Coupling lens group design method of optical gate for point ring fiber laser

Abstract

The invention discloses a design method of a coupling lens group of an optical shutter for a point ring fiber laser. The coupling lens group of the optical shutter comprises a collimating lens and a focusing lens. The collimating mirror and the focusing mirror which are sequentially arranged behind the output optical fiber of the ring laser are adopted to realize coupling of the optical gate, and the reflecting mirror is added to realize different outputs of the optical path. The lens group with the optimal coupling effect in the optical shutter is scientifically and accurately designed, so that a central circular light beam and an annular light beam emitted by a point annular laser are coupled through the coupling lens group, enter an annular optical fiber and are converged at a fiber core and an annular core, and high-efficiency coupling of the laser optical shutter is realized.

Description

Technical Field

[0001] The invention belongs to the field of high-power fiber lasers, and in particular relates to a method for designing a coupling lens group of an optical shutter for a point ring fiber laser. Background Art

[0002] The rapid development of new energy vehicles has driven the application of lasers in the production process of power batteries. In the field of new energy vehicle lithium battery welding, compared with traditional Gaussian lasers, dot ring spot lasers can significantly reduce metal spatter generated by welding, improve weld strength, and ensure welding quality. As the application of dot ring spot lasers develops towards high efficiency and integration, high-power laser shutters are urgently needed to convert the single-channel laser of this type of light source into multi-channel laser output, so as to realize the multi-purpose use of dot ring spot lasers, reduce costs, improve efficiency, and save energy.

[0003] The structure distribution of the coaxial dual-core fiber output by the point ring laser on the cross section is the structure of core + cladding + ring core + cladding. The laser output laser can be transmitted in the core and ring core respectively. The laser is not transmitted in the cladding, and if the laser enters the cladding, it will be lost. Therefore, when coupling the output laser of the point ring laser into the coaxial dual-core fiber, the core laser must be coupled into the core of the coaxial dual-core fiber, and the ring core laser must be coupled into the ring core of the coaxial dual-core fiber. Only in this way can the laser be transmitted in the optical fiber with the maximum efficiency and meet the use requirements. Since the ring core laser and the core laser need to be coupled efficiently at the same time, and according to the index requirements, the theoretical adjustment accuracy error needs to be controlled at the micron level, because there will be errors in processing and assembly. Therefore, it is necessary to strictly control the tolerance range of the simultaneous precision coupling of the core laser and the ring core laser output by the point ring laser. Based on the above situation, the present invention provides a solution, which can efficiently couple the core laser and the ring core laser output by the point ring laser, solve the problem of needing to accurately couple the core laser and the ring core laser into the coaxial dual-core output optical fiber at the same time, and propose the coupling lens group design and coupling tolerance analysis of the point ring laser to ensure the fault tolerance of the coupling and ensure the coupling efficiency. Summary of the invention

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

[0005] The technical solution to realize the present invention is: a method for designing a coupling lens group for a light gate of a point ring fiber laser, wherein the coupling lens group includes a collimator and two focusing lenses, wherein the collimator and the focusing lenses are both aspheric plano-convex lenses made of fused quartz material, and the specific design steps are as follows:

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

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

[0008] S3: According to the aperture of the collimator and the diameter of the laser beam, the focal length of the collimator is calculated, the design condition parameters of the coupling lens group and the initial structural parameters of the collimator are set, and the collimator parameter equation C(M,D c ,X C );X C The expression of the high-order coefficient of the collimating mirror type, D c Indicates the collimator aperture.

[0009] S4: Calculate the angular radius variance of the full field of view characteristic sampling point of the collimator according to the design condition parameters of the coupling lens group and the initial structure parameters of the collimator Then, the optimal parameters of the collimator are obtained, including the optimal curvature radius of the collimator, the optimal edge thickness of the collimator and the high-order coefficients.

