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24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 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 Samuel Adams, Dr. Stephane Bourquin, Dr. Mark Brezinski, Dr. Linze Duan, Dr. Jan-Molte Fischer, Dr. Matthew Grein, Dr. Katherine Hall, Christian Jirauschek, Dr. Alexander Killi, Christian Koos, Dr. Chris Kroeger, Dr. Xinbing Liu, Dr. Francisco Lopez-Royo, Dr. Shun-ichi Matsushita, Dr. Christina Manolatou, Dr. Chan H. Park, Dr. Lelia A. Paunescu, Dr. Mark Roberts, Dr. Thomas R. Schibli, Karl Schneider, Dr. Wolfgang Seitz, Prof. Alphan Sennaroglu, Dr. Luciano Socci, Dr. G. Hugh Song, Dr. Hideyuki Sotobayoshi, Dr. Debra Stamper, Dr. Yuichi Takushima, Dr. Ping Xue, Dr. Rebecca Younkin Graduate Students Desmond Adler, Aaron Aguirre, Juhi Chandalia, Marcus Dahlem, Fuwan Gan, Ravi Ghanta, Juliet Gopinath, Felix Grawert, Paul Herz, Pei-lin Hsiung, Leaf Jiang, Aristidis Karalis, Mohammed Jalal Khan, Jung-Won Kim, Tony Ko, Andrew Kowalevicz, O. Onur Kuzucu, J.P. Laine, Ryan Lang, Lia Matos, Nirlep Patel, Milos Popovic, Poh-Boon Phua, Rohit Prasankumar, Peter Rakich, Daniel Ripin, Bryan Robinson, Karen Robinson, Vikas Sharma, Shelby Savage, Hanfei Shen, Jason Sickler, Michael Watts, Jade Wang, Aurea Tucay Zare Technical and Support Staff Mary Aldridge, Donna Gale, Cindy Kopf Research Areas and Projects Ultrashort Pulse Generation and Laser Technology Octave Spanning Lasers and Dispersion Compensating Laser Optics Compact Low-Threshold Ti:Al2O3 Laser Generation of 150nJ Pulses from a Multiple-Pass Cavity KLM Ti:Al2O3 Oscillator MPC Laser Development Ultrafast Cr4+: YAG Laser Cr:LiSAF Laser System 10 fs Diode Pumped Cr:LiCAF Laser Spectral Broadening in a Tapered Fiber and High Numerical Aperture Fiber using Femtosecond Nd:Glass Laser 1Pm Stretched-Pulse Laser with Microstructured Fiber for Dispersion Compensation Timing Jitter Studies in a Passively Modelocked Regeneratively Synchronized Fiber Laser Timing Jitter and Correlations in Harmonically Modelocked Fiber Lasers Timing Jitter Reduction Using a Timing-Jitter Eater Timing Jitter Studies in Hybridly Modelocked Semiconductor Lasers Variational Analysis of Spatio-temporal Pulse Dynamics in Dispersive Kerr Media Ultrafast Phenomena and Quantum Electronics Ultrafast Pump-Probe Studies of Silicon- and III/V-based Devices Materials for Modelocking High-Speed Femtosecond Pump Probe Spectroscopy Using a Smart Pixel Detector Array Photonics and Devices Micromachined Photonic Devices 24-1 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 Development of First, Second, and Third Order Ring Resonators for Channel Dropping Filter Applications Tuning and Switching of Ring Resonator Through Perturbation of Effective Index Guiding and Band Edge Measurements of 2-Dimensional Photonic Crystal Slab Formed by Posts Integrated Tunable/Switchable Optical Add-Drop Multiplexer Grating Filters and Reduced Radiation Polarization Mode Dispersion Optical Phase Control and Stabilization Techniques Attosecond Synchronization of Modelocked Lasers Few-Cycle Nonlinear Optics and Carrier-Envelope Phase Effects Active Modelocking Using a Nonlinear Fabry-Perot Modulator 24-2 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 Ultrashort Pulse Generation and Laser Technology Octave Spanning Lasers and Dispersion Compensating Laser Optics Sponsors National Science Foundation ECS-0119452 MIT Presidential Fellowship Office of Naval Research N-00014-02-1-0717 Project Staff Christian Koos, Onur Kuzucu, Lia Matos, Dr. Thomas R. Schibli, Dr. Lingze Duan, Professor Franz X. Kaertner 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, frequency metrology 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. Kerr-lens 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. Working in collaboration with Professors Erich P. Ippen, Hermann A. Haus, and James G. Fujimoto, we have developed a theoretical model which provides a foundation for understanding and optimization of 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 Pairs for Prismless Octave Spanning Lasers Solid state lasers can have gain over extremely broad bandwidths of several hundred nanometers, enabling the generation of few cycle pulse durations or longer pulse durations with broad tunability. In addition, self-phase modulation (SPM) is a temporal nonlinear effect originating in the Kerr nonlinearity at high intensities that generates new frequencies and spectrally broadens the pulse. The broadband gain media, together with SPM, allow for emission over one octave of bandwidth. The development of compact and robust octave spanning lasers is of prime importance for optical frequency metrology and investigation of phase sensitive nonlinear optical processes. To achieve this broadband emission directly from the laser precise dispersion compensation is indispensable. Double-chirped mirrors (DCMs) have recently emerged as a powerful technology that permits intracavity dispersion management [1-6]. Using a combination of prism pairs and pairs of matched DCMs, octave spanning spectra have been obtained directly from the laser [7]. However, the prism sequence prevents a compact layout of the laser, which is also susceptible to long term drifts, because of beam variations in the prism pair. We have designed and fabricated novel DCM-pairs[8] that cover an octave of bandwidth and compensate dispersion using mirrors and thin BaF2-wedges only. BaF2 is chosen, because it has the lowest third order dispersion in comparison with other fluorides and glasses transparent in the visible to near infrared region. Figure 1 shows the calculated reflectivity of one mirror of the DCMpair, which is also transmissive for the pump light at 532nm. The dispersion compensating mirror pairs provide on average more than 99.9% reflectivity with only a small dip in the range of 800-900 nm. The angle of incidence on one mirror type of the pair is increased by 4o in comparison with the design for optimum cancellation of the dispersion oscillations without noticeable change in the average dispersion. The dispersion oscillations cancel very well, considering the high sensitivity of the overall design on fabrication tolerances. 24-3 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 1.0 100 0.8 80 0.6 60 0.4 40 0.2 20 0.0 0 600 800 1000 Wavelength, nm Group Delay (fs) Reflectivity RLE Progress Report 145 1200 Figure 1: The calculated reflectivity (red solid line) of one mirror of the DCM-pair, which is also transmissive for the pump light at 532nm. The dispersion measured from 650 nm to 1100 nm (blue solid line) follows closely the design goal (green dashed line). The dispersion measurement was limited to 1100nm because of the Si-detector used. Prismless Octave Spanning Ti:Sapphire Laser Using these novel dispersion compensating laser mirrors a compact Ti:sapphire laser design as shown in Figure 2 is possible. The laser has a standard z-cavity design with an additional BaF2 plate in one arm of the resonator and BaF2-wedges in the other arm for fine adjustment of the overall dispersion. The footprint of the laser is only 30x25 cm even at a repetition rate of only 82 MHz. In contrast to our earlier work [7], in the current setup second as well as third order dispersion is almost symmetrically balanced in both laser arms giving rise to ideal conditions for dispersion managed soliton formation and the generation of ultrabroadband spectra when the wedges are adjusted to practically zero average intracavity dispersion. Figure 2: Setup of the prismless, octave spanning Ti:sapphire laser. The possible overall footprint of the laser is only 30x25cm. 24-4 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 1.0 0 0.8 -10 -20 0.6 -30 0.4 -40 0.2 -50 0.0 -60 800 1000 Wavelength [nm] Power Spectral Density [dB] Power Spectral Density [a.u.] RLE Progress Report 145 1200 Figure 3: Typical output spectrum of the prismless Ti:sapphire laser. It covers one octave from 600-1200 nm at about –30 dB below the average power level. Figure 3 shows first spectra generated directly from this laser. It shows a pronounced peak at 670 nm due to the early roll-off of the output coupler. On a logarithmic scale the octave is reached at about –30dB below the average power level. This is already good enough to use this laser for direct optical frequency referencing using a 1f-2f technique [9]. A detailed pulse characterization is in progress. These preliminary results show that the current limitations are not the mirror bandwidth and the dispersion compensation but rather the early roll-off of the output coupler at the short wavelength side of the spectrum. More broadband output couplers may lead to significantly more output in the short and long wavelength range. 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 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. Kaertner, 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. Kaertner and U. Keller, "Theory of Double-Chirped Mirrors," IEEE J. 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. Kaertner, 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. F.X. Kaertner, 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. Opt. Soc. of Am. B 18(6): 882-5, (2001). 9. D.J. Jones, S.A. Diddams, J.K. Ranka, A. Stentz, R.S. Windeler, J.L. Hall, S.T. Cundiff, “Carrier-Envelope Phase Control of Femtosecond Modelocked Lasers and Direct Optical Frequency Synthesis,” Science 288(5466), 635-9 (2000). 24-5 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 Novel Low-Coherence Light Sources 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 for ophthalmic imaging, while SLD at longer wavelengths ~1300 nm are used for imaging of tissue. These sources are relatively inexpensive, portable 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 conducted ongoing work to develop novel low-coherence light sources. We have introduced several alternative sources for ultrahigh resolution imaging. These sources focus on reducing the cost, increasing the reliability and portability of systems, while maintaining high performance capability. Clinical compact low-threshold Ti:Sapphire laser 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 Stephane Bourquin, Aaron D. Aguirre, Ingmar Hartl, Pei-Lin Hsiung, Paul R. Herz, Tony H. Ko, Tim A. Birks, William J. Wadsworth, Udo Bünting, Daniel Kopf, and James G. Fujimoto Kerr lens modelocked (KLM) Ti:Al2O3 lasers can generate extremely short pulse durations with broad bandwidths that are particularly useful in biomedical imaging [6]. A standard Kerr lens modelocked laser operating with a 5W pump can produce output powers of 500 mW and bandwidths in excess of 150 nm [7]. 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. 24-6 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 f = -500 mm 1.3 W Max Quartz O/2 Crystal 2.05 mm Optical Path D=5 cm-1@ 514nm f = 50 mm CM1 CM2 HR L = 40 cm L = 54.5 cm M2 L = 80 cm L = 94.5 cm M1 OC Figure 1. Schematic of the low-threshold prismless Ti:Al2O3 laser. Arm lengths are 120 cm and 149 cm for the HR and OC arms respectively. Intracavity dispersion compensation and tuning is provided by solely by double chirped mirrors (DCM). In previous work we were able to develop an ultra-low-threshold modelocked Ti:Sapphire laser which reduced the modelocking threshold to under 200 mW, significantly reducing the cost of femtosecond sources [8]. Unfortunately, because standard mirrors were used, output bandwidth of only 100 nm was achievable. While this effort significantly reduced the cost of a broadband source, its spectral width, since longitudinal resolution is inversely proportional to the bandwidth of the source, was not sufficient to achieve the ultra-high resolution OCT that other lasers with double-chirped mirrors (DCMs) were capable of. By following similar design criteria as our previous work, but by using 3rd generation DCM technology to compensate higher order dispersion, we are able to develop a low-threshold modelocked Ti:Al2O3 laser which is suitable for clinical ultra-high resolution OCT imaging. The laser cavity is shown in figure 1. The entire laser cavity, as well as the pump source, has been placed on a single lightweight breadboard measuring 19” by 45”, making it compact and portable. We are using a compact pump source operating at 1.0 W output power. The modelocking is initiated by a rapid translation of the end mirror high reflector. Once modelocked, the output power is 50 mW with the broadband and smooth output spectrum with 124 nm FWHM, shown in figure 2a. Even though the output power is modest compared with a standard laser, ophthalmic imaging has exposure limitation of ~750uW, making this amount of output more than sufficient. In figure 2b we see the measured resolution from the interference fringes. The 3.9 um resolution in air corresponds to 3 um resolution in tissue, making ultrahigh resolution imaging possible. Figure 2. (a) The output spectrum showing smooth 124 nm spectrum and (b) interferometric fringes indicating a longitudinal resolution of 3.9 um in air corresponding to 3.0 um in tissue. 24-7 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 In conclusion, we have developed a low-threshold, low-cost, compact laser system. By replacing standard mirrors and prisms with 3rd generation DCMs, we are able to develop a broadband portable source to be used in the clinical setting for ultrahigh resolution OCT imaging with 3 um longitudinal resolution. References 1. Huang, D., et al., Optical coherence tomography. Science, 1991. 254(5035): p. 1178-1181. 2. Tearney, G.J., et al., In vivo endoscopic optical biopsy with optical coherence tomography. Science, 1997. 276(5321): p. 2037-9. 3. Boppart, S.A., et al., In vivo cellular optical coherence tomography imaging. Nature Medicine, 1998. 4(7): p. 861-5. 4. Drexler, W., et al., In vivo ultrahigh resolution optical coherence tomography. Optics Letters, 1999. 24: p. 1221-1223. 5. Drexler, W., et al., Ultrahigh resolution ophthalmic optical coherence tomography. Nature Medicine, 2000. in press. 6. Spence, D.E., P.N. Kean, and W. Sibbett, 60-fsec pulse generation from a self-mode-locked Ti:Sapphire laser. Optics Lett., 1991. 16: p. 42-44. 7. Zhou, J., et al., Pulse evolution in a broad-bandwidth Ti:sapphire laser. Optics Lett., 1994. 19: p. 1149-51. 8. Kowalevicz, A.M., et al., Ultralow-threshold Kerr-lens mode-locked TiAl 2 O 3 laser. Optics Letters, 2002. 27(22): p. 2037-2039. 24-8 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 Generation of 150 nJ pulses from a multiple-pass cavity KLM Ti:Al2O3 Oscillator Sponsors National Science Foundation - ECS-019452 Air Force Office of Scientific Research - F49620-98-01-0084 Air Force Office of Scientific Research (MFEL) - F49620-01-1-0186 Project Staff Andrew M. Kowalevicz, Aurea Tucay, Rohit P. Prasankumar, Professor Alphan Sennaroglu, Professor James G. Fujimoto This year, emphasis was placed on developing solid-state femtosecond lasers that are cheaper and have improved performance. In particular, laser cavity configurations that generate high pulse energies have been examined. Reducing the pulse repetition rate by increasing the cavity length allows us to achieve higher pulse energies. However, a standard 100 MHz repetition rate laser requires a 3 meter round-trip cavity length, so reducing the repetition rate to below 10 MHz requires an unrealistically large laser. A laser that contains a multi-pass cavity (MPC) allows us to obtain long cavity lengths without compromising the need to place the laser on a typical optical table. A multi-pass cavity is essentially a resonator that is comprised of two curved mirrors, into which an off-axis laser beam tilted in either or both transverse directions is introduced (see Fig. 1). In this configuration the beam bounces between the two mirrors, walking around either mirror in an elliptical spot pattern (see Fig. 2). The ellipticity of the mirror spot pattern is determined by the tilt of the input beam. The angle between the spots on successive bounces on one mirror is governed by the radii of curvature of the mirrors and the distance between them. The choice of this angle and placement of notches on the mirrors (or additional small mirrors) to inject and extract the beam allows us to control the number of round trips the beam makes in the MPC before being extracted. Hence the careful consideration of these parameters allows us to design an MPC that, when used inside a laser, increases the laser cavity length and decreases the pulse repetition rate in a controlled fashion. With the help of the ABCD matrix formalism for periodic optical systems, it is possible to design the MPC to be a “unity q” transformation – that is, upon exiting the MPC, the beam’s focused spot size and focus position (i.e., the Gaussian beam q-parameter) are the same as those it had when it entered the MPC. When such an MPC is inserted into an existing laser, the MPC increases the cavity length while leaving the focusing conditions in the laser crystal unchanged. Figure 1. A multi-pass cavity. A beam can be injected in the cavity through notches or small mirrors. 24-9 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 Figure 2. The spot pattern on one of the mirrors in the multi-pass cavity shown above. In addition to ultrafast studies, femtosecond lasers are utilized in nonlinear material processing. Even though pulse durations as short as around 5 fs [1,2] have been reached from conventional Ti:Sapphire lasers, the pulse energy is limited to a few nanojoules, because of its high repetition rate. For material processing high energy pulses at moderate repetition rates are desirable. We utilize a multiple pass cavity (MPC) based on the Herriott cell to produce a unity-q transformation [3] that facilitates the lengthening of the cavity while maintaining the operating point of a standard laser. The long cavity lengths introduced significant dispersion from air and prismatic compensators produced higher order dispersion mismatch. Our current work makes use of specially designed double-chirped mirrors (DCMs) that compensate dispersion without the need for other intracavity dispersion compensating elements. M2 M1 L1 Pump Retroreflector Pump M3 M6 M4 OC M5 M7 M8 M10 M9 M11 Multiple Pass Cavity (MPC) Figure 3. Schematic layout of the high pulse energy laser cavity. All shaded mirrors are DCMs. The pump source is a frequency doubled Nd:Vanadate capable of 10W of light at 532 nm. The crystal is 3 mm thick and absorbs ~56% of the pump light on a single pass. Figure 4 shows the schematic of the high pulse energy Ti:Al2O3 laser. The cavity length has been increased to 5.85 MHz repetition rate. Since our cavity length is approximately 20 times longer than a standard laser, we expect a similar scaling of the pulse energy for a given average output power. This substantially higher pulse energy leads to enhanced self-phase modulation (SPM). The additional frequency components from SPM would typically lead to progressively shorter pulses and considerably higher intensity in the gain medium, which, if left unbalanced, would overdrive the nonlinearities that lead to stable pulse generation. In order to balance the SPM, we increase the net negative dispersion. Our MPC adds 48 DCM bounces with approximately -46 fs2 24-10 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 / bounce, leading to a net negative dispersion of –1250 fs2 after accounting for the additional air path of 48 m. In order to produce high pulse energies, we focus 9.4 W of pump light into our laser crystal. Since the crystal absorbs approximately 56% of the incident light, there are several watts of unabsorbed pump light that gets transmitted. We retroreflect this transmitted light with a 20 cm ROC mirror back into the crystal, which increases the average output power of the laser while also allowing for the use of a 25% OC. This high percentage of output coupling, along with the large net negative dispersion, prevents the intracavity intensity from becoming too large. KLM is initiated by translating the end mirror (M11), which results in single pulsed modelocked operation with output powers as high as 877 mW. In order to verify that the laser was, in fact, producing single pulses, the output was measured with a fast photodiode, an Optical Multichannel Analyzer (OMA), as well as an intensity autocorrelator. The oscilloscope trace, shown in Figure 4a, shows spikes with 171 ns separation corresponding to the cavity roundtrip time for 5.85 MHz. At the same time, the OMA monitored the laser spectrum, shown in Figure 4b. The modelocked spectrum of the laser has 16.5 nm FWHM centered at 788nm with dual symmetric sidebands, which are due to operation at large negative dispersion. In order to establish the duration of our pulses, we performed an intensity autocorrelation with a thin 300 um KDP crystal. The measurement yielded a FWHM of 67 fs resulting in a pulse width of 43 fs (Figure 4c), which is close to the transform limit of 39 fs, assuming a sech2 pulse shape. FWHM 779.0nm - 795.5nm = 16.5nm 39 fs Limit FWHM 67 fs -> 43 fs Assuming Sech Pulse 1.0 0.8 0.8 0.8 0.6 0.4 0.2 Intensity (a.u.) 1.0 Amplitude (a.u.) Amplitude (a.u.) 171ns Separation -> 5.85 MHz 1.0 0.6 0.4 0.2 0.6 0.4 0.2 0.0 0.0 -0.2 -225 -150 -75 0 Time (ns) 75 150 225 725 0.0 750 775 800 Wavelength (nm) 825 850 -150 -100 -50 0 Delay (fs) 50 100 150 Figure 4. a) Pulse spikes from a fast photodiode at the repetition rate of the laser, b) the modelocked spectrum of the laser with 16.5 nm FWHM, and c) the measured pulse duration of 43 fs which is close to the transform limit of 39 fs. In conclusion, we have demonstrated a prismless, KLM Ti:Al2O3 laser operating at 5.85 MHz based on a Herriott-style MPC. Because of its unity transformation of the guassian beam in the MPC, we have achieved long cavity laser performance with standard cavity laser stability. We have demonstrated 150 nJ pulses with 43 fs duration corresponding to 3.5 MW peak power. We expect this never-before-achieved performance to open new avenues for the micromachining of materials previously only possible with amplified laser systems. It will also be a useful tool to eliminate thermal parasitics in pump probe and nonlinear optics experiments. References 1. Morgner, U., et al., Sub-two cycle pulses from a Kerr-Lens modelocked Ti:sapphire laser. Optics Letters, 1999. 24: p. 411 -- 413. 2. Sutter, D.H., et al., Semiconductor saturable-absorber mirror-assisted Kerr-lens mode-locked Ti:sapphire laser producing pulses in the two-cycle regime. Optics Letters, 1999. 