Large aperture beam expander for vertical use and design method

Abstract

The application discloses a large-aperture expander used in the vertical direction and a design method, relates to the fields of high-power laser systems and precise optical measurement technologies, and can solve the technical problems of wavefront aberration deterioration and difficult debugging of the expander in the vertical use state due to gravity deformation. The expander comprises a main mirror assembly, a secondary mirror assembly and a support structure, the optical axis of the expander is arranged in the vertical direction, the main mirror assembly comprises a main mirror, a main mirror frame and an elastic supporting element, the main mirror is installed in the main mirror frame through the elastic supporting element, the elastic supporting element is used for providing axial pre-tightening and controlling the micro-deformation of the main mirror under the action of gravity, the secondary mirror assembly comprises a secondary mirror and a secondary mirror frame, the secondary mirror frame is installed on the support structure through a wing spar cantilever type, the wing spars are uniformly distributed along the circumference of the secondary mirror frame, and tangential constraints are formed between each wing spar and the secondary mirror frame, and the support structure comprises an upper connecting frame, a lower connecting frame and at least six vertical supporting rods connecting the upper connecting frame and the lower connecting frame, the upper connecting frame is connected with the main mirror frame, and the lower connecting frame is connected with the secondary mirror frame.

Description

Technical Field

[0001] This application relates to the fields of high-power laser systems and precision optical measurement technology, and in particular to a large-aperture beam expander for vertical use and its design method. Background Technology

[0002] In fields such as high-power laser systems, optical measurement, and space optical communication, large-aperture beam expanders are key components for beam aperture transformation, wavefront measurement, and far-field beam quality assessment. Beam expanders can compress large-aperture parallel beams into small-aperture parallel beams to match the receiving aperture of downstream optical systems or detectors.

[0003] Existing beam expanders are typically designed for horizontal placement, with their support structures and assembly methods optimized for gravity distribution in a horizontal configuration. However, in some practical applications (such as laser towers, beam transmission channels in high-power laser devices, vertical transmission and reception systems for ground-based lidar, and ground-based gravity unloading test platforms for space payloads), beam expanders require vertical installation. In a vertical configuration, the direction of gravity is parallel to the optical axis, fundamentally altering the support methods for the primary and secondary mirrors. The axial gravity component experienced by the primary mirror increases significantly, easily leading to asymmetrical axial concavity or convexity on the mirror surface; the cantilever support structure of the secondary mirror is prone to tilting and lateral displacement under gravity. Furthermore, gravity causes changes in the axial spacing between the primary and secondary mirrors, further degrading the system wavefront. The combined effect of these factors results in a sharp deterioration of the transmission wavefront aberration, severely impacting the beam expander's performance. Moreover, the assembly and adjustment of vertical beam expanders present unique challenges: in a horizontal configuration, the adjustment mirror can be easily placed at one end of the optical path for adjustment, but in a vertical configuration, the adjustment mirror cannot be directly placed along the optical axis. Existing technologies lack dedicated debugging tools and methods for vertical beam expanders, resulting in low installation and adjustment efficiency and difficulty in guaranteeing accuracy. Summary of the Invention

[0004] This application provides a large-diameter beam expander for vertical use and its design method, which can solve the technical problems of wavefront aberration degradation and debugging difficulties caused by gravity deformation in existing beam expanders under vertical use.

[0005] To achieve the above objectives, this application adopts the following technical solution: In a first aspect, this application provides a large-aperture beam expander for vertical use, comprising: a primary mirror assembly, a secondary mirror assembly, and a support structure; The optical axis of the beam expander is arranged vertically, the primary mirror assembly is located at the upper part of the beam expander, and the secondary mirror assembly is located at the lower part of the beam expander; The primary mirror assembly includes a primary mirror, a primary mirror frame, and an elastic support element. The primary mirror is installed in the primary mirror frame via the elastic support element. The elastic support element is used to provide axial preload and control the micro-deformation of the primary mirror under gravity. The secondary mirror assembly includes a secondary mirror and a secondary mirror frame. The secondary mirror frame is cantilevered on the support structure via wing beams. There are at least three wing beams, which are evenly distributed along the circumference of the secondary mirror frame, and each wing beam forms a tangential constraint with the secondary mirror frame. The support structure includes an upper connecting frame, a lower connecting frame, and at least six vertical support rods connecting the two. The upper connecting frame is connected to the main mirror frame, and the lower connecting frame is connected to the secondary mirror frame. The primary mirror and the secondary mirror are both made of low-expansion optical glass; the primary mirror frame, the secondary mirror frame, the upper connecting frame, the lower connecting frame, and the vertical support rod are all made of low-expansion metal; wherein the linear expansion coefficient of the low-expansion metal is no greater than 1.5 × 10⁻⁶. -6 / K, and the difference between the linear expansion coefficient of the low-expansion metallic material and the linear expansion coefficient of the low-expansion optical glass does not exceed 0.5×10. -6 / K.

[0006] In one embodiment, the low-expansion optical glass is fused silica glass, and the coefficient of linear expansion of the fused silica glass is 0.55 × 10⁻⁶. -6 / K; The low-expansion metallic material is Invar, and the coefficient of linear expansion of Invar is no greater than 1.0 × 10⁻⁶. -6 / K.

[0007] In one embodiment, the primary mirror and the secondary mirror constitute a coaxial reflective Galilean telescope system with a magnification of 10 to 20 times, a field of view ≥ ±0.05°, and an effective aperture of the primary mirror ≥ 280 mm. Both the primary mirror and the secondary mirror are quadric mirrors, and the absolute value of the aspheric coefficient of the quadric mirror is 0.95~1.05. Furthermore, the surface accuracy of both the primary mirror and the secondary mirror is better than λ / 60 (RMS value), where λ is the working wavelength.

[0008] In one embodiment, the elastic support element is a flexible pressure block with an elastic thin-walled structure. The wall thickness and notch size of the elastic thin-walled structure are matched with the residual aberration of the optical system so that the additional wavefront introduced by the controllable micro-deformation of the primary mirror under vertical gravity has the opposite sign to the residual aberration, and the ratio of the RMS value of the additional wavefront to the residual aberration is 0.8 to 1.2, thereby achieving self-compensation matching of the gravity-induced wavefront.

[0009] In one embodiment, the secondary mirror is fixed to the secondary mirror frame by adhesive, and the back of the secondary mirror frame is provided with an adhesive injection hole for controlling the thickness and uniformity of the adhesive layer; The wing beam consists of three beams, each of which is connected tangentially to the secondary mirror frame. This allows the radial displacement to be converted into torsion of the secondary mirror around the optical axis when the structure undergoes radial deformation due to temperature changes or gravity, thereby suppressing the tilt and lateral displacement of the secondary mirror.

[0010] In one embodiment, the system further includes a folding mirror assembly, which includes a folding mirror and a folding mirror frame. The folding mirror is installed between the primary mirror and the secondary mirror and on the side closer to the primary mirror. The angle between the reflecting surface of the folding mirror and the optical axis is 43°~47°, which is used to fold the vertically emitted parallel beam of light by 90° into a horizontal direction.

[0011] In one embodiment, a first trimming ring is provided between the lower connecting frame and the secondary mirror frame to adjust the axial spacing between the primary mirror and the secondary mirror; A second trimming ring is provided between the folding lens frame and the main lens frame to adjust the optical axis height of the system; the thickness processing accuracy of the first trimming ring and the second trimming ring is better than ±0.002mm.

