Three-piece Passive Athermalized Long-Wave Infrared Optical System Based on Diffractive-Refractive Hybrid Lenses

Abstract

The present invention relates to a three-piece passive athermalized long-wave infrared optical system based on a refractive-diffractive hybrid lens, belonging to the field of optical design imaging structures in the infrared band; it includes a spherical cover, a first meniscus positive lens, a meniscus negative lens, and a second meniscus positive lens. Collimated light from infinity enters the first meniscus positive lens with the largest aperture, the meniscus negative lens, and the second meniscus positive lens in sequence through the spherical cover from the object plane, balancing the thermal difference, chromatic aberration, and monochromatic aberration of the system, and finally imaging on the detector focal plane. The first meniscus positive lens is a refractive-diffractive hybrid lens, and the side facing the image plane is an aspherical diffractive surface. By using the lens with the largest aperture in the optical system as a refractive-diffractive hybrid chalcogenide glass lens, the present invention maximally utilizes the low refractive index coefficient and good dispersion performance of chalcogenide glass, as well as the dispersion coefficient of the negative dispersion of the binary optical diffractive element and the large temperature compensation characteristic, solves the system focal shift caused by temperature change, better achieves the purpose of athermalization, and reaches the best imaging effect.

Description

Technical Field

[0001] The present invention belongs to the field of optical design imaging structures in the infrared band, and particularly relates to a three-piece passive athermal long-wave infrared optical system based on a refractive-diffractive hybrid lens. Background Art

[0002] With the rapid development of science and technology and the continuous development and increasing maturity of uncooled detector technology, long-wave infrared uncooled optical systems have been widely used in military and civilian fields. When an infrared optical system is used in a relatively large temperature range, the drastic change in the operating temperature will cause thermal deformation of infrared optical materials and mechanical materials, resulting in effects such as changes in the focal length of the optical system, image plane drift, and deterioration of imaging quality. Therefore, it is necessary to perform athermal design on the infrared optical system. Currently, the main athermal methods are: electromechanical active type, mechanical passive type, and optical passive type. The optical passive type realizes the matching of the focal plane position and the change in the lens barrel length by reasonably distributing the optical power and optical materials, so as to ensure the imaging quality of the lens within a specified temperature range. It has the advantages of light weight and simple structure, and is more suitable for the requirements of infrared optical systems. However, in traditional refractive optical systems, only by changing the curvature of the curved surface or using different materials can aberrations be corrected, and at least three or more materials are required, which increases the number of lenses and makes the system structure complex.

[0003] In the prior art, a transmissive uncooled passive athermal long-wave infrared optical system using 5 lenses and 4 optical materials is disclosed; another method to achieve optical passive athermalization is to adopt a binary optical diffraction element in the optical system to form a refractive-diffractive hybrid system, and use the dispersion coefficient of the negative dispersion and the large temperature compensation characteristics of the binary optical diffraction element to eliminate the thermal difference of the optical system. However, the currently designed optical systems use multiple diffractive surfaces, which seriously reduces the system efficiency and causes insufficient energy of the optical system. For example, in the infrared refractive-diffractive three-piece athermal optical imaging system structure disclosed in the prior art, 2 refractive-diffractive hybrid lenses are used; in addition, in order to obtain better optical passive athermal and achromatic performance, chalcogenide glass with a relatively low refractive index coefficient and good dispersion performance can be used as the lens material. Especially when using chalcogenide glass material on the lens with the largest aperture, the athermal and chromatic aberration performance is more significant. However, the processing and coating of chalcogenide glass have certain difficulties, and using too much chalcogenide glass in an optical system will increase the processing cost. Currently, the designed optical systems use multiple pieces of chalcogenide glass in the system. Summary of the Invention

[0004] Technical Problems to be Solved:

[0005] In order to avoid the deficiencies of the prior art, the present invention provides a three-piece passive athermalized long-wave infrared optical system based on a refractive-diffractive hybrid lens. By using the lens with the largest aperture in the optical system as a refractive-diffractive hybrid chalcogenide glass lens, the low refractive index coefficient and good dispersion performance of chalcogenide glass, as well as the dispersion coefficient of negative dispersion and large temperature compensation characteristics of the binary optical diffraction element, are maximally utilized to solve the system focal shift caused by temperature change, better achieve the purpose of athermalization, and reach the best imaging effect. It can have good imaging quality and athermalization effect in the 8-12 long-wave infrared band within the temperature range of -40°C to +60°C.

