Adjustable light spot laser based on spindle ring optical fiber

Abstract

The invention discloses an adjustable light spot laser based on a spindle annular optical fiber, which comprises a central area laser module, an annular area laser module and an annular beam laser combiner, and overcomes the problem that the spindle annular optical fiber in the prior art can only output a simple light spot light field. The high-power annular fiber laser capable of improving the non-linear threshold value is obtained, and meanwhile the quality of light beams is not reduced.

Description

Technical Field

[0001] This invention relates to a tunable spot laser based on a spindle-shaped ring fiber, used to obtain a tunable fiber ring spot with high brightness and a high nonlinear threshold at the center. Background Technology

[0002] In the field of laser welding, there are high requirements for laser energy output. The higher the energy output of the laser, the more welding fields it can be applied to. In particular, the welding of aerospace and certain special materials requires very high laser energy density. However, when a high-energy laser beam is output from an optical fiber, it is easy to excite nonlinear effects, which limits the output energy.

[0003] Currently, the main method for suppressing nonlinear effects in fiber lasers is to use chirped tilt gratings. However, this method is mainly used for fiber lasers with a core diameter ≤30 micrometers. For fibers with larger mode fields, the number of fiber modes increases with the increase of the mode field area, making the fabrication of chirped tilt gratings complex and difficult to achieve good results, thus limiting its application range. In addition, the reflection effect of chirped gratings can couple nonlinear laser components to the cladding, which can amplify the nonlinear laser in the cladding.

[0004] Existing technologies have developed techniques to address fiber nonlinearity effects using spindle-shaped optical fibers. Xi Xiaoming from the National University of Defense Technology designed an integrated oscillation amplification laser using spindle-shaped optical fibers, and Shi Jianhong from Huazhong University of Science and Technology designed a fiber laser that utilizes spindle-shaped optical fibers to enhance the Raman threshold. However, these spindle-shaped fiber designs are relatively simple, only capable of outputting extremely simple spot light fields, making them unsuitable for specific laser welding applications and complex beam scenarios. Furthermore, there is a strong demand in the welding field for low-nonlinearity, high-power output of special spot light fields. For example, ring-shaped tunable spot laser welding technology, capable of achieving deep welding depths and low spatter effects, has emerged and is now widely used in deep-penetration welding. Improving the output power of ring lasers is particularly important; however, the high-power output of ring fiber lasers (core or ring region) is severely limited by nonlinear effects. Therefore, the inventors aim to develop a new spindle-shaped optical fiber capable of outputting a high-power ring laser with a high nonlinear threshold.

[0005] Based on this, the engineers of this invention creatively conceived of designing a new tunable spot laser based on a spindle-shaped ring fiber, which can achieve high-power ring laser output with a high nonlinear threshold. Summary of the Invention

[0006] The purpose of this invention is to propose a tunable spot laser based on a spindle-shaped fiber, which overcomes the problem that the existing spindle-shaped fiber can only output a simple optical field, and obtains a high-power ring fiber laser with improved nonlinear threshold, without reducing beam quality.

[0007] To address the aforementioned problems, this invention provides an tunable spot laser based on a spindle-shaped circular fiber, comprising a central region laser module, a circular region laser module, and a circular beam laser combiner. The output fiber of the circular beam laser combiner is a spindle-shaped circular fiber, which includes a first non-conical region, a first conical region, a second non-conical region, a second conical region, and a third non-conical region connected sequentially from the incident end towards the output end. The diameter of the first conical region gradually increases along the output direction, and the diameter of the second conical region gradually decreases along the output direction. The center of the spindle-shaped circular fiber is a central core. A first cladding layer is disposed immediately outside the central fiber core, covering the central fiber core. A second cladding layer is disposed immediately outside the first cladding layer, covering the first cladding layer. A third cladding layer is disposed immediately outside the second cladding layer, covering the second cladding layer. A fourth cladding layer is disposed immediately outside the third cladding layer, covering the third cladding layer. A coating layer is disposed immediately outside the fourth cladding layer, covering the fourth cladding layer. The ring beam laser combiner uses an (N+1)*1 combiner to combine the laser beams output from the central region laser module and the ring region laser module and output them through a spindle ring fiber. N is a positive integer greater than 1.

