Distributed gas sensing using microstructured optical fibres
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3 Minute Thesis presented at the Cranfield Doctoral Network Annual Event 2018.
With a global warming potential 34 times that of CO2, as well as an explosion hazard, methane monitoring is of interest. Current technology requires the placement of a large number of sensors to cover an area. We propose a distributed technique to allow the use of a single measurement device to cover multiple sampling sites. Tunable Diode Laser Spectroscopy has been combined with optical fibre coupling of gas cells to permit a large number of cells to be interrogated simultaneously. Techniques to multiplex a number of TDLS cells together have been developed including time-division-multiplexing, Optical Time Domain Reflectometry and use of multiple fibres, each with its own detector.Range Resolved Interferometry (RRI) is an interferometric signal processing technique that has been used for position sensing and vibrometry. The principle of operation is the sinusoidal modulation of the emission wavelength of a diode laser which is input into an interferometer. The signal is demodulated to give amplitude information within each interferometer. Signals from different interferometers can be distinguished based on the optical path length difference, enabling interrogation of all measurement channels simultaneously.Here, a combination of RRI and absorption spectroscopy is performed to multiplex two gas cells arranged to form two interferometers and to recover gas concentrations from the two cells independently. The configuration is two 1m path length single-pass gas cells connected via different lengths of optical fibre in a nested Mach-Zehnder interferometer along with a common reference arm. The two gas cells were filled with different concentrations of methane. A distributed feedback diode laser was modulated with a ramp and sinusoidal wave form. The ramp sweeps the laser emission frequency across a methane absorption line at 1651nm, while the sinusoidal modulation generates interference patterns.RRI was used to recover the amplitude from the two gas cells and thereby the concentration of methane.