静止轨道全谱段宽覆盖成像光谱仪光学系统设计与高保真分光系统研制

Design of Geostationary Full-Spectrum Wide-Swath High-Fidelity Imaging Spectrometer and Development of Its Spectrometers

根据静止轨道特点,分析设计覆盖近紫外至长波红外波段的成像光谱仪光学系统,将0.3~12.5μm的光学全谱段细分为5个子谱段并集成于光学系统,每个子谱段均采用4个分光系统在视场内拼接实现宽幅所需的超长狭缝,狭缝总长最长达241.3 mm,通过内扫描实现400 km×400 km地面覆盖。重点研究满足拼接要求的紧凑型长狭缝分光系统,指出其实现高保真分光成像需满足低畸变、低杂散、高信噪比、均匀光谱响应等条件。据此,设计基于凸面闪耀光栅的Offner型和Wynne-Offner型高保真分光系统,并研制各个子谱段的原理样机。给出全谱段长狭缝和凸面闪耀光栅两种核心元件的研制结果,单条狭缝最长达61.44 mm,5个谱段的光栅槽密度范围为8.8~312.1 lp/mm,峰值效率均在70%以上,最高达86.4%。以可见近红外和长波红外两个谱段为例,给出分光系统装调和测试过程,结果表明所研制的分光系统具有高保真性能,各项指标均满足要求。

Objective It is necessary to develop full-spectrum hyperspectral remote sensing technology from geostationary orbit to fully meet the application requirements of continuous monitoring as well as fine identification and classification in the fields of disaster prevention and mitigation,environment,agriculture,forestry,ocean,meteorology,and resources.In view of the increasing demand for efficient and accurate spatial and spectral information acquisition,imaging spectrometers are driven to develop in the direction of faster response,larger width,higher resolution,and higher signal-to-noise ratio(SNR).Since the satellite in a geostationary orbit has the unique advantage of keeping relatively stationary with the ground,it has high timeliness and continuous observation capability and can obtain information from ground scenes quickly and widely.However,at present,most of the spectral remote sensing payloads serving on the geostationary orbit are multispectral payloads.They have only a dozen of spectral channels in the full spectrum,which is not sufficient to obtain fingerprint information of ground scenes,and their ability to identify types and components of substances is far inferior to that of hyperspectral payloads.The fusion with hyperspectral data in the full spectrum can greatly improve the recognition accuracy and can provide more descriptions of targets.However,due to the great difficulty and high cost of such payloads,there is little related research reported at present.In this paper,we present an optical system of the geostationary fullspectrum wide-swatch high-fidelity imaging spectrometer(GeoFWHIS).The prototypes of the spectrometers are developed to verify the correctness and feasibility of the design.Methods According to the characteristics of the geostationary orbit,the imaging spectrometer covering near-ultraviolet to long-wave infrared(LWIR)is analyzed and designed.The full spectrum from 0.3μm to 12.5μm is divided into five sub-spectral bands and integrated into the optical system.Each sub-spectral band adopts four spectrometers to splice in the field of view to realize an ultra-long slit required for the wide width.The total length of the slit is 241.3 mm,and 400 km×400 km ground coverage is realized through internal scanning.The compact long-slit spectrometer meeting the splicing requirement is studied specifically.It is pointed out that low distortion,low stray light,high SNR,and uniform spectral response are the requirements for high-fidelity spectral imaging.The high-fidelity Offner and Wynne-Offner spectrometers based on convex-blazed gratings are designed.The performance of each part of the GeoFWHIS is fully evaluated,and the prototypes of each sub-spectral band are developed.Long slits for each band based on silicon substrate are developed by semiconductor technology,and the convex-blazed gratings are manufactured by holographic lithography and ion beam etching.The properties of the long slits and blazed gratings are tested and discussed.The alignment and test of the VNIR spectrometer and LWIR spectrometer are introduced as examples.In addition,we adjust the optical elements of spectrometers by using a high-precision point source microscope and achieve micrometer-level alignment precision.The performance that affects the fidelity of the spectrometers,such as MTF,spectral resolution,spectral distortions,and stray light,are tested and discussed.Results and Discussions The optical system of GeoFWHIS is designed and evaluated.The design results show that its imaging quality is close to the diffraction limit in the full spectrum,and its spectral distortions,including the smile and keystone,are less than 1%pixel.The SNR of the system is also analyzed;the SNR of the B1 and B2 bands is higher than 250,and the SNR of the B3 band is higher than 150.Core elements of spectrometers are manufactured and tested.The results show that the maximum length of a single slit is 61.44 mm,and its width is only 15μm.The groove density of the gratings in five spectral bands ranges from 8.8 lp/mm to 312.1 lp/mm.The peak efficiency is all above 70%,and the maximum efficiency is 86.4%.The prototypes of the spectrometers are assembled and tested.The test results are consistent with the design results,which show high imaging quality,low distortion,low stray light,and uniform spectral response.Conclusions The optical system of GeoFWHIS designed in this paper can realize hyperspectral imaging with a wide swath and a full spectrum.The full spectrum of 0.3-12.5μm is divided into five sub-spectral bands,including UVIS,VNIR,SWIR,MWIR,and LWIR,which are integrated into the optical system to realize a swath of 400 km,spatial resolution of 25-100 m,and spectral resolution of 4-200 nm.The high-fidelity spectrometer is studied emphatically.The designed Offner and Wynne-Offner spectrometers can meet the requirements of a long slit and a small volume and have high imaging quality,low spectral distortion,uniform spectral response,low stray light,and high SNR.The prototypes of the spectrometers for each band are developed.The test results show that the convex-blazed grating has the desired sawtooth groove shape and high diffraction efficiency.The test results of the prototypes demonstrate high-fidelity performance,and all specifications meet the requirements.The GeoFWHIS reported in this paper provides a technical scheme for full-spectrum hyperspectral remote sensing from a geostationary orbit.The successful development of fullspectrum high-fidelity spectrometers verifies the feasibility and correctness of the design.