[0006] The technical solution of the present invention is: a three-piece passive athermalized long-wave infrared optical system based on a refractive-diffractive hybrid lens, including a dome, a first meniscus positive lens, a meniscus negative lens, and a second meniscus positive lens. The parallel light at infinity enters the first meniscus positive lens, the meniscus negative lens, and the second meniscus positive lens with the largest aperture in sequence from the object surface through the dome, balancing the system thermal difference, chromatic aberration, and monochromatic aberration, and finally imaging on the detector focal plane to complete the entire imaging process; the first meniscus positive lens is a refractive-diffractive hybrid lens, and the side facing the image surface is an aspherical diffraction surface.

[0007] A further technical solution of the present invention is: the convex surfaces of the first meniscus positive lens, the meniscus negative lens, and the second meniscus positive lens all face the object surface.

[0008] A further technical solution of the present invention is: the first meniscus positive lens is made of chalcogenide glass material.

[0009] A further technical solution of the present invention is: the first meniscus positive lens includes a first lens surface s4 and a second lens surface s5, the meniscus negative lens includes a third lens surface s6 and a fourth lens surface s7, and the second meniscus positive lens includes a fifth lens surface s8 and a sixth lens surface s9. Among them, the first lens surface s5, the fourth lens surface s7, and the fifth lens surface s8 are aspherical surfaces, and the rest are spherical surfaces.

[0010] A further technical solution of the present invention is: a phase polynomial type kinoform binary optical surface with a first-order diffraction order is arranged on the aspherical second lens surface s5 of the first meniscus positive lens.

[0011] A further technical solution of the present invention is that the radius of curvature of the surface s4 of the first lens is 73 mm, its thickness is 12.5 mm, and its effective aperture is 91 mm; the radius of curvature of the surface s5 of the second lens is 131.185 mm, its thickness is 40.5 mm, and its effective aperture is 87 mm; the radius of curvature of the surface s6 of the third lens is 43.86 mm, its thickness is 5.5 mm, and its effective aperture is 43 mm; the radius of curvature of the surface s7 of the fourth lens is 31.358 mm, its thickness is 20 mm, and its effective aperture is 36 mm; the radius of curvature of the surface s8 of the fifth lens is 66.418 mm, its thickness is 5 mm, and its effective aperture is 33 mm; the radius of curvature of the surface s9 of the sixth lens is 99.22 mm, its thickness is 20.9 mm, and its effective aperture is 31 mm.

[0012] A further technical solution of the present invention is that the total length of the optical system < 122 mm, the f-number is 1.1, and the field of view angle is 7.8°; the wavelength range of the transmitted light is 8 um to 12 um.

[0013] A further technical solution of the present invention is that the imaging resolution of the optical system is 640×512 pixels.

[0014] A further technical solution of the present invention is that the spherical cover is made of zinc sulfide material, and the meniscus negative lens and the second meniscus positive lens are both made of germanium material.

[0015] A further technical solution of the present invention is that the barrel material of the optical system is aluminum alloy material.

[0016] Beneficial effects

[0017] The beneficial effects of the present invention are as follows: By using the largest-aperture chalcogenide glass lens as a refractive / diffractive hybrid lens in the passive athermalized long-wave infrared optical system, the present invention maximally utilizes the low refractive index coefficient and good dispersion performance of chalcogenide glass, as well as the dispersion coefficient of the negative dispersion of the binary optical diffraction element and the large temperature compensation characteristic, solves the system focal shift caused by temperature change, better realizes the purpose of athermalization, and achieves the best imaging effect. Therefore, the refractive / diffractive hybrid athermalized optical system using binary optical elements can not only obtain imaging quality close to the diffraction limit within a large field of view and a very wide temperature working range, but also has less material, a simple structure, a small volume, and a light weight. Therefore, the refractive / diffractive hybrid lens has great application value in the athermalization design and chromatic aberration correction of infrared optical systems.

[0018] The present invention uses two kinds of optical materials to remove the spherical cover, reasonably allocate and combine to eliminate thermal difference, with a simple and compact structure and fewer lens elements. In the temperature range of -40°C to +60°C, the maximum defocus amount of this system is less than 1 times the focal depth. At the spatial cut-off frequency of 30 lp / mm, the optical modulation transfer function (MTF) value is 0.52, approaching the diffraction limit. The analysis results show that this system has good imaging quality and athermal effect. The designed system uses a total of three optical elements excluding the spherical cover. Through the combination of lens materials and the distribution of optical power, and by coordinating with the thermal expansion and contraction of the lens barrel material, the optical passive athermal effect is achieved. When the system operates in the environment with a temperature range of -40°C to +60°C, the MTF values of the system transfer function at each temperature are greater than 0.52 at the spatial cut-off frequency of 30 lp / mm. It has the advantages of high image quality, a wide operating temperature range, a compact structure, light weight, high imaging resolution, good imaging quality, small volume, high system transmittance, large relative aperture, and the modulation transfer function (MTF) approaching the diffraction limit.

