Design method of long-focus infrared integrated refrigeration optical imaging system

Abstract

The invention discloses a method for designing a long-focus infrared integrated refrigeration optical imaging system, which comprises the following steps of: designing the long-focus infrared integrated refrigeration optical imaging system in an assembly environment temperature and air environment by using optical design software, exporting data of a refrigeration lens group, and establishing a three-dimensional optical machine structure model of the refrigeration lens group; performing finite element analysis on the model from the assembly environment temperature to the working environment temperature, extracting optical surface displacement data, and fitting the optical surface displacement data into optical thickness and surface shape change data; obtaining the refractive index and the fitting dispersion coefficient of the refrigeration lens group material under the working environment temperature and the vacuum environment; and inputting the optical thickness, the surface type change and the material dispersion coefficient of the refrigeration lens group at the working environment temperature into optical design software, fixing the parameters of the refrigeration lens group, and optimizing the parameters of the non-refrigeration window sheet. The design accuracy of the long-focal-length infrared integrated refrigeration optical imaging system is improved, and the use scene of the long-focal-length infrared integrated refrigeration optical imaging system is enlarged.

Description

Technical Field

[0001] This invention relates to the field of infrared integrated cooled optical imaging system design, and more particularly to a design method for a long focal length infrared integrated cooled optical imaging system. Background Technology

[0002] With the continuous development of infrared optical imaging technology, practical applications have placed demands on infrared optical imaging systems for lightweight design, high signal-to-noise ratio, and high sensitivity. Cooled infrared optical imaging systems can achieve high signal-to-noise ratio and high sensitivity images, but because their cold stop is located behind the optical lens and acts as the aperture stop of the optical system, it is difficult to miniaturize the entire optical lens assembly. Integrated infrared cooled optical imaging systems, by encapsulating some lenses in a Dewar flask, can overcome the positional limitations of the cold stop and provide more design freedom for cooled optical imaging systems. However, the cooled lens assembly encapsulated in the Dewar flask is typically assembled at room temperature and then operates in an ultra-low temperature vacuum environment. During the transition from the assembled state to the operating state, temperature and pressure affect the refractive index, optical profile, optical thickness, and air gap of the optical materials of the cooled lens assembly, thus affecting the design accuracy and passive calorimetric compensation of the integrated infrared cooled optical imaging system.

[0003] Patent CN120493611A discloses a method for optomechanical-thermal integrated analysis of a space cryogenic infrared camera. It involves simulating mirror displacement and deformation through optomechanical modeling and finite element analysis, and then importing the results into optical design software for performance evaluation. While this method effectively reflects the influence of temperature on the optical system structure, it does not consider the effects of temperature and pressure on the refractive index of optical materials, nor does it include a process for further optimization after evaluating the optical system's performance. Patent CN120044698A discloses a long-wave infrared integrated cooling optical system, involving a calorimetric integrated cooling optical system designed based on optical design software, capable of maintaining high imaging quality in environments ranging from -40℃ to 60℃. This patent uses the built-in thermal analysis methods of the optical design software to simulate changes in the refractive index, optical shape, and optical thickness of the cooling lens group at ultra-low temperatures. However, it neglects the changes in the temperature gradient coefficient of the refractive index of the optical material during cooling and the influence of mechanical deformation on the surface shape caused by the compression of optical elements. This results in significant differences between the simulated refractive index, optical shape, and optical thickness and the actual situation. Furthermore, the patent does not disclose its passive heatless design method. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention proposes a design method for a long-focal-length infrared integrated cooled optical imaging system. This method, specifically for long-focal-length infrared integrated cooled optical imaging systems, fully simulates the changes in optical thickness and optical surface shape of the cooled lens group from the assembly environment to the working environment through optomechanical modeling and finite element analysis. By measuring the refractive index of the optical material of the cooled lens group in a vacuum environment at the working temperature and fitting the dispersion coefficient, the inherent properties of the optical material under working conditions are accurately reflected. Finally, the fitted changes in optical thickness, optical surface shape, and refractive index are input into optical design software. Through the selection of materials and surface shape of the uncooled window and parameter optimization, the aberrations caused by the change in the assembly environment to the working environment of the cooled lens group are compensated, achieving a locally calorimetric design.