[0012] In one embodiment, a dedicated vertical adjustment frame for vertical assembly is also included, the beam expander being detachably mounted on the vertical adjustment frame via a flange mounting surface; A vibration isolation pad is provided between the vertical adjustment frame and the beam expander; The bottom of the vertical debugging frame is provided with a debugging reflector mounting base for fixing and installing the debugging reflector. The surface accuracy of the debugging reflector is better than λ / 10.

[0013] A second aspect of this application provides a method for assembling and adjusting a large-aperture beam expander for vertical use, applied to a large-aperture beam expander for vertical use as described in the first aspect of this application. The method includes: Step A1: Install the beam expander as a whole on a dedicated vertical adjustment frame, ensuring that the optical axis of the beam expander remains vertical; Step A2: Fix the debugging reflector on the debugging reflector mounting base at the bottom of the vertical debugging frame; Step A3: Place the interferometer horizontally on one side of the beam expander's output port, aligning the optical axis of the interferometer with the horizontal output optical axis of the beam expander; Step A4: The test light emitted by the interferometer enters the beam expander along the optical path, returns along the original path after reaching the adjustment mirror, forming interference fringes, and measuring the two-way wavefront aberration of the measurement system; Step A5: Based on the two-way wavefront measurement data, decouple the independent aberration contribution terms of the primary mirror and the secondary mirror using Zernike polynomial decomposition and sensitivity matrix; Step A6: Based on the decoupling results, fine-tune the spatial positions of the primary mirror assembly and the secondary mirror assembly sequentially until the system aberrations meet the design requirements; Step A7: Remove the debugging reflector and take the beam expander off the vertical debugging frame to complete the vertical installation and adjustment.

[0014] A third aspect of this application provides a design method for a large-aperture beam expander used vertically, applied to a large-aperture beam expander used vertically as described in the first aspect of this application. The design method includes: Step B1: Determine the optical system parameters, including beam expansion magnification, design working spectrum, effective aperture of the incident beam, field of view, and operating temperature range; Step B2: Optical design is carried out using a coaxial reflective optical path structure to obtain the surface shape, spacing, and material parameters of the primary mirror and the secondary mirror; Step B3: Design the mechanical structure. The overall arrangement is vertical, with the primary mirror on top and the secondary mirror below. Elastic support elements are used to provide elastic support for the primary mirror, and a cantilever structure with tangential constraints between the wing beam and the mirror frame is used to release the stress of the secondary mirror. Step B4: Perform optomechanical-thermal integration simulation analysis, establish a finite element model under vertical gravity field conditions, apply temperature load and gravity load within the set operating temperature range, extract the surface shape changes of the primary mirror and the secondary mirror and the axial spacing changes between them, and substitute them into the optical design software to evaluate the wavefront aberration of the system. Step B5: Optimize the primary and secondary mirror spacing tolerance and the support structure stiffness based on the simulation results of Step B4, so that the transmission wavefront aberration of the system in the vertical working state and the entire working temperature range meets the usage requirements. The usage requirements are that the transmission wavefront RMS value is better than λ / 20.

[0015] The beneficial effects of the technical solutions provided in this application include at least the following: The large-aperture beam expander for vertical use provided in this application includes: a primary mirror assembly, a secondary mirror assembly, and a support structure; the optical axis of the beam expander is arranged vertically, the primary mirror assembly is located at the upper part of the beam expander, and the secondary mirror assembly is located at the lower part of the beam expander; the primary mirror assembly includes a primary mirror, a primary mirror frame, and an elastic support element, the primary mirror being installed in the primary mirror frame via the elastic support element, the elastic support element being used to provide axial preload and control the micro-deformation of the primary mirror under gravity; the secondary mirror assembly includes a secondary mirror and a secondary mirror frame, the secondary mirror frame being cantilevered to the support structure via a wing beam. Structurally, there are at least three wing spars, evenly distributed along the circumference of the secondary mirror frame, with each wing spars forming a tangential constraint with the secondary mirror frame. The supporting structure includes an upper connecting frame, a lower connecting frame, and at least six vertical support rods connecting the two. The upper connecting frame is connected to the primary mirror frame, and the lower connecting frame is connected to the secondary mirror frame. The mirror blanks of both the primary and secondary mirrors are made of low-expansion optical glass. The primary mirror frame, secondary mirror frame, upper connecting frame, lower connecting frame, and vertical support rods are all made of low-expansion metal materials; wherein, the linear expansion coefficient of the low-expansion metal material is no greater than 1.5 × 10⁻⁶. -6 / K, and the difference between the linear expansion coefficient of the low-expansion metallic material and the linear expansion coefficient of the low-expansion optical glass does not exceed 0.5×10. -6 / K.

[0016] This application arranges the optical axis of the beam expander vertically, with the primary mirror assembly at the top and the secondary mirror assembly at the bottom. By incorporating an elastic support element within the primary mirror assembly, the primary mirror undergoes controllable micro-deformation under vertical gravity. The additional wavefront introduced by this micro-deformation has the opposite sign and similar magnitude to the residual aberrations of the optical system, thus achieving self-compensation matching of the gravity-induced wavefront. Compared to existing schemes where horizontally placed beam expanders are directly mounted vertically, leading to a sharp deterioration of the wavefront due to changes in the direction of gravity, this application fully utilizes rather than counteracts vertical gravity, transforming gravity-induced deformation into a "beneficial" aberration compensation source, significantly reducing the impact of gravity on the transmitted wavefront under vertical operating conditions.

[0017] This application employs at least three circumferentially evenly distributed wing beams in the secondary mirror assembly, cantilevering the secondary mirror frame onto a supporting structure, with each wing beam forming a tangential constraint with respect to the secondary mirror frame. When temperature changes or gravity causes radial deformation of the structure, this tangential connection structure converts the radial displacement into a slight torsion of the secondary mirror around its optical axis, rather than tilting or lateral displacement, thereby effectively suppressing changes in the spatial azimuth angle of the secondary mirror in its vertical operating state. Compared to existing cantilever support structures that are prone to tilting and displacement under gravity, this application significantly reduces the impact on the system wavefront through a torsional energy release mechanism.

[0018] This application limits the linear expansion coefficient of low-expansion metallic materials to no more than 1.5 × 10⁻⁶. -6 / K, and the difference in the coefficient of linear expansion between the low-expansion metallic material and the low-expansion optical glass does not exceed 0.5×10.-6 / K achieves thermal expansion matching between optical components and structural parts over a wide temperature range. The difference in thermal expansion between the two is minimal within the operating temperature range of -40°C to +60°C. Combined with a reasonable structural gap design, the anechoic design requirements can be met without active temperature control, eliminating thermally induced wavefront distortion caused by temperature gradients and ensuring stable output of the beam expander in all weather conditions.

[0019] This application systematically solves the problem of optical performance degradation caused by gravity-induced and thermally-induced wavefront distortion in large-aperture beam expanders under vertical use by using a synergistic design of vertical layout, primary mirror gravity self-compensation support, secondary mirror tangential constraint spar, and material thermal matching. This enables the transmission wavefront aberration of the beam expander under vertical working conditions to meet the stringent requirements of vertical application scenarios such as high-power laser systems, space optical communication, and ground-based laser measurement. Attached Figure Description

[0020] Figure 1 A system schematic diagram of a large-diameter beam expander for vertical use provided in an embodiment of this application; Figure 2 This is a schematic diagram of the structure of a primary mirror assembly provided in an embodiment of this application; Figure 3 This is a schematic diagram of the structure of a secondary mirror assembly provided in an embodiment of this application; Figure 4 This is a schematic diagram of a support structure provided in an embodiment of this application; Figure 5 This is a schematic diagram of the structure of a folding mirror assembly provided in an embodiment of this application; Figure 6 An assembly diagram of a beam expander and a debugging component provided for an embodiment of this application; Figure 7 A flowchart illustrating a method for assembling and adjusting a large-diameter beam expander used vertically, as provided in an embodiment of this application; Figure 8 A flowchart illustrating a design method for a large-diameter beam expander used vertically, as provided in an embodiment of this application.

