Littrow short-wave imaging spectroscopy system based on free-form surfaces

Abstract

The present invention provides a Littrow short-wave imaging spectroscopy system based on a free-form surface, which mainly solves the technical problems that the existing Littrow spectroscopy system has different spectra for the same object during the later image processing due to the existence of spectral bending and spectral distortion. The system includes a slit, a Littrow optical module, a plane grating and a detection module arranged in sequence along the optical path. The design of combining the Littrow optical module with the free-form surface effectively suppresses the spectral bending and spectral distortion generated by the plane grating, thereby realizing the large relative aperture design of the system and improving the imaging quality of the system.

Description

Technical Field

[0001] The invention relates to a high-resolution imaging spectrometer, and in particular to a Littrow short-wave imaging spectrometer system based on a free-form surface. Background Art

[0002] The grating optical system is an optical system used to detect the target spectral information. Its imaging principle can be divided into convex reflection grating, concave reflection grating, plane transmission grating and plane reflection grating according to the existing optical model. Among them, the convex reflection grating and concave reflection grating related technologies are widely used abroad, but less involved in the domestic market; the imaging system of the plane transmission grating is slender, and the system stray radiation is very serious;

[0003] At present, the commonly used planar reflection grating spectral system is the Littrow spectral system, which has the advantages of small size, high signal-to-noise ratio, and compact structure. However, since the plane grating will produce very serious spectral bending and spectral distortion, the imaging spectral system will have the problem of different spectra for the same object during the later image processing. Based on this, an imaging spectral system with high imaging quality has not been developed by researchers. Summary of the invention

[0004] The purpose of the present invention is to solve the technical problems of the existing Littrow spectral system, such as different spectra for the same object in the later image processing, due to the existence of spectral bending and spectral distortion, and to provide a Littrow short-wave imaging spectral system based on a free-form surface. The present invention is based on the design method of the Littrow free-form surface short-wave imaging spectrometer, and utilizes the combination of the Littrow optical model and the free-form surface to effectively suppress the spectral bending and spectral distortion generated by the plane grating, thereby realizing the large relative aperture, high resolution, high signal-to-noise ratio, and lightweight and miniaturized design of the system.

[0005] To achieve the above object, the technical solution of the present invention is as follows:

[0006] A Littrow shortwave imaging spectroscopy system based on free-form surfaces, which is special in that:

[0007] It includes a slit, a Littrow optical module and a plane grating arranged in sequence along the optical path, and a detection module located above the slit;

[0008] The slit is used to receive the spectral information of the target radiation; the slit and the detection module are respectively located on both sides of the optical axis, and the distance between the central axes of the two is 7 to 11 mm;

[0009] The Littrow optical model is an optical path multiplexing lens group, including a first lens group, a folding mirror and a second lens group; the first lens group, the folding mirror and the second lens group form a collimating lens group along the incident light path, and form an imaging lens group along the reflected light path;

[0010] The first lens group comprises a first positive lens, a first negative lens and a second positive lens which are sequentially arranged along the incident light path, wherein the first positive lens is a free-form surface lens;

[0011] The folding mirror is a reflector, the plane where the folding mirror is located forms an angle of 45°±1′ with the optical axis of the first lens group, and is used to fold the light beam passing through the first lens group so that it is vertically incident on the second lens group;

[0012] The second lens group is arranged on the reflection light path of the folding mirror, the optical axis of the second lens group is perpendicular to the optical axis of the first lens group, and the second lens group includes a second negative lens and a third positive lens arranged along the incident light path;

[0013] The plane grating is located on the transmission light path of the second lens group, and is used to disperse the light beam passing through the second lens group, thereby separating different spectra, so that the light beam enters the detection module after passing through the second lens group, the folding mirror, and the first lens group;

[0014] The detection module is located on the reflection light path of the Littrow optical module (2) and is used to convert the received light signal into an electrical signal or an image signal.

