Adaptive-Optic Approach to Mitigating Aero-Optic Disturbances for a Forced Shear Layer

Non-uniform, variable-density fields, resulting from compressibility effects in turbulent flows, are the source of aero-optical distortions which cause significant reductions in optical system performance. As a laser beam transverses through an optically active medium, containing index-of-refraction variations, several optical phenomena occur including beam wander, image distortion, and beam defocus. When encountering a variation in the index field, light waves refract causing an otherwise planar wavefront of a laser beam to become aberrated, contributing to the adverse effects mentioned above. Adaptive-Optics (AO) is a technique used to correct for such spatially and temporally varying aberrations on an optical beam by applying a conjugate waveform correction prior to the beams transmission through the flow. Conventional AO systems are bandwidth limited by real-time processing issues and wavefront sensor limitations. Therefore, an alternative to the conventional AO approach has been proposed, developed and evaluated with the goal of overcoming such bandwidth limitations. The alternative AO system, presented throughout this document, consists of two main features; feed-forward flow control and a phase-locked-loop AO control strategy. Initially irregular, unpredictable large-scale structures within a shear layer are regularized using flow control. Subsequently, the resulting optical wavefront, and corresponding optical signal, emerging from the regularized flow becomes more periodic and predictable effectively reducing the bandwidth necessary to make real-time corrections. A phase-lock-loop controller is then used to perform real-time corrections. Wavefront corrections are estimated based upon the regularized flow, while two small aperture laser beams provide a non-intrusive means of acquiring amplitude and phase error measurements. The phase-lock-loop controller uses these signals as feedback to synchronize the deformable mirror's waveform to that of the shear layer by adjusting its amplitude and phase. A third-order analog phase-lock-loop controller has been designed and a prototype board assembled; the higher order controller was designed to accommodate for any step and ramp changes in phase. The control system was assessed and validated through numerical simulations. The prototype controller was then constructed and several experimental tests were run using a function generator signal as the input. The frequency and phase of the input signal was varied throughout the testing process and the phase-lock-loop controller was able to successfully synchronize its output signal with the changing sinusoidal input. This work represents a key step in the successful development of an automated AO controller capable of applying real-time corrections to an optical beam for high-speed aero-optic applications.