[0010] S5: According to the focal length of the collimator and the magnification M of the coupling lens group, establish the focusing lens parameter equation F(M,D f ,X F ), X F The expression of the high-order coefficient of focusing mirror type, D f Represents the focusing lens aperture; then the focal length of the focusing lens is obtained, and combined with the initial structural parameters of the focusing lens, the radius variance of the spot diagram under the full field of view characteristic sampling points of the coupling lens group is calculated

[0011] S6: According to and Calculate the value of the multi-objective evaluation function, the expression is as follows:

[0012]

[0013] Where W 1 , W 2 , W 3 are weight coefficients; T const is the evaluation function determined by the ray tracing limitation and the imaging quality limitation, T i represents the value of the multi-objective function with the history number i, T is the value of the multi-objective evaluation function, It 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 mirror structure parameters to the optimal focusing mirror structure parameters corresponding to this iteration; otherwise, do not update the focusing mirror structure parameters, and output the collimating mirror and focusing mirror structure parameters of the coupling lens group at this time as the optimal parameters.

[0015] S8: Perform feasibility tolerance analysis based on the optimal structural parameters of the collimator and focusing lens in the coupling lens group 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 not, 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 collimator and focusing lens structural parameters of the coupling lens group.

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

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

[0018] (3) The lens can withstand higher power lasers and improve the versatility of the optical shutter device. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0020] Figure 2 The present invention is a flow chart of a method for designing a coupling lens group of an optical shutter for a point ring fiber laser.

[0021] Figure 3 The laser output spot diagram and the structural diagram of the input and output optical fibers of the method for designing the coupling lens group of the optical shutter for the point ring fiber laser of the present invention are shown, wherein (a) represents the mathematical geometric model of the point ring light beam emitted by the point ring fiber laser, (b) represents the mathematical geometric model of the input optical fiber structure, and (c) represents the mathematical geometric model of the output optical fiber structure.

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

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

[0025] The present invention will be described in detail below according to the accompanying drawings and preferred embodiments, and the purpose and effect of the present invention will become more clear. The present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0026] First, the design concept of the embodiment of the present invention is briefly introduced.

[0027] In order to achieve efficient coupling of the core laser and the ring core laser output by the point ring laser through the optical gate output fiber, the particularity of the cross-sectional structure of the coaxial dual-core fiber must be fully considered. The cross-sectional structure of the coaxial dual-core fiber output by the point ring laser is a structure of core + cladding + ring core + cladding. The laser output by the laser can only be transmitted in the core and ring core respectively, and the laser is not transmitted in the cladding. If the laser enters the cladding, the coupling efficiency will be greatly reduced.

[0028] In order to solve the above technical problems. The present invention proposes a coupling lens group design method for a light gate for a point ring fiber laser, wherein the coupling lens group includes a collimator and two focusing lenses. The coupling lens group can effectively control the propagation path of the light beam and prevent the laser from entering the cladding, thereby reducing losses and improving coupling efficiency; the coupling lens group design has higher flexibility and can be customized according to the specific fiber structure and laser parameters to adapt to different laser output modes and application requirements.

[0029] The design process of the coupling lens group is explained below with reference to the accompanying drawings.

[0030] The optical shutter coupling lens group in the optical shutter for the fiber laser includes a collimator and two focusing lenses. Both the collimator and focusing lenses are made of plano-convex even-order aspheric lenses, and the lenses are made of fused quartz material with low thermal expansion coefficient and small infrared light absorption coefficient. The lenses are composed of a surface anti-reflection film layer and a fused quartz matrix. The purpose of multi-path output is achieved by adding reflection of a reflector to the coupling lens group. The reflector is composed of a surface anti-reflection film layer and a fused quartz matrix, wherein the fused quartz has a high melting point and can withstand high temperatures. Under high-power laser conditions, the lens will produce different degrees of thermal deformation due to differences in temperature distribution, causing thermal aberration. The requirements for lens design are: (1) cooling the temperature of each part of the lens to within an acceptable range; (2) cooling to reduce the thermal aberration of the lens caused by temperature differences. Figure 1 The figure shows a simplified optical path design diagram of the optical gate. The output optical fiber of the fiber laser serves as the input optical fiber of the optical gate. The laser output by the input optical fiber of the optical gate is collimated by a collimating mirror and approximated as parallel light. It is then deflected by a reflecting mirror and finally focused by a focusing mirror to couple the laser to the output optical fiber of the corresponding optical gate for output.