[0021] Figure label: 1-Primary mirror assembly; 2-Secondary mirror assembly; 3-Folding mirror assembly; 4-Support structure; 5-First trimming ring; 6-Second trimming ring; 101-Primary mirror; 102-Primary mirror frame; 103-Flexible pressure block; 104-Adhesive block; 201-Secondary mirror; 202-Secondary mirror frame; 301-Folding mirror; 302-Folding mirror frame; 401-Upper connecting frame; 402-Lower connecting frame; 403-Vertical support rod; 701-Vertical adjustment frame; 702-Vibration isolation pad; 703-Adjustment reflector; 704-Adjustment reflector mounting base. Detailed Implementation

[0022] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application.

[0023] Hereinafter, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of embodiments of this disclosure, unless otherwise stated, "a plurality of" means two or more.

[0024] In addition, the use of “based on” or “according to” implies openness and inclusivity, because processes, steps, calculations or other actions “based on” or “according to” one or more conditions or values ​​can in practice be based on additional conditions or values ​​beyond those conditions.

[0025] This application provides an embodiment of a large-diameter beam expander for vertical use, such as... Figure 1 As shown, the beam expander includes a primary mirror assembly 1, a secondary mirror assembly 2, and a support structure 4. The optical axis of the beam expander is arranged vertically, with the primary mirror assembly 1 located at the top and the secondary mirror assembly 2 located at the bottom. This vertical arrangement of the primary mirror on top and the secondary mirror on the bottom is an overall structural design for vertical use of the beam expander, fundamentally different from the existing horizontally placed beam expander layout.

[0026] like Figure 2 As shown, the primary mirror assembly 1 includes a primary mirror 101, a primary mirror frame 102, and an elastic support element. The primary mirror 101 is mounted within the primary mirror frame 102 via the elastic support element, which provides axial preload and controls the micro-deformation of the primary mirror 101 under gravity. The primary mirror 101 is the largest aperture optical element in the beam expander. In vertical operation, the direction of gravity is parallel to the optical axis, significantly increasing the axial gravity component borne by the primary mirror 101. If a traditional rigid support method is used, the primary mirror 101 will experience asymmetrical axial concavity or convexity under its own weight, severely affecting the wavefront quality of the system. By setting the elastic support element, the primary mirror 101 can produce controllable micro-deformation under vertical gravity, providing a structural basis for subsequent gravity self-compensation.

[0027] like Figure 3As shown, the secondary mirror assembly 2 includes a secondary mirror 201 and a secondary mirror frame 202. The secondary mirror frame 202 is cantilevered onto the support structure 4 via wing beams 203. There are at least three wing beams 203, evenly distributed circumferentially along the secondary mirror frame 202, and each wing beam 203 forms a tangential constraint with the secondary mirror frame 202. In the vertical operating state, the secondary mirror 201 is located below the beam expander, and its cantilever support structure is prone to tilting and lateral displacement under gravity. By setting at least three wing beams 203 evenly distributed circumferentially and forming a tangential constraint between each wing beam 203 and the secondary mirror frame 202, when radial deformation of the structure is caused by temperature changes or gravity, this tangential connection structure converts the radial displacement into a slight torsion of the secondary mirror 201 around the optical axis, rather than tilting or lateral translation, thereby effectively protecting the spatial azimuth angle of the secondary mirror 201.

[0028] like Figure 4 As shown, the support structure 4 includes an upper connecting frame 401, a lower connecting frame 402, and at least six vertical support rods 403 connecting the two. The upper connecting frame 401 is connected to the main mirror frame 102, and the lower connecting frame 402 is connected to the secondary mirror frame 202. This cage-like support structure 4, while ensuring axial stiffness, provides sufficient dynamic stiffness for the entire machine by rationally designing the cross-sectional shape and distribution of the vertical support rods 403, thus adapting to the bearing requirements of the vertical gravity field.

[0029] The primary mirror 101 and the secondary mirror 201 are both made of low-expansion optical glass; the primary mirror frame 102, the secondary mirror frame 202, the upper connecting frame 401, the lower connecting frame 402, and the vertical support rod 403 are all made of low-expansion metal. The linear expansion coefficient of the low-expansion metal is no greater than 1.5 × 10⁻⁶. -6 / K, and the difference between the linear expansion coefficient of the low-expansion metallic material and the linear expansion coefficient of the low-expansion optical glass does not exceed 0.5×10. -6 / K. Through the above material matching, thermal expansion matching between optical components and structural parts is achieved over a wide temperature range, which can meet the requirements of athermal design without active temperature control and eliminate thermally induced wavefront distortion caused by temperature gradients.

[0030] This application systematically solves the problem of optical performance degradation caused by gravity-induced and thermally-induced wavefront distortion in large-aperture beam expanders under vertical operation by designing the beam expander as a whole with the primary mirror above and the secondary mirror below, in conjunction with elastic support elements, tangentially constrained spars, and low-expansion material matching. The primary mirror achieves controllable micro-deformation under gravity through elastic support elements, providing a structural basis for gravity self-compensation; the secondary mirror implements a "torsional energy release" mechanism through tangentially constrained spars, effectively suppressing tilt and lateral offset; and low-expansion material matching ensures athermal operation over a wide temperature range. The synergistic effect of these three elements enables the beam expander to meet the stringent requirements of vertical applications such as high-power laser systems, space optical communication, and ground-based laser measurement in vertical operation.

[0031] Optionally, the low-expansion optical glass is fused silica glass, and the coefficient of linear expansion of the fused silica glass is 0.55 × 10⁻⁶. -6 Fused silica glass (such as JGS1 grade) possesses excellent optical uniformity, good light transmission, and an extremely low coefficient of thermal expansion, making it an ideal optical mirror blank material for high-power laser systems and precision optical measurement. The primary mirror 101 and secondary mirror 201 utilize fused silica glass blanks, which exhibit minimal surface shape changes with temperature variations, thus contributing to stable optical performance across the entire operating temperature range.

[0032] The low-expansion metallic material is Invar, and the coefficient of linear expansion of Invar is no greater than 1.0 × 10⁻⁶. -6 Invar steel (such as grades 4J32 and 4J36) is an iron-nickel alloy widely used in precision instruments and optical mechanical structures due to its extremely low coefficient of thermal expansion. The main frame 102, secondary frame 202, upper connecting frame 401, lower connecting frame 402, and vertical support rod 403 are all made of Invar steel, resulting in minimal dimensional changes in the structural components with temperature variations.

[0033] The coefficient of linear expansion of fused silica glass is approximately 0.55 × 10⁻⁶. -6 / K, the linear expansion coefficient of Invar steel is no greater than 1.0×10 -6 The thermal expansion difference between the two materials is minimal over a wide temperature range. Within the operating temperature range of -40°C to +60°C, the relative deformation caused by temperature changes is extremely low. Combined with a reasonable structural gap design and flexible connection method, this meets the requirements of a thermal-free reflective design. Athermal-free design refers to maintaining stable performance of the optical system within a set operating temperature range through material matching and structural design without relying on active temperature control measures. This application utilizes a material combination of fused silica glass and Invar, whose thermal expansion coefficients are highly matched, enabling the beam expander to maintain stable wavefront output without active temperature control under all-weather environmental temperature variations.

