Research On Design Method Of Optical System Of Large Field Of View Imaging Spectrometer With Freeform Surface

Hyperspectral imaging technology is capable of acquiring both image and "fingerprint" spectral information of a feature,which has great advantages in applications such as accurate classification and identification of features.Currently,hyperspectral imaging technology has been widely used in remote sensing observation fields such as resource survey,meteorological observation,geological survey,oceanographic research,vegetation analysis and gas monitoring,and has played an important role.With the continuous research on hyperspectral imaging technology,the application scope is expanding and the variety of installed platforms is increasing,especially the rapid development of microsatellite platforms and unmanned aircraft platforms.All these have put forward higher requirements on the size,weight,imaging quality and cost of hyperspectral imaging instruments.The structural forms and design methods of traditional imaging spectrometers have been unable to uniformly address these stringent requirements.In recent years,the burgeoning freeform optical technology provides an opportunity to comprehensively solve these problems.Therefore,this paper applies the freeform optical technology to the hyperspectral imaging spectrometer system and establishes a new method for designing the freeform imaging spectrometer system,which solves the mutual constraint problem between large field of view(Fo V),high numerical aperture(NA)and miniaturization.The main research contents are as follows:(1)Research on design method of optical system of large Fo V Offner curved prism imaging spectrometer with freeform surfaces.The optical elements of the Offner curved prism imaging spectrometer with freeform surfaces are all in decenter and tilt states.The ray tracing process of the system is complicated,and the use of freeform surface mirror increases the difficulty of system design.How to select the initial parameters of the freeform surface mirror becomes a difficult problem in the system design.Firstly,in this paper,the idea of “inverse optical design” is proposed for the structure of curved prism dispersive spectrometer.The partial differential equation for the aberration of the system and the optical surface is constructed by deriving the ray-tracing equations.Then,we fitted the discrete data points to obtain the initial structure containing the parameters of the freeform surface.In addition,a procedure for solving the initial structure of the freeform surface mirror was developed independently,and the optical system design process of the large Fo V Offner curved prism imaging spectrometer with freeform surfaces was established,the optimized design of the system was completed.Finally,the design results show that the design method in this paper reduces the total length of the optical system by 44% and the volume by 75%,while maintaining the same optical performance.The design method reduces the design difficulty of the imaging spectrometer with freeform surfaces,and contributes significantly to the miniaturization and light-weight of the imaging spectrometer system.(2)Research on the vector aberration analysis method of the compact Offner imaging spectrometer.Firstly,the vector aberration theory(VAT)is applied to the aberration analysis of the compact Offner grating imaging spectrometer,and the wavelength difference phenomenon of the Offner grating imaging spectrometer is reasonably explained.In the Offner grating spectrometer,the concept of “equivalent field decenter vector” is proposed,the expression of the third-order vector aberration of the system is established,the key factors of the system design are analyzed.Furthermore,the optimal position of the freeform surface of the compact Offner grating imaging spectrometer is determined,which is based on the analysis result of the VAT of the positional relationship of freeform surface,aperture stop and dispersive element.Finally,under the guidance of the new aberration theory,a large Fo V compact freeform surface Offner grating imaging spectrometer is designed,which has a spectral range of0.4?m-1?m,a slit size of 22.5mm,a NA of 0.167 and a spectral resolution better than2.7nm.Compared with the spherical imaging spectrometer with the same index,the total length of the imaging spectrometer designed in this paper is reduced by 37%,and the volume is reduced by 75%.The new design method proposed in this paper has effectively promoted the development of the miniaturization of the large Fo V Offner grating imaging spectrometer with freeform surfaces.(3)Research on the design method of the high numerical aperture imaging spectrometer with freeform surfaces.Firstly,based on the analysis results of the vector aberration,the proposed design methods of “decenter anamorphic stop” and “single ray aberration correction” can obtain design results of large Fo V and a high NA at the same time.According to the paraxial ray trace data and vector aberration constraints,the parameters of the decenter anamorphic stop were calculated.Then,the method of data interaction calculation and optimization between software is used to achieve fast acquisition of discrete data points of freeform surfaces by optical design software Zemax and data processing software Matlab,which reduces the design complexity of the optical system with freeform surfaces.In addition,an improved freeform surface generation strategy is proposed,which can take into account the discrete data points and normal vectors,and obtain better freeform surface fitting results.Finally,this paper uses the proposed method to develop the corresponding program module,and the NA is increased to 0.2 under the premise of the same design index and imaging quality as the previous one.The design results show that the light gathering capacity of the imaging spectrometer has been increased by 40%,and the size of the structure has not increased.The design method has important application value for the development of a large Fo V,high NA,and miniaturized imaging spectrometer.(4)Research on stray light analysis methods and suppression techniques for the imaging spectrometer with freeform surfaces.Firstly,in order to suppress the stray light formed by “multi-order and multi-diffraction” and ensure that the light gathering ability of the instrument remains unchanged,the “aperture compensation” stray light suppression design method is proposed.In addition,a large Fo V compact imaging spectrometer with a freeform surface,which can eliminate the stray light of the “multiorder and multi-diffraction”,is designed.Then,the manufacturability of the designed imaging spectrometer with freeform surface was analyzed in detail,and a feasible testing and machining plan was formulated.Finally,in this paper,the core optical element of the imaging spectrometer with freeform surface,the freeform mirror,was machined.The surface testing results show that the freeform surface meets the tolerance requirements,which further proves that the imaging spectrometer with freeform surface has a truly feasible manufacturing capability.