**3.2 Pattern-effect mitigation of turbo-switches**

As a result of the shorter gain/phase recovery time in turbo-switch, the pattern effect associated with the slow recovery is supposed to be mitigated. It should be noted that, along with the faster gain response of the turbo-switch, an overshoot in the gain/phase curve can also be clearly observed. However, the overshoot level can be controlled by adjusting the average input optical power to the SOA2. To verify the mitigation of the pattern effect, a variable optical attenuator (VOA) is experimentally applied before SOA2 in the turbo-switch configuration to optimize the output pattern of the turbo-switch (Giller et al., 2006c).

The simulation results of the turbo-switch gain dynamics under a single shot of 3 ps pump pulse are presented in Fig. 6, as a function of power levels before SOA2, which are in good agreement with the experimental results (see Fig. 6(b)) presented in (Giller et al., 2006c). In the simulation, the input CW power to SOA1 is 0 dBm and pump pulse energy is 100 fJ. It is shown that, when reducing the input power to SOA2, the recovery time becomes longer, and the level of the overshoot is lower. When the input power becomes low enough to make SOA2 unsaturated, the overall turbo-switch gain response will exhibit similar to that of a single SOA. So there is an optimum input power level for SOA2 in order to achieve the optimum effective recovery time at a specific data bit-rate.

single SOA SOA2 in TS 2 SOAs, no filter

TS

0 20 40 60 80 100 120

Time (ps)

Fig. 5. Normalized phase dynamics of a single SOA, the SOA2 in TS, 2 cascaded SOAs with

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

As a result of the shorter gain/phase recovery time in turbo-switch, the pattern effect associated with the slow recovery is supposed to be mitigated. It should be noted that, along with the faster gain response of the turbo-switch, an overshoot in the gain/phase curve can also be clearly observed. However, the overshoot level can be controlled by adjusting the average input optical power to the SOA2. To verify the mitigation of the pattern effect, a variable optical attenuator (VOA) is experimentally applied before SOA2 in the turbo-switch

The simulation results of the turbo-switch gain dynamics under a single shot of 3 ps pump pulse are presented in Fig. 6, as a function of power levels before SOA2, which are in good agreement with the experimental results (see Fig. 6(b)) presented in (Giller et al., 2006c). In the simulation, the input CW power to SOA1 is 0 dBm and pump pulse energy is 100 fJ. It is shown that, when reducing the input power to SOA2, the recovery time becomes longer, and the level of the overshoot is lower. When the input power becomes low enough to make SOA2 unsaturated, the overall turbo-switch gain response will exhibit similar to that of a single SOA. So there is an optimum input power level for SOA2 in order to achieve the

configuration to optimize the output pattern of the turbo-switch (Giller et al., 2006c).


double length of the active waveguide.

**3.2 Pattern-effect mitigation of turbo-switches** 

optimum effective recovery time at a specific data bit-rate.

no filter between them, and the TS. TS: turbo-switch.

0

0.5

Phase shift ()

1

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

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

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.

High-Speed All-Optical Switches Based on Cascaded SOAs 37

al., 2008). The DI utilizes the differential cross phase modulation (XPM) effect of the SOA to achieve the polarity-maintaining wavelength conversion. The PMF is used to introduce a delay between the TE / TM components of the probe, thus introducing a differential phase shift between the two orthogonal components. On the other hand, the polarizer acts as an interfering device to extract phase difference between the two components, which is the

Fig. 8(b) presents the 160 Gb/s wavelength-converted output trace and the corresponding eye diagram. The average powers of the CW and the pump are 10 and 3 dBm respectively, while the wavelengths of the CW and pump are 1560 and 1550 nm respectively. The input PRBS data has a length of 27-1. The PMF gives a differential delay of 2 ps. It is shown that the turbo-switch configuration expedites the recovery of the intensity and phase, which helps to mitigate the patterning of the output. The clearly opening eye diagram of the output shows the feasibility of the wavelength conversion at 160 Gb/s. More specifically, the well-known Q factor, defined for instance in (Agrawal, 2002), for the output signal is 6.8,

The wavelength conversion incorporating a turbo-switch was experimentally verified using the setup shown in Fig. 9(a), at ~85 and 170 Gb/s. The wavelength converter had the configuration of the DI. In a DI, CW light is amplitude and phase modulated in SOA1 by the action of the data pulse stream, and is then split into 'fast' and 'slow' components that travel along the two axes of a length of PM fiber. The two components experience a differential

polarization rotation when they interfere at the polarizer, and hence switching of the CW beam occurs, with a non-inverted output. The wavelength-converted output was de-

> 10.7 GHz 3ps TMLL MZM

10.7 GHz

**(a) (b)**

5nm Filter

Polariser

MZM MZM

V<sup>π</sup> 2V<sup>π</sup>

Fig. 9. 170 Gb/s wavelength conversion using DI configuration incorporating a turboswitch. (a). The setup; (b). BER curves demultiplexed to 42.6 Gb/s for back-to-back and

**Demultiplexer**

SOA2

**Wavelength Converter**

SOA2

42.6GHz Power Meter

wavelength-converted signals at 85 and 170 Gb/s (Manning et al., 2006).

3ps TMLL laser MZMPower Meter

*t* (3ps, in our experiment). The phase difference between them results in a


> -40 -35 -30 -25 -20 Received Power (dBm)

Back to Back 170 Gbit/s 85 Gbit/s


Log(BER)

wavelength-converted output.

delay,

85/170Gb/s 2 -1 PRBS <sup>7</sup>

CW Laser

BER Rx

which corresponds to the bit error rate (BER) of 6.9×10-12.

**4.1.2 Experiment of AND gate based on turbo-switch** 

multiplexed down to 42.6 Gb/s using MZ modulators.

10.7 Gbit/s

PM Fibre

SOA1

3ps CW laser

4 nm Filter

Multiplexer

SOA1

EDFA

Consequently, there is an optimum optical power level that could mitigate the pattern effect at a specific bit-rate. For instance, in this case, the optimum power to SOA2 should be -11 dBm, as shown in Fig. 7(b), where the pattern effect is mitigated.

On the contrary, the simulated modulation curves in Fig. 7(a) and 7(c) both experience a worse trend (constantly lower or higher) under a sequence of consecutive marks (ones) or spaces (zeros). This implies that, the unsaturated (-26 dBm), saturated (-11 dBm) and oversaturated (14 dBm) input power level to SOA2 has an important impact on the overall performance of turbo-switch. For instance, in the case of the modulated CW power of -26 dBm, the SOA2 cannot be saturated, and the overall recovery time is not shortened, turboswitch behaviors similarly to a single SOA, as shown in Fig. 7(a).