[0005] The specific technical solution is as follows: A design method for a long-focal-length infrared integrated cooled optical imaging system includes the following steps: S1: Determine the performance design parameters of the long focal length infrared integrated cooled optical imaging system, and use optical design software to design the long focal length infrared integrated cooled optical imaging system under assembly ambient temperature and air environment; the long focal length infrared integrated cooled optical imaging system includes a cooled lens group and an uncooled window. S2: Export the data of the cooled lens group under the assembly ambient temperature from the optical design software and establish a three-dimensional optomechanical structure model of the cooled lens group; S3: Perform finite element analysis on the three-dimensional optomechanical structure model of the cooling lens assembly to measure the change in ambient temperature from the assembly environment temperature to the working environment temperature, and extract optical surface displacement data; the finite element analysis includes steady-state thermal analysis and static analysis; S4: Fit the optical surface displacement data of the cooling lens group into optical thickness and surface shape change data; S5: Test the refractive index of the optical materials of the cooled lens assembly under working temperature and vacuum conditions, and fit the dispersion coefficient. S6: Input the optical thickness, surface shape change, and material dispersion coefficient of the cooled lens group at the operating ambient temperature into the optical design software; S7: Fix the parameters of the cooled lens group in the optical design software and optimize the parameters of the uncooled window to achieve a localized calorimetric design where the uncooled window operates at ambient temperature and the cooled lens group operates at the operating ambient temperature.

[0006] Furthermore, the assembly environment temperature is selected as a constant 20℃, and the working environment temperature is selected as -123℃; the focal length of the long focal length infrared integrated cooled optical imaging system is not less than 85mm, and the working wavelength is 7~9.5μm; the assembly and operation of the long focal length infrared integrated cooled optical imaging system are as follows: at a constant temperature of 20℃, the cooled lens group is encapsulated in a Dewar flask, and the uncooled window is connected to the Dewar flask through a mechanical structure; then the Dewar flask is cooled and evacuated, the cooled lens group is cooled to -123℃ and works stably, and the uncooled window is at the same temperature as the ambient temperature.

[0007] Furthermore, in the long focal length infrared integrated cooled optical imaging system in S1, the optical power of the uncooled window is greater than 0, and the optical power of the cooled lens group is less than 0.

[0008] Furthermore, in S2, the established three-dimensional structural model and the actual assembly method are consistent.

[0009] Furthermore, S3 is implemented through the following sub-steps: S3.1: Import the three-dimensional optomechanical structure model of the cooling lens group into the finite element analysis software, complete the definition of the thermal conductivity, density, coefficient of thermal expansion and Young's modulus parameters of the optical materials and structural materials, and perform mesh generation, boundary condition setting and solution settings; S3.2: Perform steady-state thermal analysis on the three-dimensional structural model from the assembly environment temperature to the working environment temperature, and input the results of the steady-state thermal analysis into the thermal conditions of the static analysis; S3.3: Perform static analysis to obtain and save displacement data of the optical surface.

[0010] Furthermore, in S4, the optical thickness is fitted using the least squares method, and the optical surface shape change is fitted using the Zernike polynomial.

[0011] Furthermore, in step S5, the working wavelength band of 7~9.5μm is divided into several intervals. Under the working environment temperature and vacuum environment, the refractive index of the optical material of the cooling lens group in each interval is measured and fitted using the Sellmeier dispersion formula.

[0012] Furthermore, S6 is implemented through the following sub-steps: S6.1: Convert the optical thickness of the optical surface of the cooled lens group into the lens thickness and air thickness in the optical design software; S6.2: Replace the surface shape variation of the cooling lens group with a Zernike polynomial surface shape; S6.3: Reconstruct the material library of optical materials for the cooling lens group under working environment temperature and vacuum environment based on the dispersion coefficient obtained by fitting in S5, and change the optical materials of the cooling lens group to the corresponding materials in the reconstructed material library.

[0013] Furthermore, in S7, based on the light propagation path, the side closer to the object side is defined as the front, and the side closer to the image side is defined as the rear; the uncooled window is a meniscus lens with a positive front and a negative rear, and the rear surface is a binary diffraction surface based on an even-order aspherical surface, the contour of which satisfies the following expression:

[0014] Where c is the curvature of the surface vertex, k is the conic coefficient, and r is the radial distance. The coefficients are even-order aspherical coefficients; The additional phase of the diffraction satisfies the following expression:

[0015] Where r is the radial distance and R is the normalized radius. For even-order diffraction coefficients; The refractive index temperature gradient coefficient of the non-cooled window material The negative refractive index temperature gradient coefficient of the uncooled window material is less than 0. The negative thermal difference characteristics of the binary diffraction surface and the positive thermal difference characteristics of the mechanical structure work together to achieve local calorimetry in the long focal length infrared integrated cooled optical imaging system.

