A push-broom imaging spectrometer and its imaging method

Abstract

The present invention relates to a push-broom imaging spectrometer and an imaging method thereof. In the direction of incident light, the imaging spectrometer includes a front objective lens, two plane mirrors for folding light, and a spectroscopic imaging system; the front objective lens adopts an off-axis three-reflection structure, and the mirrors are aspherical; the spectroscopic imaging system adopts a Wynne-Offner type spectroscopic device with a concentric structure, its mirrors are free-form surfaces, the grating adopts an aberration-corrected holographic convex grating, and the grating, meniscus lens and mirror are concentric. The imaging spectral system provided by the present invention has the characteristics of high spectral resolution and compact structure, better controls the aberration of the full band and full field of view, and its advantages of wide field of view, high spectral resolution and easy alignment and adjustment are suitable for airborne remote sensing applications with large swath width and high spectral resolution.

Description

Technical Field

[0001] The present invention relates to an imaging spectroscopy technology with a wide field of view and high resolution, and particularly to a push-broom imaging spectrometer and an imaging method thereof. Background Art

[0002] An imaging spectroscopy system is a combination of optical imaging technology and spectral analysis technology. It can obtain the spatial information of the target image and its corresponding spectral information at the same time, and is an optical instrument for comprehensively obtaining target information. At present, push-broom imaging spectrometers have been widely used in many fields such as aerospace remote sensing, medical detection and diagnosis, mineral resource exploration, environmental monitoring, and military reconnaissance and camouflage recognition. The demand for high spectral imaging of the earth from high altitudes by spacecraft is increasing day by day, which puts higher requirements on the field of view and spectral resolution of imaging spectrometers.

[0003] However, there are many technical problems to be solved in push-broom imaging spectroscopy systems with a wide field of view and high spectral resolution. A wide field of view requires the front objective lens to have a large field of view angle and requires the spectroscopic imaging system to have a long entrance slit. However, geometric aberrations, especially astigmatism, increase rapidly with the increase of the system field of view and the entrance slit. The spectral resolution of a hyperspectral imager is determined by the spectroscopic imaging system. High spectral resolution requires a large grating line density. However, grating spectroscopy causes the system optical path to be asymmetric, and it is difficult for the system to correct geometric aberrations, especially astigmatism. Astigmatism causes the image points in the meridian direction not to be well focused, resulting in crosstalk between adjacent spectral channels and reducing the spectral resolution of the system. Therefore, in traditional imaging spectroscopy systems, due to the existence of geometric aberrations, especially astigmatism, it is difficult to improve the field of view and spectral resolution.

[0004] Offner spectrometers with a concentric structure have been widely used in push-broom imaging spectroscopy systems due to their high performance and compact structure. In the imaging spectrometers reported in the existing literature, high spectral resolution, long slits, and compact structure usually cannot be satisfied simultaneously. Refer to the literature "Design Method of Offner Imaging Spectrometer" ([J]. Acta Optica Sinica, 2010 (4): 1148-1152), which reported a traditional convex grating Offner device with a slit length of less than 20 mm, an optical length of more than 200 mm, and a spectral resolution of 3 nm; Chinese invention patent CN 109781261 A published a compact catadioptric athermal imaging spectrometer based on the Wynne-Offner structure, with a compact volume, achieving a slit length of 32 mm, but its full immersion structure is not light enough, and the spectral resolution is 2.5 nm. Summary of the Invention

[0005] Aiming at the deficiencies existing in the prior art, the present invention provides a push-broom imaging spectrometer with a compact structure, easy to assemble and adjust, and capable of simultaneously achieving a wide field of view and high spectral resolution, as well as an imaging method thereof.

[0006] To achieve the above object of the invention, the technical solution adopted by the present invention is to provide a push-broom imaging spectrometer, whose working wavelength range is 400nm - 1000nm; along the light incident direction, it sequentially includes a front objective lens, two plane mirrors for folding light, and a spectroscopic imaging system; the front objective lens has an off-axis three-reflector structure, including a concave primary mirror, a convex secondary mirror, and a concave third mirror, the primary mirror and the third mirror are located on the same side of the secondary mirror, and the surface shapes of the three mirrors are all even aspheres; the spectroscopic imaging system includes an entrance slit, a meniscus lens, a free-form mirror, an aberration-corrected holographic convex grating with a concentric structure, and an imaging sensor; the meniscus lens bends towards the light incident direction, and its inner and outer surfaces are spherical surfaces; the entrance slit and the imaging sensor are located on one side of the inner surface of the meniscus lens, and the free-form mirror is located on one side of the outer surface of the meniscus lens; the aberration-corrected holographic convex grating is attached to the center of the outer surface of the meniscus lens;

