Long-distance intrusion sensor based on phase sensitivity optical time domain reflectometry

The influences of stimulated Brillouin scattering (SBS) on the sensing range of the phase sensitivity optical time domain reflectometry (φ-OTDR) was investigated and the limiting sensing range under different detection sensitivities of photodiodes was proposed. As the SBS threshold decreases along with the increasing of the fiber length, a novel scheme was presented to extend the sensing range. Circulators were employed in the scheme to divide the sensing fiber into several sections and the Rayleigh backscattering of different sections were detected by photodiodes, respectively. As a result, Stokes radiation of different sections would not affect each other such that the SBS become more difficult to build up. Therefore, the power injected into the sensing fiber increased and much longer sensing range was realized. In laboratory tests, three circulators were used to divide the sensing fiber into 3 sections, and 66.92 km sensing range was realized. The sensing range can be extend further by adding more circulators.