24(9): p. 631-3. 3. Herriott, D., H. Kogelnik, and R. Kompfner, Off-axis paths in spherical mirror interferometers. Applied Optics, 1964. 3(4): p. 523-526. 24-11 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 MPC Laser Development Sponsors National Science Foundation - ECS-019452 Air Force Office of Scientific Research - F49620-98-01-0084 Air Force Office of Scientific Research (MFEL)- F49620-01-1-0186 Project Staff Andrew M. Kowalevicz, Aurea Tucay, Rohit P. Prasankumar, Professor Alphan Sennaroglu, Professor James G. Fujimoto High Performance, Compact, Prismless, Low-Threshold 30 MHz Ti:Al2O3 Laser Practical femtosecond laser sources need to meet several important requirements so that they can be readily integrated in systems and used in a wide range of scientific and technological applications such as pump-probe spectroscopy, medical imaging, and communications. These include low-cost system design, compactness, and efficient all-solid-state operation with reasonably high pulse energies. One possible method to lower the overall laser cost involves the development of resonator designs that enable low-threshold laser operation [1,2]. Since the pump laser is one of the major components of a Ti:Al2O3 system, this results in a dramatic cost reduction. The resulting decrease in the average output power, however, leads to a decrease in the pulse energy and peak intensity, limiting their use in nonlinear optics experiments. Previous studies have shown that laser output pulse energy can be scaled up by reducing the pulse repetition rate. In particular, multi-pass cavity configurations have been introduced which increase the effective cavity length, while preserving the characteristics of the laser beam inside the gain medium, to generate high-energy pulses from femtosecond oscillators with low-tomoderate average output powers [3-5]. L1 M1 pump M3 xtal M6 M4 M5 M2 M7 M8 OC zR1 Fig. 1: Schematic of the compact prismless, low-threshold 30 MHz Ti:Al2O3 laser. In this project, we constructed a novel femtosecond Ti:Al2O3 laser which combines several favorable features to meet the above system requirements. A schematic of the laser is shown in Fig. 1. The resonator contains the highly reflecting mirrors M1-M8 as well as a 11% output coupler (OC). Light amplification occurs inside a 2-mm-long Ti:Al2O3 crystal (xtal) which is end pumped by a diode-pumped frequency-doubled Nd:YVO4 laser operated at 532 nm. The input lens L1 focuses the pump beam to an estimated 7-Pm radius inside the crystal. The absorption of the crystal is 74%. The laser has prismless dispersion compensation with double-chirped mirrors (M1-M6) [6]. The effective resonator length is extended by using a multi-pass cavity consisting of the highly reflecting mirrors M7 and M8, separated by 23.4 cm. Although the cavity 24-12 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 length is approximately 5 meters long, an extremely compact design measuring only 30 x 55 cm has been achieved by using the multi-pass cavity design. Autocorrelation Intensity (a.u.) 1.25 1 0.75 0.5 0.25 0 -0.25 -100 -75 -50 -25 0 25 50 75 100 Delay (fsec) Fig. 2. Intensity autocorrelation of the femtosecond pulses generated compact prismless, low-threshold 30 MHz Ti:Al2O3 laser . with the Tight focusing geometry enabled efficient low-threshold operation of the compact Ti:Al2O3 laser. With only 1.5 W of pump power, the laser generates 19-fs pulses with an average output power of 115 mW, corresponding to a pulse energy of 3.7 nJ at a repetition rate of 31 MHz. The measured autocorrelation and the spectrum of the pulses are displayed in Figs. 2 and 3, respectively. The pulses are centered at the wavelength of 780 nm with a spectral bandwidth of 42 nm, indicating that they were nearly transform-limited. The output energy of the laser is comparable to that of a conventional 100-MHz femtosecond Ti:Al2O3 laser with an average output power of 365 mW. The reduced average output power of the present design should also significantly decrease the role of unwanted thermal effects in pump probe measurements. Spectral Intensity (a.u.) 1.25 1 0.75 0.5 0.25 0 700 750 800 850 900 Fig. 3. Spectrum of the femtosecond pulses obtained from the compact Ti:Al2O3 laser. The pulse wavelength is centered around 780 nm with a spectral bandwidth of 42 nm. 24-13 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 References 1. 2. 3. 4. 5. 6. K. Read, F. Blonigen, N. Riccelli, M.M. Murnane, H.C. Kapteyn, “Low-threshold operation of an ultrashort-pulse mode-locked Ti:sapphire laser,” Optics Letters, 21, 489-491, 1996. A. M. Kowalevicz, T. R. Schibli, F. X. Kartner, and J. G. Fujimoto, “Ultra-low-threshold Kerr lens modelocked Ti:Al2O3 lasers,” Optics Letters 27, 2037, 2002. A.R. Libertun, R. Shelton, H.C. Kapteyn. M.M. Murnane, “A 36 nJ-15.5 MHz extendedcavity Ti:sapphire oscillator” CLEO ’99 Technical Digest. p.469-70. S.H. Cho, B.E. Bouma, E.P. Ippen, and J.G. Fujimoto, “A low repetition rate high peak power KLM Ti:Al2O3 laser using a multiple pass cavity,” Optics Letters, 24, 417-419, 1999. S. H. Cho, F. X. Kärtner, U. Morgner, E. P. Ippen, J. G. Fujimoto, J. E. Cunningham, W. H. Knox, “Generation of 90-nJ pulses with a 4 MHz repetition-rate Kerr-lens mode-locked Ti:Al2O3 laser operating with net positive and negative intracavity dispersion,” Optics Letters 26, 560-562, 2001. F. X. Kärtner, N.Matuschek, T. Schibli, U. Keller, H. A. Haus, C. Heine, R. Morf, V. Scheuer, M. Tilsch, T. Tschudi, "Design and fabrication of double-chirped mirrors," Opt. Lett., 22, 831, 1997. 24-14 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 Ultrafast Cr4+:YAG Laser Sponsors U.S. Air Force – Office of Scientific Research - F49620-01-1-0084 MRSEC Program of the National Science Foundation - DMR 98-08941 U.S. Navy – Office of Naval Research Project Staff Hanfei Shen, Juliet T. Gopinath, Sheila Tandon, Dr. Gale Petrich, Dr. Daniel Ripin, Dr. Alexei Erchak, Professor Franz X. Kaertner, Professor Leslie A. Kolodziejski, Professor Erich P. Ippen Several transition-metal-doped solid-state materials are useful as ultrafast laser media because of their chemical and mechanical robustness and stability, broad gain bandwidths, and high nonlinear coefficients, which allow for the design of efficient Kerr-lens modelocking. Optical sources obtained with such media can be exploited for both their short temporal pulse duration and their large spectral bandwidth. The former property makes such lasers ideal for timeresolved studies of ultrafast phenomena and devices, such as optical clocks with precise timing at the cavity repetition rate and ultra-high speed optical communications; while the latter can be used for spectroscopy, for example, to generate synchronized multi-wavelength optical sources, or for optical frequency standards in metrology. Cr4+:YAG is one such material, with broad emission from 1300 to 1600 nm. This gain spectrum makes Cr4+:YAG ideal for studying applications associated with optical telecommunications. We have previously demonstrated the generation of 20-fs pulses directly from a prismless Cr4+:YAG laser using double-chirped mirrors for dispersion compensation [1]. The modelocked spectrum peaked at 1490 nm and had a full-width at half-maximum of 190 nm, extending from 1310 to 1500 nm. The laser cavity was a standard Z-fold configuration, designed to maximize Kerr-lens modelocking (KLM). In general, however, KLM is not self-starting without precise alignment of the laser cavity. Instead, external perturbations are required to initiate modelocking by creating transient power spikes. Saturable absorber mirrors based on semiconductor quantum wells, capable of initiating modelocking without sensitive alignment, have been used to overcome this difficulty in a variety of solid-state lasers. In Cr4+:YAG lasers, saturable absorber mirrors, consisting of InGaAs quantum wells grown upon highly-reflecting mirrors, have been demonstrated [2,3]. In most cases, these mirrors were GaAs/AlAs Bragg stacks, whose bandwidth of ~100 nm typically limited the minimum pulse width due to spectral filtering. Recently, we have demonstrated a novel high-index-contrast mirror-based saturable Bragg reflector (SBR), which was used to generate self-starting pulses with duration of 35 fs directly from a Cr4+:YAG laser [4]. The SBR consisted of a broadband 7-period GaAs/AlxOy Bragg mirror substrate supporting a 10-nm InGaAs quantum well absorber in a O/2-thick InP layer [5]. The GaAs and AlxOy layers have refractive indices of 3.39 and 1.61 at 1.5 Pm, respectively, creating a high-index-contrast mirror that has a calculated reflectivity of 99.9% over the wavelength range 1220 to 1740 nm. This represents a substantial improvement in bandwidth over previous Bragg mirrors that used GaAs/AlAs stacks. Mirror reflectivity was measured using Fourier transform infrared spectroscopy (FTIR), and is shown in Figure 1. The SBR has a stopband from 1300 to 1800 nm, and its nonsaturable loss is estimated to be <0.8% using Findlay-Clay analysis. Furthermore, an absolute reflectivity greater than 99% is inferred by the successful use of the mirror in the low gain Cr4+:YAG laser itself. 24-15 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 Figure 1. Measured reflectivity of the high-index-contrast mirror-based saturable Bragg reflector. The broadband SBR was introduced into the Cr4+:YAG laser cavity to initiate modelocking. The laser was then optimized for KLM. Plots of the pulse spectrum and autocorrelation from the selfstarting Cr4+:YAG laser are shown in Figure 2. The pulse spectrum is centered at 1490 nm, and has a full-width at half-maximum of 68 nm. The bandwidth-limited pulse width is determined to be 35 fs. We believe that these parameters can be improved further, to match the performance of the previously-described 20-fs Cr4+:YAG laser. One possible hindrance in the current experiment is two-photon absorption (TPA) in the saturable absorber, which limits the minimum pulse duration. This difficulty could be overcome by focusing the laser light onto a larger spot size on the SBR, which would result in a lower beam intensity that counteracts TPA. The growth technique for the SBR used in this experiment, however, cannot provide a large enough usable mirror surface to do this. It is estimated that the usable mirror surface extends only as far as 200 Pm a side into the structure. 0.8 -35 -40 0.6 -45 -50 0.4 -55 0.2 -60 0.0 -65 1200 1300 1400 1500 1600 8 Autocorrelation -30 Intensity (dB) Intensity (Arb. Units) -25 1.0 6 W ~ 32 fs with sech fit 4 2 0 -70 1700 -100 -50 0 50 Time Delay (fs) Wavelength (nm) Figure 2. Spectrum and interferometric autocorrelation of self-starting modelocked Cr4+:YAG laser pulses. 24-16 100 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 A promising alternative growth is now being investigated that uses AlGaAs in place of GaAs in the SBR mirror substrate. Previously, the broadband GaAs/AlxOy Bragg mirrors were created by steam oxidation of GaAs/AlAs. However, lattice contraction during oxidation resulted in delamination between the GaAs and AlxOy layers, creating damaging areas of low reflectivity on the SBR. Use of AlGaAs in place of GaAs greatly extends the oxidation dimensions for stable mirror layers. A comparison of the usable mirror surface area in (a) the GaAs/AlxOy and (b) the AlGaAs/AlxOy -based SBRs is shown in Figure 3. The allowable spot size for laser light has been increased from 200 to 500 Pm. In addition, the new structures are mesas that are placed throughout the surface of the structure, as opposed to from the side before. The new larger area SBRs would allow us to focus the laser beam onto larger spot sizes and avoid significant TPA. A similar AlGaAs/AlxOy -based SBR has already been used to demonstrate self-starting in a Cr:forsterite laser [6] and will next be tested in the Cr4+:YAG. Oxidized Region Unoxidized Region Oxidized Region 500 Pm 200 P m (a) (b) Figure 3. Top-down view comparison of the usable mirror surface in (a) the GaAs/AlxOy and (b) the AlGaAs/AlxOy -based SBRs. The maximum spot size for laser light has been increased from 200 to 500 Pm. References 1. D. J. Ripin, C. Chudoba, J. Gopinath, J. G. Fujimoto, E. P. Ippen, U. Morgner, F. X. Kaertner, V. Scheuer, G. Angelow, and T. Tschudi, "Generation of 20-fs pulses by a prismless Cr4+: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 and K. Bergman, "Saturable Bragg reflector self-starting passive mode locking of a Cr4+:YAG laser pumped with a diode-pumped Nd:YVO4 laser," Opt. Lett. 21, 1171-1173 (1996). 3. S. Spalter, M. Bohm, M. Burk, B. Mikulla, R. Fluck, I. Jung, G. Zhang, U. Keller, A. Sizmann and G. Leuchs, "Self-starting soliton-modelocked femtosecond Cr4+:YAG laser using an antiresonant Fabry-Perot saturable absorber," App. Phys. B 65, 335-338 (1997). 4. D. J. Ripin, J. T. Gopinath, H. M. Shen, G. S. Petrich, L. A. Kolodziejski, F. X. Kaertner and E. P. Ippen, "Oxidized GaAs/AlAs mirror with a quantum-well saturable absorber for ultrashort-pulse Cr4+:YAG laser," Opt. Comm. 214, 285-289 (2002). 24-17 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 5. A. A. Erchak, D. J. Ripin, J. T. Gopinath, H. M. Shen, F. X. Kaertner, G. S. Petrich, L. A. Kolodziejski and E. P. Ippen, "Large scale oxidation of AlAs layers for broadband saturable Bragg reflector," CLEO 73 (2002), CTuK43 225. 6. T. R. Schibli, J. W. Kim, L. Matos, A. W. Killi, J. T. Gopinath, S. N. Tandon, G. S. Petrich,J. G. Fujimoto, E. P. Ippen, F. X. Kaertner and L. A. Kolodziejski, "300 attosecond activesynchronization of passively mode-locked lasers using balanced cross-correlation, submitted to CLEO 2003. 24-18 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 Cr:LiSaF Laser System Sponsors National Science Foundation - ECS-019452 Air Force Office of Scientific Research - F49620-98-01-0084 Air Force Office of Scientific Research (MFEL) - F49620-01-1-0186 Project Staff Rohit P. Prasankumar, Dr. Yasuyuki Hirakawa, Andrew M. Kowalewicz, Jr., Professor Franz X. Kaertner, Professor James G. Fujimoto. An 8.6 MHz extended cavity femtosecond Cr:LiSAF laser pumped by low cost diode lasers Femtosecond lasers are an essential technology for many applications including ultrafast spectroscopy, high speed measurement, laser micromachining and biomedical imaging. In order to make femtosecond technology more accessible to users outside of the laboratory, methods of reducing cost while maintaining high performance must be developed. Laser diode pumped solid state lasers are an attractive alternative to conventional pumping with expensive gas or solid state lasers. Cr:LiSAF is a well established laser material operating around 850 nm that can be pumped with red laser diodes to obtain mode-locked pulse durations as short as 10 fs (Uemura and Torizuka 2002; Wagenblast, Morgner et al. 2002). Previous efforts in diode pumping Cr:LiSAF used broad-stripe diodes with powers of hundreds of mW (Dymott and Ferguson 1995; Uemura and Torizuka 2002). These pump diodes are still relatively expensive and have poor mode quality, making efficient mode matching and Kerr lens mode-locking difficult. An attractive alternative is to pump with single spatial mode diodes, which significantly improves mode matching and laser efficiency. Single mode diodes with powers of 50-60 mW at wavelengths ranging from 660-690 nm are available for only $20 each, making this pump source extremely inexpensive. Previous work demonstrated compact mode-locked Cr:LiSAF lasers in several compact configurations pumped by single spatial mode diodes (Agate, Stormont et al. 2002; Hopkins, Valentine et al. 2002). These lasers typically generated 20 mW output power and 120 fs pulses at 430 MHz, corresponding to a pulse energy of 0.05 nJ. The maximum pulse energy achieved was 0.14 nJ (Agate, Stormont et al. 2002), which may be too low for some applications. Multi-pass cavities (MPC) providing a unity q parameter transformation have been used to reduce repetition rates from laser oscillators and thereby increase pulse energies without requiring external amplification (Cho, Bouma et al. 1999). By using an MPC in a single mode diodepumped Cr:LiSAF laser, an inexpensive source of femtosecond pulses with energies comparable to those generated by standard Ti:sapphire lasers can be achieved. The multi-pass cavity used in this work consists of one large plane mirror and one large curved mirror separated by a distance. For a desired repetition rate, the radius of the curved mirror, number of bounces on each mirror, and distance between the two mirrors can be optimized to give a unity q parameter transformation using ABCD matrix analysis. Two smaller mirrors, one plane and one curved, are used to introduce and extract the laser beam from the MPC. A schematic of the experimental setup is shown in Figure 1. The diode pump source consisted of two diodes at 663 nm (Hitachi HL6503MG) and one diode at 685 nm (Mitsubishi ML1013R). The diodes were microlensed by Blue Sky Research to provide a circular output beam. The diodes were collimated and one diode at 663 nm (D1) was combined with the 685 nm diode (D2) using a dichroic mirror (DM). The other 663 nm diode (D3) was polarization rotated using a half-wave plate (WP) and the 3 beams were multiplexed with a polarizing beam splitter (PBS). This yielded a collimated beam with a total power of 137 mW when each diode was driven by a current of 117 mA. The pump spot was focused to a minimum radius of 15 x 18 µm using a combination of a R=-100 mm diverging lens (P1) and a R=76.3 mm antireflection coated achromatic lens (P2). 24-19 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 Two curved mirrors (M1 and M2, R=10 cm) were used to tightly focus the laser mode within the 5 mm long Brewster cut, 1.5 % doped Cr:LiSAF crystal (CR). The output coupler (OC) had a transmission of 1% at 860 nm. Mirrors M3 and M4 were plane mirrors used to increase the arm length. Initially, we optimized the laser in a standard x cavity without an MPC, SBR, or prisms, obtaining a cw power of 28.5 mW. We then introduced the MPC, consisting of one plane 1.5” diameter mirror (M6) and one 1.5” diameter R=4 m mirror (M7), separated by 2 meters. These mirrors were DCMs that provided –42 fs2 group delay dispersion (GDD) per bounce around 860 nm. The beam passes through the MPC 16 times, resulting in a total added cavity length of 32m. This MPC is designed to introduce a negligible amount of GDD, since each bounce on an MPC mirror compensates the dispersion from the 2 meters of air between the mirrors. A flat mirror (M5) and a curved R=4 m DCM (M8) are used to introduce and extract the beams from the MPC. An extra fold for focusing onto the SBR was added, consisting of mirrors M9, M10, and the SBR; this extra fold was set for a unity transformation of the q parameter. The SBR was similar to that described in ref (Tsuda, Knox et al. 1996), with a reflectivity of 99.5% from 825-900 nm. D3 M1 CR D2 M2 P 2 P 1 D1 M3 PBS M6 M 5 M7 M4 PR1 DM OC PR2 M10 M9 M8 OC SBR Figure 1. Experimental setup of the extended cavity Cr:LiSAF laser. Details are given in the text. Initial experiments were performed using prisms for dispersion compensation. Two fused silica prisms (PR1 and PR2) separated by 50 cm were used for compensating the dispersion of the crystal and tuning the dispersion operating point. The prism arm was 70 cm and the arm including the MPC had an effective length of 90 cm. A standard high reflecting R=20 cm mirror (M10) was used to tightly focus the laser mode onto the SBR. We obtained 43 fs pulses with 18.5 nm bandwidth at an 8.4 MHz repetition rate (figure 2). The average power was 5.5 mW in this configuration, corresponding to a pulse energy of 0.66 nJ. 24-20 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 Intensity (arb.units) 1.0 0.8 0.6 0.4 0.2 0.0 FWHM 43 fs -200 0 delay (fs) 200 (a) Intensity (arb.units) 1.0 0.8 0.6 FWHM 18.5 nm 0.4 0.2 0.0 820 840 860 delay (fs) 880 900 (b) Figure 2. Intensity autocorrelation (a) and spectrum (b) of the extended cavity Cr:LiSAF laser with prisms. The repetition rate was 8.4 MHz and the pulse energy was 0.66 nJ. Subsequently, we configured the cavity to operate without prisms. A total of ten bounces on DCMs outside the MPC, providing -420 fs2 GDD, were used to compensate the dispersion of the crystal and excess dispersion from the MPC and air in the cavity. M1 was replaced by a R=10 cm DCM. M3, M4, and M9 were replaced by flat DCMs. M10 was replaced by a R=30 cm standard high reflecting mirror. PR1 and PR2 were removed. The arm lengths were 40 and 80 cm. All other components remained the same. With this prismless setup, we obtained 39 fs pulses with 20 nm bandwidth (figure 3). The average power was 6.5 mW at an 8.6 MHz repetition rate, corresponding to 0.75 nJ pulse energy. We believe that the improved pulse energy in this configuration is primarily due to the lower intracavity loss of the DCMs as compared to the prisms. The pulsewidth is most likely limited by the bandwidth of the SBR in both configurations; we mode-locked the short cavity with no DCMs and only prisms for dispersion compensation and obtained similar output spectra. 24-21 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 Intensity (arb.units) RLE Progress Report 145 Intensity (arb.units) 1.0 0.8 0.6 0.4 0.2 0.0 FWHM 39 fs 1.0 0.8 0.6 0.4 0.2 0.0 FWHM 20 nm 820 -200 -100 0 delay (fs) 100 200 (a) 840 860 880 900 wavelength (nm) 920 (b) Figure 3. Intensity autocorrelation (a) and spectrum (b) of pulses from the prismless extended cavity Cr:LiSAF laser. The repetition rate was 8.6 MHz and the pulse energy was 0.75 nJ. We measured the threshold and slope efficiencies for cw and mode-locked operation in the prismless configuration as a function of different combinations of the three diodes. Thresholds for both cw and mode-locked operation were typically between 69 and 81 mW, depending on the particular diode combination tested. Mode-locked slope efficiencies were between 8 and 13%, again depending on the combination of diodes. Diode D1 pumped the Cr:LiSAF crystal most efficiently, as expected since its polarization and wavelength are most strongly absorbed. Diode D3 was the least efficient, also expected since the absorption coefficient was a factor of 1.75 lower for this polarization. In conclusion, a Cr:LiSAF laser incorporating an MPC and pumped by three single spatial mode diodes has been demonstrated. Pulses as short as 39 fs with 20 nm bandwidth and 0.75 nJ energy per pulse were generated at an 8.6 MHz repetition rate using only DCMs for dispersion compensation. With prisms used for dispersion compensation, 43 fs pulses with 18.5 nm bandwidth and 0.66 nJ pulse energy were generated at an 8.4 MHz repetition rate. This laser has the potential to be significantly less expensive than conventional Ti:sapphire lasers due to the low cost of its pump source. It could be useful for many applications requiring moderate pulse energies and short pulse durations. Future work will include development of higher reflectivity MPC mirrors and development of an SBR with higher reflectivity and a wider bandwidth. References 1. Agate, B., B. Stormont, et al. (2002). "Simplified cavity designs for efficient and compact femtosecond Cr:LiSAF lasers." Opt. Comm. 205: 207-213. 