Novel modulated-master injection-locked 1 . 55-μ m VCSELS

We report a novel optical injection-locking configuration using a directly-modulated master VCSEL to injection-lock a slave VCSEL. We demonstrate an interesting RF modulation response for the modulated master laser. An RF gain up to 30 dB is attained for frequencies greater than a certain critical frequency fc, which is ~10 GHz here and increases with the injection power. In addition, an RF phase change of 2π is achieved above fc by varying the wavelength detuning. The slope of the phase-frequency curve represents an equivalent slow or fast light attained through the slave laser. We incorporate an amplifier model to explain this novel modulated-master injection-locked VCSEL configuration. Simulations show good qualitative agreement with the experimental results. ©2006 Optical Society of America OCIS codes: (140.3520) injection-locked; (250.7260) VCSELs; (060.4080) Modulation; References and links 1. S. Kobayashi and T. Kimura, “Injection locking characteristics of an AlGaAs semiconductor laser,” IEEE J. Quantum Electron. 16, 915-917 (1980). 2. H. Nakajima, “Demodulation of multi-gigahertz optical signal in an injection-locked distributed feedback laser oscillator,” IEE Electron. Lett. 26, 1129-1131 (1990). 3. A. C. Bordonalli, C. Walton, and A. J. Seeds, “High-performance homodyne optical injection phaselock loop using wide-linewidth semiconductor lasers,” IEEE Photon. Technol. Lett. 8, 1217 (1996). 4. C. H. Chang, L. Chrostowski, and C. J. Chang-Hasnain, “Injection locking of VCSELs,” J. Sel. Top. Quantum Electron. 9, 1386-1393 (2003). 5. L. Chrostowski, X. Zhao, and C. J. Chang-Hasnain, “Microwave performance of optically injection-locked VCSELs,” IEEE Trans. Microwave Theory Technol. 54, 788-796 (2006). 6. M. Ortsiefer, M. Furfanger, J. Rosskopf, G. Bohm, F. Kohler, C. Lauer, M. Maute, W. Hofmann, M. Amann, “Singlemode 1.55 μm VCSELs with low threshold and high output power,” Electron. Lett. 39, 1731 (2003). 7. H. Su and S. L. Chuang, “Room temperature slow and fast light in quantum-dot semiconductor optical amplifiers,” Appl. Phys. Lett. 88, 061102 (2006). 8. X. Zhao, P. Palinginis, B. Pesala, C. Chang-Hasnain, and P. Hemmer, “Tunable ultraslow light in vertical-cavity surface-emitting laser amplifier,” Opt. Express 13, 7899-7904 (2005) 9. B. Pesala, Z. Chen, and C. Chang-Hasnain, “Tunable pulse delay demonstration using four-wave mixing in semiconductor optical amplifiers,” presented at OSA Topical Meeting of Slow and Fast Light, Washington, DC, 23-26 July 2006. 10. X. Zhao, C. Chang-Hasnain, W. Hofmann and M. C. Amann, “Modulation efficiency enhancement of 1.55-μm injection-locked VCSELs,” in Conference Digest of IEEE 20 International Semiconductor Laser Conference (Institute of Electrical and Electronics Engineers, New York, 2006), pp.125-126. 11. E. Wong, X. Zhao, C. J. Chang-Hasnain, W. Hofmann, and M. C. Amann, “Uncooled, optical injection-locked 1.55 μm VCSELs for upstream transmitters in WDM-PONs,” presented at Optical Fiber Communications Conference, Anaheim, California, Postdeadline paper PD50, 5-10 Mar. 2006. #72362 $15.00 USD Received 26 June 2006; revised 2 October 2006; accepted 16 October 2006 (C) 2006 OSA 30 October 2006 / Vol. 14, No. 22 / OPTICS EXPRESS 10500


Introduction
For over twenty years, optical injection locking (OIL) has been studied for a wide range of applications in optical communications, optical signal processing and microwave photonics [1][2][3].Typically, the injection locking configuration uses one continuous-wave (CW) operated laser as the master laser to optically lock a second directly-modulated slave laser.With the assist of the CW external light injection from the master laser, various direct modulation characteristics of the slave laser can be greatly improved, including a reduced frequency chirp leading to an improved digital transmission [4], a significant increase of the modulation frequency response [5], and greatly reduced non-linear distortion [5].
In this paper, we explore a novel OIL scheme using a directly-modulated master laser and a CW slave laser.By modulating the master rather than the slave laser, we report the influence of an injection-locked slave laser on the direct modulation characteristics of the master laser for the first time.We demonstrate that an RF gain up to 30 dB is achieved for a modulation frequency higher than a critical frequency f c , which is approximately 10 GHz here and can be increased with injection power.A full 2π phase shift is also achieved above f c within a bandwidth of several GHz, which is effectively a slow/fast light medium and potentially useful to achieve tunable RF delay as well as optical delay.