[0015] Furthermore, the parameters of the first positive lens are expressed by a k-order XY polynomial:

[0016]

[0017] In the formula, z(x,y) represents the best fitting sag of the free-form surface; the first term of the formula is the base of the conic coefficient surface, and the second term is the sag offset relative to the base of the conic surface; c is the conic coefficient of the free-form surface; x is the coefficient component of the free-form surface in the x direction; y is the coefficient component of the free-form surface in the y direction; k is the number of terms of the free-form surface; B i,j are the coefficients of different terms of the polynomial, i and j represent the orders of variables x and y respectively, where 1≤i+j≤k.

[0018] Furthermore, the first positive lens is a meniscus free-form surface positive lens with the convex surface facing the image side;

[0019] The first negative lens is a biconcave negative lens;

[0020] The second positive lens is a meniscus positive lens with a convex surface facing the image side;

[0021] The second negative lens is a biconcave negative lens;

[0022] The third positive lens is a biconvex positive lens.

[0023] Further, the first positive lens has a front surface curvature radius of -60.1 mm, a rear surface curvature radius of -32 mm, a lens thickness of 7 mm, and a spacing of 6.5 mm;

[0024] The first negative lens has a front surface curvature radius of -40.5 mm, a rear surface curvature radius of 301 mm, a lens thickness of 4.5 mm, and a spacing of 7.4 mm;

[0025] The front surface curvature radius of the second positive lens is -71.23 mm, the rear surface curvature radius is -33.45, the lens thickness is 10 mm, and the interval is 31.5 mm;

[0026] The distance between the folding mirror and the second negative lens is 36.5 mm;

[0027] The front surface curvature radius of the second negative lens is 121.3 mm, the rear surface curvature radius is -85.3 mm, the lens thickness is 3 mm, and the interval is 4.8 mm;

[0028] The front surface curvature radius of the third positive lens is -102.1 mm, the rear surface curvature radius is 44.5 mm, the lens thickness is 7 mm, and the interval is 11.5 mm.

[0029] Further, the refractive index of the first positive lens is nd1=2.01, and the Abbe number is vd1=28.31;

[0030] The refractive index of the first negative lens is nd2=1.62, and the Abbe number is vd2=29.42;

[0031] The refractive index of the second positive lens is nd3=1.61, and the Abbe number is vd3=30.4;

[0032] The refractive index of the second negative lens is nd4=1.58, and the Abbe number is vd4=46.51;

[0033] The refractive index of the third positive lens is nd5=1.43, and the Abbe number is vd5=94.99.

[0034] Furthermore, the folding mirror is made of quartz glass.

[0035] Furthermore, the surface RMS of the folding mirror is 1 / 30λ@632.8nm.

[0036] Furthermore, the width of the slit is 30-100 μm.

[0037] Furthermore, the line density of the plane grating is 80-150 lp / mm, and the blaze angle of the grating is 5.2°.

[0038] Compared with the prior art, the present invention has the following beneficial effects:

[0039] 1. The free-form surface-based Littrow short-wave imaging spectroscopy system of the present invention comprises a slit, a Littrow optical module, a plane grating and a detection module which are sequentially arranged along an optical path. The design of combining the Littrow optical module with the free-form surface effectively suppresses the spectral bending and spectral distortion produced by the plane grating, and effectively eliminates the phenomenon of different spectra of the same object in the later image processing of the imaging spectroscopy system, thereby realizing a large relative aperture design of the system and improving the imaging quality of the system.

[0040] 2. The optical space volume of the free-form surface-based Littrow short-wave imaging spectroscopy system of the present invention is 83 mm×63 mm×42 mm, and the optical path is folded by a folding mirror, making the overall structure more compact.

