1] and highspeed optical sampling [2]. Recently, we proposed that the timing jitter could be reduced at the
expense of frequency noise using group velocity dispersion and phase modulation [3]. Here, we
report on experimental results.
Fig. 1. Experimental setup of the timing jitter eater.
The setup used to reduce the timing jitter of a modelocked laser is shown in Fig. 1. A train of
2
2
pulses with a given amount timing jitter <∆t > and frequency noise <∆f > is shown at the top of
the figure. The pulses are then sent through pre-chirp fiber and then through a phase modulator.
The offset phase of the phase modulator is chosen so that correctly timed pulses do not get
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24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24
RLE Progress Report 144
chirped. Pulses that are mis-timed so that they arrive too early in the modulator are positively (or
negatively; depends on the cycle of the phase modulator). Pulses that are too late are negatively
(or positively) chirped. If the sign of the dispersion is chosen so that the positively chirped pulses
are delayed and the negatively chirped pulses are advanced, then at the end of the dispersive
fiber, the timing jitter is reduced. The frequency noise, on the other hand, increases since each
mistimed pulse has a slightly different carrier frequency due to the modulator.
-10
-20
39.50 dB
Power (dBm)
-30
-40
Phase Mod (135 deg)
-50
43.50 dB
Phase Mod (0 deg)
-60
-70
No Mod
-80
-90
Noise Floor
RBW: 30 kHz
10.0030G 10.0035G 10.0040G 10.0045G 10.0050G
Frequency Offset (Hz)
Fig.2. RF spectra of the output pulses show 4 dB reduction.
In our particular experiment, we started with 6.7 ps pulses from a semiconductor modelocked
laser at a repetition rate of 10 GHz [4]. The gain current was 65 mA, the saturable absorber bias
was -1.5 V, and the RF drive to the saturable absorber was 11 dBm at 10 GHz. To simulate the
case of a laser with quantum-limited timing jitter with a relatively noisy RF oscillator (our
HP83732B synthesizer has an integrated jitter of 300 fs from 10 Hz to 10 MHz), it was necessary
to decrease the rf power to the saturable absorber to a low value of 11 dBm (usually operated at
25 dBm). When the modelocker is weak, spontaneous emission is the dominant contributor to
timing jitter. In a hero experiment, one would use a low noise sapphire oscillator and one would
not need to decrease the performance of the laser. Our experimental setup had no pre-chirp fiber,
a phase modulator capable of phase shifting by almost one optical cycle at maximum applied
peak-to-peak voltage, and 21 km Corning SMF-28 post-chirp fiber. The linewidth of the actively
modelocked laser was 4 MHz. The linewidth is narrow enough in this case that frequency noise
to timing jitter conversion is relatively small. An EDFA was used directly after the semiconductor
laser to increase the output power from 0.5 mW to 10 mW. The RF spectra of the output pulses
are shown in Fig. 2. The output noise is shown to decrease by 4 dB or 60% when the phase
modulator has the correct phase with respect to the pulses. When the phase of the phase
modulator is detuned by 135 degrees, there is no improvement in the rf noise spectrum as
expected.
In summary, we have demonstrated 4 dB of timing jitter reduction of a semiconductor
modelocked laser. We are currently looking into optimal designs for telecommunication
applications and optical sampling applications as well as performance improvements with
solitons.
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24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24
RLE Progress Report 144
References
[1] R. A. Barry, V. W. S. Chan, K. L. Hall, E. S. Kintzer, J. D. Moores, K. A. Rauschenbach, E. A.
Swanson, L. E. Adams, C. R. Doerr, S. G. Finn, H. A. Haus, E. P. Ippen, W. S. Wong, and M.
Haner, “All-optical network consortium- ultrafast TDM networks,” IEEE Journal on Selected Areas
in Communications 14(5): 999-1013 (1996).
[2] P. W. Juodawlkis, J. C. Twichell, G. E. Betts, J. J. Hargreaves, R. D. Younger, J. L.
Wasserman, F. J. O’Donnell, K. G. Ray, and R. C. Williamson, “Optically sampled analog-todigital converters,” IEEE Trans. Microwave Theory Tech. 49(10): 1840-1853 (2001).
[3] M. E. Grein, H. A. Haus, L. A. Jiang, and E. P. Ippen, “Action on pulse position and
momentum using dispersion and phase modulation,” Opt. Express 8(12): 664-669 (2001).
[4] L. A. Jiang, M. E. Grein, E. P. Ippen, C. McNeilage, J. Searls, and H. Yokoyama, “Quantumlimited noise performance of a mode-locked laser diode,“ Opt. Lett 27(1):49-51 (2002).
Publications
M. E. Grein, H. A. Haus, L. A. Jiang, and E. P. Ippen, “Action on pulse position and momentum
using dispersion and phase modulation,” Opt. Express 8(12): 664-669 (2001).
L. A. Jiang, M. E. Grein, B. S. Robinson, E. P. Ippen, and H. A. Haus, “Experimental
demonstration of a timing jitter eater,” submitted to the Conference on Lasers and Electro-Optics
2002.
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RLE Progress Report 144
Novel Low-Coherence Light Sources for Optical Imaging Applications
Sponsors
Air Force Office of Scientific Research (MFEL)
Grant F49620-01-1-0186
Air Force Office of Scientific Research
Grant F49620-98-01-0084
National Science Foundation
Grant ECS-019452
National Institute of Health
Grant NIH-5-R01-CA75289-04
National Institute of Health
Grant NIH-2-R01 EY11289-15
Project Staff
Paul Herz, Pei-Lin Hsiung, Tony Ko, Andrew M. Kowalevicz, Rohit P. Prasankumar, Aurea Tucay,
Dr. Thomas Schibli, Dr. Christian Chudoba, Dr. Wolfgang Drexler, Dr. Ingmar Hartl, Dr. Xingde Li,
Dr. Uwe Morgner, Dr. Ping Xue, Dr. Timothy Birks and Dr. William Wadsworth (U. Bath), Dr.
Daniel Kopf and Dr. Udo Bunting (High Q Laser Productions), Prof. Rene Salathe and Dr. Markus
Pollnau (ETH Lausane), Dr. Robert Windeler (Lucent Technologies), Professor Franz X. Kartner,
and Professor James G. Fujimoto
The need for simple, robust sources of broadband light exists in fields such as spectroscopy as
well as biomedical imaging applications such as Optical Coherence Tomography (OCT) [1-3].
The achievable longitudinal resolution is inversely related to the bandwidth of the source.
Standard, ~10 µm axial resolution OCT imaging can be performed using superluminescent
diodes (SLD) that have ~20-30 nm FWHM bandwidths centered near 800 nm. These sources
are relatively inexpensive and have turn-key operation suitable for clinical use, but provide limited
resolutions due to their narrow bandwidths. Recently, OCT imaging with axial resolutions of ~1
µm has been achieved using a Ti:Al2O3 laser with a bandwidth of ~300 nm [4, 5]. Unfortunately,
these systems are expensive and complex, limiting their widespread use. Our group has
developed several novel low-coherence light sources.
Spectral broadening in tapered fiber using a femtosecond Nd:Glass Laser
High nonlinearity, air-silica microstructure fibers [6] or tapered fibers [7] can generate an
extremely broadband continuum using low energy femtosecond pulses. The anomalous
dispersion characteristics of the fibers, which shift the zero dispersion to shorter wavelengths,
and the small core diameters, which provide tight mode confinement, help exploit the high
nonlinearities of the fiber.
We have demonstrated a new low coherence light source using a compact Nd:Glass
femtosecond laser spectrally broadened in a tapered single mode fiber. Our setup uses a
compact diode pumped femtosecond Nd:Glass laser (High Q Laser Production GmbH) which
generates pulses with 110-150 fs duration and 130 mW average power at 75 MHz repetition rate
and 1.06 µm wavelength. The Nd:Glass is pumped by two 1 W diode laser diodes. The
Nd:Glass laser is soliton modelocked [8] using a SESAM [9, 10] for self starting and intracavity
prisms for dispersion compensation. The laser pulses are coupled into a single mode fiber
(Corning SMF-28). The fiber was tapered by stretching in a flame so that after a short length
(~20 mm) of normal 125 µm diameter fiber, the fiber tapers down to a uniform waist with a
diameter of 2 µm and a length of 90 mm, before tapering up again to normal fiber. This thin
uniform waist enables the efficient generation of continuum [7]. The output of the tapered fiber is
fusion spliced to a 10 m length of dispersion shifted fiber (zero dispersion at longer wavelengths)
to reduce parasitic contributions from four wave mixing.
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24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24
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0.8
Spectral Power [dBc / Hz]
Intensity [a.u.]
1.0
(a)
0.6
132 nm
8 nm
0.4
0.2
0.0
1.00
1.20
(b)
-50
-100
Detection System Noise Level
-150
2
10
1.40
Wavelength [µm]
Nd:Glass Laser
Continuum
(Tapered Fiber)
3
10
4
10
5
10
6
10
Frequency Offset to First Harmonic [Hz]
Figure 1. (a) The optical spectrum of the femtosecond Nd:Glass laser and the optical spectrum of
the continuum generated in a tapered fiber, and (b) noise spectrum of the continuum generated in
a tapered fiber compared to the pump laser.
Figure 1 (a) shows a typical laser output spectrum and the continuum generated by the tapered
fiber with an average power of 60 mW. The continuum is asymmetrically shifted toward longer
wavelengths. The shift in the spectrum may be the result of Raman effects and the soliton self
frequency shift as well as other mechanisms. In order to measure the noise in the continuum, a
110 nm wide portion of the spectrum centered at 1.3 µm was selected and monitored with a fast
InGaAs photodiode. Figure 1 (b) shows the RF spectrum of the photodiode signal compared to
the laser. These measurements show that the continuum generation produces a negligible
increase in noise above the laser noise itself.
Continuum generation in the visible wavelength region using high nonlinearity air-silica
microstructure optical fibers
High nonlinearity, photonic crystal fibers or air-silica microstructure fibers and tapered fibers can
generate an extremely broadband continuum extending from 390 nm to 1600 nm using low
energy femtosecond pulses [7, 11]. This large bandwidth enables ultrahigh resolution OCT
2
because of the λ /∆λ dependence of the axial OCT resolution.
We demonstrate that extremely broad bandwidth of the continuum centered at 525 nm in the
spectral region 450 nm to 700 nm can be generated by femtosecond pulses from a commercial
Ti:Sapphire laser coupled into an air-silica microstructure fiber, enabling OCT resolutions below 1
um. The spectrum is limited at shorter wavelengths by the transmission properties of the
microscope objective. Longitudinal OCT resolutions below 0.9 µm are feasible using the visible
part of the continuum. Because of the high dispersion of typical optical materials in this
wavelength range and the lack of broadband fiberoptic couplers, a free space optical setup is
used to demonstrate ultrahigh resolution OCT in the visible spectral range. Figure 2 shows a
schematic of the experimental setup.
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24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24
RLE Progress Report 144
KLM
Ti:Sappire
Laser
Diode Pumped
SHG Nd:YVO4
Faraday
Isolator
λ/2
MO
MO
Spectral Filtering
Photonic Crystal Fiber
MO
BS
Reference
MO
D1
Filter
Amplifier
Demod
BS
Computer
D2
MO
Z-scan
Sample
X-scan
Figure 2. Ultrahigh resolution OCT system using continuum generation in an air-silica
microstructure fiber as the light source. MO: Microscopic Objective Lens, BS:Beam Splitter,
D1,D2: Detector, λ/2:Half-wave Plate, A: Aperture.
An isolated reflection from a silver mirror demonstrates that transverse resolutions below 0.9 µm
in air can be achieved using the supercontinuum light source. The spectrum of the collimated
continuum (Figure 5 (a)) was measured using a calibrated optical spectrum analyzer to have a
FWHM bandwidth of 100 nm centered at 525 nm. The interferometric signal is shown together
with its envelope in Figure 5 (b). A free-space axial OCT resolution of 0.86 µm was determined
by measuring the full width at half maximum of the envelope of the interferometric signal. This is
to our knowledge the highest longitudinal OCT resolution demonstrated to date in this wavelength
range.
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24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24
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0.40
Wavelength [µm]
0.60
0.80
-10
-5
Delay [µm]
0
5
(b)
(a)
0.86 µm
0.6
0.5
100 nm
0.4
0.2
0.0
0.0
(c)
Intensity [a.u.]
Intensity [a.u.]
0.8
10
1.0
400
(d)
0.8
200
0.6
190 nm
0
0.4
Group Delay Dispersion [fs2]
Intensity [a.u.]
1.0
-200
0.2
-400
0.0
0.40
0.60
0.80
Wavelength [µm]
0.40
0.60
0.80
Wavelength [µm]
Figure 3. (a) Typical output spectrum of microstructure fiber before spectral filtering. (b)
Interference fringes recorded by using an isolated reflection. The full width at half maximum was
measured to be 0.86µm. Detected optical spectrum (c) and group delay dispersion mismatch of
the interferometer arms (d) obtained from the Fourier transform of the interferometric signal. The
vertical dashed lines mark pronounced spectral features.
The detected optical spectrum was calculated by Fourier transforming the interferometric signal.
The detected spectral bandwidth is 190 nm. This broadening of the bandwidth occurs due to an
attenuation of the short wavelength part of the light source and also the spectral responsibility of
the detector. Pronounced spectral features of the light source are still visible after spectral
shaping and are marked with dashed vertical lines between Figure 3 (a) and Figure 3 (c). The
group delay dispersion mismatch between the two interferometer arms is obtained from the
phase of the Fourier transformed interferometric signal and is shown in Figure 3 (d) and is
relatively low due to the highly symmetrical setup. Pinholes of 15 microns in diameter are placed
before the two detectors respectively to eliminate the background due to multiscattering of tissue
samples. To reduce the noise level, dual balance detection is further utilized. The light intensity
on the photodiodes is balanced by adjustment the diameter of aperture A before the detector D2
and MO. The measured sensitivity is 97db with incident light power 0.5mw at sample. Imaging
studies using this system and further study of fiber system are in progress.
Broadband fluorescence sources using Ti:Al2O3 crystals
Fluorescence from Ti:Al2O3 and laser dyes have been demonstrated as sources for ultrahigh
resolution low coherence reflectometry [12, 13]. Low coherence reflectometry with <2 µm axial
resolution was first demonstrated in Ti:Al2O3 using 4.8 µW fluorescence produced from a Ti:Al2O3
crystal pumped by a 20 W Argon laser. While this result demonstrated the highest resolution low
coherence reflectometry achieved at that time, the output powers were not sufficient to permit
imaging.
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Output powers can be improved significantly using a high doping density crystal and more
efficient fluorescent coupling. By matching the numerical aperture of the single-mode fiber as
well as retroreflecting the transmitted pump and fluorescent light, we demonstrate a more than
one order of magnitude improvement in light source efficiency. Using 5 W cw laser pumping of a
Ti:Al2O3 crystal, 40.3 µW of fluorescence with bandwidths of 138 nm can be coupled into a singlemode optical fiber.
Figure 4. (a) Coupled power vs. pump power for different temperatures and comparison between
single pass power and retroreflecting. (b) Coupled spectrum with 138 nm FWHM.
Figure 4 (a) shows the fluorescent power coupled into the single mode fiber versus incident pump
power. Figure 6 (b) shows the spectrum of the fiber-coupled fluorescence. The FWHM
bandwidth was 138 nm and follows the expected fluorescence spectrum of Ti:Al2O3. Both the
power and spectrum were stable for periods of several hours without realignment. A maximum
fluorescence power of 40.3 µW was achieved at 5 W of incident pump, cooling the crystal to –15
°C. Operating at room temperature without cooling, 34.1 µW could be generated at 5 W pump. A
family of curves was obtained with the crystal cooled to different temperatures. The output power
increased sublinearly at higher pump powers and this reduced efficiency was probably the result
of thermal effects. These results suggest that higher power should be possible with better
thermal control.
Using this source enables ultrahigh resolution OCT imaging with 2.2 µm axial resolution in air (1.7
µm in tissue) and >86 dB sensitivity. This demonstrates a simple, robust light source for
spectroscopy and ultrahigh resolution OCT imaging without the need for complex femtosecond
Ti:Al2O3 lasers.
References
1.
2.
3.
D. Huang, E. A. Swanson, C. P. Lin, J. S. Schuman, W. G. Stinson, W. Chang, M. R.
Hee, T. Flotte, K. Gregory, C. A. Puliafito, and J. G. Fujimoto, “Optical coherence
tomography,” Science, 254: 1178-1181, 1991.