2. Cho, S. H., B. E. Bouma, et al. (1999). "Low-repetition-rate high-peak power Kerr-lens modelocked Ti:Al2O3 laser with a multiple-pass cavity." Opt. Lett. 24: 417-419. 3. Dymott, M. J. P. and A. I. Ferguson (1995). "18-fs-pulse generation from a diode-pumped selfmode-locked Cr:LiSAF laser". Conference on Lasers and Electro-Optics, CLEO, Baltimore. 4. Hopkins, J.-M., G. J. Valentine, et al. (2002). "Highly compact and efficient femtosecond Cr:LiSAF lasers." IEEE J. Quant. Elect. 38(4): 360-368. 5. Tsuda, S., W. H. Knox, et al. (1996). "Mode-locking ultrafast solid-state lasers with saturable Bragg reflectors." IEEE J. Sel. Top. Quant. Elect. 2: 454-464. 6. Uemura, S. and K. Torizuka (2002). Characteristics of 10-fs diode-pumped Kerr-lens modelocked Cr:LiSAF and Cr:LiSGAF lasers. Conference on Lasers and Electro-Optics. 7. Wagenblast, P. C., U. Morgner, et al. (2002). "Generation of sub-10-fs-pulses from a KerrLens mode locked Cr3+:LiCAF laser oscillator using third order dispersion compensating doublechirped mirrors." Opt. Lett. 27(19): 1726. 24-22 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 10 fs Diode Pumped Cr:LiCAF Laser Sponsors National Science Foundation - ECS-0119452 Project Staff Felix Grawert, Phillip Wagenblast, Dr. Uwe Morgner, Professor Franz X. Kaertner Cr3+-doped Colquiriite (Cr3+-:LiSAF, Cr3+-:LiSGaF, Cr3+-:LICAF) crystals are promising materials for compact femtosecond laser sources. They show high quantum efficiency, broad absorption bands in a wavelength range where high-brightness laser diodes are available, and broad-band emission from 700 nm to 1000 nm, which supports pulses substantially shorter than 10 fs. Among them, Cr3+-:LiCAF has the highest quantum efficiency and the most favorable thermal properties. Presently, Ti:sapphire lasers pumped by frequency-doubled solid-state lasers are the only systems that deliver sub-10 fs pulses directly from the oscillator. Diode pumped sub10 fs laser systems are highly desirable as an inexpensive alternative for Ti:sapphire lasers in spectroscopy, metrology, optical coherence tomography, and THz-generation. Initially. we used the diffraction limited beam of a Ti:sapphire laser to generate 9 fs pulses with 220 mW average power and 97 MHz repetition rate from Kerr-lens mode-locked Cr3+:LiCAF laser using broadband double-chirped mirrors for second- and third-order dispersion compensation [1]. Figure 1 shows the new setup using diode pumping. It is a standard z-folded resonator at 110 MHz including a fused-silica prism sequence. The pump sources are two 500 mW laser diodes (COHERENT S670C-500C) with an emitting area of 1x100 µm, where the large divergence of the fast axis is collimated by a fiber lens. The two diodes are polarization multiplexed using a polarizing beam splitter. DCM quartz prisms O 2 DCM R=75 PBS f=100 f=50 OC Pump mirror R=75 Figure1: Set-up of the laser resonator and pumping scheme. The total absorbed pump power amounts to 800 mW at maximum, and in cw mode of operation, the laser emits up to 150 mW at a wavelength of 780 nm, and operates eight times above threshold. Due to the low beam quality of the transverse multimode pump diodes Kerr-Lens mode locking in diode pumped systems is usually achieved by hard apertures in the cavity [2,3]. Here, we found that by exploiting the gain guiding effect [4] we were able to demonstrate for the first time a diode-pumped, soft-aperture KLM laser without internal aperture. In the mode-locked state of operation, the laser emits 10 fs pulses with 40 mW of average power in a near-diffraction limited beam. The mode-locked spectrum is shown in Fig. 2b. It extends from 750 to 900 nm and shows an additional peak at 980nm, which is beyond the cavity bandwidth. The modulation in the main part of the spectrum is due to the group delay oscillations of the mirrors. Assuming a flat phase the transform-limited pulse duration would be 8.4 fs. The pulse train has been characterized by spectral shearing interferometry [2] (SPIDER), a method which directly provides the spectral phase of the pulses, and by Fourier transform the exact pulse shape. 24-23 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 SPIDER interferogram / arb. RLE Progress Report 145 1 (a) 0.1 0.01 400 420 440 460 wavelength / nm 480 1.0 (b) 2 0.8 0 0.6 0.4 -2 0.2 -4 0.0 700 -6 750 800 850 900 950 wavelength / nm 1000 1.0 1050 10 (c) 'tFWHM = 9.3 fs 8 6 0.5 4 2 0 0.0 phase / radian power / arb. spectral phase / radian spectral intensity / arb. 0.001 -2 -40 -20 0 time / fs 20 40 Figure 2: Results of the SPIDER characterization of the mode-locked pulses. a): sheared interferogram, b): mode-locked spectrum and spectral phase, c): Reconstructed pulse and temporal phase. The interferogram of the upconverted, spectrally sheared pulses is shown in Fig. 2a on a logarithmic scale. Over the whole spectral range including the peak at 980 nm, the pulses show interference. The duration of the reconstructed pulse, shown in Figure 2.c is 9.3 fs (FWHM), and prepulses are suppressed by one order of magnitude. This result represents a ten fold improvement in output power compared to previous results [2]. References 1. P. Wagenblast, U. Morgner, F. Grawert, V. Scheuer, G. Angelow, M. J. Lederer, and F. X. Kaertner, “Generation of sub-10-fs pulses from a Kerr-lens modelocked Cr3+:LiCAF laser oscillator using third order dispersion compensating double chirped mirrors,” Opt. Lett. 27(19), 1726-9, 2002. 2. S. Uemura and K. Torizuka, “Development of a Diode-Pumped Kerr-Lens Mode-Locked Cr:LiSAF Laser”, IEEE JQE 39(1), 68 (2003). 3. K. M. Gäbel, P. Rußbüldt, R. Lebert, and A. Valster, “Diode pumped Cr3+:LiCAF fs-Laser,” Opt. Comm. 157, 327, (1998). 24-24 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 Spectral broadening in tapered fiber and a high numerical aperture fiber using a femtosecond Nd:Glass Laser Sponsors Air Force Office of Scientific Research (MFEL) - F49620-01-1-0186 Air Force Office of Scientific Research - F49620-98-01-0084 National Science Foundation - ECS-019452 National Institute of Health - NIH-5-R01-CA75289-04 National Institute of Health - NIH-2-R01 EY11289-15 Project Staff Andrew M. Kowalevicz, Rohit Prasankumar, Tony H. Ko, Alphan Sennaroglu, Thomas Schibli, Professor Franz X. Kaertner, Professor Erich P. Ippen, Professor James G. Fujimoto High nonlinearity, air-silica microstructure fibers [1] or tapered fibers [2] 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 150 mW average power at 75 MHz repetition rate and 1.06 µm wavelength with a 12 nm bandwidth. The Nd:Glass is pumped by two 1 W diode laser diodes. The Nd:Glass laser is soliton modelocked [3] using a SESAM [4,5] 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 [2]. 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. Figure 1 (a) shows a typical continuum generated by the tapered fiber with an average power of 50 mW. The spectrum is centered at 1.3 µm with a bandwidth of 132 nm. 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. 24-25 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 1.0 (a) (b) 0.8 0.8 Intensity [a.u.] Intensity [a.u.] 1.0 0.6 132 nm 0.4 0.2 0.6 123 nm 0.4 0.2 0.0 0.0 1000 1100 1200 1300 1400 1500 Wavelength [nm] 800 900 1000 1100 1200 1300 Wavelength [nm] Figure 1. (a) The optical spectrum of the continuum generated in a tapered fiber, and (b) optical spectrum of the continuum generated in a high numerical aperture fiber. Based on the same principle than the tapered fiber, we have demonstrated supercontinuum generation in a high numerical aperture fiber. The light from the Nd:glass laser is coupled into a 2 meter length of commercially available ultrahigh numerical aperture (NA) single mode fiber. The germanium doped, 2.5 Pm core provides enhanced nonlinear effects and efficient continuum generation. A typical optical spectrum of the continuum at the output of the ultrahigh NA fiber is presented in Figure 1b. The broadening is mainly due to the self phase modulation effect. A slight shift of the central wavelength from 1064 nm to 1080 nm is observed, which may results from Raman effect. With 90 mW of average output power, a bandwidth of 123 nm could be generated These compact and portable light sources are well suited for ultrahigh resolution optical coherence tomography. Axial resolution in air of 5.6 µm and 4.2 µm are theoretically achievable with the tapered fiber and the high numerical aperture fiber, respectively. References 1. J. K. Ranka, R. S. Windeler, and A. J. Stentz, “Visible continuum generation in air silica microstructure optical f ibers with anomalous dispersion at 800nm,” Optics Letters, vol. 25, pp. 25-27, 2000. 2. T. A. Birks, W. J. Wadsworth, and P. St. J. Russell, “Supercontinuum generation in tapered fibers,” Optics Letters, vol. 25, pp. 1415-1417, 2000. 3. 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), vol. 2, pp. 540-556, 1996. 4. D. Kopf, F. X. Kärtner, K. J. Weingarten, and U. Keller, “Diode-pumped modelocked Nd:glass lasers using an A-FPSA,” Optics Lett., vol. 20, pp. 1169-1171, 1995. 5. U. Keller, K. J. Weingarten, F. X. Kärtner, D. Kopf, B. Braun, I. D. Jung, R. Fluck, C. Hönninger, N. Matuschek, and J. Aus der Au, “Semiconductor saturable absorber mirros (SESAMs) for femtosecond to nanosecond pulse generation in solid-state lasers,” Special issue on Ultrafast Electronics, Photonics and Optoelectronics, IEEE J. Selected Topics in Quantum Electronics (JSTQE), vol. 2, pp. 435-453, 1996. 24-26 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 1Pm Stretched-Pulse Laser with Microstructured Fiber for Dispersion Compensation Sponsors U.S. Air Force – Office of Scientific Research - F49620-01-1-0084 Project Staff J. T. Gopinath, Dr. K. S. Abedin, Dr. M. E. Grein, and Professor E. P. Ippen There is considerable interest in femtosecond pulse generation at 1 Pm for medical imaging and procedures, spectroscopy and microscopy. At 1 Pm, Yb-doped silica fiber has excellent conversion efficiency, broad gain-bandwidth, can be pumped by telecomm laser diodes at 975 nm, and should produce short high-energy pulses. However, in order to produce ultrashort pulses, one must compensate the normal dispersion of the Yb fiber. Unfortunately, other conventional fiber, such as single mode fiber at 1550 nm, also have normal GVD at this wavelength. Previous femtosecond Yb fiber lasers have used intracavity prisms or grating pairs for dispersion compensation. Because these elements are lossy, the output pulsewidth and power of the laser are limited. Thus, it is desirable to replace these elements with low loss fiber. Photonic crystal and microstructure fiber can provide anomalous dispersion at 1 Pm. Microstructure fiber, fiber with a pattern of air holes, that owes its guiding properties purely due to index contrast, took the world by surprise a few years ago, with its high profile application to frequency metrology. This fiber can be used for many nonlinear processes including supercontinuum generation (applications: frequency metrology, medical imaging etc.), four-wave mixing [1], and high harmonic generation. However, the loss of these fibers, which can be orders of magnitude higher than the 0.2 dB/km loss of conventional fiber, is a major drawback and makes them generally unsuitable for use in lasers. We are collaborating with Dr. Benjamin Eggleton, Dr. Robert Windeler, and Charles Kerbage at OFS-FITEL, who make tapered air-silica microstructure fiber (TASMF), with losses as low as 0.3 dB/taper (18 cm) [2]. The fiber can be spliced reliably with relatively low losses of 0.1 dB/splice . Untapered, this fiber behaves similarly to SMF; tapered, it has a nonlinearity an order of magnitude higher and a unique dispersion profile (see Figure 1). a) b) Figure 1: a) Cross-section of microstructure fiber. b) Calculated group index and GVD vs. wavelength for the tapered air-silica microstructure fiber versus the taper diameter. We are using tapers of ~1.44 Pm diameter in the laser. 24-27 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 We plan to use this fiber to generatefemtosecond pulses with the Yb fiber laser. One group has already demonstrated 100 fs pulses with photonic crystal fiber in this system [3], Our goal is to improve on this result by extending the laser operation into the stretched-pulse regime. Figure 2 shows the schematic of the laser we are building. Figure 2: Schematic of laser Initial results, in a ring configuration, incorporating a fiber WDM, have shown that very broadband output pulses can be generated. The free space configuration shown above should provide more precise dispersion balancing and lead to shorter pulses. Two tapers will be used in the laser, in conjunction with a saturable absorber for self-starting modelocked operation. References 1. K. S. Abedin, J. T. Gopinath, E. P. Ippen, C. E. Kerbage, R. S. Windeler and B. J. Eggleton, "Highly nondegenerate femtosecond four-wave mixing in tapered microstructure fiber," Applied Physics Letters, 81(8): 1384-1387 (2002). 2. J. K. Chandalia, B. J. Eggleton, R. S. Windeler, S. G. Kosinski, X. Liu and C. Xu, "Adiabatic coupling in tapered air-silica microstructured optical fiber," IEEE Photonics Technology Letters, 13(52-54 (2001). 3. H. Lim, F. O. Ilday and F. W. Wise, "Femtosecond ytterbium fiber laser with photonic crystal fiber for dispersion control," Optics Express, 10(25): 1497-1502 (2002). 24-28 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 Timing Jitter Studies in a Passively Modelocked Regeneratively Synchronized Fiber Laser Sponsors U.S. Air Force – Office of Scientific Research - F49620-01-1-0084 DARPA – Defense Advanced Research Projects Agency - F49620-96-01266 U. S. Navy - Office of Naval Research Project Staff Jason W. Sickler, Matthew E. Grein, Leaf A. Jiang, Professor Erich P. Ippen, Professor Hermann A. Haus Currently, a strong demand exists for modelocked lasers that produce sub-picosecond pulses at high repetition rates. Such lasers would be extremely useful as clocks for high frequency, high resolution analog-to-digital optical sampling, and as sources for high-speed time-division multiplexed optical communications systems. The timing noise, or timing jitter, of these lasers often limits the performance of systems in which these lasers would be used [1], thus understanding and reducing the timing jitter in these lasers is important. A primary candidate for satisfying the desire for such lasers is harmonically modelocked fiber lasers. Previous work on modelocked fiber lasers sought to generate short pulses at high repetition rates. This includes work using polarization additive-pulse modelocking (P-APM) schemes for short pulse generation, combined with regenerative feedback as a means to harmonically modelock and thus increase the repetition rate [2, 3, 4]. In this work, we seek to reduce the timing jitter of an harmonically modelocked regeneratively synchronized fiber laser using an intracavity fiber loop. The nature of timing jitter in harmonically modelocked lasers can be described in both the time and frequency domain. Both domains, and the relationship between them, are illustrative. In the time domain, a fundamentally modelocked laser produces a pulse train where all the pulses originate from the same intracavity pulse. Because the noise of the intracavity pulse at one time is partially correlated to the noise of that same intracavity pulse at another time, the noise of the output pulses tends to be correlated. Because Fig. 1. Theoretical plot of the spectral noise power density noise is added (via of a fundamentally modelocked laser. [5] spontaneous emission, and other classical sources) and removed (via filtering, and other loss mechanisms) with each round trip, the degree of correlation between the intracavity pulse at two different times generally decreases when the times considered are further apart. One expects, then, that timing jitter in a fundamentally modelocked laser appears primarily at low frequencies, as the theory shown in Fig. 1 predicts. 24-29 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 The timing jitter in an harmonically modelocked laser differs from that of a fundamentally modelocked laser. The timing jitter of any two pulses in the output pulse train may or may not be correlated. In general, as long as effects that would lead to pulse interaction can be ignored, such as those resulting from gain recovery, the timing jitter of output pulses that originate from different intracavity pulses will not be correlated. The timing jitter of those output pulses originating from the same intracavity pulse will be tend Fig. 2. Theoretical plot of the spectral noise power density to be correlated, much like of a harmonically modelocked laser for phase modulation output pulses of a and filtering [5]. fundamentally modelocked laser. Thus, for a laser harmonically modelocked with harmonic number, N, one can think of N interleaved fundamentally modelocked laser output pulse trains. The pulses within each interleaved pulse train tend to jitter together, but the pulses contained in different interleaved trains are independent. The possibility of pulse-patterning, using this heuristic, becomes clear. The spectral noise power density will show timing jitter at low frequencies, as well as at harmonics of the fundamental frequency of the laser. The theoretical plot in Fig. 2 shows the low frequency noise pedestal, as well as N-1 noise pedestals occurring at harmonics of the fundamental frequency. We are attempting to use an intracavity fiber loop to correlate the noise of the intracavity pulses, in order to effect and hopefully reduce the timing jitter of the output pulses. The fiber loop functions as a Gire-Tournois interferometer. When the fundamental frequency of the fiber loop is at a harmonic of the cavity repetition rate, pulses exiting the fiber loop will overlap pulses passing by the loop. In the shuffling of photons among the intracavity pulses we hope to correlate their noise. The fiber loop will be placed in a regeneratively Fig. 3. Regeneratively synchronized harmonically synchronized P-APM modelocked fiber laser. harmonically modelocked fiber laser, shown in Fig. 3, harmonically modelocked at 1GHz (N~70). [2, 4] Comparisons between the spectral noise power 24-30 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 densities, using a frequency discrimination technique [6, 7] shown in Fig. 4, as well as optical correlations, will be made. Interesting to note is that the use of the frequency discriminator technique to measure timing jitter is not typically done. Rather, timing jitter measurements are more commonly made in reference to a stable local oscillator [8]. The advantage of the frequency discriminator technique is that a low-noise external oscillator is no longer Fig. 4. Diagram of a frequency discriminator setup. required. When measuring the timing jitter of extremely low noise systems, this is a valuable advantage. Results relevant to this work appear in the literature. These include experiments using intracavity Fabry-Perot etalons [9, 10, 11] and fiber loops [12]. To our knowledge, however, the nature of the fiber loop’s effect on noise has not been fully explored. We predict two general possible results from this work. The first is that the correlation of pulses will shift timing jitter power to lower frequencies (i.e. suppress the supermode). The pulses of the pulse train will be “tied together” so that they jitter together, but the total timing jitter, the integral of the noise power spectrum out to the Nyquist frequency, will not be reduced. The second possibility is that the correlation of pulses will shift timing jitter power to lower frequencies, as well as reduce the total timing jitter. The pulses of the pulse train will be “tied together” so that they jitter together, and the “inertia” of the pulse train will reduce the total timing jitter. In either case, successful correlation of the pulses in the pulse train should result in a noise spectrum that qualitatively approaches that of a fundamentally modelocked laser. References 1. P. W. Juodawlkis, et. al, “Optically Sampled Analog-to-Digital Converters,” IEEE Transactions On Microwave Theory and Techniques 49(10): 1840-1853 (2001). 2. M. Margalit, et al, “Harmonic Mode-Locking Using Regenerative Phase Modulation,” IEEE Photonics Technology Letters 10(3): 337-339 (1998). 3. C. X. Yu, et. al, “Noise of a Regeneratively Synchronized GHz Passively Modelocked Fiber Laser,” in Conference of Laser and Electro-Optics - Europe, IEEE, (Nice, France), pp. 1, IEEE, 2000. 4. C. X. Yu, et. al, “A GHz Regeneratively Synchronized Passively Mode-locked Fiber Laser for Spectrum Generation In The 1.5Pm Region,” in Conference of Laser and Electro-Optics, OSA Technical Digest – Postconference Edition, (San Francisco, CA), pp. 80, Optical Society of America, 2000. 5. M. E. Grein, Noise and Stability of Actively Modelocked Fiber Lasers, Ph.D. Thesis, Department of Electrical Engineering and Computer Science, MIT, 2002. 24-31 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 6. D. Scherer, “The Art of Phase Noise Measurement,” RF & Microwave Measurement Symposium and Exhibition (August 1985). 7. T. Decker and B. Temple, “Choosing A Phase Noise Measurement Technique: Concepts and Implementation,” RF & Microwave Measurement Symposium and Exhibition (?). 8. R. P. Scott, et. al, “High-Dynamic-Range Laser Amplitude and Phase Noise Measurement Techniques” IEEE Journal of Selected Topics in Quantum Electronics 7(4): 641-655 (2001). 9. C. M. DePriest, et. al, “Ultralow noise and supermode suppression in an actively mode-locked external-cavity semiconductor diode ring laser,” Optics Letters 27(9): 719-721 (2002). 