[0008] Preferably, the input end of the ring beam laser combiner includes a central fiber and N side fibers. The ring region laser module includes N ring region laser units, which respectively input lasers into the N side fibers. The core of the central fiber is fused with the core of the first non-conical region of the spindle ring fiber, and the central laser is injected into the core of the spindle ring fiber. The cores of the N side fibers are fused with the cores of the ring cladding of the first non-conical region of the spindle ring fiber, and the ring laser is injected into the ring cladding of the spindle ring fiber; N is greater than 3.

[0009] Preferably, in the spindle ring fiber, the refractive index of the first cladding is less than the refractive index of the central core, and the refractive index of the first cladding is less than the refractive index of the second cladding; the refractive index of the third cladding is less than the refractive index of the second cladding; the refractive index of the third cladding is less than the refractive index of the fourth cladding; the central fiber core is fused to the central core of the first non-conical region of the spindle ring fiber, and the N side fiber cores are fused to the second cladding of the first non-conical region of the spindle ring fiber.

[0010] Preferably, the central core, the second cladding, and the fourth cladding are composed of pure quartz material, while the first cladding and the third cladding are composed of fluorine-doped quartz material with low refractive index.

[0011] Preferably, both the first and second cone regions are uniformly tapered, with the same cone length and tapering ratio, and opposite tapering directions. The first and second cone regions have a symmetrical conical structure. The mode field diameter of the second non-conical region is larger than the mode field diameter of the light in the first and second cone regions, and the second non-conical region is a large mode field transmission region.

[0012] Preferably, both the first and second conical regions meet the thermal tapering condition, the length of the first conical region is greater than 3m, the length of the second conical region is greater than 3m, and the fiber length of the second non-conical region is greater than or equal to 10m.

[0013] Preferably, the output power of the central region laser module is greater than 3kW, the output power of a single ring region laser unit is greater than 2kW, and N is greater than or equal to 6.

[0014] Preferably, a cladding stripper is provided on the third non-conical region to strip the cladding light. A CO2 laser is used to etch the cladding diameter with an etching depth greater than or equal to 25 μm and a roughened region length greater than or equal to 5 cm, generating a regular groove structure on the cladding surface. An output device is provided at the output end of the third non-conical region, and a CO2 fusion splicer is used to fusion splice the output fiber to the quartz end cap.

[0015] Preferably, the fiber parameters in the first non-conical region and the third non-conical region are the same. In the first non-conical region and the third non-conical region, the fiber core diameter is 10-40 μm; the diameter of the first cladding is 15-50 μm, the diameter of the second cladding is 50-130 μm, the diameter of the third cladding is 60-150 μm, the diameter of the fourth cladding is 150-280 μm, and the diameter of the coating layer is 170-300 μm; the numerical aperture of the core is 0.06-0.22, the numerical aperture of the second cladding is 0.1-0.24, and the fiber length in the first non-conical region or the third non-conical region is 0.3m-10m.

[0016] Preferably, in the second non-conical region, the diameter of the central core is 20-80 μm, the diameter of the first cladding is 30-100 μm, the diameter of the second cladding is 100-260 μm, the diameter of the third cladding is 120-300 μm, the diameter of the fourth cladding is 300-560 μm, the diameter of the coating layer is 340-600 μm, the numerical aperture of the central core is 0.06-0.22, and the numerical aperture of the second cladding is 0.1-0.24. The length of the second non-conical region is ≥10 m.

[0017] The beneficial effects of this invention are as follows: by using a specially designed spindle-shaped ring fiber as the output power fiber of the combiner, it overcomes the problem that existing spindle-shaped fibers can only output simple optical fields, thus obtaining a high-power ring fiber laser with an improved nonlinear threshold, while not reducing beam quality. Utilizing a ring beam laser combiner with a double-grooved structure, it can suppress the nonlinear effects of specific high-power ring optical fields and effectively constrain the cladding ring light, thereby improving light extraction efficiency and ring fiber laser output power, while ensuring that the output fiber quality does not deteriorate. Based on the spindle-shaped design of the special double-grooved ring fiber, on the one hand, it significantly increases the power density of the ring laser in the core and ring region within the transmission ring fiber, suppressing the nonlinear effects caused by high peak power density and increasing the upper limit of system output power. On the other hand, it can increase the length of the special ring fiber laser transmission cable while effectively constraining the ring light, increasing the welding working space range supported by the special ring laser. Simultaneously, the combination of its side fibers and center fiber with the spindle-shaped double-grooved ring fiber also allows for adjustable ring spot. The side fibers can use elliptical cores, significantly improving coupling efficiency. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the laser structure of the present invention.