[0031] Combination Figure 2 , a method for designing a coupling lens group for an optical shutter of a point ring fiber laser, the coupling lens group includes a collimator and two focusing lenses, and both the collimator and focusing lenses are aspheric plano-convex lenses made of fused quartz. The coupling lens group proposed by the present invention couples and outputs the central circular beam and the ring beam emitted by the point ring laser at the same time, and can adapt to different laser output modes and application requirements. All parameters of the collimator and focusing lenses in the coupling lens group (including focal length, aperture, radius of curvature, edge thickness, number of high-order terms, etc.) are designed according to the parameters of the input fiber and the output fiber, and high-efficiency coupling into the ring fiber and convergence at the fiber core and the ring core are achieved under the guarantee of tolerance analysis verification, so as to achieve a simple structure and a higher flexibility and convenience in design. The specific steps are as follows:

[0032] S1: According to the input fiber and output fiber parameters of the optical shutter for the point ring fiber laser, the input condition Case of the coupling lens group is calculated as follows:

[0033] The input fiber and output fiber are both coaxial twin-core, and the input conditions of the coupling lens group are as follows:

[0034]

[0035] Among them, d 1 is the output circular core spot diameter, θ 1 is the core light spot divergence angle, d 2 is the inner diameter of the output annular spot, d 3 is the outer diameter of the output annular spot, θ 2 is the core light spot divergence angle, θout1 is the output fiber core divergence angle, D out1 is the output fiber core diameter, D out2 D is the inner diameter of the output fiber ring core, out3 is the outer diameter of the output fiber ring core, θ out2 is the divergence angle of the output fiber ring core.

[0036] S2, according to the input condition Case of the coupling lens group, the coupling lens group magnification M is designed as follows:

[0037] The calculation expression of the coupling lens group magnification M is as follows:

[0038]

[0039] Among them, D in1 is the input fiber core diameter, θ in1 is the input fiber core divergence angle, D in2 D is the inner diameter of the input fiber ring core. in3 is the outer diameter of the input fiber ring core, θ in2 is the input fiber ring core divergence angle.

[0040] S3: According to the aperture of the collimator and the diameter of the laser beam, the focal length of the collimator is calculated, the design condition parameters of the coupling lens group and the initial structural parameters of the collimator are set, and the collimator parameter equation C(M,D c ,X C );X C The expression of the high-order coefficient of the collimating mirror type, D c Indicates the collimator aperture.

[0041] The design parameters of the coupling lens group include: working band, working field, optical gate input fiber size and numerical aperture size.

[0042] The initial structural parameters of the collimator include the surface radius R of the collimator c , Collimator aperture D c , the lens material of the collimator, the lens surface shape of the collimator;

[0043] Establish the collimator parameter equation C(M,D c ,X C ):

[0044]

[0045] Among them, f c is the focal length of the collimator, n 0 is the real refractive index of the lens material fused silica at room temperature, X Cis the expression of the high-order coefficient of the collimating mirror, a, b, c are the high-order coefficients of the equation, k is the quadratic constant of the collimating mirror, 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 collimator cross section to the optical axis.

[0046] S4: Calculate the angular radius variance of the full field of view characteristic sampling point of the collimator according to the design condition parameters of the coupling lens group and the initial structure parameters of the collimator Then the optimal parameters of the collimator are obtained, including the optimal curvature radius of the collimator, the optimal edge thickness of the collimator, and the high-order coefficient; the details are as follows:

[0047] Calculate the angular radius variance of the collimator at the full field of view feature sampling points The calculation expression is as follows:

[0048]

[0049] Where h represents the horizontal coordinate of the field of view, h m represents the maximum horizontal coordinate value; v represents the vertical coordinate of the field of view, v m Indicates the maximum vertical coordinate value; θ h,v is the root mean square angular radius value under the corresponding field of view, It is the mean square angular radius value under the corresponding field of view.

