Desensitization design method for free-form surface optical system based on local curvature control

Abstract

A free-form surface optical system desensitization design method based on local curvature control relates to the field of optical free-form surface design optimization technology. The existing method needs to consume a lot of evaluation and optimization time, and cannot optimize the optical system, resulting in low efficiency and blindness. This method optimizes the image quality of the initial structure of the optical system; constructs an error sensitivity evaluation function based on local curvature control, and evaluates and optimizes the error sensitivity. This method reduces the error sensitivity of the optical system and improves the ability of the free-form surface optical system to resist error interference. Combined with the "ring-arm" reference point sampling method, by analyzing, evaluating and optimizing the local curvature of the small area around the pupil sampling point, the need to reduce the error sensitivity of the optical system is achieved under the premise of ensuring the imaging quality of the optical system.

Description

Technical Field

[0001] The present invention relates to the technical field of optical free-form surface design optimization, and in particular to a method for evaluating and optimizing the error sensitivity of a free-form surface optical system based on local curvature control. Background Art

[0002] At present, any optical system will inevitably be disturbed by various errors from theoretical design to practical realization. These errors may come from manufacturing, alignment, environment, etc. Optical systems with high error sensitivity often find it difficult to achieve the performance of the optimal design results. Therefore, reducing the error sensitivity of the optical system is an important part of the optical system design process.

[0003] Optical free-form surfaces have been gradually applied in various optical systems due to their powerful aberration correction capabilities. Currently, free-form surfaces are mostly used to improve the imaging quality of optical systems. Among the methods reported in papers and patents, there is no error sensitivity evaluation and optimization method proposed for free-form surface optical systems. Therefore, it is of great significance to propose a method to optimize the error sensitivity of free-form surface optical systems.

[0004] In the design and application of free-form surfaces, researchers have found that the error sensitivity of optical systems will be improved to varying degrees after the application of certain types of free-form surfaces. Bin Ma et al. used ordinary optical aspheric surfaces and Q-type free-form surfaces to design mobile phone camera lenses. Tolerance analysis shows that the eccentricity sensitivity and thickness error sensitivity of the optical system using the Q-type surface are significantly lower than the error sensitivity of ordinary aspheric surfaces (Source: Bin Ma, Sharma K, Thompson K P. Mobile device camera design with Q-type polynomials to achieve higher production yield. Optics Express, 2013, 21 (15): 17454-17463) (Using Q-type polynomial free-form surfaces to improve mobile camera production). KP Thompson et al. designed a non-coaxial three-mirror optical system using free-form surfaces. Compared with the traditional off-axis three-mirror optical system with non-spheric surfaces, the non-coaxial system has lower adjustment sensitivity. These comparisons show that in this design, the free-form surface plays an important role in reducing the error sensitivity of the optical system (Source: Thompson KP, Schiesser E, Rolland J P. Why are freeform telescopes less alignment sensitive than a traditional unobscured TMA?. Proceedings of SPIE, 2015, 9633: 963317.) (Why does the free-form surface off-axis three-mirror optical system have lower alignment sensitivity than the traditional unobstructed off-axis three-mirror optical system?). However, the existing desensitization design methods applicable to free-form surfaces can only prove that some complex optical surfaces have a certain positive effect on optimizing the error sensitivity of optical systems. There is currently no clear method to guide the desensitization of free-form surfaces.

[0005] At present, there is no error sensitivity evaluation and optimization method for free-form surface optical systems. The global search method and multiple structure optimization method, which are applicable to all optical systems, are still used. Neither of these two methods is guided by the error sensitivity theory, so it takes a lot of time to evaluate and optimize. The global search method searches within a certain range, evaluates the error sensitivity of all systems that meet the conditions, and selects the optical system with better error sensitivity; the multiple structure optimization method constructs multiple structures containing multiple error types based on the optimization results, and then optimizes these multiple structures. Although these two methods have certain practicality, they cannot optimize the optical system in a targeted manner, are inefficient and blind.

[0006] However, in the prior art, there is no publicly reported desensitization design method specifically for free-form surface optical systems. Summary of the invention

[0007] In order to solve the problems that the existing methods need to consume a lot of evaluation and optimization time, and cannot optimize the optical system, resulting in low efficiency and blindness, the present invention provides a free-form surface optical system desensitization design method based on local curvature control.

[0008] A desensitization design method for a free-form surface optical system based on local curvature control is implemented by the following steps:

[0009] Step 1: Optimize the image quality of the initial structure of the optical system;

[0010] Step 2: Construct an error sensitivity evaluation function based on local curvature control; the specific process is:

[0011] Step 21. Pupil sampling: Pupil sampling is performed according to the complexity of the face shape;

[0012] Step 22: Calculate the error sensitivity LC of a single optical element in a single field of view;

[0013] Step 23: Calculate the error sensitivity LC of the optical system S,F ;

[0014] Step 3: Error sensitivity evaluation and optimization;

[0015] The error sensitivity of the optical system is evaluated. If the optical system meets the error sensitivity requirements, the system is output as the final result. If it does not meet the requirements, desensitization design optimization is required until an optical system that meets both image quality requirements and error sensitivity requirements is output.

