Optical Path Structure of Immersion Echelle Grating Spectrometer and Spectrometer

Abstract

The present invention relates to the technical field of imaging spectrometers, and specifically provides an optical path structure and a spectrometer for an immersion echelle grating spectrometer. The optical path structure includes an entrance slit, a collimation system, a primary dispersion system, a secondary dispersion system, a focusing system, and a detector; the substance to be detected enters the entrance slit, and the generated incident light beam is incident on the collimation system. After being collimated by the collimation system, it becomes a parallel light beam; the parallel light beam undergoes a first dispersion through the primary dispersion system and then a second dispersion through the secondary dispersion system to obtain a two-dimensional spectrum; the two-dimensional spectrum is focused by the focusing system and imaged onto the detector. The optical path structure and spectrometer provided by the present invention not only greatly improve the efficiency and spectral resolution of the optical system, but also make the spectrometer more compact in space. At the same time, for a cooled infrared spectrometer, the stability of the system is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of imaging spectrometers, and particularly to an optical path structure of an immersion echelle grating spectrometer and a spectrometer having the optical path structure. Background Art

[0002] The echelle grating spectrometer is one of the important research fields of modern imaging spectrometers, because it has the characteristics of high resolution, low detection limit, wide band, no moving parts, compact structure, full-spectrum direct reading, etc. It has a wide range of applications in fields such as stellar observation, sewage detection, crop remote sensing, geological identification, and military defense.

[0003] However, at present, the domestic research on immersion grating spectrometers is mainly in the visible light band and the near-infrared band, and the mid-wave infrared is not designed. Its application is limited to the detection of CO2 gas concentration, and the related research on immersion spectrometers belongs to a blank field. Internationally, the research on immersion grating spectrometers is at the forefront. With the development of grating processing technology, scientific research institutions at home and abroad are committed to the processing and manufacturing of high-diffraction-efficiency immersion gratings, and have also successfully applied them to spectrometers. However, the existing optical path structures need to be improved. Some of the optical systems of the spectrometers studied are based on catadioptric hybrids. On the one hand, the aberration control of the entire optical system cannot reach an ideal effect; on the other hand, the spatial layout of the optical elements cannot be perfectly realized in a compact form.

[0004] For example, a echelle spectrometer designed by Lawrence Livermore National Laboratory (LLNL) in the United States for sewage detection, (2000, Stevens C G, Thomas N L. Immersion echelle spectrograph:, US06078048A[P]), reported an infrared echelle grating spectrometer whose optical system uses an immersion grating combined with a prism for cross-dispersion. Immediately afterwards, the paper: Ebizuka N, Oka K, Yamada A, et al. Grism and immersion grating for space telescope[J]. 5th International Conference on Space Optics, 2004., introduced an infrared spectrometer on the Subaru telescope that uses a germanium immersion grating combined with a plane grating for cross-dispersion. Therefore, it can be seen from the above existing research technologies that there are still deficiencies in the design of immersion grating spectrometers. Its main disadvantages are that the plane grating cannot compress the light beam well and is relatively divergent; at the same time, the optimization degree of the system aberration brought by the transmission system is limited. Summary of the Invention

[0005] The present invention aims to solve problems such as high resolution and miniaturization of volume in high-resolution echelle grating spectrometers, and provides an optical path structure of an immersion echelle grating spectrometer with a compact structure and high spectral resolution for spectrometers for remote material detection.

[0006] The present invention provides an optical path structure of an immersion echelle grating spectrometer, and the optical path structure includes an entrance slit, a collimation system, a primary dispersion system, a secondary dispersion system, a focusing system, and a detector;

[0007] The substance to be detected enters the entrance slit, and the generated incident light beam is incident on the collimation system. After being collimated by the collimation system, it becomes a parallel light beam; the parallel light beam undergoes a first dispersion by the primary dispersion system and then a second dispersion by the secondary dispersion system to obtain a two-dimensional spectrum; the two-dimensional spectrum is focused by the focusing system and imaged onto the detector.

[0008] Preferably, the collimation system includes an off-axis parabolic mirror.

