Research On High-resolution Liquid Crystal Adaptive Optics Technique With Phase Diversity

Nowadays,most of telescopes are equipped with adaptive optical systems in the world.Adaptive optics technology has been essential tool for astronomical detection.Deformable mirror is applied to correct the aberration in traditional adaptive optics technology,but it is confronted with numerous difficulties on the large telescopes due to the less number of actuators.In this respect,liquid crystal adaptive optics technology breaks the limit and has a good prospect.But the system still has some shortcomings such as long response time,and the resolution of the corrected image can not reach diffraction limit.Phase diversity(PD)was firstly proposed by Gonsalves in 1979.The method can not only estimate the wavefront of the pupil but also reconstruct the high-definition image.And it is suitable for point object and extended object.The thesis introduces PD to liquid crystal adaptive optical system and realizes visible light high-resolution imaging.The thesis detailedly analyses the fundamental principle of PD.Firstly,according to Fourier optics theory,the object-image relation for optical system is stated.Secondly,the physical model of PD is introduced,and the objective evaluation function and the expression of object are derived on the basis of maximum-likelihood estimation method.Then,numerical simulations for point object and extended object are achieved to validate the feasibility of PD for both of them.Finally,some error components are analysed such as defocus error,two-branch energy error and image translation.In order to increase the dynamic sensing range of PD,we research on the nonlinear optimization algorithm used in PD.An effective method is proposed here.We apply the hybrid particle swarm optimization(PSO)with chaos initialization to increase the sensing range of PD.The ergodicity of chaos theory in the algorithm ensures the variety of initial group,and the optimal selection operation can make the initial group has a better position.Besides,the global convergence of the hybrid PSO decreases the possibility of phenomenon that falling into local extremum.The use of natural selection mechanism increases the local searching accuracy of PSO.Firstly,we analyse the influence of particle number and iteration number on the convergence of hybrid PSO,and show the convergent thresholds of them.Secondly,we analyse the sensing accuracy for different-scale aberrations of hybrid PSO,the results show,compared to traditional local nonlinear optimization algorithms,hybrid PSO increases the sensing range greatly.Finally,a verification experiment is performed to validate the ability of hybrid PSO.A two-step high-resolution optical imaging strategy is proposed by applying PD to liquid crystal adaptive optics system.For the approach,we firstly use the open-loop liquid crystal adaptive optics technique to correct the wavefront distortions produced by atmosphere turbulences,then,PD is used to estimate residual aberrations to reconstruct the higher-resolution image.Besides,a new liquid crystal adaptive optical system with PD is proposed.For the system,a liquid crystal wavefront corrector is rotated for a small angle to separated two corrected beams.The open-loop correcting signal and phase diversity function are all added to this corrector to realize open-loop correction and defocused image collection meanwhile.Compared to traditional PD optical layout,the proposed one not only decrease the PD estimating error brought by zero-order stray light,but also can achieve the optimization for phase diversity function.Firstly,we analyse the influence of the zero-order light on PD estimating precision.The results show,compared to traditional PD,no matter for large or small turbulences,within the range of 1/15-1/3 in terms of energy ratio of zero-order light and first-order light,the proposed PD has higher estimating precision.Most of the error percentages are under 5% and 10% respectively and the maxima are no higher than 10% and 20%.Secondly,a monochromatic light high-resolution imaging experiment is done to prove the proposed approach.The results show,compared to the open-loop corrected image,the highest spatial resolution of PD reconstructed image is increased from 20.16lp/mm to 35.92lp/mm,the SSIM is also improved from 80.34% to 89.78%?Finally,a high-resolution imaging experiment with different signal-to-noise ratios is done.BM3 D denoising technique is used to the system for image preprocessing to solve the adverse effect brought by noise.Traditional PD is applied under the condition of monochromatic light or quasi-monochromatic light.It is necessary to revise PD for broadband illumination.Through numerical simulations,we can see that the estimating accuracy for wavefront is increased with the increase of the number of discrete sampling monochromatic lights,however,the revised broadband PD has heavy calculation load which is bad for practical application.An effective method is proposed here.We propose a best monochromatic approximation approach for broadband illumination based on spectral energy.According to the method,the best approximate monochromatic light is the one that divides the spectrum energy area into two equivalent parts.Firstly,we analyse the similarity between the monochromatic degraded image and the broadband degraded image,and the results show,the image of the monochromatic light we select has the highest similarity to the actual broadband image.Secondly,we research on the sensing precision of the method for broadband illumination,the results show,compared to other monochromatic lights within spectral coverage,the one based on the approach has higher sensing accuracy.Thirdly,we analyse the applicability of the approach,the results show,the approach always has the highest accuracy for different spectral widths which reveals the high applicability.Finally,we introduce the proposed method into liquid crystal adaptive optical system and realize the engineering application.Through revising the algorithm of PD and researching the technical problems appearing while introducing PD into liquid crystal adaptive optical system,the visible light high-resolution imaging is achieved.The research shows advantages of the new system on large telescopes,which have important meaning and application value in various fields.