A compensator and design method for imperfect imaging off-axis three-mirror optical path

Abstract

This invention discloses the structure and design method of an imperfect imaging off-axis three-reflector optical path compensator. Its core feature is the insertion of a set of lenses and a folding mirror as compensators before the imperfect imaging off-axis three-reflector optical path. Using the overall optical path and the off-axis three-reflector optical path in their designed state as constraints, multiple configurations are established to optimize the image quality of the compensator optical path, thereby determining the optimal compensator parameters. The compensator's surface shape can be one or a combination of spherical, planar, or freeform surfaces. The lens group consists of one or more lenses, and the lens materials are selected and coated according to the interferometer's wavelength range. This invention solves the problem of the difficulty in accurately assembling and testing an imperfect imaging off-axis three-reflector optical path independently. By using the aforementioned compensator to equivalently replace the optical power and aberrations of the front optical path, the imperfect imaging off-axis three-reflector optical path can be independently tested and adjusted, thereby obtaining an optical path state consistent with the design.

Description

Technical Field

[0001] This invention belongs to the field of optical design or optical inspection, and relates to a compensator for an off-axis three-reflector optical path that cannot achieve perfect imaging. Background Technology

[0002] With proper design, the off-axis three-reflector optical path can be used as a standalone imaging optical path or as part of an overall optical path, serving functions such as focal length scaling, aperture matching, and aberration correction, and together with the front optical path, it forms a complete imaging optical path.

[0003] The assembly and adjustment test of the off-axis three-reflector optical path usually adopts the self-collimating interferometry method. The misalignment solution model is established by computer-aided assembly and adjustment method. With reference to the interferometry results, the reflectors of the off-axis three-reflector optical path under test are finely adjusted to iteratively approximate the predetermined optical path state.

[0004] However, for the off-axis three-mirror backlight with imperfect imaging, a self-collimating interferometric measurement optical path cannot be directly built separately. The reasons are as follows:

[0005] First, the residual aberration of a single imperfect off-axis three-reflector path is large, making it difficult to meet the requirements of autocollimation interferometry. Even if an interferogram can be obtained, the fringe density of the interferogram corresponding to the residual aberration exceeds the resolution of the interferometer, making it impossible to calculate wave aberration and misalignment error.

[0006] Second, in the process of assembling and adjusting the off-axis three-mirror optical path for imperfect imaging, if the position and orientation of the three mirrors are not constrained, the adjustment of the position of the three mirrors will only move in the direction of minimizing wavelet aberration, get stuck in a local minimum, deviate from the optical path state expected by the design, and make it difficult to achieve the predetermined image quality requirements.

[0007] In summary, the imperfect imaging off-axis three-reflector optical path, as a part of the overall optical path, cannot work independently. If the optical path is assembled, adjusted, and tested separately according to the principle of minimizing wavelet aberration, an additional compensator is required as an auxiliary measure.

[0008] To achieve the assembly and testing of the imperfect imaging off-axis three-lens optical path, the first approach that comes to mind is to place the rear optical path of the off-axis three-lens in the overall optical path, and use the front optical path of the overall optical path as a reference or benchmark to perform assembly and testing on the rear optical path of the imperfect imaging off-axis three-lens.

[0009] However, the above approach also has two problems:

[0010] The first issue is that the processing, assembly, and testing of the front optical path and the off-axis three-lens reflex camera's rear optical path are usually performed in parallel, resulting in time overlap between the two processes. This necessitates that the off-axis three-lens reflex camera's optical path be assembled and tested independently using a third-party method before integration with the front optical path. Simultaneously, this independent third-party testing method allows for cross-validation, demonstrating the effectiveness of decoupling and cross-validation.

[0011] Only after the imperfect imaging off-axis three-reflector optical path has undergone corresponding optical testing and meets the predetermined indicators, can it be integrated and tested together with the front optical path as a whole.

[0012] The second problem is that if the front optical path of the overall optical path is used for assembly and adjustment, the misalignment aberration of the off-axis three-lens mirror optical path in the imperfect imaging is mixed with the residual aberration of the front optical path processing and assembly, which is difficult to distinguish and separate, causing great trouble to the assembly, adjustment, testing and status judgment of the rear optical path and the overall optical path of the off-axis three-lens mirror.

[0013] For independent assembly and testing of the rear optical path of off-axis three-lens reflex cameras with a front optical path, it is necessary to rely on an optical compensator to replace the optical power and aberration correction of the front optical path, thereby eliminating the need for the front optical path and enabling independent assembly and testing of the rear optical path of off-axis three-lens reflex cameras and third-party inspection.

