Design method of planar multi-order diffraction lens for cooled infrared detectors

Abstract

A design method for a planar multi-order diffraction lens suitable for cooled infrared detectors belongs to the field of optical design technology. To address the problem that existing multi-order diffraction lenses are incompatible with cooled detectors, the method includes: step 1, fitting the lens surface structure in matrix form; step 2, designing a cooled on-axis field-of-view planar multi-order diffraction lens based on the on-axis point focusing efficiency; and step 3, increasing the off-axis field of view to expand the lens aperture, optimizing based on the full field of view, and designing a cooled multi-field-of-view planar multi-order diffraction lens. This method optimizes the cooled planar multi-order diffraction lens by freely assigning weights to the full field of view. By introducing a full-band, full-field-of-view optimization design method, the imaging quality within the full field of view is significantly improved. The multi-order diffraction lens designed using this method is suitable for cooled infrared detectors. This not only significantly simplifies the overall structure of the cooled infrared system, but also has important significance for improving the portability of the cooled infrared optical system.

Description

Technical Field

[0001] The invention relates to a design method of a planar multi-order diffraction lens suitable for a refrigerated infrared detector, and belongs to the technical field of optical design. Background Art

[0002] Cooled infrared optical systems offer the advantages of reducing stray light, improving signal-to-noise ratio, increasing contrast, and enhancing the detection and recognition capabilities of light detection systems. However, current designs of cooled infrared optical systems are complex, requiring numerous optical components, resulting in large size and weight. This can be a limiting factor in applications requiring lightweight and compact designs, and the effects of cold reflections must also be considered. Existing multi-order diffraction lenses achieve achromatic aberration with a single lens, avoiding the need for multiple lens elements. Consequently, these diffraction lenses are lighter and more compact. Combined with cooled infrared detectors, they can enable miniaturization and integration of infrared optical systems.

[0003] In Chinese Patent Publication No. CN113866979A, entitled "Achromatic Method for Multi-Order Diffractive Lenses and Achromatic Multi-Order Diffractive Lenses," a global optimization algorithm is used to optimize the focusing efficiency of the on-axis focal plane to achieve a multi-order diffractive lens with achromaticity of 400nm-1100nm. The method also demonstrates that the maximum aspect ratio of the unit structure for the grayscale level distribution per ring height does not exceed 2:1. However, this method focuses only on the on-axis field of view and cannot achieve high-quality imaging across the entire field of view. Furthermore, the aperture overlaps with the element, preventing 100% cold aperture efficiency. Therefore, this method is unsuitable for use with cooled infrared detectors. Summary of the Invention

[0004] In order to solve the problem that existing multi-order diffraction lenses cannot be adapted to refrigerated detectors, the present invention proposes a design method for a planar multi-order diffraction lens suitable for refrigerated infrared detectors. This method significantly simplifies the structure of the refrigerated infrared system, realizes imaging of a single-element refrigerated infrared optical system, and can optimize the design over the entire band and full field of view.

[0005] The technical solution of the present invention to solve the technical problem is:

[0006] A method for designing a planar multi-order diffraction lens suitable for a cooled infrared detector comprises the following steps:

[0007] Step 1: Fitting the lens surface structure in matrix form;

[0008] Step 2: Based on the on-axis point focusing efficiency, a cooled on-axis planar multi-order diffraction lens is designed;

[0009] Step three: Increase the off-axis field of view to expand the lens aperture, optimize based on the full field of view, and design a cooled multi-field-of-view planar multi-order diffraction lens.

[0010] In step 1, a planar multi-order diffraction lens surface structure is fitted in a matrix form. The lens structure is an n*n microstructure. The lens radius corresponding to the field of view is r1. The side length of each microstructure is r1 / n. Assuming that there are n1 transverse microstructures and n2 longitudinal microstructures, the distance between the position of the microstructure and the center of the lens is:

[0011]

[0012] If S≤r1, it is within the field of view of the lens; if S>r1, it is outside the field of view.

[0013] In the second step, based on the on-axis point focusing efficiency, a cooled on-axis field of view planar multi-order diffraction lens is designed. The lens is composed of n*n closely arranged rectangular blocks with the same base area but different heights. According to the cold aperture F number, the lens parameters are first set, and the improvement of the average focusing efficiency of the focal plane across the entire band is used as an indicator to evaluate the lens performance. The optimal solution for the lens structure is obtained through an optimization algorithm.

