Abstract: A Sagnac interferometer (19) is defined by four port, 50:50 optical fiber coupler (20), two output ports (38, 40) of which are coupled together by a non-linear semiconductor laser amplifier (3) in series between a pair of optical fibres (22, 24). The coupling losses between the ports (38, 40) and the amplifier (30) are different. An input pulse coupled to port (34) of the coupler (20) is split into two portions which counterpropagate from one of the output ports (38, 40) to the other. The portions return to the coupler (20) with an intensity dependent relative phase shift. This provides switching of an input pulse at the input port (34) between the input ports (34 and 36) dependant on the intensity of the input pulse. The device can provide intensity dependent switching at optical power levels of less than 250 .mu.W.