Optical sensor system

An optical sensor system has at least two fiber Bragg grating (FBG) sensors 1a, 1b. An optical de-multiplexing system 21, e.g. a series chain of drop de-multiplexers 21a, 21b, routes light to associated receivers. Each receiver comprises a detector 31a, 31b and a slope filter 30a, 30b, which may have a pass-band corresponding to the drop band 25a, 25b of the de-multiplexers. The filters' wavelength gain function 32a, 32b attenuates according to wavelength, allowing identification of the light reflected by the FBG sensors. A light source 10 e.g. a LED or laser diode may be used. The system may be used to interrogate multiple sensors for e.g. aircraft ventilation monitoring or high frequency motor synchro feedback and control (figs 4-6). An interrogation fibre 9 and first circulator 13 may be used. Also disclosed is a system comprising a beam splitter forming a reference and measurement beam from a fibre grating.