Fibre-optic distributed sensing based on spontaneous Brillouin scattering
This thesis reports on the use of spontaneous Brillouin scattering for the purpose of fibre-optic distributed temperature and strain sensing based on a time-domain Landau-Placzek ratio technique. Detection system specifications are dictated by the spatial resolution, range, measurand resolution and measurement time. Pulsed sources are used in these sensors. The minimum spatial resolution depends on both the pulse width and receiver bandwidth. The range and measurand resolution depend on the peak pulse power launched into the sensing fibre as well as the Brillouin signal-to-noise characteristics at the receiver. The maximum launched pulse power is limited by the onset of nonlinear effects in the sensing fibre. Novel interferometric techniques based on low-cost, low loss all fibre Mach-Zehnder interferometric optical filters needed to separate the backscattered Rayleigh and spontaneous Brillouin signals have been developed with enhanced signal-to-noise capabilities. Used in conjunction with a newly developed low noise optical preamplifier /transimpedance receiver system, a distributed temperature sensor having 1.8m spatial resolution, 6.3 °C temperature resolution and a range of 23km is demonstrated. The strain dependence of the spontaneous Brillouin intensity has been determined. This coefficient is crucial for the development of a distributed temperature only sensor and /or a combined distributed temperature and strain sensor. Pulsed narrowband and broadband sources are necessary for resolving the Rayleigh and Brillouin signals as well as reducing coherent Rayleigh noise. The latter has been investigated and its dependence on certain parameters confirmed. A source capable of switching between narrowband and broadband operation has been demonstrated and is particularly appropriate for extended periods of data collection cycles.