Design method of diffractive optical element for large diffraction angle

Abstract

The invention provides a design method for a diffraction optical element with a large diffraction angle, and the method comprises the steps: firstly, calculating a transmission complex amplitude needed for achieving a diffraction pattern in a far field according to a target diffraction pattern and related parameters; and gradually reducing the diffraction propagation distance through iteration by taking the transmission complex amplitude of the element as an initial parameter until the transmission complex amplitude of the element meeting the requirement is obtained and is output as a result. The method is suitable for the design of a diffractive optical element working under the conditions of a large diffraction angle and a limited propagation distance; compared with an existing design method for a small diffraction angle, the design method has the advantage that the designable diffraction angle range is obviously increased. Meanwhile, the calculation complexity is effectively optimized, and the method is suitable for design of large-size diffractive optical elements in finite time. Besides, correction processing on the transmission complex amplitude of the element is added in the design process, so that the working robustness of the designed element can be effectively improved, and the designed element can stably work under the condition that a certain machining error exists.

Description

Technical Field

[0001] The present invention belongs to the technical field of optical elements, and in particular relates to a design method of a diffraction optical element for a large diffraction angle. Background Art

[0002] Diffractive optical elements are designed to produce a specific diffraction pattern distribution over a specific propagation distance, and their propagation process follows the diffraction propagation theory of light. Diffractive optical elements have various structural types, which can be divided into two-dimensional devices (elements) and three-dimensional devices. Two-dimensional devices include Fresnel diffraction devices, which consist of wavelength-scale multi-step steps, and metasurface devices, which consist of subwavelength-scale nanopillars. Three-dimensional devices include volume holographic gratings, whose refractive index or geometric shape varies according to spatial position. The design of each of these devices requires determining the shape distribution of the device based on the phase information required to form the diffraction pattern.

[0003] At present, the methods used in related industries to quickly calculate the phase information of diffraction elements are mostly aimed at small diffraction angles; and the few calculation methods for large diffraction angles mostly rely on vector theory, which requires a lot of computing time and computing resources and is less efficient. Summary of the Invention

[0004] To address the challenges of the prior art, the present invention provides a design method for diffractive optical elements with large diffraction angles. This design method incorporates innovative computational methods, enabling rapid computation of the phase information required for large-scale, high-diffraction-angle diffraction elements. Furthermore, the design method takes tolerance compatibility into account, enabling the element to accommodate a certain degree of manufacturing error, resulting in increased applicability.

[0005] A method for designing a diffractive optical element for a large diffraction angle comprises the following steps:

[0006] (1) Input target diffraction pattern , pattern size , diffraction pattern sampling number , component aperture size , Component aperture sampling number , diffraction propagation distance and operating wavelength ;in, 、 are the coordinate parameters of the diffraction plane respectively;

[0007] (2) Based on the input parameters, calculate the element transmission complex amplitude required to achieve the target diffraction pattern when the diffraction propagation distance is very long;

[0008] (3) Using the result in step (2) as the initial parameter, the diffraction propagation distance is gradually reduced in an iterative manner until the required element transmission complex amplitude is obtained and output as the design result.

[0009] As a preference, the operation process of step (2) is specifically as follows:

[0010] 2.1 Setting the Far Field Propagation Distance ; Initial far-field diffraction complex amplitude ;

[0011] 2.2 Iterate the element transmission complex amplitude and far-field diffraction complex amplitude:

[0012] According to the far-field diffraction complex amplitude corresponding to the current iteration number and the parameters in step (1), the element transmission complex amplitude is calculated and normalized;

[0013] 2.3 Use the normalized component transmission complex amplitude to calculate the current far-field diffraction complex amplitude and determine whether the deviation between it and the design target is less than the set threshold:

[0014] If so, save the normalized component transmission complex amplitude and go to step (3);

[0015] If not, the current far-field diffraction complex amplitude is updated and used as the far-field diffraction complex amplitude corresponding to the next iteration number, and the process returns to step 2.2.

[0016] As a further preferred embodiment, in step 2.2, the element transmission complex amplitude is calculated by the following formula:

[0017]

[0018] in, is the complex amplitude of the element transmission at the i-th iteration; is the far-field diffraction complex amplitude corresponding to the i-th iteration; 、 are the coordinate parameters of the plane where the component is located; is an imaginary unit;

[0019] The element transmission complex amplitude is normalized by the following formula:

[0020]

[0021] in, is the normalized complex amplitude of the element transmission; for The modulus value of .