10. J. S. Wey, et. al, “Performance Characterizaton of a Harmonically Mode-Locked Erbium Fiber Ring Laser” IEEE Photonics Technology Letters 7(2): 152-154 (1995). 11. J. S. Wey, et. al, “Active Harmonic Modelocking of an Erbium Fiber Laser with Intracavity Fabry-Perot Filters” IEEE Journal of Lightwave Technology 15(7): 1171-1180 (1997). 12. E. Yoshida, et. al, “Laser diode-pumped femtosecond erbium-doped fiber laser with a subring cavity for repetition rate control,” Applied Physics Letters 60(8): 932-934 (1992). 13. [13] M. E. Grein, et. al, “Experimental Observation of Quantum-Limited Timing Jitter in an Active, Harmonically Modelocked Fiber Laser”, in Conference of Laser and Electro-Optics, OSA Technical Digest, (Washington, D.C.), pp. 561-562, Optical Society of America, 2002. 14. M. E. Grein, et. al, Active Harmonically Modelocked Fiber Lasers, RLE 2001(Cambridge: MIT Research Laboratory of Electronics, 2001). 15. H. A. Haus, et. al, “Noise of Mode-Locked Lasers,” IEEE Journal of Quantum Electronics. 29(3): 983-996 (1993). 16. L. A. Jiang, Ultralow-Noise Modelocked Lasers, Ph.D. Thesis, Department of Electrical Engineering and Computer Science, MIT, 2001. 17. L. A. Jiang, et. al, Noise in Harmonically Modelocked Lasers, RLE 2001(Cambridge: MIT Research Laboratory of Electronics, 2001). 18. F. Rana, et. al, “Characterization of the noise and correlations in harmonically mode-locked lasers,” J. Opt. Soc. Am. B 19(11): 2609-2621 (2002). 19. D. von der Linde, “Characterization of the Noise in Continuously Operating Mode-Locked Lasers,” Applied Physics B. 39: 201-217 (1986). 20. C. X. Yu, Soliton Squeezing in Optical Fibers, Ph.D. Thesis, Department of Electrical Engineering and Computer Science, MIT, 2000. 24-32 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 Timing Jitter and Correlations in Harmonically Modelocked Fiber Lasers Sponsors U.S. Air Force – Office of Scientific Research - F49620-01-1-0084 DARPA – Defense Advanced Research Projects Agency - F49620-96-01266 U. S. Navy - Office of Naval Research Project Staff Matthew E. Grein, Leaf A. Jiang, Jason Sickler, 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 quantum-limited 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 particular goal of the present work is to understand the noise and correlations particular to harmonically modelocked fiber lasers (and harmonically modelocked lasers generally). Due to the low gain per unit length of rare-earth doped fiber (e.g., erbium and erbium-ytterbium), fiber lasers are generally very long. High repetition rates are achieved by modelocking the cavity at some harmonic, N, of the laser cavity frequency, resulting in N pulses per round trip. This results in a timing jitter spectrum that is more complicated than that for a fundamentally modelocked laser and depends strongly on the pulse-to-pulse correlations. To date, much of the published work on measuring and interpreting the timing jitter has not properly taken these correlations into account. In this work we have shown how the pulse-to-pulse correlations are related to the total timing jitter, developed a theoretical model, and demonstrated experimental confirmation of the model. 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. 24-33 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 . 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 Pm 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, described previously in Refs 1 and 2. 24-34 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 Figure 3. Timing jitter spectrum for the case of mostly AM. Upper solid curve, data; lower solid curve, measurement noise floor; dotted curve, theory. A typical phase-noise spectrum L(f) is shown in Fig. 3 for the case where the modulation is set to mostly AM. The characteristic spectrum of the jitter at low frequencies (f < 250 kHz) is well understood [3]. However, the significance and a complete explanation for the features appearing at harmonics of the fundamental round-trip frequency—referred to as supermodes--have not been explained in the literature. We have shown that the supermodes are, in fact, aliased versions of the baseband mode—as shown in Fig. 4—and contribute to the overall timing jitter by a factor of ¥N, where N is the harmonic number. This is consistent with the explanation that pulses within the laser cavity are uncorrelated with each other. We confirmed this by measuring the timing jitter by comparing the accumulated timing jitter from one pulse to the next using optical cross-correlation—as shown in Fig. 5—and comparing it with the integrated timing jitter. The width of the optical cross correlations did not change as the pulse was delayed from one to the next, and the accumulated timing jitter agreed closely with that computed by integrating the timing jitter spectrum of Fig. 3. We are currently working on a scheme to correlate each of the pulses in the laser cavity (positively correlated) using an etalon or GTI to reduce the overall timing jitter, which we believe should result in timing jitter reduction by a factor of ¥N. Figure 4. Overlap of the first, tenth, and twentieth harmonics of the timing jitter spectrum. 24-35 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 Figure 5. Background-free autocorrelation and crosscorrelation of adjacent pulses traces References 1. M. E. Grein, L. A. Jiang, H. A. Haus, and E. P. Ippen, C. McNeilage, J. H. Searls, and R. S. Windeler, “Observation of quantum-limited timing jitter in an active, harmonically mode-locked fiber laser,” Opt. Let. 27, 957-959 (2002). 2. W. F. Walls, “Cross-correlation phase noise measurements,” In IEEE Frequency Control Symposium, (Institute of Electrical and Electronics Engineers, New York, 1992), pp. 257260. 3. 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). Publications Journal Articles, Published M. E. Grein, L. A. Jiang, H. A. Haus, and E. P. Ippen, C. McNeilage, J. H. Searls, and R. S. Windeler, “Observation of quantum-limited timing jitter in an active, harmonically mode-locked fiber laser,” Opt. Let. 27, 957-959 (2002). L. A. Jiang, M. E. Grein, J. K. Chandalia, E. P. Ippen, and H. Yokoyama, “Retiming dynamics of modelocked semiconductor lasers”, Electron. Lett. 38, 1446-1447 (2002). F. Rana, H. L. T. Lee, M. E. Grein, L. A. Jiang, and R. J. Ram, “Characterization of the noise and correlations in harmonically mode-locked lasers,” J. Opt. Soc. Am. B 19, 26092621 (2002). Journal Articles, Submittted for Publication M. E. Grein, H. A. Haus, Y. Chen, and E. P. Ippen, “Quantum-limited timing jitter in actively mode-locked lasers,” submitted to IEEE J. Quantum Electronics. 24-36 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 Meeting Papers Published M. E. Grein, L. A. Jiang, H. A. Haus, E. P. Ippen, C. McNeilage, and J. H. Searls, “Observation of quantum-limited timing jitter in an actively modelocked laser,” in OSA Trends in Optics and Photonics (TOPS) Vol. 73, Conference on Lasers and ElectroOptics, OSA Technical Digest, Postconference Edition (Optical Society of America, Washington DC, 2002). L. A. Jiang, M. E. Grein, H. A. Haus, E. P. Ippen, “Experimental demonstration of a timing-jitter eater,” in OSA Trends in Optics and Photonics (TOPS) Vol. 73, Conference on Lasers and Electro-Optics, OSA Technical Digest, Postconference Edition (Optical Society of America, Washington DC, 2002). 24-37 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 Timing Jitter Reduction Using a Timing-Jitter Eater Sponsors U. S. Air Force – Office of Scientific Research - F49620-01-1-0084 U. S. Navy – Office of Naval Research Project Staff Leaf A. Jiang, Matthew E. Grein, Professor Erich Ippen, Professor Hermann A. Haus Removing the timing jitter from a train of pulses can be useful for optical communications and optical sampling. In long-haul optical soliton communication systems, where the transmitted pulses propagate through many amplifiers, the timing jitter of the pulses can become significant. Retiming the data pulses before detection can lead to improved bit-error rates. For optical sampling applications, the timing jitter of the pulse train puts an ultimate limit on the speed and resolution of the system. There have been many approaches to retiming pulses using active modulation [1-3], including the use of phase modulation and dispersion [4-6]. The novelty of the present work is the effective application of active retiming to nonsolitonic pulses with a single phase modulator, which is difficult in two respects: (1) short nonsolitonic pulses disperse easily in fiber, thereby putting a severe limitation on retiming, whereas solitons are immune to dispersive broadening; and (2) frequency-to-timing noise conversion puts a limit on the amount of timing jitter reduction. We show how to overcome these problems by introducing prechirp fiber. The experimental setup of the timing jitter eater is shown in Figure 1. A train of mistimed pulses enters the eater from the top left-hand corner. The first pulse is slightly delayed, and the third pulse is slightly advanced. The jittery pulse train propagates through prechirp fiber. Figure 1. Timing-jitter eater experimental setup. Next, the pulses enter a phase modulator that is sinusoidally modulated at the pulse repetition rate. The phase of the modulation signal is set so that the desired pulse positions are aligned to the peak of the sinusoidal phase modulation. A pulse that arrives early is redshifted, and a pulse that arrives late is blueshifted. The pulses from the phase modulator then propagate through the anomalous dispersion postchirp fiber in which blueshifted light travels faster than redshifted light. If the length of the postchirp fiber is chosen correctly, the output yields jitter-reduced pulses. Note that the eater will also work if the phase modulation is delayed by 180° and the sign of the postchirp fiber is flipped. At the output of the timing jitter eater, each mistimed pulse has a slightly 24-38 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 different color, and hence the timing jitter reduction comes at the cost of increased wavelength jitter. Fig. 2 shows the jitter reduction revealed by the directly detected spectrum of the photocurrent displayed on an rf spectrum analyzer. The reduction of the pedestals surrounding the 10.0 GHz carrier is indicative of timing jitter reduction. Figure 2. Spectrum of the first harmonic before and after the timing-jitter eater There are two additional complications: (1) the output pulse width may be significantly larger than the input pulse width due to dispersive broadening, and (2) the input wavelength jitter can turn into timing jitter at the output of the system through dispersion. The prechirp fiber ameliorates these two problems since it has the opposite dispersion of the postchirp fiber, so that the total dispersion of the timing jitter eater is close to zero. Figure 3 shows theoretical calculations of the best timing jitter reduction for a given input and output pulse width made using a single phase modulator. We computed the amount of timing jitter reduction by propagating a mistimed Gaussian pulse through the timing jitter eater and then comparing the initial and final mistimed positions. The timing jitter reduction (defined as the rms output timing jitter divided by the input timing jitter) is plotted in Fig. 3 on a decibel scale and where the input pulse is assumed to be transform limited. The solid curves show the best that can be done with both prechirp and postchirp fibers, and the dashed curves show the best reduction possible when only postchirp fiber is used. Figure 3 shows that (1) it is difficult to retime short pulses since their large spectra lead to significant dispersive broadening; (2) the prechirp fiber improves performance when the output pulse width is close to the input pulse width; (3) the output pulses can be shorter than the input pulses, as in the case of 10-ps input pulse with ±3S phase modulation, since the phase modulator adds spectrum to the input pulses; and (4) the dashed and solid curves come together at large output pulse widths, since the final pulse width and noise reduction is insensitive to the amount of prechirp dispersion when the input and output timing jitter are the same. 24-39 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 Figure 3. Theoretical calculations of the best possible jitter reduction for a given input and output pulse width made using one phase modulator with M = 0.6S and M =3S. The input pulse is transform limited, but the output pulse is not necessarily transform limited. Solid curves, best noise reduction with prechirp and postchirp fiber; dashed lines, best noise reduction with only postchirp fiber. The star corresponds to the experimental results in Fig. 2. References 1. M. Nakazawa, E. Yamada, H. Kubota, and K. Suzuki, Electron. Lett. 27, 1270 (1991). 2. N. J. Smith, K. J. Blow, W. J. Firth, and K. Smith, Opt. Commun. 102, 324 (1993). 3. J. P. King, I. Hardcastle, and H. J. Harvey, “Method and apparatus for conditioning optical solitons,” U.S. patent 6,130,767 (October 10, 2000). 4. M. E. Grein, L. A. Jiang, E. P. Ippen, and H. A.Haus, Opt. Express 8, 664 (2001), http://www.opticsexpress.org. 5. L. Mollenauer and C. Xu, in Conference on Lasers and Electro-Optics, Vol. 73 of OSA Trends in Optics and Photonics Series (Optical Society of America, Washington,D.C., 2002), postedeadline paper CPDB1-1. 6. L. A. Jiang, M. E. Grein, E. P. Ippen, and H. A. Haus, in Conference on Lasers and Electro-Optics, Vol. 73 of OSA Trends in Optics and Photonics Series (Optical Society of America, Washington, D.C., 2002), postconference edition, pp. 164–165. Publications Journal Articles, Published L. A. Jiang ,M. E. Grein, H. A. Haus, and E. P. Ippen, “Timing jitter eater for optical pulse trains”, Opt. Lett. 28, 78-80 (2003). Meeting Papers, Published L. A. Jiang, M. E. Grein, H. A. Haus, E. P. Ippen, “Experimental demonstration of a timing-jitter eater,” in OSA Trends in Optics and Photonics (TOPS) Vol. 73, Conference on Lasers and Electro-Optics, OSA Technical Digest, Postconference Edition (Optical Society of America, Washington DC, 2002). 24-40 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 Timing Jitter Studies in Hybridly Modelocked Semiconductor Lasers Sponsors U. S. Air Force – Office of Scientific Research Grant F49620-01-1-0084 U. S. Navy – Office of Naval Research Project Staff Leaf A. Jiang, Matthew E. Grein, Professor Erich Ippen, Professor Hermann A. Haus High-speed optical sampling systems and optical time-division multiplexed transmission systems have stringent timing-jitter requirements. Timing jitter less than 500 fs is required for optical timedivision multiplexed transmission at 160 Gbits/s (timing jitter should be less than 10% of the 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 Gsamples/s. External cavity modelocked laser diodes (EC-MLLD) can produce high-quality picosecond pulses at GHz repetition rates, and thus could potentially be used as a pulse source for the aforementioned applications. Beyond the state-of-the-art performance aspects, in this work we investigated the quantum-limited timing jitter of an EC-MLLD and the effects of device optimization The EC-MLLD used in our noise measurements is shown in Fig. 1. The semiconductor chip consisted of two sections [1] a 50-Pm saturable-absorber section and a 500-Pm gain section. The gain section was biased at 65 mA, and the saturable-absorber section was reverse biased at 21.4 V. The saturable absorber was modulated externally with a low-noise Poseidon Scientific Shoe Box Oscillator (SBO) at 9 GHz, amplified to give 24 dBm of rf power. The cavity length was set so that the round-trip frequency matched the rf drive frequency (the laser was fundamentally mode locked). The residual phase-noise measurement setup, shown in Fig. 2, consists of the SBO microwave oscillator, the device under test, and a delay arm for setting the microwave phase into quadrature so that the output is proportional to the phase difference between arms. The device under test consists of the MLLD and the photodetector for the noise measurement or an attenuator with equivalent loss for the noise-floor measurement. The mixer output is then observed on a vector signal analyzer for low-frequency offsets (0–10 MHz) and on a rf spectrum analyzer for high offsets (10 MHz–4.5 GHz). The delays through both arms were closely matched so that the oscillator noise was suppressed. The rf spectrum analyzer measurement was calibrated by measurement of the noise down to 1 MHz, where the noise overlapped the frequency range of the vector signal analyzer measurement. The single-sided phase noise is shown in Fig. 3a. Spurs that are due to 60-Hz wall current and harmonics thereof are apparent. Vibration spurs at harmonics of 20 and 55 Hz that are due to ac fans and other ambient vibrations are also visible. At higher offsets, radio station signals appear in the 10–100-MHz decade. A spur that is due to wireless telephones is visible in the 100-MHz–1GHz decade. In addition, contributions of noise that are due to the fast gain dynamics in the semiconductor laser at the relaxation oscillation frequency are also evident in this decade [2]. In the 100-kHz–1-MHz decade, the spurs in the laser noise are due to the switching power supply in the ILX-3207B current source (ILX Lightwave). 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 low-frequency noise is due to the current-source driver and voltage supply. For offsets greater than 5 kHz, the noise is reduced to that produced by the fundamental amplified spontaneous emission quantum noise. Since noise less than 5 kHz contributes a small fraction of the integrated timing jitter, the predominant source of the total value is amplified spontaneous emission. Figure 3b shows the integrated timing jitter in each decade. 24-41 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 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 by microwave oscillator noise. The noise of a laser diode depends on many laser parameters [3]. We have found some rules of thumb for achieving low noise: (1) reduce cavity loss, (2) use tighter optical filtering, and (3) use a quieter oscillator. Reducing cavity loss increases the ratio of signal to noise photons, while tighter optical filtering limits Gordon–Haus jitter [4]. Fig. 1. Hybridly mode-locked semiconductor laser used in our experiments. GRIN, graded-index; BPF, bandpass filter Fig. 2. Residual phase-noise experimental setup. The device under test (DUT) is the hybridly modelocked laser diode and photodetector. OSC, oscillator; AMP, amplifier 24-42 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 1 Fig. 3. a) Single-sideband phase noise of the EC-MLLD and corresponding noise floor, pieced together from the vector signal analyzer (10 Hz-10 MHz) and rf spectrum analyzer (10 MHz-5 GHz). The bandwidth of the mixer’s IF port was limited to 2 GHz. b) Integrated timing jitter in each decade of the phase noise shown in Fig. 3a. In the last decade from 1 to 4.5 GHz, the white bar corresponds to the timing jitter, assuming that the noise is equal to the noise floor; the black bar corresponds to a theoretically expected –20 dB/decade roll off. References 1. H. Yokoyama, “Highly stabilized mode-locked semiconductor diode lasers,” Rev. Laser Eng. 27, 750–755 (1999). 2. D. J. Derickson, A. Mar, and J. E. Bowers, “Residual and absolute timing jitter in actively modelocked semiconductor lasers,” Electron. Lett. 26, 2026–2028 (1990). 3. L. A. Jiang, M. E. Grein, H. A. Haus, and E. P. Ippen, “Noise of mode-locked semiconductor lasers,” IEEE J. Sel. Top. Quantum Electron. 7, 159–167 (2001). 4. J. P. Gordon and H. A. Haus, “Random walk of coherently amplified solitons in optical fiber transmission,” Opt. Lett. 11, 665–667 (1986). Publications Journal Articles L. A. Jiang, M. E. Grein, J. K. Chandalia, E. P. Ippen, and H. Yokoyama, “Retiming dynamics of modelocked semiconductor lasers”, Electron. Lett. 38, 1446-1447 (2002). L. A. Jiang, M. E. Grein, H. A. Haus, and E. P. Ippen, “Quantum-limited noise performance of a modelocked laser diode,” Opt. Lett. 27, 49-51 (2002). F. Rana, H. L. T. Lee, M. E. Grein, L. A. Jiang, and R. J. Ram, “Characterization of the noise and correlations in harmonically mode-locked lasers,” J. Opt. Soc. Am. B 19, 2609-2621 (2002). Meeting Papers L. A. Jiang, M. E. Grein, H. A. Haus, E. P. Ippen, “Experimental demonstration of a timing-jitter eater,” in OSA Trends in Optics and Photonics (TOPS) Vol. 73, Conference on Lasers and Electro-Optics, OSA Technical Digest, Postconference Edition (Optical Society of America, Washington DC, 2002). 