A coupling lens group design method for a point-ring fiber laser shutter

Abstract

This invention discloses a design method for a coupling lens group of an optical shutter for a point-ring fiber laser. The coupling lens group of the optical shutter includes a collimating lens and a focusing lens. This invention uses a collimating lens and a focusing lens sequentially positioned behind the output fiber of the ring laser to achieve coupling of the optical shutter, and adds a reflecting mirror to achieve different outputs of the optical path. By scientifically and precisely designing the lens group with optimal coupling effect in the optical shutter, it helps to couple the central circular beam and the ring beam emitted by the point-ring laser into the ring fiber through the coupling lens group and converge at the fiber core and ring core, thereby achieving high-efficiency coupling of the laser optical shutter.

Description

Technical Field

[0001] This invention belongs to the field of high-power fiber lasers, and specifically relates to a design method for a coupling lens group of an optical shutter for a point-ring fiber laser. Background Technology

[0002] The rapid development of new energy vehicles has driven the application of lasers in the manufacturing process of power batteries. In the field of lithium battery welding for new energy vehicles, point-ring lasers can significantly reduce metal spatter, improve weld strength, and ensure weld quality compared to traditional Gaussian lasers. As point-ring laser applications develop towards higher efficiency and integration, there is an urgent need for high-power laser shutters to convert the single-channel laser of this type of light source into multiple laser outputs, realizing the multi-purpose use of point-ring lasers, reducing costs, improving efficiency, and saving energy.

[0003] The coaxial dual-core fiber output from a point-ring laser has a cross-sectional structure of core + cladding + ring core + cladding. The laser output can propagate separately in the core and ring core, but not in the cladding. Furthermore, if the laser enters the cladding, it will be lost. Therefore, when coupling the point-ring laser output into the coaxial dual-core fiber, the core laser must be coupled into the core of the fiber, and the ring core laser into the ring core. This ensures maximum laser transmission efficiency within the fiber and meets operational requirements. Because high-efficiency coupling of the ring core and core lasers is required simultaneously, and the theoretical adjustment precision error needs to be controlled within the micrometer range according to specifications, and because errors exist in processing and assembly, the tolerance range for precise simultaneous coupling of the core and ring core lasers from the point-ring laser must be strictly controlled. Based on the above, this invention provides a solution that can efficiently couple the fiber core laser and the ring core laser output from a point-ring laser, solving the problem of simultaneously and accurately coupling the fiber core laser and the ring core laser into a coaxial dual-core output fiber. At the same time, it proposes a coupling lens group design and coupling tolerance analysis for the point-ring laser to ensure the fault tolerance of the coupling and the coupling efficiency. Summary of the Invention

[0004] The purpose of this invention is to provide a design method for the coupling lens group of an optical shutter for a point ring fiber laser, so as to achieve precise and accurate coupling of the optical shutter and improve processing and production efficiency.

[0005] The technical solution for achieving this invention is as follows: a design method for a coupling lens group of an optical shutter for a point-ring fiber laser. The coupling lens group includes a collimating lens and two focusing lenses. Both the collimating lens and the focusing lenses are aspherical plano-convex lenses made of fused silica material. The specific design steps are as follows:

[0006] S1: Calculate the input condition Case of the coupling lens group based on the input and output fiber parameters of the optical shutter for the point ring fiber laser.

[0007] S2: Design the magnification M of the coupling lens group based on the input condition Case of the coupling lens group.

[0008] S3: Based on the aperture of the collimator and the diameter of the laser beam, calculate the focal length of the collimator, set the design condition parameters of the coupling lens group and the initial structural parameters of the collimator, and establish the collimator parametric equation C(M,D) c ,X C ); X C D represents the expression for the coefficients of higher-order terms in a collimated mirror. c This indicates the aperture of the collimating lens.

[0009] S4: Based on the design parameters of the coupled lens group and the initial structural parameters of the collimator, calculate the variance of the angular radius at the full field-of-view characteristic sampling points of the collimator. This leads to the acquisition of the optimal parameters of the collimator, including the optimal radius of curvature, the optimal edge thickness, and the coefficients of higher-order terms.

[0010] S5: Based on the focal length of the collimating lens and the magnification M of the coupling lens group, establish the focusing lens parametric equation F(M,D) f ,X F ), X F D represents the expression for the coefficients of higher-order terms in a focusing mirror-type surface. f The aperture of the focusing lens is given; the focal length of the focusing lens is then obtained. Combined with the initial structural parameters of the focusing lens, the radius variance of the point plot at the full field-of-view feature sampling points of the coupled lens group is calculated.