G. J. Tearney, M. E. Brezinski, B. E. Bouma, S. A. Boppart, C. Pitris, J. F. Southern, and
J. G. Fujimoto, “In vivo endoscopic optical biopsy with optical coherence tomography,”
Science, 276: 2037-9, 1997.
S. A. Boppart, B. E. Bouma, C. Pitris, J. F. Southern, M. E. Brezinski, and J. G. Fujimoto,
“In vivo cellular optical coherence tomography imaging,” Nature Medicine, 4: 861-5, 1998.
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24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24
RLE Progress Report 144
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
W. Drexler, U. Morgner, F. X. Kärtner, C. Pitris, S. A. Boppart, X. D. Li, E. P. Ippen, and
J. G. Fujimoto, “In vivo ultrahigh resolution optical coherence tomography,” Optics
Letters, 24: 1221-1223, 1999.
W. Drexler, U. Morgner, R. K. Ghanta, F. X. Kaertner, J. S. Schuman, and J. G. Fujimoto,
“Ultrahigh resolution ophthalmic optical coherence tomography,” Nature Medicine, in
press, 2000.
J. K. Ranka, R. S. Windeler, and A. J. Stentz, “Visible continuum generation in air-silica
microstructure optical fibers with anomalous dispersion at 800 nm,” Optics Letters, 25:
25-7, 2000.
T. A. Birks, W. J. Wadsworth, and P. S. J. Russell, “Supercontinuum generation in
tapered fibers,” Optics Letters, 25: 1415-1417, 2000.
F. X. Kärtner, I. D. Jung, and U. Keller, “Soliton Modelocking with Saturable Absorbers,”
Special Issue on Ultrafast Electronics, Photonics and Optoelectronics, IEEE J. Selected
Topics in Quantum Electronics (JSTQE), 2: 540-556, 1996.
D. Kopf, F. X. Kärtner, K. J. Weingarten, and U. Keller, “Diode-pumped modelocked
Nd:glass lasers using an A-FPSA,” Optics Lett., 20: 1169-1171, 1995.
U. Keller, K. J. Weingarten, F. X. Kaertner, D. Kopf, B. Braun, I. D. Jung, R. Fluck, C.
Honninger, N. Matuschek, and J. A. D. Au, “Semiconductor saturable absorber mirrors
(SESAM's) for femtosecond to nanosecond pulse generation in solid-state lasers,” IEEE
Journal of Selected Topics in Quantum Electronics, 2: 435-453, 1996.
J. K. Ranka, R. S. Windeler, and A. J. Stentz, “Visible continuum generation in air-silica
microstructure optical fibers with anomalous dispersion at 800nm,” Optics Letters, 25: 2527, 2000.
X. Clivaz, F. Marquis-Weible, and R. P. Salathe, “Optical low coherence reflectometry
with 1.9 mm spatial resolution,” Electronics Letters, 28: 1553-1554, 1992.
H. H. Liu, P. H. Cheng, and J. Wang, “Spatially coherent white light interferometer based
on a point flourescent source,” Optics Letters, 18: 678-680, 1993.
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RLE Progress Report 144
Optical Phase Control and Stabilization Techniques
Control and Stabilization of the Absolute Optical Phase Evolution
Sponsors
U.S. Air Force – Office of Scientific Research
Grant F49620-01-1-0084
MIT Lincoln Laboratory
Grant ACC 334
National Science Foundation
Project Staff
Onur Kuzucu, Dr. Thomas Schibli, Richard Ell, G. Metzler, and Dr. Uwe Morgner Professor
James G. Fujimoto, Prof. Hermann A. Haus, Professor Erich P. Ippen and Professor Franz X.
Kärtner.
When pulses become only a few cycles in length the precise shape of the electric field
underneath its intensity envelope becomes significant. Then the shape of the field sensitively
depends on the phase-difference between pulse-envelope and optical carrier wave [1,2]. The 5 fs
pulses with octave-spanning spectra from a Ti:sapphire laser oscillator, demonstrated above,
have been directly used to observe this phase evolution in a pulse train emitted from a modelocked laser. Due to the difference between phase and group velocity in a laser and additional
nonlinear effects [3], the carrier-envelope phase φ slips by a given amount per roundtrip [4],
leading to an offset of the frequency comb of the modelocked laser from the origin by fφ=φ/(2πTR)
[5]. This offset is of great importance if the comb shall be used as a standard for frequency
metrology. Phase controlled lasers are considered to be the clock work of improved optical clocks
for frequency standards. Mode-locked lasers have already greatly advanced frequency metrology
over the last two years [6], [7]. The frequency shift of the comb due to the carrier-envelope phaseslip can be detected by nonlinear optical techniques. When the spectrum of the pulse is broad
enough, i.e. one octave, one can beat the second harmonic of the long wavelength part of the
pulse spectrum with the short wavelength part of the spectrum, see Fig. 1.
Fundamental
eiφ
χ(2)
e2iφ
e3iφ
eiφ
0
ω0
2ω0
χ(3)
3ω0
Figure 1: Schematic spectra of a pulse covering one octave on an optical frequency scale
(2)
(3)
together with the spectra generated by instantaneous χ - and χ -processes, given by
successive convolution of the fundamental spectrum.
Figure 2 shows our experiment. 5-fs pulses from our Ti:sapphire laser oscillator are focused into
a BBO crystal for second-harmonic generation. At the output the interference signal between
second-harmonic and fundamental is enhanced before detection by spectral filtering of the
overlap regime near 600 nm. As Figure 1 indicates, greater, phase-dependent, spectral overlap
can be obtained between second and third harmonics as has been also demonstrated [4]. Using
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24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24
RLE Progress Report 144
RF spectral power density [dB]
this phase-dependent signal we could stabilize the carrier-envelope phase evolution to a
microwave oscillator, as shown in Figure 3.
0
-10
-20
-30
-40
-50
0
10
20
30
40
RF Frequency [ MHz]
50
60
70
RF frequency [MHz]
Figure 2: Interference between fundamental and second harmonic in a compact detector. Top:
Set-up; Second harmonic light generated in a BBO-crystal interferes with the other end of the
fundamental spectrum in a polarizer. The beat signal is detected with a photomultiplier tube
behind a wavelength filter. Bottom: RF-spectrum of the signal at the photomultiplier tube. The
right peak at 65 MHz is from the repetition rate fR. The two inner peaks represent the carrierenvelope phase evolution frequency fφ and the mixing product fR -fφ . The background is due to
shot noise.
70
60
50
40
30
20
10
00
fR
f -Φ
f
R
PLL off PLL on
fφ
1
2
3
time [minutes]
4
Figure 3: Stability of the carrier-envelope phase evolution frequency fφ with and without external
stabilization to an RF-oscillator by a phase-locked loop (PLL).
So far we have demonstrated how to control the phase evolution of optical pulses and we could
stabilize it. However, we still do not know what the phase of the optical pulse is. Therefore, there
is an ongoing quest for nonlinear optical effects that reveal the value of the absolute optical phase
of a pulse.
References:
1. T. Brabec and F. Krausz, Rev. Mod. Phys. 72(2): 545-91 (2000).
2. A. Apolonski, A. Poppe, G. Tempea, C. Spielmann, T. Udem, R. Holzwarth, T. W. Hänsch,
and F. Krausz, "Controlling the Phase Evolution of Few-Cycle Light Pulses," Phys. Rev. Lett.,
85(4): 740-43 (2000).
3. H.A. Haus and E.P. Ippen, “Group velocity of solitons,“ Opt. Lett. 26(21): 1654-56 (2001).
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4. L. Xu, C. Spielmann, F. Krausz, and R. Szipöcs, “Ultrabroadband ring oscillator for sub-10-fs
pulse generation,” Opt. Lett. 21(16): 1259-61 (1996).
5. J. Reichert, R. Holzwarth, T. Udem, and T. Hänsch, “Measuring the frequency of light with
mode-locked lasers,” Opt. Comm. 172(1): 59-68 (1999).
6. S.A. Diddams, D.J. Jones, J. Ye, S.T. Cundiff, J.L. Hall, J.K. Ranka, R.S. Windeler, R.
Holzwarth, T. Udem and T.W. Hänsch, "A direct link between microwave and otpical
frequencies with a 300 THz femtosecond laser comb," Phys. Rev. Lett., 84(22): 5102-5
(2000).
7. S.A. Diddams, L. Hollberg, L.S. Ma and L. Robertsson, "Femtosecond-laser-based optical
-16
clockwork with instability ≤ 6.3x10 in 1s," Opt. Lett. 27(1): 58-60 (2002).
8. U. Morgner, R. Ell, G. Metzler, T. R. Schibli, F. X. Kärtner, J. G. Fujimoto, H. A. Haus and E.
P. Ippen, "Nonlinear optics with phase-controlled pulses in the sub-two-cycle regime," Phys.
Rev. Lett., 86(24): 5462-5 (2001).
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Few-Cycle Nonlinear Optics and Absolute Optical Phase Effects in CarrierWave Rabi Flopping
Sponsors
MIT Lincoln Laboratory
Grant ACC 334
National Science Foundation
Project Staff
Professor Franz X. Kärtner
University of Karlsruhe: Dr. Uwe Morgner, Oliver D. Mücke, Thorsten Tritschler, and Professor
Martin Wegener
As already discussed above it is important to have control over the absolute optical phase. In
frequency metrology only control over the phase-slip rate is of importance. However, there are
other areas of research, e.g. high-harmonic generation, where knowledge and stabilization of the
absolute optical phase is absolutely necessary [1] for further progress in this area. In these
experiments the outcome depends on the optical phase of the pulses in use. Therefore, there is
an ongoing quest to find nonlinear optical effects that depend on the value of the optical phase of
a pulse. Such effects have been predicted to occur in extreme nonlinear optics of atoms [2] and
have been observed recently [3].
In cooperation with researchers from the University of Karlsruhe, our sub-two-cycle laser pulses
have been used to study carrier wave Rabi flopping in GaAs [4], which was predicted to occur by
Hughes [5], a few years earlier. In short, carrier wave Rabi-flopping means, that the laser field
becomes so intense, that the Rabi frequency is on the order of the carrier frequency of the optical
pulse. For example in GaAs, electric fields as large as 1 GV/m, equivalent to intensities of 0.1
2
TW/cm , are necessary to achieve such large Rabi frequencies. In earlier work on optical
resonance phenomena, it was thought, that such a regime is not accessible [6]. Such high fields
are readily available by focusing few-cycle laser pulses directly from an oscillator to a diffraction
limited spot. Matter can sustain such strong fields for the short period of time comprising only a
few optical cycles of light. In the regime of carrier wave Rabi flopping, the rotating wave
approximation in the Bloch-Equations is not any longer valid and new phenomena, such as
breakdown of the area-theorem are expected [5]. Beyond the Rotating Wave Approximation the
two-level system also emits radiation at all odd harmonics as an inversion symmetric medium
does and which has been observed in GaAs [4]. Very recently, we found by computer
simulations, that the dipoles undergoing carrier wave Rabi-flopping also show a phase sensitive
emission [7]. Figure 1 (a-c) shows the spectral intensity of a sub-two-cycle pulse with a
rectangular pulse spectrum at the input, after propagation through a 20 nm thick layer of GaAs
transferred to a sapphire substrate and front coated with an antireflection coating. The
assumption of a square shaped pulse spectrum is close to the experimental conditions [8]. The
driven dipoles radiate at the fundamental and odd higher order harmonics. The oscillation of the
inversion with the Rabi frequency modulates the optical polarization, which generates sidebands
in the emission. When the Rabi-frequency becomes twice as large as the carrier-frequency, the
modulation sidebands of the fundamental and the third harmonic overlap at twice the fundamental
frequency. Very similar to the above discussed phase dependent nonlinear effects, where higher
order harmonics of different spectral components of the pulse overlap, to give rise to a phase
dependent interference of the sidebands. Figure 1 shows the intensity of the output light around
twice the carrier frequency for different phase values of the input pulse. As the simulation shows,
the signal is 12 orders of magnitude less intense then the incoming laser light. Thus the signal is
in the sub-pW range. Surprisingly, we also found, that the minimum and maximum of the phase
sensitive emission does not exactly occur at zero or π phase shift and the location of the
extremes depend on the Rabi frequency, see Figure 2. It remains to be investigated, whether the
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24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24
RLE Progress Report 144
signal is strong enough to lead to a practical phase detector. The availability of a compact phase
detector for few-cycle laser pulses directly from the laser may lead to much more compact and
improved frequency standards for frequency metrology and will help to synthesize arbitrary
electrical field waveforms in the optical domain [9].
Figure 1: (a) Signal intensity (linear scale, normalized to the maximum intensity, Imax, of the
incident laser spectrum) emitted into the forward direction versus spectrometer frequency ω in
units of the laser center frequency ω0 for different values of the absolute optical phase φ . The
GaAs film with L=20nm thickness on a sapphire substrate has a frontside antireflection coating.
E0 is measured in units of GV/m. The inset illustrates the interference of peaks from the different
Rabi doublets as the Rabi frequency ΩR= ħdE0. (b) as (a), but signal intensity (normalized) on a
logarithmic scale for fixed φ = 0 and for different incident electric field amplitudes as indicated. (c)
shows the signal intensity as a function of the electric field strength and frequency [7].
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24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24
RLE Progress Report 144
Figure 2: Grey scale image of the emitted intensity as a function of ω and φ for a thin GaAs film
with thickness L on a sapphire substrate. (a) L = 100 nm and E0 = 3.5 GV/m, (b) as in (a) but L =
20 nm, (c) as (b) but with an additional front-side antireflection coating (as in Fig. 9), (d) as (c) but
for an electric field amplitude of E0 = 4 GV/m [7].
References:
1. A. Durfee, A. Rundquist, S. Backus, C. Herne, M. Murnane, and H. Kapteyn, “Phase
matching of high-order harmonics in hollow waveguides,” Phys. Rev. Lett. 83(11): 2187-90
(1999).
2. P. Dietrich, F. Krausz and P.B. Corkum, "Determining the absolute carrier phase of a fewcycle laser pulse," Opt. Lett. 25(1): 16-8 (2000).
3. G.G. Paulus, F. Grasbon, H. Walther, P. Villoresi, M. Nisoli, S. Stagira, E. Priori and S. d.
Silvestri, "Absolute-phase phenomena in photoionization with few-cycle laser pulses," Nature
414: 182-4 (2001).
4. O.D. Mücke, T. Tritschler, M. Wegener, U. Morgner and F.X. Kärtner, "Signatures of CarrierWave Rabi-Flopping in GaAs," Phys. Rev. Lett. 87(057401): 1-4 (2001)
5. S. Hughes, "Breakdown of the Area Theorem: Carrier-Wave Rabi Flopping of Femtosecond
Optical Pulses," Phys. Rev. Lett. 81(16): 3363-6 (1998).
6. L. Allen, J. H. Eberly, Optical Resonance and Two-Level Atoms. (J. Wiley & Sons, New York,
1975).
7. O.D. Mücke, T. Tritschler, M. Wegener, U. Morgner and F.X. Kärtner, "Role of the absolute
optical phase of few-cycle pulses in non-perturbative resonant nonlinear optics," submitted to
Phys. Rev. Lett.
8. U. Morgner, F.X. Kärtner, S.T. Cho, H.A. Haus, J.G. Fujimoto, E.P. Ippen, V. Scheuer, G.
Angelow and T. Tschudi, "Sub-two cycle pulses from a Kerr-Lens modelocked Ti:sapphire
laser," Opt. Lett. 34(6): 411-3 (1999)
9. A. Poppe, R. Holzwarth, A. Apolonski, G. Tempea, C. Spielmann, T. W. Hänsch and F.
Krausz, "Few-cycle optical waveform synthesis," Appl. Phys. B 72(3): 373-6 (2001).