[0014] The analysis of existing technical approaches is as follows:

[0015] In 2021, Li Zhaoyang et al. reported a compensator structure for an off-axis three-mirror optical system. A set of lenses was added to the rear optical path of the off-axis three-mirror system as a compensator for system assembly and testing. This method is based on aberration analysis for compensator design.

[0016] Li Ming and others from the Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, used a computational holographic compensator to compensate the intermediate real image plane of a Rug-type off-axis three-mirror optical system for the adjustment of the primary and secondary mirrors in the off-axis three-mirror optical system.

[0017] CN115508992A describes a compensator structure and design method for using an off-axis three-lens reflex camera as the front optical path. In this case, the off-axis three-lens reflex camera serves as the front optical path, and the subsequent optical path only contains simple optical elements such as windows and filters. In this type of optical path, the off-axis three-lens reflex camera itself undertakes almost all the optical power and aberration correction.

[0018] The compensator structure proposed in CN115508992A is mainly designed for off-axis three-mirror systems as the main optical path, to compensate for and equivalently replace the optical elements such as windows and filters that follow. In this case, the aberration and optical power of the compensator are relatively small. Therefore, it is designed as a focalless compensator placed near the focal point of the interferometer, as demonstrated by the accompanying example.

[0019] Unlike existing technologies and literature reports, this invention addresses the problem of separately assembling and adjusting the optical path of the off-axis three-lens reflex camera for imperfect imaging. It proposes to directly optimize the compensation optical path and compensator of the off-axis three-lens reflex camera for imperfect imaging by using the designed overall optical path and the off-axis three-lens reflex camera's rear optical path as constraints. This not only compensates for the aberrations of the off-axis three-lens reflex camera for imperfect imaging but also replaces the optical power and aberration correction function of the front optical path. Therefore, it can obtain the optimal compensator design. Independent assembly and adjustment tests of the off-axis three-lens reflex camera for imperfect imaging are performed according to this optimal compensator, so that the state of the off-axis three-lens reflex camera for imperfect imaging is close to the design value. Summary of the Invention

[0020] Based on the application background of optical path assembly and adjustment testing of off-axis three-lens reflex cameras with imperfect imaging, a compensator structure and design method for the optical path of off-axis three-lens reflex cameras with imperfect imaging are proposed. This solves the problem of optical path consistency when performing assembly and adjustment testing of the optical path of off-axis three-lens reflex cameras with imperfect imaging independently.

[0021] This invention proposes to insert a lens group and a folding mirror in front of the off-axis three-mirror optical path with imperfect imaging, as a compensator for the assembly and adjustment of the off-axis three-mirror optical system, and then form a detection optical path with an interferometer.

[0022] An off-axis three-reflector interferometer optical path with imperfect imaging is pre-inserted with a folding compensator and a set of lenses. The detection wavefront emitted by the interferometer passes sequentially through the fifth, fourth, and third reflecting mirrors, through the compensator lens group, and then into the folding mirror. After being reflected by the folding mirror, it passes sequentially through the compensator lens group, the third, fourth, and fifth reflecting mirrors, and returns to the interferometer, forming an off-axis three-reflector interferometric measurement optical path with a compensator.

[0023] The compensator's lens group can consist of one or more lenses, and each lens can have a surface shape that is spherical, planar, aspherical, freeform, or diffractive. During the optimization process, the number of lenses can be increased or decreased appropriately, while simultaneously adjusting the complexity of the surface shape equation.

[0024] The reflective surface of the retroreflector can be optically coated according to the operating wavelength of the interferometer.

[0025] The surface shape of a reflector can be a plane, a concave spherical surface, or a convex spherical surface, and its surface shape and position parameters are determined according to the following method.

[0026] Based on the above analysis, multiple configurations are established in the optical design software to optimize the compensator. The overall optical path, which includes the front optical path and the off-axis three-mirror rear optical path, is used as a constraint. Different optical path configurations reflect the mutual influence between the compensator, the rear optical path, the front optical path, and the overall optical path. Finally, the optimal compensator structural parameters are obtained by minimizing the blur spot or wavefront aberration of the multiple configurations simultaneously.

[0027] The design steps for the compensator's catadioptric mirror and lens group parameters are as follows:

[0028] Step 1: In the optical design software, create Configuration 1. The optical path model is the pre-designed entire optical system, including the front optical path and the imperfect imaging off-axis three-reflector optical path. All parameters adopt the design values.