[0011] S6: According to and The value of the multi-objective evaluation function is calculated using the following expression:

[0012]

[0013] In the formula, W1, W2, and W3 are all weighting coefficients; T const The evaluation function T is determined by ray tracing limitations and imaging quality limitations. i Let represent the value of the multi-objective function for historical number i, and T represent the value of the multi-objective evaluation function. This is the normalized index value of the multi-objective evaluation function.

[0014] S7: Determine whether the normalized index value of the current multi-objective evaluation function is equal to 1. If not, update the focusing lens structure parameters to the optimal focusing lens structure parameters corresponding to this iteration; otherwise, do not update the focusing lens structure parameters, and output the collimating lens and focusing lens structure parameters of the coupled lens group as the optimal parameters.

[0015] S8: Based on the optimal structural parameters of the collimating lens and focusing lens in the coupling lens group, perform a feasibility tolerance analysis to determine whether the feasibility tolerance value of each optical element in the coupling lens group to be analyzed is greater than the historical maximum feasibility tolerance value. If it is not satisfied, update the feasibility tolerance of each optical element in the coupling lens group; otherwise, do not update the feasibility tolerance of each optical element in the coupling lens group, and output the optimal collimating lens and focusing lens structural parameters of the coupling lens group.

[0016] Compared with the prior art, the present invention has significant advantages: (1) it simultaneously achieves high-efficiency coupling of the fiber core and the ring core, thereby improving the accuracy of the coupling.

[0017] (2) Ensure the feasibility of the entire system through tolerance analysis.

[0018] (3) The lens can withstand higher power lasers, which improves the versatility of the light gate equipment. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the optical path design for the coupling lens group design method of the optical shutter for the circular fiber laser of the present invention.

[0020] Figure 2 This is a flowchart illustrating the design method of the coupling lens group for the optical shutter of the point ring fiber laser of the present invention.

[0021] Figure 3 The laser output beam pattern and the structural diagrams of the input and output optical fibers are shown in the design method of the coupling lens group of the optical shutter for the point ring fiber laser of the present invention. (a) represents the mathematical and geometric model of the output point ring beam of the point ring fiber laser, (b) represents the mathematical and geometric model of the input optical fiber structure, and (c) represents the mathematical and geometric model of the output optical fiber structure.

[0022] Figure 4 The diagram shows the optical trace effect of the coupling lens group design method for the optical shutter of the point ring fiber laser of the present invention. (a) represents the optical trace effect of the core beam output by the core beam emitted from the point ring fiber laser after being coupled by the coupling lens group, and (b) represents the optical trace effect of the ring beam output by the ring beam emitted from the point ring fiber laser after being coupled by the coupling lens group.

[0023] Figure 5The image shows the adjusted light trace after tolerance analysis of the coupling lens group design method for the optical shutter of the point ring fiber laser of the present invention. (a) represents the effect of adjusting the light trace after adding tolerance to the core beam output by the core beam of the point ring fiber laser coupled by the coupling lens group, and (b) represents the effect of adjusting the light trace after adding tolerance to the ring beam output by the ring beam of the point ring fiber laser coupled by the coupling lens group. Specific implementation methods

[0024] The present invention will be described in detail below with reference to the accompanying drawings and preferred embodiments. The objectives and effects of the present invention will become clearer as a result. The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0025] First, a brief introduction to the design concept of the embodiments of the present invention will be given.

[0026] To achieve high-efficiency coupling between the core laser and the ring laser output from a point-ring laser after passing through the optical gate output fiber, the unique characteristics of the coaxial dual-core fiber cross-section must be fully considered. The coaxial dual-core fiber output from the point-ring laser has a cross-sectional structure of core + cladding + ring core + cladding. The laser output can only propagate in the core and ring core separately; it does not propagate in the cladding, and if the laser enters the cladding, the coupling efficiency will be significantly reduced.

[0027] To address the aforementioned technical problems, this invention proposes a design method for a coupling lens group of an optical shutter for a point-ring fiber laser. The coupling lens group includes a collimating lens and two focusing lenses. The coupling lens group can effectively control the beam propagation path, preventing laser light from entering the cladding, thereby reducing loss and improving coupling efficiency. Furthermore, the coupling lens group design offers greater flexibility, allowing for customized design based on specific fiber structures and laser parameters to adapt to different laser output modes and application requirements.

[0028] The design process of the coupling lens group is explained below with reference to the accompanying drawings.