[0041] 3. The assembly process of the free-form surface-based Littrow short-wave imaging spectroscopy system of the present invention is simple, and there is no special requirement for the spacing and relative positions between the optical elements, as long as the assembly process is met. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 It is a structural schematic diagram of an embodiment of a Littrow short-wave imaging spectroscopy system based on a free-form surface of the present invention;

[0043] Figure 2 This is a diagram showing the bending changes of different spectral curves in the edge field of view of an embodiment of the Littrow short-wave imaging spectroscopy system based on a free-form surface of the present invention;

[0044] Figure 3 It is a color distortion curve diagram of different spectral curves of the edge field of view of the Littrow short-wave imaging spectroscopy system embodiment based on the free-form surface of the present invention.

[0045] The reference numerals are as follows:

[0046] 1-slit, 2-Littrow optical module, 3-plane grating, 4-detection module, 5-first lens group, 51-first positive lens, 52-first negative lens, 53-second positive lens, 6-fold mirror, 7-second lens group, 71-second negative lens, 72-third positive lens. DETAILED DESCRIPTION

[0047] The principle of the present invention is: the Littrow optical model has the advantages of miniaturization and large relative aperture, but the current Littrow optical model is only suitable for spectrometers, not for spectral imagers. The present invention combines the Littrow optical model with a free-form surface, and uses the advantage of the free-form surface in expanding the field of view in the sagittal direction to expand the Littrow spectrometer into a Littrow imaging spectrometer. In addition, the free-form surface lens effectively suppresses the spectral bending and spectral distortion caused by the plane grating. The traditional Littrow optical model is broken through, and the model establishment and principle simulation of the design method of the Littrow free-form surface short-wave imaging spectrometer are completed.

[0048] The present invention is described in detail below in conjunction with the accompanying drawings. The accompanying drawings are in simplified form and use non-precise proportions, which are only used to conveniently and clearly assist in explaining the purpose of the embodiments of the present invention. It should be noted that the terms "first", "second", "third", etc. used in the present invention are only used for descriptive purposes and cannot be understood as indicating or implying relative importance.

[0049] See also Figure 1 The present invention provides a Littrow short-wave imaging spectroscopy system based on a free-form surface, the system comprising a slit 1, a Littrow optical module 2, a plane grating 3 and a detection module 4 located above the slit 1, which are sequentially arranged along the optical path. The slit 1 is used to receive the spectral information of the object and limit the spectral width of the system to achieve the expected spectral resolution. The width of the slit 1 is generally 30 to 100 μm. In this embodiment, the width of the slit 1 is 30 μm, which can effectively suppress the aliasing of the light image. The slit 1 and the detection module 4 are respectively located on both sides of the optical axis, and the distance between the central axes of the two is 7 to 11 mm. This distance setting can further improve the imaging quality of the system.

[0050] The Littrow optical model 2 is a light path multiplexing lens group, including a first lens group 5, a folding mirror 6 and a second lens group 7. The first lens group 5, the folding mirror 6 and the second lens group 7 form a collimating lens group along the incident light path and form an imaging lens group along the reflected light path.

[0051] The optical space volume of the system is 83 mm×63 mm×42 mm, and the optical path is folded by a folding mirror 6, making the overall structure more compact.

[0052] The first lens group 5 includes a first positive lens 51 , a first negative lens 52 and a second positive lens 53 which are sequentially arranged along the incident light path.

[0053] In this embodiment, the first positive lens 51 is a meniscus free-form surface positive lens with the convex surface facing the image side, and the asymmetric characteristics of the free-form surface are used to correct the spectral curvature, spectral distortion and off-axis astigmatism generated by the plane grating 3. The refractive index nd1 of the first positive lens 51 is 2.01, the Abbe number vd1 is 28.31, and the radius of curvature of the front surface of the first positive lens 51 is -60.1mm, the radius of curvature of the rear surface is -32mm, the lens thickness is 7mm, and the interval is 6.5mm. The free-form surface parameters of the first positive lens 51 are expressed by a k-order XY polynomial as follows:

[0054]

[0055] In the formula, z(x, y) indicates that the free-form surface is a surface sag; the first term of the formula is the cone coefficient surface base, and the second term is the sag offset relative to the cone surface base; c is the cone coefficient of the free-form surface; x is the component of the free-form surface in the x direction of the two-dimensional space; y is the component of the free-form surface in the y direction of the two-dimensional space; k is the order of the free-form surface; B i,j are coefficients of different terms of the polynomial, i and j represent the orders of variables x and y respectively, where 1≤i+j≤k. Table 1 shows the free-form surface parameters of the first positive lens 21 in this embodiment.

