阅读说明：本技术 多阶衍射透镜的消色差方法及消色差多阶衍射透镜 (Achromatic method for multi-order diffraction lens and achromatic multi-order diffraction lens ) 是由 李涛 肖行健 祝世宁 于 2021-09-07 设计创作，主要内容包括：本发明公开了一种多阶衍射透镜的消色差方法及消色差多阶衍射透镜,首先根据初始多阶衍射透镜的环数和灰度阶数,设计光经过多阶衍射透镜后在焦点的光场强度的约束优化问题,使每一个波长处的聚焦光场一致,结合全局优化算法和搜索算法计算所述的约束优化问题的准全局最优解,然后进行平滑处理和梯度下降处理,得到实现消色差的多阶衍射透镜；本发明的消色差多阶衍射透镜每一环高度的灰度阶数分布的单元结构深宽比最大值不超过2:1。本发明实现了大尺寸宽光谱消色差平面透镜参数设计,将传统的消色差系统压缩为一个平面透镜,极大地降低了消色差成像系统的体积和成本,减小加工难度,提升了透镜性能,实现了宽光谱范围内的高质量消色差成像。(The invention discloses an achromatic method of a multi-order diffraction lens and the achromatic multi-order diffraction lens, firstly, according to the ring number and the gray level of the initial multi-order diffraction lens, designing the constraint optimization problem of the light field intensity at the focus after passing through the multi-order diffraction lens, enabling the focused light field at each wavelength to be consistent, calculating the quasi-global optimal solution of the constraint optimization problem by combining a global optimization algorithm and a search algorithm, and then performing smoothing processing and gradient descent processing to obtain the achromatic multi-order diffraction lens; the maximum value of the depth-to-width ratio of the unit structure of the gray scale order distribution of each ring height of the achromatic multi-order diffraction lens is not more than 2: 1. The invention realizes the parameter design of the large-size wide-spectrum achromatic plane lens, compresses the traditional achromatic system into a plane lens, greatly reduces the volume and the cost of the achromatic imaging system, reduces the processing difficulty, improves the performance of the lens and realizes high-quality achromatic imaging in a wide spectrum range.)

1. An achromatization method for a multiple order diffractive lens comprising the steps of:

(1) according to the ring number and the gray scale order of the initial multi-order diffraction lens, the constraint optimization problem of the light field intensity I (m, lambda) of the focus of the light passing through the multi-order diffraction lens is designed, so that the focused light field at each wavelength is consistent, wherein m is the gray scale order distribution of the multi-order diffraction lens, and lambda is the wavelength of the light;

(2) calculating a quasi-global optimal solution m' of the constraint optimization problem by combining a global optimization algorithm and a search algorithm;

(3) smoothing m ' obtained in the step (2), removing a unit structure with a large depth-width ratio in m ', and obtaining a gray level order distribution m ' after smoothing;

(4) and (4) performing gradient descending treatment on the m 'obtained in the step (3) to obtain the gray scale order distribution m' of the achromatic multi-order diffraction lens.

2. The achromatization method for multi-order diffraction lenses of claim 1 wherein the constrained optimization problem of step (1) is a maxmin optimization.

3. The method for achromatizing an optical diffraction lens of claim 1 wherein the constraint optimization problem of step (1) is formulated as:

s.t 1≤m_i≤M

i＝1,2…N

wherein m is_iThe number of gray scale orders of the ith ring height is N, the number of rings of the multi-order diffraction lens is N, and M is the maximum value of the gray scale orders.

4. An achromatization method for multi-order diffraction lenses as claimed in claim 1, wherein the aspect ratio α (m ") of the gradation order distribution m" after the smoothing in step (3) is calculated as follows:

wherein beta is a constant, beta is more than or equal to 5 and less than or equal to 10, and alpha₀Is a reference value for the aspect ratio α (m ') of m'.

5. The method for achromatizing an optical multi-order diffraction lens as recited in claim 1, wherein said step (2) is:

(21) carrying out binarization processing on the gray level order distribution of the initial multi-order diffraction lens;

(22) calculating a solution of the constraint optimization problem in the step (1) by using a binary genetic algorithm;

(23) and (3) taking the solution obtained in the step (2) as an initial value, and calculating a quasi-global optimal solution m' of the constraint optimization problem in the step (1) by using a pattern search method.