[0034] Optionally, the primary mirror 101 and the secondary mirror 201 constitute a coaxial reflective Galilean telescope system. The Galilean telescope system consists of a primary mirror with positive optical power and a secondary mirror with negative optical power. After the parallel beam is converged by the primary mirror 101, it is intercepted and collimated by the secondary mirror 201 before reaching the focal point, resulting in a reduced-aperture parallel beam output, thus achieving beam expansion. This application adopts a coaxial reflective structure, where the optical axes of the primary mirror 101 and the secondary mirror 201 coincide, avoiding the asymmetric aberrations introduced by the eccentricity and tilt of optical elements in off-axis reflective structures, which is beneficial for obtaining higher wavefront quality. The beam expansion magnification of this coaxial reflective Galilean telescope system is 10 to 20 times, preferably 15 times. The beam expansion magnification determines the proportion of beam aperture transformation; the higher the magnification, the greater the reduction in the output beam aperture relative to the incident beam aperture. The field of view is ≥ ±0.05°, ensuring good image quality of the beam expander within a certain field of view. The effective aperture of the primary mirror 101 is ≥280mm, meaning the effective imaging area of ​​the primary mirror 101 is not less than 280mm. The large-aperture primary mirror 101 is fundamental to achieving high resolution and high energy transmission capabilities, but it also places higher demands on the supporting structure.

[0035] Optionally, both the primary mirror 101 and the secondary mirror 201 are quadric mirrors. The absolute value of the aspheric coefficient of the quadric mirror is 0.95 to 1.05. The surface accuracy of both the primary mirror 101 and the secondary mirror 201 is better than λ / 60 (RMS value), where λ is the operating wavelength. Taking an operating wavelength of λ = 632.8 nm (red light from a helium-neon laser) as an example, λ / 60 ≈ 10.5 nm, meaning that the root mean square value of the deviation of the surfaces of the primary mirror 101 and the secondary mirror 201 from the ideal quadric surface does not exceed 10.5 nm.

[0036] Optionally, the elastic support element is a flexible pressure block 103, which has an elastic thin-walled structure. The flexible pressure block 103 is disposed on the light-incident end face of the main lens frame 102. Its elastic thin-walled structure, through a reasonable design of wall thickness and notch size, provides axial preload while allowing the main lens 101 to undergo controllable micro-deformation under gravity. Preferably, three adhesive blocks 104 are disposed on the back of the main lens 101, evenly distributed along the circumference of the main lens 101. The main lens 101 is fixedly connected to the main lens frame 102 through the adhesive blocks 104. The flexible pressure block 103 applies axial preload from the front end of the main lens 101, and together with the back adhesive blocks 104, constitutes the elastic support system of the main lens 101.

[0037] The wall thickness and notch size of the elastic thin-walled structure are matched to the residual aberrations of the optical system. Residual aberrations refer to the aberrations that remain after the theoretical design of the optical system, which cannot be eliminated through design due to limitations imposed by the optimization algorithms of optical design software and trade-offs in design specifications. The specific distribution and magnitude of residual aberrations can be obtained using optical design software (such as Zemax, Code V, etc.), and are usually expressed in the form of Zernike polynomials. In vertical use, the direction of gravity is parallel to the optical axis, and the axial gravitational component experienced by the primary mirror 101 is the main factor causing wavefront distortion. This application differs from the traditional design approach of suppressing gravitational deformation as much as possible; instead, it actively utilizes gravitational deformation, making it a "beneficial" source of aberration compensation.

[0038] See Figure 2 The elastic deformation field generated by the primary mirror 101 under the action of gravitational load is: The additional wavefront it introduces The relationship with the amount of deformation is as follows: (1) in, For normalized radial coordinates, It is the azimuth angle. Let be the angle of incidence of the light ray relative to the normal to the mirror. For the primary mirror in a coaxial reflection system, the light ray is incident approximately along the normal. ,therefore .

[0039] Let the residual aberration of the optical system be... The conjugate self-compensating matching condition requires: (2) Where k is a scaling factor close to 1. At this point, the total wavefront of the system... Significant suppression. The physical meaning of the scaling factor k is that the gravity-induced additional wavefront does not need to be exactly equal to the residual aberration; they only need to be similar in magnitude but opposite in sign to achieve significant cancellation. In this embodiment, the ratio of the RMS value of the additional wavefront to the residual aberration is 0.8 to 1.2, that is, the amplitude of the gravity-induced additional wavefront is 80% to 120% of the amplitude of the residual aberration. This ratio range ensures a good cancellation effect between the gravity-induced additional wavefront and the residual aberration, while also allowing for reasonable engineering tolerance.

[0040] To achieve the aforementioned self-compensating matching, this embodiment employs the finite element iterative optimization method to determine the geometric parameters (wall thickness and notch size) of the flexible pressure block 103. The specific steps are as follows: First, obtain the residual aberration distribution of the optical system in optical design software. The Zernike coefficients are used to represent this. Then, a parametric model of the primary mirror 101 and the flexible pressure block 103 is established in the finite element software. The wall thickness and notch size of the flexible pressure block 103 are used as design variables. Iterative optimization is performed with the goal of minimizing the sum of residual aberration and gravity-added wavefront. The optimization objective function is: (3) That is, convergence occurs when the change in wavefront RMS between two consecutive iterations is less than a set threshold, yielding the optimal wall thickness and notch size. Optimization results show that when the flexible block 103 is within a specific range of geometric parameters, gravity-induced additional wavefront... The main aberration terms and The results show a high degree of agreement, with the total wavefront RMS value of the system significantly reduced compared to before compensation. This principle enables the controllable micro-deformation of the primary mirror 101 under vertical gravity to become a beneficial source of aberration compensation, rather than simply a source of error.

[0041] This application utilizes the elastic thin-walled structure of the flexible pressure block 103 to actively utilize vertical gravity, enabling the controllable micro-deformation of the primary mirror 101 under gravity to cancel out the residual aberrations of the optical system, thus achieving self-compensation matching of the gravity-induced wavefront. This self-compensation mechanism requires no additional optical components or active control system; it significantly reduces the impact of vertical gravity on the system wavefront solely through passive structural design, offering outstanding advantages such as simple structure, low cost, and high reliability.

[0042] Optionally, the secondary lens 201 is fixed to the secondary lens frame 202 by adhesive. Adhesive fixing is a common method in optical lens installation, offering advantages such as uniform stress distribution, no need for additional mechanical fasteners, and no obstruction of the effective light transmission aperture. The secondary lens frame 202 has an adhesive injection hole on its back, allowing adhesive to be injected from the back of the frame, avoiding the risk of adhesive overflow and contamination of the lens surface when injected from the front. Simultaneously, by controlling the amount and pressure of adhesive injection, the thickness and uniformity of the adhesive layer can be precisely controlled, preventing changes in the surface shape of the secondary lens 201 caused by uneven adhesive layer thickness or stress concentration due to curing shrinkage.

[0043] There are three wing beams 203, each of which is tangentially connected to the secondary mirror frame 202. The three wing beams 203 are evenly distributed along the circumference of the secondary mirror frame 202 (i.e., one beam is set every 120°), and each wing beam 203 is tangential to the outer circle of the secondary mirror frame 202, forming a tangential constraint. One end of the wing beam 203 is fixedly connected to the secondary mirror frame 202 (preferably integrally formed or welded), and the other end is fixedly connected to the lower connecting frame 402, thereby realizing the cantilever installation of the secondary mirror frame 202 on the support structure 4 through the wing beams 203. The core function of the tangential connection between each wing beam 203 and the secondary mirror frame 202 is to convert the radial displacement into the torsion of the secondary mirror 201 around the optical axis when the radial deformation of the structure is caused by temperature changes or gravity, thus suppressing the tilt and lateral displacement of the secondary mirror 201.