[0056] Table 1 Free-form surface parameters of the first positive lens

[0057] R K <![CDATA[x0y1]]> <![CDATA[x2y0]]> <![CDATA[x0y2]]> <![CDATA[x2y1]]> <![CDATA[x0y3]]> <![CDATA[x4y0]]> 10.74 -0.509 0.264 -1.289 -1.136 0.059 0.08 <![CDATA[4.738e -3 ]]>

[0058] The first negative lens 52 is a double concave negative lens; it is made of glass with a refractive index nd2=1.62 and an Abbe number vd2=29.42; the radius of curvature of the front surface of the first negative lens 52 is -40.5mm, the radius of curvature of the rear surface is 301mm, the lens thickness is 4.5mm, and the interval is 7.4mm.

[0059] The second positive lens 53 is a meniscus positive lens with the convex surface facing the image side; it is made of glass with a refractive index nd3=1.61 and an Abbe number vd3=30.4; the radius of curvature of the front surface of the second positive lens 53 is -71.23mm, the radius of curvature of the rear surface is -33.45mm, the lens thickness is 10mm, and the interval is 31.5mm.

[0060] The folding mirror 6 is a complex optical path folding reflector, which mainly folds the optical path to reduce the radial size of the system, making the entire structure more compact. The folding mirror 6 is made of quartz glass, and its surface RMS is 1 / 30λ@632.8nm. The plane where the folding mirror 6 is located is at an angle of 45°±1′ with the optical axis of the first lens group 5, and is used to fold the light beam passing through the first lens group 5 so that it is vertically incident on the second lens group 7.

[0061] The second lens group 7 is arranged on the reflected light path of the folding mirror 6, the optical axis of the second lens group 7 is perpendicular to the optical axis of the first lens group 5, and the second lens group 7 includes a second negative lens 71 and a third positive lens 72 arranged along the incident light path. The interval between the folding mirror 6 and the second negative lens 71 is 36.5mm. The second negative lens 71 is a double concave negative lens; it is made of glass with a refractive index nd4=1.58 and an Abbe number vd4=46.51; the radius of curvature of the front surface of the second negative lens 71 is 121.3mm, the radius of curvature of the rear surface is -85.3mm, the thickness of the lens is 3mm, and the interval is 4.8mm. The third positive lens 72 is a double convex positive lens; it is made of glass with a refractive index nd5=1.43 and an Abbe number vd6=94.99; the radius of curvature of the front surface of the third positive lens 72 is -102.1mm, the radius of curvature of the rear surface is 44.5mm, the thickness of the lens is 7mm, and the interval is 11.5mm.

[0062] The plane grating 3 is located on the transmission light path of the second lens group 7, and is used to disperse the light beam passing through the second lens group 7, thereby separating different spectra. The line density of the plane grating 3 is generally 80 to 150 lp / mm. In this embodiment, the line density of the plane grating 3 is 150 lp / mm, and the blaze angle of the grating is 5.2°.

[0063] The working process of the free-form surface-based Littrow short-wave imaging spectroscopy system of the present invention is as follows:

[0064] The incident light beam enters the Littrow optical module 2 through the slit 1, and is collimated by the first positive lens 51, the first negative lens 52 and the second positive lens 53 to reach the surface of the folding mirror 6. The folding mirror 6 folds the incident light beam so that it vertically enters the second lens group 7, and reaches the plane grating 3 after passing through the second negative lens 71 and the third positive lens 72. At this time, the exit pupil position of the system coincides with the plane grating 3. Finally, the incident light beam returns to the original path after being dispersed by the plane grating 3, and finally converges to the target surface of the detection module 4 to complete the imaging.

