Refrigeration medium wave infrared optical system based on binary surface

Abstract

The utility model discloses a refrigeration medium wave infrared optical system based on a binary surface, and belongs to the technical field of infrared optical systems. The optical system comprises an outer optical system and six coaxial lenses installed in an optical-mechanical structure, the six lenses comprise a first lens made of silicon, a second lens made of zinc sulfide, a third lens made of germanium, a fourth lens made of silicon, a fifth lens made of germanium and a sixth lens made of zinc selenide which are sequentially arranged from the object space to the image space, the third lens is a plano-concave lens, and the fifth lens is a plano-concave lens. The sixth lens is a plano-convex lens, and the rest lenses are meniscus lenses; the two surfaces of the fifth lens are even-order aspheric surfaces, the surface, facing the image space, of the sixth lens is a binary surface, and the surfaces of the other lenses are standard surfaces. The maximum aperture of the optical system is 110mm, and the total length of the system is 180mm. The optical system provided by the utility model has the advantages of good imaging quality, high transmittance, no obvious stray light and compact structure, meets the requirements of various technical indexes, and realizes the comprehensive improvement of the comprehensive performance.

Description

Technical Field

[0001] This invention belongs to the field of infrared optical system technology, and more specifically, relates to a cooled mid-wave infrared optical system based on a binary surface. Background Technology

[0002] With the rapid development of infrared detection technology, mid-wave infrared optical systems have demonstrated irreplaceable application value in fields such as military reconnaissance, industrial inspection, environmental monitoring, and medical diagnosis. Compared to short-wave and long-wave infrared bands, mid-wave infrared combines high thermal sensitivity with strong penetration capability, enabling high-contrast imaging of targets in complex environments (such as smoke and dust), while also supporting precise detection of gas molecule characteristic spectral bands. In recent years, breakthroughs in uncooled infrared focal plane arrays and novel optical materials have further propelled the evolution of mid-wave infrared systems towards miniaturization, low cost, and high performance.

[0003] However, the design of mid-wave infrared optical systems still faces multiple technical bottlenecks. First, transmission materials suitable for this band (such as chalcogenide glasses and single-crystal germanium) are limited by their high refractive index temperature coefficients and complex fabrication processes, easily introducing thermal defocusing and aberrations. Second, anechoic designs over a wide temperature range must balance the stability of the optomechanical structure with imaging requirements, and traditional compensation methods struggle to balance system size and imaging quality. Furthermore, high diffraction efficiency and low stray light optical surface processing, as well as environmentally resistant packaging technologies, place stringent demands on practical applications. Existing research largely focuses on optimizing single performance characteristics, lacking a systematic approach to improving overall performance under complex operating conditions.

[0004] A search revealed Chinese patent application number 202011106928.X, filed on October 16, 2020, entitled "A Cooled Mid-Wave Infrared Optical System with a Large Field of View." This system comprises a front group A and a rear group B arranged sequentially along the optical path. The front group A consists of a meniscus lens A1, a meniscus lens A2, and a biconcave negative lens A3 arranged sequentially along the incident light path. The rear group B consists of a meniscus lens B1 and a meniscus lens B2 arranged sequentially along the incident light path. This optical system employs a secondary imaging structure. By combining the concave surface of the first element of the front group with the object surface, it ensures a large field of view while effectively compressing the overall outer diameter of the optical system, achieving miniaturization. It can be matched with a 320x256@30μm cooled mid-wave infrared detector, solving the problem of the small field of view in current mid-wave cooled systems. However, the technical solution in this application mainly focuses on solving the problem of a large field of view, which is suitable for detectors with low resolution. Its application scenarios are limited and it is difficult to meet the needs of high-precision infrared detectors with high resolution. Utility Model Content

[0005] 1. The problem to be solved

[0006] To address the contradiction between high-resolution imaging and compact structure in existing mid-wave infrared optical systems, this invention provides a cooled mid-wave infrared optical system based on a binary surface. By co-optimizing aberrations using both binary and aspherical surfaces, the overall system length is shortened. Simultaneously, through material thermal difference compensation and curvature radius optimization, a fully static, calorimetric design over a wide temperature range (-40~60℃) is effectively achieved.

