50 GHz Directly-Modulated Injection-Locked 1.55 µm VCSELs Lukas Chrostowski, Xiaoxue Zhao and Connie J. Chang-Hasnain 253M Cory Hall, Department of Electrical Engineering and Computer Science University of California at, Berkeley, Berkeley, CA 94720, USA E-mail: email@example.com Robert Shau, Markus Ortsiefer and Markus-Christian Amann Walter Schottky Institut,Technical University of Munich, Germany, and VERTILAS GmbH, Germany Abstract: The resonance frequency of several 1.55 µm VCSELs is enhanced from 7 GHz up to ~50 GHz with the optical injection locking technique. This is the highest value reported for directly modulated lasers. 2005 Optical Society of America OCIS codes: (250.7260) Vertical cavity surface emitting lasers, (060.4080) Modulation. 1. Introduction Semiconductor lasers are at the heart of optical communication networks. Direct modulation of semiconductor lasers for digital data transmission has been a topic of significant interest for over 20 years, with much research activity directed at improving the frequency response of the lasers to increase the data rates possible. Currently, the highest reported bandwidth for edge emitting lasers is 40 GHz , and for vertical cavity surface emitting lasers (VCSELs) is 21 GHz . The corresponding resonance frequencies for both lasers are 31 GHz and 15 GHz, respectively. These results were obtained using high-gain short wavelength materials. For the longer fiber telecommunication wavelength of 1.55 um, the material gain is lower and limits the maximum frequency response. For high-speed fiber optic communication applications using directly modulated (DM) lasers, two limitations have been encountered. First, the bandwidth is limited by the relaxation oscillation frequency, and second, the optical chirp (wavelength shift under modulating) is too severe. For these reasons, a continuous wave laser with an external modulator is used for bit-rates > 10 Gb/s. DM lasers are thus only used for networks at less than 2.5 Gb/s. In this paper, we address the frequency response limitations of DM semiconductor lasers, and demonstrate that a technique called optical injection locking can be used to enhance their frequency. The injection locking technique uses one laser (master) to optically lock a second directly modulated one (follower). Using a VCSEL as the follower and a distributed feedback (DFB) laser as the master, we have shown that such enhancements can be obtained over a large range of injection ratios and wavelength detuning (=λ DFB-λVCSEL), demonstrating the effectiveness and robustness of the technique . It has been shown to be very effective at enhancing the laser resonance frequency [3, 4], thus increasing the modulation bandwidth. For 1.55 um VCSELs, the highest previously published resonance frequency was 28 GHz, achieved with strong injection locking . Injection locking has been shown to significantly reduce optical chirp, opening the possibility for high bit-rate wavelength division multiplexed (WDM) data transmission using DM lasers. Additionally, injection locking improves other aspects of laser performance, including reducing laser noise, linewidth, and non-linear distortions. Numerical and analytic simulations have shown that the modulation frequency response of the laser is significantly enhanced. The model predicts that very high resonance frequencies (fr) are possible, with the frequency enhancement increasing as the square root of the injection ratio. No upper bound on the resonance frequency enhancement is found, though the bandwidth may have an upper bound. In this paper, we report experiments on several VCSELs, all showing resonance frequencies up to 50 GHz when injection-locked, compared to <10 GHz in free-running operation. This is to the best of our knowledge the highest resonance frequency observed through small-signal modulation for all semiconductor lasers, and for VCSELs in particular. 2. Experiments The experiments performed seek to determine if a limit to the resonance frequency enhancement can be found, by increasing the injection ratio to very high values as well as its dependence on wavelength detuning. The directly modulated follower lasers used in this experiment were buried tunnel junction 5-QW InGaAlAs/InP 1.55 µm VCSELs typically with >25 dB side mode suppression ratio under continuous wave (CW) operation . The typical laser resonance frequency is about 6 GHz at ~2X threshold. A diagram of the VCSEL is shown in Figure 1. Figure 1 – Schematic of a buried tunnel junction 1.55 um VCSEL Figure 2 – Experimental setup The experimental setup is shown in Figure 2. Two lenses in a confocal arrangement (f = 3 mm) on micro- positioning stages were used to couple light between the VCSEL and the angle-polished fiber. The master laser is an Ortel/Emcore DFB laser (relative intensity noise < -165 dB/Hz) with a polarization maintaining (PM) single mode fiber output. It is coupled to the VCSEL via a PM circulator. The wavelength detuning and injection power were adjusted by tuning the DFB temperature and current. The polarization of the DFB signal is adjusted to match that of the VCSEL by rotating the circulator port-2 fiber. The VCSEL was directly modulated and characterized using an Agilent E8364A 50 GHz network analyzer. The modulated VCSEL output was amplified using an EDFA and detected using a 50 GHz u2t Photonics waveguide photodiode. Simultaneously, we observe the optical spectrum, as well as measure the RF gain without EDFA amplification. The experiments were conducted at room temperature without VCSEL temperature stabilization. The S21 data has been calibrated, with the device/packaging parasitic response de-embedded using the free-running frequency responses, as in . 