[0065] Figure 2 This is a diagram of the bending changes of different spectral curves in the edge field of view of this embodiment. The horizontal axis in the figure is the image plane height, and the vertical axis is the spectral bending amount at different image plane heights. It can be seen from the figure that the maximum spectral bending of the system is 2.3μm, and its ratio to the slit width is much smaller than the spacecraft's requirement for spectral instruments (generally required to be 1 / 5).

[0066] Figure 3This is a color distortion curve diagram of different spectral curves of the edge field of view of this embodiment. The horizontal axis in the figure is the wavelength, and the vertical axis is the spectral distortion corresponding to different wavelengths. It can be seen from the figure that the maximum spectral distortion of the system is less than 0.5μm, and its ratio to the slit width is much smaller than the spacecraft's requirement for spectral instruments (generally required to be 1 / 5).

[0067] Initial simulations show that the system operates in an ambient temperature of -40°C to 60°C; within the wavelength range of 0.9 to 1.7μm, the system's spectral resolution is better than 6nm, the spectral curvature is less than 2.3μm, and the pixel size is 15μm, meeting international high-resolution requirements.

[0068] The present invention mainly relates to the fields of spectral detection, spectral measurement, and spectral calibration, and can also be applied to the fields of aerospace, aviation atmospheric remote sensing, earth observation, medical detection, agricultural detection, etc. The Littrow short-wave imaging spectral system based on the free-form surface uses a design method combining the Littrow optical model with the free-form surface to effectively suppress the spectral bending and spectral distortion generated by the plane grating, and at the same time uses the asymmetry of the free-form surface to effectively correct the off-axis astigmatism caused by the slit deviation, which greatly improves the imaging quality of the system. In this way, the localization, low cost and miniaturization of the spectral instrument can be achieved. Compared with similar products at home and abroad, the Littrow free-form surface optical structure has the advantages of small size, light weight, high light-gathering ability, high signal-to-noise ratio, small color distortion, small spectral line bending, etc., and is very suitable for ground, airborne, micro-satellite and other platforms.

[0069] Although the embodiments of the present invention have been shown and described above, it will be apparent to those skilled in the art that any changes or modifications to the above embodiments should be deemed to fall within the protection scope of the present invention as long as they are within the spirit of the present invention.

Claims

1. A Littrow shortwave imaging spectroscopy system based on free-form surfaces, characterized in that: It comprises a slit (1), a Littrow optical module (2) and a plane grating (3) which are sequentially arranged along an optical path, and a detection module (4) located above the slit (1); The slit (1) is used to receive spectral information of target radiation; the slit (1) and the detection module (4) are respectively located on both sides of the optical axis, and the distance between the central axes of the two is 7 to 11 mm; The Littrow optical module (2) is an optical path multiplexing lens group, comprising a first lens group (5), an axis-folding mirror (6) and a second lens group (7); the first lens group (5), the axis-folding mirror (6) and the second lens group (7) form a collimating lens group along the incident light path, and form an imaging lens group along the reflected light path; The first lens group (5) comprises a first positive lens (51), a first negative lens (52) and a second positive lens (53) which are sequentially arranged along the incident light path, wherein the first positive lens (51) is a free-form surface lens; The folding mirror (6) is a reflecting mirror, the plane on which it is located forms an angle of 45°±1′ with the optical axis of the first lens group (5), and is used to fold the light beam passing through the first lens group (5) so that it is incident vertically into the second lens group (7); The second lens group (7) is located on the reflection light path of the axis-folding mirror (6), and the optical axis of the second lens group (7) is perpendicular to the optical axis of the first lens group (5); the second lens group (7) comprises a second negative lens (71) and a third positive lens (72) arranged along the incident light path; The plane grating (3) is located on the transmission light path of the second lens group (7) and is used to reflect the light beam passing through the second lens group (7) so that the light beam enters the detection module (4) after passing through the second lens group (7), the folding mirror (6) and the first lens group (5); The detection module (4) is located on the reflection light path of the Littrow optical module (2) and is used to convert the received light signal into an electrical signal or an image signal.