[0022] As a further preferred embodiment, in step 2.3, the calculation formula for the current far-field diffraction complex amplitude is:

[0023]

[0024] in, is the current far-field diffraction complex amplitude; is the normalized complex amplitude of the element transmission; 、 are the coordinate parameters of the plane where the component is located; is the number of iterations; is an imaginary unit;

[0025] The deviation between the current far-field diffraction complex amplitude and the design target is calculated by the following formula:

[0026]

[0027] in, is the deviation between the current far-field diffraction complex amplitude and the design target; for The modulus value of

[0028] The update formula of the current far-field diffraction complex amplitude is:

[0029]

[0030] in, is the far-field diffraction complex amplitude corresponding to the next iteration; It is a preset parameter with a value range of 0.01-0.1.

[0031] Preferably, the operation process of step (3) is as follows:

[0032] 3.1 Setting the initial diffraction propagation distance ,in, is the distance reduction rate;

[0033] 3.2 Calculate the diffraction transfer function;

[0034] 3.3 Iterate the diffraction complex amplitude and diffraction propagation distance:

[0035] For the first iteration, the result in step (2) is used as the normalized element transmission complex amplitude in step 3.4 and the subsequent correction process is performed;

[0036] For non-first iterations, based on the diffraction complex amplitude and diffraction propagation distance corresponding to the current iteration number and the parameters in step (1), the diffraction surface transformation spectrum, the transformation spectrum of the element transmission complex amplitude, and the element transmission complex amplitude are calculated in sequence, and the element transmission complex amplitude is normalized;

[0037] 3.4 Correct the normalized complex amplitude of the element transmission;

[0038] 3.5 Using the corrected element transmission complex amplitude, update the transformation spectrum of the element transmission complex amplitude and update the diffraction surface transformation spectrum;

[0039] 3.6 Calculate the updated diffraction complex amplitude based on the updated diffraction surface transformation spectrum and determine whether the deviation between it and the design target is less than the set threshold:

[0040] If yes, save and output the corrected component transmission complex amplitude, and end the design;

[0041] If not, the diffraction complex amplitude and the diffraction propagation distance are further updated as the diffraction complex amplitude and the diffraction propagation distance corresponding to the next iteration number, and the process returns to step 3.2.

[0042] As a further preferred embodiment, in step 3.2, the calculation formula of the diffraction transfer function is:

[0043]

[0044]

[0045] in, is the diffraction propagation distance corresponding to the kth iteration, where k represents the number of iterations; is the diffraction transfer function from the diffraction plane to the element plane; is the diffraction transfer function from the element plane to the diffraction plane; 、 are the independent variable coordinates of the diffraction transfer function; is an imaginary unit;

[0046] As a further preferred embodiment, in step 3.3, the diffraction surface transformation spectrum is calculated by the following formula:

[0047]

[0048] in, is the diffraction surface transformation spectrum; is the diffraction complex amplitude corresponding to the kth iteration.

[0049] As a further preferred embodiment, in step 3.3, the transformation spectrum of the element transmission complex amplitude is calculated from the diffraction surface transformation spectrum and the diffraction transfer function by the following formula:

[0050]

[0051] in, is the transformation spectrum of the complex amplitude of the element transmission; is the diffraction surface transformation spectrum; k represents the number of iterations; is the diffraction transfer function from the diffraction plane to the element plane; 、 are the independent variable coordinates of the diffraction transfer function;

[0052] The calculation formula of the element transmission complex amplitude is:

[0053]

[0054] The element transmission complex amplitude is normalized by the following formula:

[0055]

[0056] in, is the complex amplitude of the element transmission; for The modulus value of is the normalized complex amplitude of the element transmission; 、 They are the coordinate parameters of the plane where the component is located.

[0057] As a further preferred embodiment, in step 3.4, the normalized element transmission complex amplitude is corrected as follows:

[0058]

[0059]

[0060] in, is the normalized complex amplitude of the element transmission; is the corrected element transmission complex amplitude; 、 are the coordinate parameters of the plane where the component is located; is the diffraction propagation distance of the kth iteration; is an imaginary unit; is the real part operation;

[0061] For the first iteration, the result saved in step (2) is used as .

