50 GHz Directly-Modulated Injection-Locked 1.55 μm VCSELs by vwp15099


									                                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: lukasc@ieee.org

                            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 [1], and for vertical cavity surface emitting lasers
(VCSELs) is 21 GHz [2]. 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 [3]. 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 [5]. 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
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 [6].
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 [7].
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

                                                                                                                                                             RF Gain Enhancement (dB)
                                                                                                            35                                                                          10
                                                                                                                                               VCSEL 2
                                                                                                            30                                      10.7dB                                5
                                    35                                                                      25                                                                            0
                                                                                                                                               VCSEL 3
                                                                                                            20                                                                           -5
                                    30                                                                      15                                      FR                                  -10
                                                                                                            10                                 VCSEL 4                                  -15
                                    25                                                                                                              FR
                                                                                                            5                                       13.3dB                              -20
a)                                                                           b) -0.5
                                                                                                                 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.
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[2]                                      K. L. Lear, M. Ochiai, V. M. Hietala, H. Q. Hou, et al., “High-speed vertical cavity surface emitting lasers,” Proc. IEEE/LEOS
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[3]                                      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).
[4]                                      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).
[5]                                      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).
[6]                                      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).
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