A position-detectable optical distributed vibration sensor using an additional sub-loop

The paper proposes a simple distributed vibration sensor system which employs an optical fibre ring interferometer having an additional sub-loop coupled to the main ring by a 3 dB coupler. Comparing multiple responses of interfered beams by the light travelling n times in the sub-loop with each other, the proposed system can detect the position as well as the amplitude of acoustic vibration occurred in the main ring. The multiple responses are easily extracted in the time domain by driving an optical source with a pulsed current. It is demonstrated theoretically and experimentally that the proposed system is able to detect both the amplitude and position of the acoustic vibration occurring in the main ring with reasonable accuracy.