IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 18, NO. 13, JULY 1, 2006
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Integrable Multimode Interference Distributed Bragg Reflector Laser All-Optical Flip-Flops Maura Raburn, Member, IEEE, Mitsuru Takenaka, Member, IEEE, Koji Takeda, Xueliang Song, Member, IEEE, Jonathon S. Barton, Member, IEEE, and Yoshiaki Nakano, Member, IEEE
Abstract—A novel fully integrable all-optical flip-flop has been created using distributed Bragg reflector multimode interference bistable laser diodes. The single metal–organic vapor phase epitaxy regrowth offset quantum well structure active/passive integrated flip-flop lases in single mode at 1554 nm and is compatible with standard fabrication methods of InP photonic integrated circuits (PICs). All-optical reset switching through cross-gain saturation was demonstrated over a 52-nm range, at 1522–1574 nm. An optical set has been achieved with 3 dBm and optical reset with less than 5-dBm external light injection. The flip-flop will be useful for integration in PICs of future photonic systems for self-routing and optical memories or buffers. Index Terms—Bistable laser diode (BLD), distributed Bragg reflector (DBR) lasers, flip-flop, gain saturation, multimode interference (MMI) coupler, optical memory.
I. INTRODUCTION
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LL-OPTICAL flip-flops will be key enabling devices for novel routing technologies such as burst and packet switching for storage and retrieval from optical memory. Optical turn-off difficulties with traditional bistable laser diode (BLD) flip-flops [1], [2] have been overcome through the use of multimode interference (MMI)-BLDs [3], [4]. MMI-BLDs feature emission at 1550 nm, have been predicted to operate at several tens of gigahertz [4], and have been successfully used for bit-length conversion [5]. They feature standard in-plane InP laser diode fabrication, and are thus advantageous for on-chip integration with other waveguide devices in comparison to vertical cavity bistable lasers [6]. However, the cleaved-facet mirrors of the original multiple-lasing-wavelength Fabry–Pérot design present a challenge for photonic integration. Passive distributed Bragg reflectors (DBRs) and input/output guides
Manuscript received February 27, 2006; revised April 21, 2006. This work was supported by the Japan Society for the Promotion of Science. M. Raburn is with Infinera, Sunnyvale, CA 94089 USA (e-mail: mraburn@ infinera.com). M. Takenaka is with the Information Devices Laboratory, Research Center for Advanced Science and Technology, University of Tokyo, Tokyo 153-8904, Japan, and also with the Optoelectronic Industry and Technology Development Association (OITDA), Tokyo 112-0014, Japan (e-mail:
[email protected]). K. Takeda and X. Song are with the Information Devices Laboratory, Research Center for Advanced Science and Technology, University of Tokyo, Tokyo 153-8904, Japan (e-mail:
[email protected]). J. S. Barton is with the Electrical and Computer Engineering Department, University of California, Santa Barbara, CA 93106 USA (e-mail: jsbarton@ engineering.ucsb.edu). Y. Nakano is with the Information Devices Laboratory, Research Center for Advanced Science and Technology, University of Tokyo, Tokyo 153-8904, Japan, and also with Japan Science and Technology-SORST (e-mail:
[email protected]). Digital Object Identifier 10.1109/LPT.2006.877569
Fig. 1. (a) Schematic of DBR-MMI-BLD. L = 660 m, L = 50 m, L = 480 m, L = 200 m, L = 110 m, W = 12 m, W = 2 m, W = 2:6 m, W = 5 m. 20-m passive region between MMI and SA, and bends and tapers for fiber coupling not shown. (b) Simplified cartoon illustration of position of high photon density for SET and RESEST states for ideal device. DBRs may be tuned for different lasing wavelengths.
have been integrated with the active MMI-BLDs through metal–organic vapor phase epitaxy (MOVPE) regrowth to allow lithographic on-chip integration with other devices [Fig. 1(a)]. The DBRs not only define the laser cavity but also provide single-mode operation, achieve a smoother wavelength dependence of the switching power through the lack of Fabry-Pérot cavity resonance, and allow tuning of the lasing wavelength through an applied bias. All-optical flip-flop operation of the device is achieved through cross-gain saturation of the two bistable MMI cross-coupler lasing modes [Fig. 1(b)] [4]. Principles of operation are similar to those of the Fabry-Pérot device, discussed in [3] and [4]. Lasing bistability, lasing spectra, and all-optical SET and RESET through external injection of light around 1550 nm are investigated. II. DEVICE FABRICATION A 300-nm nm InGaAsP waveguide core and nm InGaAsP 6 0.8% compressively strained
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 18, NO. 13, JULY 1, 2006
Fig. 2. Microscope view of device top surface. Photograph width has been expanded for clarity and is not to scale with length. Total integrable flip-flop length is 1.6 mm, but curved tapered waveguides and extra length for cleaving provide a facet-to-facet length of 2.8 mm.