[0007] 2. Technical Solution

[0008] To solve the above problems, the technical solution adopted by this utility model is as follows:

[0009] This invention discloses a cooled mid-wave infrared optical system based on a binary surface, comprising six coaxial lenses mounted in an optomechanical structure. The six lenses are arranged sequentially from the object side to the image side: a silicon lens 1, a zinc sulfide lens 2, a germanium lens 3, a silicon lens 4, a germanium lens 5, and a zinc selenide lens 6. Lens 3 is a plano-concave lens, lens 6 is a plano-convex lens, and the rest are meniscus lenses. The two surfaces of lens 5 are even-order aspherical surfaces, the image-side surface of lens 6 is a binary surface, and the surfaces of the remaining lenses are standard surfaces. The maximum aperture of the optical system is 110 mm, and the total system length is 180 mm.

[0010] Furthermore, the surface curvature radius of the lens facing the object side is 105.624 mm, the surface curvature radius of the lens facing the image side is 205.112 mm, the center thickness is 19.111 mm, and the air gap between it and the second lens is 3.123 mm.

[0011] Furthermore, the surface curvature radius of the second lens facing the object side is 250.536 mm, the surface curvature radius facing the image side is 283.789 mm, the center thickness is 29.645 mm, and the air gap between it and the third lens is 3.261 mm.

[0012] Furthermore, the object-side surface of the lens is flat on three sides, while the radius of curvature of the image-side surface is 97.052 mm, the center thickness is 25.274 mm, and the air gap between it and the fourth lens is 0.998 mm.

[0013] Furthermore, the surface curvature radius of the lens facing the object side is 89.715 mm, the surface curvature radius facing the image side is 73.486 mm, the center thickness is 7.144 mm, and the air gap between it and lens five is 0.99 mm.

[0014] Furthermore, the surface curvature radius of the fifth lens facing the object side is 70.823 mm, the surface curvature radius facing the image side is 65.513 mm, the center thickness is 35.014 mm, and the air gap between it and the sixth lens is 3.638 mm.

[0015] Furthermore, the lens has a flat surface on the object side and a radius of curvature of 41.267 mm on the image side, with a center thickness of 3.417 mm.

[0016] Furthermore, it also includes a window, a cold stop, and a filter arranged sequentially from the object side to the image side. The window is made of silicon and is 2.5 mm thick. The air gap between the object side surface of the window and the lens is 4.038 mm. The cold stop is 1.85 mm away from the image side surface of the window and 35.2 mm away from the object side surface of the filter. The filter is made of germanium and is 0.8 mm thick. It is 4 mm away from the image plane.

[0017] Furthermore, all six lenses are coated with a film, with a transmittance of 0.99.

[0018] Furthermore, the optical system has a spectral range of 3μm to 5μm, an F number of 1.65, and an MTF@42lp / mm>0.3. The optical system is suitable for mid-wave infrared detectors with a target array of 1024×1024 and a pixel size of 12μm.

[0019] 3. Beneficial effects

[0020] Compared with the prior art, the beneficial effects of this utility model are as follows:

[0021] (1) The present invention provides a cooling mid-wave infrared optical system based on a binary surface, which uses six coaxial lenses. By optimizing the structure and material selection of the lenses, especially by introducing a binary surface on the image side surface of the sixth lens, the lens aperture and optical system structure can be well controlled, achieving a maximum aperture of 110mm. Furthermore, the phase modulation of the binary surface is not sensitive to temperature changes (the geometric thermal expansion of the microstructure is negligible), which can further suppress astigmatism and field curvature caused by thermal difference and ensure imaging quality.

[0022] (2) The present invention provides a cooling mid-wave infrared optical system based on a binary surface, which adopts a single imaging structure. The six lenses are arranged in a Si-ZnS-Ge-Si-Ge-ZnSe pattern, which balances the optical imaging quality and thermal difference, lens thickness and aperture. Furthermore, the transmittance is further improved by coating. While ensuring reasonable tolerance, the processing difficulty is considered. A flat surface is used on the other side of the binary surface, which is suitable for actual production.

