Multipoint gas detection using range resolved interferometry

Abstract

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.