[0044] The specific mechanism is as follows: For the cantilevered secondary mirror 201, radial thermal deformation or gravitational shear effect will cause the mirror surface to tilt and translate. In this embodiment, the tangential stiffness of the spar is assumed to be... Radial stiffness is And achieved through structural design When the frame undergoes radial displacement Time (e.g., due to temperature changes) cause, ,in is the coefficient of linear expansion of the material. (where the radius is the mirror frame radius), the tangential constraint allows the secondary mirror to produce a small twist angle about the optical axis. Meanwhile, the tilting and translation of the mirror surface are forcibly suppressed.

[0045] Torsional-translational decoupling ratio It can be defined as: (4) in, The angle of twist of the secondary mirror about the optical axis. The radius of the frame, This refers to the tangential displacement at the edge caused by torsion. This represents the lateral translational displacement of the center point of the mirror surface. The physical meaning of this ratio is: under the same radial deformation input, The larger the value, the more the deformation tends to translate into torsion about the optical axis rather than lateral translation. When At this time, radial deformation is mainly converted into torsion, and the lateral translation of the mirror center is significantly suppressed.

[0046] By rationally designing the geometric parameters of the wing beam (length L, cross-sectional shape, connection method), the tangential stiffness can be increased. Much smaller than radial stiffness Calculation results show that the tangential structure can efficiently convert radial deformation into torsion, while significantly suppressing tilting and translation. This mechanism effectively protects the spatial azimuth of the secondary mirror and significantly reduces its impact on the system wavefront.

[0047] Optional, such as Figure 5 As shown, the beam expander in this embodiment also includes a folding mirror assembly 3. The folding mirror assembly 3 includes a folding mirror 301 and a folding mirror frame 302. The folding mirror 301 is installed inside the folding mirror frame 302, and the folding mirror frame 302 is preferably fixedly connected to the main mirror frame 102 or the upper connecting frame 401 to ensure the spatial stability of the folding mirror 301.

[0048] The folding mirror 301 is installed between the primary mirror 101 and the secondary mirror 201, and closer to the primary mirror 101. In other words, along the optical axis, from top to bottom, the primary mirror 101, folding mirror 301, and secondary mirror 201 are arranged in sequence. The folding mirror 301 is located after the primary mirror 101 and before the secondary mirror 201, that is, the folding mirror 301 is placed in the optical path reflected and converged by the secondary mirror 201, turning the optical path before the light beam reaches the primary mirror 101. This position choice allows for a relatively small aperture of the folding mirror 301 (the light beam is compressed and collimated by the secondary mirror 201, resulting in parallel light), which helps to reduce the manufacturing difficulty and cost of the folding mirror 301.

[0049] The angle between the reflecting surface of the folding mirror 301 and the optical axis is 43°~47°. When the angle between the reflecting surface of the folding mirror 301 and the optical axis is 45°, the incident light changes its propagation direction by 90° after reflection. In this embodiment, the angle is set to 43°~47°, providing a reasonable engineering tolerance range and reducing the difficulty of assembling and adjusting the folding mirror 301. The folding mirror 301 is used to fold the vertically emitted parallel beam by 90° to make it horizontal. Under normal operating conditions of the beam expander, the parallel beam expanded by the primary mirror 101 and the secondary mirror 201 propagates vertically, and after being reflected by the folding mirror 301, it becomes horizontally emitted, which facilitates docking with horizontally arranged back-end optical systems (such as spectrometers, detectors, power meters, etc.).

[0050] Among them, the folding mirror 301 is preferably a plane mirror. The surface accuracy of a plane mirror is easy to ensure, its manufacturing and assembly are simple, and it does not introduce additional optical power or chromatic aberration. The surface accuracy of the folding mirror 301 is preferably better than λ / 10 to avoid the folding mirror 301 itself introducing significant wavefront errors.

[0051] Optionally, a first trimming ring 5 is provided between the lower connecting frame 402 and the secondary mirror frame 202. The first trimming ring 5 is an annular washer structure, disposed between the mating surfaces of the lower connecting frame 402 and the secondary mirror frame 202. By replacing the first trimming ring 5 with different axial thicknesses, the axial position of the secondary mirror frame 202 relative to the lower connecting frame 402 can be changed, thereby adjusting the axial spacing between the primary mirror 101 and the secondary mirror 201. The axial spacing between the primary mirror 101 and the secondary mirror 201 is a key parameter determining the magnification and aberration correction state of the beam expander, and its deviation directly affects the wavefront quality of the system. In this embodiment, the first trimming ring 5 enables precise adjustment of the axial spacing between the primary and secondary mirrors.

[0052] A second trimming ring 6 is provided between the folding mirror frame 302 and the main mirror frame 102. The second trimming ring 6 is also a ring-shaped washer structure, positioned between the mating surfaces of the folding mirror frame 302 and the main mirror frame 102. By replacing the second trimming ring 6 with different axial thicknesses, the axial position of the folding mirror frame 302 relative to the main mirror frame 102 can be changed, thereby adjusting the height of the reflecting surface of the folding mirror 301, i.e., adjusting the optical axis height of the system. The optical axis height of the folding mirror 301 needs to be precisely aligned with the optical axis height of the rear optical system; excessive deviation will lead to decreased beam coupling efficiency and increased aberrations.

[0053] The thickness processing accuracy of the first trimming ring 5 and the second trimming ring 6 is better than ±0.002mm. This means that the deviation of the axial thickness of each trimming ring from its nominal value does not exceed 2 micrometers, and the corresponding primary and secondary mirror spacing adjustment accuracy and optical axis height adjustment accuracy both reach the micrometer level. To achieve this high precision, the trimming rings are preferably processed by precision grinding or lapping, and their actual thickness is measured using a laser interferometer or a high-precision length measuring instrument under constant temperature conditions. During the assembly and adjustment process, by pre-preparing a set of trimming rings with thicknesses distributed in an arithmetic sequence (e.g., one specification every 0.002mm), the assembly and adjustment personnel can select trimming rings of appropriate thickness for installation based on the measurement results, thereby achieving accurate compensation for system aberrations.

[0054] Optional, such as Figure 6 As shown, the beam expander also includes a commissioning assembly for vertical adjustment. The commissioning assembly includes a dedicated vertical commissioning frame 701, a vibration isolation layer 702, a commissioning reflector 703, and a commissioning reflector mounting base 704. The vertical commissioning frame 701 is a rigid frame structure with sufficient structural stiffness and stability to provide precise vertical positioning and support for the beam expander during the commissioning process.

[0055] The beam expander is detachably mounted on the vertical adjustment frame 701 via a flange mounting surface. The beam expander's support structure 4 (specifically, the upper connecting frame 401 or the lower connecting frame 402) has a flange mounting surface, and the vertical adjustment frame 701 has a corresponding flange mating surface. The two are detachably fixed together by screws or bolts. This detachable connection allows the beam expander to be removed from the vertical adjustment frame 701 after assembly and adjustment and installed in the final use environment. The vertical adjustment frame 701 can be reused for the assembly and adjustment of multiple beam expanders.