[0023] (3) The present invention provides a cooling mid-wave infrared optical system based on a binary surface. In order to achieve the goal of calorimetry in a wide temperature range of -40℃ to 60℃, the system adopts the strategy of "material thermal difference complementarity + structure optimization". It makes full use of the complementary characteristics of the temperature derivatives (dn / dt) of the refractive index of germanium and silicon, and combines the lens curvature design and matching to offset the focus drift caused by temperature changes, and can effectively maintain the stability of the image plane.

[0024] (4) The present invention provides a cooling mid-wave infrared optical system based on a binary surface, with an F number of 1.65, a matching detector target array of 1024×1024, and a mid-wave infrared detection and receiving device with a pixel size of 12 micrometers. It achieves a thermal design within a temperature range of -40℃ to 60℃, and features a small F number, compactness, small size, and high resolution. It has good imaging quality across the entire temperature range and can effectively meet the stable detection requirements in complex environments such as aviation and vehicle.

[0025] (5) The present invention provides a cooling mid-wave infrared optical system based on a binary surface. The optical system has a simple and compact structure, is easy to process and calibrate, and its various performance indicators meet the technical requirements, with good overall performance. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the structure of the mid-wave infrared optical system of this utility model;

[0027] Figure 2 This is a two-dimensional optical path diagram of the mid-wave infrared optical system of this utility model;

[0028] Figure 3 This is a dot plot of the mid-wave infrared optical system of this utility model at 10 degrees Celsius;

[0029] Figure 4 The modulation transfer function of the mid-wave infrared optical system of this invention at -40 degrees Celsius is shown in the figure.

[0030] Figure 5 The modulation transfer function of the mid-wave infrared optical system of this invention at 10 degrees Celsius is shown in the figure.

[0031] Figure 6 The modulation transfer function of the mid-wave infrared optical system of this invention at 60 degrees Celsius is shown in the figure.

[0032] Figure 7 This is a field curvature distortion diagram of the mid-wave infrared optical system of this utility model;

[0033] Figure 8 This is an incoherent irradiance diagram of the mid-wave infrared optical system of this utility model.

[0034] In the picture:

[0035] 1. Lens 1; 2. Lens 2; 3. Lens 3; 4. Lens 4; 5. Lens 5; 6. Lens 6; 7. Window; 8. Cold stop; 9. Filter; 10. Image plane. Detailed Implementation

[0036] It should be noted that the terms "center," "longitudinal," "lateral," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only used to facilitate the description of this utility model and to simplify the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limiting the scope of protection of this utility model.

[0037] In the description of this utility model, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "joining" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this utility model based on the specific circumstances.

[0038] The present invention will be further described below with reference to specific embodiments.

[0039] Example 1

[0040] Existing MWIR optical systems mostly use conventional infrared materials such as germanium (Ge) and silicon (Si) to balance aberrations by controlling the lens radius and spacing. However, such designs often face bottlenecks such as insufficient efficiency of cold apertures, difficulty in thermal design, and sensitivity to assembly and adjustment tolerances.

[0041] To address the above issues, this embodiment introduces a six-element lens optical system based on binary surface fusion. Through optimized design of the lens structure, materials, and layout, while ensuring 100% cold stop efficiency, a fully static, non-moving component design is adopted, significantly reducing processing and assembly complexity. This achieves a synergistic improvement in wide-temperature-range calorimetry and high image quality output, providing a reliable solution for engineering applications. Specifically, in conjunction with... Figure 1 This embodiment describes in detail a cooled mid-wave infrared optical system based on a binary surface.