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RLE Progress Report 144
Nonlinear Fabry-Perots
Oscillators
for
Synchronization
of
Independent
Laser
Sponsors
MIT Lincoln Laboratory
Grant ACC 334
Project Staff
Dr. Thomas Schibli, Franz X. Kärtner
University of Karlsruhe: Wolfgang Seitz and Dr. Uwe Morgner
Synchronized ultrashort laser pulses at different independently tunable wavelengths increase the
flexibility of time-resolved optical spectroscopy, allowing for sophisticated excitation and probe
techniques [1,2]. Self-synchronized two-color single crystal Ti:sapphire lasers have been reported
[3]. Other methods to synchronize two independent Ti:sapphire lasers are based on active
stabilization using electronic control-loops [4]. Here, we report a novel method of passive
synchronization of two independently mode-locked lasers at discretely separate wavelengths. In
our case a passively ps-mode-locked Nd:YVO4 laser oscillator at λ1 = 1064 nm is synchronized to
a fs-mode-locked Ti:sapphire laser at λ2 = 850 nm. Synchronization is achieved by optical
modulation of the intracavity losses of the slave oscillator (Nd:YVO4) by the master oscillator
(Ti:sapphire). The loss modulator is realized by a nonlinear semiconductor Fabry-Perot mirror
(FPM). Synchronization of the two pulse trains is observed and verified via cross correlation
measurements over a cavity length detuning range exceeding 20 µm between the two laser
oscillators.
The experimental set-up for synchronization is schematically shown in Fig.1. A diode-pumped
Nd:YVO4 laser (dashed box) is passively mode-locked using a semiconductor saturable absorber
mirror, which is mounted on a translation stage to adjust the cavity length. The free running
Nd:YVO4 laser (without FPM) produces optical pulses with a duration of 8 ps at 1064 nm
wavelength. For the master laser a commercially available Ti:sapphire laser was used, emitting
transform-limited optical pulses of 180 fs duration. For synchronization, one mirror in the
Nd:YVO4 laser cavity is replaced by the FPM.
BS
Ti:sapphire
laser
BBO (1mm)
PM
BS
DBS
filter
(472 nm)
4
OC
Fig. 1: Experimental setup of the two synchronized laser oscillators. PM, Photomultiplier;
SESAM, Semiconductor Saturable Absorber Mirror; FPM, Fabry-Perot-Modulator; OC, Output
Coupler; DBS, Dichroic Beam Splitter; BS, Beam Splitter.
The nonlinear Fabry-Perot mirror used consists of a 5-µm-thick InxGa1-xAs layer (x = 0.09) on top
of a Bragg reflector grown by solid source molecular beam epitaxy at 580 °C. The Bragg reflector
consists of 15 pairs GaAs/AlAs quarter-wave layers at 1064 nm grown on a GaAs substrate at
615°C. For external optical modulation, a fraction of the Ti:sapphire laser output power
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RLE Progress Report 144
24
a)
1000
locking range (µm)
fNd:YVO4 (Hz) - 82.6 MHz
(approximately 100 mW) is focused on the FPM in a spot overlapping with the Nd:YVO4 laser
spot. By controlling the temperature of the FPM, the resonance frequency of the FPM can be
shifted in wavelength, which allows choosing the operating point. For maximum modulation the
resonance frequency is tuned just below 1064 nm resulting in a slightly reduced reflectivity of the
linear FPM. With incident Ti:sapphire radiation, free carriers are generated inside the InGaAs
layer causing a shift of the resonance frequency to shorter wavelengths. The shift results in an
increased reflectivity of the FPM and in reduced losses for the Nd:YVO4 in sychronism with the
repetition rate of the Ti:sapphire master oscillator. To determine the tolerance in cavity length
mismatch, the round-trip frequency of the Nd:YVO4 laser versus cavity length variation is
detected. At 150 mW of control power the tolerance in cavity length mismatch exceeds 20 µm
(see Fig. 2b).
900
800
700
600
500
b)
20
16
12
8
-15
-10
-5
0
75
5
100
125
150
175
control power (mW)
cavity length variation (µm)
Fig.2: (a) Measured repetition rate of the Nd:YVO4 laser versus cavity length variation. The
synchronization regime (locking range) is emphasized by the grey bar. For clarity, the value 82.6
MHz is subtracted from the exact repetition rate. (b) Locking range plotted as a function of the
Ti:sapphire laser power driving the FPM. The dashed line is a linear fit to the measured points
(dots).
1.0
a)
b)
0.8
intensity (arb.)
cross correlation (arb.)
1.0
0.6
0.4
0.2
0.0
-25
0
25
50
75
time delay (ps)
100
0.8
0.6
0.4
0.2
0.0
-80
-40
0
40
80
time delay (ps)
Fig.3: (a) Measured cross correlation between the synchronized Ti:sapphire and Nd:YVO4 laser
which directly reveals the pulse shape of the Nd:YVO4 laser pulses. The FWHM is approximately
13 ps. (b) Measured intensity autocorrelation of the synchronized Nd:YVO4 laser (solid curve) and
calculated autocorrelation of the measured cross correlation function from (a) (dotted curve).
Fig. 3 (a) shows the cross correlation between the two laser pulse trains by sum-frequency
generation in a 1-mm-thick BBO crystal. The asymmetry of the cross correlation trace is due to
the asymmetric temporal response of the FPM as described above. The simultaneously
measured intensity autocorrelation trace of the synchronized picosecond Nd:YVO4 pulses has a
FWHM of 22.3 ps and is displayed in Fig. 3 (black line). The computed autocorrelation function of
the measured cross correlation signal, which ideally equals the autocorrelation, is also shown in
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24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24
RLE Progress Report 144
Fig. 3 (dotted curve). From the cross correlation signal, a width of about 13 ps is derived for the
pulses from the Nd:YVO4 laser. The two traces in Fig. 3 agree within the measurement accuracy.
A conservative estimation of the timing jitter between both lasers was performed by analyzing the
intensity fluctuations in the rising / falling edge of the cross correlation [4]. A time record in 100kHz bandwidth over 1 s revealed a timing jitter between the two lasers below 1 ps, which is much
smaller than the duration of the Nd:YVO4 pulses. In fact, subsequent synchronization
experiments as demonstrated in the following chapter indicate a timing jitter much shorter than 3
fs. The great advantage of the passive scheme demonstrated here, in comparison to an active
scheme is, that it removes also the fast timing-jitter components in the slave-oscillator and tightly
links the two pulses.
References:
1. J. Feldmann, S. T. Cundiff, M. Arzberger, G. Böhm and G. Abstreiter, “Carrier capture into
InAs/GaAs quantum dots via multiple optical phonon emission,” J. Appl. Phys. 89(2): 1180-4
(2001).
2. M.R.X. de Barros and P.C. Becker, “Two-color synchronously mode-locked femtosecond
Ti:sapphire laser,” Opt. Lett. 18(8): 631-3 (1993).
3. A. Leitenstorfer, C. Fürst and A. Laubereau, “Widely tunable two-color mode-locked
Ti:sapphire laser with pulse jitter of less than 2 fs,” Opt. Lett. 20(8): 916-8 (1995).
4. R. K. Shelton, L.-S. Ma, H. C. Kapteyn, M. M. Murnane, J. L. Hall and J. Ye, ”PhaseCoherent Optical Pulse Synthesis from Seperate Femtosecond Lasers,” Science 293(5533):
1286-9 (2001).
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Control of the Absolute Optical Phase in Picosecond Lasers
Sponsors
MIT Lincoln Laboratory
Grant ACC 334
National Science Foundation
Project Staff
Dr. Thomas R. Schibli and Professor Franz X. Kärtner
University of Karlsruhe: Richard W. Ell, Wolfgang Seitz and Dr. Uwe Morgner
Recently, coherent superposition of two independently mode-locked, but spectrally overlapping
Ti:sapphire laser oscillators has been demonstrated by means of external active control [1]. This
concept opens up new perspectives in metrology with femtosecond mode combs as discussed
before, and might lead to phase coherent emission over much broader bandwidth as is possible
from a single mode-locked laser (e.g. [4]). The corresponding pulses may reach the single-cycle
regime.
Ti:Sa
fR
f
fΦ1
Nd:YVO 4
fR
f
fΦ2
Fig.1 Mode combs of two lasers with equal pulse
repetition rate fR and independent field repetition
rate fφ1 and fφ2.
PD
Ti:sa - ring
(600-1200nm)
DM
1000-1200nm
SMF
G
Here, we demonstrate a possibility to
measure the carrier-envelope phase-shift
frequency of a picosecond laser with only
0.1 nm bandwidth by synchronizing the
picosecond laser to a broadband
Ti:sapphire oscillator using the FabryPerot mirror discussed before.
The master laser – a few-cycle-Ti:sapphire
ring laser emitting around 800 nm –
modulates the losses in a Nd:YVO4
oscillator, emitting 10-ps-pulses at 1064
nm, via the intracavity nonlinear FabryPerot semiconductor mirror.
600 - 1000nm
1064nm
Nd :YVO 4
Here, we demonstrate a possibility to
measure the field repetition rate of a
picosecond laser with only 0.1 nm
bandwidth by passive synchronization of
the picosecond laser to an octave
spanning Ti:sapphire oscillator. Phase
controlled picosecond lasers might
become important for particle accelerators
and in metrology. The optical spectrum of
a mode-locked laser is characterized by
two frequencies, the pulse repetition rate
fR, which is the repetition rate of the pulse
envelope and the field repetition rate
fφ that describes the carrier-envelope
phase shift rate (see Fig. 1). Recently, the
measurement of fφ in pulse trains with
octave spanning spectra has been
demonstrated [5-7].
FPM
(1064nm)
Fig.2 Experimental set-up for the measurement of
the difference field repetition rate |fφ1 - fφ2|. DM:
dichroic mirror, FPM: Fabry-Perot-Modulator,
SMF: single mode fiber, G: grating, PD:
photodiode
24-62
The set-up is shown in Fig.2: A dichroic
mirror splits up the output light of the
master laser, a Ti:sapphire ring laser
emitting octave spanning spectra from 600
to 1200 nm. The short wavelength part
24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24
RLE Progress Report 144
RF spectral power density (dB)
below 1 µm is used to drive the intracavity Fabry-Perot-Modulator (FPM) in the Nd:YVO4-slaveoscillator to provide synchronism. Within a few micron of cavity length detuning the lasers have
the same pulse repetition rate fR . An upper boundary for the timing jitter between the two pulse
trains has been estimated by cross-correlation measurements to be well below 1 ps. The long
wavelength part of the Ti:sapphire spectrum is spatially superimposed with the output of the
Nd:YVO4 -laser in a single-mode fiber
and temporally by means of an optical
delay line. A PIN photodiode detects the
0
beat signal behind a narrow band
fR
-10
spectral filter. Fig.3 shows the measured
RF-spectrum which indicates the same
-20
pulse repetition rate of both lasers fR and
-30
the position of the difference field
-40
repetition rate |fφ1 - fφ2| between the two
| fφ1 − fφ2 |
fR - | fφ1 − fφ2 |
lasers. This peak can be shifted by
-50
changing the intracavity dispersion in one
-60
of the two lasers. The measurement has
been done with a resolution bandwidth of
-70
100
kHz
which
approximately
0
10
20
30
40
50
60
70
80
corresponds to the width of the beat
RF frequency (MHz)
signal. The finite width of this line
indicates residual phase fluctuations
Fig.3 RF spectrum of the beating signal between
between the two lasers. On one hand, the
the two different lasers indicating the difference
observation time interval, 10 µs, is longer
carrier-envelope phase-slip rate |fφ1 - fφ2|. The
than the timing constant for timing
resolution bandwidth was 100 kHz.
restoration between the two lasers due to
the passive synchronization. On the other
hand it is shorter than the bandwidth of typical fluctuations in the carrier-envelope phase of both
lasers [7]. Therefore, the observation of the beat signal proofs that the timing jitter of both lasers
within the observation time is much smaller than one optical cycle equivalent to about 3 fs.
An independent measurement of the carrier-envelope phase shift frequency fφ1 of the Ti:sapphire
oscillator with the scheme shown before allows for the control of the carrier-envelope phase of
the picosecond laser.
References:
1. R.K. Shelton, L.-S. Ma, H.C. Kapteyn, M.M. Murnane, J.L. Hall, and J.Ye, “Phase-Coherent
Optical Pulse Synthesis from Seperate Femtosecond Lasers,” Science 293(5533): 1286-9
(2001).
2. J. Reichert, M. Niering, R. Holzwarth, M. Weitz, T. Udem, and T.W. Hänsch, “Phase
Coherent Vacuum-Ultraviolet to Radio Frequency Comparison with a Mode-Locked Laser,”
Phys. Rev. Lett. 84(15): 3232-5 (2000).
3. J. Stenger, C. Tamm, N. Haverkamp, S. Weyers, and H.R. Telle, “Absolute frequency
measurement of the 435.5-nm 171Yb+-clock transition with a Kerr-lens mode-locked
femtosecond laser,” Opt. Lett. 26(20): 1589-91 (2001).
4. R. Ell, U. Morgner, F. X. Kärtner, J. G. Fujimoto, E. P. Ippen, V. Scheuer, G. Angelow, T.
Tschudi, M. J. Lederer, A. Boiko, and B. Luther-Davies, “Generation of 5 fs pulses and
octave-spanning spectra directly from a Ti:sapphire laser,” Opt. Lett. 26(6): 373 (2001).
5. D. J. Jones, S. A. Diddams, J. K. Ranka, A. Stentz, R. S. Windeler, J. L. Hall, and S. T.
Cundiff, “Carrier-Envelope Phase Control of Femtosecond Modelocked Lasers and Direct
Optical Frequency Synthesis,” Science 288: 635-9 (2000).
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6. A. Apolonski, A. Poppe, G. Tempea, C. Spielmann, T. Udem, R. Holzwarth, T. W. Hänsch,
and F. Krausz, “Controlling the Phase Evolution of Few-Cycle Light Pulses,” Phys. Rev. Lett.
85(4): 740-43 (2000).
7.
U. Morgner, R. Ell, G. Metzler, T. R. Schibli, F. X. Kärtner, J. G. Fujimoto, H. A. Haus, and E.
P. Ippen, “Nonlinear optics with phase-controlled pulses in the sub-two-cycle regime,” Phys.
Rev. Lett. 86(24): 5462-5 (2001).
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Control of Q-Switching Instabilities in Mode-locked Lasers by Active
Feedback
Sponsors
MIT Lincoln Laboratory
Grant ACC 333
National Science Foundation
Project Staff
Keren Robinson, Dr. Thomas R. Schibli and Professor Franz X. Kärtner
University of Karlsruhe: Dr. Uwe Morgner
Since the early days of passive mode locking additional undesired Q-switching is a major concern
[1]. Especially in the case of fiber and solid state lasers with long upper-state lifetimes modelocked by semiconductor saturable absorbers, the suppression of the Q-switching instability is a
challenging task. Over the last year several passive schemes for the suppression of Q-switching
instabilities in passively mode-locked lasers using a variety of absorber characteristics have been
investigated [2,3,4]. Even if those methods are used, there still exists a rather limited parameter
range (i.e. pump power, absorber saturation, modulation depth of the absorber, pump volume,
and gain cross section, etc.) over which the laser is continuous wave (cw) mode-locked.
In the last year we demonstrated, that the Q-switching instability in mode-locked lasers can be
controlled by intracavity loss- or gain-modulation driven by the average output power of the laser
via a feedback loop [5]. Stabilization of an unstable system by use of a feedback loop is a well
known concept [6] but has not been applied to suppress Q-switching of mode-locked lasers to our
knowledge, even though it has been used to reduce undesired output power fluctuations. This
technique also allows reducing the energy fluence on the absorber, since the stabilization against
Q-switching is done actively and not by strong saturation of the absorber as is needed especially
in picosecond laser systems. For the case of directly diode-pumped lasers, gain modulation can
be achieved without any additional intracavity elements by direct modulation of the diode current.
The Q-switching instability can be enhanced by positive- or suppressed by negative feedback.
We have derived stability conditions, that help to design the feedback loop for the case of
intracavity gain modulation via pump current modulation in various forms [5]. To demonstrate this
concept, we used a commercially available diode-pumped Nd:YVO4-laser (High-Q-Laser: ModelSC-400) mode-locked with a semiconductor saturable absorber mirror. For the experiment the
laser is operated at low pump power levels, where mode-locked Q-switching does occur. The
feedback loop can be opened or closed by a switch. Fig. 3 shows the output power versus pump
power characteristic of the laser system extracted from a simulation and in the experiment with
and without the feedback loop closed. The feedback technique is able to stabilize the laser in cwmode-locked operation over the full range of pump power available.
This method of controlling the Q-switching instability can be applied to many other laser systems.
