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Fig.1 (a) Optical phase-shifter using 1.55 µm-VCSEL. Calculated (b) group delay and (c) nonlinear phase-shift for spot-size of 3.2 µm with acarrier lifetime of 10 ps.

Tunable Optical Delay and Nonlinear Phase Shift Using 1.55 µm VCSEL

S. Suda1, G. Hirano1, F. Koyama1, N. Nishiyama2, C. Caneau2, and C.E. Zah2

1Microsystem Research Center, Tokyo Institute of Technology 4259-R2-22, Nagatsuta, Midori-ku, Yokohama,226-8503, Japan, Email: ssuda@ms.pi.titech.ac.jp

2Corning Incorporated, SP-PR-02-3, Corning, NY 14831, USA

Abstract: We present the tunable optical delay and nonlinear phase-shift using reverse-biased 1.55µm VCSEL. The reduction of recovery time in saturable absorber with increasing reverse bias would enable to control the phase of high speed optical signals.

1. Introduction

The control of the phase of high-speed optical signals is becoming important for long-haul fiber transmission systems of 40Gbps or beyond. We proposed and demonstrated a novel nonlinear optical phase shifter based on a long wavelength VCSEL structure [1]. The intensity-dependent phase shift is induced by photo-carriers in a resonant cavity saturable absorber, which would enable us to control the transient chirp and to compensate fiber nonlinearities, since either negative or positive nonlinear phase shift can be obtained depending on the design of resonant cavities [1]. The speed of the proposed device is determined by the recovery time of photo-carriers in a saturable absorber. The addition of reverse bias in a saturable absorber reduces the recovery time and allows us to control the optical delay and nonlinear phase shift. In this paper, we present the tunable optical delay and nonlinear phase shift using a reverse-biased 1.55µm VCSEL.

2. Modeling of optical nonlinear phase shifter

Figure 1(a) shows the schematic structure of our proposed optical phase shifter based on 1.55 µm vertical-cavity surface emitting laser (VCSEL) [2]. This device consists of a top-DBR and a bottom-DBR and a saturable absorber consisting of InGaAlAs quantum wells (QWs). An intensity-dependent refractive index change is induced in the saturable absorber, which is enhanced by a resonant cavity. We reported that either positive or negative phase-shifts of reflected light can be obtained with 1.55 µm-VCSEL without bias [1]. Both positive and negative phase shifts are useful for the compensation of laser chirp and fiber nonlinearities in optical domain, respectively. We calculated the reflectance and nonlinear phase shift of the optical phase shifter by using a transfer matrix method including saturable absorption. The device we simulated consists of 4-pair Si/SiO2 top-DBR and 40-pair InGaAlAs/InP bottom-DBR with InGaAlAs QWs. Per design, the operating wavelength is close to the resonant wavelength of the cavity. Here, we assumed that the parameter of saturation coefficient is 100 kW/cm2, which corresponds to a carrier recovery time of 10 ps for 40Gb/s pulse signal. A large nonlinear phase shift of over 0.1 radian is predicted, which would be useful for chirp compensation. The fast recovery time is an important issue for high speed optical signals.

1210-7803-9560-3/06/$20.00 ©2006 IEEE

WC4 2:45 PM – 3:00 PM

3. Measurement We carried out the measurement on the tunable optical delay and phase shift with reverse bias using a 1.55

µm VCSEL [2], which was fabricated in Corning Incorporated. We measured the reflectivity and the group delay characteristics by using an optical component analyzer (Advantest, Q7761). Figures 2(a) and 2(b) show the reflectance and the group delay for a fixed input power of 0.1 mW with different reverse bias voltages, respectively. We could see the tunable optical delay with a reverse bias. Figure 2(c) shows the maximum group delay as a function of the reverse bias for different input powers. A simple saturable absorber model shows that the group delay is dependent on the amount of photo-carriers, which is determined by the product of input power and recovery time [3]. Thus, we estimate the recovery time in QWs with a fitting procedure for different bias levels (Fig.2(b)). We assumed a saturation coefficient Isat=1 kW/cm2, which corresponds to the case of carrier lifetime τ=1 ns recovery time for a saturable absorber without bias. We estimated a recovery time of about 50 ps for 0.8 V.

Figure 3 shows the measured results under different input powers with 0.8 V reverse bias, which corresponds to a carrier recovery time of 50 ps. The phase shift was estimated from the spectral integration of the measured group delay. Even with a reverse bias enabling fast recovery time, we are able to obtain a large nonlinear phase shift of over 0.1 radian for the reasonably low input power level of 2 mW. Further increase of reverse bias voltages over 1V would enable us to reduce the recovery time for high speed optical signals beyond 40 Gbps.

4. Conclusion

We presented the tunable optical delay and nonlinear phase shift using a reverse-biased 1.55µm VCSEL. We predicted a recovery time of 50 ps with a reverse bias of 0.8V. Further decrease of recovery time would enable us to control the nonlinear-phase shift for high bit-rate optical signals of 40Gbps or beyond. References [1] S. Suda, F. Koyama, N. Nishiyama, C. Caneau and C.E. Zah, CLEO/IQEC 2006, CWK3 (2006). [2] N. Nishiyama, C. Caneau, B. Hall, G. Guryanov, M.H. Hu, X.S. Liu, M.J. Li, R. Bhat and C.E. Zah, IEEE JSTQE, 11, (2005) 990. [3] T. Sizer, II, T. K. Woodward, U. Keller, IEEE Journal of Quantum Electronics, 30, (1994).

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Fig.2 Measured wavelength dependence of (a) reflectivity, (b) group delay, (c) Maximum group delay as a function of reverse bias. Themeasured device is 1.55 µm InGaAlAs VCSEL with reverse bias. For (a) and (b), the input power was fixed at 0.1 mW.

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