A multi-pass folded beam quality enhancement system

Abstract

A multi-pass folded beam quality enhancement system. It belongs to the technical field of laser optical design, and specifically to the technical field of laser beam quality control. It solves the problem of beam quality degradation caused by thermal effects in previous multi-pass folded cavities. The system includes a laser diode pump source, an M1 reflector, a gain medium crystal Tm:YLF, a combined cylindrical lens, an M2 reflector, and an output coupling mirror. By arranging a group of cylindrical lenses with different focal lengths in the multi-pass folded cavity, the light beam in the cavity can be flexibly adjusted. Due to its special focusing characteristics, the cylindrical lens can concentrate the light beam in one direction and maintain parallel propagation in another direction. Therefore, it is suitable for compensating for a specific direction of the light beam in the multi-pass cavity. By designing a group of cylindrical lenses with different focal lengths, the present invention can achieve segmented compensation of the light beam at different propagation positions, greatly improving the flexibility and accuracy of compensation.

Description

Technical Field

[0001] The invention belongs to the technical field of laser optical design, and in particular relates to the technical field of laser beam quality control. Background Art

[0002] With the widespread application of high-power lasers in industrial processing, medical treatment, scientific research and other fields, how to maintain the high quality of laser beams while outputting high power has become a key technical issue in laser design. Especially in multi-pass folded cavity lasers, the beam passes through the gain medium many times, which inevitably brings about thermal effects, mode differences, optical distortion and other effects while generating cumulative gain. These factors will lead to a significant reduction in the quality of the laser beam. In order to overcome these problems, many optical design schemes are committed to maintaining high stability and high focus of the beam in the cavity by compensating and correcting the beam quality.

[0003] In the design of high-power lasers, the multi-pass folded cavity structure is a commonly used gain enhancement method. By allowing the laser beam to travel back and forth in the gain medium multiple times, the multi-pass folded cavity can effectively increase the optical path of the beam, thereby significantly improving the gain of the laser. Especially in solid-state lasers, the use of a multi-pass folded cavity structure can achieve higher output power within a limited gain medium length.

[0004] However, while the multi-pass folded cavity brings about gain improvement, it also has an adverse effect on the beam quality. As the light beam repeatedly passes through the gain medium and reflective elements in the cavity, it is affected by the thermal effect each time it passes through the gain medium, causing the optical path of the light beam to gradually distort. This distortion not only affects the spatial mode of the light beam, but also causes problems such as spot divergence and uneven focusing, resulting in a decrease in the quality of the output beam. In addition, during the multiple reflections in the folded cavity, the focus changes between each optical path are different, further exacerbating the degradation of the beam quality. Summary of the invention

[0005] In order to solve the problem of degradation of beam quality caused by thermal effect in conventional multi-pass folded cavities, the present invention provides a multi-pass folded beam quality enhancement system.

[0006] The system includes a laser diode pump source, an M1 reflector, a gain medium crystal Tm:YLF, a combined cylindrical lens, an M2 reflector and an output coupling mirror;

[0007] The laser diode pump source generates a pump beam for exciting a gain medium crystal Tm:YLF and injects the pump beam into the gain medium crystal Tm:YLF. The gain medium crystal Tm:YLF generates laser gain under the action of the pump beam and emits a laser beam. The M1 reflector, the gain medium crystal Tm:YLF, the combined cylindrical lens and the M2 reflector form a multi-pass folding cavity, wherein the gain medium crystal Tm:YLF and the combined cylindrical lens are placed between the M1 reflector and the M2 reflector. Under the reflection action of the M1 reflector and the M2 reflector, the laser beam repeatedly passes through the gain medium crystal Tm:YLF and the combined cylindrical lens and is finally output through an output coupling mirror.

