Distributed fibre measurement using backscatter polarimetry

Fully-distributed optical-fibre sensing (FDOFS) systems are developing rapidly and are offering significant advantages for measurement functions in a variety of structural applications. 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 presents a new technique for providing this information, and thus for measuring the distribution of any parameter, external to the fibre, which can modify its polarization behaviour. As a result, 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. Distributions of electric, magnetic and acoustic fields may also be measured. The technique comprises an extension of polarization-optical time domain reflectometry (POTDR), and necessitates on-line processing. It holds promise for evolution into a generic method for distributed measurement over a wide range of measurands and application areas. Details of the physical principles, the algorithms and the polarimetry are presented, together with results illustrating the measurement accuracies which can be achieved.