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

32 Selected Topics on Optical Amplifiers in Present Scenario

Symbol Description Value *L* Length of active region 0.7 mm

Cross section area of active region 0.2 m2

 Confinement factor 0.45 *I* Injected / Bias current of the SOA 200 mA *A* Linear recombination coef. 21×108 /s *B* Bimolecular recombination coef. 10×10-16 m3/s *C* Auger recombination coef. 35×10-41 m6/s *vg* Group velocity in the active region 8.5×107 m/s *N0* Transparency carrier density 0.65×1024 /m3 *a0* Differential gain 3.13×10-20 m2

*a* Gain model coef. 1.2

*b1* Gain model coef. 0

*b0* Gain model coef. 3.17×10-32 m4

*p0* Wavelength at peak 1575 nm

*z0* Wavelength at transparency 1625 nm *z0* Gain model coef. -2.5×10-33 m4

*CH* Temperature relaxation time 700×10-15 s

*SHB* Carrier-carrier scattering time 70×10-15 s

*CH* Gain compression factor due to CH 1×10-23 m3

*ase* Equivalent SE coupling factor 3.65×10-4

*ase* Equivalent ASE wavelength 1550 nm




Output power (dBm)

*int* Internal loss 5000

*SHB* Gain compression factor due to SHB 0.5×10-23 m3

*N*

*T* Table 1. SOA parameters used in the simulation

0

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

5

10

15

Gain (dB)

20

25

30

The gain dynamics of a single SOA and turbo-switch is plotted in Fig. 4. The input CW power is 0 dBm, while the pump pulse energy (single shot) is 100 fJ.

Fig. 4. Normalized gain of a single SOA, the SOA2 in TS, 2 cascaded SOAs with no filter between them, and the TS. TS: turbo-switch.

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 that of a single SOA.

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.

High-Speed All-Optical Switches Based on Cascaded SOAs 35

Fig. 6. Normalized gain dynamics of turbo-switch with a VOA before SOA2. (a).Simulation, where the optical power input to SOA2 varies from 14 to -26 dBm, by -4 dB each step. (b). Experimental result, where the optical power input to SOA2 varies from 5.8 to -10.2 dBm

To show the pattern effect of turbo-switch, the output patterns of a CW probe beam modulated by a 40 Gb/s PRBS pump pulse train are presented in Fig. 7(a)-(c), where three different input power levels are chosen before SOA2: -26, -11, and 14 dBm. The input CW

Fig. 7. Modulated CW output patterns from the TS, with a PRBS pump pulse train at 40 Gb/s. (a)-(c) Simulation, where, the optical power input to SOA2 in simulation are -26, -11 and 14 dBm respectively from top to bottom. (d)-(f). Experimental results (Giller et al., 2006c).

It is shown in Fig. 7 that, the simulation results are in good agreement with the experimental measurements (see Fig. 7(d)-(f)) presented in (Giller et al., 2006c) for all three cases.

by a step of -2 dB (Giller et al., 2006c).

power before SOA1 is 0 dBm and the pump pulse energy is 2 fJ.

Fig. 5. Normalized phase dynamics of a single SOA, the SOA2 in TS, 2 cascaded SOAs with no filter between them, and the TS. TS: turbo-switch.

Moreover, our simulation shows that filtering the pump pulses before the SOA2 in turboswitch scheme does further reduce the gain/phase recovery time, when compared to the case of no filter between two SOAs (dash-dotted gain/phase curves in Fig. 4), as presented in (Marcenas et al., 1995). In the latter case, the pump was entered into two cascaded SOAs along with the probe CW beam, which can also be regarded as a single long SOA with a double length of the active waveguide.