[0008] Furthermore, the combined cylindrical lens includes an F1 cylindrical lens, an F2 cylindrical lens, an F3 cylindrical lens and an F4 cylindrical lens, and each cylindrical lens is arranged in sequence in the folding direction of the light path. In the multi-pass folding cavity, the laser beam passes through the F1 cylindrical lens, the F2 cylindrical lens, the F3 cylindrical lens and the F4 cylindrical lens in sequence, wherein the F1 cylindrical lens is used for preliminary focusing compensation, the F2 cylindrical lens and the F3 cylindrical lens are used for gradually adjusting the laser beam, and the F4 cylindrical lens is used for finally adjusting the laser beam.

[0009] Furthermore, in the combined cylindrical lens, the focal length of each cylindrical lens is ( =1, 2, 3, 4) meet: .

[0010] Furthermore, in the combined cylindrical lens, the focal length of each cylindrical lens is satisfy: ;

[0011] in is the equivalent radius of curvature of the laser beam after passing through the thermal lens effect area, is the refractive index of the corresponding cylindrical lens material.

[0012] Furthermore, the thermal lens effect region occurs within the range of the gain medium crystal Tm:YLF.

[0013] Furthermore, the laser diode pump source generates a 792 nm pump light beam for exciting the gain medium crystal Tm:YLF.

[0014] Furthermore, the M1 reflector and the M2 reflector are both inclined plane mirrors, and the M1 reflector is also used to guide the pump light from the laser diode pump source to the gain medium crystal Tm:YLF.

[0015] The beneficial effects of the system of the present invention are:

[0016] By setting a group of cylindrical lenses with different focal lengths in a multi-pass folded cavity, the light beam in the cavity can be flexibly adjusted. Due to its special focusing characteristics, the cylindrical lens can concentrate the light beam in one direction and maintain parallel propagation in another direction. Therefore, it is suitable for compensating for a specific direction of the light beam in the multi-pass cavity. By designing a group of cylindrical lenses with different focal lengths, the present invention can achieve segmented compensation of the light beam at different propagation positions, greatly improving the flexibility and accuracy of compensation. At the same time, the multi-focal length characteristics of the cylindrical lens enable it to be differentially adjusted according to the mode characteristics of the light beam at different positions in the cavity, thereby effectively eliminating the influence of thermal lens effect, mode difference, optical distortion, etc. on the light beam quality.

[0017] In addition, the multi-focal length cylindrical lens group of the present invention can not only compensate for the quality of the light beam, but also optimize the matching degree between the cavity mode and the pump light through a specific focal length combination, so that the laser can maintain stable output characteristics under high-power operation. This innovative design greatly broadens the application range of lasers in high-power and long-term operation, and also provides technical support for high-precision industrial processing, medical applications, and precision measurement. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 It is a structural diagram of a multi-pass folded beam quality enhancement system in an embodiment of the present invention;

[0019] Figure 2 Schematic diagram of the equivalent cavity of the multi-pass folding cavity in an embodiment of the present invention. DETAILED DESCRIPTION

[0020] The technical solution of the present invention will be described clearly and completely below in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the protection scope of the present invention.

[0021] The system is as Figure 1 As shown, it includes a laser diode pump source 1, an M1 reflector 2, a gain medium crystal Tm: YLF3, a combined cylindrical lens 10, an M2 reflector 8 and an output coupling mirror 9;

[0022] The laser diode pump source 1 generates a pump beam for exciting the gain medium crystal Tm: YLF3 and injects it into the gain medium crystal Tm: YLF3. The gain medium crystal Tm: YLF3 generates laser gain under the action of the pump beam and emits a laser beam. The M1 reflector 2, the gain medium crystal Tm: YLF3, the combined cylindrical lens 10 and the M2 reflector 8 form a multi-pass folding cavity, wherein the gain medium crystal Tm: YLF3 and the combined cylindrical lens 10 are placed between the M1 reflector 2 and the M2 reflector 8. Under the reflection action of the M1 reflector 2 and the M2 reflector 8, the laser beam repeatedly passes through the gain medium crystal Tm: YLF3 and the combined cylindrical lens 10, and is finally output through the output coupling mirror 9.