24-43 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 Variational Analysis of Spatio-temporal Pulse Dynamics in Dispersive Kerr Media Sponsors National Science Foundation ECS-0119452 Project Staff Christian Jirauschek, Dr. Uwe Morgner, Professor Franz X. Kaertner The full spatio-temporal dynamics of Kerr-lens mode-locked (KLM) lasers can be studied in numerical simulations, either by complete 3-D space modeling [1] or by various approximation schemes [2, 3]. Alternatively, simplified models, allowing for quick numerical algorithms or even handy formulas, are available for studying either the spatial or the temporal dynamics. Spatial models for understanding and optimizing the self-focusing dynamics in a KLM laser are based on an ABCD- or q-parameter analysis, neglecting the temporal breathing of the pulse in the Kerr medium. In these approaches, the Kerr lens is modeled by an intensity-dependent lens, and an iterative solution scheme is used [4, 5, 6]. As for the temporal dynamics, it has been shown [7, 8] that dispersion-managed soliton formation is the most important pulse shaping process in KLM lasers. The KLM action stabilizing the pulse against gain filtering can be considered a perturbation to this dynamics. In the model introduced in [7], the temporal dynamics is studied based on the variational principle [9], taking the nonlinearity in the Kerr medium and the actual dispersion management into account. We extended the variational principle to include the spatial effects due to self-focusing and diffraction as well as the temporal effects due to self-phase modulation and second-order dispersion. As a result, the equations of motion in the Kerr medium for the pulse parameters, which are pulse width, pulse cross-sections in x and y directions, and temporal as well as spatial chirp, are obtained [10]. Considering only the energy-preserving effects, the spatio-temporal dynamics in the laser can then be described in Gaussian approximation, and the Gaussian steady-state solutions can be extracted from the model [11]. To verify the validity of the Gaussian approximation, the Gaussian steady-state solutions of the laser system are compared to the results of spatio-temporal numerical simulations [11]. By including gain and loss in the numerical simulations, the influence of non-energy-preserving effects on the pulse shaping process can be assessed. For obtaining the numerical solutions, an initial laser pulse is propagated through the laser resonator over many roundtrips, until steady state is reached. A transversally and spectrally dependent gain is allowed in the Kerr medium, and loss is introduced by renormalizing the pulse after each roundtrip. The Gaussian steady-state solution is computed for a Ti:sapphire laser setup with a 2.5 mm thick Kerr medium and resonator arm lengths of 80 cm (left arm) and 110cm (right arm). The Gaussian approximation is compared to numerically obtained results. In order to reduce the computation time for the numerical simulation, axial symmetry has to be assumed. For the simulation, an increase in intracavity pulse energy of about 2% during one roundtrip is assumed, corresponding to a typical gain in today’s ultrashort-pulse KLM lasers. The spatial extension of the gain is chosen to be somewhat narrower than the transversal pulse profile to enable soft-aperture Kerrlens mode-locking. In Fig. 1, the pulse duration, the spectral width and the beam width are shown for the Gaussian steady-state solution subject to the position z in the Kerr medium. In Figs. 2 and 3, the Gaussian steady-state solution (dashed curve) is compared to the numerically obtained result (solid curve). Displayed are the temporal pulse shape, characterized by the instantaneous power P(t), the power spectrum S(f) and the transversal profile, described by the fluence )(r), at the right end of the Kerr medium and at the right end mirror. 24-44 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 As can be seen from Figs. 2 and 3, the Gaussian steady-state solution is in good agreement with the numerically obtained result for pulse durations down to a few optical cycles. The Gaussian model provides insight into the elementary spatio-temporal dynamics of a KLM laser system. An application of this model is to roughly determine the range of laser parameters for which a steady-state solution exists, and to get a first approximation of the solution, avoiding lengthy numerical simulations. Due to the low computational effort, even a scan over a wide range of laser parameters becomes possible, allowing for a further optimization of KLM lasers. Figure 1: Steady-state pulse dynamics in the Kerr medium: (a) Pulse duration. (b) Spectral width. (c) Beam width. Figure 2: Comparison between numerical solution (solid curves) and Gaussian approximation (dashed curves) at the right end of the Kerr medium. (a) Instantaneous power. (b) Power spectrum. 24-45 Figure 3: Comparison between numerical solution (solid curves) and Gaussian approximation (dashed curves) at the right end mirror. (a) Instantaneous power. (b) Power spectrum. (c) Fluence. 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 References 1. I.P. Christov and V.D. Stoev, “Kerr-lens mode-locked laser model: role of space-time effects,” J. Opt. Soc. Am. B 15(7): 1960-66 (1998). 2. O.E. Martinez and J.L.A. Chilla, “Self-mode-locking of Ti:sapphire lasers: a matrix formalism,” Opt. Lett. 17(17): 1210-12 (1992). 3. V.P. Kalosha, M. Müller, J. Herrmann and S. Gatz, “Spatiotemporal model of femtosecond pulse generation in Kerr-lens mode-locked solid-state lasers,” J. Opt. Soc. Am. B 15(2): 535-50 (1998). 4. H.A. Haus, J.G. Fujimoto, and E.P. Ippen, “Analytic theory of additive pulse and Kerr lens mode locking,” IEEE J. Quantum Electron. 28(10): 2086-96 (1992). 5. G. Cerullo, S. De Silvestri, and V. Magni, “Self-starting Kerr-lens mode locking of a Ti:sapphire laser,” Opt. Lett. 19(14): 1040-42 (1994). 6. A. Penzkofer, M. Wittmann, M. Lorenz, E. Siegert, and S. Macnamara, “Kerr lens effects in a folded-cavity four-mirror linear resonator,” Opt. Quant. Electron. 28(4): 423-42 (1996). 7. Y. Chen and H.A. Haus, “Dispersion-managed solitons in the net positive dispersion regime,” J. Opt. Soc. Am. B 16(1): 24-30 (1999). 8. Y. Chen, F.X. Kaertner, U. Morgner, S.H. Cho, H.A. Haus, E.P. Ippen, and J.G. Fujimoto, “Dispersion-managed mode locking,” J. Opt. Soc. Am. B 16(11): 1999-2004 (1999). 9. D. Anderson, “Variational approach to nonlinear pulse propagation in optical fibers,” Phys. Rev. A 27(6): 3135-45 (1983). 10. Ch. Jirauschek, U. Morgner, and F.X. Kaertner, “Variational analysis of spatio-temporal pulse dynamics in dispersive Kerr media,” J. Opt. Soc. Am. B 19(7): 1716 - 21 (2002). 11. Ch. Jirauschek, F.X. Kaertner and U. Morgner, “Spatio-temporal Gaussian pulse dynamics in Kerr-lens mode-locked lasers,” J. Opt. Soc. Am. B, forthcoming. 24-46 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 Ultrafast Phenomena and Quantum Electronics Ultrafast Pump-Probe Studies of Silicon- and III/V-based Devices Sponsors U.S. Air Force – Office of Scientific Research - F49620-01-1-0084 MRSEC Program of the National Science Foundation - DMR 98-08941 Project Staff J. T. Gopinath, J. M. Fischer, D. Cannon, Dr. G. S. Petrich, Professor F. Kaertner, Professor L. Kimerling, Professor L. A. Kolodziejski, Professor E. P. Ippen In order to improve designs of devices for applications in lasers, opto-electronics and telecommunications, it is necessary to understand the fundamental properties of materials and device structures. In this report, we describe the use of a femtosecond optical parametric oscillator (OPO), producing 150 fs tunable between 1400 and 1600 nm, for characterization of several important photonic materials. The pump-probe technique, which yields information about absorption and index changes, as well as recovery times, is used to study the samples. In pumpprobe, a powerful pump pulse excites a sample at time t = 0. As the sample relaxes back to equilibrium, its properties are sampled with weak, non-perturbing probe pulses at varying time delays. The pump probe setup used has a measurement sensitivity of ~ 10-4 for transmission and reflection changes. Semiconductor structures designed for several different applications were characterized with a degenerate pump-probe technique, in which the OPO signal wavelength is used for both the pump and the probe. The pump and probe are in a ratio of 10:1 in fluence, and are crosspolarized. A schematic of the setup is shown below. Figure 1: Schematic of pump probe setup. A 543-nm-thick film of crystalline germanium on an undoped silicon wafer was characterized to determine its suitability for use as a broadband ultrafast saturable absorber. The maximum absorption of the film at 1540 nm is ~ 8.5%. Pump-probe traces as a function of incident fluence are shown below. The fast component, due to spectral hole burning and thermalization of carriers recovers quickly, and is a large percentage of the overall signal. However, there is still a long-lived component of the signal, due to recombination and thermal effects. Future studies of 24-47 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 these effects as a function of fabrication conditions will help determine the optimum design for a broadband saturable absorber. Figure 2: Pump-probe trace of 543 nm Germanium film at 1540 nm. In other experiments, a high-modulation-depth InP-based saturable Bragg reflector (SBR), designed for use in a high repetition rate laser system, was studied.. The structure, fabricated by Professor Kolodziejski’s group, consists of 12 InGaAs quantum wells in a O layer of InP. This is grown on top of an MOCVD mirror consisting of a 22-pair GaAs/AlAs Bragg stack, centered at 1550 nm with a 100 nm bandwidth. Additional InP wasovergrown on the structure to enhance two-photon absorption, which can help stabilize lasers against Q-switching instabilities [1]. Structures with either antireflection or resonant coatings to enhance the modulation depth have been investigated Below is a pump-probe trace of the resonantly coated structure, in which a maximum modulation depth of 20% has been achieved. The recovery time of this device is ~43 ps, due to the high lattice mismatch between the InP and the GaAs. In addition to degenerate pump-probe, two-color pump-probe has been performed, using the leftover 800 nm pump of the OPO and the OPO signal beam. Because the OPO is synchronously pumped, these two colors have less than 20 fs of timing jitter [2] between them. In addition, it is possible to perform two-color pump probe with the OPO signal and idler. Microstructured fiber [3] and a Fabry Perot device have been studied using this technique. The Fabry Perot device consisted of 1 Pm of polysilicon on a 1 mm quartz substrate, forming a Fabry Perot. The Ti:Sapphire laser was used as the pump, and the OPO as the probe. The Ti:Sapphire generated free carriers in the sample, causing the index to change slightly. This in turn, leads to a shift of the resonances of the Fabry Perot, causing a change in transmission as a function of wavelength. From this pump-probe experiment, a carrier density of 3 x 1018 /cm3 produced an index change of ~0.15%, a number that agrees well with values in the literature. 24-48 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 Figure 3: Pump probe of high modulation depth SBR at 1575 nm. References 1. T. R. Schibli, E. R. Thoen, F. X. Kaertner and E. P. Ippen, "Suppression of Q-switched mode locking and break-up into multiple pulses by inverse saturable absorption," Applied Physics B Lasers and Optics, B70: S41-S49 (2000). 2. J. D. Kafka, M. L. Watts and J. W. Pieterse, "Synchronously pumped optical parametric oscillators with LiB3O5," Journal of Optical Society of America B, 12(11): 2147-2157 (1995). 3. K. S. Abedin, J. T. Gopinath, E. P. Ippen, C. E. Kerbage, R. S. Windeler and B. J. Eggleton, "Highly nondegenerate femtosecond four-wave mixing in tapered microstructure fiber," Applied Physics Letters, 81(8): 1384-1387 (2002). 24-49 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 Materials for Modelocking Sponsors Air Force Office of Scientific Research (MFEL) - F49620-01-1-0186 Air Force Office of Scientific Research - F49620-98-01-0084 National Science Foundation - ECS-019452 Project Staff Rohit P. Prasankumar, Paul Mak, Professor Michael Ruane, Professor James G. Fujimoto Non-epitaxially grown semiconductor-doped silica films for laser modelocking Ultrafast laser technology has matured in recent years, resulting in the widespread availability of ultrafast laser systems. However, the suitability of ultrashort pulse lasers for some applications is still limited by their reliability and high cost, necessitating novel approaches to developing stable, inexpensive ultrashort pulse laser sources. Semiconductor saturable absorbers are a well established technology for generating stable, self-starting pulses in solid-state lasers. These devices, known as semiconductor saturable absorber mirrors (SESAM) or saturable Bragg reflectors (SBR), typically consist of semiconductor quantum wells grown in a semiconductor mirror structure by molecular beam epitaxy (MBE) [1-3]. SESAMs and SBRs have been very successful in mode-locking solid-state lasers, helping start and support pulses as short as 5.5 fs in a Ti:sapphire laser [4]. 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. The goal of this project is to develop a more versatile, lower cost alternative to epitaxially grown semiconductor saturable absorbers. In previous work, we developed non-epitaxially grown saturable absorber devices and applied them to self-starting Kerr lens mode-locking (KLM) in Ti:sapphire and Cr:forsterite lasers [5, 6]. The devices consist of InAs nanocrystallites doped into SiO2 films and deposited on sapphire substrates using magnetron and non-magnetron radio frequency (RF) sputtering systems. 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) in nitrogen from 500-750qC was an effective method of controlling the absorption saturation dynamics of our saturable absorbers. In Ti:sapphire, 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 measured to be 25 mJ/cm2, which is too high to enable saturable absorber mode-locking without KLM and also limits the minimum achievable pulsewidth. Recently, we have developed guidelines for designing semiconductor-doped silica film saturable absorbers with an optimized saturation fluence for a given solid state laser system. These guidelines are based on extensive device characterization using linear and nonlinear optical techniques while varying fabrication parameters. Growth parameters including choice of semiconductor and target materials, annealing time and temperature, and ratio of semiconductor to glass were varied to determine the most important factors influencing device performance. Nonlinear optical measurements were performed using a previously developed pump-probe system based on a broadband 5.5 fs Ti:Al2O3 laser [7] to obtain 17 fs time resolution and independent pump and probe wavelength tunability over a range of 700 to 1000 nm [8]. The magnitude of the measured pump probe signal can be shown to be inversely proportional to the saturation fluence. Linear optical measurements were performed using a Cary spectrophotometer. Initially, we examined the saturation fluence near the absorption edge of the semiconductor nanocrystallites in a degenerate pump probe measurement between 750 and 925 nm (Fig. 1). 24-50 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 The measurement was performed on a 10%InAs/90% SiO2 film deposited on a sapphire substrate. The film was annealed at 600qC for 60 seconds in nitrogen. The absorption edge of this device was approximately 1100 nm. The saturation fluence of this film decreased with increasing wavelength, demonstrating that operation close to the absorption edge is desirable to minimize the saturation fluence. 10% InAs / 90% SiO Pump probe -3 3x10 925 nm -'DD 2 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 size of the InAs nanocrystallites was controlled by varying the ratio of InAs to SiO2 on the sputtering target. As expected, the absorption edge shifted to longer wavelengths for higher InAs/SiO2 ratios in linear transmission measurements. Pump probe measurements were performed at 925 nm on 10%InAs/90% SiO2 films using the Ti:sapphire based pump probe system, and on 40%InAs/60% SiO2 films at 1260 nm using a Cr:forsterite based pump probe system. From the two graphs shown below (Fig. 2), it is clear that the magnitude of the signal is significantly larger for the 40%InAs/60% SiO2 films. This data shows that the saturation fluence strongly decreases with an increase in nanocrystallite size at wavelengths approximately the same distance from the absorption edge. -3 40%InAs/60%SiO2, annealed at 600 C Pump/probe wavelength 1260 nm -'D D 30 20 3x10 -'D D 40x10 10 0 10%InAs/90%SiO2, annealed at 600 C Pump/probe wavelength 925 nm -3 2 1 0 0 10 20 30 time (fs) 40 50x10 3 0 (a) 5 10 time (fs) 15x10 3 (b) Figure 2. Pump probe measurements on (a) 40%InAs/60% SiO2 films at 1260 nm and (b) 10%InAs/90% SiO2 films at 925 nm. The saturation fluence is significantly lower for the 40%InAs/60% SiO2 films. We also varied the annealing time and temperature of 10%InAs/90% SiO2 films to determine the effect of RTA on the nonlinear absorption saturation dynamics. We observed discrete changes in the dynamics as the annealing temperature was varied. Fig. 3 depicts the changes in the 24-51 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 measured pump-probe signal at 800 nm as a function of annealing temperature for 10%InAs/90% SiO2 films annealed for 60 seconds in nitrogen. For temperatures of 350qC and below, a negative pump-probe signal was measured, indicating no absorption saturation; this was identical to measurements done on unannealed samples. In the range of 400-500qC, the absorption saturation was positive but small. At temperatures of 550qC and above, the absorption saturation was relatively large; samples annealed at these higher temperatures self-started KLM in Ti:sapphire and Cr:forsterite lasers. Samples annealed at 700qC had a faster relaxation time than those annealed at 550-600qC, although the magnitude of the signal and therefore the saturation fluence was nearly the same as that of the sample annealed at 550qC. We believe that these discrete changes in the pump-probe signal as a function of annealing temperature are related to the glass melting and transition temperatures for SiO2. We tested this by fabricating films with InAs nanocrystallites doped into a borosilicate glass matrix that had different glass melting and transition temperatures. Pump-probe measurements demonstrated that the absorption saturation did not change discretely with annealing temperature, instead increasing continuously as the RTA temperature increased. However, at high temperatures the dynamics were very similar to those displayed in Fig. 3 with a SiO2 matrix and there was no significant improvement in the saturation fluence. Temperature dependence of RTA 10%InAs/90%SiO2 -3 3x10 ' T/T 550 C 2 700 C 1 500 C 350 C 0 0 no RTA 1000 2000 time (fs) 3000 Figure 3. Pump probe measurements on 10%InAs/90% SiO2 films at 800 nm as a function of rapid thermal annealing temperature. The samples were all annealed for 60 seconds in nitrogen. From these experiments, guidelines were formulated to optimize the saturation fluence of semiconductor-doped silica film saturable absorbers for a given laser system. As shown in fig. 2, films with larger nanocrystallites have a significantly lower saturation fluence than those with smaller nanocrystallites. It is also clear that the films should have an absorption edge close to the laser wavelength, since this also lowers the saturation fluence (Fig. 1). Therefore, the semiconductor material and semiconductor/glass ratio should be chosen to satisfy these two criteria. Finally, the annealing temperature should be above 550qC for strong absorption saturation based on the results displayed in Fig. 3; however, the optimum temperature may vary for different semiconductor and glass materials. We applied these guidelines to design semiconductor-doped glass film saturable absorbers for self-starting modelocking in a Cr:forsterite laser operating at 1.26 Pm [5, 6]. We deposited thin 40%InAs/60% SiO2 films on sapphire substrates and annealed them at 600qC. Their saturation 24-52 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 fluence at 1.26 Pm was measured from pump probe experiments to be 3.35 mJ/cm2, which is nearly an order of magnitude improvement over the previous work in Ti:sapphire. Self-starting KLM was obtained, with a bandwidth of 91 nm and pulsewidth of 25 fs measured by interferometric autocorrelation. 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 also investigated pure saturable absorber mode-locking without KLM as a function of the intracavity dispersion. Self-starting saturable absorber mode-locking was observed; however, intensity autocorrelations showed that the output consisted of a short pulse with a long background pulse (Fig. 4). This is consistent with theories of fast saturable absorber modelocking with KLM and soliton-like pulse shaping. Adjustment of the intracavity dispersion towards zero by varying the prism insertion results in a shorter pulse duration. However, the low self amplitude modulation of the saturable absorber cannot support this short pulse, therefore it sheds energy to a longer background pulse with a duration determined by the recovery time of the saturable absorber. Autocorrelation measurements of the pulses for different intracavity dispersion operating points agree with theoretical expectations. For small positive dispersions, a long pulse with 5.3 ps duration is generated. The corresponding spectrum is 11 nm and the pulse is strongly chirped. As the intracavity dispersion is made increasingly negative, a short solitonlike pulse of ~150 fs duration is generated with a longer background pulse. For more negative values of dispersion, the intensity of the longer background pulse decreases. Pulse shaping would be improved if the saturable absorber saturation fluence was reduced or if the relaxation dynamics were made faster. 1.0 SHG signal (arb. units) positive 2 dispersion ~+150 fs 0.8 slightly positive dispersion 0.6 slightly negative dispersion 0.4 negative 2 dispersion ~-340 fs 0.2 0.0 -10 -5 0 time (ps) 5 Figure 4. Intensity autocorrelations at different dispersion operating points for saturable absorber modelocking without KLM. The peak intensity of the longer background pulse decreases as the negative dispersion increases. Future work will include applying non-epitaxially grown semiconductor saturable absorbers to other laser systems, designing devices in different geometries, and testing other semiconductor and glass materials to further reduce the saturation fluence. 24-53 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 References 1. U. Keller, K. J. Weingarten, F. X. Kärtner, D. Kopf, B. Braun, I. D. Jung, R. Fluck, C. Hönninger, N. Matuschek and J. Aus der Au, "Semiconductor saturable absorber mirrors (SESAMs) for femtosecond to nanosecond pulse generation in solid-state lasers," IEEE J. Sel. Top. Quant. Elect. (JSTQE), 2, pp.435-453, 1996. 2. S. Tsuda, W. H. Knox, S. T. Cundiff, W. Y. Jan and J. E. Cunningham, "Mode-locking ultrafast solid-state lasers with saturable Bragg reflectors," IEEE J. Sel. Top. Quant. Elect., 2, pp.454-464, 1996. 3. D. Jung, F. X. Kärtner, N. Matuschek, D. H. Sutter, F. Morier-Genoud, Z. Shi, V. Scheuer, M. Tilsch, T. Tschudi and U. Keller, "Semiconductor saturable absorber mirrors supporting sub10 fs pulses," Appl. Phys. B, 65, pp.137-150, 1997. 4. D. H. Sutter, G. Steinmeyer, L. Gallmann, N. Matuschek, F. Morier-Genoud, U. Keller, V. Scheuer, G. Angelow and T. Tschudi, "Semiconductor saturable-absorber mirror assisted Kerr-lens mode-locked Ti:sapphire laser producing pulses in the two-cycle regime," Opt. Lett., 24, pp.631-633, 1999. 5. P. Bilinsky, J. G. Fujimoto, J. N. Walpole and L. J. Misaggia, "Semiconductor-doped silica saturable absorber films for solid state laser mode locking," Opt. Lett., 23, pp.1766-1768, 1998. 