Optical sensing systems.

摘要

V" A multiplex optical sensing system comprises an optical fibre 5 arranged to be subjected at the longesth length to fibre deforming forces during operation of the system and means 1, 2, 3 for producing a plurality of pulse trains of coherent light in which the frequency of frequencies of F1 + AF1, F2 + AF2, F3 + AF3 of the pulses in respective pulse trains lie within a different frequency channel for transmission along said optical fibre 5. The optical fibre 5 is provided along its length with a number of equally spaced discontinuities 6 to 14 which effectively divide the fibre 5 into a plurality of discrete fibre elements so that a small proportion of each light being pulse being transmitted along the fibre 5 will be reflected back along the fibre from each of the discontinuities 7 to 14 whereby, within any one of the pulse trains, each reflected light pulse after the first is superimposed upon either the previously reflected pulse from the preceding discontinuity or a reference light signal of the same frequency or a frequency with a constant difference frequency to the said transmitted light signal to produce an electrical signal in square law photo-detection means 15, the difference between respective electrical signals corresponding to successive fibre elements being dependent upon the length of the fibre elements so that changes in optical path length of these elements produced by the incidence of deforming forces will result in changes in the electrical signals which will be detected.