[0050] S5: According to the focal length of the collimator and the magnification M of the coupling lens group, establish the focusing lens parameter equation F(M,D f ,X F ), X F The expression of the high-order coefficient of focusing mirror type, D f Represents the focusing lens aperture; then the focal length of the focusing lens is obtained, and combined with the initial structural parameters of the focusing lens, the radius variance of the spot diagram under the full field of view characteristic sampling points of the coupling lens group is calculated

[0051] The variance of the spot diagram radius under the full field of view feature sampling points The calculation expression is as follows:

[0052]

[0053] Where h represents the horizontal coordinate of the field of view, h m represents the maximum horizontal coordinate value; v represents the vertical coordinate of the field of view, v m Indicates the maximum vertical coordinate value; r h,v is the RMS point diagram radius value under the corresponding field of view, is the RMS point diagram radius value under the corresponding field of view.

[0054] S6: According to and Calculate the value of the multi-objective evaluation function, the expression is as follows:

[0055]

[0056] Where W 1 , W 2 , W 3 are weight coefficients; T const is the evaluation function determined by the ray tracing limitation and the imaging quality limitation, T i represents the value of the multi-objective function with the history number i, T is the value of the multi-objective evaluation function, It is the normalized index value of the multi-objective evaluation function.

[0057] S7: Determine whether the normalized index value of the current multi-objective evaluation function is equal to 1. If not, update the focusing mirror structure parameters to the optimal focusing mirror structure parameters corresponding to this iteration; otherwise, do not update the focusing mirror structure parameters, and output the collimating mirror and focusing mirror structure parameters of the coupling lens group at this time as the optimal parameters;

[0058] The focusing mirror structural parameters include: the surface curvature radius R of the focusing mirror f , focusing lens aperture D f , lens material of the focusing lens, lens surface shape of the focusing lens;

[0059] Establish the focusing lens parameter equation F(M,D f ,X F ):

[0060]

[0061] Among them, f f is the focal length of the focusing lens, n 0 is the real refractive index of the lens material fused silica at room temperature, X F is the coefficient expression of the focusing mirror type, A, B, C are the coefficients of the equation, K is the quadratic constant of the focusing mirror, E is the paraxial curvature of the focusing mirror, E=1 / R c , R is the distance from the focusing mirror section to the optical axis.

[0062] S8: According to the optimal structural parameters of the collimator and focusing lens in the coupling lens group, a feasibility tolerance analysis is performed 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 not, the feasibility tolerance of each optical element in the coupling lens group is updated; otherwise, the feasibility tolerance of each optical element in the coupling lens group is not updated, and the optimal collimator and focusing lens structural parameters of the coupling lens group are output, as follows:

[0063] The component tolerances of the coupling lens group are classified into eccentricity tolerance, tilt tolerance, and surface tolerance; according to the characteristics and sizes of the above tolerances, the above three tolerances are linearly combined to carry out targeted iterative analysis; firstly, the surface tolerance, eccentricity and tilt tolerance distribution of the components of the coupling lens group to be analyzed are determined, and through linear combination, while ensuring the accuracy of tolerance analysis, it is guaranteed that after the final assembly, the optical components can meet the actual feasibility after coupling adjustment; 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; otherwise, the optimal structural parameters of the collimating lens and focusing lens of the coupling lens group are output. Example 1

[0064] Combination Figure 2 and Figure 3 The method for designing a coupling lens group of a light gate for a point ring fiber laser according to the present invention comprises the following steps:

[0065] Step 1. According to the input fiber and output fiber parameters of the optical gate for the point ring fiber laser, the input condition Case of the coupling lens group is calculated. The parameters of the input and output ring fibers of the optical gate for the point ring fiber laser (input fiber core diameter 34um, input fiber core divergence angle 6.3°, input fiber ring core inner diameter 48um, input fiber ring core outer diameter 100um, input fiber ring core divergence angle 12.7°; corresponding output fiber core diameter 50um, output fiber core divergence angle 12.7°, output fiber ring core inner diameter 70um, output fiber ring core outer diameter 150um, output fiber ring core divergence angle 12.7°) meet the input condition Case.