[0019] Figure 2 This is a schematic diagram of the structure of the ring beam laser combiner of the present invention.

[0020] Figure 3 This is a schematic diagram of the structure of the spindle-shaped annular optical fiber of the present invention.

[0021] Figure 4 This is the optical transmission diagram of the first cone region of the present invention.

[0022] Figure 5 This is a light distribution diagram within the spindle-shaped annular fiber of the present invention. Detailed Implementation

[0023] The technical solution of the present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0024] like Figure 1 As shown, this invention discloses an adjustable spot laser based on a spindle-shaped ring fiber. The laser mainly includes a central region laser module 1, a ring region laser module 2, a ring beam laser combiner 3, a cladding stripper 4, and an output device 5; the output fiber of the ring laser combiner is a spindle-shaped ring fiber 8.

[0025] The central region laser module 1 preferably includes at least one high-power fiber laser built based on a MOPA or resonant cavity structure, preferably with an output power ≥ 3kW; the output fiber parameters are preferably 14 / 250 / 0.07 (the parameters adopt the common expression method of the prior art, and their format is "core diameter μm / cladding diameter μm / numerical aperture", and the fiber parameters in the following text are also expressed in the same format). The central region laser module 1 is used to inject into the central core of the ring fiber to provide the central spot laser power for the tunable spot. The central region laser module preferably includes an independent driver, which can independently control its output power.

[0026] The ring-area laser module 2 comprises N high-power fiber lasers built on MOPA or resonant cavity structures (preferably, N is a positive integer greater than 1, preferably, N is greater than or equal to 3, preferably, N is greater than or equal to 6, preferably, N is 6). In the ring-area laser module 2, the output power of a single high-power fiber laser built on MOPA or resonant cavity structures is preferably greater than or equal to 2kW. A single high-power fiber laser built on MOPA or resonant cavity structures can be referred to as a ring-area laser unit, meaning the ring-area laser module comprises N ring-area laser units. The ring-area laser module 2 includes N output fibers for outputting laser light from the N high-power fiber lasers built on MOPA or resonant cavity structures. The type of output fiber used in the ring-area laser module 2 is preferably 20 / 250 / 0.07. The ring-area laser module 2 is used to inject laser light into the annular cladding region of the annular fiber to provide an annular spot laser power with an adjustable output spot. The ring-area laser module preferably includes an independent driver, allowing independent control of its output power.

[0027] The ring beam laser combiner 3 preferably employs an (N+1)*1 combiner to combine the laser beams output from the central region laser module and the ring region laser module, and output them through the spindle-shaped ring fiber 8. The input end of the ring beam laser combiner includes one central fiber and N side fibers. See also Figure 2For example, when the number N of high-power fiber lasers in the ring region laser module 2 is 6, the ring beam laser combiner is a 7*1 combiner. The input fiber consists of 7 fibers, fabricated using a sleeve tapering technique. These 7 fibers include one central fiber and 6 side fibers. The central fiber parameters are preferably 14 / 250 / 0.07. The core of this central fiber is fused with the central core (preferably with a diameter d = 15 μm) of the first non-tapered region 81 of the spindle ring fiber, and the central laser is injected into the central core of the spindle ring fiber. The 6 side fibers are preferably of type 20 / 250 / 0.07. After tapering, the cores of these 6 side fibers are fused with the annular cladding (preferably with a diameter d = 100 μm) of the first non-tapered region of the spindle ring fiber, and the annular laser is injected into the annular cladding of the spindle ring fiber. In other words, the output fiber of the ring beam laser combiner is a single spindle-shaped annular fiber.

