Far infrared broadband athermalization folding and ultrafolding hybrid lens and optical model design method

Abstract

The invention provides a far-infrared broadband athermalization refraction and super-refraction hybrid lens and an optical model design method. The far-infrared broadband athermalization refraction and super-refraction hybrid lens comprises a diaphragm, a first lens, a second lens, a third lens and an optical window, the diaphragm is a first surface; the first lens is an aspherical lens and comprises a second surface facing the object surface and a third surface facing the image surface, and the second surface and the third surface are even-order aspherical surfaces; the second lens is an aspheric lens and comprises a fourth surface facing the object surface and a fifth surface facing the image surface, and the fourth surface and the fifth surface are even aspheric surfaces; the third lens is a metasurface lens and comprises a sixth surface facing the object plane and a seventh surface facing the image plane, the sixth surface is a standard plane, and the seventh surface is a binary plane; the optical window comprises an eighth surface facing the object surface and a ninth surface facing the image surface, and the eighth surface and the ninth surface are standard surfaces; materials of the first lens, the second lens and the third lens have different thermal expansion coefficients. Broadband athermalization can be achieved in the whole far infrared band, and the requirements for small size, light weight and compact structure are met.

Description

Technical Field

[0001] The present invention relates to the technical field of optical engineering, and in particular provides a far-infrared broadband athermal refractive-diffractive hybrid lens and an optical model design method. Background Art

[0002] Infrared imaging technology is an imaging method that uses the difference in thermal radiation intensity between the target and the background to highlight the target. This imaging technology has the advantages of not requiring additional illumination, being able to work all-weather and all-day, and having strong anti-interference ability, and is widely used in military and civilian fields. During the use of an infrared lens, the change in the external environmental temperature will affect the refractive index of the lens material, and at the same time, the materials of the infrared lens and the barrel structure will undergo thermal expansion, resulting in a change in the geometric structure of the lens. Eventually, the best focal plane position of the lens shifts, causing the imaging quality to decline.

[0003] The passive athermalization system is a system that does not rely on external energy sources or active control mechanisms. It utilizes the complementarity of the thermal characteristics of optical element materials, and through the reasonable distribution between materials with different characteristics, it cleverly reduces the influence brought by temperature changes, thereby achieving the athermal performance of the optical system. This method has the advantages of low energy consumption, low cost, and high system reliability, and has high application value. However, the existing passive athermal optical systems have complex structures, are large and heavy, are not easy to integrate, and have high costs, and are limited in applications where the installation platform is limited and the energy consumption is limited. Summary of the Invention

[0004] The purpose of the present invention is to provide a far-infrared broadband athermal refractive-diffractive hybrid lens and an optical model design method to achieve broadband athermalization in the entire far-infrared band. By introducing a metasurface lens to replace one of the lenses in the traditional lens group, at the same time, the silicon-based metasurface lens has a lower cost compared to traditional refractive lenses designed based on materials such as germanium, zinc selenide, and zinc sulfide, and can meet the requirements of small volume, small weight, and compact structure.

[0005] To achieve the above object, in a first aspect, a far-infrared broadband anastigmatic refractive-diffractive hybrid lens includes a diaphragm, a first lens, a second lens, a third lens, and an optical window sequentially arranged from the object side to the image side; the diaphragm is the first surface; the first lens is an aspherical lens, including a second surface facing the object surface and a third surface facing the image surface, and both the second surface and the third surface are even-order aspherical surfaces; the second lens is an aspherical lens, including a fourth surface facing the object surface and a fifth surface facing the image surface, and both the fourth surface and the fifth surface are even-order aspherical surfaces; the third lens is a metasurface lens, including a sixth surface facing the object surface and a seventh surface facing the image surface, the sixth surface is a standard surface, and the seventh surface is a binary surface; the optical window includes an eighth surface facing the object surface and a ninth surface facing the image surface, and both the eighth surface and the ninth surface are standard surfaces; the materials of the first lens, the second lens, and the third lens have different thermal expansion coefficients.

