Study On Increasing The Correction Bandwidth Of Liquid Crystal Adaptive Optics Systems

Liquid Crystal Adaptive Optics have drawn more and more attention fromscientists and engineers as a new adaptive optics technology. On one hand, liquidcrystal wavefront corrector can achieve high wavefront distortion correction accuracywith millions of driving units. On the other hand, liquid crystal wavefront correctorcan gain a correction range of tens of wavelengths with a modulation depth of onlyone wavelength, by means of generating wavefront with binary optics diffraction. Theresponsing speed can be improved significantly as the modulation depth reduced.Moreover, with open-loop control mode, the intrinsic polarization dependenceproblem can be solved effectively. Hence, open-loop liquid crystal adaptive opticssystems (LCAOS) has a great potential to be applied to large aperture telescopes.Currently, the main problem of the open-loop LCAOS is that the correctionspeed is not enough, which limits the application of the LCAOS on large telescopes.For the problem of low correction frequency, the detecting frequency ofHartmann wavefront sensor is improved. With these renovations, the detectingfrequency increases from500Hz to909Hz. The system bandwidth is accordinglyincreased from13Hz to19Hz. By utilizing of a parallel sequential control and anoptimized proportional correction factor, the system bandwidth is further increasedfrom19Hz to23Hz. Through the research of the evolution of the atmospheric turbulence, we foundthat the correlation between adjacent turbulence wavefront within9ms (turbulencefrequency is55Hz) is strong. Based on this, a correlative modal predictive (CMP)algorithm is developed to further solve the problem of large time delay. Using theCMP to predict the real55Hz atmospheric turbulence, the result shows animprovement of10%compared to direct correction.By improving the CMP so that the predictive coefficients can adaptively fit to theturbulence, an adaptive modal predictive (AMP) algorithm is developed. Thecorrection result showed that it could improve23%compared to the direct correctionor the bandwidth is increased from22Hz to30Hz. Finally, the spatial correlation isintroduced to the AMP. It shows that the correction result could improve26%compared to the direct correction, or the bandwidth is increased from25Hz to36Hz.The resolution of the system reaches1.8times of the diffraction limit. Fine imagingresults were acquired in field adaptive correction of celestial objects.The work is a tentative research on the practicability of open-loop liquid crystaladaptive optics systems, and will have a significant influence on the astronomicalapplication of liquid crystal adaptive optics.