Optimization Of Long-range Brillouin Optical Time-domain Analysis

Distributed strain and temperature fiber-optic sensors based on Brillouin scattering in optical fibers can measure changes in both temperature and strain along the sensing fiber.This thesis focused on long range measurement based-BOTDA sensor. Nonlinear effects including modulation instability(MI), self-phase modulation(SPM) and cross-phase modulation(XPM) constituting the main limitation factors in long range measurements have been widely discussed. Simplex coding technique and differentialpulsewidth-pair Brillouin optical time domain analysis(DPP-BOTDA) technique are used to avoid the nonlinear effects and thus obtaining a good performance over long-range sensing fiber.Optical pulse coding uses sequences of short pulses, which are launched into the fiber in bursts at a low repetition rate fixed by the fiber length, as in a standard single-pulse scheme. The method requires a suitable decoding process based on a linear transformation to retrieve the single-pulse fiber response, which is obtained with an improved SNR. A measurement with a higher SNR offers the possibility to improve the spatial resolution as well as extending the sensing range and reducing the measurement time and uncertainty along the entire sensing fiber.Simplex-coding-based long-range BOTDA has been both theoretically and experimentally studied. It allows delivering the signal avoiding the potential distortions caused by nonlinear phenomena and consequently offers a good SNR quality. Distributed temperature and strain measurement using 25 km single mode- fiber(SMF) with 15-bit in return-to-zero modulation format has been conducted. One meter(1 m) spatial resolution with temperature and strain resolution of 0.52 oC and 10.83 με respectively has been demonstrated.DPP-BOTDA technique has also been implemented by achieving 0.5, 1 and 2 m spatial resolution over 50 km SMF. The influence of both the original pulsewidth as well as pulsewidth difference and that of the spatial resolution on the quality of the SNR are both analyzed. Optimized parameters have been established from experiment, where a 50-km SMF has been used to realize measurements with 0.5 m, 1 m and 2 m spatial resolution.