[0009] Preferably, the primary dispersion system includes an immersion echelle grating.

[0010] Preferably, the immersion material of the immersion echelle grating is germanium, the refractive index of the immersion material is 4; the groove density of the immersion echelle grating is 16 grooves / mm; the blaze angle of the immersion echelle grating is 75°.

[0011] Preferably, the secondary dispersion system includes a concave grating.

[0012] Preferably, the radius of curvature of the concave grating is 1000 mm, the groove density of the concave grating is 80 grooves / mm; the grating frequency of the concave grating is 85 l / mm.

[0013] Preferably, the focusing system includes an off-axis three-reflection anastigmat optical system.

[0014] Preferably, the off-axis three-reflection anastigmat optical system includes a first off-axis mirror, a second off-axis mirror, and a third off-axis mirror; total internal reflection is achieved through the first off-axis mirror, the second off-axis mirror, and the third off-axis mirror.

[0015] Preferably, the focal length of the off-axis three-reflection anastigmat optical system is 70 mm.

[0016] The present invention also provides a spectrometer, and this spectrometer includes the above optical structure.

[0017] The optical path structure of the immersion echelle spectrometer provided by the present invention proposes an immersion echelle spectrometer scheme (Immersion Echelle Spectrometer with Three-Mirror Anastigmat, IESTMA) combining an off-axis three-mirror anastigmat optical system in the design of the mid-wave infrared spectrometer optical system. By using a concave grating as the cross-dispersive element, the difficulties brought by using a prism for spectroscopy in the infrared immersion echelle spectrometer are solved, enabling the beam to have a certain degree of focusing when passing through the concave grating, and the beam aperture is also reduced compared to using a prism or a plane grating. At the same time, an off-axis three-mirror anastigmat optical system is used to replace the imaging system composed of refractive elements, fundamentally reducing the aberration of the optical system. And the IESTMA scheme also provides the possibility for the compactness of the immersion spectrometer.

[0018] The spectrometer adopting the optical path structure of the present invention not only greatly improves the efficiency and spectral resolution of the optical system, but also makes the spectrometer more compact in space. At the same time, for a cooled infrared spectrometer, the stability of the system is improved. Brief Description of the Drawings

[0019] Figure 1 It is a schematic diagram of the optical path structure of the immersion echelle spectrometer in the specific embodiment of the present invention.

[0020] Reference Signs:

[0021] Entrance slit 1, off-axis paraboloidal mirror 2, immersion echelle grating 3, concave grating 4, off-axis three-mirror anastigmat optical system 5, first off-axis mirror 51, second off-axis mirror 52, third off-axis mirror 53, detector 6. Detailed Description of the Invention

[0022] In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and do not constitute a limitation to the present invention.

[0023] The present invention provides an optical path structure of an immersion echelle spectrometer. The optical path structure includes an entrance slit, a collimation system, a primary dispersion system, a secondary dispersion system, a focusing system and a detector; the substance to be detected enters the entrance slit through a pre-optical system, and the generated incident beam is incident on the collimation system. After being collimated by the collimation system, it becomes a parallel beam; the parallel beam undergoes a first dispersion through the primary dispersion system and then a second dispersion through the secondary dispersion system to obtain a two-dimensional spectrum; the two-dimensional spectrum is focused by the focusing system and imaged onto the detector.

[0024] In the specific implementation manner, the substance to be detected can enter the incident slit through a pre - optical system, and the pre - optical system can be determined according to the application scenario. Taking the airborne missile warning system as an example, the pre - optical system can be a telescopic system. During the spectrometer test stage, the substance to be detected can directly enter through the incident slit.

