A method and system for spectrally imaging based on blazed gratings

Abstract

The present application relates to a kind of based on spectrometer imaging method and system of blaze grating.From the far field light of incident slit, into plano-convex lens, refract by plano-convex lens and meniscus lens, after again passing through thick catadioptric mirror concave surface reflection, with convergent light beam form is incident to convex blaze grating, after grating diffraction and reflection, form multiple beam single wavelength divergent light beam, in turn through thick catadioptric mirror reflection, meniscus lens and plano-convex lens refract, image on image plane, realize spectral imaging.The present application adopts immersion littrow-Offner structure, incident slit is off-axis in meridian and sagittal direction, so that incident slit and image plane are separated, facilitate engineering implementation;Spectrometer imaging system uses full immersion optical path, so that imaging spectrometer structure is more simple and compact, more convenient to assemble and use;Grating selects convex blaze grating, has high diffraction efficiency, so that the present imaging system can play a role in more extensive application field.

Description

TECHNICAL FIELD

[0001] The present application relates to a kind of based on blaze grating spectroscopic imaging method and its system, belong to spectral imaging technical field. BACKGROUND

[0002] Spectral imaging technology combines imaging technology and spectral technology, which can obtain the two-dimensional spatial structure and one-dimensional spectral information of the detected object, and obtain the spectral image of the detected target. Through the spectral information of the object, data that cannot be obtained by traditional observation methods can be obtained. Imaging spectrometer technology has important applications in mineral exploration, military intelligence, disaster prevention and control, and precise mapping.

[0003] The Offner relay imaging system is a three-reflection concentric system composed of two concentric spherical mirrors, which has a simple and symmetrical structure and can obtain high-resolution high-quality image. In the 1970s, Thevenon first suggested replacing the convex reflective mirror in the Offner relay imaging system with a convex diffraction grating to obtain an Offner spectroscopic imaging system. The Littrow spectroscopic imaging system is a collimation system, which uses a concave mirror as a collimation and imaging objective lens. The structure is simple and compact, the incident angle on the grating is equal to the diffraction angle, a high diffraction efficiency can be obtained, the entrance slit and the exit slit are located on the same side of the dispersion system, and the centers of the three are in the main section.

[0004] The Littrow-Offner spectroscopic imaging system is a concentric system with a convex diffraction grating as a spectroscopic element. It has the advantages of both Offner spectroscopic imaging system and Littrow spectroscopic imaging system. Compared with the classic Offner spectroscopic imaging system, the Littrow-Offner spectroscopic imaging system is smaller and lighter, which meets the development direction and trend of modern imaging spectrometer, i.e. large relative aperture, high resolution, small volume, light weight and simple and compact structure. The existing Littrow-Offner spectroscopic imaging system is suitable for high-resolution, narrow working waveband and wide spectral dimension background spectral instruments, and has been successfully applied to high-resolution imaging remote sensing spectrometers for detecting and analyzing atmospheric composition in the visible light waveband of the geostationary orbit. However, there are still limitations such as insufficient small size, high processing requirements, high cost and difficulty. For example, Chinese invention patent CN103268017B discloses a Littrow-Offner spectroscopic imaging system for spectral instruments. The system adopts a concentric Littrow-Offner structure, the length of the entrance slit is only 6mm, which cannot meet the requirements of large field of view imaging of imaging spectrometers; the total length of the system is 195mm, which limits the further application of imaging spectrometers. SUMMARY

[0005] This invention addresses the shortcomings of existing technologies by providing an immersion littrow-Offner imaging spectroscopic system and its imaging method based on a blazed grating, which features high diffraction efficiency, high resolution, simple and compact structure, and ease of fabrication and assembly.

[0006] The technical solution to achieve the purpose of this invention is to provide a beam splitting imaging system based on a blazed grating. Its optical elements include an entrance slit, a plano-convex lens, a meniscus lens, a thick reflector, a convex blazed grating, and an image plane. The optical system has an immersion littrow-Offner structure. The entrance slit is off-axis in the meridional and sagittal directions.

