A portable laser system for the remote detection of methane gas
We have developed an open-path hand-held gas detector incorporating a distributed feedback InGaAs laser diode at 1.65μm. Incorporated into a hand-held transceiver unit, the emitted laser beam is backscattered from nearby surfaces, collected and focussed onto an amplified InGaAs detector using a 150mm diameter plastic Presnel lens. At ranges of 4-5metres, a typical backscattered signal is tens of nanowatts of laser light. Applying second derivative wavelength modulation spectroscopy (WMS) gives a sensitivity to methane of better than 10 parts per million over a one metre path length. Chapter 1 gives background information on existing detection methods, and explains why we chose to implement WMS. Chapter 2 discusses the various models created to justify the decision to use WMS. It also describes techniques used to help visualise escaping gas. Chapter 3 discusses the various stages of design and build of the actual prototypes, starting with a laboratory based prototype and finishing with a fully portable technology demonstrator. Chapter 4 give details of the laboratory testing undertaken in order to characterise and benchmark the system performance. Chapter 5 is concerned with the design and construction of an additional add-on scanning platform which scans the pointer instrument in order to build a 2-dimensional image of the gas escape. Chapter 6 mirrors chapter 3 in format, and discusses the various field-trials where the instruments were tested in representative conditions. Finally Chapter 7 highlights the work performed in developing a pre-production prototype and advertising the instrument to a wider market.