**4.2.1 Principle of all-optical XOR gate**

All-optical XOR logic is regarded as one of the fundamental logic gates in signal processing, which plays an important role in applications such as bit pattern recognition (Webb et al., 2009), pseudorandom bit sequence (PRBS) generation, parity checking and optical computing. A high-speed all-optical XOR gate has potential applications for on-the-fly digital serial processing of optical signals, for example, in packet header recognition, error detection and coding/decoding. In addition, the XOR function has been used recently as a wavelength converter and regenerator for signals in a differential phase shift keying (DPSK) format (Sartorius et al., 2006; Kang et al., 2005).

The scheme of the XOR logic gate is shown in Fig. 10. Two ultrafast nonlinear interferometer (UNI) elements are cascaded to allow two data pulse streams A and B to be input into SOAs 1 and 2 respectively as control pulses. The input probe pulses are launched into a polarization-maintaining (PM) fiber with equal intensities on the fast and slow fiber axes, which are coupled to the TM and TE axes of SOA1 respectively. As a result, the TE pulse lags the TM pulse by *Δt*. The control pulse A is introduced between the two probe pulse components before they are input into SOA1, in which it induces a π-radian phase shift experienced by the TE pulse alone. The probe pulses are then injected into another PM fiber with a differential delay of -2*Δt*. The fast and slow axes of this PM fiber are orthogonal to those of the first section, resulting in a reversal of the delay between TE and TM pulses so that the TE pulse is now *Δt* ahead of the TM pulse. The control pulse B is then introduced

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

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

Section 4.2.1 gives operation principle of the 85 Gb/s XOR gate, while the experimental results including eye diagram and the spectrum of the output are present in Section 4.2.2.

All-optical XOR logic is regarded as one of the fundamental logic gates in signal processing, which plays an important role in applications such as bit pattern recognition (Webb et al., 2009), pseudorandom bit sequence (PRBS) generation, parity checking and optical computing. A high-speed all-optical XOR gate has potential applications for on-the-fly digital serial processing of optical signals, for example, in packet header recognition, error detection and coding/decoding. In addition, the XOR function has been used recently as a wavelength converter and regenerator for signals in a differential phase shift keying (DPSK)

The scheme of the XOR logic gate is shown in Fig. 10. Two ultrafast nonlinear interferometer (UNI) elements are cascaded to allow two data pulse streams A and B to be input into SOAs 1 and 2 respectively as control pulses. The input probe pulses are launched into a polarization-maintaining (PM) fiber with equal intensities on the fast and slow fiber axes, which are coupled to the TM and TE axes of SOA1 respectively. As a result, the TE pulse lags the TM pulse by *Δt*. The control pulse A is introduced between the two probe pulse components before they are input into SOA1, in which it induces a π-radian phase shift experienced by the TE pulse alone. The probe pulses are then injected into another PM fiber with a differential delay of -2*Δt*. The fast and slow axes of this PM fiber are orthogonal to those of the first section, resulting in a reversal of the delay between TE and TM pulses so that the TE pulse is now *Δt* ahead of the TM pulse. The control pulse B is then introduced

*t* is non-optimal. The

too long for 170 Gb/s data, and implied that our differential delay

measured OSNR was 40 dB, referred to a 0.1 nm noise bandwidth.

polarization controller)

**4.2 High-speed XOR gate based on turbo-switch** 

**4.2.1 Principle of all-optical XOR gate** 

format (Sartorius et al., 2006; Kang et al., 2005).

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 is crossed with respect to the un-rotated probe.

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


Table 2. Truth table of XOR gate

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.