[0016] Beneficial effects of the present invention:

[0017] The present invention is a free-form surface optical system desensitization design method based on local curvature control. The method decomposes the complex free-form surface shape into multiple simple quadratic surface shapes based on the idea of ​​microelement, and adopts the method of local area curvature control to evaluate and optimize the local error sensitivity. The purpose is to solve the problem of reducing the error sensitivity of the optical system and improving the ability of the free-form surface optical system to resist error interference. The method combines the "ring-arm" reference point sampling method, analyzes, evaluates and optimizes the local curvature of the tiny area around the pupil sampling point, and realizes the demand for reducing the error sensitivity of the optical system under the premise of ensuring the imaging quality of the optical system.

[0018] Compared with the prior art, the method of the present invention can evaluate and optimize the error sensitivity of the free-form surface optical system while ensuring the imaging quality of the optical system, and quantify the error sensitivity of the optical system. This method can significantly reduce the amount of calculation in the desensitization optimization process of the free-form surface optical system, and perform targeted desensitization optimization design on the optical system, thereby improving the error sensitivity optimization efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is a schematic diagram of a pupil sampling method and a local curvature control method in the free-form surface optical system desensitization design method based on local curvature control described in the present invention;

[0020] Figure 2 It is a flow chart of the free-form surface optical system desensitization design method based on local curvature control according to the present invention;

[0021] Figure 3 (a) is a schematic diagram of the initial structure of the optical system; (b) is a schematic diagram of the image quality optimization result; (c) is a schematic diagram of the error sensitivity optimization result.

[0022] In the figure: 1. Free-form surface, 2. Quadratic surface basis, 3. Deviation between the free-form surface and the quadratic surface basis along the z direction, 4. Pupil sampling point, 5. Pupil sampling arm, 6. Pupil sampling ring, 7. Quadratic surface approximated in the local area. DETAILED DESCRIPTION

[0023] Specific implementation method 1. Combination Figure 1 and Figure 2 The present embodiment is described as follows: a free-form surface optical system desensitization design method based on local curvature control, which is implemented by the following steps:

[0024] 1. Construct the initial structure of the optical system according to the design index requirements;

[0025] 2. Optimize the image quality of the initial structure of the optical system described in step 1; during the optimization process, set the curvature, spacing, quadratic surface coefficients and high-order coefficients of the reflector as optimization variables. In order to achieve better imaging quality, the surface type can be gradually upgraded from spherical surface, aspherical surface to free-form surface. Optimize the imaging quality. If the image quality requirements are not met, continue to optimize the image quality. If the image quality requirements are met, execute step 3 to enter the error sensitivity evaluation link.

[0026] 3. Based on local curvature control, an error sensitivity evaluation function is constructed to perform error sensitivity evaluation and optimization. If the error sensitivity requirements are met, the system can be output as the final result. If the requirements are not met, desensitization design optimization is required until an optical system that meets both image quality requirements and error sensitivity requirements is output.

[0027] In this embodiment, the image quality requirement and the error sensitivity requirement are not fixed standards. The image quality requirement is determined according to the evaluation standards of different systems, and the error sensitivity requirement is also determined according to the evaluation standards of different systems.

[0028] The specific process is:

[0029] (1) Pupil sampling: Pupil sampling is performed according to the complexity of the face shape;

[0030] Taking the three-mirror free-form surface optical system as an example, the pupil sampling method and the local curvature control method are shown in the figure. Figure 1 As shown. The figure includes a free-form surface 1, a quadratic surface base 2, the deviation between the free-form surface and the quadratic surface base along the z direction 3, a pupil sampling point 4, a pupil sampling arm 5, a pupil sampling ring 6 and a quadratic surface 7 approximated in the local area; in the figure, z(y) is the free-form surface vector height, and Arm1-Armn is the number of sampled arms. Based on the idea of ​​infinitesimal elements, the tiny area around each sampling point is analyzed by the pupil sampling method, and each tiny area can be approximated into a simple quadratic surface shape, and its error sensitivity is evaluated and optimized;

[0031] (2) Calculate the error sensitivity LC of a single optical element in a single field of view;

[0032]

[0033] At each sampling point, the radius of curvature of the small area around the sampling point is defined as LC u,v , where u is the ring number, there are NOR rings in total; v is the arm number, there are NOA arms in total. First, calculate the error sensitivity of a free-form surface mirror corresponding to a single field of view,

[0034] (3) Calculate the error sensitivity evaluation function of the three free-form surface reflectors corresponding to all fields of view. That is, calculate the error sensitivity LC of the optical system S,F ;

[0035] Error sensitivity LC of the three free-form mirrors corresponding to all fields of view S,F :

[0036]

[0037] Where, S is the surface number, and there are NOS optical surfaces in total; F is the field number, and there are NOF fields in total. Figure 3 This embodiment is described as an example of the method for evaluating and optimizing the error sensitivity of a free-form surface optical system based on local curvature control described in the first embodiment.