[0066] Step 2: Design the coupling lens group magnification M according to the input condition Case of the coupling lens group. If the spatial ring laser beam is coupled with the optical fiber, it needs to be transformed (enlarged or reduced) by the optical coupling lens group to constrain the laser beam within the output range. This is the spot size d output by the point ring laser fiber. in and beam convergence angle and output fiber diameter D out And the numerical aperture angle of the optical fiber, that is,

[0067]

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

[0069] Step 3: According to 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 parameter equation C(M,D c ,X C ), as follows:

[0070]

[0071] X C The expression of the high-order coefficient of the collimating mirror type, D c Indicates the collimator aperture.

[0072] Step 4: Calculate the angular radius variance of the full field feature sampling point of the collimator according to the design condition parameters of the coupling lens group and the initial structural parameters of the collimator. Then the optimal parameters of the collimator are obtained, including the optimal curvature radius of the collimator, the optimal edge thickness of the collimator, and the high-order coefficient. According to the requirements of the coupling device and the common collimator lens aperture parameters on the market, 30mm is selected as the final parameter of the collimator aperture, the core numerical aperture of the input optical fiber is 0.11, and the ring core numerical aperture is 0.22. Simulation software is selected for optimization research. Zemax is an optical simulation analysis software. The content in the steps is implemented in the Zemax software. Establish in the Zemax software Figure 3 The geometric model and physical model of the collimator are shown in the figure. The geometric model includes the collimator (the material property is set to fused silica), the surface type is an even-order aspheric lens, the collimator lens is a plano-convex lens, the output fiber of the ring fiber laser is used as the input fiber of the optical shutter, and the light emitted is approximately a uniform conical light with a certain divergence angle emitted from a point. When the point is located at the focus of the collimator according to the optimization, the conical light is collimated by the collimator and becomes parallel light. The angular radius variance of the full field of view characteristic sampling point of the coupling lens group is calculated. minimum, at this time, the focal length of the collimator is the optimal focal length R c .

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

[0074] R c is the curvature radius of the collimator. The collimator can be approximated as a thin plano-convex lens. Its curvature radius corresponds to its focal length. The curvature radius is obtained by transforming the focal length formula of the thin plano-convex lens:

[0075] R c =(n 0 -1)f c

[0076] where n 0 is the real refractive index of the lens material fused quartz at room temperature.

[0077] Taking into account the loss caused by diffraction, the aperture of the collimator is generally more than twice the beam diameter. After collimation, the fiber laser beam diameter D is proportional to the fiber numerical aperture NA and the collimator focal length f.c The relationship is as follows

[0078] D≈2NA×f c

[0079] f c 56.5mm is more appropriate.

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

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

[0082]

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

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

[0085] Step 5: According to the focal length of the collimator and the magnification M of the coupling lens group, establish the focusing lens parameter equation F(M,D f ,X F ), X F The expression of the high-order coefficient of the collimating mirror type, D f Indicates the collimator aperture, as follows:

[0086]

[0087] Combined with the collimator lens parameters obtained in step 4 and the initial structural parameters of the focusing lens, the focusing lens aperture is the same as the collimator lens aperture, which is 30 mm. In order to facilitate processing, the edge thickness of the focusing lens is the same as that of the collimator lens. The focal length of the focusing lens is calculated based on the calculated coupling magnification M, and the point diagram radius variance under the full field of view characteristic sampling points of the coupling lens group is calculated.

[0088] Step 6: According to and Calculate the value of a multi-objective evaluation function.

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

[0090] At this point, all the parameters of the focusing mirror have been known.

[0091] Step 8: Perform feasibility tolerance analysis based on the optimal structural parameters of the collimator and focusing lens in the coupling lens group 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 not, 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 collimator and focusing lens structural parameters of the coupling lens group.

[0092] In related technologies, the RMS or PV value of the optical component surface shape is generally used to evaluate the manufacturing accuracy and assembly accuracy of the optical component. The tolerance analysis method is a probability and statistical analysis method based on Monte Carlo Monte Carlo, which is suitable for analyzing the tolerance of optical components manufactured in large quantities. 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 following table, and the element processing and adjustment tolerances are substituted into the optical design software, and the RMS diffuse spot radius is used as the evaluation standard. The sensitivity analysis model is used for analysis, and it is found that 90% of the RMS diffuse spot obtained by error superposition is concentrated in the fiber core and the ring core.

