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Optical Engineering
OPTICS IN 2008
Repetition Rate Multiplication Using
All-Pass Optical Structures
Miguel A. Preciado and Miguel A. Muriel
(a)
Input periodic pulse train
a1(t)
Intensity [a.u.]
(b)
Output periodic pulse train
All-pass
optical
structure
a2(t)
Proposed structures
0.5
2x
0.25
0
0 1 2 3 4 5 6 7 8 9 10
Time [ps]
(c)
0.3
Intensity [a.u.]
T
echniques for creating ultrahigh
repetition rate pulse trains are
highly sought after for future ultrahighspeed optical communication systems.
Researchers have explored several strategies for generating periodic pulse trains
at repetition rates beyond those achievable by mode locking or direct modulation. One alternative is pulse repetition
rate multiplication (PRRM) of a lower
rate source by applying phase-only
spectral filtering, usually based on the
temporal Talbot effect.1
We have recently proposed several
all-pass structures based on optical
cavities; these perform phase-only
spectral filtering for the implementation of repetition-rate multipliers of a
periodic pulse train with uniform output
train envelopes.2,3 We found optimum
solutions for 23, 33, 43, 63 and 123
multiplication factors. As can be seen
in part (a) of the figure, the proposed
optical structures are composed of 1-4
ring resonators (RRs). We found that a
single RR structure can achieve three
factors of repetition-rate multiplication
(23, 33 and 43), being specially suited
for 23 in terms of accuracy and robustness.2 We presented two structures that
achieve accurate and robust solutions for
33 and 43 PRRM, both composed of
two identical RRs in cascade or coupled
configuration.3 We have also proposed
several optical structures for 63 and 123
PRRM by combining filters of 23, 33
and 43 PRRM.3
Parts (b) and (c) show the results
numerically for two of our studies.2,3
Part (b) shows the output pulse train
intensity numerically obtained for 23
repetition rate multiplication, where an
input repetition rate of 100 GHz was
assumed. Part (c) shows the output pulse
train intensity numerically obtained
for 33, 43, 63 and 123 multiplication
factors, with an input repetition rate of
0.2
0.1
0
0 10 20 30 40 50 60 70 80 90100
Time [ps]
(a) Schematic of the system. The periodic pulse train is processed by the all-pass optical
structure. We propose nine optical structures composed of multiple RRs. (b) Output pulse
train intensity of examples for 23 multiplication with a single all-pass RR. (c) Output pulse
train intensity of examples for 33 (blue), 43 (red), 63 (green) and 123 (yellow) multiplication
and the respective optical structures.
10 GHz. The RR parameters obtained in
these examples are readily feasible. We
have also analyzed the effect of RR losses
on the energetic efficiency and the output pulse train envelope uniformity and
the effect of the frequency deviations on
the envelope uniformity.3
In conclusion, these structures are
readily feasible and present an intrinsic
high energetic efficiency, ideally of 100
percent, that is only limited by internal RR losses. It is worth noting that,
like other spectrally periodic filtering
techniques based in optical cavities,4
the system requires the locking of the
spectrum of the input signal, which is
typically composed of the mode comb of
the laser, to the spectral response of the
optical structure. t
Miguel A. Preciado ([email protected]) and
Miguel A. Muriel are with the Universidad Politecnica
de Madrid in Madrid, Spain.
References
1. J. Azaña and M.A. Muriel. “Temporal Talbot effect in fiber
gratings and its applications,” Appl. Opt. 38, 6700-4
(1999).
2. M.A. Preciado and M.A. Muriel. “Repetition rate multiplication using a single all-pass optical cavity,” Opt. Lett. 33,
962-4 (2008).
3. M.A. Preciado and M.A. Muriel. “All-pass optical structures
for repetition rate multiplication,” Opt. Express 16, 11162-8
(2008).
4. J. Chen et al. “Generation of low-timing-jitter femtosecond
pulse trains with 2 GHz repetition rate via external repetition rate multiplication,” Opt. Lett. 33, 959-61 (2008).