[0056] A vibration isolation pad 702 is provided between the vertical adjustment frame 701 and the beam expander. The vibration isolation pad 702 is located between the vertical adjustment frame 701 and the flange mounting surface of the beam expander, and is made of vibration isolation rubber, cork, or composite vibration isolation material. The vibration isolation pad 702 is used to reduce the influence of environmental vibrations (such as ground vibration, air disturbance, personnel movement, etc.) on the interference measurement results during the assembly and adjustment process, and to ensure the stability and repeatability of the measurement data.

[0057] The bottom of the vertical calibration frame 701 is provided with a calibration reflector mounting base 704 for fixing the calibration reflector 703. The calibration reflector mounting base 704 is located at the center of the bottom of the vertical calibration frame 701, and its position is coaxial with the optical axis of the beam expander. After the calibration reflector 703 is fixedly installed on the calibration reflector mounting base 704, its reflective surface is perpendicular to the optical axis of the beam expander, and it is used to reflect the test light propagating vertically back along the original path to form a folded self-calibration optical path.

[0058] The surface shape accuracy of the adjustment mirror 703 is better than λ / 10. The adjustment mirror 703 is a reference element in the assembly and measurement optical path, and its surface shape error is directly added to the measurement result. A surface shape accuracy better than λ / 10 ensures that the adjustment mirror 703 itself does not introduce significant measurement errors, guaranteeing the accuracy of the assembly and measurement. The adjustment mirror 703 is preferably a plane mirror with a high-reflectivity coating on its reflective surface to achieve high reflectivity within the test light wavelength range.

[0059] The large-aperture beam expander for vertical use provided in this application is designed with an overall vertical arrangement for vertical applications. Through optomechanical-thermal integration simulation optimization under a vertical gravity field, gravity-induced wavefront distortion in the vertical state is significantly reduced. Furthermore, this application includes a dedicated vertical adjustment frame, utilizing the beam expander's own folding mirror to bend the optical path to a horizontal direction, achieving interference adjustment in the vertical state and solving the problem of difficult assembly and adjustment of the vertical beam expander. The secondary mirror frame of this application adopts a cantilever structure with multiple wing beams tangent to the frame and a "torsional energy release" mechanism, which effectively reduces tilting deformation caused by its own weight and temperature changes during vertical use. The primary mirror clamp adopts a flexible structure and gravity self-compensation design, reducing the impact of gravity and assembly stress on the surface shape.

[0060] The method for assembling and adjusting a large-aperture beam expander used vertically provided in this embodiment is based on constructing a folding self-calibrating optical path using the beam expander's own folding mirror 301 and the adjustment mirror 703 at the bottom of the vertical adjustment frame 701. It also decouples the independent aberration contributions of the primary mirror 101 and the secondary mirror 201 based on two-way wavefront measurement data, achieving precise assembly and adjustment. Figure 7 As shown, the assembly and adjustment method includes the following steps: Step A1: Install the beam expander as a whole on the dedicated vertical adjustment frame 701, so that the optical axis of the beam expander remains vertical.

[0061] Specifically, the beam expander flange mounting surface is aligned with the flange mating surface of the vertical adjustment frame 701 and secured with screws or bolts, with the vibration isolation pad 702 positioned between them. After installation, the optical axis of the beam expander coincides with the vertical axis of the vertical adjustment frame 701, ensuring that the optical axis is perpendicular to the horizontal plane.

[0062] Step A2: Fix the debugging mirror 703 on the debugging mirror mounting base 704 at the bottom of the vertical debugging frame 701.

[0063] The reflecting surface of the adjustment mirror 703 is perpendicular to the optical axis of the beam expander, and its surface shape accuracy is better than λ / 10 to ensure that no significant measurement error is introduced. After the adjustment mirror 703 is fixed, it should be initially adjusted so that the perpendicularity of its reflecting surface to the optical axis is better than the specified value.

[0064] Step A3: Place the interferometer 8 horizontally on one side of the beam expander's output port, aligning the optical axis of the interferometer 8 with the horizontal output optical axis of the beam expander.

[0065] Since the beam expander refracts the vertical beam into a horizontal direction via the folding mirror 301 during normal operation, the interferometer 8 can receive the emitted beam simply by placing it horizontally. The interferometer 8 is preferably a laser interferometer (such as a Zygo or 4D interferometer), which has high-precision wavefront measurement capabilities.

[0066] Step A4: The test light emitted by the interferometer 8 enters the beam expander along the optical path, reaches the debugging mirror 703, and returns along the original path to form interference fringes, thus measuring the two-way wavefront aberration of the measurement system.

[0067] The specific optical path is as follows: The collimated test light emitted by interferometer 8 propagates horizontally, is reflected by folding mirror 301 and then becomes vertically propagating. It propagates downwards along the beam expander's optical axis, passing sequentially through secondary mirror 201 and primary mirror 101 before reaching adjustment mirror 703. After being reflected by adjustment mirror 703, it returns along the original path, passing through primary mirror 101 and secondary mirror 201 again before reaching folding mirror 301. After being reflected by folding mirror 301, it becomes horizontal again and returns to interferometer 8, forming interference fringes with the reference light of interferometer 8. After acquiring the interference fringes, interferometer 8 calculates the two-way wavefront aberration using Fourier transform or phase-shifting algorithms. .

[0068] Step A5: Based on the two-way wavefront measurement data, the independent aberration contribution terms of the primary mirror 101 and the secondary mirror 201 are decoupled using Zernike polynomial decomposition and sensitivity matrix.

[0069] Two-way wavefront measured by interferometer The relationship between the aberrations of each optical element and the aberrations of the components is as follows: (5) in: One-way aberration introduced by the primary mirror; One-way aberrations introduced by the secondary mirror; One-way aberration introduced by the folding mirror; Aberrations introduced by the two reflections of the folding mirror in its return path; : Adjusting the aberrations introduced by the reflecting mirror.

[0070] The aberrations of the folding mirror and the adjustment mirror can be pre-determined through offline calibration, denoted as... After subtracting it, we get: (6) To decouple and Pattern decomposition is performed using the orthogonality of Zernike polynomials: (7) Simultaneously, the sensitivity matrices of the primary and secondary mirrors for each unit Zernike mode are calculated in advance using optical design software. , of which elements Indicates the first of the primary mirrors The contribution of the Zernike coefficient variation to the system wavefront. The principal and secondary mirror difference coefficients can then be obtained by solving the system of linear equations: (8) in, The principal image difference coefficient vector. Let be the aberration coefficient vector of the secondary mirror. By solving the above system of linear equations, the independent aberration contribution terms of the primary mirror 101 and the secondary mirror 201 can be decoupled and separated.

[0071] Step A6: Based on the decoupling results, fine-tune the spatial positions of the primary mirror assembly 1 and the secondary mirror assembly 2 sequentially until the system aberrations meet the design requirements.

[0072] Specifically, based on the decoupled and Determine the source of aberration: If defocusing or spherical aberration exists, it is usually related to the axial spacing error of the primary and secondary lenses, and can be adjusted by replacing or regrinding the first trimming ring 5. If coma or eccentricity exists, it is usually related to the lateral offset or tilt of the primary mirror 101 or the secondary mirror 201, and can be corrected by adjusting the installation attitude of the primary mirror assembly 1 or the secondary mirror assembly 2. If astigmatism exists, it is usually caused by the asymmetric deformation or surface error of the mirror itself. It is necessary to check the preload uniformity of the elastic support element or replace the optical element. If higher-order aberrations account for a large proportion of the system aberrations, the tilt angle of the folding mirror 301 can be slightly adjusted to introduce a small amount of reverse aberrations for compensation.