[0094] Tolerance range of each optical component

[0095] Face type Surface aperture number Center thickness error / mm Surface off-axis amount / mm Surface tilt / ° Aspheric Lenses 0.4 ±0.01 ±0.01 ±0.0167 Plane mirror 0.4 ±0.01 ±0.01 ±0.0167

[0096] Face type X eccentricity / mm Y eccentricity / mm X tilt / ° Y tilt / ° Aspheric Collimator ±0.05 ±0.05 ±0.03 ±0.03 Fixed-plane reflector 1 ±0.1 ±0.1 ±0.03 ±0.03 Fixed-plane reflector 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 coupling lens group design method for the optical shutter of the point ring fiber laser in this embodiment is a Zemax simulation light trace diagram after the error superposition and coupling adjustment. It can be seen from the figure that the core spot size is 45um and the ring spot size has an inner diameter of 63um and an outer diameter of 136.1um, which meets the coupling conditions, and the coupling efficiency formula is used again. 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 is subjected to tolerance analysis. At this time, the feasibility tolerance values ​​of each optical element in the coupling lens group meet the requirements. The optimal collimator parameters of the output coupling lens group are: the optimal focal length of the collimator is 56.5 mm, the optimal aperture of the collimator is 30 mm, the optimal radius of curvature of the collimator is -25.44, and the optimal edge thickness of the collimator is T c , the fourth-order coefficient is -5.772×10 -6 , the sixth-order coefficient is -4.262×10 -10 , focusing mirror structural parameters: the optimal focal length of the focusing mirror is 66.5mm, the optimal aperture of the focusing mirror is 30mm, the optimal curvature radius of the focusing mirror is 29.88, and the optimal edge thickness of the focusing mirror is T f , the fourth-order coefficient is -1.245×10 -6 , the sixth-order coefficient is -1.796×10 -10 .

[0098] By combining the design and optimization of the above 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 coupling lens and coupled into the ring optical fiber through a fiber connector and converged at the fiber core and the ring core, thereby achieving high-efficiency coupling of the laser light gate.

Claims

1. A method for designing a coupling lens group for a light gate of a point ring fiber laser, characterized in that: The above coupling lens group includes a collimator and two focusing lenses. Both the collimator and the focusing lenses are aspheric plano-convex lenses made of fused quartz. The coupling lens group is characterized in that the specific steps are as follows: S1: Calculate the input condition Case of the coupling lens group according to the input fiber and output fiber parameters of the optical shutter for the point ring fiber laser; S2: Design the coupling lens group magnification M according to the input condition Case of the coupling lens group; S3: According to the aperture of the collimator and the diameter of the laser beam, the focal length of the collimator is calculated, the design condition parameters of the coupling lens group and the initial structural parameters of the collimator are set, and the collimator parameter equation C(M,D c ,X C );X C The expression of the high-order coefficient of the collimating mirror type, D c Indicates the collimator aperture; S4: Calculate the angular radius variance of the full field of view characteristic sampling point of the collimator according to the design condition parameters of the coupling lens group and the initial structure parameters of the collimator Then, the optimal parameters of the collimator are obtained, including the optimal curvature radius of the collimator, the optimal edge thickness of the collimator and the high-order coefficient; S5: According to the focal length of the collimator and the magnification M of the coupling lens group, establish the focusing lens parameter equation F(M,D f ,X F ), X F The expression of the high-order coefficient of focusing mirror type, D f Represents the focusing lens aperture; then the focal length of the focusing lens is obtained, and combined with the initial structural parameters of the focusing lens, the radius variance of the spot diagram under the full field of view characteristic sampling points of the coupling lens group is calculated S6: According to and Calculate the value of the multi-objective evaluation function, the expression is as follows: Where W1, W2, and W3 are weight coefficients; T const is the evaluation function determined by the ray tracing limitation and the imaging quality limitation, T i represents the value of the multi-objective function with the history number i, T is the value of the multi-objective evaluation function, is 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 not, update the focusing mirror structure parameters to the optimal focusing mirror structure parameters corresponding to this iteration; otherwise, do not update the focusing mirror structure parameters, and output the collimating mirror and focusing mirror structure parameters of the coupling lens group at this time as the optimal parameters; S8: Perform feasibility tolerance analysis based on the optimal structural parameters of the collimator and focusing lens in the coupling lens group 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 not, 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 collimator and focusing lens structural parameters of the coupling lens group.