[0007] The surface shape of the free-form mirror is an XY polynomial free-form surface, and the coordinate system is a Cartesian space rectangular coordinate system constructed with the vertex of the free-form mirror as the origin O, the light incident direction is the positive direction of the Z axis, the positive direction of the Y axis is upward, and the positive direction of the X axis is perpendicular to the paper and into the paper. In the coordinate system, the equation of the XY polynomial free-form surface z(x, y) is:

[0008] ;

[0009] wherein, is the radius of the free-form mirror; c is the curvature, c = -1.17×10 -2 ; k is the conic coefficient, k = 0.387; A1 - A 20 are the coefficients of each monomial respectively. Among them, -1 ≤ A2 ≤ 1, -20 ≤ A3 ≤ 20, -20 ≤ A5 ≤ 20, -1 ≤ A7 ≤ 1, -1 ≤ A9 ≤ 1, -2 ≤ A 10 ≤ 2, -3 ≤ A 12 ≤ 3, -2 ≤ A 14 ≤ 2, -1 ≤ A 16 ≤ 1, -1 ≤ A 18 ≤ 1, -1 ≤ A 20 ≤ 1, and the rest are 0.

[0010] The technical solution of the present invention also includes an imaging method of the push-broom imaging spectrometer. Using the push-broom imaging spectrometer provided by the present invention, the imaging method includes the following steps:

[0011] (1)Light with a wavelength of 400 nm to 1000 nm is incident on a front objective lens with an off-axis three-reflection structure. First, it is reflected by a concave primary mirror to form a first reflected light. The convex secondary mirror reflects the first reflected light a second time to form a second reflected light. A concave third mirror is arranged on the reflection light path of the secondary mirror to reflect the second reflected light again to form a third reflected light. Then, the light path of the third reflected light formed by the front objective lens is folded by two plane mirrors to form a telecentric light beam on the entrance slit.

[0012] (2)The telecentric light beam formed in step (1) is incident from the entrance slit, and after being refracted by a meniscus lens and reflected by a free-form mirror in sequence, it is incident on an aberration-corrected holographic convex grating in the form of a converging light beam.

[0013] (3)The converging light beam is diffracted by the aberration-corrected holographic convex grating to achieve spectral splitting.

[0014] (4)The diverging light beam obtained after spectral splitting is reflected by the free-form mirror and refracted by the meniscus lens in sequence, and then converges on the imaging sensor to achieve hyperspectral imaging.

[0015] The principle of the present invention is as follows: An off-axis three-reflection front objective lens is adopted, which is beneficial to energy utilization and improves image quality. The free form is applied to the concentric structure Wynne-Offner spectroscopic imaging system. By utilizing the non-rotation symmetry of the free form and the advantage of flexibly controlling the light direction, the aberration caused by the asymmetry of the system light path due to grating spectroscopy is corrected. At the same time, an aberration-corrected holographic convex grating is adopted to further correct the system aberration, especially astigmatism, so as to simultaneously achieve a wide field of view, high spectral resolution and compact structure of the imaging spectroscopic system.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0017] 1. The present invention adopts an off-axis three-reflection front objective lens, and the surface shapes of the mirrors are all even aspheres, with excellent image quality and high energy utilization rate.

[0018] 2. The Wynne-Offner type spectroscopic system with a concentric structure adopted in the present invention has a compact structure and extremely high imaging performance. The spectral resolution of the hyperspectral imager is determined by the spectroscopic imaging system. Compared with other existing spectroscopic systems, under the same wavelength band, numerical aperture and slit length conditions, the structure of the present invention can achieve higher spectral resolution.

[0019] 3. The aberration-corrected convex holographic grating adopted in the present invention not only has a dispersion effect, but also can use its own holographic aberration to compensate for the geometric aberration introduced by the structure, further improving the spectral resolution of the system.