[0006] Further, the material used for the first lens is Ge, and its thermal expansion coefficient is 6.1×10 -6 / ℃; the material used for the second lens is ZnSe, and its thermal expansion coefficient is 7.1×10-6 / ℃; the material used for the third lens is Si, and its thermal expansion coefficient is 2.5×10 -6 / ℃; the barrel structure of the optical system uses an aluminum metal material, and its thermal expansion coefficient is 23.6×10 -6 / ℃; the optical window is a germanium window with a thickness of 1 mm, and the physical distance from the germanium window to the photosensitive surface is 1.2 mm.

[0007] Further, the metasurface lens structure of the third lens is arranged on a silicon substrate with a thickness of 0.3 mm, 0.5 mm, or 1 mm.

[0008] Further, the surface equation of the aspherical lens is:

[0009]

[0010] where z is the vertical height from the aspherical vertex to the aspherical surface at the radial distance r, c is the curvature of the aspherical surface, related to the radius of curvature R of the surface, c = 1 / R, k is the conic coefficient, which determines the deviation degree between the aspherical surface and the spherical surface. When k = 0, the equation degenerates into a spherical surface equation. A is the fourth-order aspherical coefficient, B is the sixth-order aspherical coefficient, C is the eighth-order aspherical coefficient, D is the tenth-order aspherical coefficient, and E is the twelfth-order aspherical coefficient.

[0011] Further, the phase distribution equation of the binary surface is:

[0012]

[0013] Among them, M is the diffraction order of the metasurface lens, and the diffraction order is set to "-1". ρ is the radial coordinate of each meta - unit in the metasurface lens, R0 is the normalized radius, and its value is set to 1 mm. a, b, c, d, e, f are the optimization coefficients of the quadratic term, quartic term, sextic term, octic term, decic term, and dodecic term of the first metasurface lens respectively.

[0014] To achieve the above - mentioned purpose, in a second aspect, an optical model design method for a far - infrared broadband athermalized refractive - diffractive hybrid lens includes:

[0015] Establish an optical model. Starting from behind the object surface, insert the first surface as a standard surface, set it as the aperture surface, and set its thickness as a variable; insert the second surface as an even - order aspheric surface, and the third surface as an even - order aspheric surface to form the first lens. Set the radius of curvature and thickness of the second surface and the third surface as variables, and define the material of the second surface as Ge material; insert the fourth surface as an even - order aspheric surface, and the fifth surface as an even - order aspheric surface to form the second lens. Set the radius of curvature and thickness of the fourth surface and the fifth surface as variables, and define the material of the fourth surface as Ge material; insert the sixth surface as a standard surface, and the seventh surface as a binary surface to form the third lens. Set the thickness of the sixth surface as 0.5 mm, and set the thickness of the seventh surface as a variable; insert the eighth surface as a standard surface, and the ninth surface as a standard surface to form the optical window. Set the thickness of the eighth surface as 1 mm, and set the thickness of the ninth surface as 1.2 mm.

[0016] Furthermore, set the surface - shape equation of the aspheric lens as:

[0017]

[0018] Among them, z is the vertical height from the vertex of the aspheric surface to the aspheric surface at the radial distance r, c is the curvature of the aspheric surface, which is related to the radius of curvature R of the surface, c = 1 / R, k is the conic coefficient, which determines the deviation degree between the aspheric surface and the spherical surface. When k = 0, the equation degenerates into a spherical - surface equation. A is the fourth - order aspheric coefficient, B is the sixth - order aspheric coefficient, C is the eighth - order aspheric coefficient, D is the tenth - order aspheric coefficient, and E is the twelfth - order aspheric coefficient.

[0019] Furthermore, set the fourth - order, sixth - order, eighth - order, tenth - order, and twelfth - order aspheric coefficients of the first lens and the second lens as variables;

[0020] The phase - distribution equation of the binary surface is:

[0021]

[0022] Among them, M is the diffraction order of the metasurface lens, and the diffraction order is set to "-1". ρ is the radial coordinate of each meta-unit in the metasurface lens. R0 is the normalized radius, and its value is set to 1 mm. a, b, c, d, e, and f are the optimization coefficients of the quadratic term, quartic term, sextic term, octic term, decic term, and dodecic term of the first metasurface lens, respectively.