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

3. The method for designing a coupling lens group for a light gate for a point ring fiber laser according to claim 2, characterized in that: In S2, according to the input condition Case of the coupling lens group, the coupling lens group magnification M is designed as follows: The calculation expression of the coupling lens group magnification M is as follows: Among them, D in1 is the input fiber core diameter, θ in1 is the input fiber core divergence angle, D in2 D is the inner diameter of the input fiber ring core. in3 is the outer diameter of the input fiber ring core, θ in2 is the input fiber ring core divergence angle.

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

5. The method for designing a coupling lens group for a light gate for a point ring fiber laser according to claim 4, characterized in that: In S3, the initial structural parameters of the collimator include the surface radius R of the collimator c , Collimator aperture D c , the lens material of the collimator, the lens surface shape of the collimator; Establish the collimator parameter equation C(M,D c ,X C ): Among them, f c is the focal length of the collimator, n0 is the real refractive index of the lens material fused quartz at room temperature, X C is the expression of the high-order coefficient of the collimating mirror, a, b, c are the high-order coefficients of the equation, k is the quadratic constant of the collimating mirror, 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 collimator cross section to the optical axis.

6. The method for designing a coupling lens group for a light gate for a point ring fiber laser according to claim 5, characterized in that: In S4, the angular radius variance of the full field of view characteristic sampling point of the collimator is calculated according to the design condition parameters of the coupling lens group and the initial structure parameters of the collimator. Then the optimal parameters of the collimator are obtained, as follows: Calculate the angular radius variance of the collimator at the full field of view feature sampling points The calculation expression is as follows: Where h represents the horizontal coordinate of the field of view, h m represents the maximum horizontal coordinate value; v represents the vertical coordinate of the field of view, v m Indicates the maximum vertical coordinate value; θ h,v is the root mean square angular radius value under the corresponding field of view, It is the mean square angular radius value under the corresponding field of view.

7. The method for designing a coupling lens group for a light gate for a point ring fiber laser according to claim 6, characterized in that: In S5, the radius variance of the spot diagram under the full field of view feature sampling points The calculation expression is as follows: Where h represents the horizontal coordinate of the field of view, h m represents the maximum horizontal coordinate value; v represents the vertical coordinate of the field of view, v m Indicates the maximum vertical coordinate value; r h，v is the RMS point diagram radius value under the corresponding field of view, is the RMS point diagram radius value under the corresponding field of view.

8. The method for designing a coupling lens group for a light gate for a point ring fiber laser according to claim 7, characterized in that: In S7, the focusing mirror structural parameters include: the surface curvature radius R of the focusing mirror f , focusing lens aperture D f , lens material of the focusing lens, lens surface shape of the focusing lens; Establish the focusing lens parameter equation F(M,D f ,X F ): Among them, f f is the focal length of the focusing lens, n0 is the real refractive index of the lens material fused quartz at room temperature, X F is the coefficient expression of the focusing mirror type, A, B, C are the coefficients of the equation, K is the quadratic constant of the focusing mirror, E is the paraxial curvature of the focusing mirror, E=1 / R c , R is the distance from the focusing mirror section to the optical axis.

9. The method for designing a coupling lens group for a light shutter for a point ring fiber laser according to claim 8, characterized in that: In S8, a feasibility tolerance analysis is performed based on the optimal structural parameters of the collimating lens and the 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 component tolerances of the coupling lens group are classified into eccentricity tolerance, tilt tolerance, and surface shape tolerance; according to the characteristics and sizes of the above tolerances, the above three tolerances are linearly combined to carry out targeted iterative analysis; first, the surface shape tolerance, eccentricity and tilt tolerance distribution of the components of the coupling lens group to be analyzed are determined, and through linear combination, while ensuring the accuracy of tolerance analysis, it is ensured that after the final assembly, the optical components can meet the actual feasibility after coupling adjustment; 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; On the contrary, the output coupling lens group collimator and focusing lens have optimal structural parameters.

Images