[0019] The present invention is applicable to uncooled long-wave infrared focal plane detectors with 640×512 pixels and a pixel size of 17 μm. When an aluminum alloy lens barrel is used, through the paired combination of lens materials and the matching of the linear expansion coefficient and length of the lens barrel material, arranged in sequence, within the temperature range of -40°C to +60°C, no focusing adjustment is required, and the average change of the MTF value does not exceed 10% at the spatial cut-off frequency of 30 lp / mm. Description of the Drawings

[0020] Figure 1 It is the optical structure diagram of the three-piece athermal long-wave infrared optical system based on refractive-diffractive hybrid lenses of the present invention.

[0021] Figure 2 It is the ray aberration curve of the three-piece athermal long-wave infrared optical system based on refractive-diffractive hybrid lenses of the present invention at -40°C.

[0022] Figure 3 It is the ray aberration curve of the three-piece athermal long-wave infrared optical system based on refractive-diffractive hybrid lenses of the present invention at 20°C.

[0023] Figure 4 It is the ray aberration curve of the three-piece athermal long-wave infrared optical system based on refractive-diffractive hybrid lenses of the present invention at 60°C.

[0024] Figure 5 It is the astigmatism and distortion curve of the three-piece athermal long-wave infrared optical system based on refractive-diffractive hybrid lenses of the present invention at -40°C.

[0025] Figure 6 It is the astigmatism and distortion curve of the three-piece athermal long-wave infrared optical system based on refractive-diffractive hybrid lenses of the present invention at 20°C.

[0026] Figure 7 These are the astigmatism and distortion curves of the three - element passive athermalized long - wave infrared optical system based on refractive - diffractive hybrid lenses of the present invention at - 60°C.

[0027] Figure 8 These are the spot diagrams of the three - element passive athermalized long - wave infrared optical system based on refractive - diffractive hybrid lenses of the present invention at - 40°C.

[0028] Figure 9 These are the spot diagrams of the three - element passive athermalized long - wave infrared optical system based on refractive - diffractive hybrid lenses of the present invention at 20°C.

[0029] Figure 10 These are the spot diagrams of the three - element passive athermalized long - wave infrared optical system based on refractive - diffractive hybrid lenses of the present invention at 60°C.

[0030] Figure 11 These are the MTF curves of the three - element passive athermalized long - wave infrared optical system based on refractive - diffractive hybrid lenses of the present invention at - 40°C.

[0031] Figure 12 These are the MTF curves of the three - element passive athermalized long - wave infrared optical system based on refractive - diffractive hybrid lenses of the present invention at 20°C.

[0032] Figure 13 These are the MTF curves of the three - element passive athermalized long - wave infrared optical system based on refractive - diffractive hybrid lenses of the present invention at 60°C.

[0033] Explanation of reference numerals: 1. Object surface, 2. Dome, 3. First meniscus positive lens, 4. Meniscus negative lens, 5. Second meniscus positive lens, 6. Detector focal plane; s1. Object surface, s2. Outer surface of the dome, s3. Inner surface of the dome, s4. First lens surface, s5. Second lens surface, s6. Third lens surface, s7. Fourth lens surface, s8. Fifth lens surface, s9. Sixth lens surface, s10. Detector focal plane surface. Detailed implementation manners

[0034] The embodiments described below by referring to the accompanying drawings are exemplary and are intended to explain the present invention, and should not be construed as limiting the present invention.

[0035] In the description of the present invention, it should be understood that the orientation or positional relationship indicated by terms such as "center", "longitudinal", "transverse", "length", "width", "thickness", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise", etc. is based on the orientation or positional relationship shown in the drawings. It is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation. Therefore, it should not be construed as a limitation to the present invention.