[0007] The convex surface of the plano-convex lens is cemented to the concave surface of the meniscus lens; the convex surface of the meniscus lens is cemented to the concave surface of the thick folding mirror, and the reflecting surface of the thick folding mirror is a convex surface; the working surface of the convex blazed grating is set on the convex surface of the meniscus lens, and the highest point of the convex blazed grating surface coincides with the convex vertex of the meniscus lens.

[0008] radius of curvature of the concave surface of a meniscus lens The radius of curvature of the convex surface of the meniscus lens Condition met: 0.75≤ ≤0.85; Radius of curvature of the convex surface of a meniscus lens The radius of curvature of the convex surface of the thick-reflected mirror Condition met: 0.45≤ ≤0.55; Center thickness of plano-convex lens With respect to the center thickness of the thick-reflecting mirror Condition met: 0.75≤ ≤0.8;

[0009] The convex blazed grating has an incident angle of [missing information]. for vertex The blaze angle is 90°. for The scribing density is 1100 lp / mm;

[0010] Plano-convex lenses and thick-reflecting mirrors are made of the same material and have the same refractive index. 1.4≤ ≤1.5, the refractive index of the material of the meniscus lens (3) 1.6≤ ≤1.7.

[0011] The blazed grating-based beam-splitting imaging system of this invention has an incident slit with off-axis distances of x and y in the meridional and sagittal directions, respectively, satisfying the condition: 15mm ≤ ≤20mm, 25mm≤y≤30mm.

[0012] The technical scheme of the present application also comprises a spectrometer imaging method based on a blazed grating, comprising the following steps:

[0013] Step 1: light is incident from an entrance slit, and forms a divergent light beam after being refracted by a plano-convex lens and a meniscus lens in turn, and enters a thick catadioptric mirror; the entrance slit is away from the optical axis in both meridional and sagittal directions, the convex surface of the plano-convex lens is glued to the concave surface of the meniscus lens, and the convex surface of the meniscus lens is glued to the concave surface of the thick catadioptric mirror;

[0014] Step 2: the divergent light beam formed in step 1 continues to propagate in the thick catadioptric mirror, and forms a convergent light beam after being reflected by the concave surface of the thick catadioptric mirror;

[0015] Step 3: the working surface of a convex blazed grating is arranged on the convex surface of the meniscus lens, and the convex vertex of the surface of the convex blazed grating coincides with the convex vertex of the meniscus lens; the convergent light beam obtained in step 2 is incident onto the working surface of the convex blazed grating, and forms a plurality of divergent light beams of single wavelength after being diffracted and reflected by the grating;

[0016] Step 4: the plurality of divergent light beams of different wavelengths formed in step 3 form a plurality of convergent light beams of different wavelengths after being reflected by the concave surface of the thick catadioptric mirror, and the plurality of convergent light beams of different wavelengths pass through the meniscus lens and the plano-convex lens in turn, and are imaged onto different positions on an image plane, so that spectral imaging is realized.

[0017] The immersion-type Littrow-Offner spectrometer imaging system provided by the present application has the following characteristics: concentricity is adopted, and spherical aberration is not introduced; based on the principle of a Rowland circle, a concave mirror with a proper radius of curvature and a convex diffraction grating are selected to eliminate field curvature, so that a flat field is realized; since the main residual aberration of the system is positive astigmatism, a meniscus lens with negative astigmatism is introduced to balance the positive astigmatism of the system, so that the image quality is improved; the entire optical system is glued by a plano-convex lens, a meniscus lens and a thick catadioptric mirror, and is of an immersion structure, so that the system structure is more stable, and the volume is further reduced.

[0018] Meanwhile, the aberration correction capability of the convex blazed grating is used to further increase the relative aperture of the system and improve the imaging quality; the ruling density of the convex diffraction grating affects the spectral resolution, the higher the ruling density, the greater the dispersion degree, and thus the higher the spectral resolution; and the blazed grating can obtain a diffraction efficiency greater than 50% in a spectral range, and thus the energy utilization rate of the system is improved. Therefore, the convex blazed grating with high ruling density is introduced in the present application, so that the spectral resolution is improved and the diffraction efficiency is also improved.

