Planar multi-order diffraction lens design method suitable for refrigeration infrared detector

Abstract

A planar multi-order diffraction lens design method suitable for a refrigeration infrared detector belongs to the technical field of optical design, and aims to solve the problem that an existing multi-order diffraction lens cannot be matched with a refrigeration detector, and the method comprises the following steps: step 1, fitting a lens surface structure in a matrix form; 2, based on the on-axis point focusing efficiency, designing a planar multi-order diffraction lens of a refrigeration type on-axis view field; and step 3, increasing an off-axis view field to enlarge the aperture of the lens, performing optimization based on a full view field, and designing a refrigeration type multi-view-field planar multi-order diffraction lens. According to the method, aiming at the full-view-field free distribution weight optimization refrigeration type plane multi-order diffraction lens, by introducing the full-wave-band full-view-field optimization design method, the imaging quality in the full-view-field range is remarkably improved. The multi-order diffraction lens designed through the method can be suitable for a refrigeration type infrared detector. The integral structure of the refrigeration infrared system is obviously simplified, and the refrigeration infrared optical system has important significance in improving the portability.

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 have the advantages of reducing stray light, improving signal-to-noise ratio, increasing contrast, and improving the detection and recognition capabilities of light detection systems. However, the design of cooled infrared optical systems is relatively complex and requires more optical components, resulting in a larger size and heavier weight, which may become a limiting factor in some applications that require lightweight and compact design, and the influence of cold reflection needs to be considered. The existing multi-order diffraction lens can achieve the purpose of achromatism with only one lens, avoiding the combination of multi-faceted lenses. Therefore, this diffraction lens is lighter and smaller in size. Combined with a cooled infrared detector, it can realize the miniaturization and integration of infrared optical systems.

[0003] In the Chinese patent publication number "CN113866979A" entitled "Achromatic method of multi-order diffraction lens and achromatic multi-order diffraction lens", the method combines the global optimization algorithm to optimize the focusing efficiency of the focal plane of the on-axis field of view to obtain a multi-order diffraction lens with an achromatic range of 400nm-1100nm, and derives that the maximum depth-to-width ratio of the unit structure of the grayscale order distribution of each ring height does not exceed 2:1. The above method is only designed for on-axis field of view focusing, and cannot achieve high-quality imaging within the entire field of view. At the same time, the aperture overlaps with the element, and 100% cold aperture efficiency cannot be achieved, so the above method cannot be applied to cooled infrared detectors. Summary of the invention

[0004] In order to solve the problem that the existing multi-order diffraction lens cannot be adapted to the refrigerated detector, the present invention proposes a design method for a planar multi-order diffraction lens suitable for a refrigerated infrared detector. The 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 of the full 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 planar multi-order diffraction lens with a cooled on-axis field of view 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] The step 1 fits the plane multi-order diffraction through-surface structure in 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 step 2, based on the on-axis point focusing efficiency, a planar multi-order diffraction lens with a refrigerated on-axis field of view is designed. The lens is composed of n*n closely arranged rectangular blocks with the same bottom 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 in the entire band is used as an indicator for evaluating the lens performance. The optimal solution of the lens structure is obtained through an optimization algorithm.

[0014] In the step 3, based on the on-axis point focusing efficiency, a refrigerated on-axis field of view plane multi-order diffraction lens is designed, the lens radius (field of view radius) is r1, and the lens aperture is enlarged by increasing the off-axis field of view. The enlarged lens radius is derived by 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 center distance between 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 after increasing the field of view and the center distance between the fields of view: 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 increasing the field of view. According to the method of step 1, the obtained field of view position and the corresponding microstructure are represented by a matrix. Based on one off-axis field of view, it is extended 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 cooling type off-axis field of view plane multi-order diffraction lens in step 3, the lens aperture is enlarged by increasing the off-axis field of view, and the lens microstructure affected by each field of view is optimized. 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 in each field of view, ω1, ω2, ... 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 in the whole field of view, and significantly improves the imaging quality in the whole field of view by introducing the optimization design method of the whole band and the whole field of view.

[0026] The 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 suitable for cooling 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 The present invention is a flow chart of a design method for a planar multi-order diffraction lens suitable for a cooled infrared detector.