[0036] A three-piece passive anastigmatic long-wave infrared optical system based on a refractive-diffractive hybrid lens in this embodiment includes a dome 2, a first meniscus positive lens 3, a meniscus negative lens 4, and a second meniscus positive lens 5. The barrel material of the whole system is aluminum alloy material. The infrared light emitted by the object sequentially passes through the dome 2 and enters the first meniscus positive lens 3 with the largest aperture, the meniscus negative lens 4, and the second meniscus positive lens 5, and finally forms an image on the detector focal plane 6. The convex surfaces of the first meniscus positive lens 3, the meniscus negative lens 4, and the second meniscus positive lens 5 all face the object surface. The first meniscus positive lens 3 is a refractive-diffractive hybrid lens, and a refractive-diffractive hybrid lens is used to correct the chromatic aberration and thermal aberration of the infrared optical system; the diffractive surface of the refractive-diffractive hybrid lens is located on the side facing the image surface, its preparation substrate is an aspherical surface, and a chalcogenide glass material is used; the aspherical surface is used to coordinately correct the coma and spherical aberration of the system.

[0037] The first meniscus positive lens 3 includes a first lens surface s4 and a second lens surface s5. The meniscus negative lens 4 includes a third lens surface s6 and a fourth lens surface s7. The second meniscus positive lens 5 includes a fifth lens surface s8 and a sixth lens surface s9. Among them, the lens surfaces s5, s7, and s8 are aspherical surfaces, and the rest are spherical surfaces. A phase polynomial type kinoform binary optical surface with a first-order diffraction order is arranged on the aspherical surface of the second lens surface s5; the material of the lens is the chalcogenide glass IRG206 of Xinhua Optoelectronics Company, and it can be thermally molded under the condition of not exceeding 300°, reducing the lens cost.

[0038] The specific parameters of each lens are as follows: the radius of curvature of the surface s4 of the first lens is 73 mm, its thickness is 12.5 mm, and its effective aperture is 91 mm; the radius of curvature of the surface s5 of the second lens is 131.185 mm, its thickness is 40.5 mm, and its effective aperture is 87 mm; the radius of curvature of the surface s6 of the third lens is 43.86 mm, its thickness is 5.5 mm, and its effective aperture is 43 mm; the radius of curvature of the surface s7 of the fourth lens is 31.358 mm, its thickness is 20 mm, and its effective aperture is 36 mm; the radius of curvature of the surface s8 of the fifth lens is 66.418 mm, its thickness is 5 mm, and its effective aperture is 33 mm; the radius of curvature of the surface s9 of the sixth lens is 99.22 mm, its thickness is 20.9 mm, and its effective aperture is 31 mm.

[0039] In the optical system, the first meniscus positive lens 3, the meniscus negative lens 4, and the second meniscus positive lens 5 can be coated with an antireflection film. The total length of the lens does not exceed 122 mm. To improve the light transmission, the f-number of this optical system example is 1.1, the ratio of the total length to the focal length is less than 2, the field of view angle is 7.8°, the resolution of the lens can reach 640×512 pixels, and the pixel size of the uncooled infrared sensor used is 15 um, with a diagonal length of 13.932 mm.

[0040] The wavelength range of the light transmitted through the optical system is 8 um to 12 um.

[0041] In this embodiment, the spherical cover 2 is made of zinc sulfide material, and the meniscus negative lens 4 and the second meniscus positive lens 5 are both made of germanium material.

[0042] Figures 2 to 4 Respectively characterized the ray aberration curve, astigmatism, distortion, spot diagram, and modulation transfer function MTF value of the three-piece passive athermalized long-wave infrared optical system based on refractive-diffractive hybrid lenses of the present invention. It can be seen that the distortion of the three-piece passive athermalized long-wave infrared optical system based on refractive-diffractive hybrid lenses of the present invention is very small, less than 1%, the ray aberration is less than 17 um, the MTF of all field of view angles is greater than 0.52, and the imaging quality is very good.

[0043] The present invention adopts athermalization design scheme containing diffraction elements, and uses a three-piece infrared refractive / diffractive system without a spherical cover in the temperature range of -40°C to +60°C. Among them, the first meniscus positive lens 3 is a refractive / diffractive hybrid element, and chalcogenide glass IRG206 material is used.

[0044] Through optimization by CODE V optical design software, the three-piece passive athermalized long-wave infrared optical system based on refractive-diffractive hybrid lenses of the present invention is obtained. Among them, the surfaces s7 of the fourth lens and s8 of the fifth lens are high-order aspherical surfaces. The purpose of adding high-order aspherical surfaces is to be more conducive to athermalization.

[0045] Figure 4 This shows the optical transfer function of the optical system of the present invention at -40°C, 20°C, and 60°C, indicating that this system can operate normally within a temperature range of 100°C. Figure 4 It shows that the aberrations of the system at -40°C and 60°C are very small, indicating that the binary optical diffraction element has a very good chromatic aberration correction effect.