6. R. P. Prasankumar, C. Chudoba, J. G. Fujimoto, P. Mak and M. F. Ruane, "Self-starting mode locking in a Cr:forsterite laser by use of non-epitaxially-grown semiconductor-doped silica films," Optics Letters, 27, pp.1564-66, 2002. 7. 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," Opt. Lett., 24, pp.411 -- 413, 1999. 8. 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," Quantum Electronics and Laser Science Conference, 2001. 24-54 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 High-Speed Femtosecond Pump-Probe Spectroscopy Using a Smart Pixel Detector Array Sponsors Air Force Office of Scientific Research (MFEL) - F49620-01-1-0186 Air Force Office of Scientific Research - F49620-98-01-0084 Project Staff Stephane Bourquin, Rohit P. Prasankumar, Professor Franz X. Kaertner T. Lasser, R.-P. Salathe, Professor James G. Fujimoto The standard pump-probe technique uses an ultrashort pulse, narrow-band pump to excite the sample and a continuum probe and spectrometer to acquire the wavelength-dependent dynamics. This technique usually requires a femtosecond amplifier to achieve intensities necessary for continuum generation and sufficient pump-probe signal magnitudes. With the development of few-cycle, ultra broad bandwidth lasers which can generate spectra that span nearly one octave, spectrally resolved pump-probe measurements can be performed without the need for amplifiers [1, 2, 3]. Although the pulse repetition rate from a laser oscillator is extremely high, the pulse energies are low and therefore the signal levels are small. Standard CCD detectors cannot detect modulated signals, thus it is not possible to take advantage of high signal-to-noise measurements that are theoretically possible with high repetition rate laser sources [4]. We have developed a new technique that enables the parallel acquisition of pump-probe measurements at multiple wavelengths simultaneously, using a novel, two-dimensional, smart pixel detector array, which was originally developed for high-speed optical coherence tomography (OCT) [5, 6]. Each pixel performs amplitude demodulation, and, in combination with a spectrometer, probe transmission signals can be acquired in parallel for multiple wavelengths. The smart pixel array can achieve sensitivities comparable to lock-in amplification while simultaneously demodulating probe transmission signals at multiple wavelengths, thus enabling time- and wavelength-resolved femtosecond pump-probe spectroscopy. Figure 1 left shows a schematic of the experiment. A 100 MHz repetition rate, 5 fs, 250 nm bandwidth Ti:sapphire mode-locked laser [1] is used with a femtosecond pump-probe system. A beamsplitter (BS) separates the output of the laser into a 90 mW pump beam and 0.4 mW probe beam. A pair of prisms is used in both pump and probe beams to permit independent adjustment of pulse duration and chirp. A conventional delay line (DL) is used to scan the pump beam. The pump and the probe beams are focused onto the sample by a parabolic mirror (PM) which is used to avoid dispersion and to preserve pulse duration. An aperture stop (AS) is inserted into the probe beam after the sample to prevent pump scattering into the detector. The probe beam is then collimated by a lens (L1) and spectrally spread by a diffraction grating (DG). A lens (L2) focuses the spread beam onto one row of the two-dimensional smart pixel detector array (DA). The chopper (CH) modulates the pump beam at a frequency of 3.8 kHz. Each pixel in the row demodulates the transmitted probe signal at a specific wavelength. The row is read multiple times for each pump 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. 24-55 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 5 fs Ti:Sapphire laser BS Probe Prism compressor PD PM Pump M Prism compressor M EC Sample CH L1 DL AS O1 … On DA DG L2 Computer RD CD 100 Pm Figure 1. (left) Schematic diagram of the experiment: BS, beamsplitter; M, mirror; DL, delay line; CH, chopper; PM, parabolic mirror; L1,L2, lenses; DG, diffraction grating; DA, detector array. (right) Photograph of a section of the smart pixel detector array. PD; photodiode; EC, electrical circuit; RD, row address decoder; CD, column address decoder. A photograph of a section of the detector array is shown in Figure 1 right. 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 generated amplitude modulation signals are selected sequentially by a row and a column address decoder. The analog output signal is read out of the chip, digitized by a 12-bit data acquisition card and transferred to a computer. Measurements were performed in a thin sample of bulk GaAs. A 3D data set acquired by the smart pixel detector array is shown in Figure 2 left. The normalized differential probe transmission is plotted versus the time delay and the probe wavelength. The pump beam spectrum was set to a bandwidth of 700 nm to 770 nm to excite conduction band states above the band gap of ~870 nm. The detected probe wavelength ranges from 700 nm to 900 nm and the pump-probe signals are recorded over a 5 ps time delay. For each delay, the pixels are read 5000 times and averaged, yielding a sensitivity to normalized differential probe transmission signal changes as small as 2 x 10-4. The pixels are read at 625 kHz, corresponding to 188 seconds of total measurement time. This time is a factor of 58 times faster than possible using a conventional single detector pump-probe spectroscopy system. Figure 2 right presents pump-probe measurements at selected wavelengths selected from the 3D data set. The dynamics are a strong function of wavelength. Measurements performed far above the band edge of 870 nm show a rapid relaxation of absorption saturation 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. 24-56 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 3.0 879 nm 834 nm 2.5 807 nm 2.0 -2 'T/T (10 ) 'T/T (10-2) 1.0 0.5 0.0 av 850 ele 800 ng th 750 [n m 700 ] 1.5 772 nm 1.0 900 W 779 nm 769 nm 755 nm 0.5 0 1 Tim 2 3 4 728 nm 0.0 s] ay [p e del 703 nm 0 1 2 3 4 5 Time delay [ps] Figure 2. (left) Spectrally resolved femtosecond pump-probe measurements of a thin sample of bulk GaAs as a function of probe wavelengths and time delay. The data are acquired simultaneously by the smart pixel detector array. (right) Pump-probe measurements at selected probe wavelengths extracted from the three-dimensional data set. The traces are separated for clarity. These results demonstrate the feasibility to perform parallel spectrally resolved pump-probe measurements across 58 simultaneous wavelengths with a detection sensitivity to normalized differential probe transmissions as small as ~2 x 10-4 with a measurement time of only ~3 minutes for a 5 ps scan. The current limitations of the technique are set by the low pixel readout rate and low sensitivity due to the first-order filters implemented in the detector array. A redesign of the detector array with second-order bandpass filters and with a higher readout rate to increase the number of averages per time delay point would improve the signal-to-noise ratio of the system and sensitivities to differential probe signal changes in the 10-5 to 10-6 range should be achievable. References 1. U. Morgner, F.X. Kärtner, S.H. Cho, Y. Chen, H.A. Haus, J.G. Fujimoto, E.P. Ippen, V. Scheurer, G. Angelow, and T. Tschudi, Opt. Lett. 24, 411 (1999). 2. D.H. Sutter, G. Steinmeyer, L. Gallmann, N. Matuschek, F. Morier-Genoud, U. Keller, V. Scheuer, G. Angelow, T. Tschudi, Opt. Lett. 24, 631 (1999). 3. 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, Opt. Lett. 26, 373 (2001). 4. G. P. Wakeham and K. A. Nelson, Opt. Lett. 25, 505 (2000). 5. S. Bourquin, P. Seitz, and R. P. Salathé, Opt. Lett. 26, 512 (2001). 6. S. Bourquin, P. Seitz, and R. P. Salathé, Electron. Lett. 37, 975 (2001). 24-57 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 Photonics and Devices Micromachined Photonic Devices using Nonlinear Materials Processing Sponsors Air Force Office of Scientific Research (MFEL)- F49620-01-1-0186 Air Force Office of Scientific Research - F49620-98-01-0084 Project Staff Andrew M. Kowalevicz, Dr. Ingmar Hartl, Dr. Kaoru Minoshima, Professor Erich P. Ippen, and Professor James G. Fujimoto Nonlinear materials processing for photonic device fabrication using near-IR femtosecond pulses has emerged as an active area of research because it is possible to fabricate localized, clean, three-dimensional structures in a wide range of materials without the need for linear absorption. A variety of devices in glasses such as waveguides [1, 2] [3], couplers [4] [5, 6], gratings [7], 3D structures [8] [6], active waveguides [9], and void structures [10-12] have been successfully fabricated. While laser amplifiers have often been used for fabrication, laser oscillators have many advantages over amplifier systems, and moreover, the higher pulse repetition rate enables faster and more efficient waveguide fabrication. Here we demonstrate the fabrication of coupled mode devices. The behavior of device function is investigated by varying structural parameters such as interaction length and separation. Mach-Zehnder interferometers were also fabricated and spectral filtering was demonstrated using an unbalanced path length interferometer. These devices constitute the basic building blocks of photonic devices. Studies were performed using a novel, extended cavity modelocked titanium sapphire laser [13]. Since the total output power of a laser is limited, pulse energies can be increased by increasing the laser cavity length and reducing the pulse repetition rate. Using 4 MHz repetition rate titanium sapphire laser, pulses of up to 100 nJ can be generated with 80 fs pulse duration. Waveguides were fabricated inside glass by focusing the femtosecond laser beam and translating the glass perpendicular to the incident beam to write the waveguides. The refractive index differentials were measured using OCT to be 10-3 to 10-2 depending upon exposure parameters. The edges of the glass substrate were polished to optical quality in order to enable efficient coupling. (a) Input Coupling He-Ne laser (b) Separation Output Input Interaction Output 25 Fig. 1. (a) Phase contrast microscopic image of one of the directional couplers. (b) Schematic of the coupler. Separation, d and interaction length, L are varied with the fixed total length, 25 mm. Directional couplers were fabricated in which two single mode waveguides have an interaction region without intersecting. Figs. 1(a) and (b) show the phase contrast microscopic top view and 24-58 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 the schematic of a directional coupler. The coupler has an interaction region of length L where the two waveguides are parallel and closely spaced with a separation d. Each waveguide has two bends of 1o. When a He-Ne laser beam (633 nm) was coupled into one input port, power was transferred to the two output ports, suggesting coupling between the two waveguides. Coupling ratio, R Directional couplers operate using coupled mode effects. When two waveguides are brought into close proximity with a small separation between them, the mode that is guided in one of the waveguides can couple into the other waveguide by evanescent field interaction. The operation of these devices can be described by coupled mode theory [14]. This oscillation of power between the two waveguides is characteristic of coupled mode behavior. When the separation d between the waveguides becomes larger, the overlap between the eigenmodes is reduced and the coupling coefficient is smaller, so the oscillation of power between the two waveguides becomes slower. 0.6 0.4 0.2 (a) 0.0 0 2 4 6 8 10 0.3 Coupling ratio, R Coupling ratio, R Interaction length, L (mm) (b) 0.2 0.1 0.0 0 2 4 6 8 0.08 (c) 0.06 0.04 0.02 0.00 10 0 Interaction length, L (mm) 2 4 6 8 10 Interaction length, L (mm) Fig. 2. (a) Interaction length dependence of the coupling ratio for d = 8 um, (b) d = 10 um, and (c) d = 12 um. Experimental results (dots) and their best fit results to sinusoidal curves (lines) are shown. The period of the oscillation increases from (a) 5, to (b) 9, to (c) 14 mm, which is consistent with the coupled mode theory. Figure 2 shows the measured results from our directional couplers. When the separation, d, between the waveguides increases from (a) 8 um, to (b) 10 um to (c) 12 um, the oscillation period of the coupling ratio, R, increases from (a) 5 mm, to (b) 9 mm, to (c) 14 mm. This behavior is consistent with coupled mode theory which predicts that decreasing in coupling coefficient, yields an increase in the oscillation of the coupling ratio with interaction length. The oscillatory behavior of the coupling ratio as a function of interaction length, L, that is predicted by coupled mode theory is clearly evident. The lines in the figures show best-fit sinusoidal functions. To the best of our knowledge, this is the first demonstration of the oscillatory behavior of the coupling coefficient and confirms coupled mode operation in devices fabricated by femtosecond nonlinear material processing. 24-59 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 The fabrication and characterization of a Mach-Zehnder interferometer filter is demonstrated as another example of a more complex photonic device. The interferometer consists of two Xcouplers placed back-to-back, with crossing angles of 2o as shown in Fig. 3(a). The path length difference between the two arms is approximately 10 um. Light coupled into the interferometer is split into the two arms of the interferometer at the first X-coupler, travels different path lengths, and will either constructively or destructively interfere at the second X-coupler. The unbalanced path length Mach-Zehnder interferometer functions as a wavelength dependent filter. The frequency or wavelength dependence of the interferometer can be measured using a broadband light source[15]. Fig. 3(b) shows the input spectrum with a FWHM of 130 nm, together with the output spectrum. Fig. 3(c) shows the wavelength transfer function in the crossed interferometer arm. The transfer function is constructed by normalizing the output spectrum by the input spectrum. The red line shows the theoretically predicted wavelength dependence of the transfer function for a path length difference of 14.1 um. The experimental measurements are in close agreement with the theory. The difference between design arm length and actual arm length of ~4.1 um over 16.5 mm is quite small and is probably due to inaccurate translation (overshoot) of the motorized stages used for fabrication and can be corrected with more precise mechanical control. (a) Modelocked Ti:Al2O3 Laser 1.0 Intensity (a. u.) Intensity (a. u.) 1.0 0.8 0.6 0.4 0.2 0.0 700 (b) 2O Crossing 16.5 750 800 850 Wavelength (nm) 900 0.8 0.6 0.4 0.2 0.0 700 (c) 750 800 850 900 Wavelength (nm) Fig. 3. (a) Schematic representation of Mach-Zehnder Interferometer with phase-contrast microscope images of waveguides; (b) Input (red line) and output (black line) spectra from broadband Ti:Al2O3 and, (c) Normalized output spectrum (black line) demonstrating the filtering effect of the interferometer compared to theoretical model (red line). In conclusion, we have demonstrated the fabrication of coupled mode devices and interferometers using nonlinear femtosecond materials processing in glass. Oscillation of the power between the two waveguides has been observed as a function of the waveguide interaction length and coupling coefficient as well as a function of wavelength. These results are 24-60 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 consistent with coupled mode theory and demonstrate that the directional couplers operate by mode coupling. The fabrication of an unbalanced Mach-Zehnder interferometer is also demonstrated as an example of a more complex device. The operation of the unbalanced interferometer as a wavelength filter is demonstrated and is in agreement with theory. These results demonstrate the practical photonic device fabrication is possible using femtosecond nonlinear materials processing. References 1. Davis, K.M., et al., Writing waveguides in glass with a femtosecond laser. Optics Letters, 1996. 21: p. 1729-1731. 2. Miura, K., et al., Photowritten optical waveguides in various glasses with ultrashort pulse laser. Applied Physics Letters, 1997. 71: p. 3329-3331. 3. Streltsov, A.M. and N.F. Borrelli, Fabrication and analysis of a directional coupler written in glass by nanojoule femtosecond laser pulses. Optics Letters, 2001. 26: p. 42-43. 4. Homoelle, D., et al., Infrared photosensitivity in silica glasses exposed to femtosecond laser pulses. Optics Letters, 1999. 24: p. 1311-1313. 5. Schaffer, C.B., et al., Micromachining bulk glass by use of femtosecond laser pulses with nanojoule energy. Optics Letters, 2001. 26: p. 93. 6. Minoshima, K., et al., Photonic device fabrication in glass by use of nonlinear materials processing with a femtosecond laser oscillator. Optics Letters, 2001. 26: p. 1516-1518. 7. Kondo, Y., et al., Fabrication of long-period fiber gratings by focused irradiation of infrared femtosecond laser pulses. Optics Letters, 1999. 24: p. 646-648. 8. Glezer, E.N., et al., Three-dimensional optical storage inside transparent materials. Optics Letters, 1996. 21: p. 2023-2025. 9. Sikorski, Y., et al., Optical waveguide amplifier in Nd doped glass written with near-IR femtosecond laser pulses. Electronic Letters, 2000. 36(226). 10. Varel, H., et al., Micromachining of quartz with ultrashort laser pulses. Applied Physics A, 1997. 65: p. 367-373. 11. Glezer, E.N. and E. Mazur, Ultrafast-laser driven micro-explosions in transparent materials. Applied Physics Letters, 1997. 71: p. 882-884. 12. Watanabe, W., et al., Optical seizing and merging of voids in silica glass with infrared femtosecond laser pulses. Optics Letters, 2000. 25: p. 1669-1671. 13. Cho, S.H., et al., High energy pulse generation using a 4 MHz repetition rate KLM Ti:Al2O3 laser operating with positive and negative dispersion. Optics Letters, 2001. 26: p. 560-562. 14. Saleh, B.E.A. and M.C. Teich, Fundamentals of Photonics. 1991: John Wiley & Sons, Inc. 15. Morgner, U., et al., Sub-two cycle pulses from a Kerr-Lens modelocked Ti:sapphire laser. Optics Letters, 1999. 24: p. 411 -- 413. 24-61 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 Development of First, Second, and Third Order Ring Resonators for Channel Dropping Filter Applications Sponsor Pirelli Project Staff Peter T. Rakich, Tymon Barwicz, Milos Popovic, Christina Manolatou, Michael R. Watts, Giacomo Gorni, Professor Henry I. Smith, Professor Herman A. Haus, Professor Erich P. Ippen Ring resonators are particularly interesting for many applications because they can be designed to function as efficient, high-Q filters which are extremely small in size. Ring resonators have been used in microwave electronics for years. However, only recently have materials processing and micro-fabrication become precise enough to demonstrate such devices at optical wavelengths in dielectric waveguides [1-3]. As the demand for high data transmission in telecommunications increases, the need for a fully integrated means of filtering and switching telecommunications channels is ever increasing. Ring resonators appear to be a promising solution for the integration problem. The goal of our current research is to construct second and third order ring resonator filters which would be suitable for telecommunications applications. A SiN2/SiO2 material system has been chosen for the rings fabricated in this study. First, second and third order ring resonators have been fabricated through e-beam lithography. A small ring radius of 20Pm produces a free spectral range which extends over the C-band of telecommunications channels. This is important to ensure that only one resonance from a given filter can overlap with the C-band at a given time. A schematic of the device layout can be seen in Figure 1. The SiN2 waveguides are supported by a SiO2 undercladding, and a bus waveguide couples horizontally to the ring resonator through evanescent coupling. The resonant wavelengths preferentially couple to the drop port while all other wavelengths pass, unaffected, through the bus waveguide. In second and third order ring resonators, the first ring transfers its power to one or two more rings before it transfers power to the drop waveguide; however, the same concepts of the first order ring resonators still apply. Additionally second and third order ring resonators offer added flexibility of the transfer function. These devices were designed to more closely approximate top hat like transfer functions which can serve as telecommunications filters. The filter responses of the first, second and third order ring resonators can be seen in figures 1, 2, and 3 respectively. The spectrum in Figure 1 exhibits a Lorenzian-type transfer function which is characteristic of first-order ring resonators. In comparing the through and drop ports, a 3db transfer of power occurs on resonance. The second order ring resonator exhibits a symmetric double-peak structure. This can be understood as a symmetric breaking of degeneracy through the coupling of the two rings to one-another. It is important to notice that the roll-off of the second order and third order rings is significantly faster for off-resonance wavelengths. This is an important aspect of the higher order coupled ring structures in order to reduce cross-talk between telecommunications channels. The third order ring resonator exhibits a somewhat more complicated spectrum. In particular, the Through-port signal exhibits a sharp dip at longer wavelengths. This can be attributed to the fact that the middle ring is slightly detuned from the neighboring rings due to its environment. Despite this fact, the third order device exhibits a 120GHz spectral width and a 3db transfer of power from the through waveguide. In conclusion, we have demonstrated respectable performance of first, second and third order ring resonator devices in a SiN2 material system. Currently we work to calibrate our device simulations with experiments and fabrication to improve the device performance and obtain filters which could conceivably be useful as telecommunications channel dropping filters for large scale integration in telecommunications networks. 