**5.1 Switch of multiple cascaded SOAs**

For the multiple cascaded SOAs, the simulations are carried out. The gain recovery time, overshoot level (normalized by the initial power level), and noise figure as a function of SOA stage are plotted in Fig. 14, where the powers of input CW probe and pump pulse to SOA1 are the same as Fig. 4. The noise figures of the cascaded switches are obtained using the equations presented in (Baney et al., 2000).

The results are actually encouraging, since the recovery time is reduced to ~10 ps when three SOAs are cascaded, which implies that more SOAs after turbo-switch, faster recovery could be expected. However, the degree of the overshoot and noise figure also rise almost linearly as the numbers of SOA increases, whereas the recovery time is not reduced significantly any more after the stage number exceeding 5. Moreover, the ASE noise and the complexity of the device are also expected to increase when more SOAs are cascaded. Therefore a trade-off has to be considered accordingly when choosing an optimum structure of turbo-switch for a specific application. Nevertheless, our simulation suggests that, the optimum number of SOA should be in the range of 2 to 5.

High-Speed All-Optical Switches Based on Cascaded SOAs 43

0.5

Normalized gain (a.u.)

Fig. 15. Normalized gain dynamics of turbo-switch, by varying the injected current level of

A detailed theoretical model to simulate the gain/phase dynamics of the input optical signals propagating through turbo-switch has been presented. The simulation results have been shown in excellent agreement with experimental measurements incorporating the turbo-switch, in terms of CW modulation, pattern effect mitigation at 40 Gb/s, and eye diagram of 160 Gb/s wavelength conversion based on turbo-switch. The introduction of turbo-switch has the capability of increasing the overall switch speed by a factor of four if compared with the case of a single SOA switch, which is also confirmed by the previous

Moreover, the theoretical analysis based on the SOA model suggests that, higher bit-rate operation can be expected, if using an extended turbo-switch structure for instance with three or more cascaded SOAs. In addition, optimized configurations of turbo-switch with the differential XPM scheme as well as the bias current of SOAs can also be implemented to

The authors would like to thank our colleagues Dr. R. J. Manning, Dr. R. P. Webb, and Dr. R. Giller from Tyndall National Institute, Ireland, for the experiment facilities and results used

Agrawal G. P. and Olsson N. A. (1989). Self-phase modulation and spectral broadening of

optical pulses in semiconductor laser amplifiers. *IEEE J. Quantum Electron.*, vol. 25,

1

(b)

<sup>0</sup> <sup>20</sup> <sup>40</sup> <sup>60</sup> <sup>80</sup> <sup>100</sup> <sup>120</sup> <sup>0</sup>

From 200 mA to 40 mA by a step of 20 mA

Time (ps)

<sup>0</sup> <sup>20</sup> <sup>40</sup> <sup>60</sup> <sup>80</sup> <sup>100</sup> <sup>120</sup> <sup>0</sup>

From 200 mA to 80 mA by a step of 20 mA

Time (ps)

0.2 0.4 0.6 0.8 1 1.2

(a)

(a) SOA1, (b) SOA2.

**6. Conclusions** 

experimental demonstration.

**7. Acknowledgments** 

**8. References** 

in the paper for comparisons.

achieve a potential higher switch speed.

no.11, pp. 2297-2306, ISSN 0018-9197.

Normalized gain (a.u.)

It should be mentioned that, SOAs can be cascaded directly without any filter between them as presented in (Marcenac & Mecozzi, 1997), where it requires ten SOAs to achieve a speed of 110 Gb/s, whereas our simulation shows that the same operation speed can be achieved with a three-SOA-switch if a filter is implemented, as indicated in Fig.14.

Fig. 14. Gain recovery time, overshoot level, and noise figure as a function of the number of the SOA stages.
