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Fiber optic intrusion sensing based on coherent optical time domain reflectometry

The Raileigh back scattering of a highly coherent laser pulse in a single mode optical fiber has been simulated. An array of retro reflectors was used to simulate a scattering medium. Both amplitude and phase of the reflected waves were random. It has been shown that the reflectometry signal is not smooth (exponential) as in a usual non-coherent reflectometer. About 100 percent contrast in a chaotic signal is achievable provided the laser has single frequency, i.e. generation spectrum is limited by pulse duration. This chaotic signal nevertheless is stable in time if phase changes in the sensing fiber are negligible. Any mechanical impact leads to a phase shift in the sensing fiber. Due to local interference scattering signal changes, the location of impact can be easily calculated knowing light velocity in the sensing fiber. In our model, a harmonic in time mechanical influence was applied to the fiber pointwise. The sensitivity in different points of the fiber is quite random, and the same low amplitude impact may be transformed to a positive, negative or nearly zero signal. The reasons of such behavior are discussed as well as the methods to enhance the system sensitivity. The results of computer simulations were compared with experimental data and satisfactory conformity was obtained. In conclusion, the coherent optical time domain reflectometry seems to be very prospective for development of intrusion sensing systems of high (interference) sensitivity, precise impact localization, low error rate and low cost.