Distributed optical fibre sensing system based on Brillouin scattering

Abstract

Fiber optic sensing systems offer many advantages comparing with it electric counterpart such as small size, light weight, free from electromagnetic interference, easy to be multiplexed and ability of remote sensing over long distances. Systems based on the Brillouin optical fibre time domain analysis (BOTDA) technique enable simultaneous distributed strain and temperature measurement over a moderate range with high spatial resolution. They have found applications in many different areas. The performance limitation of current BOTDA systems is addressed in this thesis. The factors limiting both the spatial resolution and the sensing range are analyzed. A scheme for measuring acoustic lifetime of different type of fibres has been proposed and results obtained. Coding technique and phase modulation are shown to be effective to enhance the signal to noise ratio of a BOTDA system and enable the realization of distributed sensing with high spatial resolution. The impact of optical pulse coding techniques on phase modulated Brillouin optical time domain analysis (BOTDA) systems is evaluated theoretically and experimentally. Theoretical and experimental studies have been carried out to analyze and compare the performance of BOTDA systems employing simplex coded and complementary coded pump pulses. Results obtained through the study show that both schemes demonstrate significant enhancement to SNR performance of the BOTDA systems. Key factors affecting the performance of a distributed sensing system with complementary code are identified. Using complementary coded pump pulses with pulse duration of 0.1ns, strain sensing over 20m of single mode fiber with 1 cm spatial resolution has been realized.