[0062] As a further preferred embodiment, in step 3.4, the transformation spectrum of the element transmission complex amplitude is updated using the following formula:

[0063]

[0064] in, is the transformed spectrum of the complex amplitude of the element transmission after the update; is the corrected element transmission complex amplitude; 、 are the independent variable coordinates of the diffraction transfer function; 、 are the coordinate parameters of the plane where the component is located; is the diffraction propagation distance of the kth iteration; is an imaginary unit;

[0065] The calculation formula for updating the diffraction surface transformation spectrum is:

[0066]

[0067] in, is the updated diffraction surface transformation spectrum; is the diffraction transfer function from the element plane to the diffraction plane.

[0068] As a further preferred embodiment, in step 3.6, the calculation formula for updating the diffraction complex amplitude is:

[0069]

[0070] in, is the updated diffraction complex amplitude; is the updated diffraction surface transformation spectrum; 、 are the independent variable coordinates of the diffraction transfer function; is the diffraction propagation distance of the kth iteration.

[0071] As a further preferred embodiment, in step 3.6, the deviation between the updated diffraction complex amplitude and the design target is calculated by the following formula:

[0072]

[0073] in, is the deviation between the updated diffraction complex amplitude and the design target; is the updated diffraction complex amplitude; for The modulus value; k is the number of iterations;

[0074] The further update formula of the updated diffraction complex amplitude is:

[0075]

[0076] in, is the diffraction complex amplitude corresponding to the next iteration; It is a preset parameter with a value range of 0.01-0.1;

[0077] The updated formula for the diffraction propagation distance is:

[0078]

[0079] in, is the diffraction propagation distance of the k+1th iteration; is the distance reduction rate, and its value range is 0.01-0.1.

[0080] The present method for designing a diffractive optical element for large diffraction angles allows, after determining the expected diffraction pattern, to design the phase distribution of the diffractive optical element so that the diffraction pattern matches the design requirements. Once the transmission complex amplitude (phase distribution) of the element meets the requirements, the type of diffractive optical element can be selected based on the requirements. Electromagnetic field numerical simulation methods (such as the finite-difference time-domain method, the Fourier modal method, and the frequency-domain difference method) are then used to obtain a numerical solution from the phase distribution to the nanopillar size, thereby determining the specific structure of the diffractive optical element.

[0081] Compared with the prior art, the present invention has the following beneficial effects:

[0082] The design method of a diffraction optical element for a large diffraction angle of the present invention first calculates the element transmission complex amplitude required to realize the diffraction pattern in the far field based on the target diffraction pattern and related parameters; then, using the element transmission complex amplitude as the initial parameter, the diffraction propagation distance is gradually reduced through iteration until the element transmission complex amplitude that meets the requirements is obtained and output as a result. The design method of the present invention is suitable for the design of diffraction optical elements working under conditions of large diffraction angles and limited propagation distances; compared with existing design methods for small diffraction angles, the designable diffraction angle range is significantly increased. At the same time, its computational complexity is effectively optimized, and it is suitable for the design of large-scale diffraction optical elements within a limited time. In addition, the method incorporates a correction process for the element transmission complex amplitude into the design process, which can effectively improve the working robustness of the designed element and enable the designed element to operate stably in the presence of certain processing errors. BRIEF DESCRIPTION OF THE DRAWINGS

[0083] Figure 1 is a design flow chart of an embodiment of the present invention;

[0084] Figure 2 A schematic diagram of symbol definitions in an embodiment of the present invention;

[0085] Figure 3 This is the target diffraction pattern in Application Example 1;

[0086] Figure 4 A schematic structural diagram of the diffractive optical element type selected in Application Example 1;

[0087] Figure 5 is the diffraction pattern of the optical element designed in Application Example 1;

[0088] Figure 6 This is the target diffraction pattern in Application Example 2;

[0089] Figure 7 A schematic structural diagram of the diffractive optical element type selected in Application Example 2;

[0090] Figure 8 This is the diffraction pattern of the optical element designed in Application Example 2. DETAILED DESCRIPTION

[0091] like Figure 1 As shown, a method for designing a diffractive optical element for a large diffraction angle includes the following steps:

[0092] 1. Input basic design parameters

[0093] Input diffraction pattern , diffraction pattern size , diffraction pattern sampling number , component aperture size , Component aperture sampling number , diffraction propagation distance and operating wavelength ;in, 、 are the coordinate parameters of the diffraction plane respectively.