[0016] A long-focal-length infrared integrated cooled optical imaging system, manufactured according to the design method of the long-focal-length infrared integrated cooled optical imaging system, includes: an uncooled window, a cold aperture, a cooled lens group, and a detector arranged sequentially along the optical axis along the light propagation direction; with the light propagation path as a reference, the side closer to the object side is defined as the front, and the side closer to the image side is defined as the rear. The uncooled window is made of chalcogenide glass with a refractive index temperature coefficient of less than 0. Its front and back surfaces are both spherical, and its shape is a meniscus lens with a positive focal length. It is used to compress infrared light into the cold aperture and reduce the diameter of the cooling lens group. The focal length of the cooling lens group is negative and is used to correct aberrations and transmit light to the detector. The cooled lens group includes: cooled lens one, cooled lens two, cooled lens three, and a filter arranged sequentially along the optical axis in the direction of light propagation; the surface shape of cooled lens one, cooled lens two, and cooled lens three are all spherical meniscus lenses with a convex front surface and a concave rear surface.

[0017] The beneficial effects of this invention are: (1) Unlike simple finite element analysis methods or design methods based solely on optical design software, this invention fully considers the changes in material refractive index, optical thickness, and optical surface shape of the cooling lens group during the process of cooling from assembly temperature to working temperature, thereby improving the design accuracy of long focal length infrared integrated cooling optical imaging system.

[0018] (2) Unlike conventional calorimetric design of optical systems, this invention proposes a local calorimetric optimization method after finite element analysis. By combining the negative refractive index temperature gradient coefficient of the material, the negative thermal difference characteristics of the binary diffraction surface and the positive thermal difference characteristics of the mechanical structure, the working temperature range of the uncooled window is improved, thus expanding the application scenarios of the long focal length infrared integrated cooled optical imaging system. Attached Figure Description

[0019] Figure 1 This is a flowchart of the design method for a long focal length infrared integrated cooled optical imaging system in an embodiment of the present invention.

[0020] Figure 2 This is a ray tracing diagram of the long focal length infrared integrated cooled optical imaging system in the assembled state according to an embodiment of the present invention.

[0021] Figure 3 This is a three-dimensional structural model of the cooling lens group in the long focal length infrared integrated cooled optical imaging system in this embodiment of the invention.

[0022] Figure 4 The following are the optical surface displacement results of the cooling lens in the long focal length infrared integrated cooled optical imaging system of the present invention before and after the change from the assembly environment to the working environment, wherein (a) is the front optical surface displacement result and (b) is the back optical surface displacement result.

[0023] Figure 5 The following are the optical surface displacement results of the cooling lens 2 in the long focal length infrared integrated cooled optical imaging system of the present invention before and after the change from the assembly environment to the working environment, wherein (a) is the front optical surface displacement result and (b) is the back optical surface displacement result.

[0024] Figure 6 The following are the optical surface displacement results of the cooling lens three in the long focal length infrared integrated cooled optical imaging system of the present invention before and after the change from the assembly environment to the working environment, wherein (a) is the optical surface displacement result before the change and (b) is the optical surface displacement result after the change.

[0025] Figure 7The following are fitting results of the changes in surface shape of the cooling lens in the long focal length infrared integrated cooled optical imaging system of the present invention from the assembly environment to the working environment, wherein (a) is the fitting result of the changes in surface shape of the front optical surface and (b) is the fitting result of the changes in surface shape of the rear optical surface.

[0026] Figure 8 The following are fitting results of the changes in surface shape of the cooling lens 2 in the long focal length infrared integrated cooled optical imaging system of the present invention from the assembly environment to the working environment. Among them, (a) is the fitting result of the changes in surface shape of the front optical surface, and (b) is the fitting result of the changes in surface shape of the rear optical surface.

[0027] Figure 9 The figures are fitting results of the changes in surface shape of the cooling lens three in the long focal length infrared integrated cooled optical imaging system of the present invention from the assembly environment to the working environment. Among them, (a) is the fitting result of the change in surface shape of the front optical surface and (b) is the fitting result of the change in surface shape of the rear optical surface.

[0028] Figure 10 This is an MTF curve of the long focal length infrared integrated cooled optical imaging system in this embodiment of the invention at an ambient temperature of -40℃.

[0029] Figure 11 This is an MTF curve of the long focal length infrared integrated cooled optical imaging system in this embodiment of the invention at an ambient temperature of 20°C.