[0025] As Figure 1 Shown in the figure is a schematic diagram of the optical path structure of an immersion echelle grating spectrometer in a specific implementation manner of the present invention. Its spectral range is 2.2 - 4.3 μm, excluding the water vapor molecular absorption band of 2.4 - 3.3 μm. It can be seen from the figure that in this implementation manner, the optical path structure includes an incident slit 1. The collimation system includes an off - axis parabolic mirror 2. The main dispersion system includes an immersion echelle grating 3. The secondary dispersion system includes a concave grating 4. The focusing system includes an off - axis three - mirror anastigmatic optical system 5. The optical path structure further includes a detector 6. Specifically, the incident slit 1 is located on the focal plane of the off - axis parabolic mirror 2, and the size of the incident slit 1 determines the amount of light energy entering the optical system. The incident light beam emitted from the incident slit 1 is collimated by the off - axis parabolic mirror 2 and then becomes a parallel light beam and is projected onto the immersion echelle grating 3 for the first dispersion. The immersion echelle grating 3 uses multiple high diffraction orders to disperse the incident collimated light beam, and then passes through the concave grating 4 for spectral order separation, that is, secondary spectroscopy, to form a two - dimensional spectrum.

[0026] In the specific implementation manner, the substance to be detected enters the incident slit 1 through the pre - optical system, and the generated incident light beam is incident on the off - axis parabolic mirror 2. After being collimated by the off - axis parabolic mirror 2, it becomes a parallel light beam. The parallel light beam undergoes the first dispersion through the immersion echelle grating 3 and then the second dispersion through the concave grating 4 to obtain a two - dimensional spectrum. The two - dimensional spectrum is focused by the off - axis three - mirror anastigmatic optical system 5 and imaged onto the detector 6.

[0027] In the specific implementation manner, the off - axis angle of the off - axis parabolic mirror 2 is 15°, and the focal length is f col= 272 mm. The immersion material of the immersion echelle grating 3 is germanium, and the refractive index of the immersion material is 4; the groove density of the immersion echelle grating 3 is 16 grooves / mm; the blaze angle of the immersion echelle grating 3 is 75°; when the immersion echelle grating 3 operates under the quasi-Littrow condition, the off-plane angle is γ = 5.6°, using 100 - 125, corresponding to 2.2 - 2.4 μm; 148 - 180, corresponding to 2.4 - 4.3 μm; a total of 59 orders. After the first dispersion by the immersion echelle grating 3, there may be serious spectral overlap between each order. Therefore, secondary dispersion is performed by the concave grating 4. The concave grating 4 plays a dual role of focusing and imaging in the spectrometer. Specifically, the radius of curvature of the concave grating 4 is 1000 mm, the groove density of the concave grating is 80 grooves / mm; the grating frequency of the concave grating is 85 l / mm, using the +1 order of the grating, and the concave surface operates in Wadsworth. Parallel light irradiates the concave grating, and the astigmatism of the system brought by the concave grating is the smallest in this state; that is, by using the first-order concave grating under the Wadsworth device as the secondary dispersion element, the aperture divergence of the beam passing through the collimating and main dispersion elements can be effectively reduced, so that the dispersed beam can be effectively compressed, thereby also reducing the requirements for the sizes of subsequent components; moreover, using the concave grating 4 increases the optical power of the optical system, so that the beam will not be overly expanded before entering the focusing system.

[0028] In the specific implementation manner, the parameters of the off-axis paraboloidal mirror 2, the immersion echelle grating 3, and the concave grating 4 can be designed in cooperation with each other to achieve the final required resolution of the spectrometer.

[0029] In the specific implementation manner, by adopting the off-axis three-mirror anastigmatic optical system 5, the focusing of the light beam is achieved, and a two-dimensional spectrogram in which the light intensity varies with the wavelength is obtained through detection and recording by the area array detector. The off-axis three-mirror anastigmatic optical system 5 includes a first off-axis mirror 51, a second off-axis mirror 52, and a third off-axis mirror 53; the three off-axis mirrors can all be free-form mirrors, and the specific parameters can be designed differently according to actual needs. The total reflection of the optical system is realized through the first off-axis mirror 51, the second off-axis mirror 52, and the third off-axis mirror 53, reducing the astigmatism of the image formed after passing through the dispersion system and effectively improving the spectral resolution of the spectrometer. The focal length of the off-axis three-mirror anastigmatic optical system is 70 mm. Such an optical device is very compact, and the optical system can cover almost the entire detection band (2.33 - 4.35 μm) in a single exposure without structures such as acousto-optic tunable bandpass filters or grating rotators. Software simulation shows that the optical system reaches the diffraction limit performance (Strehl ratio > 0.99), and the optical system provides high efficiency by combining the immersion echelle grating 3 and the minimum number of mirrors.

