Optical Switching using the Nonlinear Optical Loop Mirror

Since the proposal of the nonlinear optical loop mirror (NOLM), or the Sagnac interferometer, about six years ago [1] and its first experimental demonstration [2], there has been a very rapid progress in the field of all-optical switching using the NOLM. The reason for this is the development of erbium doped fiber amplifiers (EDFA), which allow the signals to reach power levels required for performing switching based on fiber nonlinearities. The main incentive for studying the NOLM, as well as other fiber based switches that rely on the third order nonlinear susceptibility, is that the intrinsic nonlinear response time is extremely fast. The major application of the NOLM which has been studied so far is the all-optical demultiplexer (or a logic AND gate), but other possible applications include: exclusive-OR gates [3], all-optical sampling [4], loop memories [5], and all-optical data regeneration [6]. Apart from the NOLM, two other all-optical techniques for demultiplexing high-speed data using fiber nonlinearities have been demonstrated. In the soliton dragging gate [7], the timing of the data pulses is controlled by a gating pulse, i.e. demultiplexing is obtained by moving the data in and out of a prescribed bit slot. In the third optical demultiplexer, which is based on optical four-wave mixing [8], the gating pulses create new frequency components,one of which is filtered out to give the demultiplexed data. While most early NOLM experiments were been made at relatively low pulse repetition rates and with high power solid state lasers, recent experiments have shown the feasibility of using the NOLM with semiconductor lasers for demultiplexing ultrahigh speed (> 30 Gb/s) data [9-12]. This is important, since the NOLM is not likely to be implemented at lower speeds where standard electronic circuits are capable of handling the data.