An optical fiber sensor assembly comprises a source of a plurality of different frequency substantially monochromatic signals (1, 2, 3, 4); a modulator connected to the output of the source to produce a train of output pulses of the monochromatic signals; a plurality of sensor sub-assemblies connected to the output of the modulator, each sub-assembly comprising an optical drop multiplexer (ODM) (13), a sensor array (14) comprising a plurality of sensor elements and an optical add multiplexer (OAM) (15); a wavelength demultiplexer (WDM), having an input coupled to the sub-assemblies output; a plurality of detectors, each detector having an input connected to receive a respective output of the WDM and providing at an output thereof a signal corresponding to a respective frequency of the modulated monochromatic signals; and an interrogation system, having a plurality of inputs connected such that each input receives the output signal from a respective detector.