[0038] The initial structure of the optical system is an off-axis three-mirror optical system with a focal length of 850mm, F#4.25, a field of view of 20°×30°, and an operating wavelength of 588nm. The layout diagram is as follows Figure 3 As shown in (a), the error sensitivity optimization process of the system is taken as an example to verify the effectiveness of the present invention.

[0039] 1. Optimize the image quality of the initial structure. Upgrade the three-sided reflective mirror surface to a free-form surface. After image quality optimization, the image quality optimization result is obtained. The layout diagram is as follows: Figure 3 As shown in (b) in the figure, the wave aberration is 0.088λ.

[0040] 2. Evaluate and optimize the error sensitivity of the image quality optimization results. Set the image quality allowable range to 0.088λ±10%, where λ is the wavelength and the error sensitivity requirement is less than 0.400. After error sensitivity analysis of the image quality optimization results, the error sensitivity is 0.638, which does not meet the error sensitivity requirements and needs to be optimized.

[0041] 3. In the error sensitivity analysis process, an 8-ring 10-arm pupil sampling method is used, with a total of 80 reference points, to control and optimize the curvature of 80 tiny local areas. After error sensitivity optimization, the error sensitivity optimization result is obtained. The layout diagram is shown in the figure below. Figure 3 As shown in (c), at this time, the wave aberration of the optical system is 0.096λ, and the error sensitivity is 0.400. The image quality and error sensitivity of the optical system meet the requirements at the same time and can be output as the optimization design result.

[0042] The error sensitivity analysis of the initial structure and the design results was carried out using the Monte Carlo method. The error form was the tilt and eccentricity of the optical element. The error amount was 0.01° tilt and 0.01mm eccentricity. The comparison results are shown in Table 1. Table 1 shows the error sensitivity analysis results of the initial structure and the design results:

[0043] Table 1

[0044] Initial structure Optimized design results Root mean square wavefront aberration ΔRMS WFE / λ 0.088 0.096 ΔRMS WFE average value / λ 0.068 0.034 ΔRMS WFE max. / λ 0.024 0.119 Standard deviation / λ 0.049 0.024

[0045] According to the error sensitivity analysis results, the error sensitivity of the optical system is effectively reduced. Under the same error interference, the change in wave aberration of the optical system after error sensitivity optimization is half of that before optimization, and the image quality of the optical system after error sensitivity optimization still meets the requirements of the optical system, which proves the correctness and effectiveness of the present invention.

[0046] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A desensitization design method for a free-form surface optical system based on local curvature control, characterized by: The method is implemented by the following steps: Step 1: Optimize the image quality of the initial structure of the optical system; Step 2: Construct an error sensitivity evaluation function based on local curvature control; the specific process is: Step 21. Pupil sampling: Pupil sampling is performed according to the complexity of the face shape; Step 22: Calculate the error sensitivity of a single optical element in a single field of view; Step 23: Calculate the error sensitivity of the optical system; Step 3: Error sensitivity evaluation and optimization; The error sensitivity of the optical system is evaluated. If the optical system meets the error sensitivity requirements, the system is output as the final result. If it does not meet the requirements, desensitization design optimization is required until an optical system that meets both image quality requirements and error sensitivity requirements is output.

2. The method for desensitization design of a free-form surface optical system based on local curvature control according to claim 1, characterized in that: In step 1, the curvature, spacing, quadratic surface coefficients and high-order coefficients of the reflector are set as optimization variables to optimize the image quality.

3. The method for desensitization design of a free-form surface optical system based on local curvature control according to claim 1, characterized in that: In step 21, a small area around each sampling point is analyzed by pupil sampling, and each small area is approximated into a quadratic surface shape for pupil sampling.

4. The method for desensitization design of a free-form surface optical system based on local curvature control according to claim 1, characterized in that: In step 22, the formula for the error sensitivity LC of a single optical element is: At each sampling point, the radius of curvature of the small area around the sampling point is LC u,v , where u is the ring number, there are NOR rings in total; v is the arm number, there are NOA arms in total.

5. The method for desensitization design of a free-form surface optical system based on local curvature control according to claim 1, characterized in that: In step 3, calculate the error sensitivity LC of the optical system S,F The formula is: Where S is the surface number, there are NOS optical surfaces in total; F is the field of view number, there are NOF fields of view in total.

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