[0023] Furthermore, set the optimization coefficients of the quadratic term, quartic term, sextic term, octic term, decic term, and dodecic term of the metasurface lens as variables; use the spot diagram as the evaluation and optimization function of the optical system, and optimize to obtain the initial optical model that meets the technical indicators without thermal compensation optimization.

[0024] Furthermore, the principle of thermal compensation optimization is to endow the optical models at different temperatures with different parameter structures, set the parameters at one temperature as variables, and endow the parameters at other temperatures with thermal pick-up. Therefore, the parameters will change with the change of temperature, and the change amount is related to the thermal expansion coefficient of the lens material itself, the thermo-optic coefficient (the degree of change of the material refractive index with temperature), and the thermal expansion coefficient of the structural parts. Through optimization, the performance of the optical model at each temperature is balanced, and the situation of rapid decline in the performance of the optical model when the temperature changes is avoided.

[0025] Furthermore, create a thermal analysis multi-structure in the multi-structure editor, set the number of multi-structures to 5, set the lowest temperature to -50 °C, and the highest temperature to 50 °C, that is, it includes six structures of -50 °C, -25 °C, 0 °C, 20 °C, 25 °C, and 50 °C.

[0026] Furthermore, set the radius of curvature of the second surface, S3 - the third surface, S4 - the fourth surface, and the fifth surface of the optical model at the ambient temperature of 20 °C as variables, set the thicknesses of the second surface, S3 - the third surface, S4 - the fourth surface, the fifth surface, and the seventh surface as variables, and at the same time set the above parameters at -50 °C, -25 °C, 0 °C, 25 °C, and 50 °C as thermal pick-up. Optimize to obtain the optical model with thermal compensation optimization in the range of -50 °C to 50 °C.

[0027] Compared with the prior art, the far-infrared broadband athermalized refractive-metamaterial hybrid lens provided by the present invention has the following technical advantages:

[0028] First, for the provided hybrid lens, the aspherical lens, the metasurface lens, and the optical window are a combination of optical materials with different refractive index temperature coefficients and thermal expansion coefficients. Through the method of optical passive thermal compensation, broadband athermalization is achieved in the entire far-infrared band. In the temperature range of -50 °C to +50 °C, the imaging quality of the optical system does not decrease significantly due to extreme temperature changes and can operate in the entire band of 8 - 12 μm.

[0029] In a second aspect, the provided hybrid lens uses a metasurface lens to replace the traditional refractive lens and consists of only three optical elements. The thickness of the metasurface lens is only at the wavelength level, and its volume and weight are significantly reduced, realizing the compactness and light weight of the far-infrared athermal optical system;

[0030] In a third aspect, the metasurface lens uses low-cost single-crystalline silicon material and can be fabricated relying on semiconductor processes. Compared with traditional refractive lenses fabricated based on materials such as germanium, zinc selenide, and sulfide, it has a lower manufacturing cost. Description of the Drawings

[0031] In order to make the objectives, technical solutions, and advantages of the present invention clearer and more understandable, the present invention will be further described in detail below with reference to the drawings. It should be understood that the drawings described herein are used to provide a further understanding of the present invention, form a part of this application, and the illustrative embodiments and descriptions of the present invention are used to explain the present invention and do not constitute an improper limitation of the present invention. In the drawings:

[0032] Figure 1 is the optical path diagram of the optical system embodiment of the far-infrared broadband athermal refractive-metamaterial hybrid lens provided by the present invention.

[0033] Figure 2 is the MTF curve diagram of the optical system at a temperature of -50°C for the 8 - 12μm broadband.

[0034] Figure 3 is the MTF curve diagram of the optical system at a temperature of -25°C for the 8 - 12μm broadband.

[0035] Figure 4 is the MTF curve diagram of the optical system at a temperature of 0°C for the 8 - 12μm broadband.