[0029] Step 2: Establish Configuration 2. The optical path model consists of a retroreflector, a compensator lens group, and an imperfect imaging off-axis three-reflector optical path, which together form an imperfect imaging off-axis three-reflector optical path with a compensator, simulating the compensation and verification state of the imperfect imaging off-axis three-reflector optical path.

[0030] Among them, the surface shape of the retroreflector and the relative pose parameters from the retroreflector to the compensator, the surface shape and thickness parameters of the compensator lens group, the pose of the compensator lens group to the retroreflector and the pose of the compensator lens group to the imperfect imaging off-axis three-reflector path, a total of four sets of parameters are used as design variables for configuration two.

[0031] Step 3: Establish Configuration 3. The optical path model consists of a retroreflector, a compensator lens group, and an imperfect imaging off-axis three-reflector optical path, representing the state of the imperfect imaging off-axis three-reflector optical path being assembled, adjusted, and tested individually.

[0032] The surface shape of the retroreflector, the relative pose of the retroreflector to the compensator lens group, the thickness and surface shape parameters of the lens group are the same as the corresponding parameters of configuration two. At the same time, the pose parameters of the two mirrors in the imperfect off-axis three-reflector path, the pose of the compensator lens group to the retroreflector, and the pose of the compensator lens group to the imperfect off-axis three-reflector path, a total of four sets of parameters are used as design variables for configuration three.

[0033] Step 4: Establish Configuration 4. The optical path model is a pre-designed entire optical system, including the front optical path and the off-axis three-mirror rear optical path. It represents the coupling state of the imperfect imaging off-axis three-mirror optical path and the front optical path after separate assembly and adjustment.

[0034] The pose parameters of the front optical path and the imperfect imaging off-axis three-reflector path are the same as those of configuration three. The relative position parameters of the front optical path and the imperfect imaging off-axis three-reflector path are used as design variables for configuration four.

[0035] Step 5: Simultaneously optimize the design variables of Configuration 1, Configuration 2, Configuration 3 and Configuration 4 to minimize wave aberration or blurring, and obtain the surface shape and position parameters of the reflector and lens group.

[0036] The optimized lens group and the folding mirror are used as compensators. Together with the off-axis three-reflector optical path with imperfect imaging and the interferometer, they form a self-collimating interferometric test optical path, which can be used for independent assembly and testing. At the same time, it can also replace the front optical path for system integration.

[0037] The above compensator parameters are the result of optimization under the constraints of the overall optical path and the local optical path, achieving the best equivalent and approximation effect of the front optical path. Therefore, it can ensure that the assembly and testing state of the rear optical path of the imperfect imaging off-axis three-lens reflex camera is consistent with the design state.

[0038] Based on the above technical features and implementation methods, this invention solves the detection problem of imperfect imaging off-axis three-reflector optical paths, and can also achieve the following technical effects:

[0039] (a) It avoids the limitations of relying on the front optical path and can obtain the state where the rear optical path of the imperfect imaging off-axis three-lens mirror is closest to the designed optical path, realizing the separate testing of the optical path of the imperfect imaging off-axis three-lens mirror.

[0040] (b) The compensator lens group is a small-diameter optical lens. It is assembled into a lens group using existing optical clamping equipment. After independent assembly, it becomes a module. It is then combined with the imperfect imaging off-axis three-reflector light path to form an interference test light path, which facilitates the relatively independent assembly, adjustment and testing of the imperfect imaging off-axis three-reflector light path.

[0041] (c) The compensator lens group has light rays passing back and forth twice in the optical path, and the same-direction error can be canceled out. Therefore, the processing accuracy and testing requirements of the compensator lens group can be appropriately optimized. Attached Figure Description

[0042] Figure 1 This is an assembly and testing optical path diagram consisting of an imperfect imaging off-axis three-reflector optical path and a compensator.

[0043] Figure 2 This is an embodiment of the imperfect imaging off-axis three-reflector path assembly and adjustment test described in this invention.

[0044] Figure 3 yes Figure 2 A schematic diagram of the overall optical path of the off-axis three-reflector path in imperfect imaging.