Experiment
The experimental setup is shown in Fig. 1.Two 1.55-µm VCSELs [6] are used in the experiment as master and slave lasers.A polarization maintaining (PM) erbium-doped fiber amplifier (EDFA) is used to adjust the power of the master VCSEL.The master VCSEL, directly modulated by a network analyzer (HP 8270B), injection-locks the slave VCSEL through a PM fiber circulator.A small amount of the output light goes into an optical spectrum analyzer (OSA) to monitor the locking process, while the majority of the light goes to a receiver and then back to the network analyzer.Therefore, the small-signal amplitude and phase frequency response (S21 in both amplitude and phase formats) of a modulated-master OIL VCSEL are measured.
The amplitude response at various wavelength detuning values (Δλ = λ masterλ slave ) is shown in Fig. 2. Figure 2(a) shows the raw data taken by the network analyzer.The black curve shows the unlocked master frequency response including all parasitics, which lead to an overall roll-off at frequencies > 6 GHz.The modulated-master OIL VCSEL frequency response is shown at different Δλ, and color-coded as Δλ increases (the master moves from the blue to the red side relative to the slave wavelength).A dip is observed at a critical frequency f c , which is 9.2 GHz in this injection power condition, and followed by a peak at a higher frequency, ~13 GHz.As Δλ is increased, the depth of the dip first increases with detuning until a certain value (0.206 nm in this case), and subsequently decreases with further increasing detuning.The peak, on the other hand, drops monotonically with increasing Δλ.To better illustrate these features induced by injection locking; these frequency response curves can be simply calibrated by subtracting out the unlocked master laser S21 response.The calibrated response is shown in Fig. 2(b).More pronounced dips and gain peaks can be seen.Gain as high as 18 dB is demonstrated at this injection power level.injection locking.An interesting feature is that at f c , a phase change, φ, flip occurs at the detuning value corresponding to the maximum dip.At frequencies greater than f c , a phase shift of 2π can be achieved over a bandwidth as large as 8 GHz.The phase change can also be easily tuned by just simply tuning Δλ between the master and the slave laser.The slope of the phase change response curve, dφ/dω, represents the delay time τ.Centered at f c with a frequency bandwidth of 2 GHz, dφ/dω changes from negative (0.2 ns) to positive (-0.2 ns) value with increasing the detuning.Hence, effectively, we can tune the slave laser medium from a slow to a fast light medium, with a total delay of ~0.4 ns.Since the VCSEL contains 5×6-nm quantum wells (QWs) and the slow/fast light effects occurs in the active region, a slow down factor S (speed up for the fast light case) can be obtained as large as 2×10 6 .For frequencies above f c , dφ/dω is always negative, which always results in slow light with an effective S~4×10 5 .The slow/fast light effect shown here is different from those using semiconductor optical amplifiers where carrier density pulsation was the primary cause for the slow/fast light [7,8].However, it is similar to [9], where the VCSEL was biased below threshold and used as a narrow-band amplifier.
Although the slave VCSEL here is biased well above threshold, and under a strong external light injection, it acts like a narrow-band amplifier [10].In this case, the slow/fast light occurs at a frequency several GHz away from the master laser frequency.Therefore, utilizing it as an optical buffer for a train of light pulses requires subcarrier modulation at or above f c .Nevertheless, the tunable phase shift is also intriguing to achieve a flexible phase control of a narrowband RF signal.Figure 4 shows both the amplitude and phase response at a fixed detuning but various master injection power levels.All the features are still well maintained, while f c increases as the injection power is increased.This shows that RF gain and phase change is possible for even higher frequencies with higher injection power levels.To show that these observations are a general trend of modulated-master OIL VCSELs and not due to any peculiar characteristics of any particular device or artifacts during the measurement, the same experiment is performed using different VCSELs for the master and slave lasers.Table 1 shows the amplitude and phase change response for three different device combinations at various injection power levels as well as detuning values.All of them show similar features and behaviors with detuning.
For the case of VCSEL 4 as master laser and VCSEL 5 as slave laser, experiments are also performed at three different power levels -10.70 dBm, -4.65 dBm and -2.27 dBm as shown in Table 2.At each power level a set of curves for various detuning conditions are presented.All these results show that the features of the frequency response of modulated-master OIL VCSELs are quite repeatable.
RF phase shift can also be measured in the time domain as shown in Fig. 5. Figure 5(a) shows the phase change response at two different detuning conditions.The blue one has negative phase shift at frequencies greater than f c , while the green one has positive phase shift.The time-domain measurement is performed by modulating the master laser at single tones 14 GHz, 15 GHz and 16 GHz separately, while keeping the same detuning conditions as shown in Fig. 5(a).The oscilloscope traces are shown in Fig. 5(b).The same amount of phase delay and advancement are observed for both cases at those single-tone frequencies as compared with the frequency-domain measurement.