6. The achromatization method of multi-order diffraction lenses of claim 1, wherein step (4) uses the steepest descent method to perform gradient descent processing on m "obtained in step (3).

7. An achromatic multiorder diffractive lens produced by a method as claimed in any one of claims 1 to 6, comprising annular structures of different heights, the maximum value of the aspect ratio of the unit structures of the gray scale order distribution of each annular height not exceeding 2: 1.

Technical Field

The invention relates to an achromatic method of a multi-order diffraction lens and the achromatic multi-order diffraction lens.

Background

In the field of optical imaging, obtaining broad-spectrum achromatic imaging is one of the important targets of optical imaging. In a conventional imaging system, a compound lens made of a plurality of different materials is usually used to eliminate refractive chromatic aberration generated by material dispersion, so as to realize clear imaging in the whole spectrum, or a plurality of refractive lenses are combined into a lens group to realize suppression of chromatic aberration.

Achromatic compound lenses made of various materials are currently roughly classified into three types: achromats, apochromats, superachromats; the achromatic lens is made of at least two materials, so that chromatic aberration at two wavelengths (red light and blue light) can be eliminated; the apochromatic lens adopts at least three materials to eliminate chromatic aberration at three wavelengths; the super-achromatic lens adopts at least four materials, and eliminates chromatic aberration at four wavelengths. Because the achromatic performance of the lens needs to be improved by increasing the types of materials, compared with the common lens, the lens is larger in volume, high in processing precision requirement and expensive to manufacture.

By using different materials and different curvatures of the refraction lenses, different dispersion laws, achromatization can be realized by combining a plurality of refraction lenses into a lens group. The proposal increases the volume and the cost of the whole system in multiples, and simultaneously, the alignment between the lenses is a process with high precision requirement, thereby increasing the difficulty of manufacturing and greatly limiting the popularization and the application of the achromatic imaging device.

Disclosure of Invention

The purpose of the invention is as follows: the invention aims to provide an achromatic method of a multi-order diffraction lens, which can clearly image in a wide spectrum range, and the achromatic multi-order diffraction lens.

The technical scheme is as follows: the achromatization method of the multi-order diffraction lens is characterized by comprising the following steps:

(1) according to the ring number and the gray scale order of the initial multi-order diffraction lens, the constraint optimization problem of the light field intensity I (m, lambda) of the focus of the light passing through the multi-order diffraction lens is designed, so that the focused light field at each wavelength is consistent, wherein m is the gray scale order distribution of the multi-order diffraction lens, and lambda is the wavelength of the light;

(2) calculating a quasi-global optimal solution m' of the constraint optimization problem by combining a global optimization algorithm and a search algorithm;

(3) smoothing m ' obtained in the step (2), removing a unit structure with a large depth-width ratio in m ', and obtaining a gray level order distribution m ' after smoothing;

(4) and (4) performing gradient descending treatment on the m 'obtained in the step (3) to obtain the gray scale order distribution m' of the achromatic multi-order diffraction lens.

Further, the formula of the constraint optimization problem is as follows:

s.t 1≤m_i≤M

i＝1,2…N

wherein m is_iThe number of gray scale orders of the ith ring height is N, the number of rings of the multi-order diffraction lens is N, and M is the maximum value of the gray scale orders.

The achromatic multi-order diffraction lens consists of annular structures with different heights, and the maximum value of the depth-to-width ratio of the unit structure with the gray scale order distribution of each annular height is not more than 2: 1.

The step (2) is as follows:

(21) carrying out binarization processing on the gray level order distribution of the initial multi-order diffraction lens;

(22) calculating a solution of the constraint optimization problem in the step (1) by using a binary genetic algorithm;

(23) and (3) taking the solution obtained in the step (2) as an initial value, and calculating a quasi-global optimal solution m' of the constraint optimization problem in the step (1) by using a pattern search method.

And (4) performing gradient descending treatment on the m' obtained in the step (3) by adopting a steepest descent method.

