Distributed optical sensing

An optical sensing system comprising an optical sensing fibre having at least one optical sensor 17. Optical components of a sensor consist of only optical fibre with a sensor utilising mechanical enhancement of phase sensitivity to the measurand of interest. The system further comprises an optical source 13 and an optical receiver 14, each being optically coupled to the sensing fibre. A sensor is interrogated by the optical source transmitting a pair of optical pulses into the sensing fibre; and the optical receiver receives a returning composite optical signal, which is sampled and comprises light scattered from at least a region of a sensor. A sensor may be positioned within an optical sensor body comprising a plurality of serially connected optical. A sensor may be sensitive to acceleration in a different direction. The pair of optical pulses may be the same or different in frequency. Polarisation of each pulse may be different. Time between adjacent pulses may be greater than the optical transit time of the pulses through a sensor. Sample rate may be improved by using a plurality of pairs of pulses where the pulses are of a plurality of wavelengths. Optical fibre between sensors may also be interrogated.