Time-domain reflectometry techniques for optical communications

Abstract

Time Domain Reflectometry Techniques are applied to measurements in optical fibres. The study covers broadly two areas of interest. Firstly, the Backscatter Technique for attenuation measurements is considered. The resolution requirements of instrumentation are studied and a new 2-channel approach to backscatter waveform analysis is proposed. An optimum operating strategy is described such that the sensitivity and range are maximized. The fundamental accuracy of the backscatter measurement is determined by a comparison with the more-standard cutback technique over a wide spectral range and variations in OH- impurity along the length of a fibre are tracked by measurements at different wavelengths. The effects on the backscattered power of variations in longitudinal fibre parameters are also demonstrated. In particular, backscatter-loss signatures are presented which clearly show correlation with programmed fibre defects and indicate that in many cases diameter variations are the cause of the previously unidentified features in some backscatter waveforms. In addition, the backscatter method is used for the first time to track the state of polarization of a pulse propagating in a monomode fibre. The second application of Time Domain Reflectometry is to the assessment of pulse delay stability against variations in temperature and external stresses. Data from measurements on unjacketed fibres is used to analyse the time delay variations found in jacketed fibres. It is shown that the application of a close-fitting plastic jacket results in a level of residual compressive stress in the fibre and the delay stability is considerably degraded. This may also be influenced by environmental factors.

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