It can be used to scale up the output power, repetition rate and the active volume of this and
other laser systems, which is of great importance for diode-pumped laser systems because it
relaxes the requirement on the brightness of the pump diodes. Furthermore, it enables lower
power-densities on the saturable absorber mirror which is beneficial in respect to longterm
stability of the laser system. Fig. 2 shows the microwave spectrum of a higher power, highrepetition rate version of the laser studied above. Without a feedback stabilization, the laser only
operates in the Q-switch modelocking regime at any pump power level. With the feedback loop
on, the Q-switching is highly suppressed, see Fig. 2. The remaining sidebands at -76dBc are due
to a parasitic resonance in the described current source and can be further suppressed by
appropriate shielding. The noise floor at -82dB is due to the available RF-spectrum analyzer.
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24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24
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a)
b)
c)
d)
cw
cw
QSM cwM
L
L
cwM
L
Fig. 1: Output power of a diode-pumped, mode-locked Nd:YVO4 laser as a function of pump
power. Results of a numerical simulation with (a) and without (b) the feedback loop. The feedback
loop stablized the laser system in the cw-modelocked opteration for all pump conditions. (c) and
(d) show the corresponding experimental results, which agree well with the simulations.
(a)
(b)
Fig. 2: Microwave spectra of a feedback stabilized high-power, high-repetition rate Nd:YVO4laser: a) feedback loop open, strong Q-switched mode locking occurs; b) feedback loop closed,
Intensity fluctuations are suppressed by 80 dB.
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RLE Progress Report 144
Ultrafast Phenomena and Quantum Electronics
Resonance Raman Studies on 0.4nm Single Wall Carbon Nanotubes
Sponsors
U.S. Air Force – Office of Scientific Research
Grant F49620-01-1-0084
MRSEC Program of the National Science Foundation
Award DMR 98-08941
Project Staff
Peter Rakich, Dr. Ado Jorio, Professor Gene Dresselhaus, Professor Mildred Dresselhaus,
Professor Erich Ippen, Professor Katrin Kneipp, Professor Z. M. Li, Professor Z. K. Tang
Due to their remarkable physical properties, carbon nanotubes have been studied extensively
during the past several years [1]. Single wall carbon nanotubes (SWNTs) exhibit a wide range of
electronic structures as a result of their reduced dimensionality. Depending on the specific
diameter and chirality, a nanotube may be either semiconducting or metallic. As a result, they are
promising for many future applications.
Recently, fabrication of very small diameter SWNT's has been made possible by growth inside
channels of AlPO4-5 (AFI) zeolite single crystals [2]. Through high-resolution transmission
electron microscopy studies, fabricated nanotubes have been found to have a diameter of 4.2 ±
0.2 Å, which is consistent with the diameter of the pores inside of the AFI crystal (AFI having an
inner diameter of 7.3 Å). Nanotubes of this size may prove to be among the smallest theoretically
possible. It is note worthy that the smallest possible fullerene C20 are of the same diameter, and
recent reports show that they exhibit superconductivity below 15K.
Only three different SWNT structures could be consistent with the measured diameter: metallic
(5,0) and (3,3) and semiconducting (4,2) structures. (Here the indices (m,n) specify chiral vector
and the unit cell of the SWNT.[1]) To experimentally determine the structure of these nanotubes,
both polarized absorption studies [2] and polarized micro-Raman measurements [3] have been
performed, however neither have yielded conclusive evidence of the exact nanotube structures.
Shown in Figure 1 are the polarized absorption spectra of a zeolite crystal with imbedded
nanotubes. The several absorption bands labeled as S, A, B and C in these spectra are
consistent with dipole transitions between Van Hove singularities of the nanotube structures.
Thus, one can only conclude that all of the structures could be present [2].
Alternatively, resonant Raman spectra can yield detailed information about the structure of
nanotubes. A unique chirality can be assigned if the radial-breathing mode is identified through
resonance Raman measurement. Resonance Raman measurements yield two pieces of
information that are necessary to classify the structure of any given nanotube: the precise
measurement of an electronic resonance as well as the frequency of the radial breathing mode.
The absorption peak identified as peak A (at 1.37 eV) in Figure 1 would be consistent with an
absorption resonance of either (5,0) or the (4,2) structure. In order to determine which of these
structures has formed in the zeolite crystal, we have developed a confocal Raman microscope
that is capable of tuning from 1.3 eV to 1.77 eV. Through resonant Raman measurements over
these energies we will identify the structure of these nanotubes, which will allow further study of
these new molecules.
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References
[1] R. Saito, G. Dresselhaus, and M. S. Dresselhaus, in Physical Properties of Carbon Nanotubes
(Imperial College Press, London, 1998)
[2] Z. M. Li, Z. K. Tang, H. J. Lui, N. Wang, C. T. Chan, R. Saito, S. Okada, G. D. Li, and J. S.
Chen, "Polarized Absorption Spectra of Single-Walled 4 Å Carbon Nanotubes Aligned in
Channels of an ALPO4-5 Single Crystal"; Phys Rev. Lett. 87, 127401(1-4) (2001)
[3] A. Jorio, A. G. Souza Filho, G. Dresselhaus, M. S. Dresselhaus, A. Righi, F. M. Matinaga, M.
S. S. Dantas, M. A. Pimenta, J. Mendes Filho, Z. M. Li, Z. K. Tang and R. Saito, "Raman studies
on 0.4nm diameter single wall carbon nanotubes", Chem. Phys. Lett. 351, 27-34 (2002)
Fig. 1 The polarized optical absorption spectra of the SWNT containing AFI crystal. The Curves
Labeled by θ = 0° and θ = 90° are for the electric field 7 AFI channels and the electric field ⊥ AFI
channels respectively. The dotted Curve is the spectrum of the AFI crystal [2].
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Enhanced Light Extraction and Lasing in Two Dimensional Photonic
Crystals
Sponsors
U.S. Air Force – Office of Scientific Research
Grant F49620-01-1-0084
MRSEC Program of the National Science Foundation
Award DMR 98-08941
Project Staff
Peter Rakich, Alexei Erchak, Professor Shanhui Fan, Dr. Daniel Ripin, Dr. Gale Petrich,
Professor Erich P. Ippen, Professor John D. Joannopoulos, Professor Leslie A. Kolodziejski
Semiconductor light-emitting diodes (LEDs) are used in a variety of displays, indicators, lighting
applications, and even short distance communication systems. They are popular because of the
high brightness, low power consumption, and long lifetime. Depending on the geometry of a
particular structure, semiconductor LEDs can be plagued by low efficiencies due to poor light
extraction. In a typical structure, only 2.5% of the light generated will be extracted out of the
device's top surface, while the rest is trapped by guided modes in the high-index semiconductor
material.
Photonic crystals (PC), materials with a spatially periodic refractive index, can be designed to
efficiently couple light from the dielectric guided modes into free space. [1] In particular, twodimensional photonic crystals with the proper dimensions can coherently couple light from
waveguide modes into highly directional radiation modes through diffraction. To demonstrate this
effect we have fabricated a InGaP/InGaAs quantum well LED, with emission centered around 980
nm. A two-dimensional photonic crystal microstructure is then fabricated within the LED by
electron beam lithography, and is designed to inhibit waveguide modes at the emission
frequency. Surface normal photoluminescence (PL) is observed to increase by as much as a
factor of 8 in the photonic crystal region compared to regions on the same LED structure without
photonic crystals.
The two-dimensional photonic crystal LED under study consists of an InGaP/InGaAs active
quantum well region, a low refractive index AlxOy spacer layer, and an AlxOy/GaAs distributed
Bragg reflector (DBR) with a calculated stop band from 800nm to 1400nm. The structure is
fabricated using gas source molecular beam epitaxy, direct-write electron beam lithography,
reactive-ion etching, and oxidation processes [2]. The two-dimensional photonic crystal consists
of a triangular lattice of holes etched within the upper InGaP cladding layer, with a nominal holeto-hole spacing of 382nm and a nominal hole diameter of 193nm. The holes do not penetrate
into the InGaAs quantum well layer, to avoid creating additional non-radiative surface
recombination. Each two-dimensional photonic crystal LED is a 30µm by 30µm region within a
50µm by 50µm LED mesa. Structure schematics and scanning electron micrographs of a
structure are shown in Figure 1.
Room temperature PL measurements have been performed with use of a tunable Ti:Al2O3 laser
and a cooled CCD spectrometer. 785nm laser light was focused with a microscope objective
onto the photonic crystal and the resulting photoluminescence was coupled through the same
objective into the spectrometer. Total PL enhancement factors as high as 100 have been
achieved at individual wavelengths, and the total light extraction efficiency has been improved by
a factor of 8 [1]. As pump powers were increased a sharp spike in the PL spectrum was
observed at 1005nm indicating laser action (see Figure 2a). In order to understand the nature of
the lasing, careful band structure calculations and simulations were necessary. In addition,
several new measurements were needed to connect theory to experiment.
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Understanding the Band structure of a photonic crystal is crucial to understanding how it interacts
with and manipulates light. Several spectroscopic measurements can yield evidence of the
photonic crystal band structure. In our study we have performed both photoluminescence
measurements and white light reflection measurements. Photoluminescence measurements give
a good indication of the integrated density of modes in the vicinity of the Gamma point. One
expects that where the density of modes (above the light line) is highest, the PL will be enhanced
the most. White-light reflection measurements may also yield similar information through phase
matching of incident light into guided modes. Coupling of the radiation into these modes will
result in dips in the reflection spectrum, provided that the appropriate phase matching conditions
are met. These dips tell us where the photonic crystal density of modes is large. In many ways
these two measurements yield the same information, however photoluminescence
measurements are limited to the wavelengths of emission whereas white-light is not.
In order to understand the origin of the lasing, detailed band structure calculations based solely
on the SEM measurements of the device parameters were performed yielding the band diagram
shown in Figure 2b. The horizontal dotted line indicates the lasing frequency. As can be seen,
this line matches well with bending of the bands at the M-point, which occurs at 1007nm.
Flattening of the bands indicates that the group velocity goes to zero, producing a high density of
modes, and likewise a lower threshold for lasing. Therefore, this band diagram predicts the
lasing frequency with good accuracy.
To further test the validity of this band diagram, narrow collection angle (5 degree) PL and whitelight reflection measurements were performed, which precisely determine the positions of the
bands at the Γ point. As can be seen in Figure 3a, plots of both PL and white-light reflection have
sharp spectral features labeled (A) and (B) indicating a large density of modes in the vicinity of
the Γ point. Due to the high symmetry of the photonic crystal at the Γ point, phase matching of
the guided modes to radiation can only occur where bands are degenerate. Upon inspection of
Figure 3b, one finds that there are only two degenerate bands that one would expect to phase
match at the Γ point, these include bands 2 and 3 as well as bands 5 and 6. The frequencies of
these degenerate points are in close agreement with features (A) and (B) identified in the PL and
white-light reflection spectrum seen in Figure 3a.
In conclusion, we have observed PL enhancement factors as high as 100 at individual
wavelengths. As pump powers are increased, two-dimensional distributed feedback results in
laser action at 1005nm. Through band structure calculations based entirely on device
parameters, the lasing was found to be consistent with bending of the bands at the M point. To
our knowledge this is the first observation of lasing at the M point in a photonic crystal.
References
[1] Alexei A. Erchak, Daniel J. Ripin, Shanhui Fan, Peter Rakich, John D. Joannopoulos, Erich P.
Ippen, Gale S. Petrich and Leslie A. Kolodziejski, "Enhanced coupling to vertical radiation using a
two-dimensional photonic crystal in a semiconductor light-emitting diode"; APL., 78, 563-565
(2001).
[2] See RLE Progress Report 2002, L. A. Kolodziejski.
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b
a
Frequency (2πc/a)
Fig. 1. a) The two-dimensional PC Structure b) Scanning electron micrograph of fabricated PC
structure.
b
a
Fig. 2. a) Plot of measured PL at higher pump powers vs. wavelength and frequency.
Pronounced peak visible at 1005 nm. b) Calculated band diagram showing frequency vs. wave
number. The horizontal dotted line indicates the frequency of observed lasing
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Frequency (2πc/a)
400
0.449
0.424
0.402
0.382
1.2
350
1
P
h
o
ot
n
0.8 ci
C
ysr
at
0.6 Rl
e
efl
c
vti
0.4 yti
300
B
t
n 250
e
m
e
c
n 200
a
h
n
E
L 150
P
A
100
0.2
50
0
0
850
900
950
1000
Wavelength (nm)
a
b
Fig. 3 a) Top (bottom) curve is reflectivity (PL enhancement) versus wavelength for 5 degree
Collection angle. b) Calculated band diagram in the vicinity of the Γ point showing frequency vs.
wave number. The line crossing bands indicates cutoff for a collection angle of 5 degrees.
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RLE Progress Report 144
Highly Nondegenerate Four-Wave Mixing in Tapered Microstructure Fiber
Sponsors
U.S. Air Force – Office of Scientific Research
Grant F49620-01-1-0084
Japanese Science and Technology Agency
Project Staff
Dr. K. S. Abedin, J. T. Gopinath, J. Chandalia, Professor E. P. Ippen, and Professor H. A. Haus
Four-wave mixing (FWM) in fiber and semiconductor amplifiers/lasers is a promising technique
for wavelength conversion in telecommunications. Fiber is attractive because it is a passive
device with a large bandwidth. Non-degenerate FWM has previously been observed in 50 m of
SMF for a frequency difference of 96 THz (wavelength separation of 600 nm) [1], and in 6.1 m of
microstructure fiber for a frequency difference of 32 THz (wavelength separation of 20 nm) [2].
We have achieved highly non-degenerate FWM, involving wavelengths of 1540 nm, 800 nm, and
537-575 nm, with conversion efficiencies up to 10%, in an 18 cm tapered air silica microstructure
fiber (T-ASMF) [3]. In the tapered region, the core diameter is reduced to ~3 µm resulting in an
enhancement of the nonlinear effect by an order of magnitude. The large core-clad index
contrast significantly helps the phase matching for FWM.
Figure 1. Experimental setup for highly non-degenerate FWM.
The experimental setup is shown in Figure 1. Light from an femtosecond optical parametric
oscillator (OPO, 1540 nm) with repetition rate of 80 MHz, and a Ti:Sapphire laser (810 nm), was
focused into an 18 cm-long T-ASMF, using an aspheric lens with a focal length of 18 mm.
Coupling efficiencies for the OPO and Ti:Sapphire ranged from 30-40 %. Because of the large
wavelength separation of the Ti:Sapphire and the OPO, the pulses walk-off as they propagate
through the microstructure fiber, due to the difference in the group index. We performed
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numerical simulations to determine the group index and the GVD of the T-ASMF. We obtain a
group index difference ng(1540 nm)-ng(810 nm) of 0.0125 for the lowest order LP mode in the TASMF. This gives an interaction length of 15 mm in the T-ASMF, the distance over which the
pulses overlap. The zero dispersion wavelength was estimated to be ~884 nm.
When the Ti:Sapphire pulse is launched alone, self phase modulation caused the 10 nm
spectrum to broaden to 55 nm. When the OPO is added, we observed generation of strong green
light from the T-ASMF with wavelengths ranging from 537 nm to 575 nm. With a maximum
incident Ti:Sapphire power of 180 mW and OPO power of 80 mW, we were able to generate
about 4 mW of green light (idler) from the fiber. Assuming coupling efficiencies of 32.5% and
43.5%, respectively, for the pump and signal beams, an efficiency Pidler/Psignal of about 10 % was
obtained.
The parametric gain of four-wave mixing process can be expressed as:
g = γPp 2 − (κ / 2 )2
[( )
]
(
)(
where γ = 2π / λ p n 2 / Aeff
wavelength,
)
is the nonlinearity coefficient of the T-ASMF,
λp
is the pump
n2 is the nonlinear refractive index, Aeff is the effective mode-field area, and Pp is
the pump power. The parameter κ is the phase mismatch and is defined as:
κ = ∆k + 2γPp
Here, the term
2γPp is the induced pump nonlinearity, and ∆k = k s + k i − 2k p is the wave vector
mismatch between the signal, idler and the pump beam involved in the FWM process. In an
optical fiber, the effective refractive index of the waves (hence, also ∆ k ), is determined by the
dispersion of the bulk medium (core) as well as the dispersive properties of the waveguide. For
the T-ASMF fiber used in the experiment, the large index difference and the small core size can
result in a waveguide contribution which is comparable to the material contribution and
significantly affects the value of ∆ k .
