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

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 delay, *t* (3ps, in our experiment). The phase difference between them results in a 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 demultiplexed down to 42.6 Gb/s using MZ modulators.

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

High-Speed All-Optical Switches Based on Cascaded SOAs 39

between the TE and TM probe pulses before entering SOA2 where now the induced radian phase shift affects only the TM pulse. The third PM fiber, with differential delay, *Δt*, resynchronizes the TE and TM probe pulses in time. A -radian phase shift between TE and TM pulses gives a polarization rotation of /2 when they recombine at the polarizer, which

When both of the control pulses A and B are present, the nonlinear phase difference between TE and TM will be zero (first , then -), the same result as the case when both A and B are absent. In the cases of either A or B alone being present, the phase shift will be ±π. The system is biased OFF (no output) in the absence of the control pulses. A pulse is generated after the polarizer only when one of A and B is present. Thus the operation of the device satisfies the XOR logic truth table as shown in Table 2. As with the conventional UNI gate, the probe pulses may be replaced by a continuous wave (CW) beam, in which case the

The transmission of control pulse A is blocked by a filter (not shown) placed before SOA2. SOA1 and SOA2 are therefore configured as a turbo-switch (Manning et al., 2006) and the effective switching speed by control pulse A is enhanced. The addition of a third SOA (also after a filter) to the original dual ultrafast nonlinear interferometer XOR gate (DUX) forms a second turbo-switch that similarly enhances the speed of switching by control pulse B.

An experiment was carried out to evaluate the performance the proposed XOR scheme at 85 Gb/s, whose experimental setup is shown in Fig. 11. A CW laser with a wavelength of 1552 nm was employed as the probe beam instead of a pulse train, so the first PM fiber in Fig. 10 was not required. The 3 ps, 1557 nm control pulses A and B were obtained from a 10.645 GHz mode-locked laser The control pulse stream was optically modulated with a 27-1 pseudo-random bit sequence (PRBS) and the pulses were passively multiplexed to 85 Gb/s before being injected into the SOAs 1 and 2. An optical delay-line was used to present different parts of the sequence to each SOA. Two variable optical attenuators (VOAs) were employed to adjust the control pulse energies. Another VOA was used to optimize the input

All the three (Kamelian) SOAs were biased at 400mA, where their unsaturated gain was greater than 30 dB. The differential delays of PM fibers were 11.5 ps (2*Δt*) and 5.75 ps (*Δt*) respectively, where *Δt* is one half of the bit period at 85 Gb/s. 5 nm band-pass filters blocked the control pulses and allowed the propagation of the probe beam. The polarization controllers (PC) in front of each PM fiber were adjusted to launch approximately equal amplitudes into the TE and TM modes. The two polarization states were also aligned with the TE and TM modes of the active layer at the input to SOAs 1 and 2 with further PCs. This

Data A Data B XOR 0 0 0 0 1 1 1 0 1 1 1 0

is crossed with respect to the un-rotated probe.

output takes the form of pulses of width *Δt*.

**4.2.2 Experiment of XOR gate based on turbo-switch** 

was to prevent the control pulses causing polarization rotation.

power of the probe beam injected to SOA2.

Table 2. Truth table of XOR gate

Fig. 9(b) shows the BER curves for the 42.6 Gb/s channels for wavelength conversion at 85 and 170 Gb/s. We observed no power penalty at 85 Gb/s, and a 3 dB penalty at 170 Gb/s, which we believe was due to the pulse width of the converted channels of 3 ps being slightly too long for 170 Gb/s data, and implied that our differential delay *t* is non-optimal. The measured OSNR was 40 dB, referred to a 0.1 nm noise bandwidth.

Fig. 10. Principle of dual-UNI XOR logic gate (PM: polarization maintaining fiber; PC: polarization controller)
