Research On Stray Light In Hybrid Diffractive/Refractive Imaging Optical System

As apertures increase,the telescopes developed by traditional approaches become increasingly weight and huge,difficult to control and increasingly costly to develop,launch and deploy.In order to explore the new construction scheme of large-aperture lightweight space telescope,the new diffractive imaging technology that breaks through the traditional imaging mechanism has attracted the attention of many research institutions.The main idea of diffraction imaging technology is using diffractive optical element(DOE)instead of the conventional refractive,reflective optical elements as the primary lens of telescope.The diffractive element modulates the light waves with a surface microstructure pattern and can be implemented on a very thin substrate.However,the DOE has multiple diffraction orders and only one diffraction order can be used for imaging.Other non-designed-order diffracted rays will become stray light if they propagate to the image plane,which will degrade the imaging performance of the system or even overwhelm the target signal.In addition,strict analysis and control of stray light must be carried out for space optical systems.This article focuses on the stray light characteristics in diffraction imaging systems.The main research work includes the following sections:1.The effects of fabrication error on the point spread function(PSF)and diffraction efficiency of the diffractive lens were studied.Firstly,the effects of some dominant errors,including etch depth error,line-width error and mask alignment error,on the diffractive lens's complex amplitude transmittance function were analyzed by the scalar diffraction theory.Then,the PSFs of diffractive lens with different errors were calculated by the diffraction field numerical calculation.Finally,the relationships between fabrication errors and the changes of PSF were established.2.A PSF test system was constructed and calibrated.The PSF of a 4-level diffractive lens with diameter of 80 mm was tested and compared with the calculated result.The calculated result is in accordance with the test result in distribution and diffraction efficiency.It verifies the validity of the analysis model for fabrication errors.3.Method of low-order ghost image analysis for diffraction telescope system was proposed.Based on this method,the low-order ghost images of a diffraction telescope prototype with diameter of 80mm were analyzed and tested.The source,propagation path and other characteristics of ghost images has been summarized.Furthermore,the effective measures for suppressing ghost image of diffractive telescope was explored.4.The effect of multi-order diffraction of DOEs on the modulus transfer function(MTF)of diffractive telescope system has been analyzed.Firstly,non-designed order diffraction was introduced into the system analysis by coherent superposition of multi-order diffraction.Then the effect of multi-order diffraction on the PSF of the diffraction telescope system was obtained by wave optical simulation technology.The MTF is the Fourier transform of the PSF.Combining with conventional non-sequential ray tracing techniques,the factors that have the greatest impact on the MTF in the non-design were found out.The optimization idea of hybrid-level diffractive lens has been put forward,and the lowest cost solution was explored.5.The suppression characteristics for out of field view stray light source of the diffractive telescope have been studied.Due to the existence of multi-order diffraction of the diffractive primary lens,stray light sources outside the field of view can directly illuminate the image plane.The quantitative analysis was performed by ray tracing technology and Monte Carlo method.To evaluate the influence of stray light under different paths,normalized illumination was propssed.According to the value of normalized illumination,the stray light paths were divided into major contradictions and minor contradictions.6.The veiling glare index(VGI)of the prototype system was calculated according to the point source transmittance(PST)and tested in laboratory environment.Combining the analysis and test results,some measures for suppressing stray light of diffractive telescope system have been discussed.In this paper,a comprehensive and detailed study of the stray light characteristics of the diffractive telescope system based on Schupmann achromatic model were conducted.A great deal of knowledge and experience on stray light sources,propagation paths,effects on system imaging performance,and stray light control methods have been summarized and accumulated.This will provide reference for the analysis and control of stray light and optimization of imaging performance in the process of practical application of large-aperture diffraction telescope systems.