An optical switch comprises an optical fibre Sagnac loop interferometer (2) having an input port (10) and an output port (12). A source of optical signals (26) at 1.53 mu m is coupled to the input port (10) and a source of control optical signals (18, 20) of wavelengths 1.30 mu m and 1.31 mu m are coupled to the interferometer (2) by a dichroic coupler (8). The coupler (8) splits the 1.53 mu mn input optical signal into two signal portions which counterpropagate round the loop (6) and couples the control signal to propagate in one direction only round the loop (6). The phase difference between the signal portions at the output port (12) is dependent on the intensity of the control optical signal when they recombine at the coupler (8). The optical control signals are formed by combining at least two optical pulse streams, the pulses of distinct optical pulse streams being non-interferometric, to provide a logical AND gate.