All-Optical Switching in a Fibre Nonlinear Mach-Zehnder Interferometer

All-optical switching is of increasing interest for ultrafast transmission systems and logic applications. Silica fibres are particularly attractive for fabrication of switching devices since the nonlinearity is ultrafast and losses are low. A number of fibre interferometric devices with long fibre lengths have been investigated in order to minimise the switching power. A major constraint is the stability of the device against external perturbations. To date the nonlinear fibre loop mirror (NOLM) is the only interferometer to show insensitivity to external perturbations[1]. However, as the NOLM is a two-port device it is necessary to use additional optical elements e.g. isolators, to improve its functionality[2-3]. In contrast the nonlinear Mach-Zehnder interferometer (NLMZ) is a simple four-port device and can be directly concatenated for all-optical switching/logic applications. However, in the NLMZs reported to date instabilities occurred having time constants of the order of milliseconds[4] and active stabilisation was necessary to maintain the linear operating point. Also, the temporal overlap was not an issue as a CW signal was switched using control pulses