In Fig. 2, we plot the wave vector mismatch for T-ASMF with core diameter of 2 µm and 3 µm as
a function of signal wavelength (pump wavelength kept constant at 810 nm). The material
contribution of ∆ km is also shown for comparison. The figure clearly indicates that the T-ASMF
with large index difference and small core size can considerably reduce the phase mismatch, and
through proper design it may even be possible to make ∆ k small enough to be compensated for
by the induced pump nonlinearity to ensure κ = 0 .
This work is being carried out in collaboration with C. E. Kerbage and Dr. B. J. Eggleton at Lucent
Technologies.
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RLE Progress Report 144
-1
∆ k m , ∆ k m+w (cm )
600
∆km
400
200
∆ k m+w
φ = 3 µm
0
∆ k m+w
-200
-400
φ = 2 µm
0.8
1
1.2
1.4
1.6
1.8
Signal Wavelength (µm)
Fig. 2. Calculated wave-vector mismatch (waveguide and material contribution combined)
plotted a function of signal wavelength (pump wavelength kept constant at 810 nm) for T-ASMF
with core diameters of 2 µm and 3 µm. The dotted curve shows the contribution of material
dispersion to wave-vector mismatch.
References
1. C. Lin, W. A. Reen, A. D. Pearson, and H.-T. Shang, "Phase matching in the minimum
chromatic-dispersion region of single-mode fibers for stimulated four-photon mixing," Opt.
Lett. 6: 493-495 (1981).
2. J. E. Sharping, M. Fiorentino, A. Coker, P. Kumar, and R. S. Windeler, "Four-wave mixing
in microstructure fiber," Opt. Lett. 26: 1048-1051 (2001).
3. J. K. Chandalia, B. J. Eggleton, R. S. Windeler, S. G. Kosinski, X. Liu, C. Xu, "Adiabatic
coupling in tapered air-silica microstructured optical fiber," IEEE Phot. Tech. Lett. 13: 5254 (2001).
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Femtosecond Pump-Probe Spectroscopy Using a Two-Dimensional Smart
Pixel Detector Array
Sponsors
Air Force Office of Scientific Research (MFEL)
Grant F49620-01-1-0186
Air Force Office of Scientific Research
Grant F49620-98-01-0084
Project Staff
Dr. Stephane Bourquin, Rohit P. Prasankumar, Dr. Rene Salathe (ETH Lausane) and Professor
James G. Fujimoto
Femtosecond pump-probe spectroscopy is a well-known method used to investigate ultrafast
excited state dynamics in condensed matter, chemical, and biological systems. The standard
technique uses a narrowband pump and continuum probe to excite the sample, and employs a
spectrometer to acquire the wavelength dependent absorption saturation dynamics. This
technique often requires the use of a femtosecond amplifier to achieve sufficient intensities for
continuum generation and sufficient pump-probe signal magnitudes. With the development of 5
fs lasers, which can generate spectra spanning one octave, spectrally resolved pump-probe
measurements can be performed without the need for amplifiers [1-3]. Although the pulse
repetition rate from a laser oscillator is extremely high, pulse energies are low and signal levels
are small. Standard CCD detectors cannot detect modulated signals and thus it is not possible to
take advantage of high signal to noise measurements that are possible with high repetition rate
laser sources. Here, we present a new technique that enables the parallel acquisition of pumpprobe measurements for multiple wavelengths. This is made possible using a novel, twodimensional smart pixel detector array, which was originally developed for high speed optical
coherence tomography (OCT) [4, 5]. Each pixel performs amplitude demodulation, and in
combination with a diffraction grating, probe transmission signals can be acquired in parallel for
multiple wavelengths. The smart pixel array can achieve sensitivities comparable to lock-in
amplification but can perform demodulation of probe transmission signals at multiple wavelengths
simultaneously, enabling time and wavelength resolved femtosecond pump-probe spectroscopy.
In this work, we demonstrate spectrally resolved measurement of carrier dynamics in a thin
sample of GaAs using this smart pixel detector array in combination with a broadband Ti:sapphire
laser and femtosecond pump-probe scheme. This system uses a 100 MHz, 5 fs, 275 nm
bandwidth Ti:sapphire laser, the output of which is separated into a 71 mW pump beam and 0.3
mW probe beam. A schematic of the beam configuration is shown in Figure 1. Reflective optics
are used to preserve pulse duration. A 3.8 kHz modulation of the pump beam is performed by use
of a chopper CH. A diffraction grating DG spectrally spreads the probe beam after the sample
and a lens L2 focuses the beam onto one row of the detector array DA. A row of this detector is
read out multiple times for each pump-probe time delay and averaging is performed to increase
the signal to noise ratio. The probe beam is subsequently chopped and the same row is read to
acquire the linear transmission of the sample for data normalization.
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Pump
CH
M
S
L1
DL
DG
Probe
L2
λ1 …
λn
DA
Figure 1. Schematic diagram of the experiment: M, mirror; DL, delay line; CH, chopper; S,
sample; L1,L2, achromat lenses; DG, diffraction grating; DA, detector array.
Figure 1 shows a photograph of a section of the detector array and a schematic diagram. The
silicon detector chip is realized with a 2 µm complementary metal-oxide semiconductor process
with a bipolar transistor option. The die size is 7.2 mm x 7.2 mm, which allows a 58 x 58 pixel
array. Each pixel is 110 µm x 110 µm and contains a 35 µm x 35 µm photodiode and electronic
circuitry for performing amplification, band-pass filtering centered at the modulation frequency,
rectification and low-pass filtering. The filter cut-off frequencies are adjusted by off-chip reference
voltages. The generated amplitude modulation signals are selected sequentially by a row and a
column address decoder, via the pixel and the column buffers. The analog output signal is read
out of the chip, digitized by a 12-bit data acquisition card and transferred to a computer.
Reference
voltages
Pixel cell Photodiode
Row address decoder
Row
address
Pixel buffer
Electronic
circuitry
Column
buffer
Chip
output
Column
address
Column address decoder
Figure 2. (left) Photograph of a section of the smart pixel detector array. The address decoders
are visible on the left and bottom. (right) Detector array architecture corresponding to the
photograph. Each pixel performs amplitude demodulation of the modulated optical signal and its
output is addressed sequentially by a row and a column address decoder.
Results of initial experiments are shown in Figure 3. It can be seen that the dynamics are a
strong function of wavelength. Measurements performed far above the band edge of 875 nm
have a rapid relaxation due to carrier-carrier and carrier-phonon scattering, which remove carriers
from their initial optically excited states. Measurements closer to the band edge show increased
absorption saturation as a function of time from carrier relaxation and state filling.
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2
854 nm
2 .5
1 .5
735 nm
2
1
1 .5
1
0 .5
794 nm
∆T/T
(x10-2)
880 nm
∆T/T 0
(x10-2)
0 .5
0
-0 .5
-0 .5
828 nm
-1
-1
897 nm
-1 .5
-1 .5
-1
0
1
2
d e la y (p s )
3
-1
4
0
1
2
d e la y (p s )
3
4
Figure 3. Femtosecond pump-probe measurements at selected probe wavelengths as a function
of time, extracted from the 3-dimensional data acquired from the smart pixel detector array. The
traces are separated for clarity.
This measurement demonstrates the feasibility of performing parallel spectrally resolved pumpprobe measurements over 58 simultaneous wavelengths with high modulation sensitivities when
averaging each pixel 1000 times, corresponding to 52 seconds of measurement time for a 5.67
ps scan. Even higher sensitivities are possible with increased averaging. This technology can be
applied to a wide range of pump-probe measurements in condensed matter, chemistry, and
biology. Using the two dimensional imaging capabilities of the smart pixel array, an additional
dimension of detection is also possible, enabling for example, one-dimensional spatial imaging
combined with spectrally and time resolved measurement.
References
1.
2.
3.
4.
5.
U. Morgner, F. X. Kärtner, S. H. Cho, H. A. Haus, J. G. Fujimoto, E. P. Ippen, V. Scheuer,
G. Angelow, and T. Tschudi, “Sub-two cycle pulses from a Kerr-Lens modelocked
Ti:sapphire laser,” Optics Letters, 24: 411 -- 413, 1999.
D. H. Sutter, G. Steinmeyer, L. Gallmann, N. Matuschek, F. Morier-Genoud, U. Keller, V.
Scheuer, G. Angelow, and T. Tschudi, “Semiconductor saturable-absorber mirrorassisted Kerr-lens mode-locked Ti:sapphire laser producing pulses in the two-cycle
regime,” Optics Letters, 24: 631-3, 1999.
R. Ell, U. Morgner, F. X. Kärtner, J. G. Fujimoto, E. P. Ippen, V. Scheuer, G. Angelow,
and T. Tschudi, “Generation of 5 fs pulses and octave-spanning spectra directly from a
Ti:sapphire laser,” Optics Letters, 26: 373-5, 2001.
S. Bourquin, P. Seitz, and R. P. Salathe', “Two-dimensional smart detector array for
interferometric applications,” Electronics Letters, 37: 975-976, 2001.
S. Bourquin, P. Seitz, and R. P. Salathe', “Optical coherence topography based on a twodimensional smart detector array,” Optics Letters, 26: 512-514, 2001.
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Photonics and Devices
Resonant Channel Add/Drop Filters
Sponsor
The MIT-Pirelli Lab S.p.A. Research Agreement
Project Staff
Professor H. A. Haus, Dr. C. Manolatou, M. Popović
Channel add/drop filters are basic components in WDM systems. Their function is to add/extract
a channel to/from a WDM stream without affecting the other channels. The filters that we are
studying are based on resonators evanescently coupled to two waveguides: the bus that carries
all the WDM channels and the receiver that carries the selected channel. If the resonant
wavelength coincides with the wavelength of the selected channel and with appropriate
waveguide-resonator couplings, all the channel power is transferred between the bus and the
receiver. To satisfy the WDM system specifications, (e.g. channel bandwidths 10GHz, channel
spacings 50-100GHz, crosstalk from adjacent channels <-25dB), the shape of the filter’s
frequency response must be almost square. Using a number of identical resonators with
appropriate quality factors and couplings the filter response can be engineered to satisfy these
constraints in analogy with filter design using lumped circuit elements.
The design of these filters is tested by Finite Difference Time Domain (FDTD) simulations. An
analytical method that approximates well the filter response around the resonance for given the
resonant frequency and quality factors is the Coupled-Mode Theory (CMT). Fitting the
numerically obtained response with the curves given by the CMT expressions we can extract
basic filter parameters, (i.e. radiation and external quality factors) and understand how we can
improve the filter performance. Our design starts from the first order filter. A basic implementation
of first order filter consists of a ring symmetrically placed between the bus and the receiver. The
bandwidth and the peak of the filter response depend on the ring-waveguide coupling which in
turn depends on their mutual separation and the interaction length. Because a circular ring has a
very short interaction length, its separation from the waveguide is too small to be successfully
fabricated. For this reason we have preferred to use a racetrack instead which, due to its straight
waveguide segments, has a longer interaction length and can thus be placed at a practical
distance from the waveguide. A potential problem with the racetrack is a mode mismatch at the
transition from the straight to the curved section which can result in reflection into the counterpropagating mode as well as radiation in addition to that produced by the bend itself, thus
degrading the filter performance. Specifically if the racetrack has circular bends then at the
transition point the radius of curvature changes discontinuously from infinite to that of the circular
bend. Mode-matching can be improved and the reflection and radiation reduced by designing the
bend to be non-circular with a continuous curvature change. Our designs will be implemented
with silicon nitride waveguides (refr. index 2.2) buried in SiO2 (refr. index 1.46). The core crosssection is chosen 600nmx600nm to ensure single mode operation and polarization
independence. In our FDTD simulations we use a 2D model of this waveguide that has the same
effective index and mode profile in the horizontal direction.
st
An example of such a 1 -order filter simulated by FDTD is shown in Fig.1(a) and its numerically
obtained response fitted by CMT is shown in Fig.1(b). The racetrack fits 40 optical wavelengths
rd
and its separation from the waveguides is 320nm. A 3 -order filter made up of 3 such racetracks
placed between the bus and receiver is shown in Fig. 3(a) with the electric field found by FDTD.
The numerical response curves at the drop and through ports are shown in Fig. 3(b) and (c). CMT
provides the relation between the resonator-waveguide coupling and the resonator-resonator
couplings for maximum received power and Butterworth response; the corresponding theoretical
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curves are also shown in Fig. 3(b) and (c) (dotted line). Comparing the numerical and the
theoretical curve it is clear that the coupling between the racetracks is stronger than desired; in
fact their separation is about 40-60nm too small. Our FDTD simulations have also revealed
st
rd
certain interesting effects, as we go from 1 to 3 order filter, that we can quantify by fitting the
rd
3 -order response obtained by FDTD with the expressions of the CMT.
For this particular example:
a) There is a 21GHz downward shift of the spectrum center.
b) The radiation loss of the resonant modes of 3 coupled racetracks drops almost to one
third of the single racetrack.
c) The spectrum appears skewed which is an indication that each of the 3 modes has a
different radiation loss.
Our explanation for a) is that the resonator-waveguide and the resonator-resonator couplings
bring about different shifts of the propagation constants in the racetracks and thus shift the
resonant frequencies. We attribute b) to radiation cancellations occurring when 3 resonators are
coupled together and c) to the fact that these are different for each of the excited modes of the 3
racetrack system. Our goal is to develop a theory that can predict these effects quantitatively.
(a)
λo =1550.92nm
Q d = 7600
(b)
st
Figure 1: a) Electric field amplitude in a 1 -order resonant channel dropping filter based on a
racetrack with non-circular bends, b) Frequency response obtained by FDTD and fitted by CMT
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(a)
(b)
(c)
rd
Figure 2: a) Electric field amplitude found in a 3 -order channel add/drop filter based on 3
racetracks identical to that of Fig.1 and, b), c) drop and throughput spectra, respectively, found by
rd
FDTD and compared with the desired 3 -order response found by CMT.
Fiber-Chip Coupling
The high index-contrast (HIC) integrated waveguides used in our structures have sub-micron
cross-sections while standard optical fibers have diameters of 8-10 microns. The large mode
mismatch makes it impossible to couple light efficiently from the fiber directly to the optical chip
(more than 80% of the power is lost to radiation).To improve the coupling efficiency a spot-size
converter must be placed between the fiber and the integrated waveguide with certain desirable
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characteristics such as: compactness, broadband operation and low polarization dependence. In
order to ensure compactness and avoid misalignment problems it is preferable to have the spotsize converter integrated on the chip. In the literature such devices are hundreds of microns long,
e.g. adiabatic tapers, and the coupling losses are 1-3dB.
We propose an integrated 3D mode-size converter based on two lensing mechanisms, as shown
in the schematic of Fig. 3(a): The refractive index in the vertical direction is graded quadratically
2
2
as n(y) = ni(1-y /2h ), where ni is the index at the center, by deposition of thin layers of varying
index. A material system suitable for this is structure is silicon oxynitride (SiON) which allows an
index variation from 1.45 (SiO2 index) up to 2.2 (silicon nitride index). It is well known that a
Gaussian beam propagating along the axis of this index distribution contracts and expands
periodically, the period and the minimum beam width depending on the parameter h. If the length
of this layered structure is half a period long it acts as a lens. In the lateral direction the curved
interface also acts a lens with focal length determined by the curvature and the index contrast of
the interface. Both lensing structures are preceded by a quarter-wave thick antireflecting layer.
These two lensing mechanisms combined as in the schematic of Fig. 3(b) will result in a 3D
mode-size conversion.
vertical conversion
lateral conversion
graded index
HIC lens
waveguide mode
fiber mode
(a)
y
x
z
AR coating
(b)
Figure 3: (a) Schematic of 3D mode-size conversion using a graded index distribution
in the vertical direction and high index-contrast (HIC) curved interface in the lateral
direction (b) Combination of the two mechanisms to form a single, compact 3D modesize converter.