[0030] Figure 12 This is an MTF curve of the long focal length infrared integrated cooled optical imaging system in this embodiment of the invention at an ambient temperature of 60°C.

[0031] Figure 13 This is a photograph of the actual optical element fabricated in an embodiment of the present invention.

[0032] Figure 14 This is a real-world photograph of the long-focal-length infrared integrated cooled optical imaging system in an embodiment of the present invention. Detailed Implementation

[0033] The present invention will be described in detail below with reference to the accompanying drawings and preferred embodiments. The objectives and effects of the present invention will become clearer as a result. The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0034] like Figure 1 As shown, a design method for a long-focal-length infrared integrated cooled optical imaging system includes the following steps: S1: Determine the performance design parameters of the long focal length infrared integrated cooled optical imaging system, and use optical design software to design the long focal length optical system under the assembly environment air; the preferred assembly environment temperature is 20℃.

[0035] The performance design parameters of the long focal length infrared integrated cooled optical imaging system in this embodiment are shown in Table 1.

[0036] Table 1 Performance design parameters of the long focal length infrared integrated cooled optical imaging system in this embodiment

[0037] The assembly and operation of the long-focal-length infrared integrated cooled optical imaging system are as follows: In an air environment with a constant temperature of 20℃, the cooled lens group is encapsulated in a Dewar flask, and the uncooled window is connected to the Dewar flask through a mechanical structure; then the Dewar flask is cooled and evacuated, and the cooled lens group is cooled to the working ambient temperature (-123℃) and operates stably. The uncooled window is at the same temperature as the ambient temperature and can work normally in a temperature range of -40℃ to 60℃.

[0038] Since both the cooled lens group and the uncooled lens are assembled in an air environment of 20°C, the long-focal-length infrared integrated cooled optical imaging system needs to be simulated under the assembly environment in this embodiment. After reasonable design optimization, the ray tracing diagram of the long-focal-length infrared integrated cooled optical imaging system in the assembly state in this embodiment is as follows. Figure 2 As shown. The optical system includes, arranged sequentially along the optical axis in the direction of light propagation: an uncooled window (NCW), a cold stop (CS), and a cooled lens group (CLG). The cooled lens group (CLG) includes, arranged sequentially along the optical axis in the direction of light propagation: cooled lens 1 (CL1), cooled lens 2 (CL2), cooled lens 3 (CL3), and a filter (CLP). Using the light propagation path as a reference, the side closer to the object side is defined as the front, and the side closer to the image side as the rear. The uncooled window (NCW) has a positive focal length and is used to compress infrared light into the cold stop and reduce the diameter of the cooled lens group (CLG). The cooled lens group (CLG) has a negative focal length and is used for aberration correction and to transmit light to the detector.

[0039] At this point, all optical elements in the cooled lens group (CLG) are made of germanium, and the three cooled lenses are spherical meniscus lenses with a convex front surface and a concave rear surface. The front and rear surfaces of the filter (CLP) are both flat. The uncooled window (NCW) is made of a material with a refractive index temperature coefficient of... Chalcogenide glass with a refractive index less than 0 (where n is the refractive index and T is the temperature) has spherical surfaces on both the front and back, and is shaped like a meniscus lens.

[0040] S2: Export the data of the cooled lens group under the assembly ambient temperature (20℃) from the optical design software and establish a three-dimensional optomechanical structure model of the cooled lens group.

[0041] The data and model of the cooled lens assembly (CLG) were exported from the optical design software. The mechanical structure of the CLG was modeled according to the outer circular surface bonding method, consistent with the actual assembly method. Mechanical structures such as the cooling screen, spacers, and cooling apertures were established based on the dimensions of the CLG. The final three-dimensional structural model of the cooled lens assembly (CLG) in this embodiment is shown below. Figure 3 As shown.

[0042] S3: Perform finite element analysis on the three-dimensional optomechanical structural model of the cooling lens assembly (CLG) under varying ambient temperatures from the assembly environment temperature of 20℃ to the operating environment temperature of -123℃. This includes steady-state thermal analysis and static analysis, and extracts optical surface displacement data. S3 is implemented through the following sub-steps: S3.1: Import the three-dimensional structural model of the cooling lens assembly into the finite element analysis software, define the thermal conductivity, density, coefficient of thermal expansion, and Young's modulus parameters of the optical and structural materials, and perform mesh generation, boundary condition setting, and solution settings.

[0043] S3.2: Perform steady-state thermal analysis on the three-dimensional structural model from 20℃ to -123℃, and input the results of the steady-state thermal analysis into the thermal conditions of the static analysis.

