Study On Improving The Imaging Quality Of Diffractive Optical Elements

Diffractive Optical elements(DOE)can modulate the amplitude and phase of light wave,and has the advantages of light weight and easy copying.Elements have broad application prospects in beam shaping and diffraction imaging,etc.But the elementss designed by the present algorithm are affected by noise during imaging,and the intensity distribution of the reconstructed images is uneven,so how to improve the quality of reconstructed images is one of the focal point of research.The design of DOE is implemented by the phase recovery algorithm,but the current improvement of DOE is mostly based on the algorithm,and rarely considers the influence of physical factors in diffraction.In this thesis,we start from the phenomenon of light wave leakage in light field propagation.A method to add amplitude degree of freedom is proposed to suppress the uneven distribution of light intensity,and improve the quality of DOE imaging.The specific work contents are:(1)In this thesis,the basic theoretical principle of designing DOE is studied,including scalar diffraction theory and straight-edge Fresnel diffraction,etc.The evaluation functions of elements performance are introduced,such as non-uniformity of light intensity and amplitude distribution,diffraction efficiency,mean square error,etc.Several phase recovery algorithms for DOE are studied,and Fourier transform DOE,Fresnel diffraction DOE and lensless Fourier transform DOE are designed by GS algorithm and simulated annealing algorithm based on matlab.(2)How the light diffracts into areas outside the signal window during propagation and the intensity distribution form of the leaking light are studied.Based on this characteristic,the uniform and non-uniform amplitude degrees of freedom are added to the target image used in the design of the DOE.Through the design of Fourier transform DOE by matlab,it is found that intensity ununiformity of the reconstructed image of the elements designed by this method reduces from about 14% to less than 1%,and the mean square error is reduced from about 10% to less than 0.1%,which verify the practicability of the method.In summary,the method of improving DOE imaging quality is mainly studied in this thesis,based on the characteristics of light field propagation,a method for adding different amplitude degrees of freedom for the target images is proposed.The Fourier DOE is designed by GS algorithm and simulated annealing algorithm.The results show that the ununiformity and mean square error of the DOE reconstructed image are significantly reduced,which improves that the method of increasing the amplitude degrees of freedom is effective.