[0045] Figure 4 yes Figure 2 Self-collimating interference optical path for off-axis three-reflector optical path with imperfect imaging. Detailed Implementation

[0046] This invention provides a compensator structure for separate assembly and adjustment testing of an off-axis three-reflector optical path for imperfect imaging. Its core feature is that a compensator is placed in front of the off-axis three-reflector optical path for imperfect imaging. The compensator includes a folding mirror and a lens group, which together with the interferometer and the off-axis three-reflector optical path to be tested form a self-collimating interference optical path.

[0047] The surface shape, thickness, and position of the compensator's catadioptric mirror and lens group are key parameters in the compensator design. The specific implementation steps are as follows:

[0048] Step 1: In the optical design software, create Configuration 1. The optical path model is the pre-designed optical system, including the front optical path and the off-axis three-mirror rear optical path. All parameters remain unchanged, that is, Configuration 1 has no design variables.

[0049] Step 2: Establish configuration 2. The optical path model is an off-axis three-reflector optical path, a compensator lens group (2) and a folding mirror (1). The thickness and surface parameters of the compensator lens group (2), the pose of the compensator lens group (2) to the folding mirror (1) and the distance from the compensator lens group (2) to the vertex of the third mirror (3) are three sets of parameters used as design variables for configuration 2.

[0050] Step 3: Establish Configuration 3. The optical path model is the optical path and compensator after the off-axis three-mirror reflection. The thickness and surface parameters of the compensator lens group (2) are the same as the corresponding parameters of Configuration 2. At the same time, the two interval parameters from the third mirror (3) and the fifth mirror (5) to the fourth mirror (4), the pose of the compensator lens group (2) to the folding mirror (1), and the distance from the compensator lens group (2) to the vertex of the third mirror (3) are the four sets of parameters used as design variables for Configuration 3.

[0051] Step 4: Establish the original system of the off-axis three-mirror rear optical path of configuration 4, including the front optical path and the off-axis three-mirror rear optical path. The two pose parameters from the third mirror (3) and the fifth mirror (5) to the fourth mirror (4) are the same as the corresponding parameters of configuration 3. Only the pose of the fifth mirror (5) to the image plane, i.e. the back intercept, is used as the design variable of configuration 4.

[0052] Step 5: By minimizing wavefront aberration or speckle, optimize all design variables of Configuration 1, Configuration 2, Configuration 3 and Configuration 4 simultaneously to obtain the optimal compensator surface shape and thickness parameters as well as the parameters of the reflector.

[0053] Based on the above steps, the optimal parameters of the compensator lens group, such as thickness, surface shape, and aperture, are directly optimized. Suitable glass materials and coatings are selected according to the interferometer's operating wavelength. The lens group and the folding mirror are installed at appropriate positions in the imperfect imaging off-axis three-mirror optical path, forming a self-collimating interferometric test optical path together with the interferometer. This allows for independent assembly and adjustment testing of the imperfect imaging off-axis three-mirror optical path, achieving equivalent replacement and aberration compensation for the front optical path.

[0054] In summary, the key step in the design of the imperfect imaging off-axis three-mirror optical path compensator of the present invention is to establish a reference and constraint relationship between the pre-designed overall optical path and the optical path after the off-axis three-mirror, and the optical path after the off-axis three-mirror with the compensator. Using the designed overall optical path and the optical path after the off-axis three-mirror as constraints, the lens group and the folding mirror parameters of the compensator for the imperfect imaging off-axis three-mirror optical path are optimized, thereby achieving the optimal design of the compensator and the compensation optical path.

[0055] Figure 2 , Figure 3 and Figure 4 This is one embodiment of the present invention, wherein Figure 2 This is a schematic diagram of the overall optical path. Figure 3 This is an imperfect imaging off-axis three-mirror path and a magnified view of a portion of the image, consisting of the third, fourth, and fifth mirrors. Figure 4 yes Figure 3 The optical path diagram of the separate test of the optical path after the off-axis three-lens reflex camera with imperfect imaging.

[0056] Figure 2 The overall optical path is divided into the front optical path and the rear optical path by the field stop. Figure 2 The front optical path consists of a coaxial double-reflection optical path composed of the first and second reflecting mirrors, while the rear optical path consists of an imperfect imaging off-axis triple-reflection optical path composed of the third, fourth, and fifth reflecting mirrors.

[0057] Figure 2 The first and second reflecting mirrors in the front optical path have large apertures, long processing cycles, and are time-consuming. Figure 3 The imperfect imaging shown has overlapping processing cycles in the off-axis three-reflector optical path, making it unusable. Figure 2 Front optical path Figure 3 The optical path of the off-axis three-mirror system with imperfect imaging was tested.