[0044] S3.3: Perform static analysis to obtain and save displacement data of the optical surface.

[0045] In this embodiment of the invention, the material of the cooling lens assembly is germanium, and the material of the mechanical structural components is a copper-based alloy. After inputting the key characteristic parameters of these two materials, a finite element analysis is performed on the optomechanical structure model. Due to temperature changes and the squeezing effect of the mechanical structure on the cooling lens, the optical surfaces of the cooling lens assembly CLG all undergo varying degrees of displacement. Using millimeters as the unit of surface displacement data, the displacement data results of the front and rear optical surfaces of the cooling lens CL1 are as follows: Figure 4 As shown in (a) and (b), the displacement data of the front and rear optical surfaces of the cooling lens CL2 are as follows: Figure 5 As shown in (a) and (b), the displacement data of the front and rear optical surfaces of the cooling lens CL3 are as follows: Figure 6 As shown in (a) and (b). Since the CLP filter does not have optical power, the effects of its surface displacement and shape changes on the optical system can be considered negligible.

[0046] S4: Fit the optical surface displacement data of the cooling lens assembly from the assembly ambient temperature of 20℃ to the working ambient temperature of -123℃ into optical thickness and surface shape change data.

[0047] The displacement data of the optical surface are fitted to optical thickness and optical shape changes. The optical thickness is fitted using the least squares method, and the optical shape change is fitted using Zernike polynomials. The shape expression is as follows:

[0048] In the formula, c is the curvature of the surface vertex, k is the conic coefficient, and r is the radial distance. For even-order aspherical coefficients, Let be the coefficients of the i-th Zernike polynomial. For normalized radial ray coordinates, Let N be the angular ray coordinate, and N represent the total number of terms in the Zernike polynomial used for fitting the surface deformation. i Let represent the i-th Zernike orthogonal polynomial.

[0049] In the embodiments of the present invention, the first two terms on the right side of the surface shape expression can be regarded as the surface shape profile of the cooling lens at an assembly ambient temperature of 20°C, and the third term can be regarded as the fitting result of the surface shape change of the cooling lens from 20°C to -123°C.

[0050] During the process of the ambient temperature changing from 20℃ to -123℃, the optical thickness changes of the front and rear surfaces of the cooling lens CL1 are 2.425E-02mm and 2.327E-02mm, respectively. The surface shape changes in micrometers are as follows: Figure 7 As shown in (a) and (b) in the figure. For the front and rear surfaces of the cooling lens CL2, the optical thickness changes are 2.163E-02 mm and 2.065E-02 mm, respectively, and the surface shape changes in micrometers are as follows. Figure 8 As shown in (a) and (b) in the figure. For the front and rear surfaces of the cooled lens CL3, the optical thickness variations are 1.840E-02 mm and 1.761E-02 mm, respectively, and the surface shape variations in micrometers are as follows. Figure 9 As shown in (a) and (b) in the figure.

[0051] S5: Perform refractive index testing and fit the dispersion coefficient of the optical materials of the cooled lens assembly under an operating environment temperature of -123℃ and a vacuum environment.

[0052] Specifically, the working wavelength range of 7–9.5 μm was divided into several intervals. Under a vacuum environment at -123 °C, the refractive index of the optical material of the cooled lens group in each interval was measured, and the Sellmeier dispersion formula was used for fitting. The expression is as follows:

[0053] Where n is the refractive index of the material, λ is the wavelength, and A, B, C, D, and E are all dispersion coefficients.

[0054] Optical design software is based on the temperature gradient coefficient of the refractive index of optical materials at room temperature (20°C). The refractive index is used to calculate the refractive index of the material at other temperatures. In fact, the temperature gradient coefficient of the refractive index of the same material at different temperatures is... There are differences, which means that optical design software is only sufficiently accurate in processing the refractive index at other temperatures when the temperature difference is small. However, the cooled lens assembly (CLG) needs to operate from an assembled 20°C air environment to a vacuum environment at -123°C. Therefore, testing the refractive index of the optical materials of the cooled lens assembly at -123°C in a vacuum environment and fitting the dispersion coefficient is of great significance for simulating the inherent refractive index properties of optical materials under working conditions.

[0055] In this embodiment of the invention, the optical material of the cooling lens group CLG is germanium. The refractive index test results of the CLG at -123℃ and in a vacuum environment are shown in Table 2, and the Sellmeier dispersion coefficient fitting results are shown in Table 3.

