PURPOSE:To greatly improve the reproducibility of sensor sensitivity and to increase the number of sensing fibers which can be arrayed by detecting an arrival signal when the outputs of photodetectors reach the same level. CONSTITUTION:A circuit 18 applies a saw-tooth wave whose repetitive frequency is =2 times that of an acoustic signal to be detected to a light source to polarize light linearly, and then the light is made incident while aligned with the characteristic axis of a lead fiber 12 for polarization plane maintaining type optical transmission. A stationary phase difference between two characteristic modes due to a propagation speed difference is generated at the output terminal of a sensing fiber 13 of the same type which is welded to the fiber 12 so that the characteristic polarization optical axis is rotated by 45 deg.. Parallel light is obtained at the output terminal of a return lead fiber 24 which is welded to the fiber 13 so that the characteristic polarization axis is rotated by 45 deg. and a splitter 20 whose characteristic polarization axis is aligned with that of the fiber 242 splits components, which are converted by the photodetectors 15A and 15B photoelectrically. The time of the output coincidence between both elements is detected by a circuit 16 and the level of the saw-tooth wave is sampled 17 with a specific phase difference to obtain pulses of output levels proportional to a signal variation quantity. The pulses are demodulated 19 to regenerate the acoustic signal to be detected.
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