[0073] Repeat steps A4 and A5 after each fine-tuning to verify the adjustment effect until the total wavefront aberration of the system meets the design requirements.

[0074] Step A7: Remove the debugging reflector 703 and remove the beam expander from the vertical debugging frame 701 to complete the vertical installation and adjustment.

[0075] After removing the debugging mirror 703, the internal optical path of the beam expander returns to normal operation. The beam expander can then be removed from the vertical debugging frame 701 and installed in the final use environment for use.

[0076] This application utilizes the beam expander's own folding mirror 301 and the bottom-fixed adjustment mirror 703 to construct a folding self-calibration optical path. The interferometer 8 can be placed horizontally to realize the assembly and adjustment measurement of the vertical beam expander, solving the problem of the difficulty in assembling and adjusting the interferometer when the vertical beam expander cannot be directly placed along the optical axis. Based on the aberration decoupling method of Zernike polynomial and sensitivity matrix, the independent aberration contributions of the primary mirror 101 and the secondary mirror 201 can be accurately separated, guiding the direction and amplitude of the assembly and adjustment operation and avoiding blind adjustment. The selection and replacement of the trimming ring realizes the precise adjustment of the primary and secondary mirror interval, enabling the system wavefront to converge quickly to the design requirements. The vibration isolation pad 702 ensures the stability and repeatability of the measurement data.

[0077] This application also provides a design method for a large-aperture beam expander used vertically. This method organically combines optical design, mechanical structure design, and finite element simulation analysis. Under the coupled effects of a vertical gravity field and temperature field, closed-loop optimization of the system wavefront is performed to ensure that the beam expander meets usage requirements in vertical operation and across the entire operating temperature range. Figure 8 As shown, the design method includes the following steps: Step B1: Determine the optical system parameters, including beam expansion ratio, design working spectrum, effective aperture of incident beam, field of view, and operating temperature range.

[0078] The beam expansion ratio determines the proportion of beam aperture transformation, which is set to 10-20 times in this embodiment, preferably 15 times. The design covers the visible to near-infrared band (e.g., 400nm-1100nm) to meet the needs of different laser sources and detectors. The effective aperture of the incident beam is ≥280mm, meaning the diameter of the parallel beam incident on the primary mirror 101 is not less than 280mm. The field of view is ≥±0.05° to ensure good image quality within a certain field of view. The operating temperature range covers -40°C to +60°C, meeting the environmental temperature requirements of most ground and airborne applications. The transmission wavefront index has an RMS value better than λ / 20.

[0079] Step B2: Optical design is carried out using a coaxial reflective optical path structure to obtain the surface shape, spacing, and material parameters of the primary mirror 101 and the secondary mirror 201.

[0080] The optical design employs a coaxial reflective Galilean optical path structure. Both the primary and secondary mirrors are designed as quadratic mirrors with aspheric coefficients close to 1. The curvature radii, spacing, and aspheric coefficients of the primary and secondary mirrors are optimized to minimize theoretical wavefront aberrations. The residual aberration distribution of the optical system is then obtained. This serves as input for subsequent mechanical structure optimization.

[0081] Step B3: Design the mechanical structure. The overall arrangement is vertical, with the primary mirror 101 on top and the secondary mirror 201 on the bottom. Elastic support elements are used to provide elastic support for the primary mirror 101, and a cantilever structure with wing beam 203 forming a tangential constraint with the mirror frame is used to release the stress of the secondary mirror 201.

[0082] Specifically, a three-dimensional model is performed based on the structure of a vertically used large-aperture beam expander provided in the embodiments of this application. The stiffness distribution of the primary mirror flexible pressure block is optimized based on the aforementioned conjugate self-compensation principle, so that... The geometric parameters of the secondary mirror spar were designed based on the torsional-translational decoupling mechanism, enabling... Ensure torsional-translational decoupling ratio At the same time, a cage-type support structure was designed to ensure that the whole machine has sufficient dynamic stiffness.

[0083] Step B4: Perform optomechanical-thermal integration simulation analysis. Establish a finite element model under vertical gravity field conditions. Apply temperature load and gravity load within the set operating temperature range. Extract the surface shape changes of the primary mirror 101 and the secondary mirror 201 and the axial spacing changes between them. Substitute these data into the optical design software to evaluate the wavefront aberration of the system.

[0084] Treating the JGS1 fused silica optical glass and Invar support structure as linear elastic materials, their constitutive relations describe the relationship between stress, strain, and temperature changes: (9) in, For stress tensor, For strain tensor, For the elasticity matrix, The coefficient of linear expansion is 1 / 3. This represents the change in temperature. The coefficient of linear expansion of JGS1 is 0.55 × 10⁻⁶. -6 / K, the linear expansion coefficient of Invar steel is no greater than 1.0×10 -6 / K, the difference in thermal expansion between the two is minimal over a wide temperature range, which can meet the requirements of reflective calorimetric design.

[0085] Step B5: Optimize the primary and secondary mirror spacing tolerance and the support structure stiffness based on the simulation results of Step B4, so that the transmission wavefront aberration of the system in the vertical working state and the entire working temperature range meets the usage requirements. The usage requirements are that the transmission wavefront RMS value is better than λ / 20.

[0086] The specific implementation method for this step is as follows: B5a: Create a model in finite element software and apply a vertical gravity load ( Multiple temperature loads (along the optical axis) and within the operating temperature range are used to extract rigid body displacement and higher-order surface distortion of the primary and secondary mirror surfaces.

[0087] B5b: Fitting the nodal displacements of the finite element method to Zernike polynomial coefficients (Z1~Z37) yields the gravity-induced wavefront. and thermal wavefront Zernike stated.

[0088] B5c: Import Zernike coefficients into optical design software, construct a virtual assembly, and calculate the system wavefront aberrations. The total system wavefront is: (10) B5d: Adjust the structural parameters in reverse according to the optical sensitivity matrix to form a closed-loop iteration. Establish the optimization objective function: (11) The convergence criterion is that the change in wavefront RMS between two consecutive iterations is less than λ / 100.

[0089] Step B6: Tolerance Optimization Based on simulation results, key tolerances were established: primary and secondary mirror eccentricity better than 0.01 mm, primary and secondary mirror center spacing deviation better than 0.005 mm, and primary mirror surface shape better than λ / 10 (PV). Monte Carlo tolerance analysis was performed to verify that the assembly yield rate within the given tolerance range was not less than 90%, ensuring that the transmission wavefront aberration of the system met the usage requirements in vertical operating conditions and across the entire operating temperature range.

[0090] Existing beam expander designs typically separate optical design, mechanical structure design, and simulation analysis into independent steps. The optical design stage neglects the influence of gravitational and temperature fields on the mirror surface shape, and the mechanical design stage fails to compensate for residual optical aberrations. This results in a significant deviation between the wavefront of the final assembled system and the theoretical design value. This invention connects steps B1 to B5 into a unified iterative process, addressing the residual aberrations in the optical design. The results of force-thermal coupling simulation serve as input conditions for mechanical structure optimization. + W T The feedback was then fed back to the optical design for verification and correction, achieving synergistic optimization across multiple disciplines including optics, structure, and thermodynamics, ensuring the wavefront quality of the system in vertical operation from the design source.

[0091] Furthermore, this invention, through parameterized optimization of the flexible block wall thickness and notch size in step B3, enables the additional wavefront introduced by the controllable micro-deformation of the primary mirror under vertical gravity. Residual aberrations of the optical system With opposite symbols and similar sizes, the originally harmful gravitational deformation is transformed into a beneficial source of aberration compensation. This "softness overcomes rigidity" design concept breaks through the technical bias of traditional rigid suppression and provides a brand-new technical path for the design of large-aperture optical systems used vertically.