[0046] The three-piece passive athermalized long-wave infrared optical system based on a refractive-diffractive hybrid lens of the present invention uses a refractive-diffractive hybrid chalcogenide glass material lens as the largest aperture lens, and adopts a diffraction surface on an aspherical substrate, which simplifies the system structure and weight, reduces the types of optical materials and the number of lenses; the system has a large field of view angle and a small f-number design, greatly improving the system's detection and recognition ability; at the same time, the refractive-diffractive hybrid lens material with the largest aperture uses chalcogenide glass that can be mass-produced by rapid molding, greatly reducing the manufacturing cost of the lens. Currently, 8 sets of products have been put into use. After comparing the actual imaging effects, the three-piece passive athermalized long-wave infrared optical system based on an infrared refractive / diffractive hybrid lens has excellent imaging quality and a high technology maturity level.

[0047] Although the embodiments of the present invention have been shown and described above, it can be understood that the above embodiments are exemplary and should not be construed as limitations on the present invention. Those of ordinary skill in the art can make changes, modifications, substitutions, and variations to the above embodiments within the scope of the present invention without departing from the principles and purposes of the present invention.

Claims

1. A three - element passive athermalized long - wave infrared optical system based on a refractive - diffractive hybrid lens, characterized in that: It includes a spherical cover, a first meniscus positive lens, a meniscus negative lens, and a second meniscus positive lens. Parallel light from infinity enters the first meniscus positive lens with the largest aperture, the meniscus negative lens, and the second meniscus positive lens in sequence through the spherical cover, balancing the thermal difference, chromatic aberration, and monochromatic aberration of the system. Finally, the image is formed on the detector focal plane, completing the entire imaging process. The first meniscus positive lens is a refractive-diffractive hybrid lens, and the side facing the image plane is an aspherical diffractive surface. The first meniscus positive lens includes a first lens surface s4 and a second lens surface s5. The meniscus negative lens includes a third lens surface s6 and a fourth lens surface s7. The second meniscus positive lens includes a fifth lens surface s8 and a sixth lens surface s9. Among them, the first lens surface s4, the fourth lens surface s7, and the fifth lens surface s8 are aspherical surfaces, and the rest are spherical surfaces. A phase polynomial type kinofrom binary optical surface with a first-order diffraction order is set on the aspherical second lens surface s5 of the first meniscus positive lens. The radius of curvature of the first lens surface s4 is 73 mm, its thickness is 12.5 mm, and its effective aperture is 91 mm. The radius of curvature of the second lens surface s5 is 131.185 mm, its thickness is 40.5 mm, and its effective aperture is 87 mm. The radius of curvature of the third lens surface s6 is 43.86 mm, its thickness is 5.5 mm, and its effective aperture is 43 mm. The radius of curvature of the fourth lens surface s7 is 31.358 mm, its thickness is 20 mm, and its effective aperture is 36 mm. The radius of curvature of the fifth lens surface s8 is 66.418 mm, its thickness is 5 mm, and its effective aperture is 33 mm. The radius of curvature of the sixth lens surface s9 is 99.22 mm, its thickness is 20.9 mm, and its effective aperture is 31 mm. The first meniscus positive lens is made of chalcogenide glass material, and the meniscus negative lens and the second meniscus positive lens are both made of germanium material.

2. The three-piece passive athermalized long-wave infrared optical system based on a refractive-diffractive hybrid lens according to claim 1, wherein: The convex surfaces of the first meniscus positive lens, the meniscus negative lens, and the second meniscus positive lens all face the object surface.

3. The three-piece passive athermalized long-wave infrared optical system based on a refractive-diffractive hybrid lens according to claim 1, characterized in that: The total length of the optical system is < 122 mm, the f-number is 1.1, and the field of view angle is 7.8°. The wavelength range of the transmitted light is 8 μm to 12 μm.

4. The three-piece passive athermalized long-wave infrared optical system based on a refractive-diffractive hybrid lens according to claim 1, wherein: The imaging resolution of the optical system is 640×512 pixels.

5. The three-piece passive athermalized long-wave infrared optical system based on a refractive-diffractive hybrid lens according to claim 1, characterized in that: The spherical cover is made of zinc sulfide material.

6. The three-piece passive athermalized long-wave infrared optical system based on a refractive-diffractive hybrid lens according to any one of claims 1-5, characterized in that: The barrel material of the optical system is aluminum alloy material.

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