[0023] The combined cylindrical lens 10 includes an F1 cylindrical lens 4, an F2 cylindrical lens 5, an F3 cylindrical lens 6 and an F4 cylindrical lens 7, and each cylindrical lens is arranged in sequence in the folding direction of the light path. In the multi-pass folding cavity, the laser beam passes through the F1 cylindrical lens 4, the F2 cylindrical lens 5, the F3 cylindrical lens 6 and the F4 cylindrical lens 7 in sequence, wherein the F1 cylindrical lens 4 is used for preliminary focusing compensation, the F2 cylindrical lens 5 and the F3 cylindrical lens 6 are used for gradually adjusting the laser beam, and the F4 cylindrical lens 7 is used for finally adjusting the laser beam.

[0024] The core goal of the preliminary focusing compensation is to make preliminary adjustments to the beam divergence caused by the thermal lens effect and other reasons, to ensure that the laser beam propagates in the cavity with good quality. The ultimate goal of the preliminary focusing compensation is to ensure that when the laser beam enters the gain medium crystal Tm: YLF3, its spot is moderate and of high quality, and it can effectively control the divergence caused by the thermal lens effect, thereby improving the efficiency and stability of the entire system. Through the F1 cylindrical lens 4, it can be ensured that the propagation state of the laser beam meets the requirements of the intracavity mode during the operation of the system, avoiding excessive divergence or distortion of the beam.

[0025] The gradual adjustment of the laser beam is a key adjustment step in the designed multi-pass folded beam quality enhancement system, which is achieved by combining the F2 cylindrical lens 5 and the F3 cylindrical lens 6 in the cylindrical lens group. The core goal of this process is to further adjust the quality, size and divergence angle of the beam due to the thermal effect and the gradual change of beam propagation during the propagation of the beam in the multi-pass folded cavity, so as to ensure that the laser beam can maintain good mode and beam quality after passing through the gain medium crystal Tm:YLF3 and the entire optical path.

[0026] The final adjustment of the laser beam is completed by the F4 cylindrical lens 7 (the last lens in the combined cylindrical lens 10). The core goal of this process is to optimize the beam quality and propagation characteristics of the laser beam after passing through the gain medium crystal Tm: YLF3 and multiple folding reflections, and finally passing through the output coupling mirror 9. By finely adjusting the focal length of the F4 cylindrical lens 7, the error of the previous cylindrical lens can be compensated, the transmission of the beam can be optimized, and the final output laser beam can be guaranteed to have high quality, high efficiency and stable performance.

[0027] In the multi-pass folding cavity, the laser beam is reflected and transmitted multiple times from the gain medium crystal, and passes through cylindrical lenses of different focal lengths in sequence to achieve the focusing adjustment of the beam. The optical path in the cavity starts from the initial M2 reflector 8, passes through the combined cylindrical lens 10 and the gain medium crystal Tm:YLF3, and then reflects to the M1 reflector 2. The four-pass transmission is achieved through the cooperation of the reflector and the cylindrical lens group, and finally the laser is output by the output coupling mirror 9.

[0028] M1 reflector 2 and M2 reflector 8: used for multiple reflections in the cavity, and cooperate with the combined cylindrical lens 10 to realize a four-pass folded optical path.

[0029] In order to achieve effective beam quality compensation, the focal length of the combined cylindrical lens 10 needs to be designed according to the beam divergence angle caused by the thermal lens effect and the cavity mode structure. The focal length of each cylindrical lens is set to ( =1, 2, 3, 4) to compensate for the divergence and focusing effects of the light beam at different positions. By adjusting the focal length of each cylindrical lens, the focusing effect of the light beam at different propagation sections can be precisely controlled so that the mode radius of the light beam in the cavity remains stable.

[0030] In order to achieve effective compensation, the focal length of each cylindrical lens The following conditions are met:

[0031]

[0032] in is the equivalent radius of curvature of the light beam after passing through the thermal lens effect area, is the refractive index of the corresponding cylindrical lens material.