2. The free-form surface-based Littrow shortwave imaging spectroscopy system according to claim 1, characterized in that: The parameters of the first positive lens (51) are expressed by a k-order XY polynomial: In the formula, z(x,y) represents the best fitting sag of the free-form surface; the first term of the formula is the base of the conic coefficient surface, and the second term is the sag offset relative to the base of the conic surface; c is the conic coefficient of the free-form surface; x is the coefficient component of the free-form surface in the x direction; y is the coefficient component of the free-form surface in the y direction; k is the number of terms of the free-form surface; B i,j are the coefficients of different terms of the polynomial, i and j represent the orders of variables x and y respectively, where 1≤i+j≤k.

3. The free-form surface-based Littrow short-wave imaging spectroscopy system according to claim 1 or 2, characterized in that: The first positive lens (51) is a meniscus free-form surface positive lens with the convex surface facing the image side; The first negative lens (52) is a biconcave negative lens; The second positive lens (53) is a meniscus positive lens with a convex surface facing the image side; The second negative lens (71) is a biconcave negative lens; The third positive lens (72) is a biconvex positive lens.

4. The free-form surface-based Littrow shortwave imaging spectroscopy system according to claim 3, characterized in that: The first positive lens (51) has a front surface curvature radius of -60.1 mm, a rear surface curvature radius of -32 mm, a lens thickness of 7 mm, and a spacing of 6.5 mm; The first negative lens (52) has a front surface curvature radius of -40.5 mm, a rear surface curvature radius of 301 mm, a lens thickness of 4.5 mm, and a spacing of 7.4 mm; The second positive lens (53) has a front surface curvature radius of -71.23 mm, a rear surface curvature radius of -33.45, a lens thickness of 10 mm, and a spacing of 31.5 mm; The interval between the folding mirror (6) and the second negative lens (71) is 36.5 mm; The second negative lens (71) has a front surface curvature radius of 121.3 mm, a rear surface curvature radius of -85.3 mm, a lens thickness of 3 mm, and a spacing of 4.8 mm; The third positive lens (72) has a front surface curvature radius of -102.1 mm, a rear surface curvature radius of 44.5 mm, a lens thickness of 7 mm, and a spacing of 11.5 mm.

5. The free-form surface-based Littrow short-wave imaging spectroscopy system according to claim 4, characterized in that: The refractive index of the first positive lens (51) is nd1=2.01, and the Abbe number vd1 is 28.31; The refractive index of the first negative lens (52) is nd2=1.62, and the Abbe number is vd2=29.42; The refractive index of the second positive lens (53) is nd3=1.61, and the Abbe number vd3=30.4; The refractive index of the second negative lens (71) is nd4=1.58, and the Abbe number is vd4=46.51; The refractive index of the third positive lens (72) is nd5=1.43, and the Abbe number vd5=94.

99.

6. The free-form surface-based Littrow short-wave imaging spectroscopy system according to claim 5, characterized in that: The folding mirror (6) is made of quartz glass.

7. The free-form surface-based Littrow short-wave imaging spectroscopy system according to claim 6, characterized in that: The surface RMS of the folding mirror (6) is 1 / 30λ@632.8nm.

8. The free-form surface-based Littrow short-wave imaging spectroscopy system according to claim 7, characterized in that: The width of the slit (1) is 30 to 100 μm.

9. The free-form surface-based Littrow short-wave imaging spectroscopy system according to claim 8, characterized in that: The line density of the plane grating (3) is 80-150 lp / mm, and the blaze angle of the grating is 5.2°.

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