A Laser Based Sensor for Airborne Methane Detection

Abstract

Mitigating climate change and its detrimental impact to the Earth is a critical issue facing modern science. To reduce greenhouse gas emissions there is growing pressure to transition from coal-based energy sources to cleaner alternatives. Natural gas is considered the fossil fuel of choice to facilitate this transition and maintain energy production. However, natural gas predominantly consists of methane (CH₄) - a greenhouse gas with a global warming potential 25 times greater than carbon dioxide (CO₂). If the total methane emissions throughout the production, storage and transport exceeds 3% the benefits of natural gas from a global warming perspective are nullified. Thus, there are financial and environmental incentives to minimise fugitive leaks across the global network of natural gas pipelines. Methane sensors mounted in aircraft provide a means of wide-area scanning with a high spatial resolution and sensitivity for leak detection and repair. Current airborne systems use high complexity sensors and prohibitively long average times: increasing the operating costs and reducing the spatial resolution. There is thus a commercial need for a compact sensor with a high single-shot measurement sensitivity for airborne methane detection. In this thesis I describe the development of an Integrated Path Differential Absorption (IPDA) LIDAR system optimised for methane sensing from a fixed-wing light aircraft. A new dual-pulsed Q-switched Er:YAG laser source capable of producing injection-seeded pulses at the `online' and `offline' methane absorption wavelengths is developed. It uses a novel adaptation of the Pound-Drever-Hall (PDH) frequency stabilisation technique to simultaneously `lock' two low-power diode lasers to a Q-switched Er:YAG resonator for reliable injection-seeding. The high control bandwidth of this system will enable stable wavelength control in a high-vibration aircraft. The online and offline pulses have a temporal separation of 2:3 μs, which to our knowledge is the smallest reported in any dual-wavelength Q-switched laser, thus maximising system commonality and reducing potential sources of error. I also present the design and construction of custom low-noise, high-bandwidth photodetectors for measuring the return LIDAR signal. These are used in conjunction with a commercial telescope to demonstrate a ground-based measurement of methane through a 300m atmospheric column above Adelaide, Australia. The concentration was measured to be 1:979  0:003 ppm with a single-shot sensitivity of 0.124 ppm. To our knowledge the sensitivity reported in this thesis is the highest achieved by a methane IPDA LIDAR system.