[0029] The optical shutter coupling lens group for the fiber laser includes a collimating lens and two focusing lenses. Both the collimating lens and the focusing lens are plano-convex even-order aspherical lenses. The lenses are made of fused silica material with low thermal expansion coefficient and low infrared absorption coefficient. The lenses are composed of a surface antireflection coating and a fused silica substrate. The purpose of multi-path output is achieved by reflecting the light through a mirror added to the coupling lens group. The mirror is composed of a surface antireflection coating and a fused silica substrate. Fused silica has a high melting point and can withstand high temperatures. Under high-power laser conditions, the lens will undergo different degrees of thermal deformation due to temperature distribution differences, causing thermal aberrations. The design requirements for the lens are: (1) to keep the temperature of all parts of the lens within a tolerable range through cooling; (2) to reduce the thermal aberrations caused by temperature differences in the lens through cooling. Figure 1 The diagram shows a simplified optical path design for an optical shutter. The output fiber of the fiber laser serves as the input fiber of the optical shutter. The laser output from the input fiber of the optical shutter is collimated by a collimating mirror, becoming approximately parallel light. It is then refracted by a reflecting mirror and finally focused by a focusing mirror, coupling the laser into the corresponding output fiber of the optical shutter for output.

[0030] Combination Figure 2 This invention discloses a design method for a coupling lens group of an optical shutter for a point-ring fiber laser. The coupling lens group includes a collimating lens and two focusing lenses, both of which are aspherical plano-convex lenses made of fused silica material. The proposed coupling lens group simultaneously couples the central circular beam and the ring beam emitted by the point-ring laser, adapting to different laser output modes and application requirements. The method involves designing all parameters of the collimating lens and focusing lens in the coupling lens group (including focal length, aperture, radius of curvature, edge thickness, and higher-order terms) based on the parameters of the input and output fibers. After tolerance analysis verification, high-efficiency coupling into the ring fiber is achieved, with convergence at the fiber core and ring core. This results in a simple structure with greater design flexibility and convenience. The specific steps are as follows:

[0031] S1: Based on the input and output fiber parameters of the optical shutter for the point ring fiber laser, calculate the input condition case of the coupling lens group, as follows:

[0032] Both the input and output optical fibers are coaxial dual-core. The input conditions for the coupling lens group are as follows:

[0033]

[0034] Where d1 is the diameter of the output circular fiber core spot, θ1 is the divergence angle of the fiber core spot, d2 is the inner diameter of the output annular spot, d3 is the outer diameter of the output annular spot, and θ2 is the divergence angle of the fiber core spot. out1 To determine the divergence angle of the output fiber core, D out1 To determine the diameter of the output optical fiber core, D out2D is the inner diameter of the output fiber loop core. out3 θ is the outer diameter of the output fiber loop core. out2 To determine the divergence angle of the output fiber loop core.

[0035] S2, based on the input condition Case of the coupling lens group, design the magnification M of the coupling lens group as follows:

[0036] The formula for calculating the magnification M of the coupled lens group is as follows:

[0037]

[0038] Among them, D in1 θ is the diameter of the input fiber core. in1 D is the input fiber core divergence angle. in2 D is the inner diameter of the input fiber loop core. in3 θ is the outer diameter of the input fiber loop core. in2 The divergence angle of the input fiber loop core.

[0039] S3: Based on the aperture of the collimator and the diameter of the laser beam, calculate the focal length of the collimator, set the design condition parameters of the coupling lens group and the initial structural parameters of the collimator, and establish the collimator parametric equation C(M,D) c ,X C ); X C D represents the expression for the coefficients of higher-order terms in a collimated mirror. c This indicates the aperture of the collimating lens.

[0040] The design parameters for the coupling lens group include: operating wavelength, operating field of view, size of the optical shutter input fiber, and numerical aperture.

[0041] The initial structural parameters of the collimating lens include the surface radius R of the collimating lens. c Collimating lens aperture D c The lens material and surface shape of the collimating lens;

[0042] Establish the collimating mirror parametric equation C(M,D) c ,X C ):

[0043]

[0044] Among them, f c Let n be the focal length of the collimating lens, n0 be the real part of the room-temperature refractive index of the lens material fused silica, and X be the focal length of the collimating lens. C This is the expression for the coefficients of the higher-order terms in the collimating mirror type, where a, b, and c are the higher-order coefficients of the equation, k is the quadratic surface constant of the collimating mirror, and e is the paraxial curvature of the collimating mirror, e = 1 / R. c NA is the numerical aperture of the input fiber, and r is the distance from the collimating lens cross-section to the optical axis.