[0036] Figure 5 is the MTF curve diagram of the optical system at a temperature of 25°C for the 8 - 12μm broadband.

[0037] Figure 6 is the MTF curve diagram of the optical system at a temperature of 50°C for the 8 - 12μm broadband.

[0038] Figure 7 is the spot diagram of the optical system at a temperature of -50°C for the 8 - 12μm broadband.

[0039] Figure 8 is the spot diagram of the optical system at a temperature of -25°C for the 8 - 12μm broadband.

[0040] Figure 9 is the spot diagram of the optical system at a temperature of 0°C for the 8 - 12μm broadband.

[0041] Figure 10 The spot diagram of the optical system at a temperature of 25°C for a broadband of 8 - 12μm.

[0042] Figure 11 The spot diagram of the optical system at a temperature of 50°C for a broadband of 8 - 12μm.

[0043] The reference numerals are as follows:

[0044] 1 - diaphragm, 2 - first lens, 3 - second lens, 4 - third lens, 5 - optical window, 6 - image plane;

[0045] S1 - first surface, S2 - second surface, S3 - third surface, S4 - fourth surface, S5 - fifth surface, S6 - sixth surface, S7 - seventh surface, S8 - eighth surface, S9 - ninth surface, S10 - tenth surface. Detailed implementation manners

[0046] The present invention provides a far - infrared broadband athermalized refractive - diffractive - metasurface hybrid lens and an optical model design method, including a diaphragm, a first lens, a second lens, a third lens and an optical window; the diaphragm is the first surface; the first lens is an aspherical lens, including a second surface facing the object plane and a third surface facing the image plane, both of which are even - order aspherical surfaces; the second lens is an aspherical lens, including a fourth surface facing the object plane and a fifth surface facing the image plane, both of which are even - order aspherical surfaces; the third lens is a metasurface lens, including a sixth surface facing the object plane and a seventh surface facing the image plane, the sixth surface is a standard surface, and the seventh surface is a binary surface; the optical window includes an eighth surface facing the object plane and a ninth surface facing the image plane, both of which are standard surfaces; the materials of the first lens, the second lens and the third lens have different thermal expansion coefficients.

[0047] The far - infrared broadband athermalized refractive - diffractive - metasurface hybrid lens and the optical model design method provided by the present invention can solve the problem that the existing far - infrared broadband athermalized lenses cannot meet the requirements of compact structure, small volume, small weight and low cost.

[0048] Next, through the drawings and embodiments, the technical solutions of the present invention will be further described in detail. The description of the exemplary embodiments is only illustrative and in no way limits the present disclosure and its application or use. The present disclosure can be implemented in many different forms and is not limited to the embodiments described herein. These embodiments are provided to make the present disclosure thorough and complete and to fully convey the scope of the present disclosure to those skilled in the art.

[0049] The present invention provides a far-infrared broadband athermalized refractive-diffractive hybrid lens. The optical lens is sequentially arranged from the object side to the image side: a diaphragm 1, a first lens 2, a second lens 3, a third lens 4, and an optical window 5. Among them, the diaphragm 1 is the first surface S1; the first lens 2 is an aspherical lens, including a second surface S2 facing the object surface and a third surface S3 facing the image surface, both of which are even aspheres; the second lens 4 is an aspherical lens, including a fourth surface S4 facing the object surface and a fifth surface S5 facing the image surface, both of which are even aspheres; the third lens 4 is a metasurface lens, including a sixth surface S6 facing the object surface and a seventh surface S7 facing the image surface. The sixth surface is a standard surface, and the seventh surface S7 is a binary surface; the optical window 5 is the window surface in the detector, including an eighth surface S6 facing the object surface and a ninth surface S9 facing the image surface, both of which are standard surfaces. The athermalized optical system adopts a passive athermalization method, which does not rely on an external adjustment mechanism and compensates for the influence brought by temperature changes through material selection and optical design itself.

