Aero-optical Distorted Wavefront Measurement And Control

The motive of aero-optical study is to improve or avoid the reduction of beam quality induced by the turbulence. As the high frequency property of aero-optical aberration, feed-back control adaptive optics system can not reach the bandwidth requirements for aero-optical aberration adaptive correction. So exploring the new methods for aero-optical aberration wavefront measurement, seeking the new approach for reduction the wavefront processor delay time and controlling the coherent structure in the flowfield by acoustic forcing to reduce the aero-optical effect are main content in this paper.Based on the scaling law, a jet facility is designed and produced. The jet's temperature and the velocity are adjustable. The jet provides the flowfield enviorment for the aero-optical experiment.The development of the coherent structure is simulated by solving the unsteady N-S equations. Geometric optics method is applied to analysis the interaction between the rays and the flowfield. The model of the interaction between the beam and the coherent structure(s) is established. The model provides the theory basis for flowfield optical property controlling and the explaining some experimental phenomena. Based on the model, by acoustic forcing to reduce the optical aberration induce by the flowfield, so called flowfield optical property active control.A fast wavefront reconstruction arithmetic named SABT has been applied in aero-optics study. In this arithmetic, the introduction of scale-independent convection velocity hypothesis will reduce the wavefront reconstruction precision. Wavelet analysis and the scale-dependent convection velocities are computed with correlation method, the scale-dependent convection velocities are used to reconstruct the wavefront. The results indicate that the scale-dependent SABT can improve the precision of wavefront reconstruction than single convection velocity SABT. Because only the beam jitters need to measured, so SABT can reach a high frequency bandwidth limit. Furthermore, this idea is useful for improvement the Hartmann bandwidth.When a 0.65 um otherwise planar wave propagates through the heated jet, the wavefront emerges aberrated. The time series of aberration wavefront are measured by Hartmann wavefront sensor. POD analysis and low dimensional approximation are made in the time series. The results indicate: the aberration wavfronts can be express with the POD modes and a few modes can capture the main characters of the aberration wavefront. Especially, only 2 modes are sufficient to construct the distorted wavefront when the flowfield is phase-locked by the acoustic force. So POD analysis is benefit to reduce the modes number required to reconstruct the aberration wavefront and the wavefront processor delay time in adaptive optics system. Wave optics study results indicate: the conjugate correction based on the low dimensional approximation wavefront can improve the strehl ratio, corresponding, improve the optics system performance.In the measurement the high temporal frequency aero-optical distorted wavefront, by reducing the quantity of the detector beam to improve the sensor's temporal frequency is the usually approach, but will reduce the spatial resolution. POD analysis is applied to improve the measured wavefront's spatial resolution based on the limited, sparse measured point data. Compare to interpolation of the sparse measured wavefront time series directly only a few POD basis function need to be interpolated, so latter has less computation amount but the higher interpolation precision.