[0045] S4: Based on the design parameters of the coupled lens group and the initial structural parameters of the collimator, calculate the variance of the angular radius at the full field-of-view characteristic sampling points of the collimator. This leads to the optimal parameters of the collimator, including the optimal radius of curvature, optimal edge thickness, and higher-order coefficients; specifically as follows:

[0046] Calculate the variance of the angular radius at the full field-of-view characteristic sampling points of the collimating lens. The calculation expression is as follows:

[0047]

[0048] In the formula, h represents the horizontal coordinate of the field of view. m v represents the maximum horizontal coordinate value; v represents the vertical coordinate of the field of view. m Represents the maximum vertical coordinate value; θ h,v This corresponds to the root mean square angle radius value under the field of view. It is the mean square radius value of the corresponding field of view.

[0049] S5: Based on the focal length of the collimating lens and the magnification M of the coupling lens group, establish the focusing lens parametric equation F(M,D) f ,X F ), X F D represents the expression for the coefficients of higher-order terms in a focusing mirror-type surface. f The aperture of the focusing lens is given; the focal length of the focusing lens is then obtained. Combined with the initial structural parameters of the focusing lens, the radius variance of the point plot at the full field-of-view feature sampling points of the coupled lens group is calculated.

[0050] Radius variance of point plot under full field of view feature sampling points The calculation expression is as follows:

[0051]

[0052] In the formula, h represents the horizontal coordinate of the field of view. m v represents the maximum horizontal coordinate value; v represents the vertical coordinate of the field of view. m Represents the maximum vertical coordinate value; r h,v It is the radius value of the root mean square point plot under the corresponding field of view. It is the radius value of the root mean square point array diagram under the corresponding field of view.

[0053] S6: According to and The value of the multi-objective evaluation function is calculated using the following expression:

[0054]

[0055] In the formula, W1, W2, and W3 are all weighting coefficients; T const The evaluation function T is determined by ray tracing limitations and imaging quality limitations. i Let represent the value of the multi-objective function for historical number i, and T represent the value of the multi-objective evaluation function. This is the normalized index value of the multi-objective evaluation function.

[0056] S7: Determine whether the normalized index value of the current multi-objective evaluation function is equal to 1. If it is not satisfied, update the focusing lens structure parameters to the optimal focusing lens structure parameters corresponding to this iteration; otherwise, do not update the focusing lens structure parameters, and output the collimating lens and focusing lens structure parameters of the coupled lens group as the optimal parameters.

[0057] The structural parameters of the focusing lens include: the surface curvature radius R of the focusing lens. f Focusing lens aperture D f The lens material and surface shape of the focusing lens;

[0058] Establish the focusing lens parameter equation F(M,D) f ,X F ):

[0059]

[0060] Among them, f f Let n be the focal length of the focusing lens, n0 be the real part of the room-temperature refractive index of the lens material fused silica, and X be the focal length of the focusing lens. F This is the expression for the coefficients of the higher-order terms of the focusing mirror, where A, B, and C are the coefficients of the higher-order terms of the equation, K is the quadratic surface constant of the focusing mirror, and E is the paraxial curvature of the focusing mirror, E = 1 / R. c R is the distance from the focusing mirror cross section to the optical axis.

[0061] S8: Based on the optimal structural parameters of the collimating lens and focusing lens in the coupled lens group, perform a feasibility tolerance analysis to determine whether the feasibility tolerance value of each optical element in the coupled lens group to be analyzed is greater than the historical maximum feasibility tolerance value. If not, update the feasibility tolerance of each optical element in the coupled lens group; otherwise, do not update the feasibility tolerance of each optical element in the coupled lens group, and output the optimal collimating lens and focusing lens structural parameters of the coupled lens group, as follows:

[0062] The tolerances of the components in the coupling lens group are classified into eccentricity tolerance, tilt tolerance, and surface tolerance. Based on the characteristics and magnitude of these tolerances, a linear combination of these three types of tolerances is performed to conduct targeted iterative analysis. First, the distribution of surface tolerance, eccentricity, and tilt tolerances of the components in the coupling lens group to be analyzed is determined. Through linear combination, while ensuring the accuracy of the tolerance analysis, it is ensured that the optical components can meet the actual feasibility after coupling adjustment after final assembly. Through multiple Monte Carlo tolerance analysis calculations, the tolerance analysis results are obtained and compared with preset values. If they are not satisfied, the tolerances of each optical component in the coupling lens group need to be updated; otherwise, the optimal structural parameters of the collimating lens and focusing lens of the coupling lens group are output.