[0056] Table 2. Refractive index test results of the cooled lens assembly in this embodiment at -123°C and in a vacuum environment.

[0057] Table 3. Fitting results of the Sellmeier dispersion coefficient of the cooling lens group material in this embodiment at -123℃ and under vacuum.

[0058] S6: Input the optical thickness, surface shape change, and material dispersion coefficient of the cooled lens assembly (CLG) at an operating ambient temperature of -123℃ into the optical design software. S6 is implemented through the following sub-steps: S6.1: Convert the optical thickness results of the CLG optical surface fitted in S4 into the lens thickness and air thickness in the optical design software.

[0059] S6.2: Replace the surface shape of the cooling lens assembly with a Zernike polynomial surface shape, keeping the radius of curvature unchanged. Figure 7 , Figure 8 and Figure 9 The surface shape change results were used to solve for the Zernike coefficients using Sigfit software and then sequentially input into the optical design software.

[0060] S6.3: Reconstruct the material library of optical materials for the cooled lens group under vacuum conditions at -123℃. Input the dispersion coefficient of the germanium material in S5 into the optical design software, and update the material of the cooled lens group CLG from the germanium material in the optical design software material library to the fitted and reconstructed germanium material.

[0061] S7: Fix the parameters of the cooled lens group (including optical thickness, surface shape change, and material dispersion coefficient) in the optical design software, and optimize the parameters of the uncooled window (including optical thickness, surface shape change, material dispersion coefficient, parameters of even-order aspherical surfaces, even-order diffraction coefficient, etc.) to achieve a local anechoic design where the uncooled window operates at -40℃ to 60℃ and the cooled lens group operates at the operating ambient temperature.

[0062] After the first six steps, the long-focal-length infrared integrated cooled optical imaging system in this embodiment of the invention has fully simulated the changes in material refractive index, optical thickness, and optical surface shape of the cooled lens group CLG from the assembly environment to the working environment. At this point, the imaging quality of the optical system decreases to a certain extent compared to S1.

[0063] To avoid changes in the finite element analysis of the cooled lens assembly (CLG), all parameters of the CLG are fixed, and system optimization (i.e., optimizing the parameters of the NCW) is performed only based on the design degrees of freedom of the uncooled window (NCW). To compensate for additional aberrations caused by changes in the CLG's assembly environment to its operating environment, the rear surface of the uncooled window (NCW) is changed from spherical to aspherical, allowing for greater design freedom to correct these additional aberrations. The aspherical base profile satisfies the following expression:

[0064] Where c is the curvature of the surface vertex, k is the conic coefficient, and r is the radial distance. The coefficients are even-order aspherical coefficients; After optimizing the dual-order aspherical coefficients, compensation is achieved for the additional aberrations caused by the change in the cooling lens group (CLG) from the assembly environment to the working environment. At this point, the long focal length infrared integrated cooling optical imaging system in this embodiment of the invention can perform normal imaging at an external environment temperature of 20°C.

[0065] In conventional anechoic optical system designs, ambient temperature typically affects the operating temperature of the entire lens, meaning all mechanical structures and lenses operate at the same temperature. This invention achieves localized anechoicity by combining the thermal properties of the NCW optical materials, the thermal difference properties of the NCW's rear surface (diffraction plane), and the thermal difference properties of the mechanical structures. This system enables smaller apertures and better image quality.

[0066] However, in this embodiment of the invention, the ambient temperature only affects the operating temperature of the uncooled window NCW and its mechanical structure. Since only one optical element is affected by the ambient temperature, there is no situation where multiple optical materials have complementary thermal properties and multiple lenses have air gaps to achieve calorification.

[0067] Therefore, to achieve a locally athermalized design—that is, for the uncooled window (NCW) to operate in an air environment of -40°C to 60°C and the cooled lens group (CLG) to operate in a vacuum of -123°C—it is necessary to introduce the negative thermal difference property of the diffraction surface and combine it with the negative refractive index temperature gradient coefficient of chalcogenide glass to coordinate with the mechanical structure of the uncooled window (NCW) which has a positive thermal difference property. In this embodiment of the invention, the rear surface of the uncooled window (NCW) is given diffraction surface characteristics based on an even-order aspherical surface, which can effectively achieve a locally athermalized effect. Its additional diffraction phase satisfies the following expression:

[0068] Where r is the radial distance and R is the normalized radius. For even-order diffraction coefficients.

[0069] After local anechoic design, the structural parameters of the long focal length infrared integrated cooled optical imaging system in this embodiment of the invention are shown in Table 4, and the even-order aspherical coefficient and diffraction coefficient of the uncooled window are shown in Tables 5 and 6 respectively.