[0014] In step 3, based on the on-axis point focusing efficiency, a cooled on-axis field of view plane multi-order diffraction lens is designed, with a lens radius (field of view radius) of r1. By increasing the off-axis field of view and expanding the lens aperture, the expanded lens radius is derived from the following calculation formula;

[0015] Assume that the distance between the off-axis focus and the on-axis focus is L1, the length of the cold stop radius is L2, and the length from the cold stop to the image plane is L3. After increasing the field of view, the radius of the lens after considering the field of view is r2, the distance between the lens and the cold stop is L4, and the distance between the structure determined by the off-axis field of view and the structure determined by the on-axis field of view (the distance between the centers of the on-axis field of view and the off-axis field of view) is L5. After increasing the field of view, the radius r2 of the lens is determined by the following formula:

[0016]

[0017] The distance between the centers of the two fields of view is:

[0018]

[0019] The method for calculating the lens radius and the center distance between the fields of view after the field of view is increased: the range of the field of view can be determined by the obtained center distance between the fields of view, the field of view radius, and the lens radius after the field of view is increased. According to the method of step 1, the obtained field of view position and the corresponding microstructure are represented by a matrix. Based on the generalization from one off-axis field of view to multiple off-axis fields of view, there are n fields of view. The distance between the center of the lens structure determined by the maximum field of view and the center of the circle on the axis is not greater than 2r1, then:

[0020]

[0021] In the design of the cooled off-axis field of view planar multi-order diffraction lens in step 3, the lens aperture is expanded by increasing the off-axis field of view, and the lens microstructure is optimized according to the influence of each field of view. For n fields of view, the relationship between the area of ​​each part (from top to bottom) and the weight of the efficiency is:

[0022] η=ω1η1+ω2η2+…+ω n η n

[0023] Among them, η1, η2, ... η n are the focusing efficiencies under different fields of view, ω1, ω2…ω n are the weight coefficients corresponding to each field of view.

[0024] Beneficial effects of the present invention:

[0025] The present invention optimizes the refrigerated planar multi-order diffraction lens with free weight distribution over the entire field of view, and significantly improves the imaging quality within the entire field of view by introducing an optimization design method for the entire band and the entire field of view.

[0026] This method overlaps the aperture stop with the cold stop, thereby achieving 100% cold stop efficiency. The multi-order diffraction lens designed by this method can be applied to cooled infrared detectors.

[0027] The present invention not only significantly simplifies the overall structure of the refrigerated infrared system, but also has important significance for improving the portability of the refrigerated infrared optical system. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 This is a flow chart of the design method of a planar multi-order diffraction lens suitable for a cooled infrared detector according to the present invention.

[0029] Figure 2 is a schematic diagram of the matrix representation of the lens.

[0030] Figure 3 This is the design principle diagram of the cooled planar multi-order diffraction lens.

[0031] Figure 4 It is the lens microstructure affected by the on-axis field of view and off-axis field of view of the multi-order diffraction lens.

[0032] Figure 5 This is the final structure diagram of the planar multi-order diffraction lens.

[0033] Figure 6 The following are PSF diagrams for wavelengths of 3.7μm, 3.97μm, 4.25μm, 4.52μm, and 4.8μm at 0° field of view.

[0034] Figure 7 It is the PSF diagram of the full band 0° field of view.

[0035] Figure 8 This is the PSF diagram for a 4.5° field of view at wavelengths of 3.7μm, 3.97μm, 4.25μm, 4.52μm, and 4.8μm.

[0036] Figure 9 This is the PSF diagram of the full-band 4.5° field of view.

[0037] Figure 10 This is the PSF diagram for a 9° field of view at wavelengths of 3.7μm, 3.97μm, 4.25μm, 4.52μm, and 4.8μm.

[0038] Figure 11 This is the PSF diagram of the full-band 9° field of view. DETAILED DESCRIPTION

[0039] The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0040] like Figure 1 As shown in FIG, a design method for a planar multi-order diffraction lens suitable for a cooled infrared detector is shown, and the steps of the method are as follows.

[0041] The parameters of the planar multi-order diffraction lens are set as follows: F number = 2, EFL = 304 μm, lens diameter, i.e. field radius r1 = 156 μm, microstructure size 7.6 μm*7.6 μm, unit height 0.04 μm, and wavelength range 3.7 μm to 4.8 μm.

[0042] Step 1: Fit the lens surface structure in matrix form. Based on the lens diameter and microstructure size, a 20*20 matrix can be used to represent it.

[0043] According to the formula of the distance between the location of the microstructure and the center of the lens:

[0044]

[0045] Get as Figure 2 Schematic diagram of the lens matrix shown.

[0046] Step 2: Based on the on-axis point focusing efficiency, a cooled, on-axis field-of-view multi-order diffraction lens (microstructure 20*20) was designed. The focal length and aperture of the lens were set based on the F-number of the cold stop. Design was performed only for the on-axis field of view in the infrared band, and the increase in average focusing efficiency across the focal plane of the entire band was used as the metric for lens performance improvement. An optimization algorithm was used to determine the optimal lens structure.

[0047] Step three: Increase the off-axis field of view to expand the lens aperture. According to the method in step one, the lens after the expanded field of view is represented by a matrix, and a cooled off-axis field of view multi-order diffraction lens is designed. By increasing the off-axis field of view by 4.5° and 9°, the lens aperture after expansion is 182.4μm (microstructure 24*24).

