A new type of stress location sensor is discussed, in which the frequency-modulated continuous wave (FMCW) technique is used to detect the difference in propagation time between two optical paths in an optical fibre interferometer. The interferometer is formed by the polarization modes in a birefringent fibre, and the location of stress experienced by the fibre may be measured to within + 1-2 m. A variation of the interferometer using the LPo 0 1 and LPll mode-sets in a step-index fibre is investigated, and it is found that dispersion within the LP,, mode-set limits the accuracy of the system.

A new type of stress location sensor is discussed in which the FMÇW technique is used to detect the difference in propagation time between two optical paths in an optical fiber due to stress induced modal coupling.Two versions of the system are included, and experimental results are presented for each system.