[0042] The optical system of this embodiment includes six coaxial lenses mounted in an optomechanical structure. These six lenses, arranged sequentially from object to image, are: a silicon lens 1, a zinc sulfide lens 2, a germanium lens 3, a silicon lens 4, a germanium lens 5, and a zinc selenide lens 6. Lens 3 is a plano-concave lens, lens 6 is a plano-convex lens, and the others are meniscus lenses. Lens 5 has two even-order aspherical surfaces, and the image-facing surface of lens 6 is a binary surface. The surfaces of the remaining lenses are standard surfaces. The expression for an even-order aspherical surface is as follows:

[0043]

[0044] In the formula, c is the curvature. r0 is the radius of curvature of the optical surface; k is the conic coefficient; r is the normalized radius coordinate; A, B, C, D, and E are coefficients of higher-order terms, and the specific parameters are shown in the table below.

[0045]

[0046] The diffraction order of the binary surface is 1, and the second-order term is -0.017.

[0047] More specifically, the surface curvature radius of lens 1 facing the object side is 105.624 mm, and the net aperture is 55.031 mm; the surface curvature radius of lens 1 facing the image side is 205.112 mm, and the net aperture is 50.644 mm; the center thickness of lens 1 is 19.111 mm, and the air gap between lens 1 and lens 2 is 3.123 mm.

[0048] Lens 2 has a surface curvature radius of 250.536 mm facing the object side and a net aperture of 48.929 mm; its surface curvature radius facing the image side is 283.789 mm and its net aperture is 35.824 mm; the center thickness of lens 2 is 29.645 mm, and the air gap between it and lens 3 is 3.261 mm.

[0049] Lens 3 has a flat surface facing the object side with a net aperture of 34.3 mm; the surface facing the image side has a radius of curvature of 97.052 mm and a net aperture of 28.369 mm; the center thickness of lens 3 is 25.274 mm, and the air gap between it and lens 4 is 0.998 mm.

[0050] Lens 4 has a surface curvature radius of 89.715 mm facing the object side and a net aperture of 27.244 mm; its surface curvature radius facing the image side is 73.486 mm and its net aperture is 26.883 mm; the center thickness of lens 4 is 7.144 mm, and the air gap between it and lens 5 is 0.99 mm.

[0051] Lens 5 has a surface curvature radius of 70.823 mm facing the object side and a net aperture of 27.226 mm; its surface curvature radius facing the image side is 65.513 mm and its net aperture is 17.466 mm; the center thickness of lens 5 is 35.014 mm, and the air gap between it and lens 6 is 3.638 mm.

[0052] Lens 6 has a flat surface facing the object side with a net aperture of 17.079 mm; the surface facing the image side has a radius of curvature of 41.267 mm and a net aperture of 16.820 mm; the center thickness of lens 6 is 3.417 mm.

[0053] It should be noted that the air gap mentioned in this embodiment refers to the distance between the centers of two adjacent lenses. For example, the air gap between lens 5 and lens 6 is 3.638mm, which means that the distance from the center of the image-facing surface of lens 5 to the center of the object-facing surface of lens 6 is 3.638mm.

[0054] In this embodiment, the image-side surface of lens 6 adopts a binary surface design, resulting in optimal imaging performance. Combined with lenses of different materials and curvatures, i.e., "material thermal difference complementarity + structural optimization," it balances optical imaging quality and thermal difference, lens thickness, and aperture. The two-dimensional optical path diagram of this embodiment is shown below. Figure 2 As shown.

[0055] The system has a total length of 180.002mm, a maximum lens radius of 55.030mm, a reasonable ratio between the lens radius and thickness, and a reasonable optical path structure.

[0056] The dot plot of the optical system in this embodiment at 10°C is shown below. Figure 3 As shown, the size of the diffuse spots is less than 12 micrometers, which matches the size of the detector pixels.

[0057] Furthermore, the optical system of this embodiment also includes a window 7, a cold stop 8, and a filter 9 arranged sequentially from the object side to the image side. The window 7 is made of silicon and has a center thickness of 2.5 mm. The object side surface of the window 7 is flat, with a net aperture of 14.237 mm, compared to 4.038 mm for the lens 6. The image side surface of the window 7 is also flat, with a net aperture of 13.904 mm and an air gap of 1.85 mm between it and the cold stop 8. The cold stop 8 has a net aperture of 11.768 mm and an air gap of 35.2 mm between it and the filter 9.