Explanation and simulation
The experimental results with emphasis on the RF gain, absorption and phase shift flip can be explained by treating the VCSEL as an amplifier under injection-locking condition.
The optical spectrum of a modulated-master OIL laser is shown in Fig. 6 in order to explain the high frequency RF gain peaks.External injection light will enhance the stimulated emission in the slave cavity, thus depleting the carrier density to a level that is below its lasing threshold.Therefore an OIL VCSEL acts like an amplifier while lasing.Since the linewidth enhancement factor couples the gain and the phase in semiconductor lasers, the carrier density reduction will red-shift the slave cavity resonance [10].Therefore, amplified spontaneous emission from the red-shifted slave cavity can be seen in the optical spectrum on the longer wavelength side to the master mode.When the master laser is modulated by a single tone, it has two sidebands associated with it.If the modulation frequency coincides with the spacing between the master and the shifted cavity resonance, one of the sidebands will get dramatically enhanced by the cavity, thus resulting in the RF gain as shown in Fig. 6.The increasing of Δλ further depletes the slave carrier density [10], which results in a decreasing gain peak.
To explain the dip and its trend as a function of Δλ on the amplitude response, as well as the phase change response behavior which is usually associated with the amplitude response, a VCSEL amplifier model is used to perform some simulations using transmission matrices as shown in Fig. 7(a).The simulation is done in the optical domain and CW mode, which means no modulation is involved.A probe beam is incident on an active VCSEL cavity, which is biased below its lasing threshold.The relative intensity and the phase change of the output beam after interacting with the VCSEL amplifier are the quantities of interests.The VCSEL design is the same as in [6].If we define the electromagnetic field of the probe and the output beam as p and r, Figs.7(b) and 7(c) shows the simulated relative intensity (|r/p| 2 ) and phase change (arg(r)-arg(p)) as functions of the probe frequency at different amplifier gain levels, respectively.
The simulation results agree with the experimental results very well.First of all, there is a dip in relative intensity spectra.The depth of the dip increases with increased gain initially until a certain value, and subsequently, it decreases with further increase of gain.The phase change response for gain values below and above the turning point also    shows a 2π phase change.To connect these optical domain results to the RF signal measurements that are shown previously, modulated master light including two sidebands can be considered as the incident probe beam as shown in Fig. 7(a).Therefore, the features in the optical domain can be transferred to the RF domain through these sidebands.In addition, the slave laser gain level decreases as Δλ increases [10].Hence the set of curves shown in Figs.7(b) and 7(c) from high gain to low gain can be viewed equivalently from small to large Δλ.Comparing Figs.7(b) and 7(c) with Figs. 2 and 3, the trend of the simulation results are also consistent with the experimental results.
The simulation shown here is done in the optical domain only.Modulation sidebands and the influence of the frequency chirp of the master VCSEL need to be included to fully understand the mechanism that causes these features.Both are beyond the scope of this paper and further investigation is currently under way.

Conclusion
We report a novel modulated-master injection-locking scheme under which the high-speed characteristics of a directly-modulated master VCSEL are altered by a slave VCSEL it externally locks.An RF gain as high as 30 dB is obtained for frequencies higher than a certain critical frequency, which depends on the injection power.We provide a qualitative explanation of this behavior with the support of numerical simulations.This interesting behavior may find application as high pass amplifiers in a passive optical network [11] or as tunable RF gain for all-optical communications or switching.We also demonstrate slow and fast light on single-tone signals with total RF phase shift of 2π beyond 10 GHz.This device may be useful for narrow band RF applications, such as phase shifters, phase-array antennas and beam steering.It may be also used as an all-optical time delay in subcarrier-multiplexed networks.

Fig. 2 .
Fig. 2. (a).Raw amplitude response of a modulated-master OIL VCSEL under a fixed injection power at various detuning values.

Fig. 2 .
Fig. 2. (b).Calibrated amplitude response of a modulated-master OIL VCSEL under a fixed injection power at various detuning values.

Fig. 3 .
Fig. 3. Phase change response of a modulated-master OIL VCSEL under a fixed injection power at various detuning values.For a bandwidth of ~2 GHz centered at f c , the slope dφ/dω, equivalent to the delay time τ for each detuning, changes from -0.2 ns to 0.2 ns, representing slow and fast light regimes.

Fig. 4 .
Fig.4.Amplitude and phase change response of a modulated-master OIL VCSEL at a fixed detuning but at three different injection power levels.RF gain and phase change is possible for even higher frequencies with higher injection power levels.

Figure 3
Figure 3 shows the corresponding phase change response of the same modulated-master OIL VCSEL.Phase change response presents the locked phase response calibrated with the unlocked one, thus showing only the phase shift induced by

Fig. 7 .Fig. 7 .
Fig. 7. (b).Simulated relative intensity of the output of a probe beam incident on a VCSEL biased below threshold (VCSEL amplifier) at different material gain levels.

Fig. 7 .
Fig. 7. (c).Simulated phase change of the output of a probe beam incident on a VCSEL biased below threshold (VCSEL amplifier) at different material gain levels.

Table 1 .
Amplitude and phase change frequency response of different modulated-master OIL VCSELs.

Table 2 .
Amplitude and phase change frequency response at various detuning values using VCSEL 4 as master and VCSEL 5 as slave at three different power levels.