[0019] The immersion littrow-Offner spectroscopic imaging system provided by this invention improves the diffraction efficiency of the system by using a blazed grating, which is far from the optical axis in both the meridional and sagittal directions, thus separating the incident slit and the image plane and facilitating engineering implementation; the use of an immersion optical path further reduces the size of the system, thereby enabling it to play a role in a wider range of application scenarios.

[0020] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0021] 1. The spectroscopic imaging method provided by the present invention adopts an immersion littrow-Offner structure for its imaging system; the entrance slit is off-axis in the meridional and sagittal directions, which separates the entrance slit from the image plane. The structure is compact, stable, easy to process and assemble, and convenient for engineering implementation, which greatly reduces the production and development cost and is conducive to the miniaturization and portability of imaging spectrometers.

[0022] 2. The spectroscopic imaging system provided by the present invention uses a blazed grating with high line density, and has a diffraction efficiency of more than 50% in the spectral range, thereby improving the energy utilization rate of the system.

[0023] 3. The beam splitting imaging system is made of optical elements bonded together, without any mechanical parts. This integrated and modular component is very convenient to assemble, adjust and use. Moreover, all lenses are made of domestically produced glass materials, which reduces the difficulty and cost of lens processing and has practical application value. Attached Figure Description

[0024] Figure 1 , 2 3 and 4 are respectively front view, top view and side view schematic diagrams of the structured optical path of the beam splitting imaging system provided in the embodiments of the present invention;

[0025] Figure 4 This is a schematic diagram of the structure of the convex blazed grating of the spectroscopic imaging system provided in the embodiment of the present invention;

[0026] Figure 5 This is a ray tracing point array diagram of the spectroscopic imaging system provided in this embodiment of the invention;

[0027] Figure 6 This is the energy concentration curve at the center wavelength of the spectroscopic imaging system provided in this embodiment of the invention;

[0028] Figure 7 This is a graph of the MTF (Mean Transfer Function) curve of the spectroscopic imaging system provided in this embodiment of the invention.

[0029] Figure 8 This is a full-band diffraction efficiency curve of the convex blazed grating of the spectroscopic imaging system provided in this embodiment of the invention.

[0030] In the figure: 1. incident slit; 2. plano-convex lens; 3. meniscus lens; 4. thick catadioptric mirror; 2.1 plano-convex lens plane; 2.2 plano-convex lens convex surface; 3.1 meniscus lens concave surface; 3.2 meniscus lens convex surface; 3.3 convex surface blazed grating; 4.1 thick catadioptric mirror concave surface; 4.2 thick catadioptric mirror convex surface; 5. image plane; 6. optical axis. DETAILED DESCRIPTION

[0031] The specific embodiments of the present application are further described below in conjunction with the accompanying drawings and examples.

[0032] Example 1:

[0033] The technical solution of the present embodiment is an immersed Littrow-Offner spectroscopic imaging system and an imaging method thereof. The working wavelength of the system is 400 nm to 700 nm, the optical lens is made of a plano-convex lens, a meniscus lens and a thick catadioptric mirror, and the F number is F / #=3.3.

[0034] Referring to the accompanying drawings Figure 1 and 2 , which are respectively the main view optical path diagram and the top view optical path diagram of the immersed Littrow-Offner spectroscopic imaging system provided by the present embodiment; in the rectangular coordinate system in the figure, the positive direction of the y-axis is the meridional direction, the negative direction of the x-axis is the sagittal direction, and the positive direction of the z-axis is the light incident direction. The imaging spectrometer comprises an incident slit 1, a plano-convex lens 2, a meniscus lens 3, a thick catadioptric mirror 4, a convex surface blazed grating 3.3 and an image plane 5; 2.1 is the plano-convex lens plane, 2.2 is the plano-convex lens convex surface; 3.1 is the meniscus lens concave surface, 3.2 is the meniscus lens convex surface; 4.1 is the thick catadioptric mirror concave surface, 4.2 is the thick catadioptric mirror convex surface; the plano-convex lens convex surface 2.2 is cemented with the meniscus lens concave surface 3.1; the meniscus lens convex surface 3.2 is cemented with the thick catadioptric mirror concave surface 4.1; the convex surface blazed grating 3.3 is directly engraved on the meniscus lens convex surface 3.2; the length direction of the incident slit 1 and the engraved line direction of the convex surface blazed grating 3.3 are both perpendicular to the paper surface; the incident slit 1 and the image plane 5 are symmetrical about the optical axis 6.