[0058] The filter 9 is made of germanium. The net aperture of the object surface of the filter 9 is 9.73 mm, the net aperture of the image surface is 9.711 mm, the center thickness of the filter 9 is 0.8 mm, the air gap between the filter 9 and the image surface 10 is 4 mm, and the net aperture of the image surface 10 is 8.161 mm.

[0059] The optical system of this embodiment is suitable for mid-wave infrared detectors with a target array of 1024×1024 and a pixel size of 12 micrometers. The optical system uses common optical materials such as Si, Ge, ZnSe, and ZnS, overcoming the limitation of lens aperture while meeting imaging quality requirements. It also rationally considers lens structure and practical manufacturing feasibility, employing a single-image structure with a six-element Si-ZnS-Ge-Si-Ge-ZnSe layout. This balances optical imaging quality with thermal difference, lens thickness, and aperture. Transmittance is further improved through coating (each lens surface is coated with a film with a transmittance of 0.99). While ensuring reasonable tolerances, the manufacturing difficulty is also considered, resulting in lenses that meet the following specifications:

[0060]

[0061] Traditional anechoic solutions for optical systems rely on mechanical compensation (such as moving lens groups) or complex aspherical designs. While these can partially offset thermal differences, they suffer from high structural complexity, low reliability, and difficulty in simultaneously controlling aberrations across the entire field of view, failing to meet the dual requirements of high-precision imaging and low-cost mass production. This invention achieves a fully static anechoic design over a wide temperature range (-40 to 60°C) through material thermal difference compensation and curvature radius optimization.

[0062] Furthermore, traditional anechoic designs are prone to focal drift, astigmatism, and field curvature due to thermal expansion and contraction over a wide temperature range, resulting in a significant decrease in imaging quality and making it difficult to meet the stable detection requirements in complex environments such as aviation and automotive applications. This invention significantly improves imaging stability over a wide temperature range through the coordinated correction of binary surfaces and even-order aspherical surfaces.

[0063] The image quality of the optical system in this embodiment is evaluated as follows:

[0064] (1) Modulation Transfer Function (MTF) and Thermal Difference Analysis

[0065] The modulation transfer function (MTF) is a core indicator for quantifying the imaging performance of an optical system, characterizing its ability to transfer contrast at different spatial frequencies (lp / mm). For mid-wave infrared optical systems, the MTF threshold requirement is typically related to the detector pixel size and the imaging task. In this embodiment, the optical system has an MTF ≥ 0.3 at 42 lp / mm, effectively avoiding image blurring caused by aliasing. The MTF graphs of the optical system at different temperatures are shown below. Figures 4-6 As shown.

[0066] Combination Figures 4-6It can be seen that, after the anechoic design, the MTF value is greater than 0.3 at a cutoff frequency of 42 lp / mm at -40℃, 10℃, and 60℃, and the center MTF even reaches 0.5. At the same time, with the coating, the full field of view transmittance can reach 0.8515, which meets the requirements of use. The overall design has potential in terms of resolution and aberration control.

[0067] (2) Field distortion analysis

[0068] Field curvature distortion of optical systems, such as Figure 7 As shown, due to the introduction of aspherical and binary surface design, the overall field curvature value is low (<0.2mm), indicating good image plane flatness, which is beneficial to the full field of view imaging sharpness. The maximum distortion is less than one percent, and the image geometric deformation is negligible, making it suitable for high-precision measurement or calibration scenarios.

[0069] (3) Incoherent irradiance analysis

[0070] Stray light mainly refers to the incoherent irradiance of three optical systems, such as Figure 8 As shown, with 5000 analyzed rays and polarization settings, the incoherent irradiance of the detector is as follows. The infrared lens performs well in stray light control. No obvious stray light is seen in the figure. The irradiance distribution is concentrated and the edge energy decays rapidly, which meets the requirements of a high-precision optical system. Its low stray light characteristics have important application value in the field of infrared imaging, and the imaging quality is excellent and stable.

