An Er:YAG Laser for Airborne Methane Detection

Methane is the cleanest burning fossil fuel, and makes up the majority of natural gas and shale gas. With its low carbon output in comparison to coal and oil, natural gas is increasingly considered to be a fuel to bridge the gap from fossil fuels to renewables. However, unburnt methane has a global warming potential 25 times that of CO2 over a 100-year period, and 62 times over a 20-year period. Thus, if as little as 3% of the natural gas escapes from the gas well or during transport then the world is no better off than if we continued using coal. With the increasing use of natural gas as an energy source, and increasing global government regulation on carbon emission, there is a proportionally increasing financial and legal incentive to reduce fugitive emissions. High spatial resolution methaneleak measurements can be made using portable ground-based short-range Differential Absorption LIDAR (DIAL) systems or mobile point detectors. Average emission rates of a gas field can be determined using ground-based horizontal DIAL systems that use topographical or aerosol back-re ection. Neither of these approaches are suitable for wide area survey of the thousands of kilometres of gas pipelines, which are often in inaccessible locations. The MERLIN satellite, which will be launched in 2019, will measure global methane concentrations, providing a regulatory eye over the world for those responsible for leaks, but will not have the spatial resolution to pinpoint the source. Airborne platforms can provide higher resolution, wide area measurements, filling the gap between high spatial resolution ground measurements, and low spatial resolution satellite measurements. Current airborne systems use either large aircraft at high altitudes, due to the need for high-power transmitters or lower-power systems mounted on helicopters both of which are expensive to operate. There is thus a need for a compact DIAL system that can be mounted on a fixed-wing aircraft flying at low altitudes, as they could scan pipelines quickly and at low cost. Such a system would provide companies with a financially viable and rapid method of finding natural gas leaks that result in both decreased revenue and increased greenhouse footprint. In this thesis I describe the development of a new, compact, injection seeded, Q-switched Er:YAG laser source for a DIAL system. The source is optimised for producing pulses for an integrated-path methane DIAL system, which uses the ground as the reflector, mounted on a fixed-wing light aircraft. It uses a novel Q-switching technique to produce two pulses, the first at the `online' wavelength and the second at the `offline' wavelength, from within the same laser and during the same pump interval. This technique halves the required DIAL source hardware and provides a short (~2 μs) temporal separation between the pulses, ensuring that the pulses see the same ground back-scatter coefficient, thereby reducing a potentially significant source of random error. I also present a laboratory based demonstration of the dual-pulsed source using a methane cell. The measured data is in good agreement with the transmission expected from HITRAN 2012 data.Thesis (Ph.D.) -- University of Adelaide, School of Physical Sciences, 201