[0063] Example 1

[0064] Combination Figure 2 and Figure 3 The present invention discloses a method for designing a coupling lens group for an optical shutter in a point-ring fiber laser, comprising the following steps:

[0065] Step 1: Based on the parameters of the input and output fibers of the optical shutter for the point-ring fiber laser, calculate the input condition case of the coupling lens group. The parameters of the input and output ring fibers of the optical shutter for the point-ring fiber laser are as follows: input fiber core diameter 34µm, input fiber core divergence angle 6.3°, input fiber ring core inner diameter 48µm, input fiber ring core outer diameter 100µm, input fiber ring core divergence angle 12.7°; corresponding output fiber core diameter 50µm, output fiber core divergence angle 12.7°, output fiber ring core inner diameter 70µm, output fiber ring core outer diameter 150µm, output fiber ring core divergence angle 12.7°. The input condition case is satisfied.

[0066] Step 2: Based on the input condition Case of the coupling lens group, design the magnification M of the coupling lens group. If a spatial ring laser beam is coupled to an optical fiber, it needs to be transformed (magnified or reduced) by the optical coupling lens group to confine the laser beam within the output range. This is determined by the spot size d output from the point-ring laser fiber. in and beam convergence angle and output fiber diameter D out And determined by the numerical aperture angle of the optical fiber, i.e.

[0067]

[0068] At this point, the magnification M of the coupling lens group is set to 1.1 based on the range.

[0069] Step 3: Based on the aperture of the collimator and the laser beam diameter, calculate the focal length of the collimator, set the design condition parameters of the coupling lens group and the initial structural parameters of the collimator, and establish the collimator parametric equation C(M,D) c ,XC The details are as follows:

[0070]

[0071] X C D represents the expression for the coefficients of higher-order terms in a collimated mirror. c This indicates the aperture of the collimating lens.

[0072] Step 4: Based on the design parameters of the coupled lens group and the initial structural parameters of the collimating lens, calculate the variance of the angular radius at the full field-of-view characteristic sampling points of the collimating lens. This leads to the acquisition of optimal parameters for the collimator, including the optimal radius of curvature, optimal edge thickness, and higher-order coefficients. Based on the coupling device requirements and common collimator aperture parameters on the market, 30mm was selected as the final collimator aperture parameter. The core numerical aperture of the input fiber was 0.11, and the ring core numerical aperture was 0.22. Optimization was performed using simulation software; Zemax is an optical simulation analysis software, and the steps described were implemented in Zemax. A system was built in Zemax. Figure 3 The diagram shows the geometric and physical models of the collimating lens. The geometric model includes the collimating lens (material property set to fused silica), an even-order aspherical lens, a plano-convex lens, and the output fiber of a ring fiber laser serving as the input fiber of the optical shutter. The emitted light is approximated as a uniform conical beam with a certain divergence angle emitted from a single point. The model optimizes the collimation of the conical beam into parallel light when the point is at the focal point of the collimating lens. The angular radius variance at the full field-of-view characteristic sampling points of the coupled lens group is calculated. At its minimum, the focal length of the collimating lens is the optimal focal length R. c .

[0073] The output wavelength of the ring fiber laser is generally between 1030nm and 1090nm, and the output power is 2kW for the fiber core and 2kW for the ring core.

[0074] R c Let be the radius of curvature of the collimating lens. The collimating lens can be approximated as a thin plano-convex lens, and its radius of curvature corresponds one-to-one with its focal length. The radius of curvature is obtained by transforming the formula for the focal length of a thin plano-convex lens:

[0075] R c =(n0-1)f c

[0076] Where n0 is the real part of the refractive index at room temperature of the lens material fused silica.

[0077] Considering the losses caused by diffraction, the aperture of the collimating lens is generally more than twice the beam diameter. After collimation, the diameter D of the fiber laser beam is related to the fiber numerical aperture NA and the focal length f of the collimating lens. cThe relationship is as follows:

[0078] D≈2NA×f c

[0079] f c A value of 56.5mm is more suitable.

[0080] The incident surface of the first positive power lens is an aspherical surface, and the exit surface of the first positive power lens is a plane.

[0081] The aspherical surface of the first positive power lens satisfies the aspherical equation:

[0082]

[0083] In summary, the optimal focal length of the collimating lens is 56.5mm, the optimal aperture is 30mm, the optimal radius of curvature is -25.44, and the optimal edge thickness is T. c The coefficient of the fourth-order term is -5.772 × 10⁻⁵. -6 The coefficient of the sixth-order term is -4.262 × 10⁻⁶. -10 .

[0084] At this point, all parameters of the collimator are known.

[0085] Step 5: Based on the focal length of the collimating lens and the magnification M of the coupling lens group, establish the focusing lens parametric equation F(M,D) f ,X F ), X F D represents the expression for the coefficients of higher-order terms in a collimated mirror. f The aperture of the collimating lens is indicated as follows:

[0086]

[0087] Combining the collimating lens parameters obtained in step 4 with the initial structural parameters of the focusing lens, the focusing lens aperture is the same as the collimating lens aperture, which is 30mm. To facilitate manufacturing, the edge thickness of the focusing lens is the same as that of the collimating lens. Based on the calculated coupling magnification M, the focal length of the focusing lens is calculated, and the radius variance of the point plot under the full field-of-view characteristic sampling points of the coupled lens group is calculated.