Has the advantages that: compared with the prior art, the invention has the advantages that: (1) the focal point intensity is used as an achromatic optimization target, so that the calculation complexity is low and the time consumption is short; (2) by combining the global optimization algorithm and the local optimization algorithm, compared with the method only adopting the global optimization algorithm, the optimization speed is higher, the performance of the lens is improved, and high-quality achromatic imaging in a wide spectral range is realized; (3) the design of a large-size wide-spectrum achromatic plane lens is realized, the traditional achromatic system is compressed into a plane lens, and the volume and the cost of the achromatic imaging system are greatly reduced; (4) the structure height distribution is smoothed, so that the processing difficulty is greatly reduced, and the processing of the multi-order diffraction lens with larger thickness becomes feasible;

drawings

FIG. 1 is an initial gray scale order distribution m according to the present invention⁰A schematic diagram of (a);

FIG. 2 is a diagram illustrating a gray scale order distribution m' of a multi-order diffractive lens after a search process according to the present invention;

FIG. 3 is a diagram illustrating gray scale distribution of heights from the 1341 th ring to the 1400 th ring in FIG. 2;

FIG. 4 is a schematic diagram of a gray scale order distribution m' of the smoothed multi-order diffraction lens;

FIG. 5 is a schematic illustration of the gray scale order distribution m' "of an achromatic multiple order diffractive lens;

FIG. 6 is a diagram illustrating gray scale distribution of heights from the 1341 th ring to the 1400 th ring in FIG. 5;

FIG. 7 is a schematic diagram of an initial multiple order diffractive lens of the present invention;

FIG. 8 is a schematic diagram of a multi-order diffractive lens after a search process according to the present invention;

FIG. 9 is a schematic view of a smoothed multiple-order diffractive lens of the present invention;

FIG. 10 is a schematic diagram of an achromatic multi-order diffractive lens of the present invention;

FIG. 11 is a graph showing the results of a focusing experiment for an achromatic multi-order diffractive lens of the present invention;

FIG. 12 is a graph of experimental results of white light imaging with an achromatic multi-order diffractive lens of the present invention;

wherein, the graph (a) is the result of the white light imaging of the resolution plate by the achromatic multi-order diffraction lens;

and (b) is the imaging result of the achromatic multi-order diffraction lens on siemens star white light.

Detailed Description

The achromatization method of the multi-order diffraction lens comprises the following steps:

(1) achromatic optimization targets for designing multi-order diffractive lenses

The multi-order diffractive lens is composed of a series of ring structures of different heights, and the phase of the lens can be expressed as:

where φ (r) represents the phase at radial coordinate r, λ is the wavelength, n (λ) is the refractive index at the corresponding wavelength, and h (r) is the structure height at r.

In this embodiment, the width of each ring is a fixed value, which can be expressed by Δ r, and the number of rings constituting the lens is N. Because the gray level laser photoetching process is needed in actual processing, the height h is also in a discrete distribution, the height of each order is delta h, m is the gray level order distribution of the multi-order diffraction lens, and m is { m ═ m { (m {)_i}，i＝1,2,3……N，m_iThe number of gray levels, referred to as the ith ring height, is between 1 and M, M being the maximum of the number of gray levels, such that the height h (i Δ r) at the ith ring can be expressed as M_iΔ h. After introducing the number of rings and the number of gray levels, the optimization objective is expressed as follows:

s.t 1≤m_i≤M

i＝1,2…N

wherein, I (m, lambda) is the light field intensity converged at the focus after the light with wavelength lambda passes through the lens with the gray scale order distribution of m, and the maxmin optimization means is adopted to ensure the consistency of the focused light field at each wavelength. The focal intensity I is used as the objective function, and the computational complexity, i.e. the number of multiplications to be computed, is O (N), whereas if other objective functions are used, e.g. the optical field intensity of the focal plane as a whole is used as the objective function, the computational complexity is O (N)²) The focal intensity I as an objective function is therefore less time consuming. The initial multi-order diffraction lens in this embodiment comprises 2560 rings in total, and the gray scale number is at most 192. Therefore, the temperature of the molten metal is controlled,to realize the achromatic design of the multi-order diffraction lens, the following constraint optimization problem is solved equivalently:

s.t 1≤m_i≤192

i＝1,2…2560

(2) solving a constrained optimization problem for a multi-order diffractive lens

In order to solve the above constraint optimization problem, the optimization design scheme is divided into three parts: search, smoothing and gradient descent.