[0050] In some embodiments, the material used for the first lens 2 is Ge, and its thermal expansion coefficient is 6.1×10 -6 / °C. The material used for the second lens 3 is ZnSe, and its thermal expansion coefficient is 7.1×10-6 / °C. The material used for the third lens 4 is Si, and its thermal expansion coefficient is 2.5×10 -6 / °C. The metasurface lens structure of the third lens 3 is completed on a silicon substrate, and its common optional thicknesses are: 0.3 mm, 0.5 mm, 1 mm. The barrel structure of the optical system uses an aluminum metal material, and its thermal expansion coefficient is 23.6×10 -6 / °C, that is, the thermal expansion coefficient of all air gaps is this value. The technical parameters of the uncooled infrared detector used are: working band: 8 - 12 μm; resolution: 640×512; pixel size: 12 μm. The optical window 5 of the uncooled infrared detector used is a germanium window, its thickness is 1 mm, and its physical distance to the photosensitive surface of the detector is 1.2 mm. The technical parameters achieved by the optical system are: working band: 8 - 12 μm; F number: 1.8; optical focal length: 13 mm; entrance pupil diameter: 7.22 mm; field of view: 26°×26°. The F number calculation formula is f / D, where f is the focal length of the optical system and D is the entrance pupil diameter of the optical system focal length.

[0051] For the provided hybrid lens, the aspherical lens, the metasurface lens, and the optical window are a combination of optical materials with different refractive index temperature coefficients and thermal expansion coefficients. Through the optical passive athermalization method, broadband athermalization is achieved in the entire far-infrared band. In the temperature range of -50°C to +50°C, the imaging quality of the optical system does not significantly degrade due to extreme temperature changes and can operate in the entire band of 8 - 12 μm.

[0052] The optical system provided by the present invention sequentially includes a diaphragm 1, a first lens 2, a second lens 3, a third lens 4, and an optical window 5 from the object side to the image side. By means of optical passive athermalization achieved through a combination of optical materials with different refractive index temperature coefficients and thermal expansion coefficients, the imaging quality of the optical system does not significantly degrade due to extreme temperature changes within the temperature range of -50°C to +50°C, and it can operate in the entire wavelength band of 8 to 12 μm. The far-infrared broadband athermalized refractive and metasurface hybrid lens of this application has the advantages of miniaturization, light weight, and low cost due to the use of a metasurface lens instead of a traditional refractive lens.

[0053] The invented far-infrared broadband athermalized refractive and metasurface hybrid system as Figure 1 shown is composed of a diaphragm 1, a first lens 2, a second lens 3, a third lens 4, an optical window 5, and an image plane 6 sequentially arranged from the object side to the image side.

[0054] The technical indicators of the optical system of the present invention are as shown in Table 1 below:

[0055] Table 1 Technical Indicators of the Optical System

[0056] Full field of view Optical focal length F-number Operating wavelength band 26°×26° 13 mm 1.8 8 - 12 μm

[0057] An optical model is established in the ZEMAX optical simulation software according to the above optical parameters. Starting from behind the object surface, insert the first surface S1 as the reference surface, set it as the diaphragm surface, and set its thickness as a variable; insert the second surface S2 as an even aspheric surface, and the third surface S3 as an even aspheric surface to form the first lens 2. Set the curvature radii and thicknesses of the second surface S2 and the third surface S3 as variables, and define the material of the second surface S2 as Ge material; insert the fourth surface S4 as an even aspheric surface, and the fifth surface S5 as an even aspheric surface to form the second lens 3. Set the curvature radii and thicknesses of the fourth surface S4 and the fifth surface S5 as variables, and define the material of the fourth surface S4 as Ge material; insert the sixth surface S6 as the reference surface, and the seventh surface S7 as a binary surface to form the third lens 4. Set the thickness of the sixth surface S6 to 0.5 mm, and set the thickness of the seventh surface S7 as a variable; insert the eighth surface S8 as the reference surface, and the ninth surface S9 as the reference surface to form the optical window 5. Set the thickness of the eighth surface S8 to 1 mm, and set the thickness of the ninth surface S9 to 1.2 mm.