[0033] The thermal lens effect region mainly occurs within the range of the gain medium crystal Tm:YLF3. Inside the crystal, due to the non-uniform absorption of the pump beam and heat deposition, the temperature distribution presents a gradient change, thus forming an equivalent "thermal lens". In this region: the accumulation of heat causes the refractive index in the crystal to be unevenly distributed along the radial direction, forming a parabolic refractive index distribution, which is equivalent to a positive lens, resulting in the divergence or focusing of the laser beam. It is the main source of beam divergence and mode distortion.

[0034] According to the optical path length and transmission characteristics of the four-pass cavity, the focal length of the cylindrical lens must be selected to meet the requirements for reducing the equivalent thermal lens effect in multi-pass transmission, so that the divergence angle of the laser on each path in the cavity is always effectively suppressed.

[0035] The combined cylindrical lens 10 is designed to include four cylindrical lenses, which are respectively configured as: F1 cylindrical lens 4, F2 cylindrical lens 5, F3 cylindrical lens 6 and F4 cylindrical lens 7.

[0036] Due to the problem of beam divergence caused by thermal lens effect, the equivalent lens method is used to describe the influence of thermal effect of gain medium on the beam. According to the characteristics of thermal effect, the gain medium can be equivalent to a focusing lens with a focal length of Determined by thermal effect parameters and pump power, the specific formula is as follows:

[0037] ;

[0038] in is the thermal conductivity of the material, is the area of ​​the crystal, is the pump power.

[0039] By adding the thermal lens effect equivalent focal length into the design of the combined cylindrical lens 10 and combining the cylindrical lens compensation with different focal lengths, the beam divergence can be effectively reduced and the beam quality can be improved.

[0040] Laser diode pump source 1 is a gain medium crystal Tm:YLF3 pump source, generating a 792nm pump beam for exciting the gain medium crystal Tm:YLF3. The beam is focused and transmitted through a lens group in the optical path. M1 reflector 2 is an inclined plane mirror that guides the pump light from laser diode pump source 1 to the gain medium crystal Tm:YLF3. M1 reflector 2 adjusts the beam of laser diode pump source 1 to the correct direction so that it can effectively enter the gain medium crystal Tm:YLF3 in the subsequent optical path.

[0041] Under the action of pump light, the gain medium crystal Tm:YLF3 generates laser gain and emits laser output in the near-infrared band. The laser beam passes through the crystal multiple times to increase the laser gain. The combined cylindrical lens 10 is used to compensate for the focusing and divergence of the beam. Each cylindrical lens has a different focal length and is arranged in order from short to long. When the beam passes through these cylindrical lenses, the spot is adjusted so that it gradually compensates for the wavefront distortion caused by the thermal effect in the gain medium. The F1 cylindrical lens 4 is used for preliminary focusing compensation, the F2 cylindrical lens 5 and the F3 cylindrical lens 6 are used to gradually adjust the beam, and the F4 cylindrical lens 7 is used for the final fine adjustment. The M2 reflector 8 is a reflector that further guides the beam so that it returns after passing through the combined cylindrical lens 10 in the cavity to achieve multi-pass beam transmission. Through the reflection of the M2 reflector 8, the beam can pass through the gain medium multiple times, thereby increasing the laser gain and output power. The output coupling mirror 9 is the output mirror of the multi-pass folded cavity, allowing part of the beam to pass through to form laser output. The curvature radius and transmittance design of the output coupling mirror 9 help control the beam quality and achieve efficient laser output.

[0042] FIG2 is a schematic diagram of a multi-pass folded cavity equivalent cavity. It can be seen that the present invention also saves the space of the laser to a great extent. The design idea of ​​the system of the present invention is further described:

[0043] When designing the focal length of the combined cylindrical lens 10, it is necessary to fully consider the changing characteristics of the light beam in the multi-pass folded cavity to achieve effective compensation for the beam quality. The basic principle of this design relies on the focusing and divergence effects experienced by the light beam during transmission in the cavity. By introducing cylindrical lenses with different focal lengths, the wavefront distortion and defocusing effects accumulated due to multiple transmissions of the light beam can be gradually offset.