The size of the structure is prohibitive for a 3D simulation so for the analysis we decompose the
problem beam optics which gives a very good prediction for the evolution of the beam. The beamwidth variation of into two 2D problems linked by effective index. The initial design is carried out
analytically using Gaussian a Gaussian beam is associated with an effective index variation along
the propagation direction. From he analysis of the vertical mode-conversion (yz-plane) we obtain
the index to use in our analysis and design of the lateral mode-conversion (xz-plane). The FDTD
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simulations for each plane and the 2D coupling efficiencies are shown in Fig. 4(a). At the input we
launch a 10micron wide Gaussian beam that represents the fiber mode and at the output we
calculate the power transmitted into the fundamental mode of a 0.3 micron wide silicon nitride
waveguide. Due to reciprocity the coupling efficiency is the same as in the opposite direction (i.e.
from the fundamental mode of the waveguide to the gaussian - shaped mode of the fiber). The
estimated 3D coupling loss is found as the sum of the 2D coupling losses from the vertical and
the lateral mode conversion. In this example the coupling loss is less than 1dB achieved within
only within 11 microns. As shown in the wavelength response curves in Fig.4(b) this result holds
over a broad bandwidth and for both polarizations.
x
y
TE: -0.31dB (93%)
TM: -0.29dB (93.5%)
x
TE: -0.4dB (91%)
TM: -0.3dB (93.3%)
z
y
z
12 µm
(a)
11 µm
11 µm
(b)
Transmission (dB)
3D estimate
-0.59dB (87%)
-0.71dB (85%)
Figure 4: (a) FDTD simulation of vertical and lateral mode-size conversion and the respective
2D coupling efficiencies and b) 3D estimate for the coupling efficiencies showing broadband
operation and small polarization dependence.
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Polarization Mode Dispersion
Sponsor
MRSEC Program of the National Science Foundation
Grant DMR 98-08941
Project Staff:
Professor Hermann A. Haus, Professor Erich P. Ippen, John M. Fini, P. B. Phua
Polarization mode dispersion (PMD) is a limiting factor in long distance ultrahigh speed optical
telecommunication systems. It broadens and distorts the signal propagating through the fiber.
This leads to inter-symbol interference, which causes detection errors. By performing appropriate
compensation to this pulse broadening effect, one can reduce the bit error rate of the digital
transmission.
st
Our group’s previous work on real-time estimation of 1 order PMD was based on scrambling the
input state of polarization (SOP) so that the output time-averaged state of polarization (SOP) is
distributed on an ellipsoid in the Stokes space representation. The ellipsoid is in contact with the
unit Poincaré sphere at the two points corresponding to the principal states of polarization.
Recently, we investigated a new PMD estimation technique which provides first and second order
PMD information by exploiting the bandwidth dependence of the averaged SOP measured by a
polarimeter [1]. By optically filtering the tapped signal at the receiver end, we measure the
averaged SOP for different spectral bandwidths and deduce the PMD parameters. The technique
allows real-time PMD monitoring in a feed-forward PMD compensation. In addition, this technique
is easily implemented since the PMD diagnosis is done at the receiver end and it utilizes
components that are installed in existing telecommunication systems.
In a feedforward PMD compensation scheme, once the PMD parameters are characterized, a
compensator needs to be set so as to compensate for the observed PMD. For full PMD
nd
compensation up to 2 order, we need at least three concatenated first order PMD segments,
since a two-segment PMD compensator cannot compensate the second order PMD completely
because of its frequency-independent differential group delay (DGD). In [2], we studied a
particular compensator consisting of three first order PMD segments, one of which is adjustable,
concatenated via polarization rotators. Since this is a deterministic approach, we have solved
analytically the required individual rotation matrices of the polarization rotators and the required
st
DGD for the variable delay line in the compensator in order to compensate completely any 1 and
nd
2 order PMD of the transmission cable.
For PMD emulation in the laboratory, emulators have been built by physically concatenating as
many as 15 segments of polarization maintaining fiber with polarization scramblers at every PMF
junction. The reason for using so many segments is to generate statistical distributions of PMD
parameters that resemble those present in the real optical communication fiber. From Figure 1, it
is obvious that a small number of concatenated PMF segments are not adequate to produce the
tail in the Maxwellian distribution of the differential group delay, if only the polarization is randomly
scrambled between these segments. In [3], we propose a novel design of a first and second
order PMD emulator based on a configuration of 4 segments: one variable DGD segment
concatenated with 3 fixed DGD segments. There are three polarization rotators, one at each
junction of the segments. Instead of polarization scrambling at each junction, we set these
polarization rotators according to a statistical schedule so as to produce the probability density
functions (pdf) of first and second order PMD vectors that resemble those present in a long haul
transmission cable (see Figure 2). The essence of this approach is to randomly select a pair of
st
nd
isotropically distributed 1 and 2 order PMD vectors in Stokes space chosen to follow the pdf of
real transmission cables. The 4-segment emulator is set to produce these PMD vectors. Thus,
st
nd
over a large number of independent samplings, the 1 and 2 order PMD vectors generated by
the emulator, will follow the pdf of the real transmission cable.
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Equipment for this project has been provided by The 3M Company.
References
P.B. Phua, J.M. Fini, H.A. Haus, E.P. Ippen, “New 1st and 2nd order PMD
characterization using time-averaged state-of-polarization variation with signal’s
bandwidth”, OSA Annual Meeting 2001, Long Beach, TuY4
nd
P.B. Phua and H. A. Haus, “Deterministic Approach to Pure 2 Order Polarization Mode
Dispersion Compensation”, to be presented at Optical Fiber Communication Conference
2002, Anaheim.
P.B. Phua and H. A. Haus, “A Deterministically Controlled Four-Segment PMD
Emulator”, to be submitted for publication.
[2]
Prob. Density (ps -1)
[3]
0.0002
Prob. Density (ps -2)
[1]
0.015
100
200
5000
r
τ (ps)
10000
r
τ ω (ps2)
st
nd
0.1
Prob. Density (ps -2)
Prob. Density (ps -1)
Figure 1: Monte Carlo simulation of 1 and 2 order PMD generated by a concatenation of 4DGD segments with polarization scramblers between them
0.05
10
20
r
30
40
τ (ps)
st
nd
0.015
50
100
r
150
200
250
τ ω (ps2)
Figure 2: Monte Carlo simulation of 1 and 2 order PMD generated by the same concatenated
4-DGD segments. However, instead of polarization scrambling, the polarization rotators and
adjustable DGD segment, are controlled deterministically to produce the realistic pdfs of
transmission fiber.
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Suppression of Radiation from Quarter-Wave Shifted Bragg Resonators
Sponsor
MRSEC Program of the National Science Foundation
Grant DMR 98-08941
Project Staff
Professor H. A. Haus, Professor J. D. Joannopoulos, Dr. S. G. Johnson, M. R. Watts.
Resonators formed of an optical waveguide Bragg grating with a “defect", a quarter wave shift,
have been proposed as channel dropping filters [1,2]. The free spectral range of such filters
depends on the strength or index contrast of the Bragg grating which determines the bandgap.
As the bandgap is increased, the effective volume of the resonator decreases. A decrease of the
effective volume is generally accompanied by an increase of radiation into the surrounding media.
We propose designs that, in principle, suppress all radiation, and promise in practice to operate
with greatly reduced radiation loss.
~
ε2
n
ε2
z
ε1
~
ε1
ε2
~
ε2
Figure 1: Cross-section of a junction of two step index waveguides.
Our approach is to suppress all radiation at the junction of two waveguides and then form a Bragg
resonator from a series of such junctions. We show that under certain conditions radiation can be
eliminated at the junction of a pair of step index waveguides (depicted in Figure 1) : one with a
ε1 and a cladding
core of dielectric constant ε1 and a cladding of ε 2 , and the other with a core of ~
of ~
ε2 . To demonstrate this result, we begin by using the wave equation to develop necessary
conditions on both the materials and the modes for the junction to be radiation-free. We then
show that a superposition of forward and backward propagating guided modes satisfy the
boundary conditions entirely, precluding any coupling to radiation modes.
The propagation directions are defined along z and the guided modes e and ~
e with z
~
dependences exp( − jβz ) and exp( − j β z ) obey the respective wave equations
and
(∇
(∇
)
~
− β )~
e=0
2
T
+ µ 0 εω2 − β 2 e = 0
(1)
2
T
+ µ0~
ε ω2
(2)
2
Since the transverse field profiles must be continuous at the junction of the two waveguides, the
transverse mode profiles must have the same functional dependence (i.e. e Τ ∝ ~
e Τ ) if the field
solutions are to be composed solely of guided modes. This requirement in conjunction with (1)
and (2) leads directly to the condition that the dielectric contrast must be conserved across the
junction
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ε1 − ε 2 = ~
ε1 − ~
ε2
(3)
It is important to note that this condition and the boundary condition at the waveguide walls
n ⋅ (ε1E1 − ε 2E 2 ) = 0
(4)
cannot both be satisfied except in the trivial case for which ε1 = ~
ε2 or when the
ε1 and ε 2 = ~
normal component of the electric field is zero. Therefore, we restrict our attention to transverse
electric (TE) modes.
Matching of the transverse E-field profile of a TE wave automatically matches the transverse Hfield profile. Indeed, from Faraday’s law
∂
z × E T = − jωµ 0HT
(5)
∂z
we conclude that the matching is automatic without any further constraints. Mode-matching
assures that at any junction of two waveguides no radiation modes are excited, only reflection
and transmission of the propagating mode occurs. A series of mode-matched junctions can then
be spaced at quarter wave separations and a central quarter wave defect introduced to generate
a high-Q cavity. Since the subsections are mode-matched, the field is reflected and transmitted
at each junction, but no radiation occurs since the mode-match is complete with the guided-mode
solutions alone. With no radiation at any interface within the structure, the resonator is radiationfree and can be designed to possess arbitrarily high-Q. A schematic of an ideal two-dimensional
resonator and finite-difference time domain (FDTD) simulation of the field are superimposed in
Figure 2.
defect-layer
n2
n1
a T
~
n
1
~
n
2
Figure 2: Field profile and schematic of an ideal slab resonator
As discussed above, in three dimensions the TE01 modes of dissimilar cylindrical waveguides can
be matched as well, and an analogous resonant structure may be formed. We do so and
calculate the Q's of both structures as a function of the cladding thicknesses (Figure 3). As the
cladding thickness is increased, the Q's increase until they are limited by the external Q imposed
by the limited number of layer pairs in the Bragg mirrors.
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6
7
10
10
105
6
10
4
cavity Q
cavity Q
10
5
10
N=10
N=14
4
10
3
10
N=5
N=10
2
10
101
103
1
2
3
4
5
2
6
3
4
5
6
cladding diameter T (a)
cladding thickness T (a)
Figure 3: Cavity Q plotted as a function of cladding width for (a) slab and (b) cylindrical cases with
N layer pairs on both sides of the “defect". In both cases the indices are n1 = 2,n 2 = 1, ~
n1 = 3 and
~
n = 6 . Note that the effective index contrast is higher in the cylindrical case leading to higher
2
Q for a given number of layer pairs.
Practical integrated optic structures typically require rectangular waveguide geometries. Although
it does not appear to be possible to achieve a perfect mode-match with a rectangular geometry,
strong mode matches can be obtained and radiation substantially suppressed by manipulating
the aspect ratio and index contrast of the guide sections.
References
[1] H. A. Haus and Y. Lai, ``Narrow-band optical channel dropping filter," J. Lightwave
Technology, 10, 57-62 (1992)
[2] J. N. Damask and H. A. Haus, ``Wavelength division multiplexing using channel-dropping
filters," J. Lightwave Technology, 11, 424-428 (1993)
[3] H. A. Haus,``Waves and Fields in Optoelectronics," Prentice Hall, 1984
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Micron-size bending radii in silica-based waveguides
Sponsor
MRSEC Program of the National Science Foundation
Grant DMR 98-08941
Project Staff
Prof. H. A. Haus, Dr. K. Wada, M. Popović
Low index contrast silica bench technology is widely employed in the fabrication of passive
integrated optical components, by virtue of its use of well-tested IC industry manufacturing
processes and technology. Large waveguide cross-sections offer low fiber-to-chip coupling and
propagation losses. A major drawback is the relatively large component size and thus low density
of integration, where a limiting factor is the minimum waveguide bend radius. Bend radii are of
1
millimeter size in the low index contrasts (∆ = 0.25-1.5%) found in silica. On the other hand, high
index contrast such as in silicon-on-insulator (SOI), while offering dense integration, poses
challenges of fiber-to-chip coupling due to mode mismatch and misalignment, and sensitivity to
fabrication defects.
A technology that allows a drastic reduction in the bending radius would overcome one of silica’s
major obstacles to attaining large-scale optical integration. We propose a scheme using tapered
air trenches around bends to provide locally enhanced lateral mode confinement. We use
adiabatic tapering to avoid mode mismatch and Fresnel reflection losses at abrupt junctions in
2
order to miniaturize waveguide bends while maintaining broadband low-loss performance.
3, 4
Air trenches have been proposed for suppressing bend radiation in several ways.
When they
replace the cladding to enhance lateral mode confinement, mode mismatch-induced junction loss
is incurred at points of abrupt change in refractive index and limits the success of the approach in
low index contrast. To our knowledge, no attempt has been made to use air trenches to design
small, low-loss bends in low index contrast by properly addressing the mode mismatch issue
introduced by the present air trench.
We introduce a pair of “inverted” or “cladding” tapers as an integral part of the air trench at the
bend (Fig. 1), in order to provide fast mode transition to and from the high index contrast trench
region with low radiation loss and low reflection.
Polarization insensitivity was not a design consideration. It is inevitably poor in the high aspect
ratio trench in Fig. 1d. Polarization insensitivity of optical processing is to be achieved by splitting
the polarizations and rotating one of them, the signal being processed by identical circuits.
Following, the polarizations are recombined after another rotation. We use the vertical (out of the
wafer plane, Fig. 1) electric field polarization for our designs.
In a junction of two straight waveguides (with a junction similar to the one in Fig. 2a) the trench
waveguide width can be optimized for surprisingly low junction loss (<0.1dB even for fiber-like ∆ ~
0.25%). At optimum, it is wider than the low index input guide. Unfortunately, with a bent trench
waveguide as in Fig. 2a this requirement for low junction loss is in direct conflict with the objective
of minimizing the bend radius. At small radii, the modal width is determined by the bend radius,
not by the waveguide width (whispering gallery regime, e.g. see Ref. 5). Bending loss may still be
very small, but a good mode match to the straight waveguide is no longer possible. A straight
waveguide has a minimum mode width determined by the index contrast, and thus a priori cannot
match well to much narrower modes in the air trench bend. Thus, junction loss sets a lower
bound on the bend radius of a trench bend with abrupt junctions (e.g. 15mm for ∆ = 0.1%). To
make use of small radii, a low loss mechanism to compress the input mode is needed. We
propose using an adiabatic “cladding taper” (Figs. 1, 2c). The taper allows reduction of the radius
in the trench region to the point where it is limited by bending loss rather than the modal
distribution width required for acceptable junction loss.
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A progression from an abrupt junction to a cladding taper is illustrated in Fig. 2. An abrupt junction
(Fig. 2a) requires a large bending radius. A taper after the abrupt junction can be used to
compress the mode so that a tighter bend radius can be used (Fig. 2b), but a more natural way to
build the transition is by introducing the taper as a slow perturbation of the waveguide’s index
profile, as shown in Fig. 2c. This eliminates the Fresnel reflection at the junction of Fig. 2b, and
allows an adiabatic evolution of the mode to a shape that is better matched to the leaky bend
mode.
To demonstrate a reduction in overall size, we compare conventional waveguide bends and air
trench bends designed for the same total loss (98% or 0.1dB/90°). For the chosen transmission,
optimization of the present structure - consisting of a bend and two tapers - for minimum total size
is a complex problem in many parameters. For simplicity, we choose a bend first, and optimize
the taper parameters with respect to the chosen bend design. In lower index examples, the bend
contributes little to overall structure size, so we make the radius larger than needed for low bend
loss in order to also lower the taper-bend junction loss (Fig. 1a).
In a conventional curved waveguide, bending loss is minimized by coupling to the lowest order
5–10
leaky mode.
The total loss in a 90° waveguide bend comes from the bending loss of the
chosen leaky mode in the bend and from mode mismatch (“junction loss”) at the interfaces
between the straight and bent waveguides. In the air trench bend, additional loss is incurred by
propagation through each cladding taper, and at the junction where each taper meets the low
index contrast waveguide (Fig. 1). Radiation loss due to curvature in regular and air trench bends
is evaluated numerically according to the approaches in Refs. 7–9, by the WKB method or by Airy
functions. Conventional bend radii for 98% transmission and ATB radii in the bending region are
shown in Table 1. In low index contrast conventional waveguide bends, bending loss is dominant
and the junction loss can be ignored.
