Distributed Measurement of Strain using Optical‐fibre Backscatter Polarimetry

Abstract

Abstract: Fully‐distributed optical‐fibre sensing (FDOFS) systems are developing rapidly and are offering significant advantages for measurement functions in a variety of structural applications, especially in the oil industry, the power supply industry, the aerospace industries and civil engineering construction.Polarization techniques are well established in FDOFS, and in the analysis of polarization‐mode dispersion (PMD) for optical‐fibre telecommunications. However, a major problem has been that of determining, from one end of the fibre, the distribution of the full polarization properties of a monomode optical fibre, along its length, with some specific spatial resolution.This paper will present a new technique for providing this full information, and thus for measuring the distribution of any parameter, external to the fibre, which can modify its polarization behaviour. As a result, for example, it becomes possible to measure simultaneously the distributions of temperature and of a strain field comprising, in the latter case, the longitudinal and the two transverse components of direct strain, plus the transverse shear strain.The technique comprises an extension of polarization‐optical time domain reflectometry (POTDR) [16], and necessitates on‐line processing. Details of the physical principles, the algorithms and the polarimetry will be presented, together with results illustrating the measurement accuracies which can be achieved.