[0070] Table 4. Structural parameters of the long focal length infrared integrated cooled optical imaging system in the embodiments of the present invention.

[0071] Table 5. Even-order aspherical parameters in this embodiment under normal temperature assembly conditions.

[0072] Table 6. Diffraction surface parameters in this embodiment under normal temperature assembly conditions.

[0073] After localized calorimetry design, the MTF curves of the long-focal-length infrared integrated cooled optical imaging system in this embodiment of the invention at ambient temperatures of -40℃, 20℃, and 60℃ are shown in the following figures. Figure 10 , Figure 11 and Figure 12 As shown, at a spatial frequency of 17 lp / mm, the optical imaging system designed using the method of this invention exhibits high MTF values ​​at ambient temperatures ranging from -40℃ to 60℃, with an average MTF value exceeding 0.3 across the entire field of view.

[0074] According to the design in this embodiment of the invention, the optical elements are fabricated, and the actual fabrication diagrams of NCW, CL1, CL2, and CL3 are shown below. Figure 13 As shown. The optical components and mechanical structure were assembled and tested as a system. The actual imaging results of the optical system are as follows. Figure 14 As shown, the distant building complex is clearly identifiable with a high degree of detail retention.

[0075] In summary, the design method for a long-focal-length infrared integrated cooled optical imaging system proposed in this invention, through finite element analysis and refractive index measurement, can fully reflect the changes in refractive index, optical thickness, and optical surface shape of the cooled lens group from the assembly environment to the working environment. By inputting the fitting results of these changes back into optical design software, the uncooled window is further optimized, achieving aberration correction. Simultaneously, by utilizing the negative refractive index temperature gradient coefficient of the uncooled window material, the negative thermal difference characteristics of the binary diffraction surface, and the positive thermal difference characteristics of the mechanical structure in combination, localized calorimetry of the long-focal-length infrared integrated cooled optical imaging system is achieved, expanding the application scenarios of such systems.

[0076] In this invention, the terms "first," "second," "third," etc., are used to distinguish similar objects and should not be construed as indicating or implying relative importance.

[0077] It will be understood by those skilled in the art that the above descriptions are merely preferred examples of the invention and are not intended to limit the invention. Although the invention has been described in detail with reference to the foregoing examples, those skilled in the art can still modify the technical solutions described in the foregoing examples or make equivalent substitutions for some of the technical features. All modifications and equivalent substitutions made within the spirit and principles of the invention should be included within the scope of protection of the invention.

Claims

1. A design method for a long-focal-length infrared integrated cooled optical imaging system, characterized in that, Includes the following steps: S1: Determine the performance design parameters of the long focal length infrared integrated cooled optical imaging system, and use optical design software to design the long focal length infrared integrated cooled optical imaging system under assembly ambient temperature and air environment; the long focal length infrared integrated cooled optical imaging system includes a cooled lens group and an uncooled window. S2: Export the data of the cooled lens group under the assembly ambient temperature from the optical design software and establish a three-dimensional optomechanical structure model of the cooled lens group; S3: Perform finite element analysis on the three-dimensional optomechanical structure model of the cooling lens assembly to measure the change in ambient temperature from the assembly environment temperature to the working environment temperature, and extract optical surface displacement data; the finite element analysis includes steady-state thermal analysis and static analysis; S4: Fit the optical surface displacement data of the cooling lens group into optical thickness and surface shape change data; S5: Test the refractive index of the optical materials of the cooled lens assembly under working temperature and vacuum conditions, and fit the dispersion coefficient. S6: Input the optical thickness, surface shape change, and material dispersion coefficient of the cooled lens group at the operating ambient temperature into the optical design software; S7: Fix the parameters of the cooled lens group in the optical design software and optimize the parameters of the uncooled window to achieve a localized calorimetric design where the uncooled window operates at ambient temperature and the cooled lens group operates at the operating ambient temperature.

2. The design method for a long-focal-length infrared integrated cooled optical imaging system according to claim 1, characterized in that, The assembly environment temperature is selected as a constant 20℃, and the working environment temperature is selected as -123℃. The focal length of the long focal length infrared integrated cooled optical imaging system is not less than 85mm, and the working band is 7~9.5μm. The assembly and operation of the long focal length infrared integrated cooled optical imaging system are as follows: at a constant temperature of 20℃, the cooled lens group is encapsulated in a Dewar flask, and the uncooled window is connected to the Dewar flask through a mechanical structure. Then, the Dewar flask is cooled and evacuated, and the cooled lens group is cooled to -123℃ for stable operation. The uncooled window is at the same temperature as the ambient temperature.