24-62 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 First Order Ring Resonator: Comparison of Drop and Through Ports For TE Polarization (Device E1) Power (a.u.) 0.1 0.01 0.001 0.0001 1564 1566 1568 1570 1572 1574 1576 Wavelength (nm) Figure 1. Schematic and transmission spectrum of first order ring resonator. The through-port and drop-port signals are displayed in red and blue respectively. Second Order Ring Resonantor (F6) Comparitive Measurement TE Launch and Analyzer 0.1 Power (a.u.) 0.01 0.001 0.0001 10-5 1553 1554 1555 1556 1557 1558 1559 Wavelength (nm) Figure 2. Schematic and transmission spectrum of second order ring resonator. The throughport and drop-port signals are displayed in red and blue respectively. Third Order Ring Resonator Comparison of Through and Drop Port Transmission for TE Polarization (Device G2) 0.1 Power (a.u.) 0.01 0.001 0.0001 10-5 1565 1566 1567 1568 1569 1570 1571 1572 1573 Wavelength (nm) Figure 3. Schematic and transmission spectrum of third order ring resonator. The through-port and drop-port signals are displayed in red and blue respectively. 24-63 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 References 1. B. E. Little, T. S. Chu, W. Pan, D. Ripin, T. Kaneko, Y kokubun, and E. Ippen “ Vertically coupled Glass Microring Resonator Channel Dropping Filters” IEEE Photonics Tech. Lett. 11(2): 215-217 (1999) 2. B. E. Little, J. S. Foresi, G. Steinmeyer, E. R. Thoen, S. T. Chu, H. A. Haus, E. P. Ippen, L. C. Kimmerling, W. Greene “Ultra-compact Si-SiO2 Microring Resonator Optical Channel Dropping Filters,” IEEE Photonics Tech. Lett. 10(4): 549-551 (1998) 3. J. V. Hryniewicz, P. P. Absil, B. E. Little, R. A. Wilson and P. T. Ho, “Higher Order Filter Response in Coupled Microring Resonators” IEEE Photonic Tech. Lett. 12(3): 320-322 (2000) 24-64 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 Tuning and Switching of Ring Resonator Through Perturbation of Effective Index Sponsors Pirelli National Science Foundation - ECS 0119452 MRSEC Program of the National Science Foundation - DMR 98-08941 US Air Force – Office of Scientific Research - F49620-01-1-0084 Project Staff Peter T. Rakich, Danielle Faccio, Professor Herman A. Haus, Professor Erich P. Ippen Since their conception, ring resonators have demonstrated their promise as narrow band channel dropping filters, and as laser cavities for integrated quantum well lasers. However, trimming and tuning of such filters, for use in precisely defined telecommunications channels, proves to be a challenging task. Several methods of trimming and tuning have been developed [1-2]; however, none of them can promise tuning and switching on the microsecond timescale over large wavelength ranges. We demonstrate a new means of tuning and switching which is based on the external perturbation of a ring resonator with a slab of high index material. The mode of an unclad ring resonator (index guided) has an evanescent field which extends outside of the guide in the direction normal to the plane of the ring. A slab of high index material, whose surface is parallel to the plane of the ring, can be placed in the mode such that the guided mode of the ring uniformly penetrates it (see Figure 1). This produces in increase in effective index of the mode, and results in tuning of the resonance frequency of the ring. Figure 1. Diagram depicting tuning concept. Glass perturbing body penetrates the evanescent field which extends outside of the guide, changing its effective index and resulting in a tuning of the resonance frequency of the ring. For the purposes of this demonstration, a vertically coupled unclad ring resonator composed of Ta2O5 used with a SiO2 lower cladding material. For device specifics see ref [3]. Tuning-fork based shear-force feedback [4] was implemented to monitor and control the distance between a cleaved piece of fiber-optic, which produces the perturbation, and the ring resonator. The apparatus can be seen in Figure 2 (a). A small piece of fiber optic is attached to the end of a quartz tuning fork. The fiber’s interaction with the surface is sensed through mechanical coupling to the tuning-fork feedback system. The strength of the fiber-surface interaction is used as a set point for the feedback system. Various transmission spectra were acquired from the through-port of the ring resonator as a function of the fiber-ring distance as seen in figure 2 (b). The shortest wavelength resonance corresponds to the unperturbed ring. The series of red-shifted spectra correspond to increasing 24-65 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 C R Figure 2. (a) Simple schematic of tuning apparatus. Tuning fork signal is monitored to maintain the distance of probe from surface of ring. (b) Transmission data from the through port of ring resonator for various positions of the probe. The left-most resonances are those with no perturbation, while the progression of red-shifted resonances demonstrates increasing perturbation of the ring resonator. perturbations of the ring, where greater shifts correspond to greater shifts correspond to greater perturbations. Notice that the resonance which corresponds to contact of the fiber to the ring is sharper than the other intermediate spectra. This is due to the fact that the feedback mechanism which maintains the fiber-ring distance is too slow to compensate for the high frequency (kHz) ambient vibrations on the optical table. This tends to artificially broaden the transmission spectra. However, when the fiber makes contact with the ring the fluctuations in distance are removed and a significantly sharper spectrum is observed. Additionally, when the fiber is raised again the unperturbed resonance is fully recovered. In conclusion, we have demonstrated a continuous and reversible tuning mechanism based on external perturbation of the ring resonator mode which can be implemented on the microsecond timescale. A total tuning range of 1.8nm has been achieved with a relatively low index probe. Future research will seek to improve tuning ranges to as much as 20nm using higher index probes with a multilayer design. References 1. Yasuo Kokubun, Hirofumi Haeiwa, Hiroaki Tanaka, “Precise Center Wavelength Trimming of Vertically Coupled Microring Resonator Filter by Direct UV Irradiation to Ring Core” Proceedings LEOS Annual, Edinburgh, 2002 2. Heimala,-P.; Katila,-P.; Aarnio,-J.; Heinamaki,-A. “Thermally tunable integrated optical ring resonator with poly-Si thermistor,”J.Lightwave Technol. 14(10): 2260-7 (1996) 3. B. E. Little, T. S. Chu, W. Pan, D. Ripin, T. Kaneko, Y Kokubun, and E. Ippen “ Vertically coupled Glass Microring Resonator Channel Dropping Filters” IEEE Photonics Tech. Lett. 11(2): 215-217 (1999) 4. Khaled Karai, Robert D. Grober “ Piezoelectric tip-sample distance control for near field optical microscopes” Appl. Phys. Lett. 66(14): 1842-44 (1995) 24-66 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 Guiding and Band Edge Measurements of 2-Dimensional Photonic Crystal Slab Formed by Posts Sponsors National Science Foundation ECS 0119452 MRSEC Program of the National Science Foundation - DMR 98-08941 US Air Force Office of Scientific Research - F49620-01-1-0084 Project Staff Peter T. Rakich, Solomon Assefa, Dr. P. Bienstman, Dr. Steven G. Johnson, Juliet Gopinath, Dr. Hideyuki Sotobayashi, Dr. Gale S. Petrich, Professor John D. Joannopoulos, Professor Leslie A. Kolodziejski, Professor Erich P. Ippen, Professor Henry I Smith Photonic crystals provide a unique way of developing materials with novel optical properties for the manipulation of light. For this reason, photonic crystals have generated a great deal of interest as a scientific pursuit and for numerous applications. During the past several years, many studies have examined the guidance of light in photonic crystals formed by a periodic array of holes in slab waveguides [1-2]; however, little work has been done on photonic crystals formed by a periodic array of posts. We demonstrate guidance and present evidence of a photonic band gap in a two-dimensional photonic crystal, at optical wavelengths, formed by posts [3]. The two types of photonic crystals vary in several respects. However, the most important feature to note is that a complete photonic band gap can only occur for TE light in structures made with holes, while this is true only for TM light in photonic crystals formed by posts. This is because the energy density inside of a post is highly confined when the electric field is aligned with the post, resulting in a much larger splitting of electromagnetic states. This allows the formation of a complete photonic band gap with TM polarized light. The device under study is an asymmetric two-dimensional waveguide formed from posts. The high index material which forms the guide is composed of GaAs which is grown through molecular beam epitaxy, while the low index undercladding material AlxOy is formed through an oxidation of epitaxially formed AlAs. The material surrounding the posts is purposefully over etched to make the structure as symmetric as is possible. Fabrication specifics can be found in ref [4]. A schematic of the device can be seen in Figure 1. The device dimensions were chosen to produce a complete photonic band gap from 1540nm -1740nm for TM polarized light, while transmitting for TE polarization at these same wavelengths. Figure 1. Diagram of photonic crystal post devices used for experiment. Green material is GaAs (n = 3.4) and light blue is AlxOy (n = 1.6). 24-67 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 Measurements were performed with the use of tunable lasers spanning wavelengths 1430 1610nm. A fiber lens assembly and piezo controlled stages were used for coupling at the input. Collection was performed with nondispersive imaging optics, video cameras and a Ge detector. The photocurrent signal was processed with a lock-in amplifier and acquired point by point as the tunable laser was scanned. During all measurements, the guided mode at the output end facet was simultaneously imaged and measured. In all cases, the polarization of the input-coupled light was calibrated with a free space polarizer, and the collected light was analyzed with a polarizer before detection. Figure 2 is a plot showing the transmittance of the guided light through the photonic crystal for both TE and TM guided light. Notice that the TM polarization is transmitting at shorter wavelengths and the transmission rolls off at 1540nm while the TE spectrum is transmitting over all wavelengths. These initial observations appear to be consistent with the formation of a band gap for TM light. Additionally, oscillations in the transmitted spectrum appear to be consistent with the Fabry-Perot cavity formed by the interfaces of the photonic crystal. This is further evidence of guidance in the photonic crystal. Nearly identical results were observed in three identically fabricated devices, demonstrating that these measurements and devices are reproducible. Figure 2. Transmission spectrum for both TM and TE polarization through photonic crystal device shown in Figure 1. In conclusion, preliminary measurements show strong evidence of guidance and the formation of a photonic band gap in a photonic crystal formed by an array of high index posts. A 20db suppression of the transmission is observed for TM polarization near the theoretical wavelengths of the band gap. However, for the purposes of completeness we work to obtain a measurement of both bandages. Currently under development is a measurement system which is based on a supercontinuum white light source extending the measurement range from 1200nm-1800nm. A characteristic supercontinuum spectrum can be seen in figure 3 which was produced by a novel nonlinear fiber, the properties of which are currently under study. This will provide a more complete measurement of the photonic band structure, and thus a more thorough analysis of these devices. Additionally, we have developed a near-field optical microscope which will enable the detection of the evanescent field of the waveguide mode which extends outside of the photonic crystal waveguide. As a near-field fiber probe is brought in close proximity to the waveguide a small amount of light can tunnel through the probe into a fiber optic. As the probe is scanned over the waveguide surface, phase and amplitude information can be obtained. This technique promises to allow many new experimental studies of Bloch waves, lensing and negative refraction. 24-68 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 Figure 3. Supercontimuum generated by novel nonlinear optical fiber. The spectrum was generated with 150fs pulses at 1550nm. References 1. S. Rowson, A. Chelnokov, J.-M. Lourtioz, “Two-Dimensional Photonic Crystals in Macroporous Silicon: From Mid-Infrared to Telecommunications Wavelengths,” J. Lightwave Tech. 17(11):1989-95 (1999) 2. T. F. Krauss, R. M. De La Rue, and S. Brand, “Two-Dimensional photonic-bandgap structures operating at near-infrared wavelengths,” Nature 383(6602): 699-702 (1996) 3. S. Lin, E. Chow, V. Hietala, P. Villeneuve, J. D. Joannopoulos, “Experimental Demonstration of Guiding and Bending of Electromagnetic Waves in a Photonic Crystal” Science, 282(5387): 274-6 (1998) Reports Solomon Assefa, “Coupling into Photonic Crystal Waveguides” RLE TR-566 (Cambridge: MIT Research Laboratory of Electronics, 1998) 24-69 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 Integrated Tunable/Switchable Optical Add-Drop Multiplexer Sponsors Pirelli Project Staff Dr. Matteo Cherchi, Dr. Daniele Faccio, Dr. Giacomo Gorni, Professor Hermann A. Haus, Dr. Christina Manolatou, Miloš Popoviü, Dr. Maurizio Tormen, Michael R. Watts This research covers the functional and electromagnetic design of tunable, switchable integrated optical add/drop filters. It is part of a project to design a fully integrated optical add/drop module (OADM) within the wavelength division multiplexing (WDM) framework. The OADM is intended to add/drop a selected number of channels on a C-band WDM bus within prescribed drop and thru channel response specifications such as insertion loss, cross-talk, dispersion, polarization dependent loss (PDL), etc. Finally, the design is to be integrated on a chip, using high index contrast design for miniaturization. Polarization-independent performance is extremely difficult to obtain in a high-index contrast device. The TE-like and TM-like modes in these structures exhibit marked differences in coupling, propagation in bends, and sensitivity to fabrication errors. In order to obtain polarization independence, we developed a polarization independent topology (Fig. 1) in which we split the polarization states at the input, rotate one in order to obtain identical on chip polarizations, and operate on the signals in parallel with identical structures. The signals dropped from the chip are recombined electronically, and those continuing in the optical domain are recombined at the output using the reverse process. Figure 1. Polarization-independent device topology for a polarization-dependent optical circuit using a symmetric configuration of integrated polarization splitters and rotators. The required elements of an integrated polarization splitter and polarization rotator have been concept proven and are entering the fabrication stage. Diagrams of the basic structures are presented in Fig. 2. A three layered core was adopted in order to rotate and split the polarizations using adiabatic following. Adiabatic following makes the structures to be both fabrication tolerant and wavelength insensitive. For rotator and splitter device lengths of 400Pm and 150Pm, respectively, a core index of 2.2 and a cladding index of 1.445, a = 0.25Pm and b = 0.75Pm, greater than 99% of the power is transmitted over the entire 1.5-1.6Pm band. 24-70 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 Figure 2. (a) Polarization rotator, (b) polarization splitter, and (c) integrated polarization splitter and rotator. Microring resonator channel add/drop filters are being designed to be tolerant to fabrication error. A third-order drop-port response in Fig. 3 shows 100 overlaid filters with a uniform statistical error distribution in the coupling coefficients between +6% and -6%, always meeting the prescribed specification. A dielectric slab MEMS is placed at a controlled distance above the filter rings to alter the propagation constant within the rings and tune the filter across the frequency spectrum. Numerical simulation tools have been developed in order to accurately analyze and predict the performance of the OADM device. These include fully numerical 3D FDTD (temporal) and leaky waveguide modesolver (spectral) methods, as well as semi-analytic methods based on coupledmode theory. The most reliable numerical tool for electromagnetic modeling of our dielectric structures is the Finite Difference Time Domain (FDTD) method. In 3D, the full-wave Maxwell's equations are propagated by discretization in space and time. No approximations other than the discretization lead to very accurate results for arbitrary structures over wide frequency spectra. However, large time and memory requirements of 3D FDTD make it impractical to simulate devices larger than a few microns in each direction. Hence, the structure under consideration is divided into critical segments which are simulated individually, and the results are assembled for a full simulated response. Figure 3. Fourth-order inline microring filter drop channel response function with a +/-6% uniform error distribution applied to the ring-ring coupling coefficients. The filter meets specifications. 24-71 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 In the case of complex resonant structures such as the add/drop filters with vertically coupled ring resonators and with MEMS tuning slabs, full 3D simulations are required. While final tests of an optimized structure are best verified by 3D FDTD, the optimization process is done in parts because of the large computational cost of FDTD. Instead, the parameters of analytic models of the device under considerations are extracted from smaller FDTD simulations during the design and optimization process. Fig. 4 shows a logarithmically-scaled electric field map of a ring-ring coupler simulated in FDTD. Figure 4. Ring-ring coupling simulation detail (3D FDTD). Logarithmicallyscaled intensity map is shown. In the case of large non-resonant, nearly reflectionless structures such as the polarization splitter/rotator, 3D simulations are possible with alternatives such as a "sliding window" FDTD method. Electromagnetic design of the ring resonators themselves (radiation Q contrast of leaky modes), of the MEMS tuning mechanism, and of some couplers is most efficiently done using spectral methods. To this end a full-vector field modesolver was developed to find the guided and leaky modes of arbitrarily shaped waveguides with a straight or cylindrical propagation axis. Fig. 5 shows the bend mode of a high loss ring for illustration. The combination of semi-analytic methods to guide design, and numerical FDTD and modesolvers has allowed our OADM designs to develop quickly, and the first fabricated filters are forthcoming. 24-72 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 Figure 5. Leaky mode of primarily horizontal polarization in a 5.8Pm circular ring resonator showing radiation loss (complex cross-section modesolver computation). The contours of constant |EU| are non-linearly spaced to better show the small radiation loss. 24-73 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 Grating Filters with Reduced Radiation Sponsors National Science Foundation MRSEC Program - DMR 98-08941 Project Staff Aristidis Karalis, Professor Hermann A. Haus The goal of this research project is the design of a radiation-free integrated broad-band FabryPerot filter using deeply etched high index contrast gratings separated by a defect. The radiation from the defect is to be investigated and means for its reduction or elimination are sought. A defect between two semi-infinite gratings causes radiation, in general, due to the mismatch of the mode patterns on either side of the defect. An exception was found for some ideal structures, when perfect mode-matching between segments is enforced[1]. Ideal structures cannot be fabricated. We are searching for structures that approach the ideal and can be fabricated. An infinite periodic guiding structure does not radiate. In the cut-off regime, the electric and magnetic field are in quadrature throughout the entire cross-section, if the structure supports one single bounded mode. The aim is to find a defect configuration that matches the mode patterns of the electric and magnetic fields of the two grating structures to either side of the defect. Reference 1. M. R. Watts, S. G. Johnson, H. A. Haus, and J. Joannopoulos,” Electromagnetic cavity with arbitrary Q and small modal volume without a complete photonic band-gap,” Opt. Lett., 27, pp. 1785-1787, (2002) 24-74 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 Polarization Mode Dispersion Sponsor 3M Company, DSO National Laboratories Project Staff Professor Hermann A. Haus, Professor Erich P. Ippen, Poh Boon Phua Polarization mode dispersion (PMD) limits long distance ultrahigh-speed optical telecommunication systems. Due to the statistical nature of PMD, PMD compensators and emulators have to be adjustable. A variable Differential Group Delay (DGD) module is an important component in these applications. The common approach to generate variable DGD is to separate the two orthogonal polarization components using a polarization beam splitter and to introduce a path difference between them. The two polarization components are then recombined using a polarization beam combiner. This approach requires mechanical movements, and tends to suffer from slow speed (sub-second), large output polarization fluctuation and poor control stability. Alternatively, one can generate variable DGD by concatenating two fixed DGD segments via a polarization controller. However, this results in a second order PMD vector perpendicular to the resultant 1st order PMD vector, which causes rotation of the principal state of polarization as one moves away from the center wavelength. Recently, we proposed a symmetrical way of concatenating 4 identical fixed DGD segments [1] so that the resultant DGD is variable while no second order PMD is produced. In addition, the third order PMD produced is only half the value of the one produced in the concatenation of two fixed segments with the same DGD tuning range. The schematic of the module is shown in Figure 1. Two identical blocks are concatenated via a tunable phase-plate, C1 , whose rotation axis is the x-direction in Stokes space (equivalent to a horizontal linearly polarized birefringence axis). Each block itself consists of two identical fixed DGD segments of negligible second order PMD. They are concatenated via C 0 , which is a tunable phase-plate whose rotation axis is the ydirection (equivalent to a 45 degree linearly polarized birefringence axis). In Stokes space representation, each fixed DGD segment has first order PMD, U W matrix, R , whose rotation axis is the x-direction and rotation angle is U {W ,0,0} , and a rotation TR . Since the module is constructed with tunable phase-plates, which can either be electro-optic or magneto-optic, high speed tuning of DGD is possible. Moreover, instead of using polarizationmaintaining fibers as fixed DGD segments, one could use birefringent crystals, which provide compactness and stability. Alternatively, one could integrate the whole variable DGD module on a wafer based on MEMS technology, since the fixed DGD segments are simply fixed delay lines in free space while the tunable phase-plates are finely-adjustable delay lines in free space. Based on similar concept of coherent 4-segment, we have also proposed another module [2] (see Figure 2) that can be deterministically controlled to produce variable magnitude of second order PMD without generating any first order PMD. When it is used with an additional polarization controller, it can generate an arbitrary second order PMD vector within its designed operation range. By applying both modules in Figure 1 and 2 together, we can achieve deterministically controlled PMD compensation and emulation up to second order. 