Optical fibre instrumentation for environmental monitoring applications

Abstract

We report our research on the development of optical fibre trace gas sensors for environmental applications. We describe the operation of a 64-point fibre-optic methane sensor, which has been installed on a landfill site in Glasgow, UK, where methane is used for power generation as part of the current trend for renewable energy programmes. Although the environmental conditions are harsh, the sensor has performed satisfactorily, detecting methane in the range of ∼50 ppm to 100% methane. Another area of our current research is the application of erbium-doped fibre lasers and amplifiers in gas spectroscopy. One system under investigation consists of an all-fibre cavity ring-down loop employing a fibre amplifier for the compensation of loop loss. We have been able to obtain ring-down times as long as 0.2 ms, corresponding to ∼1100 pulses in the loop, producing an effective increase in a gas cell length from 5 cm to 55 m. The mode-locked operation of fibre lasers is also under investigation and, using dispersion effects, we demonstrate fine tuning of the wavelength which is important for absorption line scanning, with a typical tuning rate of ∼0.014 nm kHz−1 at the third harmonic, closely matching the theoretical predictions. Techniques for extending fibre laser systems to form multi-point, multi-species gas sensors are explored.