[0058] The surface equations of the above aspheric lenses are as follows:

[0059]

[0060] Wherein, z is the vertical height from the aspherical vertex to the aspherical surface at the radial distance r, c is the curvature of the aspherical surface, which is related to the radius of curvature R of the surface, c = 1 / R, k is the conic coefficient, which determines the deviation degree between the aspherical surface and the spherical surface. When k = 0, the equation degenerates into the spherical surface equation. a is the fourth-order aspherical coefficient, B is the sixth-order aspherical coefficient, C is the eighth-order aspherical coefficient, D is the tenth-order aspherical coefficient, and E is the twelfth-order aspherical coefficient.

[0061] Furthermore, set the fourth-order, sixth-order, eighth-order, tenth-order, and twelfth-order aspherical coefficients of the first lens and the second lens as variables.

[0062] The phase distribution equation of the binary surface is as follows:

[0063]

[0064] Wherein, M is the diffraction order of the metasurface lens, set the diffraction order as "-1", ρ is the radial coordinate of each meta-unit in the metasurface lens, R0 is the normalized radius, and set its value as 1 mm. a, b, c, d, e, f are the optimization coefficients of the quadratic term, quartic term, sextic term, octic term, decic term, and dodecic term of the first metasurface lens respectively.

[0065] Furthermore, set the optimization coefficients of the quadratic term, quartic term, sextic term, octic term, decic term, and dodecic term of the metasurface lens as variables.

[0066] Furthermore, use the spot diagram as the evaluation and optimization function of the optical system. Optimize to obtain an initial optical model that meets the technical indicators without thermal aberration optimization. Create a thermal analysis multi-structure in the multi-structure editor, set the number of multi-structures as 5, set the lowest temperature as -50 °C, and the highest temperature as 50 °C. Set the radius of curvature of the second surface S2, the third surface S3, the fourth surface S4, and the fifth surface S5 of the optical model at the ambient temperature of 20 °C as variables, set the thicknesses of the second surface S2, the third surface S3, the fourth surface S4, the fifth surface S5, and the seventh surface S7 as variables, and at the same time set the above parameters at -50 °C, -25 °C, 0 °C, 25 °C, and 50 °C as thermal pickups. In this way, optimize to obtain an optical model with thermal aberration optimization in the range of -50 °C to 50 °C. The detailed data of each optimized optical element are shown in Table 2 below:

[0067] Table 2 Detailed data of each optimized optical element

[0068]

[0069] Furthermore, the high-order aspherical coefficients of the first lens 2 and the second lens 3 are as shown in Table 3 below:

[0070] Table 3 High-order aspherical coefficients of the first lens and the second lens

[0071]

[0072]

[0073] Furthermore, the polynomial optimization coefficients of the third lens 4 are as shown in Table 4 below:

[0074] Table 4 Polynomial Optimization Coefficients of the Third Lens

[0075] a B c d e f 2.068 -0.353 0.036 -4.775e-3 6.233e-4 -2.999e-5

[0076] Furthermore, the performance of the anastigmatic refractive-diffractive hybrid lens is evaluated, as Figure 2 、 3 、4, 5, and 6 are the MTF curves of the above-mentioned refractive-diffractive hybrid lens after anastigmatic optimization under the conditions that the ambient temperatures are -50°C, -25°C, 0°C, 25°C, and 50°C respectively.

[0077] Furthermore, as Figure 7 、 8 、9, 10, and 11 are the spot diagrams of the above-mentioned refractive-diffractive hybrid lens after anastigmatic optimization under the conditions that the ambient temperatures are -50°C, -25°C, 0°C, 25°C, and 50°C respectively.

[0078] Proved by the above embodiments, the provided hybrid lens realizes broadband anastigmatism in the entire far-infrared band by means of optical passive anastigmatism. In the temperature range of -50°C to +50°C, the imaging quality of the optical system does not decrease significantly due to extreme temperature changes and can operate in the entire band of 8 - 12 μm.