[0071] Although embodiments of the present invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the present invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A cooled mid-wave infrared optical system based on a binary surface, characterized in that, The system includes six coaxial lenses mounted in the optical-mechanical structure. The six lenses are arranged sequentially from the object side to the image side: a silicon lens (1), a zinc sulfide lens (2), a germanium lens (3), a silicon lens (4), a germanium lens (5), and a zinc selenide lens (6). Among them, the third lens (3) is a plano-concave lens, the sixth lens (6) is a plano-convex lens, and the rest are meniscus lenses. The two surfaces of the fifth lens (5) are even-order aspherical surfaces, the surface of the sixth lens (6) facing the image side is a binary surface, and the surfaces of the remaining lenses are standard surfaces. The maximum aperture of the optical system is 110 mm, and the total length of the system is 180 mm.

2. The cooled mid-wave infrared optical system based on a binary surface according to claim 1, characterized in that: The surface curvature radius of lens 1 (1) facing the object side is 105.624 mm, the surface curvature radius of lens 1 facing the image side is 205.112 mm, the center thickness is 19.111 mm, and the air gap between it and lens 2 (2) is 3.123 mm.

3. The cooled mid-wave infrared optical system based on a binary surface according to claim 1, characterized in that: The surface curvature radius of lens 2 (2) facing the object side is 250.536 mm, the surface curvature radius of lens 2 facing the image side is 283.789 mm, the center thickness is 29.645 mm, and the air gap between it and lens 3 (3) is 3.261 mm.

4. The cooled mid-wave infrared optical system based on a binary surface according to claim 1, characterized in that: The object-facing surface of lens three (3) is flat, the radius of curvature of the image-facing surface is 97.052 mm, the center thickness is 25.274 mm, and the air gap between it and lens four (4) is 0.998 mm.

5. A cooled mid-wave infrared optical system based on a binary surface according to claim 1, characterized in that: The surface curvature radius of lens 4 (4) facing the object side is 89.715 mm, the surface curvature radius of lens 4 (4) facing the image side is 73.486 mm, the center thickness is 7.144 mm, and the air gap between it and lens 5 (5) is 0.99 mm.

6. The cooled mid-wave infrared optical system based on a binary surface according to claim 1, characterized in that: The surface curvature radius of lens 5 (5) facing the object side is 70.823 mm, the surface curvature radius of lens 5 (5) facing the image side is 65.513 mm, the center thickness is 35.014 mm, and the air gap between it and lens 6 (6) is 3.638 mm.

7. A cooled mid-wave infrared optical system based on a binary surface according to claim 1, characterized in that: Lens 6 (6) has a plane surface facing the object side and a radius of curvature of 41.267 mm facing the image side, with a center thickness of 3.417 mm.

8. A cooled mid-wave infrared optical system based on a binary surface according to any one of claims 1-3, characterized in that: It also includes a window (7), a cold stop (8) and a filter (9) arranged sequentially from the object side to the image side. The window (7) is made of silicon and has a thickness of 2.5 mm. The air gap between the object side surface of the window (7) and the lens (6) is 4.038 mm. The cold stop (8) is 1.85 mm away from the image side surface of the window (7) and 35.2 mm away from the object side surface of the filter (9). The filter (9) is made of germanium and has a thickness of 0.8 mm. It is 4 mm away from the image plane.

9. A cooled mid-wave infrared optical system based on a binary surface according to any one of claims 1-3, characterized in that: All six lenses, as well as the windows and filters, are coated with a film with a transmittance of 0.

99.

10. A cooled mid-wave infrared optical system based on a binary surface according to any one of claims 1-3, characterized in that: The optical system operates in the wavelength range of 3μm to 5μm, has an F-number of 1.65, and an MTF of 42lp / mm > 0.

3. The optical system is suitable for mid-wave infrared detectors with a target array of 1024×1024 and a pixel size of 12μm.

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