[0094] 2. Calculate the initial phase distribution of the element (element transmission complex amplitude)

[0095] When calculating the initial phase of the element, we first assume that the diffraction pattern is in the far field. Under this condition, we find the phase distribution that the element should satisfy. Based on this, we proceed to the subsequent step 3. Specifically:

[0096] 2.1 Setting the Far Field Distance

[0097] Far field distance Should meet ,and ; Usually you can take .

[0098] 2.2 Setting the initial far-field complex amplitude

[0099] The diffraction pattern As the initial state, assign the complex amplitude field of far-field diffraction as the parameter of the first step iteration, that is, let:

[0100]

[0101] in, is the complex amplitude of the diffraction field in the first iteration, denoted as the initial far-field diffraction complex amplitude.

[0102] 2.3 Iteratively update the element transmission complex amplitude and far-field diffraction complex amplitude:

[0103] According to the far-field diffraction complex amplitude and related parameters corresponding to the current iteration number i, the component transmission complex amplitude is calculated:

[0104]

[0105] in, is the far-field diffraction complex amplitude corresponding to the i-th iteration of the diffraction field, is the element transmission complex amplitude of the diffraction element at the i-th iteration; 、 are the coordinate parameters of the plane where the component is located; Is an imaginary unit.

[0106] The definition diagram of the symbols in this article is as follows Figure 2 shown.

[0107] 2.4 Normalize the complex amplitude of the element transmission:

[0108]

[0109] in, is the normalized complex amplitude of the element transmission; is the modulus of the complex amplitude of the element transmission.

[0110] Considering that the transmittance of the diffraction element is approximately equal everywhere, it is necessary to normalize the complex amplitude of the element transmission.

[0111] 2.5 Calculate the current far-field diffraction complex amplitude using the normalized component transmission complex amplitude:

[0112]

[0113] in, is the current far-field diffraction complex amplitude.

[0114] 2.6 Calculate the current far-field diffraction complex amplitude Deviation from design target:

[0115]

[0116] in, is the deviation between the current far-field diffraction complex amplitude and the design target; for The modulus value of

[0117] like , then go to step 2.7;

[0118] like , then save , and go to step 3;

[0119] in, is the preset threshold (set threshold) used to determine the termination of the iteration in step 2.

[0120] 2.7 Update far-field diffraction complex amplitude

[0121] If this step is reached, it means that the far-field diffraction pattern achieved by the current design element is still significantly different from the expected one, and further iteration and updating are required.

[0122] The far-field diffraction complex amplitude should be updated iteratively as follows:

[0123]

[0124] in, is the updated far-field diffraction complex amplitude; It is a preset parameter with a value range of 0.01-0.1.

[0125] Will get Take the far-field diffraction complex amplitude corresponding to the i+1th iteration and return to step 2.3.

[0126] 3. Calculate the element phase (element transmission complex amplitude) under near-field and large diffraction angle conditions

[0127] When the design process reaches Step 3, it indicates that Step 2.6 has yielded reliable far-field design results (element transmission complex amplitude). The results of Step 2.6 serve as high-quality initial conditions, accelerating convergence in Step 3 and avoiding local minima. Step 3 iteratively compresses the diffraction propagation distance, achieving the transition from far-field to near-field conditions.

[0128] 3.1 Setting the initial diffraction propagation distance ,in, is the distance reduction rate.

[0129] 3.2 Calculate the diffraction transfer function based on the diffraction propagation distance corresponding to the current iteration number k:

[0130]

[0131]

[0132] in, is the diffraction propagation distance of the kth iteration; is the diffraction transfer function from the diffraction plane to the element plane under the current propagation distance; is the diffraction transfer function from the element plane to the diffraction plane under the current propagation distance conditions.

[0133] After completion, if the current iteration is the first iteration, since the data inherited from 2.6 has been obtained , so directly replace the step 2.6 Go to step 3.6 as the normalized element transmission complex amplitude; in other cases (the current iteration is not the first iteration), go to step 3.3.