quantum wells were MOVPE grown separated by a 10-nm InP etch stop layer to form an offset quantum well structure [7]. A 100-nm InP cap was grown to prevent damage to the quantum wells during the grating formation steps. After patterning to preserve the quantum wells in the MMI and saturable absorber (SA) regions, the quantum wells in the passive regions were removed through wet etching. Gratings were e-beam written and wet etched using saturated bromine water. Shallow gratings were necessary for single-moded lasing. Then, the InP cap above the quantum wells and the InP etch stop in the passive regions were removed and samples were cleaned for regrowth. Since the top-most layer in both the active and passive regions nm InGaAsP, both terprior to regrowth was tiary-butyl arsine (TBA) and tertiary-butyl phosphine (TBP) were used during the regrowth reactor heating process. After regrowth of the unintentionally doped (uid)- and p-InP waveguide upper cladding layer, p-InP ridge layer, and p InGaAs contact layer, waveguides were patterned and wet etched. Next, the InGaAs was removed from the passive regions through further patterning and wet etching, and deposition and contact hole opening of an SiO insulation layer took place. Ti/Au contacts were deposited, wet etched, and finally annealed at 385 C. The MMI waveguide design was based on that of [4], but many additions were required for the active/passive integration as discussed in [8]. Notable ones are the 7 -curved and 2-to-5- m tapered passive waveguides concatenated to the flip-flops to reduce unwanted reflections and improve fiber coupling [Fig. 1(a)]. As can be seen in Fig. 2, the total flip-flop length for on-chip integration is only 1.6 mm, but the facet-to-facet length provided by the curved waveguide regions and additional length for cleaving is 2.8 mm. Antireflective coatings were not used. III. EXPERIMENTAL RESULTS First, light output as a function of current was measured, as shown in the upper plot of Fig. 3, to view hysteresis behavior and find an appropriate current bias for all-optical flip-flop operation. Note that, for clarity, Figs. 3 and 4 show only one of the cross-coupled lasing modes, which we define to be switched on via “SET” and off via “RESET” light. A 4-mA-wide hysteresis centered at 162.5 mA and an extinction ratio of 22 dB are achieved, using a 1-nm bandpass filter (BPF). A 162 mA is used as the bias current for all remaining measurements. The optical spectrum for the lasing and nonlasing states is plotted in
Fig. 3. Upper plot is light output from a single port of the flip-flop versus current injected to the MMI gain region, exhibiting hysteresis behavior. 1-nm BPF centered at 1554 nm was used. Extinction ratio of SET (lasing) versus RESET (not lasing) states was 22 dB. Lower plot is spectra of laser output corresponding to SET and RESET states at 162 mA current. Mode suppression ratio (MSR) in SET state was 26 dB. Optical powers are fiber-coupled values.
the bottom of Fig. 3. Lasing occurs for both modes at the Bragg wavelength of 1554 nm, with a mode suppression ratio of 26 dB. Optical SET and RESET behavior were measured by injecting 1550-nm light from an external tunable laser into the device; see Fig 4(a) and (b). Optical SET was achieved at 3-dBm injection and RESET at 5-dBm injection through cross-gain saturation. The polarization of the injected light was adjusted for coupling to the TE waveguide modes. The switching power is a function of the cavity gain and absorber loss. The wavelength dependence of the optical RESET behavior is shown in Fig. 5. Optical RESET was achieved over 1522 to 1574 nm, a 52-nm range. The wavelength dependence of the switching power is much smoother than that of the original Fabry-Pérot MMI-BLD, because of the lack of Fabry-Pérot cavity resonance. An increase in the required RESET power is expected at 1554 nm due to higher reflectivity of the DBRs near the Bragg wavelength. Intensities have not been adjusted for fiber-to-chip coupling losses. It is suggested in [8] that higher extinction ratios, output powers, and even amplification-free cascadability may be possible with reverse biased absorbers. Higher output powers are also possible with higher gain centered quantum wells and quantum-well intermixing in lieu of the offset quantum well structure. Amplification-free cascadability through the above methods would be desirable for high extinction ratios, as amplifiers may be a source of amplified spontaneous emission noise.
RABURN et al.: INTEGRABLE MULTIMODE INTERFERENCE DBR LASER ALL-OPTICAL FLIP-FLOPS
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of 22 dB, SET and RESET switching at external injection intensities of 3 dBm and less than 5 dBm, and a 52-nm 1522–1574-nm switching range covering the entire C-band. Advantages over the previous all-active Fabry-Pérot MMI-BLD [4] feature include not only integrability and single-mode lasing, but also a much smoother wavelength dependence of the switching power through the lack of Fabry-Pérot resonance. Future work includes shorter, more deeply etched devices, wavelength tuning through DBR bias, and flip-flops designed for higher output powers and extinction ratios with reverse biased absorbers [8] as well as through quantum-well intermixing for amplification-free cascadability. ACKNOWLEDGMENT
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Fig. 4. (a) All-optical SET behavior of flip-flop. SET achieved at 3 dBm external injection. (b) All-optical RESET behavior of flip-flop. RESET achieved at 5 dBm external injection. Externally injected light was 1550 nm. Flip-flop MMI gain region was biased at 162 mA. Output powers are fiber-coupled values.
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Photomasks were made using the University of Tokyo VLSI Design and Education Center (VDEC)’s 8-in EB writer , donated by ADVANTEST Corporation. The authors would like to thank Dr. H. Shimizu, Dr. F. C. Yit, T. Shioda, Dr. T. Nakano, Dr. T. Ohtsuka, T. Amemiya, J. Darja, K. Nishide, Dr. E. Gourdes, Dr. R. Nakane, Dr. D. Cohen, Dr. B. Thibeaut, Prof. L. Coldren, Dr. A. Al Amin, Prof. M. Sugiyama, K. Horiguchi, K. Sakurai, Dr. H. Binsma, K. Ikeda, S. Narata, and R. Onitsuka. REFERENCES
Fig. 5. Optical intensity required for RESET versus wavelength. Flip-flop exhibited all-optical RESET capability over wavelength range of 52 nm. Increase in required RESET intensity is expected at DBR Bragg wavelength of 1554 nm due to increased DBR reflectivity. MMI gain region injected current is 162 mA.
IV. CONCLUSION A novel fully integrable all-optical MMI flip-flop has been demonstrated. It features lasing at 1554 nm, an extinction ratio
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