The “cladding tapers” adiabatically shape the fundamental mode of the low index contrast input
waveguide (at the input of section I, Fig. 1a) to the shape of the fundamental mode of the high
index contrast output (at the output of section II, Fig. 1a). In low index contrast, this output mode
can be much smaller than the minimum possible width of the input waveguide mode. The output
waveguide width is fixed by the optimal bend design. The cladding tapers we consider are
piecewise linear.
The tapers are designed using some simple ray optics as a starting point and characterized by
2D FDTD simulations. Individual taper efficiencies are listed in Table 2, with equal throughput in
either direction dictated by reciprocity (assuming lossless dielectrics). We present 2D simulation
results for several example air trench bends, designed in the manner described above and
chosen to demonstrate the proposed idea. Examples A-C range in index contrast from 0.25% to
7% (Table 1), where the last example (Fig. 3a) is above the range of typical index contrasts found
1
in silica waveguides (∆< 1.5%), but was our first structure, used because of its small size in
terms of wavelengths and thus short simulation time. The refractive indices that are chosen for all
examples are ones that could be produced in SiOxNy core/SiO2 cladding waveguides. Waveguide
cross-sections are chosen to be square (in the low index contrast region), and at the cutoff of the
second guided mode.
Finite Difference-Time Domain (FDTD) results for transmission loss are obtained by launching a
short pulse (on the order of 50fs) into the input waveguide with enough spectral width to span the
wavelength spectrum of interest. A discretization of ~20 points per wavelength is used, with the
perfectly matched layer (PML) absorbing boundary condition imposed on the edges of the 2D
computational domain.
Figures 3a-b are plan view plots of the electric field amplitude superimposed on the outline of the
structure. The absence of clear radiation patterns gives qualitative evidence that these bends
exhibit low loss. The new bend radii and total edge lengths including tapers (i.e. effective radii)
are listed in Table 1 alongside regular bend data, and show drastic size reduction for the same
performance. As a metric for the size of bends, the “total” size, we use a box placed around the
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bend which encompasses the bend structure as well as >99.9% of the input and output
waveguide modes (Fig. 3a-b).
Figure 4 shows the transmission and back-scattering over the C-band communications window of
1530-1570nm, exhibiting little wavelength dependence as expected, because the structure makes
no use of either resonance or multi-mode/path interference. The exception is backscattering in
the 180° turn (Example A), where weak Fabry-Perot resonances show up due to the proximity of
the input and output waveguides. Markers placed in Fig. 4 at the central wavelength (1550nm)
show the values of transmission and reflection into the fundamental waveguide mode (obtained
using overlap integrals). These represent the bend transmission loss values of interest, where
98% was targeted in order to compare the bend sizes. Reflection is well below -30dB in all cases,
and thus does not present a problem, at least in theoretical design.
For our 2D simulations, effective indices were obtained assuming a guided mode, hence a
waveguide cross-section in which the air trench and cladding regions extend infinitely above and
below the core (Fig. 5b). The air trench and cladding are in reality of finite vertical extent (Fig. 5a)
and there will be substrate loss if the modal index is lower than the index of the bulk cladding
below the trench. This effect is enhanced in lower index contrast structures, where the trench is of
high aspect ratio, and the mode has a proportionately larger vertical extent (Fig. 5c). In all of
these cases, an ideal air trench of infinite extent would be guiding, and it is its truncation at a finite
depth that results in leakage loss.
For low loss values, we calculate, using a perturbative method, the required depth of the air
trench in Fig. 5a given an acceptable substrate loss (chosen to be much less than total ATB loss).
We define an equivalent current sheet at the trench-bulk cladding interface to replace the source
mode (of the infinite trench in Fig. 5a), that exactly reconstructs the field in the region of interest
below the interface. We then integrate a cylindrical vector Green’s function (e.g., Ref. 11) over the
source for the perturbed substrate region (changed from Fig. 5a to Fig. 5b) to evaluate the far
field radiation, and thus loss per unit length. In this calculation, we assume that silica cladding
continues below the trench - a Si substrate (n = 3.2) below the trench would provide lower loss
due to enhanced reflection. We also assume the mode shape remains unperturbed above the
trench-bulk cladding interface with the appearance of the leaky substrate region.
We show substrate loss results obtained for ATB examples A, B and C in Fig. 6, where 2D crosssection modal solutions from a vectorial mode solver (e.g. see Fig. 5c) were used in the
calculation. From these plots we choose a trench depth for which the substrate loss is much
lower than the ATB loss of 0.1dB.
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Figure 1. Air trench bend schematic: (a) plan view; and cross-sectional views in the (b) low index and (c) air
trench regions. Contour plots representative of the mode pattern are superimposed on cross-sections.
Figure 2. Air trenches in waveguide bends: a) an abrupt junction, b) an abrupt junction followed by a taper,
and c) the proposed adiabatic cladding taper. The bend radius is junction loss limited in (a), but curvature
loss limited in (b),(c). Junction loss is present in (a) and (b), but is virtually eliminated in (c).
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Figure 3. Electric field plot from FDTD simulation: (a) Example C (Table 1), (b) Example A (Table 1). The
total size, as referred to in the text, is outlined by a box (dash-dot). The inset in (b) shows the 180°bend
simulation used for the throughput efficiency of the low index contrast Example A. The key in the lower right
corner refers to transmission data in Fig. 4.
Table 1: Regular and Air Trench Bend radii and total sizes for 98% transmission.
a
Defined as the edge length of a box enclosing the entire bend structure and accommodating >99.9% of the input and
output mode power. The minimum possible bend size is a square with an edge equal to this 99.9% mode width.
Table 2: Air Trench Bend loss budget.
a
Total loss is due to 2 tapers, 2 junctions and 90°of bending. Estimated individual losses do not exactly add to equal the
total loss, because the latter is the result of a simulation of the entire structure and as such is more accurate. Junction loss
at the interface between the low index waveguide and the cladding taper is deemed negligible and ignored.
b
Loss from 180°ATB (note that bending loss is only 0.002 dB/90°).
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Figure 4. (a) Transmitted and (b) backscattered power ratio spectra for examples A (dash-dot), B (dash) and
C (solid). True insertion loss and return loss (into the fundamental mode) are shown at the central
wavelength (A - triangle, B -circle, C - square).
Figure 5. Air trench cross-section: (a) actual air trench of finite depth; (b) idealized trench, free of substrate
loss; and (c) the dominant E-field of the fundamental quasi-TE and -TM modes of the ideal trench for
example C.
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Figure 6. Bulk cladding (substrate) loss: loss per unit length for examples A, B and C (from Fig. 3), as a
function of displacement of the air trench-bulk cladding interface from the core axis (measured in core
heights).
References
1. T. Miya, “Silica-based planar lightwave circuits: passive and thermally active devices,” IEEE
J. of Sel. Topics in Quan. Electron. 6, 38–45, Jan 2000.
2. K. Wada, M. Popovic, S. Akiyama, H. A. Haus, and J. Michel, “Micron-size bending radii in
silica-based waveguides,” in Proc. IEEE/LEOS Summer Topical Meeting on WDM
Components, 13–14, (Copper Mountain, Colorado), Aug 2001.
3. L. H. Spiekman, Y. S. Oei, E. G. Metaal, F. H. Groen, P. Demeester, and M. K. Smit,
“Ultrasmall waveguide bends: the corner mirrors of the future?,” IEE Proc.-Optoelectron.
142,. 61–65, Feb 1995.
4. J. Yamauchi, M. Ikegaya, and H. Nakano, “Bend loss of step-index slab waveguides with a
trench section,” Microwave and Optical Technol. Lett. 5, 251–254, Jun 1992.
5. E. C. M. Pennings, Bends in Optical Ridge Waveguides: Modeling and Experiments. PhD
thesis, T. U. Delft, The Hague, Netherlands, 1990.
6. E. A. J. Marcatili, “Bends in optical dielectric waveguides,” Bell Sys. Tech. J. 48, 2103–2132,
Sep 1969.
1. M. Heiblum and J. H. Harris, “Analysis of curved optical waveguides by conformal
transformation,” IEEE J. of Quan. Electron. QE-11, 75–83, Feb 1975.
2. D. Rowland, “Nonperturbative calculation of bend loss for a pulse in a bent planar
waveguide,” IEE Proc.-Optoelectron. 144, 91–96, Apr 1997.
3. I.C. Goyal, R.L. Gallawa, and A.K. Ghatak, “Bent planar waveguides and whispering gallery
modes:a new method of analaysis,” J. Lightwave Technol. 8, 768-774, May 1990.
4. M. K. Smit, E. C. M. Pennings, and H. Blok, “A normalized approach to the design of low-loss
optical waveguide bends,” J. Lightwave Technol. 11, 1737–1742, Nov 1993.
5. J. A. Kong, Electromagnetic Wave Theory, EMW Publishing, Cambridge, MA, USA, 1999.
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Micromachined Photonic Devices using Nonlinear Materials Processing
Sponsors
Air Force Office of Scientific Research (MFEL)
Grant F49620-01-1-0186
Air Force Office of Scientific Research
Grant F49620-98-01-0084
Project Staff
Andrew M. Kowalevicz, Dr. Ingmar Hartl, Dr. Kaoru Minoshima, Professor Erich P. Ippen, and
Professor James G. Fujimoto
Photonic device fabrication using nonlinear material processing with near-IR femtosecond pulses
can generate localized, clean, three-dimensional (3D) structures in many materials [1-3].
Recently, fabrication of glass waveguides using pulses directly from femtosecond lasers
oscillators, without the need for amplification, has been reported [4-6]. Unamplified laser
oscillators have several advantages over amplified systems for device fabrication including lower
cost and complexity as well as greater control of exposure parameters. Because of their high
repetition rates, multiple low intensity shots interact within the focal volume, leading to well
controlled cumulative effects. Several photonic devices have been demonstrated including
single-mode waveguides [5], X-couplers [6], and directional couplers [4]. Our group has
developed a high-power femtosecond Ti:Sapphire laser oscillator using a novel long cavity design
[7]. Because of its high pulse energies and high repetition rate, this laser enables the versatile
fabrication of a wide range of photonic devices.
We demonstrate the fabrication and characterization of directional couplers and what is, to our
knowledge, the first Mach-Zehnder interferometer. Interferometers are of interest because of
their use in practical devices such as sensors and switches. These devices are characterized
using a variety of techniques including near field and far field mode measurements, interaction
length dependence analysis, and wavelength dependent device properties. Laser processing
enables rapid prototyping and fabrication of three dimensional device geometries.
Coupling ratio
The laser used to perform the nonlinear material processing is a high pulse energy Ti:Sapphire
laser oscillator. The laser cavity has been extended using a Herriott type multipass cell to reduce
the repetition rate to 4MHz and increase the pulse energy. Operating in net negative dispersion
with a saturable bragg reflector (SBR) to stabilize operation, 80 fs pulses with up to 100 nJ
energies can be generated [7]. The laser is focused into a 1 mm thick glass plate using a 100X
oil-immersion microscope objective with an effective 0.6 NA. Device structures were fabricated
by 2D translation of the sample perpendicular to the incident light using precision computer
controlled stages.
0.6
0.4
0.2
(C)
0.0
0
2
4
6
8
Interaction length (mm)
10
Figure 1. (a) Phase contrast image of directional waveguide couple with interaction length shown
and (b) coupling ratio vs. interaction length for directional waveguide couplers that demonstrates
coupled-move behavior.
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Couplers such as X couplers as well as coupled mode devices have been demonstrated,
however detailed characterization requires more than only output spot observation. Here we
analyzed mode-coupling behavior. Figure 1 (a) shows a two waveguide directional coupler
fabricated inside a glass plate. Waveguides were fabricated by scanning the plate in the X-Y
plane at 10 mm/s with an incident pulse energy of ~10 nJ. Evaluation of the mode profile at the
output of the coupler shows that the waveguides have diameters of 4-5 µm and are single mode
in the near-IR. We fabricated a series of couplers with several different interaction lengths
(indicated with an arrow in Figure 1 (a). Figure 1 (c) shows the coupling ratio as a function of the
interaction length with 8 µm separation. The data shows that the coupling ratio oscillates as a
function of interaction length. The period of oscillation changes as a function of the separation.
The oscillatory behavior is the first clear evidence of mode coupling between two waveguides
fabricated by the nonlinear laser processing.
As an example of a more complex device, we demonstrate, what we believe is for the first time,
the fabrication and characterization of a Mach-Zehnder interferometer by the nonlinear laser
processing. The interferometer, consisting of two X-couplers placed back-to-back, with crossing
angles of 2 degrees. The path length difference between the two arms is approximately 10 µm.
In order to characterize the operation of the interferometer, we used the output of a KLM
Ti:Sapphire laser which can generate ~5 fs pulses and bandwidths of ~300 nm [8]. This laser
source provides a high brightness, single mode beam, which can be easily coupled into the
waveguides by using a microscope objective. The edges of the glass substrate were polished to
optical quality in order to enable efficient coupling. The output spectrum from the interferometer
waveguide was collimated using a second microscope objective.
The spectral data was
measured with an optical multichannel analyzer (OMA).
If a given frequency of light is coupled into the interferometer, it will be split into the two arms of
the interferometer at the first X-coupler, travel different path lengths, and will either constructively
or destructively interfere after being recombined at the second X-coupler. Thus, the unbalanced
path length Mach-Zehnder functions as a filter. The frequency or wavelength dependence of the
interferometer can be measured using a broadband light source. Figure 2 (a) shows the input
spectrum with a FWHM of 130 nm. Figure 2 (b) shows the wavelength transfer function in the
crossed interferometer arm. The transfer function is constructed by normalizing the output
spectrum by the input spectrum. With an arm length difference of 10 µm and a spectrum
centered at 800 nm, oscillations in the output should occur with periodicity of ~65 nm. We
observe a full modulation of the transfer function with a periodicity of approximately 50 nm, in
good agreement with theory.
Figure 2. Broad input spectrum, and normalized output spectrum demonstrating the
interferometer is acting as a filter with periodic modulation of the spectrum.
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We demonstrate photonic device fabrication using nonlinear materials processing in glass with a
high power femtosecond laser oscillator. The building blocks of photonic devices such as X
couplers, coupled mode devices, as well as interferometers can be fabricated. Interaction length
dependence and wavelength dependent characteristics, using femtosecond broadband lasers as
light sources, can be measured. Femtosecond materials processing will enable rapid and
versatile device prototyping as well as the fabrication of three-dimensional structures not possible
with planer techniques.
References
1.
2.
3.
4.
5.
6.
7.
8.
K. M. Davis, K. Miura, N. Sugimoto, and K. Hirao, “Writing waveguides in glass with a
femtosecond laser,” Optics Letters, 21: 1729-1731, 1996.
D. Homoelle, S. Wielandy, A. L. Gaeta, N. F. Borrelli, and C. Smith, “Infrared
photosensitivity in silica glasses exposed to femtosecond laser pulses,” Optics Letters,
24: 1311-1313, 1999.
Y. Sikorski, A. A. Said, P. Bado, R. Maynard, C. Florea, and K. A. Winick, “Optical
waveguide amplifier in Nd=doped glass written with near-IR femtosecond laser pulses,”
Electronic Letters, 36, 2000.
A. M. Streltsov and N. F. Borrelli, “Fabrication and analysis of a directional coupler written
in glass by nanojoule femtosecond laser pulses,” Optics Letters, 26: 42, 2001.
C. B. Schaffer, A. Brodeur, J. F. Garcia, and E. Mazur, “Micromachining bulk glass by
use of femtosecond laser pulses with nanojoule energy,” Optics Letters, 26: 93, 2001.
K. Minoshima, A. M. Kowalevicz, I. Hartl, E. P. Ippen, and J. G. Fujimoto, “Photonic
device fabrication in glass by use of nonlinear materials processing with a femtosecond
laser oscillator,” Optics Letters, 26: 1516-1518, 2001.
S. H. Cho, F. X. Kärtner, U. Morgner, E. P. Ippen, J. G. Fujimoto, J. E. Cunningham, and
W. H. Knox, “High energy pulse generation using a 4 MHz repetition rate KLM Ti:Al2O3
laser operating with positive and negative dispersion,” Optics Letters, 26: 560-562, 2001.
U. Morgner, F. X. Kärtner, S. H. Cho, H. A. Haus, J. G. Fujimoto, E. P. Ippen, V. Scheuer,
G. Angelow, and T. Tschudi, “Sub-two cycle pulses from a Kerr-Lens modelocked
Ti:sapphire laser,” Optics Letters, 24: 411 -- 413, 1999.