OPN December 2008 | 37
Optical Engineering
Electromagnetic Arbitrary Waveform Generation
with Broadband Incoherent Light Sources
V. Torres-Company, J. Lancis, P. Andrés and L.R. Chen
I
n recent years, advances in pulseshaping technology have shown great
potential for applications in microwave
photonics.1,2 Researchers’ interest is
partly motivated by the fact that the
generation of arbitrary electromagnetic
signals with 1-50 GHz frequency content is a challenge for purely electronic
systems. Broad bandwidth signals could
have a positive impact on high-speed
wireless communication systems and find
interesting applications in radar, remote
sensing and electronic-equipment test
measurements.1
Previously demonstrated photonicbased arbitrary waveform generators
(AWGs) fall easily within the desired
frequency range. Their principle of operation is based on the following general
scheme. First, a broadband coherent
signal (e.g., from a mode-locked laser) is
synthesized in a user-defined way in the
optical domain. Usually, pulse shapers
based on spatial light modulators are preferred to all-fiber configurations because
they provide reconfiguration capabilities.2 Once the synthesis is performed,
the light intensity is transferred to the
electrical domain simply by using a highspeed photodetector.
In this way, the detector sets the upper
limit on the achievable electrical bandwidth. Therefore, while obtaining highfrequency electrical signals is a relatively
straightforward task with mode-locked
lasers, reaching the low-frequency regime
remains a challenge.
In 2003, this problem was circumvented thanks to the coherent “wavelength-to-time mapping technique”
demonstrated by Jalali’s group at UCLA.3
This scheme consists of synthesizing the
energy spectrum of a coherent broadband signal with a Fourier-transform
pulse shaper and later transferring the
designed spectral shape into the electrical
domain by stretching the optical pulse
38 | OPN December 2008
Lens
Reflective SLM
Grating
Synthesized
spectrum
Mirror
ASE light
1,554 1,552 1,560
Wavelength (nm)
Pattern
generator
EOM
Bias
Experimental results
Clock
0.5 ns
Fiber
O/E
conversion
Example of generating a chirped sinusoidal signal: The incoherent radiation is spectrally
shaped with a Fourier transform pulse shaper. Once the spectrum is synthesized, the radiation
is modulated with an external modulator. Finally, the light is stretched in a fiber long enough
so that the output-averaged intensity becomes a scaled replica of the synthesized energy
spectrum. The scaling factor is exactly the same as in the coherent version and, therefore,
the previous advantageous features are preserved.
in a dispersive medium (e.g., fiber) and
subsequently detecting it. Accordingly,
the scaling factor of the resulting electromagnetic waveform can be tunable by
adjusting the amount of dispersion.
We have gone one step further and
shown that this widely used system can
be operated with a spectrally incoherent
light source such as amplified spontaneous emission (ASE). The physical mechanism behind this configuration relies on
the temporal version of the vanCittertZernike theorem formulated by Dorrer in 2004.4 This theorem extends the
previous wavelength-to-time mapping to
the incoherent regime. In 2006, we suggested a theory of how this could be used
for AWG. This year, we achieved the first
experimental results.5
The setup is shown in the figure.
The main goal is to avoid the use of a
mode-locked laser. Apart from being an
interesting economic alternative, this
allows us to control the repetition rate
of the electromagnetic waveform with
an external clock. This is a key feature to
perform continuously operating radiofrequency waveforms. To date, we have
generated arbitrary electrical signals with
frequency content around 1-10 GHz
using standard 10 Gb/s telecommunications equipment. t
V. Torres-Company ([email protected]) and J. Lancis
are with the departament de fisica, Universitat Jaume
I, in Castello, Spain. P. Andrés is with the departamento de Optica, Universitat Valencia, in Burjassot,
Spain. L.R. Chen is with the department of electrical
and computer engineering at McGill University in
Montreal, Canada.
References
1. J. Capmany and D. Novak. Nature Photon. 1, 319 (2007).
2. J.D. McKinney et al. Opt. Photon. News 17, 24 (2006).
3. J. Chou et al. IEEE Photon. Technol. Lett. 15, 581 (2003).
4. C. Dorrer. J. Opt. Soc. Am. B 21, 1417 (2004).
5. V. Torres-Company et al. J. Lightwave Technol. 26,
2476 (2008).