Distributed optical fibre temperature sensor using spread-spectrum techniques

The authors describe a Raman-based distributed temperature sensor which uses spread-spectrum pseudorandom sequences to obtain high-resolution, rapid-response measurements of temperature. The sensor operates by launching a semiconductor laser beam modulated with a pseudorandom bit sequence (PRBS) into an optical fibre where the average launch power into the fibre was 5 mW. The backscattered light is detected via a fibre coupler which was built with 1 km pigtail to prevent unwanted reflections. The optical signal is then filtered using a Fabry-Perot stack to measure the Stokes and the anti-Stokes Raman lines and detected using an avalanche photodiode transimpedance amplifier combination. The output signal is then multiplied by a delayed version of the pseudorandom signal, which because of the autocorrelation function allows the spatial information to be obtained without ambiguity.