[0020] 4. The spectroscopic system of the present invention uses a mirror with a free-form surface, which can improve the aberrations that cannot be balanced by various spherical systems. Its high degree of freedom further corrects geometric aberrations, especially astigmatism, improves the spectral resolution of the system, and can achieve a high spectral resolution under a wide field of view, wide wavelength band, and high numerical aperture. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 FIG. 6 is a schematic structural diagram of a push-broom imaging spectrometer provided by an embodiment of the present invention;

[0022] Figure 2 FIG. 10 is a schematic diagram of the recording optical path principle of an aberration-corrected convex holographic grating of the imaging spectrometer provided by an embodiment of the present invention;

[0023] Figure 3 FIG. 14 is a curve graph of the modulation transfer function (MTF) of the imaging spectrometer provided by an embodiment of the present invention for the entire field of view and entire wavelength band;

[0024] Figure 4 FIG. 18 is a spot diagram for the entire field of view and entire operating wavelength band provided by an embodiment of the present invention;

[0025] Figure 5 FIG. 22 is a curve graph of the RMS spot radius for the entire field of view and entire operating wavelength band provided by an embodiment of the present invention.

[0026] In the figures, 11. primary mirror; 12. secondary mirror; 13. third mirror; 21. entrance slit; 22. meniscus lens; 23. free-form mirror; 24. aberration-corrected convex holographic grating; 25. imaging sensor; 3. plane mirror. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0027] The technical solution of the present invention will be further described below in conjunction with the drawings and embodiments.

[0028] Embodiment 1

[0029] This embodiment provides a push-broom imaging spectrometer with a wide field of view and high resolution and its imaging method.

[0030] See the attached Figure 1, which is a schematic structural diagram of the imaging spectrometer provided in this embodiment. It includes a front objective lens, two planar mirrors 3 for folding light, and a spectroscopic imaging system. The specific structure is as follows: The front objective lens is an off-axis three-reflection structure, and the components include a concave primary mirror 11, a convex secondary mirror 12, and a concave third mirror 13. The primary mirror 11 and the third mirror 13 are on the same side of the secondary mirror 12, and the surface shapes of the three mirrors are all even aspheres; The spectroscopic imaging system includes an entrance slit 11, a meniscus lens 22, a free-form mirror 23, an aberration-corrected holographic convex grating 24 in a concentric structure, and an imaging sensor 25; The meniscus lens bends towards the light incident direction, and both the inner and outer surfaces are spherical surfaces; The entrance slit and the imaging sensor are on one side of the inner surface of the meniscus lens, and the free-form mirror is on one side of the outer surface of the meniscus lens; The aberration-corrected holographic convex grating is attached to the center of the outer surface of the meniscus lens; The centers of the inner and outer spherical surfaces of the meniscus lens coincide with the center of the spherical surface of the aberration-corrected holographic convex grating on the optical axis.

[0031] The imaging method of the imaging spectrometer provided in this embodiment includes the following steps:

[0032] (1) Light with a wavelength range of 400nm to 1000nm is incident on the front objective lens with an off-axis three-reflection structure. First, it is reflected by the concave primary mirror 11 to form a first reflected light; the convex secondary mirror 12 reflects the first reflected light a second time to form a second reflected light; the concave third mirror 13 is arranged on the reflection optical path of the secondary mirror to reflect the second reflected light again to form a third reflected light, and then the optical path of the third reflected light is folded by two planar mirrors 3 to form telecentric light on the entrance slit 21.

[0033] (2) The telecentric light formed by the front objective lens is incident from the entrance slit 21, refracted by the meniscus lens 22, and reflected by the free-form mirror 23, and then enters the aberration-corrected holographic convex grating 24 in the form of a converging beam.

[0034] (3) The converging beam is diffracted by the aberration-corrected holographic convex grating 24 to achieve spectral splitting.

[0035] (4) The diverging beam obtained after spectral splitting is reflected by the free-form mirror 23 and refracted by the meniscus lens 22 in sequence, and then converges on the imaging sensor 25 to achieve hyperspectral imaging.

[0036] The performance parameters of the imaging spectrometer provided in this embodiment meet the conditions in Table 1.

[0037] Table 1

[0038] .