[0035] Referring to the accompanying drawings Figure 3It is a side view schematic diagram of the optical path of the immersion Littrow-Offner spectroscopic imaging system provided in the embodiment, and the positive direction of the y-axis of the rectangular coordinate system in the diagram is the meridional direction, and the negative direction of the x-axis is the sagittal direction. As can be seen from the diagram, the incident slit 1 is off-axis in the meridional and sagittal directions at the same time, and the off-axis amount ensures that the convex blazed grating will not block the incident slit beam and the outgoing beam. The incident slit 1 and the image plane 5 are located on both sides of the convex blazed grating 3.3. In the embodiment, the length of the incident slit is 20 mm, the incident slit is off-axis by 27 mm in the meridional direction, and off-axis by 19 mm in the sagittal direction. The highest point of the surface of the convex blazed grating 3.3 coincides with the convex vertex of the meniscus lens 3, and the meniscus lens 3 is cemented with the thick catadioptric mirror 4.

[0036] Referring to FIG. 1, Figure 4 It is a schematic diagram of the convex blazed grating structure of the immersion Littrow-Offner spectroscopic imaging system provided in the embodiment, the grating incidence angle is 7°, the grating top angle is 90°, the blaze angle is 21°, a positive first-order diffraction grating is used, and the ruling density of the convex blazed grating is 1100 lp / mm.

[0037] The performance parameters of the imaging spectrometer provided in the embodiment meet the conditions of Table 1.

[0038] Table 1

[0039] .

[0040] The optical elements (surfaces) of the embodiment meet the conditions of Table 2.

[0041] Table 2:

[0042] .

[0043] The spectroscopic imaging device provided in the embodiment is used, and the spectroscopic imaging method includes the following steps:

[0044] Step 1: light is incident from the incident slit, forms a divergent beam after being refracted by the plano-convex lens and the meniscus lens in turn, and enters the thick catadioptric mirror; the incident slit is away from the optical axis in the meridional and sagittal directions, the convex surface of the plano-convex lens is cemented with the concave surface of the meniscus lens, and the convex surface of the meniscus lens is cemented with the concave surface of the thick catadioptric mirror;

[0045] Step 2: the divergent beam formed in step 1 continues to propagate in the thick catadioptric mirror, and is reflected on the concave surface of the thick catadioptric mirror to form a convergent beam;

[0046] Step 3: The working surface of the convex blazed grating is set on the convex surface of the meniscus lens, and the convex vertex of the convex blazed grating coincides with the convex vertex of the meniscus lens; the converging beam obtained in Step 2 is incident on the working surface of the convex blazed grating, and after diffraction and reflection by the grating, it forms multiple single-wavelength diverging beams.

[0047] Step 4: The multiple divergent beams of different wavelengths formed in Step 3 are reflected by the concave surface of the thick refracting mirror to form multiple converging beams of different wavelengths. These beams are then refracted sequentially by the meniscus lens and the plano-convex lens, forming images at different positions on the image plane, thus achieving spectral imaging.

[0048] See appendix Figure 5 It is a ray tracing point diagram of light passing through the immersion littrow-Offner spectroscopic imaging system provided in this embodiment. The root mean square radius of the point diagrams corresponding to the three different wavelengths in the figure is less than 2μm, indicating good imaging quality and meeting the system usage requirements.

[0049] See appendix Figure 6 This is the energy concentration curve at the center wavelength of the immersion littrow-Offner spectroscopic imaging system provided in this embodiment. The horizontal axis is the distance from the centroid, and the vertical axis is the proportion of diffraction energy concentrated within a single pixel size. The top curve is the diffraction limit, and the other curves represent the energy concentration curves of different fields of view. It can be seen that 80% of the energy is concentrated within the detector pixel area.