[0088] Step 6, according to and Calculate the value of the multi-objective evaluation function.

[0089] Step 7: Determine whether the normalized index value of the current multi-objective evaluation function is equal to 1. This is done by determining if... Figure 4As shown, the Zemax simulation light trace of the coupling lens group in the optical shutter design method for the point-ring fiber laser of this embodiment is shown. At this time, the core spot size is 44.7 μm, and the ring spot size is 65 μm inner diameter and 129.9 μm outer diameter. This is obtained through the coupling efficiency formula. The calculations are performed, where Φ(x,y) represents the spatial mode distribution in free space (LG mode), and Ψ(x,y) represents the normalized LP mode within the fiber. The final calculated core coupling efficiency is 99.6%, and the ring-core coupling efficiency is 82%. At this point, the coupling requirements are met, and the focusing lens structure parameters of the coupling lens group are output as optimal parameters: optimal focal length 66.5mm, optimal aperture 30mm, optimal radius of curvature 29.88, and optimal edge thickness T. f The coefficient of the fourth-order term is -1.245 × 10⁻⁶. -6 The coefficient of the sixth-order term is -1.796 × 10⁻⁶. -10 .

[0090] At this point, all the parameters of the focusing lens are known.

[0091] Step 8: Based on the optimal structural parameters of the collimating lens and focusing lens in the coupling lens group, perform a feasibility tolerance analysis to determine whether the feasibility tolerance value of each optical element in the coupling lens group to be analyzed is greater than the historical maximum feasibility tolerance value. If it is not satisfied, update the feasibility tolerance of each optical element in the coupling lens group; otherwise, do not update the feasibility tolerance of each optical element in the coupling lens group, and output the optimal collimating lens and focusing lens structural parameters of the coupling lens group.

[0092] In related technologies, the RMS or PV values ​​of the optical element surface shape are generally used to evaluate the manufacturing and assembly accuracy of the optical element. Tolerance analysis is a probabilistic statistical analysis method based on Monte Carlo, which is suitable for analyzing the tolerances of mass-produced optical elements. This method has been considered the most effective method for more than half a century.

[0093] In this embodiment, the tolerance sensitivity of the coupling lens group is as follows: the parameters of each optical element are set as shown in the table below, and the element processing and assembly tolerances are specified. Substituting these parameters into optical design software, using the RMS blur radius as the evaluation standard, and employing a sensitivity analysis model, it is found that 90% of the RMS blur obtained from the superposition of errors is concentrated in the fiber core and ring core.

[0094] The tolerance ranges for each optical element are set.

[0095] face shape Aperture number Center thickness error / mm Surface off-axis measurement / mm Surface tilt / ° Aspherical lens 0.4 ±0.01 ±0.01 ±0.0167 plane mirror 0.4 ±0.01 ±0.01 ±0.0167

[0096] face shape X eccentricity / mm Y eccentricity / mm X Tilt / ° Y tilt / ° Aspherical collimating lens ±0.05 ±0.05 ±0.03 ±0.03 Fixed-plane mirror 1 ±0.1 ±0.1 ±0.03 ±0.03 Fixed-plane mirror 2 ±0.1 ±0.1 ±0.03 ±0.03 Aspherical focusing lens ±0.05 ±0.05 ±0.03 ±0.03

[0097] like Figure 5 As shown in the figure, the coupling lens group of the optical shutter for the point-ring fiber laser in this embodiment, after error superposition and coupling adjustment, is shown in the Zemax simulation light trace diagram. The figure shows that the core spot size of 45µm and the ring spot size of 63µm inner diameter and 136.1µm outer diameter satisfy the coupling conditions. Furthermore, the coupling efficiency formula is used to verify the coupling. The calculated core coupling efficiency is 99.2% and the ring core coupling efficiency is 81%, which is roughly the same as the effect before the system tolerance analysis. Furthermore, the feasibility tolerance values ​​of each optical element in the coupling lens group meet the requirements. The optimal collimator parameters for the output coupling lens group are: optimal focal length of 56.5mm, optimal aperture of 30mm, optimal radius of curvature of -25.44, and optimal edge thickness of T. c The coefficient of the fourth-order term is -5.772 × 10⁻⁵. -6 The coefficient of the sixth-order term is -4.262 × 10⁻⁶. -10 Focusing lens structural parameters: optimal focal length 66.5mm, optimal aperture 30mm, optimal radius of curvature 29.88, optimal edge thickness T. f The coefficient of the fourth-order term is -1.245 × 10⁻⁶. -6 The coefficient of the sixth-order term is -1.796 × 10⁻⁶. -10 .