[0079] The metasurface lens is used to replace the traditional refractive lens, and it consists of only three optical elements. The thickness of the metasurface lens is only at the wavelength scale, and its volume and weight are greatly reduced, realizing the compactification and lightweight of the far-infrared anastigmatic optical system; the metasurface lens uses low-cost single-crystalline silicon material and can be fabricated relying on semiconductor processes. Compared with traditional refractive lenses fabricated based on materials such as germanium, zinc selenide, and sulfide, it has a lower manufacturing cost.

[0080] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them; although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that it is still possible to modify the specific implementation manners of the present invention or perform equivalent replacements for some technical features; without departing from the spirit of the technical solutions of the present invention, they should all be covered within the scope of the technical solutions claimed by the present invention.

Claims

1. A far-infrared broadband anastigmatic catadioptric hybrid lens, characterized in that, It includes a diaphragm (1), a first lens (2), a second lens (3), a third lens (4) and an optical window (5) arranged in sequence from the object side to the image side; The diaphragm (1) is the first surface (S1); The first lens (2) is an aspherical lens, including a second surface (S2) facing the object surface and a third surface (S3) facing the image surface (6), and both the second surface (S2) and the third surface (S3) are even aspheres; The second lens (3) is an aspherical lens, including a fourth surface (S4) facing the object surface and a fifth surface (S5) facing the image surface (6), and both the fourth surface (S4) and the fifth surface (S5) are even aspheres; The third lens (4) is a metasurface lens, including a sixth surface (S6) facing the object surface and a seventh surface (S7) facing the image surface (6), the sixth surface (S6) is a standard surface, and the seventh surface (S7) is a binary surface; The optical window (5) includes an eighth surface (S8) facing the object surface and a ninth surface (S9) facing the image surface (6), and both the eighth surface (S8) and the ninth surface (S9) are standard surfaces; The materials of the first lens (2), the second lens (3) and the third lens (4) have different thermal expansion coefficients.

2. The far-infrared broadband anastigmatic catadioptric hybrid lens according to claim 1, wherein The material of the first lens (2) is Ge, and its coefficient of thermal expansion is 6.1×10 -6 / °C; the material of the second lens (3) is ZnSe, and its coefficient of thermal expansion is 7.1×10-6 / °C; the material of the third lens (4) is Si, and its coefficient of thermal expansion is 2.5×10 -6 / °C; the barrel structure of the optical system is made of aluminum metal material, and its coefficient of thermal expansion is 23.6×10 -6 / °C; the optical window (5) is a germanium window with a thickness of 1 mm, and the physical distance from the germanium window to the photosensitive surface is 1.2 mm.

3. The far-infrared broadband anastigmatic catadioptric hybrid lens according to claim 2, wherein The metasurface lens structure of the third lens (4) is arranged on a silicon substrate, and the thickness is 0.3 mm, 0.5 mm or 1 mm.

4. The far-infrared broadband anastigmatic diffractive-refractive hybrid lens according to claim 1, characterized in that, The surface equation of the aspherical lens is: Where z is the vertical height from the aspherical vertex to the aspherical surface at the radial distance r, c is the curvature of the aspherical surface, which is related to the radius of curvature R of the surface, c = 1 / R, k is the conic coefficient, which determines the deviation degree between the aspherical surface and the spherical surface. When k = 0, the equation degenerates into a spherical surface equation, A is the fourth-order aspherical coefficient, B is the sixth-order aspherical coefficient, C is the eighth-order aspherical coefficient, D is the tenth-order aspherical coefficient, and E is the twelfth-order aspherical coefficient.

5. The far-infrared broadband anastigmatic catadioptric hybrid lens according to claim 1, characterized in that, The phase distribution equation of the binary surface is: Where M is the diffraction order of the metasurface lens, the diffraction order is set to "-1", ρ is the radial coordinate of each meta-unit in the metasurface lens, R0 is the normalized radius, and its value is set to 1 mm, and a, b, c, d, e, f are the quadratic term, quartic term, sextic term, octic term, decic term, and dodecic term optimization coefficients of the first metasurface lens respectively.