[0039] All three reflecting mirrors of the front objective lens are even aspheres, and the sagittal height equation z of the even aspheres of each reflecting mirror in the corresponding coordinate system is:

[0040] ;

[0041] Among them, r is the paraxial curvature radius of the lens; c is the curvature; k is the conic coefficient, -10 ≤ k ≤ 10; a1 to a8 are the coefficients of each monomial, and the value range is a1 = 0, -1 ≤ a2 ≤ 1, -1 ≤ a3 ≤ 1, -1 ≤ a4 ≤ 1, a5 = 0, a6 = 0, a7 = 0, a8 = 0.

[0042] The main reflecting mirror of the spectroscopic imaging system is a fifth-order XY polynomial free-form surface, and the surface shape expression z is:

[0043] ;

[0044] Among them, is the radius of the free-form surface reflecting mirror; the curvature c = -1.17×10 -2 ; the conic coefficient k = 0.387;

[0045] A1 to A 20 are the coefficients of each monomial respectively. Among them, A2 = -5.1×10 -2 , A3 = 10.814, A5 = 10.707, A7 = 2.8×10 -2 , A9 = 8.4×10 -2 , A 10 = 1.482, A 12 = 2.767, A 14 = 1.550, A 16 = 8.26×10 -3 , A 18 = 0.145, A 20 = 2.3×10 -2 , and the rest are 0.

[0046] See Appendix Figure 2 , which is a schematic diagram of the recording optical path principle of the aberration-corrected convex holographic grating of the imaging spectrometer provided in this embodiment; in the figure, 0 is the center of the aberration-corrected convex holographic grating, the dotted circle is the Rowland circle, and the angles between the holographic recording points C, D and the optical axis are δ, γ respectively. The effective grating constant d of the aberration-corrected holographic grating is:

[0047] ;

[0048] Among them, l0 is the recording wavelength, the angles of the recording points C, D are δ, γ respectively, the recording angle δ is always greater than the recording angle γ, and the recording arm lengths are r C and rD 。

[0049] In this embodiment, the recorded parameters are: δ = 3.02°, γ = -1.68°, r C = 50.21 mm, r D = 50.19 mm.

[0050] Each optical element (surface) in this embodiment meets the conditions in Table 2.

[0051] Table 2:

[0052] 。

[0053] The spectroscopic imaging system of the present invention uses an aberration-corrected holographic convex grating, which can balance the inherent geometric aberrations of the system, especially astigmatism. The aberration-corrected holographic convex grating is optimized and designed based on the optical path function theory with an objective function established by aberration coefficients. By adjusting the positions of two holographic recording points on the Rowland circle, an aberration-corrected holographic grating that meets the parameter and performance requirements of the imaging spectrometer can be obtained. The two recording wave sources of the grating are spherical waves, and the grating groove pattern is a variable-pitch curve type.

[0054] The present invention applies a free-form surface to the concentric-structured Wynne-Offner spectroscopic imaging system. Utilizing the non-rotational symmetry of the free-form surface and the advantage of flexibly controlling the light direction, it corrects the aberrations caused by the asymmetry of the system optical path due to grating spectroscopy. At the same time, an aberration-corrected holographic convex grating is adopted to further correct the system aberrations, especially astigmatism, so as to simultaneously achieve a wide field of view, high spectral resolution, and compact structure of the imaging spectral system.

[0055] See Appendix Figure 3 , which is the full-band transfer function MTF curve graph of the imaging spectrometer provided in this embodiment; in the figure, figures (a), (b), (c), and (d) are the transfer function MTF curves of all fields of view on the corresponding image planes of the imaging spectrometer provided in this embodiment at wavelengths of 400 nm, 600 nm, 800 nm, and 1000 nm respectively. It can be Figure 3 seen that in the working band from 400 nm to 1000 nm at 67 lp / mm, the optical transfer functions of the full field of view are all greater than 0.3, and the curves are smooth and compact, indicating that the imaging of this system is clear and uniform, and the system has good imaging quality and resolution in the full band and the full field of view.

[0056] See Appendix Figure 4 , which is the spot diagram of the imaging spectrometer provided in this embodiment in the full field of view and the full working band. It can be Figure 4 seen that in the full field of view and the full working band, the spots in the meridional and sagittal directions are uniform, the aberrations of the system, especially astigmatism, are well corrected, and the energy is concentrated, meeting the usage requirements.