3. Results Representative experimental small-signal responses are shown in Figure 3. The data shown in Figure 3a shows the free-running VCSEL S21 as well as injection-locked VCSEL under various detuning conditions for a moderate injection ratio of 13.8 dB. The VCSEL resonance frequency is enhanced from a free-running frequency of 6 GHz to up to 50 GHz. The lowest damping (sharpest resonance peak) occurs for the lowest detuning values (i.e. the master laser is on the blue side of the free-running VCSEL). As the detuning is increased (i.e. the master laser is tuned to the red side), the resonance peak is gradually damped out, and flatter S21 responses are observed, with an increasing RF gain. The highest injection-locked relaxation oscillation frequency (fr) peak observed for this injection power is ~50 GHz, limited by the 50 GHz RF network analyzer. a) b) Figure 3 – Small-signal response of free running and injection-locked VCSELs, for varied detuning. a) for an injection ratio of 13.8 dB, VCSEL-1, b) injection ratio of 12.5 dB, VCSEL-4. The free-running fr (~7 GHz, 2 mA bias) is also shown as the reference. Thick curves are calibrated S21 data; the thin curves are curve-fitted data. Figure 3b shows the frequency response for another laser. Again, the highest resonance frequency observed is ~50 GHz. Higher injection ratios yielded resonance frequencies beyond 50 GHz. In total, four lasers were tested, and all exhibited resonance frequencies over 45 GHz, limited by measurement instrumentation. Figure 4a summarizes the results, showing the maximum resonance frequency, which is obtained for the lowest negative detuning. The injection-locked optical side-mode suppression ratio is ~30 dB. The variation in the data for the devices is attributed to optical coupling differences, and to the variation in laser threshold and biasing currents. In agreement with simulations, increasing the injection ratio increases the resonance frequency; no upper bound has been observed. Figure 4b summarizes the resonance frequencies observed for the lasers for several injection ratios, versus the wavelength detuning. The highest resonances are observed for negative detuning values. An interesting observation on the experimental data is that for certain conditions, there occurs a very large modulation efficiency increase (or RF gain enhancement at low GHz frequencies). For a large injection ratio (14 dB), the enhancement is up to 20 dB (for a detuning of 1.2 nm). This RF gain varies with detuning, as shown in Figure 4c, and is minimal for the curves showing a sharp resonance peak. The maximal RF gain is accompanied by frequency response curves with a high effective damping rate, under a high injection condition with a large positive wavelength detuning (master wavelength is shorter than the follower laser). Also, there exists a tradeoff between resonance frequency (and bandwidth) and RF gain. For the cases of small RF gain enhancement, ex. 0.26 nm detuning in Figure 3b, a nearly flat frequency response is observed up to 50 GHz. 50 50 25 VCSEL 1 VCSEL 1 45 FR 20 Maximum Resonance Frequency (GHz) VCSEL 2 Max Resonance Frequency (GHz) 45 VCSEL 3 13.8dB VCSEL 4 40 13.3dB 15 12.9dB RF Gain Enhancement (dB) 35 10 VCSEL 2 40 FR 30 10.7dB 5 9.7dB 35 25 0 VCSEL 3 FR 20 -5 14.3dB 30 15 FR -10 11.8dB 10 VCSEL 4 -15 25 FR 5 13.3dB -20 12.9dB a) b) -0.5 0 0 0.5 1 1.5 2 c) -25 -0.5 0 0.5 1 1.5 2 8 10 12 14 Detuning (nm) Detuning (nm) Injection Ratio (dB) Figure 4 – a) Maximum resonance frequency (determined from measured small-signal S21) for four VCSELs, versus injection ratio. b) Resonance frequency of four VCSELs, versus wavelength detuning. (FR: Free running, dB in legend are injection ratio) All lasers exhibit >45 GHz operation for high injection ratio and low to negative detuning. c) RF Gain measured at 1 GHz for four VCSELs, versus wavelength detuning. As high as ~20 dB RF gain is found for large positive detuning cases. 4. Conclusion A record resonance frequency of 50 GHz is achieved with polarization-maintained injection locking of a buried tunnel junction 1.55 µm VCSEL. We demonstrated this behavior for 4 lasers. The intrinsic bandwidth of the injection-locked VCSEL is also > 50 GHz. We show that the resonance frequency scales with increasing injection ratio, and no upper bound has thus far been found. As high as 20 dB RF gain is also attained for the injection-locked VCSELs under strong injection conditions. These results suggest that injection locking may be a highly effective and potentially low cost path for upgrading existing transmitters to higher data rates, broader bandwidths or longer transmission distances. 4. Acknowledgments This work was supported by NSF Award ECS-0123512. References  S. Weisser, E. C. Larkins, K. Czotscher, W. Benz, et al., “Damping-limited modulation bandwidths up to 40 GHz in undoped short- cavity In0.35Ga0.65As-GaAs Multiple-quantum well lasers,” IEEE Photonics Technology Letters, 8 (5), 608-610 (1996).  K. L. Lear, M. Ochiai, V. M. Hietala, H. Q. Hou, et al., “High-speed vertical cavity surface emitting lasers,” Proc. IEEE/LEOS Summer Topical Meetings, 53-4 (1997).  C. H. Chang, L. Chrostowski, and C. J. Chang-Hasnain, “Injection Locking of VCSELs,” Journal of Selected Topics in Quantum Electronics, 16 (3), 888-890 (2003).  X. J. Meng, C. Tai, and M. C. Wu, “Experimental demonstration of modulation bandwidth enhancement in distributed feedback lasers with external light injection,” Electronics Letters, 34 (21), 2031-2 (1998).  X. Zhao, M. Moewe, L. Chrostowski, C.-H. Chang, et al., “28 GHz Optical Injection Locked 1.55 um VCSELs,” Electronics Letters, 40 (8), 476-478 (2004).  M. Ortsiefer, R. Shau, F. Mederer, R. Michalzik, et al., “High-speed modulation up to 10 Gbit/s with 1.55 um wavelength InGaAlAs VCSELs,” Electronics Letters, 38 (20), 1180-1 (2002).  J. C. Cartledge and R. C. Srinivasan, “Extraction of DFB laser rate equation parameters for system simulation purposes,” Journal of Lightwave Technology, 15 (5), 852-860 (1997).
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