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Jun 15, 2009 - Abstract—Subnanosecond electrooptical switching times with. 26-dB extinction ratio were obtained by using semiconductor optical amplifiers ...
IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 21, NO. 12, JUNE 15, 2009

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Rise Time and Gain Fluctuations of an Electrooptical Amplified Switch Based on Multipulse Injection in Semiconductor Optical Amplifiers Napoleão S. Ribeiro, Student Member, IEEE, Adriano L. Toazza, Cristiano M. Gallep, Member, IEEE, and Evandro Conforti, Senior Member, IEEE

Abstract—Subnanosecond electrooptical switching times with 26-dB extinction ratio were obtained by using semiconductor optical amplifiers driven by a multipulse injection current. The multipulse switching current was generated by superimposing fast electronic pulse signals in a microwave resistive combiner. Although very fast switching is achievable, nonlinear behavior and circuits parasitic induce gain fluctuations and overshooting during the OFF–ON process. Theoretical and experimental results show that the reduction of parasitics is an important parameter for improving the switching performance. The multipulse injection technique can improve the switching speed for a chosen degree of overshoot. Index Terms—Electrooptical switches, optical switches, semiconductor optical amplifiers (SOAs).

very high optical gain excursion from 8 dB (optical attenuation, SOA OFF) up to 18 dB (net optical gain, SOA ON), with subnanosecond switching time over a huge optical band (60 nm to around 1550 nm). However, the experimental and simulated results show large current variations, with overshoot and fluctuations in the optical gain after OFF–ON switching. It is shown that these SOA dynamic gain fluctuations may degrade the switch performance, and they are the consequence of small capacitances and inductances in the SOA’s driver line and packaging. It is also shown that the design of high-performance electrooptical SOA-based switches must reduce those parasitic capacitances and inductances and consider the compromise between the transition speed and gain overshoot. II. SOA SWITCH IMPLEMENTATION

I. INTRODUCTION

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HE semiconductor optical amplifier (SOA) is a promising device in optical packet buffering and for next-generation optical network technologies [1], [2]. The reduction of the SOA electrooptical switching times would be helpful to improve those techniques [3]. Predistortion signals were therefore first proposed to boost the speed of laser diode switches [4], followed by a technique to improve the wavelength switching speed of tunable lasers [5], and of SOAs [6]. Further advancements in these techniques can be achieved by using a multiport resistive combiner, raising the SOA injected current excursion and thereby leading to a substantial increase in the optical gain excursion. It was experimentally implemented with signals from three pulse generators [7], simultaneously providing the desired voltage-to-current conversion for the SOA turning-ON, with a

Manuscript received December 16, 2008; revised March 02, 2009. First published March 27, 2009; current version published May 22, 2009. This work was supported in part by CNPq and Fapesp, Brazil under Grants 2007/56024-4 and 2005/51689-2 (CePOF). N. S. Ribeiro and E. Conforti are with the Department of Microwaves and Optics, Faculty of Electrical and Computer Engineering, University of CampinasUnicamp, DMO-FEEC-Unicamp, Caixa Postal 6101, Campinas, SP 13083-970, Brazil (e-mail: [email protected]; [email protected]). A. L. Toazza was with the DMO-FEEC, University of Campinas, Campinas, SP 13083-970, Brazil. He is now with the Department of Electrical Engineering, University of Passo Fundo, Passo Fundo, RS, Brazil (e-mail: [email protected]). C. M. Gallep is with the DMO-FEEC, University of Campinas, Campinas, SP 13083-970, Brazil and also with the Division of Telecommunication Technology-CESET, University of Campinas, Limeira, SP 13484-370, Brazil (e-mail: [email protected]). Digital Object Identifier 10.1109/LPT.2009.2017731

The experimental apparatus is shown in Fig. 1(a). The optical part of the setup has a tunable semiconductor laser, an optical isolator, a commercial SOA, a bandpass filter, and a 40-GHz digital communication analyzer with an optical input port. The injected SOA switching current is provided by a sequence of three pulse generators. The voltage step and impulse generator sigof 47 since the nals are combined using resistors SOA impedance is very low (in the order of 3 ), providing the 50- impedance matching. It is important to note that the large difference between the 47- and the low SOA impedance acts as a voltage-to-current converter, since a 10% variation in the SOA impedance during the switching action (from 3 to 3.3 ) will change the injected current by only 0.6% (overall resistance change from 50 to 50.3 ). The stray capacitances and inducand are related to the 47- resistors, and , to tances the low impedance line from the resistors to the SOA electrical gate of Fig. 1(a). The voltages of the synchronized generators are applied to the microwave combiner producing a formatted . They are appulse in conjunction with a current bias to plied to the SOA electrical gate, generating a current . The equivalent cirfeed the active region, represented by cuit based on [8] has been estimated by computer-aided fitting of the modeled microwave circuit and the measured data up to pF; pF; nH; 20 GHz [Fig. 1(b)]: nH; pF; nH; ; pF; ; pF; ; pH; m , . The circuit dimm; mensions are: semirigid coax strip line mm; connection to the SOA mm. gate

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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 21, NO. 12, JUNE 15, 2009

Fig. 2. Current pulse obtained with the setup shown in Fig. 1. The ideal pulse (no stray capacitances/inductances) contains an impulse whose width (W ), amplitude (3.3 V), and time position (W ) have been optimized for the SOA OFF–ON fast switching (solid line). The simulated results consider the stray capacitances/inductances (dotted line).

OFF–ON (ON–OFF)

Fig. 1. (a) Experimental setup used to implement the SOA current injection switching, with the resistive combiner and voltage-to-current converter, including stray capacitances, inductances, and the SOA equivalent circuit; (b) impedance module of SOA-feeding line (at points A, C , or E ) measured and simulated.