[0057] See the appendix Figure 5 , which is a graph showing the variation of the RMS spot radius of the imaging spectrometer provided in this embodiment with wavelength. Curve (a) is the RMS radius of different fields of view of the system, and line (b) is the RMS radius under the diffraction limit. It can be seen from Figure 5 that in the full field of view and the full working wavelength band, the RMS spot radius of the system is less than 3.5 μm, and is less than the diffraction limit RMS radius, with the energy concentrated, providing extremely high resolution.

[0058] The results prove that the imaging spectrometer system provided by the present invention has a working F number of up to 3, a slit length of up to 40 mm, a working wavelength band of 400 nm to 1000 nm, a field of view angle of up to 23°, and a spectral resolution better than 1 nm. Under a compact structure, the optical transfer function in the full working wavelength band and the full field of view is greater than 0.3, with good imaging quality, meeting the requirements of wide field of view, high spectral resolution and miniaturization of the airborne spectral imaging system.

Claims

1. A push-broom imaging spectrometer, characterized in that: Its working wavelength range is 400nm to 1000nm; in the light incident direction, it sequentially includes a front objective lens, two plane mirrors (3) for folding light, and a spectral imaging system; the front objective lens is an off-axis three-mirror structure, including a concave primary mirror (11), a convex secondary mirror (12), and a concave tertiary mirror (13). The primary mirror (11) and the tertiary mirror (13) are on the same side of the secondary mirror (12). The surface profiles of the three mirrors are all even aspheres; the spectral imaging system includes an entrance slit (21), a meniscus lens (22) with a concentric structure, a free-form mirror (23), an aberration-corrected holographic convex grating (24), and an imaging sensor (25); the meniscus lens (22) bends towards the light incident direction, and its inner and outer surfaces are spherical surfaces; the entrance slit (21) and the imaging sensor (25) are on one side of the inner surface of the meniscus lens, and the free-form mirror (23) is on one side of the outer surface of the meniscus lens; the aberration-corrected holographic convex grating (24) is attached to the center of the outer surface of the meniscus lens (22). The surface profile of the free-form mirror is an XY polynomial free-form surface. The coordinate system is a Cartesian space rectangular coordinate system with the vertex of the free-form mirror as the origin O. The light incident direction is the positive direction of the Z axis, the positive direction of the Y axis is upward, and the positive direction of the X axis is perpendicular to the paper and into the paper. The equation of the XY polynomial free-form surface z(x,y) in this coordinate system is: ； Among them, is the radius of the free-form surface mirror; c is the curvature, c = -1.17×10 -2 ; k is the conic coefficient, k = 0.387; A1 to A 20 are the coefficients of each monomial respectively. Among them, -1 ≤ A2 ≤ 1, -20 ≤ A3 ≤ 20, -20 ≤ A5 ≤ 20, -1 ≤ A7 ≤ 1, -1 ≤ A9 ≤ 1, -2 ≤ A 10 ≤ 2, -3 ≤ A 12 ≤ 3, -2 ≤ A 14 ≤ 2, -1 ≤ A 16 ≤ 1, -1 ≤ A 18 ≤ 1, -1 ≤ A 20 ≤ 1, and the rest are 0.

2. An imaging method for a push-broom imaging spectrometer, characterized in that Using the pushbroom imaging spectrometer described in claim 1, the imaging method includes the following steps: (1) Light with a wavelength range of 400nm to 1000nm is incident on the front objective lens with an off-axis three-mirror structure. First, it is reflected by the concave primary mirror (11) to form a first reflected light; the convex secondary mirror (12) reflects the first reflected light a second time to form a second reflected light; the concave tertiary mirror (13) is arranged on the reflection light path of the secondary mirror to reflect the second reflected light again to form a third reflected light. Then, the optical path of the third reflected light formed by the front objective lens is folded by two plane mirrors (3) to form telecentric light on the entrance slit (21). (2) The telecentric light formed in step (1) enters from the entrance slit (21), is refracted by the meniscus lens (22) in sequence, and then reflected by the free-form mirror (23), and then enters the aberration-corrected holographic convex grating (24) in the form of a converging light beam. (3) The converging light beam is diffracted by the aberration-corrected holographic convex grating (24) to achieve spectral splitting. (4) The diverging light beam obtained after spectral splitting is reflected by the free-form mirror (23) and refracted by the meniscus lens (22) in sequence, and then converges on the imaging sensor (25) to achieve hyperspectral imaging.

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