24-75 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 U WR U (W ZR C1 Block Block UU U U W C0 ( RB Tunable PhasePlate W ' W Z' W RC0 R) UU W ' W Z' U C1 W R R 0) U C0 W R Tunable PhasePlate R Tunable PhasePlate Figure 1 C Block 1 U W U W U W R C0 R C0 R Block 2 R R Tunable PhasePlate U W C0 R Tunable PhasePlate Fixed PhasePlate 1 Fixed PhasePlate 2 Tunable PhasePlate Figure 2 References [1] P.B. Phua and Hermann. A. Haus, “Variable Differential Group Delay Module Without Second Order PMD”, J. Lightwave Technol., 20, pp.1788-1794 (2002) [2} P.B. Phua and Hermann. A. Haus, “Variable Second Order PMD Module Without First Order PMD”, J. Lightwave Technol., 20, pp.1951-1956 (2002) 24-76 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 Optical Phase Control and Stabilization Techniques Attosecond Synchronization of Modelocked Lasers Sponsors MIT Lincoln Laboratory - ACC 334 National Science Foundation - ECS-0119452 Office of Naval Research - N-00014-02-1-0717 Project Staff Dr. Thomas R. Schibli, Jung-Won Kim, Alex Killi, Onur Kuzucu, Juliet T. Gopinath, Sheila N.Tandon, Dr. Gale S. Petrich, Professor Leslie A. Kolodziejski, Professor James G. Fujimoto, Professor Erich P. Ippen, Professor Franz X. Kaertner The synchronization of pulse trains from independent modelocked lasers with sub-cycle timing fluctuations is the most important step for the coherent synthesis of optical single-cycle pulses. Ideally, the relative timing jitter should be less than a tenth of an optical cycle for a high quality synthesized pulse stream. At a wavelength of 1 Pm, this requires a timing jitter of 330 attoseconds or less, measured over the full Nyquist bandwidth, i.e. half the laser repetition rate. Several groups have investigated the possibility of active [1] and/or passive [2,3] synchronization of multiple lasers. However, a sub-femtosecond timing jitter over the Nyquist bandwidth has never been achieved. We demonstrated a new method of synchronization for modelocked lasers, in which the timing jitter between the two lasers is detected by a balanced cross-correlator, see Fig. 1, the optical equivalent of a balanced microwave phase-detector. The signal is then fed back via an electronic control loop in order to keep the two lasers synchronized. This method enables a drift-free and temperature-independent synchronization between two individual lasers, a task that is difficult to achieve with all-electronic schemes. To ensure the long-term stability of the system, the two laser beams are combined inside the control loop. Figure 1: Experimental setup of the synchronized lasers. Cr:fo: passively mode-locked Cr:forsterite laser, Ti:sa: passively mode-locked Ti:sapphire laser; SFG: sum-frequency generation; all bandpass filters transmit only the sumfrequency (1/496nm = 1/833nm+1/1225nm). The two beam splitters consist of a thin fused silica glass substrate coated with a semi-transparent metal film. The third correlator is used to generate the graphs shown in Fig. 3a. 24-77 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 The coherent synthesis of a single cycle pulse demands an optical spectrum as wide as 1.5 octaves. This requires both lasers to generate an ultra-wide and very stable spectrum. For long term stable operation, we use the octave spanning [4], prism-less Ti:sapphire laser [5,6] described above. The repetition rate of the laser is 82 MHz, and it emits about 100 mW of modelocked power through a 1% output coupler when pumped with 3.8 W of 532 nm light. To obtain stable modelocking in the Cr:forsterite laser, we used a novel broadband InGaAs saturable absorber on a large area, high-index contrast AlGaAs/AlxOy mirror [7], which enables the generation of sub-30 fs pulses at 1230 nm wavelength in a self-starting configuration. This laser emits about 60 mW of modelocked power through a 3% output coupler when pumped with approximately 2 W of 1064 nm light. Fig. 2 shows the spectrum of the two lasers (solid line) at the output. The dashed lines indicate the extent of the individual laser spectra in the vicinity of the overlap region. The shaded region indicates the spectral region filtered out to record the difference in carrier envelope offset frequency between the two lasers (see. Fig. 4). For phase coherent superposition of the two lasers, the pulse envelopes of the two lasers must be synchronized; and, in addition, the Figure 2. Optical spectra of the mode-locked difference in the carrier envelope offset Ti:sapphire and Cr:forsterite laser. The dashed frequency between the two lasers must be lines indicate the spectra of the individual stabilized. Synchronizing the pulse trains lasers in the vicinity of the spectral overlap, with sub-cycle precision is the most and the shaded region indicates the spectral challenging part of this synthesis process. region used to detect the difference in carrier To overcome the typical problems posed by envelope offset frequency between the two balanced microwave mixers previously used lasers shown in Figure 4. for this task [1], we employ the optical equivalent of such a device: a balanced cross-correlator. As shown in Fig. 1, the outputs of the two lasers are combined on a broadband metallic beam splitter grown by P. O’Brien at MIT Lincoln Laboratory. One part of the combined beam is directed to two nearly identical cross-correlators using 1 mm-thick LBO-crystals phase matched for SFG of 833nm and 1225nm light. The only difference between the two correlators is a 3 mm-thick fused silica window in the optical path of one of them. This glass inserts a group delay between 833 nm and 1225 nm to offset the pulses emitted by the Cr:forsterite and Ti:sapphire lasers by about 45 fs with respect to each other. The balanced detector output is then proportional to the time difference between both laser pulses, and in the vicinity of zero timing offset this detector acts like a balanced phase detector operating in the range of tens of THz. The output of this balanced cross-correlator as a function of time difference between the Cr:forsterite and the Ti:sapphire pulses is shown in Fig. 3b). We used the signal from this balanced correlator to lock the repetition rates of the two lasers by controlling the cavity length of the Ti:sapphire laser with cavity mirrors mounted on piezo-electric transducers in a manner similar to that of ref. [1]. The first beam splitter used to combine the two output beams from the lasers is inside this control loop. Since the output beam shown in Fig. 1 originates from this beam splitter, temperature drifts, acoustic noise or beam-fluctuations always affect both laser beams in the same way as they both travel along identical paths. Therefore, external noise cannot corrupt the relative jitter, and the output behaves as if it originated from the same source. Fig. 3a) shows the resulting timing jitter measurement made with the out-of-loop cross-correlator shown in Fig. 2. The residual timing jitter over the detector’s bandwidth of 2.3 MHz is 299 as ±104 as. The stated error is determined from the amplitude noise measured at the peak of the cross-correlation. As in most passively modelocked laser systems, the main contribution to the timing jitter has frequency components up to a few times the relaxation oscillation frequency of the laser [8]. In the current system, the relaxation oscillation frequencies 24-78 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 are roughly 70 kHz for the Ti:sapphire laser and 140 kHz for the Cr:forsterite laser. Therefore, noise above 2.3 MHz can usually be neglected. Figure 3. a) Timing jitter determined from the amplitude noise of the SFG of the out of loop cross-correlator (see Fig.1). The rms-jitter measured in a 2.3 MHz BW results in 299 as ±104 as. b) The output of the balanced crosscorrelator as a function of time difference between the two laser pulses. Figure 4. Heterodyne beat between the Cr:forsterite and the Ti:sapphire lasers obtained from the spectral region shown in Fig. 2. The two beat signals below the repetition rate of 82 MHz represent the difference in the carrier envelope offset frequency of both lasers. The RF-analyzer filter-bandwidth is 30kHz. The noise-floor is caused by the trans-impedance amplifier and poses only a technical limitation. As soon as the repetition rates of the two lasers are locked together, we observe a strong beat signal in the overlap region of the optical spectrum (see Fig. 4). To avoid saturation of the detector only a small part of the optical spectrum was directed to the diode (shaded region in Fig. 2). As described in ref. [9], the beat signal represents the difference in the carrier-envelope offset frequency 'fCEO between the two lasers. In contrast to previous results, it is now possible to obtain this beat without the use of spectral broadening of the mode combs, which helps to provide an exceptionally large signal-to-noise ratio of about 50 dB in a 30 kHz bandwidth. This signal can also be used to lock the optical frequencies in the overlap region together, and consequently, we can generate a fully coherent mode comb consisting of the sum of the two laser spectra that currently span more than an octave in bandwidth. References 1. 2. 3. 4. 5. 6. R. K. Shelton, S. M. Foreman, L. Ma, J. L. Hall, H. C. Kapteyn, M. M. Murnane, M. Notcutt, and J. Ye, “Subfemtosecond timing jitter between two independent, actively synchronized, mode-locked lasers,” Opt. Lett. 27(5): 312-4 (2002). A. Leitensdorfer, C. Fuerst, 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). Z. Wei, Z. Kobayashi, Z. Zhang, and K. Torizuka, "Generation of two-color femtosecond pulses by self-synchronizing Ti:sappire and Cr:forsterite lasers," Opt. Lett. 26(22): 1806-8 (2001). R. Ell, U. Morgner, F. X. Kaertner, J. G. Fujimoto, E. P. Ippen, V. Scheuer, G. Angelow, T. Tschudi, M. J. Lederer, A.Boiko, B. Luther-Davies, "Generation of 5-fs pulses and octavespanning spectra directly from a Ti:sapphire laser," Opt. Lett. 26(6): 373-5 (2001). T. R. Schibli, L. M. Matos, F. J. Grawert, and F. X. Kaertner, "Continuum generation in a prism-less Ti:sapphire laser," Proceedings of the Ultrafast Phenomena, Vancouver, Canada, May 12-17, 2002. F. X. Kaertner, 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," JOSA B 18(6): 882-5 (2001). 24-79 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 7. 8. 9. D. J. Ripin, J. G. Gopinath, H. M. Shen, A. A. Erchak, G. S. Petrich, L. A. Kolodziejski, F. X. Kaertner, and E. P. Ippen, "Oxidized GaAs/AlAs mirror with a quantum-well saturable absorber for ultrashort-pulse Cr4+:YAG laser," Opt. Comm. 214: 285-9 (2002). J. Son, J. V. Rudd, and J. F. Whitaker, “Noise characterization of a self-mode-locked Ti:sapphire laser,” Opt. Lett. 17(10): 733-5 (1992). Z. Wei, Y. Kobayashi, and K. Torizuka, “Relative carrier-envelope phase dynamics between passively synchronized Ti:sapphire and Cr:forsterite lasers,” Opt. Lett. 27(23): 2121-3 (2002). 24-80 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 Few-Cycle Nonlinear Optics and Carrier-Envelope Phase Effects Sponsor National Science Foundation – ECS 0217358 Project Staff Christian Jirauschek, Juhi Chandalia, Dr. Uwe Morgner, Oliver D. Mücke, Thorsten Tritschler, Professor Martin Wegener, Professor Franz X. Kaertner The carrier-envelope offset (CEO) phase, ĭ, is the relative phase between the rapidly oscillating carrier wave of a laser pulse and its electric field envelope. The time derivative of ĭ is the CEO frequency, fCEO. As mentioned above, control over fCEO is of great importance for frequency metrology. Several approaches have been reported to measure the CEO frequency for pulses directly emitted from mode-locked oscillators [1-5]. The underlying idea is to consider the interference resulting from the superposition of the fundamental pulse spectrum, which by definition has a phase 1ĭ, and its second (third) harmonic spectrum, which has a phase 2ĭ (3ĭ). The resulting interference on a photo detector shows the difference phase 1ĭ (2ĭ), which can be used to determine the CEO frequency fCEO (2fCEO). For extremely short pulses whose spectrum covers one octave, such interference can be immediately realized by generating the second harmonic in a suitable crystal [4]. However, if the laser does not yet reach an octave, the fundamental needs to be spectrally broadened to generate sufficient spectral overlap. This has been realized by using self-phase modulation (SPM) in optical fibers [1, 2]. However, such an approach has the inherent drawback that the CEO phase changes within the setup due to the fiber dispersion. Ideally, one would like to generate the spectral components, for example due to SPM and SHG, in a medium (see Figure 1), which is so thin that ĭ does not change within the medium. Ultimately, this would allow us not only to measure the CEO frequency but also to determine the CEO phase directly. eiI Fundamental F(2) e2iI SPM eiI 0 Z0 2Z 0 3Z0 Figure 1: Schematic spectra of a pulse covering slightly less than one octave on an optical frequency scale (top), together with the spectra generated by instantaneous F(2)processes (middle) and self-phase modulation (bottom). The red shaded area is where the interference occurs. In collaboration with researchers from the University of Karlsruhe, Germany, a simple technique of determining the CEO frequency [6] has been developed. In the experiment, 5-fs optical pulses from one of our broadband Ti:sapphire lasers with an average power of several tens of milliwatts were tightly focused onto a ZnO sample, which has no inversion symmetry. The light emitted into the forward direction was filtered to remove the prominent fundamental beam and spectrally analyzed. Two sample thicknesses, 100 µm and 350 nm, were tested in the experiment. The optical and electrical spectra of the output are shown in Figure 2 and Figure 3, respectively. These results reveal the following two aspects: 1) Using about 100 µm thick ZnO single crystals, 24-81 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 we find a large peak (> 30 dB above noise level at 10 kHz bandwidth and 64 mW average power) at the CEO frequency due to the overlap of the broadened spectrum via SPM and the generated SHG. The simplicity and robustness of the approach make it attractive for applications in frequency metrology or for stabilization of the CEO frequency. 2) A peak at the CEO frequency is still observed using a 350 nm thin ZnO film, within which the CEO phase has negligible change. This might pave the way for measuring the CEO phase itself and ultimately completing the characterization of laser pulses. A further interesting result is the observation of a beat signal at twice the carrier-envelope frequency. The origin of this signal could be related to the recently discovered phenomenon of Carrier-Wave-Rabi-Flopping [5]. However, several competing processes might be at its origin and further investigations are necessary. Figure 3: RF spectra, 10 kHz resolution and video bandwidths. (a) 100 µm ZnO single crystal corresponding to Fig. 2, 455-480 nm optical filter, (b) 350 nm ZnO epitaxial layer, 455-500 nm optical filter. The peaks at the repetition frequency fr, the CEO frequency fĭ, its second harmonic 2 fĭ and the mixing Figure 2: The output spectra from the 100-µm ZnO vs. time delay of the Michelson interferometer. (a) I = 0.15 I0, (b) I = 2.04 I0, and (c) I = 2.04 I0 with different saturation of the grey scale. The curve labeled IAC is the independently measured interferometric References 1. 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). 2. 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 mode-locked laser and direct optical frequency synthesis,” Science 288(5466): 635 (2000). 3. H. R. Telle, Steinmeyer, A. E. Dunlop, J. Stenger, D. H. Sutter, and U. Keller, “Carrierenvelope offset phase control: a novel concept for absolute optical frequency measurement and ultrashort pulse generation,” Appl. Phys. B 69(4): 327-332 (1999). 24-82 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 4. U. Morgner, R. Ell, G. Metzler, T. R. Schibli, F. X. Kaertner, 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). 5. O.D. Mücke, T. Tritschler, M. Wegener, U. Morgner and F.X. Kaertner, "Role of the absolute optical phase of few-cycle pulses in non-perturbative resonant nonlinear optics," Phys. Rev. Lett. 89(16), 127401-04, (2002). 6. O. D. Mücke, Th. Tritschler, M. Wegener, U. Morgner, and F. X. Kaertner, “Determining the Carrier-Envelope Offset Frequency of 5fs Pulses Using Extreme Nonlinear Optics in ZnO,” Opt. Lett. 27(23): 2127-2129 (2002). 24-83 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 Active Modelocking Using a Nonlinear Fabry-Perot Modulator Sponsor National Science Foundation ECS-0217358 Project Staff Richard Ell, Wolfgang Seitz, Dr. Uwe Morgner, Dr. Thomas Schibli, Professor Franz X. Kaertner The dynamics of laser oscillators can be directly controlled by modulating the intracavity losses. Over the last two years we developed a new approach of optically driven loss modulation by means of a nonlinear semiconductor mirror based on a Fabry-Perot structure (Fabry-Perot modulator, FPM). The structure of the device is shown in the inset of Figure 1. 100 reflectivity (%) 80 60 (1) 40 (2) 20 (3) (4) 0 0.98 1.00 1.02 1.04 1.06 1.08 1.10 1.12 1.14 wavelength (µm) Figure 1. Measured reflectivity of the type-I-FPM (solid curve) and calculated reflectivity (dashed curve). The structure of the FPM is shown below the measured curve: (1) Fresnel reflection at the air-InGaAs interface, (2) 5 µm InGaAs layer, (3) 15 pair GaAs/AlAs quarterwave Bragg stack centered at 1.064 µm, (4) GaAs(100)-substrate. The modulation depth can be several percent and the response time is dominated by the recombination time of the generated free carriers inside the semiconductor, which can be reduced by ion-implantation. The optical characteristics have been studied via spectrally resolved two-color pump-probe spectroscopy. Figure 2 shows the differential reflectivity change of the Fabry-Perot. Applications of the FPM are the synchronization of the pulse trains of independently mode-locked laser oscillators, i.e. a ps-Nd:YVO4 laser locked to a fs-Ti:sapphire laser in a master-slave configuration[1]. However, the device can have a large variety of applications. We demonstrated active mode locking of a solid state laser by an optically driven FPM [2] as shown in Figure 3. The resulting pulse widths of the actively mode-locked Nd:YVO4 laser are as short as 6 ps, which is comparable to passively generated pulses. So far, these experiments have been performed using a Ti:sapphire laser as the optical drive for the modulation. However, in many cases this expensive source can be replaced by a cheap, actively modulated diode laser. Then the FPM together with the drive might become an interesting alternative to other mode locking schemes. 24-84 24 - Photonic Materials, Devices and Systems – Optics and Quantum Electronics – 24 RLE Progress Report 145 delta R / R (%) 2% timedelay: 4.25 ps 3.5 ps 2.75 ps 2 ps 1.25 ps 0.5 ps -0.25 ps -1 ps -1.75 ps -2.5 ps -3.25 ps -4 ps 1016.0 1018.0 1020.0 1022.0 wavelength (nm) Figure 2. Temporally and spectrally resolved measurement of reflectivity change of the Fabry-Perot modulator during the carrier generation process. The time delay between adjacent curves is 750 fs. The dots indicate the shift of the resonance. (For clarity the curves are vertically displaced.) 1 (b) WFWHM (ps) 0.1 0.01 -25 0 25 pulse width intensity (arb.) (a) 12.5 (c ) 10.0 7.5 5.0 5 time delay (ps) 10 Wr1/2 15 20 1/2 ) ( ps Figure 3. (a) Measured intensity autocorrelation traces of an actively mode-locked Nd:YVO4 laser on a logarithmic scale for FPMs with different carrier life times. (b) Measurement of the pulse width dependence on the modulating power on the FPM1 on a double logarithmic scale. The dashed line is a linear fit of the measured data points (filled circles) representing the dependence WFWHM v Pm-1/4 in agreement with active modelocking theory. (c) Shortest pulse widths WFWHM achieved with the different samples plotted as a function of the square root of the carrier lifetime Wr of the samples. The dashed line is a linear fit of the measured data points (filled circles). References 1. W. Seitz, T. R. Schibli, U. Morgner, F. X. Kaertner, C. Lange, and W. Richter, “Passive synchronization of two independent laser oscillators with a Fabry-Perot modulator,” Opt. Lett. 27(6), 454-456, 2002. 2. W. Seitz, R. Ell, U. Morgner, T. R. Schibli and F. X. Kaertner, M. J. Lederer and B. Braun, “Alloptical Active Mode-locking with a Nonlinear Semiconductor Modulator,” Opt. Lett. 27(24), 220911 (2002). 24-85