[0058] against Figure 3 The independent assembly and adjustment test of the optical path after off-axis three-mirror switching was conducted using the compensator structure and design method described in this invention. A corresponding compensator was designed and implemented, forming a complete system. Figure 4 The imperfect imaging off-axis three-way interference optical path is shown. Figure 4 The optical path includes a retroreflector, a lens, and an off-axis three-reflector optical path with imperfect imaging. The parameters of the lens group and the retroreflector are designed according to the aforementioned method.

Claims

1. An off-axis three-reflector optical path compensator for imperfect imaging, characterized in that: A retroreflector (1) and a lens group (2) are inserted in front of the off-axis three-reflector path for imperfect imaging; the retroreflector (1) and the lens group (2) serve as a compensator. The detection wavefront emitted by the interferometer (6) passes through the fifth mirror (5), the fourth mirror (4), and the third mirror (3) in sequence, passes through the lens group (2), and is incident on the retroreflector (1); then, after being reflected from the retroreflector (1), it passes through the lens group (2), the third mirror (3), the fourth mirror (4), and the fifth mirror (5) in sequence, and returns to the interferometer (6) to form a self-collimating interference optical path.

2. The imperfect imaging off-axis three-reflector optical path compensator according to claim 1, characterized in that: The surface shape of the reflector (1) is a plane, a sphere, a quadric surface, an even-order aspherical surface, a free-form surface, or a diffraction surface. The reflecting surface is optically coated according to the working band of the interferometer.

3. The imperfect imaging off-axis three-reflector optical path compensator according to claim 1, characterized in that: The lens material of the lens group (2) is selected according to the working band of the interferometer. The lens surface is optically coated. The number of lenses is one or more, and their surface shape is a plane, a sphere, a quadratic surface, an even-order aspherical surface, a free-form surface or a diffraction surface.

4. A design method for an imperfect imaging off-axis three-reflector optical path compensator, characterized in that... The steps are as follows: Step 1: In the optical design software, establish Configuration 1. The optical path model is the pre-designed entire optical system, including the front optical path and the imperfect imaging off-axis three-reflector optical path composed of the third mirror (3), the fourth mirror (4), and the fifth mirror (5). All parameters adopt the design values, that is, Configuration 1 has no design variables. Step 2: Establish configuration 2. The optical path model consists of a folding mirror (1), a lens group (2), a third mirror (3), a fourth mirror (4), and a fifth mirror (5), which together form an imperfect imaging off-axis three-reflector optical path with a compensator, simulating the compensation test state of the imperfect imaging off-axis three-reflector optical path. Among them, the surface shape of the retroreflector (1) and the relative pose parameters of the retroreflector (1) to the lens group (2), the surface shape and thickness parameters of the lens group (2), the pose of the lens group (2) to the retroreflector (1) and the pose of the lens group (2) to the third reflector (3), a total of four sets of parameters are used as design variables for configuration two. Step 3: Establish configuration 3. The optical path model consists of a folding mirror (1), a lens group (2), and an imperfect imaging off-axis three-reflection optical path, representing the state of the imperfect imaging off-axis three-reflection optical path being assembled, adjusted, and tested individually. Among them, the surface shape of the retroreflector (1), the relative pose of the retroreflector (1) to the lens group (2), the thickness and surface shape parameters of the lens group (2) are the same as the corresponding parameters of configuration two. At the same time, the two pose parameters of the third mirror (3) and the fifth mirror (5) to the fourth mirror (4), the pose of the lens group (2) to the retroreflector (1), and the pose of the lens group (2) to the third mirror (3) are a total of four sets of parameters as design variables for configuration three. Step 4: Establish Configuration 4. The optical path model is a pre-designed entire optical system, including the front optical path and the rear optical path of the off-axis three-lens reflex camera. It represents the coupling state of the imperfect imaging off-axis three-lens reflex camera optical path and the front optical path after separate assembly and adjustment. Among them, the two pose parameters of the third reflector (3) and the fifth reflector (5) to the fourth reflector (4) are the same as the corresponding parameters of configuration three. The relative position parameters of the front optical path and the imperfect imaging off-axis three-reflection path composed of the third reflector (3), the fourth reflector (4) and the fifth reflector (5) are used as the design variables of configuration four. Step 5: To minimize wave aberration or blur, simultaneously optimize the design variables of configuration 1, configuration 2, configuration 3 and configuration 4 to obtain the surface shape and position parameters of the reflector (1) and lens group (2).

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