[0092] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0093] The above embodiments merely illustrate several implementation methods of this application, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this patent application should be determined by the appended claims.

Claims

1. A large-diameter beam expander for vertical use, characterized in that, include: Primary mirror assembly, secondary mirror assembly, and supporting structure; The optical axis of the beam expander is arranged vertically, the primary mirror assembly is located at the upper part of the beam expander, and the secondary mirror assembly is located at the lower part of the beam expander; The primary mirror assembly includes a primary mirror, a primary mirror frame, and an elastic support element. The primary mirror is installed in the primary mirror frame via the elastic support element. The elastic support element is used to provide axial preload and control the micro-deformation of the primary mirror under gravity. The secondary mirror assembly includes a secondary mirror and a secondary mirror frame. The secondary mirror frame is cantilevered on the support structure via wing beams. There are at least three wing beams, which are evenly distributed along the circumference of the secondary mirror frame, and each wing beam forms a tangential constraint with the secondary mirror frame. The support structure includes an upper connecting frame, a lower connecting frame, and at least six vertical support rods connecting the two. The upper connecting frame is connected to the main mirror frame, and the lower connecting frame is connected to the secondary mirror frame. The primary mirror and the secondary mirror are both made of low-expansion optical glass; the primary mirror frame, the secondary mirror frame, the upper connecting frame, the lower connecting frame, and the vertical support rod are all made of low-expansion metal; wherein the linear expansion coefficient of the low-expansion metal is no greater than 1.5 × 10⁻⁶. -6 / K, and the difference between the linear expansion coefficient of the low-expansion metallic material and the linear expansion coefficient of the low-expansion optical glass does not exceed 0.5×10. -6 / K.

2. The beam expander according to claim 1, characterized in that, The low-expansion optical glass is fused silica glass, and the coefficient of linear expansion of the fused silica glass is 0.55 × 10⁻⁶. -6 / K; The low-expansion metallic material is Invar, and the coefficient of linear expansion of Invar is no greater than 1.0 × 10⁻⁶. -6 / K.

3. The beam expander according to claim 1, characterized in that, The primary mirror and the secondary mirror constitute a coaxial reflective Galilean telescope system with a magnification of 10 to 20 times, a field of view ≥ ±0.05°, and an effective aperture of the primary mirror ≥ 280 mm. Both the primary mirror and the secondary mirror are quadric mirrors, and the absolute value of the aspheric coefficient of the quadric mirror is 0.95~1.

05. Furthermore, the surface accuracy of both the primary mirror and the secondary mirror is better than λ / 60 (RMS value), where λ is the working wavelength.

4. The beam expander according to claim 1, characterized in that, The elastic support element is a flexible pressure block with an elastic thin-walled structure. The wall thickness and notch size of the elastic thin-walled structure are matched with the residual aberration of the optical system so that the additional wavefront introduced by the controllable micro-deformation of the primary mirror under vertical gravity has the opposite sign to the residual aberration, and the ratio of the RMS value of the additional wavefront to the residual aberration is 0.8 to 1.2, thereby achieving self-compensation matching of the gravity-induced wavefront.

5. The beam expander according to claim 1, characterized in that, The secondary mirror is fixed to the secondary mirror frame by adhesive. The back of the secondary mirror frame is provided with an adhesive injection hole to control the thickness and uniformity of the adhesive layer. The wing beam consists of three beams, each of which is connected tangentially to the secondary mirror frame. This allows the radial displacement to be converted into torsion of the secondary mirror around the optical axis when the structure undergoes radial deformation due to temperature changes or gravity, thereby suppressing the tilt and lateral displacement of the secondary mirror.

6. The beam expander according to claim 1, characterized in that, It also includes a folding mirror assembly, which includes a folding mirror and a folding mirror frame; The folding mirror is installed between the primary mirror and the secondary mirror and on the side closer to the primary mirror. The angle between the reflecting surface of the folding mirror and the optical axis is 43°~47°, which is used to fold the vertically emitted parallel beam of light by 90° into a horizontal direction.

7. The beam expander according to claim 6, characterized in that, A first trimming ring is provided between the lower connecting frame and the secondary mirror frame to adjust the axial spacing between the primary mirror and the secondary mirror; A second trimming ring is provided between the folding lens frame and the main lens frame to adjust the optical axis height of the system; the thickness processing accuracy of the first trimming ring and the second trimming ring is better than ±0.002mm.

8. The beam expander according to claim 1, characterized in that, It also includes a dedicated vertical adjustment frame for vertical assembly and adjustment, wherein the beam expander is detachably mounted on the vertical adjustment frame via a flange mounting surface; A vibration isolation pad is provided between the vertical adjustment frame and the beam expander; The bottom of the vertical debugging frame is provided with a debugging reflector mounting base for fixing and installing the debugging reflector. The surface accuracy of the debugging reflector is better than λ / 10.

9. A method for assembling and adjusting a large-diameter beam expander for vertical use, applied to a large-diameter beam expander for vertical use as described in any one of claims 1-8, the method comprising: Step A1: Install the beam expander as a whole on a dedicated vertical adjustment frame, ensuring that the optical axis of the beam expander remains vertical; Step A2: Fix the debugging reflector on the debugging reflector mounting base at the bottom of the vertical debugging frame; Step A3: Place the interferometer horizontally on one side of the beam expander's output port, aligning the optical axis of the interferometer with the horizontal output optical axis of the beam expander; Step A4: The test light emitted by the interferometer enters the beam expander along the optical path, returns along the original path after reaching the adjustment mirror, forming interference fringes, and measuring the two-way wavefront aberration of the measurement system; Step A5: Based on the two-way wavefront measurement data, decouple the independent aberration contribution terms of the primary mirror and the secondary mirror using Zernike polynomial decomposition and sensitivity matrix; Step A6: Based on the decoupling results, fine-tune the spatial positions of the primary mirror assembly and the secondary mirror assembly sequentially until the system aberrations meet the design requirements; Step A7: Remove the debugging reflector and take the beam expander off the vertical debugging frame to complete the vertical installation and adjustment.

10. A design method for a large-aperture beam expander for vertical use, applied to a large-aperture beam expander for vertical use as described in any one of claims 1-8, the design method comprising: Step B1: Determine the optical system parameters, including beam expansion magnification, design working spectrum, effective aperture of the incident beam, field of view, and operating temperature range; Step B2: Optical design is carried out using a coaxial reflective optical path structure to obtain the surface shape, spacing, and material parameters of the primary mirror and the secondary mirror; Step B3: Design the mechanical structure. The overall arrangement is vertical, with the primary mirror on top and the secondary mirror below. Elastic support elements are used to provide elastic support for the primary mirror, and a cantilever structure with tangential constraints between the wing beam and the mirror frame is used to release the stress of the secondary mirror. Step B4: Perform optomechanical-thermal integration simulation analysis, establish a finite element model under vertical gravity field conditions, apply temperature load and gravity load within the set operating temperature range, extract the surface shape changes of the primary mirror and the secondary mirror and the axial spacing changes between them, and substitute them into the optical design software to evaluate the wavefront aberration of the system. Step B5: Optimize the primary and secondary mirror spacing tolerance and the support structure stiffness based on the simulation results of Step B4, so that the transmission wavefront aberration of the system in the vertical working state and the entire working temperature range meets the usage requirements. The usage requirements are that the transmission wavefront RMS value is better than λ / 20.

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