[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 that affects the on-axis field of view and off-axis field of view of a multi-order diffraction lens.

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

[0033] Figure 6 It is the PSF diagram of 0° field of view with wavelengths of 3.7μm, 3.97μm, 4.25μm, 4.52μm, and 4.8μm.

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

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

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

[0037] Fig.10 It is the PSF diagram of 9° field of view with wavelengths of 3.7μm, 3.97μm, 4.25μm, 4.52μm, and 4.8μm.

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

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

[0040] like Figure 1 As shown, a design method for a planar multi-order diffraction lens suitable for a cooled infrared detector is provided, 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 band 3.7 μm to 4.8 μm.

[0042] Step 1: Fit the lens surface structure in matrix form. According to the diameter of the lens and the size of the microstructure, it can be represented by a 20*20 matrix.

[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 representation shown.

[0046] Step 2: Based on the on-axis point focusing efficiency, design a cooled on-axis field of view multi-order diffraction lens (microstructure 20*20). Set the focal length and aperture of the lens according to the F number of the cold aperture. Design only the on-axis field of view in the infrared band, and use the increase in the average focusing efficiency of the focal plane in the full band as a measure of lens performance improvement. The optimal solution for the lens structure is obtained through the optimization algorithm.

[0047] Step three, increase the off-axis field of view to expand the lens aperture. According to the method of step one, the lens after the field of view is expanded 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, according to the matrix representation of the lens microstructure affected by the on-axis field of view and the off-axis field of view of the multi-order diffraction lens, 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, then:

[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. 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. Finally, the average focusing efficiency of the whole band and the whole field of view is 71.0%.

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

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

[0058] The PSF of wavelengths of 3.7μm, 3.97μm, 4.25μm, 4.52μm, and 4.8μm in a 9° field of view is as follows: Fig.10 As shown, the full-band 9° field of view PSF is as follows Fig.11 As shown, light of different wavelengths is focused at the same position of the optical axis. Except for obvious side lobes at 4.52μm and 4.8μm, the other wavelengths have 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 planar multi-order diffraction lens with a cooled on-axis field of view is designed; 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.

2. The method for designing a planar multi-order diffraction lens suitable for a cooled infrared detector according to claim 1, characterized in that: The step 1 fits the plane multi-order diffraction through-surface structure in 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, characterized in that: In the step 2, based on the on-axis point focusing efficiency, a planar multi-order diffraction lens with a refrigerated on-axis field of view is designed. The lens is composed of n*n closely arranged rectangular blocks with the same bottom 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 in the entire band is used as an indicator for evaluating the lens performance. The optimal solution of the lens structure is obtained through an optimization algorithm.

4. The method for designing a planar multi-order diffraction lens suitable for a cooled infrared detector according to claim 1, characterized in that: In the step 3, based on the on-axis point focusing efficiency, a refrigerated on-axis field of view plane multi-order diffraction lens is designed, the lens radius (field of view radius) is r1, and the lens aperture is enlarged by increasing the off-axis field of view. The enlarged lens radius is derived by the following calculation formula; 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 center distance between 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: The distance between the centers of the two fields of view is:

5. The method for designing a planar multi-order diffraction lens suitable for a cooled infrared detector according to claim 4, characterized in that: The method for calculating the lens radius after increasing the field of view and the center distance between the fields of view: 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 increasing the field of view. According to the method of step 1, the obtained field of view position and the corresponding microstructure are represented by a matrix. Based on one off-axis field of view, it is extended 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:

6. The method for designing a planar multi-order diffraction lens suitable for a cooled infrared detector according to claim 1, characterized in that: In the design of the cooling type off-axis field of view plane multi-order diffraction lens in step 3, the lens aperture is enlarged by increasing the off-axis field of view, and the lens microstructure affected by each field of view is optimized. For n fields of view, the relationship between the area of ​​each part (from top to bottom) and the weight of the efficiency is: η=ω1η1+ω2η2++ω n or n Among them, η1, η2, ... η n are the focusing efficiencies in each field of view, ω1, ω2, ... n are the weight coefficients corresponding to each field of view.

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