Research On Space Cryogenic Infrared Spectrometer Based On Integrated Structural/Thermal/Optical Analysis

The hyperspectral imaging system in geostationary orbit is characterized by wide monitoring area,high time resolution,continuous monitoring and short repetition period.To apply the high-resolution hyperspectral detection technology in the geostationary orbit will contribute to achieving continuous observation in a large area throughout the day,solving the problems arising from the identification and classification of remote sensing applications,as well as meeting the needs for remote sensing in various settings like resource,forestry,environment,ocean,agriculture,and disaster prevention.In order to ensure the high signal-to-noise ratio of the space cryogenic infrared spectrometer while reducing the infrared radiation inside the instrument,it is often necessary for the instrument to work at extremely low temperatures.Due to the need to install and assemble the infrared spectrometer at room temperature,there is a significant difference in the temperature at which the infrared spectrometer is assembled and work.In case of changes to the temperature,the relative positions of optical elements,optical surface curvature,surface shape and refractive index of transmission elements will change as well.Therefore,the most critical problem is how to ensure the best possible performance of the instrument in cryogenic environment.This study is aimed to solve the problems arising from the application of curved prism dispersive infrared spectrometers in cryogenic environments,for example,the change in refractive index of prism materials with temperature gradient and stress,the mismatch in the linear expansion coefficient of curved prism component materials,as well as the shrinkage and deformation of optical and mechanical structures caused by the large temperature differences.In this sense,the research on the related integrated structural/thermal/optical simulation analysis technology would be of much practical significance to the development of cryogenic space infrared spectrometers.Firstly,the high-precision optical-structrual interface model intended for integrated optical-structrual analysis is constructed to address the significant rigid body displacement occurring to the optical surface as a result of the clear temperature difference in the environment.It relies on the two-time area-weighted least square algorithm to calculate the optical surface rigid body displacement,which is effective in distinguishing the displacement of optical surface rigid body from the deformation of optical surface.With the parabolic mirror optical system as the research object,the result of finite element analysis can be obtained for the displacement of know rigid body based on the method of actively imposing rigid body displacement.Additionally,comparative analysis was carried out using the optical-structrual interface model and Sig Fit.The calculation results are consistent with each other,which evidences the effectiveness of the high-precision rigid body displacement calculation method.In this way,the high-precision coupling of the structural finite element analysis results and optics design is made achievable.Taking the infrared spectrometer with CaF₂ curved prism as the dispersive element,an evaluation method is developed based on the ray tracing method to model the consistency of performance achieved by the infrared spectrometer affected by the temperature gradient and stress of the prism.Then,a comparative analysis is conducted on the residual accuracy of the standard Zernike phase fitting of the prism component path difference with full optical path and double optical path(transmission and reflection).According to the analytical result,the PV value of the optical path difference caused by the temperature gradient of 0.12 K is 12 nm,and the PV value of the optical path difference resulting from the stress of 0.12 Mpa is 1nm.Based on the quantitative index,it is easier to assess the infrared spectrometer for the consistency of its low-temperature performance.In order to address the shrinkage and deformation of the structure resulting from the significant difference between the low working temperature and the room temperature required for assembly,a self-centering flexible support structure intended for curved prisms is proposed in this study,so as to eliminate the risk of thermal stress caused by the mismatch of the linear expansion coefficient.The design of opto-mechanical structure is verified as reasonable by the integrated structural/thermal/optical analysis.The impact of linear expansion coefficient on the surface deformation of the optical surface is analyzed according to the design of experimental method,while the effect of the material linear expansion coefficient on the defocus of the optical system is analyzed according to the automated integrated structural/thermal/optical analysis program.Moreover,the thermal control scheme intended for the low-temperature infrared spectrometer is designed and the rationality of the thermal control scheme is verified by analyzing the changes to instantaneous optical performance as produced by the low-temperature infrared spectrometer in the steady thermal control state.In order to assess the rationality of the integrated structural/thermal/optical analysis conducted by the prism dispersive infrared spectrometer,a study is conducted on the methods of production and assemby for those critical components of the spectrometer at first,with the whole spectrometer assembled by restricting the centration of the optical surfaces.With filters used to generate narrow-band light sources,the ideal focal plance position of the detector is determined with the minimum full-width at half-height of the pixels discrete sampling curve of the slit image as the target.Then,the MTF of the spectrometer is tested under the in-lab conditions.According to the test results,the MTF at the Nyquist frequency of the center field of view based on the edge method exceeds 0.43,which meets the requirements for practical use.In addition,the spectrometer is tested at room temperature and under the cryogenic environment using a narrow linewidth laser.The spectral resolution of the prism dispersive infrared spectrometer at room temperature is consistent with that at low temperatures,and the actual performance meets the requirements for practical use,which confirms the reasonability of the integrated structural/thermal/optical analysis by the prism dispersive infrared spectrometer.