Low Noise Φ-OTDR Based on Frequency-Jitter-Free Heterodyne Demodulation Scheme

In this paper, a low noise Φ-OTDR is proposed which utilizes frequency-jitter-free heterodyne modulation scheme to retrieve the phase variations of Rayleigh backscattered light-waves along a sensing fiber. The heterodyne modulation scheme employs only one acousto-optic modulator (AOM), and thus is immune to the jitter of frequency shift of an AOM. Benefiting from that, the noise level of a Φ-OTDR is reduced. Multiple vibration events occurring simultaneously at different positions along the fiber are experimental restored by the proposed Φ-OTDR. The signal-to-noise ratio of 57.78 dB and the noise level as low as -65.4 dB (re 1rad/sqrt(Hz)) at 1 kHz are achieved.