**6. Conclusions**

42 Selected Topics on Optical Amplifiers in Present Scenario

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.

Recovery time Overshoot level Noise figure

<sup>2</sup> <sup>4</sup> <sup>6</sup> <sup>8</sup> <sup>10</sup> <sup>0</sup>

SOA stage

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

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

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

5.5

6

20

40

60

Overshoot level (%)

80

100

6.5

Noise figure (dB)

7

20

40

60

Recovery time (ps)

the SOA stages.

the turbo-switch.

**5.2 Impact of SOA injected currents** 

current of the SOA1 and SOA2 in turbo-switch scheme.

80

100

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 experimental demonstration.

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 achieve a potential higher switch speed.