(21) Search processing

A quasi-global optimal solution is obtained in a short time by combining a global optimization algorithm and a search algorithm, if the global optimization algorithm is only adopted, the search speed is one to two orders of magnitude slower, and the search speed ratio is increased along with the increase of the structure size. In this embodiment, a binary genetic algorithm and a Hooke-Jeeves algorithm (pattern search method) are used. The specific process is as follows: initial gray scale order distribution m for a given initial multi-order diffractive lens⁰，m⁰Is a random vector with the length of 2560, each component takes the value of 1-192, and the gray level order distribution m is initialized⁰Is shown in figure 1. And then binarizing the population to be used as an initial population optimized by a binary genetic algorithm, and optimizing by adopting the binary genetic algorithm. And then, using the result obtained by the binary genetic algorithm as an initial value, further optimizing by adopting a Hooke-Jeeves algorithm, and calculating a quasi-global optimal solution, namely, a schematic diagram of the gray scale order distribution m ' after search processing, namely, the gray scale order distribution m ' after search processing, is shown in figure 2, the gray scale order distributions from the 1341 ring to the 1400 ring in figure 3 are shown, the rectangular protrusions in figure 3 are called as a unit structure, and the lenses corresponding to the distributions realize the 400-plus-1100 nm waveband achromatization function, namely, the light of the 400-plus-1100 nm waveband is focused to the same position after passing through the lenses distributed as m '.

(22) Smoothing process

The gray scale order distribution m 'of the multi-order diffraction lens after search processing has a larger depth-to-width ratio (more than 2:1), and in order to meet the processing requirement, the m' needs to be smoothed to remove the structure with the larger depth-to-width ratio in the distribution. The specific process can be represented by the following formula:

where m ' is the distribution of the gradation levels after the smoothing process, α (m ') and α (m ') are the aspect ratios of the unit structures in the distribution of m ' and m ', i.e., the height to width ratios of the unit structures, α₀For reference, 2 is taken here, and β is a constant, typically 5 to 10. When the input alpha (m') is larger than alpha₀Then α (m ") of the output of the above equation will be significantly less than α (m'); and when the input alpha (m') is less than alpha₀When the output of the above equation is α (m ″) almost equal to α (m'). Therefore, the structure with a large depth-to-width ratio in the distribution m 'can be changed into the structure with a small depth-to-width ratio through the formula, and the structure with the small depth-to-width ratio is almost kept unchanged, so that the obtained gray scale order distribution is m'; fig. 4 shows a schematic diagram of the smoothed grayscale order distribution m ″, and the smoothed grayscale order distribution m ″ does not have a structure with an aspect ratio > 2:1, which greatly reduces the processing difficulty.

(23) Gradient descent processing

Because the step (22) can cause the reduction of the optimization objective function value, the gradient reduction is adopted for processing, so that the optimization objective function value is further improved, and the integral lower depth-to-width ratio is kept; in this embodiment, a steepest descent method is adopted, and this step can be expressed as:

wherein m' ″ is the gray scale order distribution after the gradient descent processing, that is, the gray scale order distribution of the achromatic multi-order diffraction lens, and a schematic diagram thereof is shown in fig. 5, and fig. 6 is the gray scale order distribution of the 1341 st ring to the 1400 th ring; f is the objective function, where f is min_λI (m', lambda), gamma isOne constant, representing the step down, is given by γ, which is a positive number since the optimization goal is to find the maximum value.

The achromatic multi-order diffraction lens is prepared according to the method, the initial multi-order diffraction lens before optimization processing is shown in figure 7, and the multi-order diffraction lens after search processing is shown in figure 8; the smoothed multiple-order diffractive lens is shown in FIG. 9; FIG. 10 shows an achromatic multi-order diffractive lens according to the present invention after gradient-down processing. The diameter of the achromatic multi-order diffraction lens is 1.024cm, the achromatic multi-order diffraction lens totally comprises 2560 rings, the maximum height is 15 mu m, the maximum height order is 192 orders, the gray-scale order distribution is shown in figure 5, wherein the maximum value of the depth-to-width ratio of the unit structure is not more than 2:1, the focal length of the lens is 5cm, and the achromatic wavelength band is 400nm-1100 nm.

The achromatic performance of the achromatic multi-order diffraction lens can be verified through focusing experiments and imaging experiments, and FIG. 11 shows the result of the focusing experiments, wherein lights of different wave bands are focused on the same position on an optical axis; FIGS. 12(a) - (b) are experimental results of white light imaging on resolution plates and Siemens stars, showing that the achromatic multi-order diffractive lens of the present invention does not have significant chromatic aberration.