[0030] In the specific implementation manner, the area array detector 6 is adopted to receive the two-dimensional spectrogram. The area array scale of the used detector can be 320 elements × 256 elements, and the pixel size can be 30 μm × 25 μm. Preferably, an area array detector is adopted to receive the spectral information on the focal plane of the imaging system.

[0031] The optical path structure of the immersion echelle spectrometer provided by the present invention proposes an immersion echelle spectrometer scheme with an off-axis three-mirror anastigmatic optical system (Immersion Echelle Spectrometer with Three-Mirror Anastigmat, IESTMA) in the design of the mid-wave infrared spectrometer optical system. By adopting a concave grating as the cross-dispersion element, the difficulties brought by using a prism for spectroscopy in the infrared immersion echelle spectrometer are solved, enabling the light beam to be focused to a certain extent when passing through the concave grating, and the beam aperture is also reduced compared with that using a prism or a plane grating. At the same time, an off-axis three-mirror anastigmatic optical system is adopted to replace the imaging system composed of refractive elements, fundamentally reducing the aberration of the optical system. And the IESTMA scheme also provides the possibility for the compactness of the immersion spectrometer.

[0032] The spectrometer adopting the optical path structure of the present invention greatly improves the efficiency and spectral resolution of the entire optical system by replacing the transmission elements in the optical system with reflection elements. Compared with a plane reflection grating, the entire system and each component are more compact, achieving high resolution and miniaturization of the spectrometer, with a size of 180mm x 100mm x 70mm. At the same time, for a cooled infrared spectrometer, the stability of the system is improved.

[0033] Although the embodiments of the present invention have been shown and described above, it can be understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those of ordinary skill in the art can make changes, modifications, substitutions, and variations to the above embodiments within the scope of the present invention.

[0034] The above specific embodiments of the present invention do not constitute a limitation on the protection scope of the present invention. Any other corresponding changes and deformations made according to the technical concept of the present invention should be included within the protection scope of the claims of the present invention.

Claims

1. An optical path structure of an immersion echelle grating spectrometer, characterized in that, The optical path structure includes an incident slit, a collimation system, a primary dispersion system, a secondary dispersion system, a focusing system, and a detector; The substance to be detected enters the incident slit, and the generated incident light beam is incident on the collimation system. After being collimated by the collimation system, it becomes a parallel light beam. The parallel light beam undergoes the first dispersion by the primary dispersion system and then the second dispersion by the secondary dispersion system to obtain a two-dimensional spectrum. The two-dimensional spectrum is focused by the focusing system and imaged onto the detector. The primary dispersion system includes an immersion echelle grating. The immersion material of the immersion echelle grating is germanium, the refractive index of the immersion material is 4, the groove density of the immersion echelle grating is 16 grooves / mm, and the blaze angle of the immersion echelle grating is 75°. The secondary dispersion system includes a concave grating, the radius of curvature of the concave grating is 1000 mm, the groove density of the concave grating is 80 grooves / mm, and the grating frequency of the concave grating is 85 l / mm.

2. The optical path structure according to claim 1, characterized in that, The collimation system includes an off-axis parabolic mirror.

3. The optical path structure according to claim 1, wherein The focusing system includes an off-axis three-mirror anastigmat optical system.

4. The optical path structure according to claim 3, wherein The off-axis three-mirror anastigmat optical system includes a first off-axis mirror, a second off-axis mirror, and a third off-axis mirror. Total internal reflection is achieved through the first off-axis mirror, the second off-axis mirror, and the third off-axis mirror.

5. The optical path structure according to claim 3, characterized in that The focal length of the off-axis three-mirror anastigmat optical system is 70 mm.

6. A spectrometer, characterized in that, The spectrometer includes the optical path structure according to any one of claims 1 to 5.

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