Atmospheric Simulation Using a Liquid Crystal Wavefront Controlling Device

摘要

Test and evaluation of laser warning devices is important due to the increased use of 1aser devices in aerial applications. In this thesis, an atmospheric aberrating system is deve1oped to enable in-1ab testing of laser warning devices. This system employs laser 1ight at 632.8nm from a He1ium-Neon source and a spatial light modulator (SLM) to cause phase changes using a birefringent liquid crystaJ material. Before the system can be used, the SLM phase response must be quantified to ensure proper manipulation of index of refrnction. Additionally, diffraction from the SLM and rea1-world system scaling are addressed. Once completed, the atmospheric simulator is demonstmted and verified. Control of the SLM is achieved by 1oading 256 1evel bitmaps which dictate the desired index of refraction changes (called phase screens). Phase screens are created using a Fourier series technique applied to an atmospheric model in the form of a power spectrum. Five laser propagation scenarios are created, each with a set of screens describing turbulence for a particular case. Outgoing radiation from the SLM is then measured using a CCD targetboard for intensity and a Shack-Hartmann wavefront sensor for phase. Comparing system output phase statistics to atmospheric theory reveals a moderate correlation in a11 turbu1ence cases indicating desired performance. Intensity statistics are compared to the log normal distribution governed by the weak fluctuation regime. An error analysis reveals that strong turbu1ence data matches theory but that weak turbulence data is inconclusive due to measurement precision issues. As an additional check on performance, a wave optics computer simu1ation is created ana1ogous to the 1ab-bench design. Phase and intensity data affirm lab-bench results so that the aberrating SLM system can be operated confidently.