[0048] like Figure 3 As shown, the distance between the off-axis focus and the on-axis focus is L1 = 9.728 μm, the length of the cold stop radius is L2 = 30 μm, and the length from the cold stop to the image plane is L3 = 120 μm. After increasing the field of view, the distance between the lens and the cold stop is L5 = 184 μm. According to:

[0049]

[0050] Considering the radius of the lens after the field of view is r2 = 91.2μm, FH is the distance between the structure determined by the off-axis field of view and the structure determined by the on-axis field of view

[0051] like Figure 4 As shown, the lens microstructure affected by the on-axis field of view and the off-axis field of view of the multi-order diffraction lens is represented by a matrix. The distance between the center of the lens structure determined by the maximum field of view and the center of the circle on the axis is no more than 2r1, so:

[0052]

[0053] like Figure 4 As shown, for the three fields of view, the relationship between the efficiency weights is:

[0054] η=ω1η1+ω2η2+ω3η3

[0055] According to the weight relationship, the final structure is obtained through symmetry and rotation, such as Figure 5 As shown. Among them, η 1、 η 2、 η3 is the focusing efficiency in each field of view, and its values ​​are 69.7%, 73.9%, and 69.3%, respectively. ω1, ω 2、 ω3 is the weight coefficient corresponding to each field of view, and its value is 1 / 3. The final average focusing efficiency of the entire band and field of view is 71.0%.

[0056] The PSFs of wavelengths 3.7μm, 3.97μm, 4.25μm, 4.52μm, and 4.8μm at 0° field of view are as follows: Figure 6 As shown, the full-band 0° field of view PSF is as follows Figure 7 As shown in the figure, light of different wavelengths is focused on the same position of the optical axis. Except for obvious side lobes at 4.8μm, light of other wavelengths has a good focusing effect.

[0057] The PSFs of wavelengths 3.7μm, 3.97μm, 4.25μm, 4.52μm, and 4.8μm at a 4.5° field of view are as follows: Figure 8 As shown, the full-band 4.5° field of view PSF is as follows Figure 9 As shown in the figure, light of different wavelengths is focused on the same position of the optical axis. Except for obvious side lobes at 4.8μm, light of other wavelengths has a good focusing effect.

[0058] The PSFs of wavelengths 3.7μm, 3.97μm, 4.25μm, 4.52μm, and 4.8μm at a 9° field of view are as follows: Figure 10 As shown, the full-band 9° field of view PSF is as follows Figure 11 As shown in the figure, light of different wavelengths is focused on the same position of the optical axis. Except for obvious side lobes at 4.52μm and 4.8μm, light of other wavelengths has a good focusing effect.

Claims

1. A design method for a planar multi-order diffraction lens suitable for a cooled infrared detector, characterized in that: The method comprises the following steps: Step 1: Fitting the lens surface structure in matrix form; Step 2: Based on the on-axis point focusing efficiency, a cooled, on-axis, planar multi-order diffraction lens with an on-axis field of view is designed. The lens consists of n*n closely arranged rectangular blocks with the same base area but different heights. The lens parameters are first set based on the cold stop F-number. The improvement in the average focal plane focusing efficiency across the entire wavelength range is used as the indicator for evaluating lens performance. An optimization algorithm is then used to determine the optimal solution for the lens structure. Step three: Increase the off-axis field of view to expand the lens aperture. Based on the full field of view, optimize and design a cooled multi-field-of-view planar multi-order diffraction lens. Assume that the distance between the off-axis focus and the on-axis focus is L1, the length of the cold stop radius is L2, and the length from the cold stop to the image plane is L3. After increasing the field of view, the radius of the lens after considering the field of view is r2, the distance between the lens and the cold stop is L4, and the distance between the structure determined by the off-axis field of view and the structure determined by the on-axis field of view is L5, that is, the center distance between the on-axis field of view and the off-axis field of view. After increasing the field of view, the radius r2 of the lens is determined by the following formula: The distance between the centers of the two fields of view is: The method for calculating the lens radius and the center distance between the fields of view after the field of view is increased: the range of the field of view can be determined by the obtained center distance between the fields of view, the field of view radius, and the lens radius after the field of view is increased. According to the method of step 1, the obtained field of view position and the corresponding microstructure are represented by a matrix. Based on the generalization from one off-axis field of view to multiple off-axis fields of view, there are n fields of view. The distance between the center of the lens structure determined by the maximum field of view and the center of the circle on the axis is not greater than 2r1, then:

2. The method for designing a planar multi-order diffraction lens suitable for a cooled infrared detector according to claim 1, characterized in that: In step 1, a planar multi-order diffraction lens surface structure is fitted in a matrix form. The lens structure is an n*n microstructure. The lens radius corresponding to the field of view is r1. The side length of each microstructure is r1 / n. Assuming that there are n1 transverse microstructures and n2 longitudinal microstructures, the distance between the position of the microstructure and the center of the lens is: If S≤r1, it is within the field of view of the lens; if S>r1, it is outside the field of view.

3. The method for designing a planar multi-order diffraction lens suitable for a cooled infrared detector according to claim 1, wherein: In the third step, the design of the cooled off-axis field of view planar multi-order diffraction lens is performed by increasing the off-axis field of view to expand the lens aperture. The lens microstructure is optimized according to the influence of each field of view. For n fields of view, the relationship between the weight of each area and efficiency is: η=ω1η1+ω2η2+…+ω n or n Among them, η1, η2, ... η n are the focusing efficiencies under different fields of view, ω1, ω2…ω n are the weight coefficients corresponding to each field of view.

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