[0098] By combining the above design and optimization steps, under high power conditions, the central circular beam and the ring beam emitted by the point-ring laser can be coupled through a double-coupled lens and coupled into the ring fiber through an optical fiber connector, and converge at the fiber core and the ring core, thereby achieving high-efficiency coupling of the laser shutter.

Claims

1. A method for designing a coupling lens group for an optical shutter in a point-ring fiber laser, characterized in that: The aforementioned coupling lens group includes a collimating lens and two focusing lenses. Both the collimating lens and the focusing lenses are aspherical plano-convex lenses made of fused silica material. The specific steps are as follows: S1: Calculate the input condition Case of the coupling lens group based on the input and output fiber parameters of the optical shutter for the point ring fiber laser. S2: Design the magnification M of the coupling lens group based on the input condition Case of the coupling lens group; S3: Based on the aperture of the collimator and the diameter of the laser beam, calculate the focal length of the collimator, set the design condition parameters of the coupling lens group and the initial structural parameters of the collimator, and establish the collimator parametric equation C(M,D) c ,X C ); X C D represents the expression for the coefficients of higher-order terms in a collimated mirror. c Indicates the aperture of the collimating lens; S4: Based on the design parameters of the coupled lens group and the initial structural parameters of the collimator, calculate the variance of the angular radius at the full field-of-view characteristic sampling points of the collimator. This leads to the acquisition of the optimal parameters of the collimator, including the optimal radius of curvature, the optimal edge thickness, and the coefficients of higher-order terms. S5: Based on the focal length of the collimating lens and the magnification M of the coupling lens group, establish the focusing lens parametric equation F(M,D) f ,X F ), X F D represents the expression for the coefficients of higher-order terms in a focusing mirror-type surface. f The aperture of the focusing lens is given; the focal length of the focusing lens is then obtained. Combined with the initial structural parameters of the focusing lens, the radius variance of the point plot at the full field-of-view feature sampling points of the coupled lens group is calculated. S6: According to and The value of the multi-objective evaluation function is calculated using the following expression: In the formula, W1, W2, and W3 are all weighting coefficients; T const The evaluation function T is determined by ray tracing limitations and imaging quality limitations. i Let represent the value of the multi-objective function for historical number i, and T represent the value of the multi-objective evaluation function. The normalized index value of the multi-objective evaluation function; S7: Determine whether the normalized index value of the current multi-objective evaluation function is equal to 1. If it is not satisfied, update the focusing lens structure parameters to the optimal focusing lens structure parameters corresponding to this iteration; otherwise, do not update the focusing lens structure parameters, and output the collimating lens and focusing lens structure parameters of the coupled lens group as the optimal parameters. S8: Based on the optimal structural parameters of the collimating lens and focusing lens in the coupling lens group, perform a feasibility tolerance analysis to determine whether the feasibility tolerance value of each optical element in the coupling lens group to be analyzed is greater than the historical maximum feasibility tolerance value. If it is not satisfied, update the feasibility tolerance of each optical element in the coupling lens group; otherwise, do not update the feasibility tolerance of each optical element in the coupling lens group, and output the optimal collimating lens and focusing lens structural parameters of the coupling lens group.

2. The design method for the coupling lens group of the optical shutter for a point-ring fiber laser according to claim 1, characterized in that, In S1, based on the parameters of the input and output fibers of the optical shutter for the point ring fiber laser, the input condition Case of the coupling lens group is calculated as follows: Both the input and output optical fibers are coaxial dual-core. The input conditions for the coupling lens group are as follows: Where d1 is the diameter of the output circular fiber core spot, θ1 is the divergence angle of the fiber core spot, d2 is the inner diameter of the output annular spot, d3 is the outer diameter of the output annular spot, and θ2 is the divergence angle of the fiber core spot. out1 To output the fiber core divergence angle, D out1 To determine the diameter of the output optical fiber core, D out2 D is the inner diameter of the output fiber loop core. out3 θ is the outer diameter of the output fiber loop core. out2 To determine the divergence angle of the output fiber loop core.

3. The design method for the coupling lens group of the optical shutter for a point-ring fiber laser according to claim 2, characterized in that, In S2, the magnification M of the coupling lens group is designed based on the input condition Case of the coupling lens group, as follows: The formula for calculating the magnification M of the coupled lens group is as follows: Among them, D in1 θ is the diameter of the input fiber core. in1 D is the input fiber core divergence angle. in2 D is the inner diameter of the input fiber loop core. in3 θ is the outer diameter of the input fiber loop core. in2 The input fiber loop core divergence angle.