[0134] 3.3 Calculate the diffraction surface transformation spectrum based on the diffraction complex amplitude and diffraction propagation distance corresponding to the current iteration number k and the parameters in step (1):

[0135]

[0136] in, is the diffraction complex amplitude corresponding to the kth iteration; After completing the diffraction surface transformation spectrum, proceed to step 3.4.

[0137] 3.4 Calculate the transformation spectrum of the complex amplitude of the element transmission:

[0138]

[0139] According to diffraction theory, the transformation spectrum of the complex amplitude of the element transmission can be calculated from the above formula: , and proceed to step 3.5 after completion.

[0140] 3.5 Calculate and normalize the component transmission complex amplitude:

[0141]

[0142]

[0143] in, is the complex amplitude of the element transmission; for The modulus value of is the normalized component transmission complex amplitude. After completion, proceed to step 3.6.

[0144] 3.6 Corrected and normalized component transmission complex amplitude:

[0145]

[0146]

[0147] For the first iteration is the result saved in step 2.3, that is , k=1;

[0148] For non-first iterations, is the normalized complex amplitude of the element transmission.

[0149] Consider the potential presence of noise in the component's transmission phase, which manifests as dramatic phase variations within a neighborhood. Such drastic phase variations significantly increase processing difficulty. By processing the component's transmission complex amplitude according to the two equations above, the component's transmission phase can be made relatively smooth, thus reducing processing complexity. Once completed, proceed to step 3.7.

[0150] 3.7 Update the transformation spectrum of the complex amplitude of the element transmission:

[0151]

[0152] in, is the transformed spectrum of the complex amplitude of the element transmission after the update; Transmitted complex amplitude for the corrected component; upon completion, proceed to step 3.8.

[0153] 3.8 Calculate the transformation spectrum of the diffraction surface under diffraction conditions (update the diffraction surface transformation spectrum):

[0154]

[0155] in, Transform the spectrum into the updated diffraction surface; when completed, proceed to step 3.9.

[0156] 3.9 Calculate the updated diffraction complex amplitude based on the updated diffraction surface transformation spectrum:

[0157]

[0158] in, is the updated diffraction complex amplitude; after completion, proceed to step 3.10.

[0159] 3.10 Calculate the updated diffraction complex amplitude Deviation from design target:

[0160]

[0161] in, is the deviation between the updated diffraction complex amplitude and the design target; for The modulus value of .

[0162] like , then go to step 3.11;

[0163] like , then save And end the design process.

[0164] in, To set the threshold.

[0165] 3.11 Further update of the diffraction complex amplitude:

[0166]

[0167] in, is the diffraction complex amplitude corresponding to the next iteration; This is a preset parameter with a value range of 0.01-0.1. After completing the current calculation, proceed to step 3.12.

[0168] 3.12 Update diffraction propagation distance:

[0169]

[0170] in, is the diffraction propagation distance for the k+1th iteration; the above result is the diffraction propagation distance corresponding to the i+1th iteration. After completion, return to step 3.2 and perform the i+1th iteration.

[0171] Application Example 1

[0172] Using the above design method to Figure 3 The diffraction image in is used as the target to obtain the phase information (complex amplitude of element transmission) that meets the conditions. During the design process, the operating parameters are set. Based on the phase information obtained, this application example uses Figure 4 In the super surface optical element, the morphological distribution of the diffractive optical element is determined by electromagnetic field numerical simulation to obtain a specific optical element. The actual diffraction pattern of the diffractive optical element is as follows Figure 5 As shown. Figure 5 With the goal Figure 3 The graphics in are basically the same.

[0173] Metasurface optical elements are characterized by the distribution of nanopillars of different sizes and shapes on a two-dimensional plane. The spacing between each pillar is usually smaller than the design wavelength. The phase response of this element is achieved by designing nanopillars of different sizes.

[0174] Application Example 2

[0175] Using the above design method to Figure 6 The diffraction image in is used as the target to obtain the phase information (complex amplitude of element transmission) that meets the conditions. During the design process, the operating parameters are set. Based on the phase information obtained, this application example uses Figure 7 In the Fresnel optical element, the morphological distribution of the diffractive optical element is determined by electromagnetic field numerical simulation to obtain a specific optical element. The actual diffraction pattern of the diffractive optical element is as follows: Figure 8 As shown. Figure 8 With the goal Figure 6 The graphics in are basically the same.

