Browsing by Author "Bremner, James"
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Item Open Access Distributed gas sensing using microstructured optical fibres(Cranfield University, 2018-10-22 10:51) Bremner, James3 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.Item Open Access Fibre-coupled, multiplexed methane detection using range-resolved interferometry(Institute of Physics, 2021-02-08) Bremner, James; Kissinger, Thomas; Hodgkinson, Jane; Tatam, Ralph P.We describe the first use of range-resolved interferometric signal processing for measurement of spectral transmission. This was applied to gas sensing using tunable diode laser spectroscopy, allowing the simultaneous and independent measurement of methane concentrations in multiple gas cells. The system uses a single injection-current modulated diode laser and a single photodetector. For three gas cells, we show the ability of the system to measure methane at noise equivalent concentrations of less than 200 ppm for a 0.5 s measurement period and a potential noise equivalent concentration (1σ) of <20 ppm with 150 s averaging time. We further show that cross-talk between cells is below the experimental uncertainty for the systemItem Open Access Multipoint gas detection using range resolved interferometry(2021-07) Bremner, James; Hogdkinson, Jane; Kissinger, Thomas; Tatam, Ralph P.The ability to detect and quantify gas in multiple locations is important in environmental and safety monitoring situations. This thesis describes the first application of Range Resolved Interferometry to the problem of gas sensing at multiple locations. Range resolved interferometry (RRI) is an interferometric signal processing technique that allows the separation of individual interferometric signals from superpositions of multiple interferometers and the rejection of interferometers other than those of interest. This allows the interrogation of the light intensity passing through each interferometer of interest which in turn allows a measure of the absorption of light by gas present within the interferometer arms. The application of the Beer-Lambert Law allows the measurement of a gas concentration from this information. Unlike previous interferometric techniques for multipoint gas measurement, RRI uses injection current modulation of a DFB laser and is therefore, cost effective. The process of applying a ramp modulation to RRI in order to extract spectroscopic information is described along with the post-processing needed to extract gas concentrations from multiple locations simultaneously. Three sensing regions ² < 0.95) and with the ability to measure methane at a concentration of 200ppm with no averaging time. Allen-Werle analysis showed that with sufficient averaging time, a limit of detection as low as 4ppm could be achieved. Cross talk experiments showed that the presence of gas in other sensing regions had no effect on gas concentration measurements. The first use of RRI for spectroscopic measurements required extensive postprocessing to account for the DFB laser’s non-uniform response to sinusoidal modulation as the driving injection current was varied to sweep the laser output wavelength. Application of an envelope function to the sinusoidal modulation provided a stable wavelength response to the sinusoidal modulation and so allowed real-time gas detection with no post processing required. Experiments were performed to establish that the most suitable deployment topology for multipoint sensing is a serial-bus topology and that the amplitude of the sinusoidal modulation must be chosen to provide the chosen balance between the spatial resolution of the system and the signal strength provided by the measurement of light absorption by the gas under test. The ability of RRI to distinguish between interferometers of interest and parasitic interferometers was used to extract the absorption measurements from a gas detection system with optical fringing and was shown to reduce the unwanted signal by a factor of 18.