4. The design method for the coupling lens group of the optical shutter for a point-ring fiber laser according to claim 3, characterized in that, In S3, the design parameters for the coupling lens group include: operating band, operating field of view, size of the optical shutter input fiber, and numerical aperture.

5. The design method for the coupling lens group of the optical shutter for a point-ring fiber laser according to claim 4, characterized in that, In S3, the initial structural parameters of the collimating lens include the surface radius R of the collimating lens. c Collimating lens aperture D c The lens material and surface shape of the collimating lens; Establish the collimating mirror parametric equation C(M,D) c ,X C ): Among them, f c Let n be the focal length of the collimating lens, n0 be the real part of the room-temperature refractive index of the lens material fused silica, and X be the focal length of the collimating lens. C This is the expression for the coefficients of the higher-order terms in the collimating mirror type, where a, b, and c are the higher-order coefficients of the equation, k is the quadratic surface constant of the collimating mirror, and e is the paraxial curvature of the collimating mirror, e = 1 / R. c NA is the numerical aperture of the input fiber, and r is the distance from the collimating lens cross-section to the optical axis.

6. The design method for the coupling lens group of the optical shutter for a point-ring fiber laser according to claim 5, characterized in that, In S4, based on the design parameters of the coupled lens group and the initial structural parameters of the collimator, the variance of the angular radius at the full field-of-view characteristic sampling points of the collimator is calculated. The optimal parameters for the collimating lens are then obtained, as follows: Calculate the variance of the angular radius at the full field-of-view characteristic sampling points of the collimating lens. The calculation expression is as follows: In the formula, h represents the horizontal coordinate of the field of view. m v represents the maximum horizontal coordinate value; v represents the vertical coordinate of the field of view. m Represents the maximum vertical coordinate value; θ h,v This is the root mean square angle radius value under the corresponding field of view. It is the mean square radius value of the corresponding field of view.

7. The design method for the coupling lens group of the optical shutter for a point-ring fiber laser according to claim 6, characterized in that, In S5, the radius variance of the point plot under full-field feature sampling points The calculation expression is as follows: In the formula, h represents the horizontal coordinate of the field of view. m v represents the maximum horizontal coordinate value; v represents the vertical coordinate of the field of view. m Represents the maximum vertical coordinate value; r h，v It is the radius value of the root mean square point plot under the corresponding field of view. It is the radius value of the root mean square point array diagram under the corresponding field of view.

8. The design method for the coupling lens group of the optical shutter for a point-ring fiber laser according to claim 7, characterized in that, In step S7, the focusing mirror structural parameters include: the surface curvature radius R of the focusing mirror. f Focusing lens aperture D f The lens material and surface shape of the focusing lens; Establish the focusing lens parameter equation F(M,D) f ,X F ): Among them, f f Let n be the focal length of the focusing lens, n0 be the real part of the room-temperature refractive index of the lens material fused silica, and X be the focal length of the focusing lens. F This is the expression for the coefficients of the higher-order terms of the focusing mirror, where A, B, and C are the coefficients of the higher-order terms of the equation, K is the quadratic surface constant of the focusing mirror, and E is the paraxial curvature of the focusing mirror, E = 1 / R. c R is the distance from the focusing mirror cross section to the optical axis.

9. The design method for the coupling lens group of the optical shutter for a point-ring fiber laser according to claim 8, characterized in that, In step S8, a feasibility tolerance analysis is performed based on the optimal structural parameters of the collimating lens and focusing lens of the coupling lens group to determine the feasibility tolerance range of each optical element of the coupling lens group to be analyzed, as follows: The tolerances of the components in the coupling lens group are classified into eccentricity tolerance, tilt tolerance, and surface tolerance. Based on the characteristics and magnitude of these tolerances, a linear combination of these three types of tolerances is performed to conduct targeted iterative analysis. First, the distribution of surface tolerance, eccentricity, and tilt tolerances of the components in the coupling lens group to be analyzed is determined. Through linear combination, while ensuring the accuracy of the tolerance analysis, it is ensured that the optical components can meet the actual feasibility after coupling adjustment after final assembly. Through multiple Monte Carlo tolerance analysis calculations, the tolerance analysis results are obtained and compared with the preset values. If they are not satisfied, the tolerances of each optical component in the coupling lens group need to be updated. Conversely, the optimal structural parameters of the collimating lens and focusing lens of the output coupling lens group are obtained.

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