Polarization effect on optical xor performance based on four-wave mixing

Polarization sensitivity of an optical exclusive or (xor) gate based on four-wave mixing (FWM) in highly nonlinear fiber is investigated. FWM is a polarization-sensitive process, and the optical xor gate can be built using this polarization-sensitive characteristic. The FWM-based optical xor does not require the input signals to have narrow pulsewidths, and works well for nonreturn-to-zero (NRZ) signals. We experimentally demonstrate the xor of two NRZ polarization-modulated (PolM) signals using a 35-cm highly nonlinear bismuth oxide fiber, and analyze the effect of polarization mismatch on the xor output. The bit-error rate is measured with different polarization settings between the two inputs, and a power penalty of only 0.5 dB results from 10° of polarization misalignment. The polarization modulation depth accuracy of the optical xor inputs are found to be important to avoid uncertainty in the xor logic levels. The difference in the xor logic level cannot prevent the xor input logic being visible from the output, which is not favorable in certain xor applications.