[0050] See appendix Figure 7 This is a full-band optical transfer function (MTF) curve of the immersion littrow-Offner spectroscopic imaging system provided in this embodiment. In the figure, (a), (b), and (c) are the MTF curves of all fields of view on the image planes corresponding to wavelengths of 400 nm, 550 nm, and 700 nm, respectively, provided in this embodiment. As can be seen from the figure, at 83 lp / mm, the optical transfer function of the entire field of view in the working band from 400 nm to 700 nm is greater than 0.6, close to the diffraction limit. The curves are smooth and compact, indicating that the system provides clear and uniform imaging, and the system has good imaging quality across the entire band and field of view.

[0051] See appendix Figure 8 This is a full-band diffraction efficiency curve of the convex blazed grating of the immersion littrow-Offner spectroscopic imaging system provided in this embodiment. It can be seen that the positive first-order diffraction efficiency of the grating is greater than 50% in the spectral range, and can reach up to 80%.

[0052] The results prove that the immersion Littrow-Offner spectral imaging system provided by the application can reach 3.3 in F number, can reach 20 mm in incident slit length, can reach 0.6 in optical transfer function of full working waveband and full field of view at a sampling frequency of 83 lp / mm, can reach 0.5 nm in spectral resolution, can reach 1 mu m in spectral line bending and color distortion, and meets the requirements of large field of view, miniaturization and high resolution of the spectral imaging system.

[0053] The immersion Littrow-Offner spectral imaging system provided by the application improves the diffraction efficiency of the system through the blazed grating, is far away from the optical axis in the meridian and sagittal directions, separates the incident slit and the image plane, and is convenient for engineering implementation; the immersion optical path is used, the volume of the system is further reduced, and the system plays a role in a wider application scene.

Claims

1. A spectrometric imaging system based on blazed gratings, characterized by: The optical element comprises an entrance slit (1), a plano-convex lens (2), a meniscus lens (3), a thick catadioptric mirror (4), a convex blazed grating (3.3) and an image plane (5), and the optical system is in an immersed Littrow-Offner structure; the entrance slit is off-axis in the meridional and sagittal directions; The convex surface (2.2) of the plano-convex lens is cemented with the concave surface (3.1) of the meniscus lens (3); the convex surface (3.2) of the meniscus lens (3) is cemented with the concave surface (4.1) of the thick catadioptric mirror, and the reflecting surface of the thick catadioptric mirror is a convex surface (4.2); the working surface of the convex blazed grating is arranged on the convex surface of the meniscus lens, and the surface highest point of the convex blazed grating coincides with the convex surface vertex of the meniscus lens; Curvature radius of meniscus lens concave surface R 1Curvature radius of meniscus lens convex surface R 2Satisfies condition: 0.75≤ R 1 / R 2≤0.85; Curvature radius of meniscus lens convex surface R 2Curvature radius of thick catadioptric convex surface (4.2) R 3Satisfies condition: 0.45≤ R 2 / R 3≤0.55; Center thickness of plano-convex lens d 1Center thickness of thick catadioptric d 2Satisfies condition: 0.75≤ d 1 / d 2≤0.8; The convex blazed grating has a grating incidence angle α of 5°≤α≤10°, a top angle β of 90°, a blazed angle γ of 20.5°≤γ≤21.5° and a ruling density of 1100 lp / mm. The material of the plano-convex lens (2) and the thick catadioptric mirror (4) is the same, the refractive index n 1 is 1.4 ≤ n 1 ≤ 1.5, the material of the meniscus lens (3) has a refractive index n 2 is 1.6 ≤ n 2 ≤ 1.

7.

2. The imaging system of claim 1, wherein: The off-axis distances of the entrance slit in the meridional and sagittal directions correspond to x and y respectively, and satisfy the conditions: 15mm≤x≤20mm and 25mm≤y≤30mm.

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