[0044] In a multi-pass folded cavity, each time the light beam passes through the gain medium, it will be affected by the thermal lens effect, which will change the focusing state of the light beam, causing it to defocus or focus in the gain medium. Therefore, the first step in designing the combined cylindrical lens 10 is to study the mode evolution and divergence trend of the light beam in multi-pass transmission through ABCD transmission matrix analysis, and then determine which positions require focusing compensation of the light beam.

[0045] In order to effectively compensate for the defocus or focus changes experienced by the light beam on different transmission paths, the focal length design of the combined cylindrical lens 10 needs to follow two main principles. The first is the gradual focal length, that is, the focal length of the cylindrical lens should gradually transition from short to long, or be nonlinearly distributed according to the changing requirements of the light beam during transmission. This gradual focal length design can gradually compensate for the divergence or focusing trend of the light beam. Secondly, a "compensation chain" is formed by combining lenses with different focal lengths, so that the beam transformation effects of each lens are superimposed on each other, thereby gradually correcting the beam quality.

[0046] The focal length design of the cylindrical lens group is based on the changes in the Rayleigh length and spot radius of the light beam. The Rayleigh Range describes the range of changes when the light beam is focused or defocused. During the design process, for positions with large beam divergence, a cylindrical lens with a shorter focal length should be selected to quickly converge the beam; for positions with small divergence, a lens with a longer focal length should be used for slight adjustments. This can alleviate beam distortion and ensure that the quality of the spot on different paths remains uniform.

[0047] By configuring cylindrical lenses with different focal lengths step by step, the beam quality can be gradually compensated, so that the beam gradually stabilizes during transmission. This design not only effectively offsets the wavefront distortion caused by the multi-pass folded cavity, but also ensures that the output beam is close to the Gaussian mode or other ideal mode, thereby improving the overall performance of the laser.

[0048] In a specific design, the combined cylindrical lens 10 can be composed of a plurality of lenses with different focal lengths to form a progressive compensation structure. For example, assuming that the combined cylindrical lens 10 needs to include four lenses, the focal lengths from short to long are , , and . Each lens is designed and positioned according to specific principles. The F1 cylindrical lens 4 has a short focal length and is used primarily for initial beam convergence, especially at the first reflection position after the beam leaves the gain medium. The F2 cylindrical lens 5 has a slightly longer focal length than the F1 cylindrical lens 4 and further corrects the divergence of the beam, and the F3 cylindrical lens 6 has a medium focal length and is used to gradually adjust the partially converged beam. Finally, the F4 cylindrical lens 7 has the longest focal length and is used primarily for fine-tuning the beam to ensure that the beam maintains a good pattern and divergence before entering the output mirror.

[0049] This progressive focal length design allows the compensation effects of each lens to be superimposed on each other, gradually reducing the distortion of the light beam. When designing a specific focal length, the changes in the Rayleigh length and spot radius of the light beam must also be considered. In locations where the light beam diverges greatly, a cylindrical lens with a shorter focal length is usually selected to achieve rapid convergence; in areas where the light beam diverges less, a cylindrical lens with a longer focal length can further stabilize the light beam. When designing the focal length, the transmission of the light beam can be simulated by the ABCD matrix transmission method, and the spot size and propagation parameters in each pass can be calculated to determine the appropriate lens focal length. The step-by-step compensation design method helps to maintain the consistency and stability of the beam quality in a multi-pass folded cavity.

[0050] The installation position and angle of the cylindrical lens are also crucial to the compensation effect. Each lens should be precisely arranged according to the propagation path and direction of the light beam to ensure that the light beam can be effectively converged by the cylindrical lens F14 in the first pass after passing through the gain medium, and other lenses follow in sequence. In order to avoid optical aberrations caused by angular errors, the surface of the lens should be perpendicular to the propagation direction of the light beam, and an adjustable lens bracket can be designed to facilitate fine-tuning of the lens angle.

[0051] In terms of material selection, the cylindrical lens should be made of materials with high thermal stability and low absorption coefficient, such as fused quartz or high-quality K9 glass, to ensure thermal stability under high-power lasers. At the same time, the installation of the lens group requires the use of a high-precision positioning system to ensure that the position and angle of each lens accurately meet the design requirements and ensure the consistency of the compensation effect.

