**5.2 Impact of SOA injected currents**

For all the figures presented before this subsection, the SOA currents are fixed at 200 mA. Since the pattern effect can be mitigated using a VOA before the SOA2, as shown in Fig. 6. One could consider that whether the variation of the SOA current level can introduce a similar effect as the VOA? The simulation is carried out by varying one of the injected current of the SOA1 and SOA2 in turbo-switch scheme.

As shown in Fig. 15(a) and 15(b), the current variation curves reveal a similar effect as that of Fig. 6, where the nonlinearity of the SOA2 is gradually diminishing as the current keep decreasing, which consequently results in the overall response of turbo-switch similar to that of a single SOA. However, an interesting phenomenon is that, the gain dynamic is quite different when the current of SOA1 is reduced, as shown in Fig. 15(a). In the latter case, even though an overshoot is obvious when the current level is high, the full recovery time is generally longer than the case of Fig. 15(b). Apart from that, the gain compression induced by the pump pulse is smaller as well, which will potentially affect the extinction ratio (ER) of the output signal. To summarize, it is better to set the current level of SOA1 high, while the current level of SOA2 can be employed to optimize the overall gain/phase recovery time of the turbo-switch.

Fig. 15. Normalized gain dynamics of turbo-switch, by varying the injected current level of (a) SOA1, (b) SOA2.