3. The design method for a long-focal-length infrared integrated cooled optical imaging system according to claim 1, characterized in that, In the long-focal-length infrared integrated cooled optical imaging system in S1, the optical power of the uncooled window is greater than 0, and the optical power of the cooled lens group is less than 0.

4. The design method for a long-focal-length infrared integrated cooled optical imaging system according to claim 1, characterized in that, In S2, the established three-dimensional structural model is consistent with the actual assembly method.

5. The design method for a long-focal-length infrared integrated cooled optical imaging system according to claim 1, characterized in that, S3 is achieved through the following sub-steps: S3.1: Import the three-dimensional optomechanical structure model of the cooling lens group into the finite element analysis software, complete the definition of the thermal conductivity, density, coefficient of thermal expansion and Young's modulus parameters of the optical materials and structural materials, and perform mesh generation, boundary condition setting and solution settings; S3.2: Perform steady-state thermal analysis on the three-dimensional structural model from the assembly environment temperature to the working environment temperature, and input the results of the steady-state thermal analysis into the thermal conditions of the static analysis; S3.3: Perform static analysis to obtain and save displacement data of the optical surface.

6. The design method for a long-focal-length infrared integrated cooled optical imaging system according to claim 1, characterized in that, In S4, the optical thickness is fitted using the least squares method, and the optical surface shape change is fitted using the Zernike polynomial.

7. The design method for a long-focal-length infrared integrated cooled optical imaging system according to claim 1, characterized in that, In step S5, the working wavelength band of 7~9.5μm is divided into several intervals. Under the working environment temperature and vacuum environment, the refractive index of the optical material of the cooling lens group in each interval is measured and fitted using the Sellmeier dispersion formula.

8. The design method for a long-focal-length infrared integrated cooled optical imaging system according to claim 1, characterized in that, S6 is achieved through the following sub-steps: S6.1: Convert the optical thickness of the optical surface of the cooled lens group into the lens thickness and air thickness in the optical design software; S6.2: Replace the surface shape variation of the cooling lens group with a Zernike polynomial surface shape; S6.3: Reconstruct the material library of optical materials for the cooling lens group under working environment temperature and vacuum environment based on the dispersion coefficient obtained by fitting in S5, and change the optical materials of the cooling lens group to the corresponding materials in the reconstructed material library.

9. The design method for a long-focal-length infrared integrated cooled optical imaging system according to claim 1, characterized in that, In S7, based on the light propagation path, the side closer to the object side is defined as the front, and the side closer to the image side is defined as the rear. The uncooled window is a meniscus lens with a positive front and a negative rear, and the rear surface is a binary diffraction surface based on an even-order aspherical surface. The aspherical substrate profile satisfies the following expression: ； Where c is the curvature of the surface vertex, k is the conic coefficient, and r is the radial distance. The coefficients are even-order aspherical coefficients; The additional phase of the diffraction satisfies the following expression: ； Where r is the radial distance and R is the normalized radius. For even-order diffraction coefficients; The refractive index temperature gradient coefficient of the non-cooled window material The negative refractive index temperature gradient coefficient of the uncooled window material is less than 0. The negative thermal difference characteristics of the binary diffraction surface and the positive thermal difference characteristics of the mechanical structure work together to achieve local calorimetry in the long focal length infrared integrated cooled optical imaging system.

10. A long-focal-length infrared integrated cooled optical imaging system, manufactured according to the design method of any one of claims 1-9, characterized in that, It includes the following components arranged sequentially along the optical axis: uncooled window, cold aperture, cooled lens group, and detector; Based on the path of light propagation, the side closer to the object is defined as the front, and the side closer to the image is defined as the back. The uncooled window is made of chalcogenide glass with a refractive index temperature coefficient of less than 0. Its front and back surfaces are both spherical, and its shape is a meniscus lens with a positive focal length. It is used to compress infrared light into the cold aperture and reduce the diameter of the cooling lens group. The focal length of the cooling lens group is negative and is used to correct aberrations and transmit light to the detector. The cooled lens group includes: cooled lens one, cooled lens two, cooled lens three, and a filter arranged sequentially along the optical axis in the direction of light propagation; the surface shape of cooled lens one, cooled lens two, and cooled lens three are all spherical meniscus lenses with a convex front surface and a concave rear surface.

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