Fibre optic distributed sensors for industrial applications

Distributed fibre optic sensors using modified Sagnac/Mach-Zehnder and Sagnac/Michelson interferometers are presented. These sensors locate the positions of perturbations applied to the fibre. They are intended for locating fire in power cables, leakage in cryogenic lines, and to guard the perimeter of industrial plants. The second interferometer that shares the fibre with the Sagnac interferometer and that has a different response to the perturbation, allows one to obtain the position of the disturbance, as well as its magnitude. The experimentally obtained resolution for a 100 m long sensor is better than 0.6 m for the first sensor, and 5 m in a 200 m loop for the second simpler configuration