6. An optical model design method for a far-infrared broadband athermalized catadioptric hybrid lens according to any one of claims 1-5, characterized in that, It includes: An optical model is established. Starting from behind the object surface, the first surface (S1) is inserted as a reference surface, set as the aperture stop (1) surface, and its thickness is set as a variable; the second surface (S2) is inserted as an even aspheric surface, and the third surface (S3) is inserted as an even aspheric surface to form the first lens (2). The radius of curvature and thickness of the second surface (S2) and the third surface (S3) are set as variables, and the material of the second surface (S2) is defined as Ge material; the fourth surface (S4) is inserted as an even aspheric surface, and the fifth surface (S5) is inserted as an even aspheric surface to form the second lens (3). The radius of curvature and thickness of the fourth surface (S4) and the fifth surface (S5) are set as variables, and the material of the fourth surface (S4) is defined as Ge material; the sixth surface (S6) is inserted as a reference surface, and the seventh surface (S7) is inserted as a binary surface to form the third lens (4). The thickness of the sixth surface (S6) is set to 0.5 mm, and the thickness of the seventh surface (S7) is set as a variable; the eighth surface (S8) is inserted as a reference surface, and the ninth surface (S9) is inserted as a reference surface to form the optical window (5). The thickness of the eighth surface (S8) is set to 1 mm, and the thickness of the ninth surface (S9) is set to 1.2 mm.

7. The optical model design method of the far-infrared broadband anastigmatic diffractive-refractive hybrid lens according to claim 6, characterized in that, The surface equation of the aspheric lens is set as: where z is the vertical height from the aspheric vertex to the aspheric surface at the radial distance r, c is the curvature of the aspheric surface, which is related to the radius of curvature R of the surface, c = 1 / R, k is the conic coefficient, which determines the deviation degree between the aspheric surface and the spherical surface. When k = 0, the equation degenerates into a spherical surface equation. A is the fourth-order aspheric coefficient, B is the sixth-order aspheric coefficient, C is the eighth-order aspheric coefficient, D is the tenth-order aspheric coefficient, and E is the twelfth-order aspheric coefficient.

8. The optical model design method of the far-infrared broadband anastigmatic diffractive-refractive hybrid lens according to claim 7, characterized in that, The fourth-order, sixth-order, eighth-order, tenth-order, and twelfth-order aspheric coefficients of the first lens (2) and the second lens (3) are set as variables; The phase distribution equation of the binary surface is: where M is the diffraction order of the metasurface lens, and the diffraction order is set as "-1". ρ is the radial coordinate of each meta-unit in the metasurface lens, R0 is the normalized radius, and its value is set to 1 mm. a, b, c, d, e, and f are the optimization coefficients of the quadratic term, quartic term, sextic term, octic term, decic term, and dodecic term of the first metasurface lens, respectively.

9. The optical model design method of the far-infrared broadband anastigmatic diffractive-refractive hybrid lens according to claim 8, characterized in that The optimization coefficients of the quadratic term, quartic term, sextic term, octic term, decic term, and dodecic term of the metasurface lens are set as variables; Taking the spot diagram as the evaluation and optimization function of the optical system, an initial optical model that meets the technical indicators without thermal aberration optimization is optimized. A thermal analysis multi-structure is created in the multi-structure editor. The number of multi-structures is set to 5, the lowest temperature is set to -50 °C, and the highest temperature is set to 50 °C.

10. The optical model design method of the far-infrared broadband anastigmatic diffractive-refractive hybrid lens according to claim 9, characterized in that Set the radii of curvature of the second surface (S2), the S3 - third surface (S3), the S4 - fourth surface (S4), and the fifth surface (S5) of the optical model at an ambient temperature of 20°C as variables, set the thicknesses of the second surface (S2), the S3 - third surface (S3), the S4 - fourth surface (S4), the fifth surface (S5), and the seventh surface (S7) as variables, and at the same time set the above parameters at temperatures of -50°C, -25°C, 0°C, 25°C, and 50°C as thermal pickups, and optimize to obtain an optical model for anastigmatic optimization in the range of -50°C to 50°C.

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