The application of Brillouin scattering to distributed fibre optic sensing
This thesis reports on an investigation into the application of Brillouin scattering for the purpose of distributed fibre optic sensing. The main focus of the work has been centred on a Brillouin optical time domain reflectometer (BOTDR) system. The behaviour of short (3m), medium (60m) and long (6km) erbium doped fibre amplifiers using narrow bandwidth pulsed signals has been investigated and the most suitable configuration for the source requirement of the BOTDR identified. The operation of a (BOTDR) system has been demonstrated at the low loss window of 1.5µm wavelength. Multiple Stokes orders of stimulated Brillouin scattering in a medium length erbium doped fibre amplifier using pulsed excitation are reported. The observed stimulated Brillouin threshold power is significantly reduced as a result of optical gain. The points of origin within the fibre of the generated Stokes pulses are located using space-time diagrams and are observed to depend on the Brillouin and Erbium pump powers. This has therefore been identified as a possible mechanism for sensing applications utilizing the novel technique of varying the pump powers to spatially interrogate the fibre. Measurements of the Brillouin scattering coherence length in silica fibre using a fibre Mach Zehnder interferometer are presented. As the Brillouin pump power is increased from below to above stimulated threshold, the line shape narrows and changes from that of a Lorentzian to a Gaussian. It is also shown that the Brillouin bandwidth approaches a limiting value. It is shown experimentally that the ratio of the intensities of Rayleigh and Brillouin backscattered light (Landau Placzek ratio) in an optical fibre has a temperature dependence which may be used for the basis of a distributed temperature sensor. This result, combined with the known frequency dependence of the Brillouin backscattering on temperature and strain, indicates spontaneous Brillouin backscatter may be used for the unique determination of either temperature or strain in a distributed fibre optic sensing system. Because of the coherent nature of Rayleigh scattering, use of the same narrow bandwidth source as required for the Brillouin signal in the Landau Placzek ratio method, results in significant coherent noise in the Rayleigh signal. A novel technique is demonstrated whereby the amplified spontaneous emission noise and amplifying properties of an erbium doped fibre amplifier may be exploited to reduce the coherent noise on the Rayleigh backscatter signal. This results in a significant improvement in both temperature and spatial resolution.