Study Of Optical Design Method For Imaging System Based On Stitching And Fusion Of Segmented Surfaces

With the development of optical precision manufacturing and improvement of metrology technology,the idea of using complex surfaces such as annularly stitched aspheric surfaces and freeform surfaces in the designs of optical imaging systems in many applications has become reality,including space remote sensing,infrared reconnaissance,head-mounted display,ultra-thin projection,and mobile phone camera.Because of its larger number freedom of design and stronger aberration correction capabilities,the use of complex surfaces allows the designs of optical imaging systems with compactness,large FOV,small F-number and high image quality.However,compared with conventional surfaces such as planar,spheric,and conic surfaces,there are fewer theory and methods which can support and guide the the design of complex surface.The key issuse is the solving method for the initial structure of the optical system.In this thesis,the optical system with the full FOV is regarded as a combination of multiple single-FOV or small-FOV sub-system units,and the aberration-free or diffraction-limited classic structure composed of conventional surfaces directly generates each sub-system unit.Furthermore,the segmented conventional surfaces from the sub-system units are stitched and fused to form a continuous and smooth complex surface to establish the initial structure of the optical system.Based on the above stitching and fusion theory,the research of optical design method for imaging system with complex surfaces evolved from segmented surfaces are elaborated.The main contents are as follows:The mathematical model and fusion mechanism of stitching and fusion of segmented surfaces are derived for for two types of system arrangements,coaxial and off-axis.In the coaxial catadioptric system,the mathematical descriptions of the annularly stitched aspheric surface with continuity and smoothness are analyzed.In the off-axis reflective system,the stitching and fusion theory using segmented surfaces based on the Fermat's principle,the principle of Abbe's imaging,and ray tracing with featured sampling points are compared.The design concept of utilizing the aberration-free characteristics of off-axis conic surface geometry to construct sub-mirror unit for single-FOV segmentation is put foward,in which the freeform surface is evolved from the fusion of off-axis connic surfaces for sub-mirrors.Taking the construction of a single off-axis mirror as an example,the freeform surface expression with polynomials is presented,which is realized by fitting the coordinate and normal data of discrete sampling feature points.Confronting the requirement of both large FOV and compactness,a dual-view compact imaging system based on the combining of front FOV and side FOV is proposed.The front FOV is constructed by the fisheye objective lens,and the side FOV is constructed by panoramic annular lens.Therefore,a continuous large FOV is realized by stitching both.The two FOVs are combined by a hybrid lens that consists of center refractive portion and side catadioptric portion.In order to improve the as-built performance of the aspheric surface and the optimization efficiency,both the front and rear surfaces of the hybrid lens are discribled by using the annularly stitched Q-type aspheres.A design of compact endoscope with a FOV of ±80° and F/# of 3.4 is successfully accomplished.Its total length is 11.5 mm and the maximum diameter is 5.5 mm.The modulation transfer function at 167 lp/mm is larger than0.4 over the full FOV.The relative illumination is more than 0.65,the optical distortion is within 10%,and the near telecentric condition is fulfilled.The freeform surface construction method in the off-axis system which images objects from infinity to finite distance is proposed,based on the analysis of the aberration-free imaging characteristics of off-axis parabolic.The full FOV of the off-axis imaging system is divided into multiple single FOV units.The aberration-free imaging unit is constructed by off-axis parabolic surfaces,and these units are then fused to form a single freeform surface.The design of a Czerny-Turner imaging spectrometer with a bandwidth of 400 nm and a spectral resolution of 0.1 nm is taken as an example to elaborate the construction method.The dispersive beams diffracted by the grating are regarded as a linear FOV composed of multiple single FOV units.Firstly,the parameters such as the reflection focal length and off-axis angle for the off-axis parabolic segments on the collimating mirror and the focusing mirror are calculated.Secondly,the off-axis parabolic segments corresponding to the center wavelength is taken as the reference,the parameters of the off-axis parabolic segments of adjacent wavelengths are adjusted and updated,to reduce the sag differences of the neighboring off-axis parabolic segments,and then obtain a continuous and smooth fused surface.Thirdly,the above steps are repeated to gradually update the parameters of each off-axis parabolic segments,and then expand and grow the fusion surface,until the collimator mirror and the focusing mirror are fused to generate a single freeform surface mirror which covers the full spectrum range.The initial structure of the spectrometer is composed of freeform surface,pinhole,plane grating and detector.Each element is aberration free or aberration constrained,which provides the significant advantages of high image quality and easy aligment.The stitching and fusion theory using multi-off-axis parabolic segments is further analyzed,and the contradiction between distortion correction and telecentricity control when constructing a single freeform surface is discovered.A design method of two-mirror freeform imaging system by fusing multiple surface-pair is proposed,which still divides the full FOV into single-field units,and a pair of plane and off-axis parabolic constitute a aberration-free single-field unit structure.Multiple plane segments are merged to construct the freeform correction mirror,which effectively reduces imaging distortion.Multiple off-axis parabolic segments are merged to construct the freeform main reflector,which realizes the control of beam convergence and telecentricity.A diffraction-limited off-axis telecentric scanning system is established by using the proposed design method,with a working distance of 44 mm and a linear FOV of ±10.4°.The scanning error is less than 5 ?m,and the telecentricity angle is less than 0.2°.