[0052] By rationally designing and optimizing the focal length of the combined cylindrical lens 10, the beam quality can be effectively improved and the overall performance of the laser can be enhanced. Compared with the traditional single-focal-length lens group, this progressive focal length design has obvious advantages in the following aspects: first, it enhances the beam quality, and makes the beam quality close to the ideal mode through gradual compensation; second, it improves the adaptability of the system, and different focal length combinations adapt to the complex beam path in the multi-pass folding cavity, and can better cope with the distortion and divergence of the beam; third, it supports high-power output, and the optimized material and focal length design enable the lens group to maintain stable optical properties under high-power conditions, thereby supporting the high-power operation of the laser.

[0053] In summary, by rationally designing the combined cylindrical lens 10 with different focal lengths, effective compensation for the beam quality in the multi-pass folded cavity can be achieved, and this design has important value in engineering applications. By optimizing and improving the design of the combined cylindrical lens 10, the beam quality of the laser can be significantly improved, providing a reliable solution for high-power laser applications.

Claims

1. A multi-pass folded beam quality enhancement system, characterized in that: The system comprises a laser diode pump source (1), an M1 reflector (2), a gain medium crystal Tm:YLF (3), a combined cylindrical lens (10), an M2 reflector (8) and an output coupling mirror (9); The laser diode pump source (1) generates a pump beam for exciting the gain medium crystal Tm:YLF (3) and injects the pump beam into the gain medium crystal Tm:YLF (3); the gain medium crystal Tm:YLF (3) generates laser gain under the action of the pump beam and emits a laser beam; the M1 reflector (2), the gain medium crystal Tm:YLF (3), the combined cylindrical lens (10) and the M2 reflector (8) form a multi-pass folding cavity, wherein the gain medium crystal Tm:YLF (3) and the combined cylindrical lens (10) are placed between the M1 reflector (2) and the M2 reflector (8); under the reflection action of the M1 reflector (2) and the M2 reflector (8), the laser beam repeatedly passes through the gain medium crystal Tm:YLF (3) and the combined cylindrical lens (10) and is finally output through the output coupling mirror (9); The combined cylindrical lens (10) comprises an F1 cylindrical lens (4), an F2 cylindrical lens (5), an F3 cylindrical lens (6) and an F4 cylindrical lens (7), wherein the cylindrical lenses are arranged in sequence in the direction of light path folding, and in the multi-pass folding cavity, the laser beam passes through the F1 cylindrical lens (4), the F2 cylindrical lens (5), the F3 cylindrical lens (6) and the F4 cylindrical lens (7) in sequence, wherein the F1 cylindrical lens (4) is used for preliminary focus compensation, the F2 cylindrical lens (5) and the F3 cylindrical lens (6) are used for stepwise adjustment of the laser beam, and the F4 cylindrical lens (7) is used for final adjustment of the laser beam; In the combined cylindrical lens (10), the focal length of each cylindrical lens is , =1, 2, 3, 4, satisfying: ; ;in, represents the focal length of the F1 cylindrical lens (4), represents the focal length of the F2 cylindrical lens (5), represents the focal length of the F3 cylindrical lens (6), represents the focal length of the F4 cylindrical lens (7); in is the equivalent radius of curvature of the laser beam after passing through the thermal lens effect area, is the refractive index of the corresponding cylindrical lens material; the thermal lens effect region occurs within the range of the gain medium crystal Tm:YLF (3).

2. The multi-pass folded beam quality enhancement system according to claim 1, characterized in that: The laser diode pump source (1) generates a 792 nm pump beam for exciting the gain medium crystal Tm:YLF (3).

3. The multi-pass folded beam quality enhancement system according to claim 2, characterized in that: The M1 reflector (2) and the M2 reflector (8) are both inclined plane mirrors. The M1 reflector (2) is also used to guide the pump light from the laser diode pump source (1) to the gain medium crystal Tm:YLF (3).

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