The simplest version for current-injection shape is a fast step signal, applied to the SOA current gate in order to turn it ON (change from attenuation to large optical gain) or OFF (opposite: from gain to attenuation). There is a limited time for the device to turn ON (or OFF), since its active region must be populated (or depleted) by the microwave signal. As the step signal rises, the SOA optical gain increases slowly with a time constant comparable to its (electrical) carrier lifetime. Therefore, the SOA finite response does not permit instantaneous optical gain variations. In addition, the carrier lifetime is not really a constant since it decreases as the carrier density increases inside the active region. Therefore, a current pulse could be used to quickly increase the carrier population and so decrease the carrier lifetime. It is possible to find optimized current formats to minimize the mean value of the carrier lifetime and the SOA

switching time. This process is not straightforward, since the SOA dynamics are nonlinear. An empirical fine-tuning using the setup of Fig. 1 led to the signal presented in Fig. 2, where the solid line would be the SOA current if no stray capacitances or inductances were present (ideal case), and the SOA electrical resistive impedance would be represented by . The best empirical formatted signal was obtained by adding an impulse with ps (generator 3) and a time lag ps width after the step signal. The current excursion was set from 40 mA (SOA current at the OFF state) to 210 mA at the top of the im. In addition, the used signal had a small step of width pulse ps, due to the addition of pulses from generators 1 and 2. The rise and fall times ( , , and ) are 60 ps. However, when the stray capacitances and inductances are considered, the appears with a simulated active region injected current smooth decrease in the rise time, and amplitude and current fluctuations are noted. The SOA optical response to the Fig. 2 signal is shown in Fig. 3(a), where a fast rise time is noted. Simulations were conducted using the Z-SOA software, described in detail elsewhere [9]. The SOA length is 0.65 mm and is transparent, including mA. The OFF-state level of 40 mA insertion loss, with is a compromise between SOA speed and the OFF-state insertion loss (equal to 8 dB), providing an optical output signal of 1.5 W. With the injection of the current signal as shown in Fig. 2, the SOA changed to a 600- W output signal, with a net gain above 18 dB, after a rise time of 650 ps (from 10% to 90%). The simulated results shown in Fig. 3(a) include the ideal case (circles) and the effects of parasitics (diamonds), by using the microwave circuit of Fig. 1(a). Note that both experimental and simulated results (with parasitics) show an overshoot of 15% at OFF–ON switching, and a time dependence of the optical gain during ON-state operation (damping). However, if no parasitics were present (ideal case), simulations show that a much faster rise time (around 400 ps) might be obtained with a higher overshoot but with less damping, as indicated by the results of Fig. 3(a). Further simulation results (not shown) predict that the driver circuit and the packaging parasitics of the encapsulated device used here are the main causes of the induced damping and speed reduction. If their parasitics are reduced, the

RIBEIRO et al.: RISE TIME AND GAIN FLUCTUATIONS OF AN ELECTROOPTICAL AMPLIFIED SWITCH

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Fig. 3. (a) Experimental and simulated (ideal circuit—circles; and including parasitics—diamonds) SOA OFF–ON switching with the injected signal of Fig. 2: response of 650 ps (from 10% to 90%) with an optical gain variation from 8 to 18 dB; (b) SOA ON–OFF switching process with the addition of a 40-mA impulse of width equal to 250 ps, and without the impulse; (c) simulated rise time (from 10% to 90%) and gain overshoot as a function of the injected impulse voltage amplitude.

0

tuning in the SOA chip capacitances—space charge and —will be the next challenge parameters to obtain diffusion faster SOA electrooptical switches. The experimental and simulated results for the SOA ON–OFF operation are shown in Fig. 3(b). The simple step signal response, with an ON-state of 170 mA and an OFF-state of 40 mA, produced a fall time of 2 ns. The switching improvement with a fall time of 450 ps was obtained by adding a negative impulse of 40 mA, with a width of 250 ps and time lag of 350 ps, just after the step signal. The response without the impulse clearly shows the advantage of using the proposed impulse technique. Moreover, the faster fall time (450 ps) relative to the rise time (650 ps) was expected [1]. The SOA simulated optical response to the signal of Fig. 2, ps and including parasitics, is shown in Fig. 3(c) for a time-lag ps, in relation to the impulse voltage (generator 3, Fig. 1), ranging from 0 (no impulse) to 10 V. Note that the rise time decreases from 2200 ps (no impulse) to 450 ps, but in this latter case the optical gain is deteriorated by a very strong overshoot. For the SOA used here, the result of Fig. 3 is a compromise between overshoot and sharp rise time. The theoretical studies have shown that the overshoot is mainly a consequence of the impulse current injection and the SOA nonlinear behavior. However, the damping and rise time are also influenced by the capacitances and inductances of the SOA microwave driver, packaging, and chip characteristics. The SOA driver and packaging parasitics have an important effect on the deterioration of the rise time of the injected current and on the reduction of the injected carriers, including reflection by imperfectly matched load. If these parameters are optimized, the SOA chip intrinsic parameters gain in importance. In this case, the SOA length and active region parameters could be designed to reduce space and diffusion capacitances, and to increase the extinction/gain of the SOA. III. CONCLUSION We describe an amplified electrooptical switch based on an SOA, driven by a formatted injection current impulse signal

(added to the normally used step signal). The formatted signal was obtained by the addition of individual pulses using a multiport resistive combiner. An SOA switching OFF–ON (ON–OFF) time of 650 ps (450 ps) was obtained, with a high contrast ratio (26 dB). The reduction of parasitics is an important parameter for improving the switching performance. However, it was shown that a decrease in the OFF–ON switching time creates an increase in the SOA gain overshoot, even if parasitics are absent. The multipulse injection technique improved the switching speed for a chosen degree of overshoot.

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