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Publications
1. L.A. Jiang, M.E. Grein, E.P. Ippen, C. McNeilage, J. Searls, and H. Yokoyama, “Quantumlimited noise performance of a mode-locked laser diode,“ Opt. Lett 27(1):49-51 (2002).
2. M.E. Grein, H. A. Haus, L. A. Jiang, and E. P. Ippen, “Action on pulse position and
momentum using dispersion and phase modulation,” Opt. Express 8(12): 664-669 (2001).
3. C. Chudoba, J.G. Fujimoto, E.P. Ippen, H.A. Haus, U. Morgner, F.X. Kärtner, V. Scheuer, G.
“
Angelow, and T. Tschudi, All solid-state Cr:forsterite laser generating 14 fs pulses at 1.3
µm,” Opt. Lett. 26(5): 292-4 (2001).
4. R. Ell, U. Morgner, F.X. Kärtner, J.G. Fujimoto, E.P. Ippen, V. Scheuer, G. Angelow, T.
Tschudi, M.J. Lederer, A. Boiko, and B. Luther-Davies, “Generation of octave filling spectra,
5-fs-pulses directly from a Ti:Sapphire oscillator with enhanced dispersion-managed-soliton
formation,” Opt. Lett. 26(6): 373-5 (2001).
5. F.X. Kärtner, U. Morgner, T.R. Schibli, E.P. Ippen, J.G. Fujimoto, V. Scheuer, G. Angelow
and T. Tschudi, “Ultrabroadband double-chirped mirror pairs for generation of octave
spectra,'' J. of the Opt. Soc. of Am. B 18(6), 882-5, (2001).
6. U. Morgner, R. Ell, G. Metzler, T.R. Schibli, F.X. Kärtner, J.G. Fujimoto, H.A. Haus and E.P.
Ippen, ''Nonlinear optics with phase-controlled pulses in the sub-two-cycle regime,'' Phys.
Rev. Lett. 86(24): 5462-65 (2001).
7. O.D. Mücke, T. Tritschler, M. Wegener, U. Morgner and F.X. Kärtner, ''Signatures of CarrierWave Rabi-Flopping in GaAs,'' Phys. Rev. Lett. 87 (057401): 1-4 (2001).
8. T.R. Schibli, T. Kremp, U. Morgner, F.X. Kärtner, R. Butendeich, J. Schwarz, H. Schweizer,
F. Scholz, J. Hetzler and M. Wegener ''CW-operation and Q-switched mode locking of
4+
Cr :YAG-microchip lasers,'' Opt. Lett. 26(12): 941-3 (2001).
9. D. J. Ripin, C. Chudoba, J. T. Gopinath, J. G. Fujimoto, E. P. Ippen, U. Morgner, F. X.
Kärtner, V. Scheuer, G. Angelow and T. Tschudi, ''Generation of 20 fs pulses by a prismless
4+
Cr :YAG laser,'' Opt. Lett. 27(1): 61-3 (2002).
10. W. Drexler, U. Morgner, R. K. Ghanta, F. X. Kärtner, J. S. Schuman and J. G. Fujimoto,
"Ultrahigh resolution ophthalmic optical coherence tomography," Nature for Medicine 7(4):
502-7 (2001)
11. S. H. Cho, F.X. Kärtner, U. Morgner, E.P. Ippen, J.G. Fujimoto, J.E. Cunningham and W.H.
Knox, “90 nJ pulse generation using a 4 MHz repetition rate KLM Ti:sapphire laser operating
with net positive and negative intracavity dispersion,“ Opt. Lett. 26(8): 560-562, (2001).
12. W. Drexler, U. Morgner, R. K. Ghanta, F. X. Kärtner, J. S. Schuman, J.G. Fujimoto, “Ultrahigh
resolution ophthalmic optical coherence tomography,” Nature Medicine 7, 502-507, (2001).
13. C. Chudoba, J. G. Fujimoto, E. P. Ippen, H. A. Haus, U. Morgner, F. X. Kartner, V. Scheuer,
G. Angelow, and T. Tschudi, “All-solid-state Cr:forsterite laser generating 14 fs pulses at 1.3
µm,” Opt. Lett. 26, 292-294, (2001).
14. R. Ell, U. Morgner, F. X. Kärtner, J. G. Fujimoto, E. P. Ippen, V. Scheuer, G. Angelow, T.
Tschudi, M. J. Lederer, A. Boiko, and B. Luther-Davies, “Generation of 5-fs pulses and
octave-spanning spectra directly from a Ti:Sapphire laser,” Opt. Lett. 26, 373-375, (2001).
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15. I. Hartl, X.D. Li, C. Chudoba, R. Ghanta, T. Ko, J.G. Fujimoto, J. K. Ranka, R. S. Windeler,
and A. J. Stentz, “Ultrahigh resolution optical coherence tomography using continuum
generation in an air-silica microstructure optical fiber,” Opt. Lett. 26, 608-610, (2001).
16. F. X. Kärtner, U. Morgner, R. Ell, T. Schibli, J. G. Fujimoto, E. P. Ippen, V. Scheuer, G.
Angelow, and T. Tschudi, “Ultrabroadband double-chirped mirror pairs for generation of
octave spectra,” J. Opt. Soc. Am.B, 18, 882-885, (2001).
17. U. Morgner, R. Ell, G. Metzler, T. R. Schibli, J. G. Fujimoto, E. P. Ippen, and F. X. Kärtner,
“Nonlinear optics with phase-controlled pulses in the sub-two-cycle regime,” Phys. Rev. Lett.,
86, 5462-5465, (2001).
18. K. Minoshima, A. M. Kowalevicz, I. Hartl, E. P. Ippen, and J. G. Fujimoto, “Photonic device
fabrication in glass by use of nonlinear materials processing with a femtosecond laser
oscillator,” Opt. Lett. 26, 1516-1518, (2001).
Conference Papers
1. L. A. Jiang, M. E. Grein, J. M. Fini, E. P. Ippen and H. A. Haus, “Noise of harmonically
modelocked lasers,” Gordon Research Conference on Nonlinear Optics and Lasers, ColbySawyer College, New London, New Hampshire, July 29 - August 3, 2001.
2. L. A. Jiang, M. E. Grein, B. S. Robinson, E. P. Ippen, and H. A. Haus, “Experimental
demonstration of a timing jitter eater,” submitted to the Conference on Lasers and ElectroOptics 2002
3. M. E. Grein, H. A. Haus, E. P. Ippen, and Y. Chen, “The quantum limit of timing jitter in
actively mode-locked soliton fiber lasers”, in OSA Trends in Optics and Photonics (TOPS)
Vol. 56, Conference on Lasers and Electro-Optics (CLEO 2001), pp. 243-244.
4. M. E. Grein, L. A. Jiang, H. A. Haus, and E. P. Ippen, “Timing jitter in modelocked lasers,”
IEEE Lasers and Electro Optics Society Annual Meeting, November 12-14, San Diego CA,
USA, paper MWP. Invited Talk
5. J. J. Hargreaves, P. W. Juodawlkis, J. J. Plant, J. P. Donnelly, J. C. Twichell, F. Rana, M. E.
Grein, R. J. Ram, E. P. Ippen, “Timing jitter in modelocked lasers,” IEEE Lasers and Electro
Optics Society Annual Meeting, November 12-14, San Diego CA, USA, paper MWQ. Invited
Talk
6. C. Chudoba, J.G. Fujimoto, E.P. Ippen, H.A. Haus, U. Morgner, F.X. Kärtner, V. Scheuer, G.
Angelow and T. Tschudi, ''All-solid-state Cr:forsterite laser generating 14-fs pulses with 250
nm bandwidth at 1.3 µm,'' Photonics West, San Jose, California, January 20-26, 2001.
7. O. D. Mücke, T. Tritschler, M. Wegener, U. Morgner and F.X. Kärtner, ''Carrier-Wave RabiFlopping in GaAs,'' Quantum Electronics and Laser Science Conference (QELS 2001),
Baltimore, USA, May 6-11, 2001.
8. U. Morgner, R. Ell, G. Metzler, T.R. Schibli, F.X. Kärtner, J.G. Fujimoto and E.P. Ippen,
''Nonlinear Optics with Phase-Conrolled Pulses in the Sub-Two-Cycle regime,'' Quantum
Electronics and Laser Science Conference, Baltimore, Maryland, May 6-11, 2001.
9. C. Jirauschek, U. Morgner and F.X. Kärtner, ''Spatio-temporal Gaussian Pulse Dynamics in
Sub-10-fs Lasers,'' paper presented at Quantum Electronics and Laser Science Conference,
Baltimore, Maryland, May 6-11, 2001.
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10. O. D. Mücke, T. Tritschler, M. Wegener, U. Morgner and F.X. Kärtner, ''Signatures of carrierWave Rabi-Flops in GaAs,'' DFG-Spring-Meeting, Hamburg, Germany, March 26-30, 2001.
11. T.R. Schibli, T. Kremp, U. Morgner, F.X. Kärtner, R. Butendeich, J. Schwarz, H. Schweizer,
F. Scholz, J. Hetzler and M. Wegener, ''CW and Q-switched mode-locked Cr:YAG micro-chip
lasers,'' Advanced Solid-State Lasers 2001, Seattle, Washington, January 28-31, 2001.
12. U. Morgner, R. Ell, F.X. Kärtner, J.G. Fujimoto, E.P. Ippen, V. Scheuer, G. Angelow, T.
Tschudi, M.J. Lederer, A. Boiko and B. Luther-Davies, ''Octave-spanning Spectra Directly
from a two-foci Ti:sapphire laser with enhanced self-phase modulation,'' Conference on
Lasers and Electro-Optics (CLEO 2001), Baltimore, Maryland, May 6-11, 2001.
13. F.X. Kärtner, R. Butendeich, J. Schwarz, H. Schweizer, F. Scholz, J. Hetzler and M.
Wegener, ''Mode-locking of Cr:YAG Microchip Lasers,'' at Conference on Lasers and ElectroOptics, Baltimore, Maryland, May 6-11, 2001.
14. F.X. Kärtner, U. Morgner, E.P. Ippen, J.G. Fujimoto, V. Scheuer, G. Angelow and T. Tschudi,
''Few-Cycle-Pulse Generation and its Applications'', International Conference on Lasers and
Electro-Optics (CLEO Pacific 2001), Tokyo, Japan, July 15-19, 2001.
15. W. Drexler, U. Morgner, F.X. Kärtner, C. Spielmann, A. Stingl, F. Krausz, E.P. Ippen, J.G.
Fujimoto and A.F. Fercher, ''Ultrahigh resolution, functional optical coherence tomography
using state of the art femtosecond laser technology,'' International Conference on Lasers and
Electro-Optics (CLEO Pacific 2001), Tokyo, Japan, July 15 - 19, 2001.
16. U. Morgner, F.X. Kärtner, J.G. Fujimoto and E. P.; Ippen, ''Sub-Two-Cycle Pulse Generation
and Phase-sensitive Nonlinear Optics,'' Gordon Research Conference, New London, New
Hampshire, USA, July 30-- August 2, 2001. Invited Talk
17. F. X. Kärtner, ''Noise in optical systems,'' paper presented at Workshop on Modeling,
Simulation and Optimization of Integrated Circuits, Oberwolfach, Germany, 2000.
18. Hartl, X. Li, C. Chudoba, R. K. Ghanta, T. H. Ko, J. G. Fujimoto, J. K. Ranka, R. S. Windeler,
and A. J. Stentz, “Ultrahigh resolution OCT using continuum generation in an air-silica
microstructure optical fiber,” International Biomedical Optics Symposium, BIOS’2001, San
Jose, CA, January 20-26, 2001, paper 4251-07.
19. X. Li, C. Chudoba, T. H. Ko, C. Pitris, R. K. Ghanta, and J. G. Fujimoto, “Imaging solid
tissues with an OCT imaging needle,” International Biomedical Optics Symposium,
BIOS’2001, San Jose, CA, January 20-26, 2001, paper 4251-09.
20. P. Hsiung, X. Li, I. Hartl, C. Chudoba, T. H. Ko, R. K. Ghanta, and J. G. Fujimoto, “Highspeed path length scanning for optical coherence tomography,” International Biomedical
Optics Symposium, BIOS’2001, San Jose, CA, January 20-26, 2001, paper 4251-15.
21. R. Ell, U. Morgner, F. X. Kärtner, J. G. Fujimoto, E. P. Ippen, V. Scheuer, G. Angelow, T.
Schudi, M. J. Lederer, A. Boiko, and B. Kuther-Davies, “Octave-spanning spectra directly
from a two-foci Ti:sapphire laser with enhanced self-phase modulation,” Conference on Laser
and Electro-Optics, CLEO’01, Baltimore, MD, May 6-11, 2001, invited paper CMF1.
22. R. P. Prasankumar, I. Hartl, J. T. Gopinath, E. P. Ippen, J. G. Fujimoto, P. Mak, and M. F.
Ruane, “Ultrafast dynamics of non-epitaxially grown semiconductor-doped silica films
saturable absorbers,” Quantum Electronics and Laser Science Conference, QELS’01,
Baltimore, MD, May 6-11, 2001, paper QtuF1.
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RLE Progress Report 144
23. K. Minoshima, I. Hartl, E. P. Ippen, and J. G. Fujimoto, “Versatile photonic device fabrication
using nonlinear processing in glass with a femtosecond laser oscillator,” Quantum Electronics
and Laser Science Conference, QELS’01, Baltimore, MD, May 6-11, 2001, paper QTuH2.
Invited Talk
24. Hartl, C. Chudoba, T. Ko, X. D. Li, J. G. Fujimoto, M. Brezinski, and R. S. Windeler, “Imaging
water absorption with spectroscopic optical coherence tomography,” Conference on Laser
and Electro-Optics, CLEO’01, Baltimore, MD, May 6-11, 2001, paper CWE2.
25. U. Morgner, R. Ell, G. Metzler, T. R. Schibli, F. X. Kärtner, J. G. Fujimoto, and E. P. Ippen,
“Nonlinear optical with phase-controlled pulses in the sub-two-cycle regime,” Quantum
Electronics and Laser Science Conference, QELS’01, Baltimore, MD, May 6-11, 2001, paper
QFC2.
26. P. Hsiung, X. D. Li, I. Hartl, T. Ko, C. Chudoba, and J. G. Fujimoto, “High-speed path length
scanning using a Herriott cell delay line,” Conference on Laser and Electro-Optics, CLEO’01,
Baltimore, MD, May 6-11, 2001, poster CWA62.
27. Hartl, C. Chudoba, T. Ko, X. D. Li and J. G. Fujimoto, R. S. Windeler, “Optical coherence
tomography using continuum generation in an air-silica microstructure optical fiber,”
European Conference on Biomedical Optics, ECBO, Munich, June 17-22, 2001, paper MH2.
28. X. Li, I. Hartl, C. Chudoba, W. Drexler, T. Ko, P. Hsiung, C. Pitris, R. Ghanta, F. X. Kärtner,
U. Morgner, M. E. Brezinski, and J. G. Fujimoto, “Ultrahigh resolution and spectroscopic
th
optical coherence tomography,” OSA Annual Meeting, ILS-XVII: 17 Interdisciplinary Laser
Science Conference, Long Beach, CA, October 14-18, 2001, paper WV1, Invited Talk.
29. J. G. Fujimoto, I. Hartl, P. Xue, T. H. Ko, C. Chudoba, W. J. Wadsworth, T. A. Birks, and R.
S. Windeler, “Supercontinuum generation in photonic crystal fibers as light sources for OCT,”
Photonics West, SPIE, San Jose, CA, January 19-25, 2002, paper 4633-26, Invited Talk.
30. A. M. Kowalevicz, T. R. Schibli, R. P. Prasankumar, F. X. Kärtner, and J. G. Fujimoto, “Ultralow-threshold, low cost, Kerr lens modelocked Ti:Al2O3 laser,” Conference on Lasers and
Electro-Optics (CLEO), Long Beach, CA, May 19-24, 2002, to be presented.
31. Hartl, P. Hsiung, T. H. Ko, J. G. Fujimoto, T. A. Birks, W. J. Wadsworth, U. Bünting, and D.
Kopf, “High resolution OCT imaging using a spectrally broadened femtosecond Nd:Glass
laser,” Conference on Lasers and Electro-Optics (CLEO), Long Beach, CA, May 19-24, 2002,
to be presented.
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