Improving the response linearity of a fibre phase-OTDR by averaging responses over a set of fibres in a fibre cable

A method is proposed for improving the response linearity of a fibre phase-OTDR to external perturbations. The method is based on the scheme of connecting sensitive fibres in a multifibre cable in the form of a chain. This connection makes it possible to average the responses of various spatially separated sections of an optical fibre subject to the same external perturbation. The resulting averaged response contains less nonlinear distortions, i.e. it is linearised with respect to external impacts. Three series of experiments are performed. To evaluate the efficiency of linearisation, the nonlinear distortion coefficient is calculated for single responses and an average response. In the experiment with harmonic acoustic impact on a fibre-optic cable, the decrease in the nonlinear distortion coefficient is 12 dB on average, and in the experiment with harmonic seismic impact on a cable buried in the ground, it amounts to 11 dB. For the experiment with a pulsed impact by a sledgehammer, a decrease in the effect of phase jumps on the resulting signal is observed. In all cases, the averaging of responses results in an increase in the signal-to-noise ratio by 7 – 8 dB.