Optimising Transmittance Measurements of Lasers Propagating Through Localised Optical Turbulence By Varying Focal Geometry

Abstract

As the global consensus of the impact of climate change converges, it is more important now then ever to ensure that emissions from industrial and automotive applications are regulated and that systems are adequately diagnosed. Current diagnostic methods are quickly being left behind but with advances in technology, laser diagnostic systems are becoming increasingly feasible, providing more accurate and reliable measurements. The environments that laser diagnostics systems have to work in is often unsuitable for optical equipment and measurements. This study looks at a specific use case for an optical diagnostic system; gas measurements from a jet engine exhaust situated in the centre of the measurement path. The high temperatures and velocities of the exhaust is deleterious to optical measurements. This study investigated whether an optical system can be designed to take advantage of the localised aberrant region by looking at the geometrical characteristics of the laser travelling through the medium. In particular looking at the focal geometry of the laser. In order to achieve this goal a lab based optical turbulence generator was designed using the combustion of propane and air. The system was characterised by comparison of the received laser profile after having passed through the turbulence with that of a simulated laser model based on turbulence theory and Fourier optics. The turbulence was characterised to have an approximate refractive index constant of 5 x 10^10(-2/3) m and an inner scale in the region of 3 millimetres, characteristics comparable to a jet engine exhaust. The crux of this study was then to see how the temporal transmittance performance, key to some optical diagnostic systems, differed between three different laser beam focal regimes; collimated, divergent and focused (at the centre of the turbulence and hence measurement path). Using the Monte Carlo statistical method, the spatial analysis indicated that a divergent beam would provide the lowest variance of transmittance over the aperture extenuated by having the lowest levels of average angular displacement with a maximum of 157 microradians. A temporal analysis painted a contrasting picture which showed that the mid-focused beam would actually have half the transmittance variance of a collimated beam, the next highest case. The findings of this research are pertinent to the design and implementation of optical diagnostic system and give credence to the effort of contemplating focal geometry in order to optimise the signal efficacy of time sensitive diagnostic system.

Date of Award	31 Dec 2020
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Krikor Ozanyan (Supervisor) & Paul Wright (Supervisor)