[0176] The Fresnel optical element chosen for this application example features a transparent medium with varying thicknesses distributed across a two-dimensional plane. Typically, the thickness of the medium is discrete, with common thickness variations occurring in 2nd, 4th, and 8th order. The phase response of this element is achieved by designing the thickness at different locations.

[0177] Variant 2.1 When the flat substrate and the step are both transparent media and the component is placed in air or vacuum environment, the step height is It can be calculated according to the formula:

[0178]

[0179] Variant 2.2 When the flat substrate is metal, the step is a transparent medium, and the component is placed in air or vacuum environment, the step height is It can be calculated according to the formula:

[0180]

[0181] Variant 2.3 When both the planar substrate and the step are made of metal and the component is placed in air or vacuum, the step height is It can be calculated according to the formula:

[0182]

[0183] in, is the phase calculated according to the process, n is the refractive index of the transparent medium, is the wavelength in vacuum.

Claims

1. A method for designing a diffractive optical element for a large diffraction angle, characterized in that: The following steps are involved: (1) Input target diffraction pattern , pattern size , diffraction pattern sampling number , component aperture size , Component aperture sampling number , diffraction propagation distance and operating wavelength ;in, 、 are the coordinate parameters of the diffraction plane respectively; (2) Based on the input parameters, calculate the element transmission complex amplitude required to achieve the target diffraction pattern when the diffraction propagation distance is very long; (3) Using the result in step (2) as the initial parameter, the diffraction propagation distance is gradually reduced in an iterative manner until the required element transmission complex amplitude is obtained and output as the design result.

2. The method for designing a diffractive optical element for a large diffraction angle according to claim 1, wherein: The specific operation process of step (2) is as follows: (2.1) Setting the far-field propagation distance ; Initial far-field diffraction complex amplitude ; (2.2) Iterate the element transmission complex amplitude and far-field diffraction complex amplitude: According to the far-field diffraction complex amplitude corresponding to the current iteration number and the parameters in step (1), the element transmission complex amplitude is calculated and normalized; (2.3) Use the normalized component transmission complex amplitude to calculate the current far-field diffraction complex amplitude and determine whether the deviation between it and the design target is less than the set threshold: If so, save the normalized component transmission complex amplitude and go to step (3); If not, update the current far-field diffraction complex amplitude and use it as the far-field diffraction complex amplitude corresponding to the next iteration number, and return to step (2.2).

3. The method for designing a diffractive optical element for a large diffraction angle according to claim 2, wherein: In step (2.2), the element transmission complex amplitude is calculated by the following formula: ； in, is the complex amplitude of the element transmission at the i-th iteration; is the far-field diffraction complex amplitude corresponding to the i-th iteration; 、 are the coordinate parameters of the plane where the component is located; is an imaginary unit; The element transmission complex amplitude is normalized by the following formula: ； in, is the normalized complex amplitude of the element transmission; for The modulus value of .

4. The method for designing a diffractive optical element for a large diffraction angle according to claim 2, wherein: In step (2.3), the calculation formula for the current far-field diffraction complex amplitude is: ； in, is the current far-field diffraction complex amplitude; is the normalized complex amplitude of the element transmission; 、 are the coordinate parameters of the plane where the component is located; is the number of iterations; is an imaginary unit; The deviation between the current far-field diffraction complex amplitude and the design target is calculated by the following formula: ； in, is the deviation between the current far-field diffraction complex amplitude and the design target; for The modulus value of The update formula of the current far-field diffraction complex amplitude is: ； in, is the far-field diffraction complex amplitude corresponding to the next iteration; It is a preset parameter with a value range of 0.01-0.

1.

