Polarisation optical-time-domain reflectometry : experimental results and application to loss and birefringence measurements in single-mode optical fibres

Abstract

Optical time-domain reflectometry is now a well established means of examining the length-dependence of attenuation in multimode optical fibres. Theoretical work has shown that the technique is similarly applicable to monomode fibres and experimental results have lately appeared in the literature. It has recently been noted that in monomode fibres the scatter return contains additional information concerning the local state of polarisation (SOP) in the fibre, since at any point the scattered light mirrors the SOP at that position. A polarisation optical time-domain reflectometry technique (POTDR) has thus been proposed, in which the backscattered light is analysed by means of a polariser to reveal the variation of polarisation along a mononode fibre. Such a technique would be invaluable in the study of the properties of polarisation-maintaining fibres for sensor applications (e.g. gyroscopes, hydrophones) and of low-birefringence fibres for the Faraday Current Monitor and telecommunications. In the latter case, information could be obtained relating to the polarisation dispersion in the fibre and to the variation of the SOP with time, an effect which is expected to produce problems when interfacing to integrated-optics devices. We present here the first measurement to be made using POTDR. Our results clearly establish the feasibility of the technique and, furthermore, show that polarisation effects can cause substantial errors in conventional backscatter attenuation measurements on monomode fibres. Consequently, care must be taken to eliminate the polarisation sensitivity of the measurement and caution exercised in the interpretation of backscatter curves