**3. Simulation results**

The parameters used in our model are list in Table I. Two identical SOAs are applied in all the turbo-switch simulation, as implemented in the reported experiments. The SOAs are 0.7 mm long, which have a relatively high gain and short carrier lifetime, as well as an acceptable noise figure. A 200 mA bias current is consistently used unless specifically described. A 100% of the injected current utilization is supposed in the model. In the following simulations, the input CW and pump pulse are at wavelengths of 1560 and 1550 nm respectively, and the pulsewidth is 3 ps (FWHM) if not otherwise specified.

A steady state numerical algorithm presented in (Connelly, 2001) is used to obtain the SOA gain saturation characteristics, as illustrated in Fig. 3, where the wavelength of the CW input beam is 1560 nm. It is shown that, the small-signal gain of the amplifier is 25 dB, while the saturation output power is 12 dBm.

High-Speed All-Optical Switches Based on Cascaded SOAs 33

The gain dynamics of a single SOA and turbo-switch is plotted in Fig. 4. The input CW

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

single SOA SOA2 in TS 2 SOAs, no filter

TS

Time (ps)

An obvious reduction of the gain recovery time is shown in the turbo-switch gain curve, comparing to the single SOA case, from about 100 ps to 20 ps, which is four times shorter than a single SOA. The simulation result is consistent with the corresponding experimental results presented in (Giller et al., 2006a). To get a better understand of the operating mechanism of turbo-switch, it is essential to know the gain response of SOA2, as plotted in Fig. 4. It is shown that, the gain curve of SOA2 has a completely different dynamics if compared with a single SOA. The gain of SOA2 increases firstly as the decrease of modulated CW input, and then starts to fall slowly back to the initial gain level. As a consequence, the slow recovery tail of the single SOA is somehow compensated, thus making the overall gain recovery of turbo-switch several times faster than that of a single SOA mechanism of turbo-switch, it is essential to know the gain response of SOA2, as plotted in Fig. 4. It is shown that, the gain curve of SOA2 has a completely different dynamics if compared with a single SOA. The gain of SOA2 increases firstly as the decrease of modulated CW input, and then starts to fall slowly back to the initial gain level. As a consequence, the slow recovery tail of the single SOA is somehow compensated, thus making the overall gain recovery of turbo-switch several times faster than

On the other hand, the phase dynamics curves are plotted in Fig. 5. It is shown that, turboswitch also reduces the phase full recovery time from 100 ps to ~20 ps, about four times shorter than the case of a single SOA. It should be noted that the ultrafast effect of the SOA has much less impact on the phase change (Giller et al., 2006b), thus the phase recovery is mainly attributed to the inter-band processes, which makes it slightly different from the gain curve. To summarize, the turbo-switch scheme has shortened the overall gain/phase response time to a large scale compared with the case of a single SOA and has the capability

of improving the overall operation speed of the switch to higher bit-rates.

Fig. 4. Normalized gain of a single SOA, the SOA2 in TS, 2 cascaded SOAs with no filter

**3.1 Gain and phase dynamics of turbo-switches** 

0.5

between them, and the TS. TS: turbo-switch.

1

Normalized gain (a.u.)

that of a single SOA.

1.5

power is 0 dBm, while the pump pulse energy (single shot) is 100 fJ.


Table 1. SOA parameters used in the simulation

Fig. 3. Gain as a function of output power of a single SOA.
