Distributed optical-fibre sensors for monitoring mechanical strain and disturbance

Abstract

This thesis reports work carried out to develop an optical sensor capable of detecting disturbance of optical fibre cables, for application in protection of telecommunications infrastructure. Initially, three types of sensor were investigated, however after preliminary experimental and theoretical work it was decided to concentrate on a novel coherent-optical time-domain reflectometer (C-OTDR).

The operation of the C-OTDR has been investigated theoretically, using various models to describe coherent scattering in the optical fibre. The assumptions used in the models were applied to a numerical simulation of coherent scattering, obtaining results in good agreement with theoretical predictions and experimental measurements. Having developed a clear explanation of the sensor's operation, a detailed noise analysis is carried out, enabling the limiting factors to be identified and minimised.

The design and development of a portable sensor system is then described, and the results from a number of field trials are presented. Seeking to explain unexpected observations during these trials, a detailed experimental and theoretical analysis of the limits due to optical non-linearity was carried out, showing that the pulse power is limited by the need to prevent spectral broadening due to self-phase modulation, which adds noise to the sensor output.

Applying the findings of these investigations, further field trials and laboratory tests were carried out with improved experimental C-OTDR sensors. By optimising the pulse power and carefully controlling the noise sources, it has been possible to demonstrate operation at longer ranges, with superior spatial resolution, than has been reported for other sensors of this type.

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