5. The method for designing a diffractive optical element for a large diffraction angle according to claim 1, wherein: The specific operation process of step (3) is as follows: (3.1) Set the initial diffraction propagation distance ,in, is the distance reduction rate; (3.2) Calculate the diffraction transfer function; (3.3) Iterate the diffraction complex amplitude and diffraction propagation distance: For the first iteration, the result in step (2) is used as the normalized complex amplitude of the element transmission in step (3.4) and the subsequent correction process is performed; For non-first iterations, based on the diffraction complex amplitude and diffraction propagation distance corresponding to the current iteration number and the parameters in step (1), the diffraction surface transformation spectrum, the transformation spectrum of the element transmission complex amplitude, and the element transmission complex amplitude are calculated in sequence, and the element transmission complex amplitude is normalized; (3.4) Correct the normalized complex amplitude of the element transmission; (3.5) Using the corrected element transmission complex amplitude, update the transformation spectrum of the element transmission complex amplitude and update the diffraction surface transformation spectrum; (3.6) Calculate the updated diffraction complex amplitude based on the updated diffraction surface transformation spectrum and determine whether the deviation between it and the design target is less than the set threshold: If yes, save and output the corrected component transmission complex amplitude, and end the design; If not, the diffraction complex amplitude and the diffraction propagation distance are further updated as the diffraction complex amplitude and the diffraction propagation distance corresponding to the next iteration number, and the process returns to step (3.2).

6. The method for designing a diffractive optical element for a large diffraction angle according to claim 5, wherein: In step (3.2), the calculation formula of the diffraction transfer function is: ； ； in, is the diffraction propagation distance corresponding to the kth iteration, where k represents the number of iterations; is the diffraction transfer function from the diffraction plane to the element plane; is the diffraction transfer function from the element plane to the diffraction plane; 、 are the independent variable coordinates of the diffraction transfer function; Is an imaginary unit.

7. The method for designing a diffractive optical element for a large diffraction angle according to claim 5, wherein: The diffraction surface transformation spectrum described in step (3.3) is calculated by the following formula: ； in, is the diffraction surface transformation spectrum; is the diffraction complex amplitude corresponding to the kth iteration; is the diffraction propagation distance of the kth iteration; is an imaginary unit; The transformation spectrum of the complex amplitude of the element transmission is calculated from the diffraction surface transformation spectrum and the diffraction transfer function using the following formula: ； in, is the transformation spectrum of the complex amplitude of the element transmission; is the diffraction surface transformation spectrum; k represents the number of iterations; is the diffraction transfer function from the diffraction plane to the element plane; 、 are the independent variable coordinates of the diffraction transfer function; The calculation formula of the element transmission complex amplitude is: ； The element transmission complex amplitude is normalized by the following formula: ； in, is the complex amplitude of the element transmission; for The modulus value of is the normalized complex amplitude of the element transmission; 、 They are the coordinate parameters of the plane where the component is located.

8. The method for designing a diffractive optical element for a large diffraction angle according to claim 5, wherein: In step (3.4), the normalized element transmission complex amplitude is corrected as follows: ； ； in, is the normalized complex amplitude of the element transmission; is the corrected element transmission complex amplitude; 、 are the coordinate parameters of the plane where the component is located; is the diffraction propagation distance of the kth iteration; is an imaginary unit; is the real part operation; For the first iteration, the result saved in step (2) is used as .

9. The method for designing a diffractive optical element for a large diffraction angle according to claim 5, wherein: In step (3.5), the transformation spectrum of the element transmission complex amplitude is updated using the following formula: ； in, is the transformed spectrum of the complex amplitude of the element transmission after the update; is the corrected element transmission complex amplitude; 、 are the independent variable coordinates of the diffraction transfer function; 、 are the coordinate parameters of the plane where the component is located; is the diffraction propagation distance of the kth iteration; is an imaginary unit; The calculation formula for updating the diffraction surface transformation spectrum is: ； in, is the updated diffraction surface transformation spectrum; is the diffraction transfer function from the element plane to the diffraction plane.

10. The method for designing a diffractive optical element for a large diffraction angle according to claim 5, wherein: In step (3.6), the calculation formula for updating the diffraction complex amplitude is: ； in, is the updated diffraction complex amplitude; is the updated diffraction surface transformation spectrum; 、 are the independent variable coordinates of the diffraction transfer function; is the diffraction propagation distance of the kth iteration; The deviation between the updated diffraction complex amplitude and the design target is calculated by the following formula: ； in, is the deviation between the updated diffraction complex amplitude and the design target; is the updated diffraction complex amplitude; for The modulus value; k is the number of iterations; The further update formula of the updated diffraction complex amplitude is: ； in, is the diffraction complex amplitude corresponding to the next iteration; It is a preset parameter with a value range of 0.01-0.1; The updated formula for the diffraction propagation distance is: ； in, is the diffraction propagation distance of the k+1th iteration; is the distance reduction rate, and its value range is 0.01-0.1.

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