[0028] See Figure 3The transverse cross-sectional structure of the spindle-shaped ring optical fiber 8 preferably consists of a central core, first, second, third, and fourth cladding layers, and a coating layer, all of which are ring-shaped. Preferably, the total length of the spindle-shaped ring optical fiber is ≥22m. Its longitudinal structure includes three non-conical regions and two conical regions. That is, the spindle-shaped ring optical fiber 8 includes a first non-conical region 81, a first conical region 82, a second non-conical region 83, a second conical region 84, and a third non-conical region 85, which are sequentially connected from the incident end towards the emission end. The diameter of the first conical region gradually increases along the light emission direction, and the diameter of the second conical region gradually decreases along the light emission direction. The spindle-shaped ring optical fiber has a central core 61 at its center. A first cladding 62 is disposed immediately outside the central core, covering the central core 61. A second cladding 63 is disposed immediately outside the first cladding 62, covering the first cladding 62. A third cladding 64 is disposed immediately outside the second cladding 63, covering the second cladding 63. A fourth cladding 65 is disposed immediately outside the third cladding 64, covering the third cladding 64. A coating layer 66 is disposed immediately outside the fourth cladding 65, covering the fourth cladding 65. In this structure, the refractive index of the first cladding 62 is less than that of the central core 61, and the refractive index of the first cladding 62 is less than that of the second cladding 63; the refractive index of the third cladding is less than that of the second cladding; and the refractive index of the third cladding is less than that of the fourth cladding, thus forming a double-groove ring structure. Preferably, the central fiber core is fused with the central core of the first non-conical region 81 of the spindle ring fiber, and the N side fiber cores are fused with the second cladding of the first non-conical region 81 of the spindle ring fiber. The inventors realized that light propagates in a ring-shaped optical field within the second cladding, while light in the N side fibers is essentially a circular spot. Therefore, when light enters the second cladding from the side fibers, there will be some mode mismatch, which will affect the coupling efficiency and overall optical efficiency. To improve the coupling efficiency and achieve better mode matching when light enters the second cladding from the side fibers, the inventors considered making certain improvements to the fiber structure at the coupling point. The inventors discovered that when the cores of the N side fibers are elliptical, and the major axis of the elliptical core of each side fiber is perpendicular to the line connecting the central fiber core and the side fiber core, the coupling efficiency between the optical field of the side fiber and the second cladding can be significantly improved, thereby improving the overall optical coupling efficiency. Preferably, the ratio of the major axis a to the minor axis b of the elliptical core of each side fiber conforms to the following range: 1.3

[0029] ​The fiber parameters in the first and third non-conical regions are identical. The diameter of the fiber core can be 10-40 μm, preferably 15 μm; the diameter of the first cladding can be 15-50 μm, preferably 30 μm; the diameter of the second cladding can be 50-130 μm, preferably 100 μm; the diameter of the third cladding can be 60-150 μm, preferably 115 μm; the diameter of the fourth cladding can be 150-280 μm, preferably 200 μm; and the diameter of the coating layer can be 170-300 μm, preferably 250 μm. The core, second cladding, and fourth cladding are composed of pure quartz material, while the first and third cladding are composed of low-refractive-index fluorine-doped quartz material (i.e., the refractive indices of the first and third cladding are significantly lower than those of the core, second cladding, and fourth cladding, used to form a double-groove ring structure). For numerical aperture, for example, the numerical aperture of the central core can be 0.06-0.22, preferably 0.1, and the numerical aperture of the second cladding (annular cladding) can be 0.1-0.24, preferably 0.22. For example, the fiber length of the first or third non-conical region can be 0.3-10m, preferably 1m.

[0030] The second non-conical region is a large mode field transmission region, mainly used to reduce the power density of the transmitted laser to suppress and reduce the nonlinear effect of the transmitted laser. For the fiber parameters of the second non-conical region, for example, preferably, the central core diameter can be 20-80μm, preferably 30μm; the first cladding diameter can be 30-100μm, preferably 60μm; the second cladding diameter can be 100-260μm, preferably 200μm; the third cladding diameter can be 120-300μm, preferably 230μm; the fourth cladding diameter can be 300-560μm, preferably 400μm; and the coating diameter can be 340-600μm, preferably 500μm. For the numerical aperture of the second non-conical region, the central core numerical aperture can be 0.06-0.22, preferably 0.1; and the second cladding (annular cladding) numerical aperture can be 0.1-0.24, preferably 0.22. Preferably, the fiber length of the second non-conical region is ≥10m.

[0031] For the first and second conical regions, preferably, both the first and second conical regions are uniformly tapered. Preferably, the first and second conical regions have the same length and tapering ratio, but opposite tapering directions. The lengths of the first and second conical regions can range from 3m to 10m, generally greater than 3m, and preferably greater than or equal to 6m. Due to the existence of the symmetrical double conical regions, the transmission angle of the laser in the fiber core gradually decreases as it passes through the first conical region (see appendix). Figure 4When the laser passes through the second non-conical region and is transmitted to the second conical region, it has the opposite effect to passing through the first conical region, which causes the transmission angle of the laser to gradually increase. The inventors found that due to the symmetrical structure of the two conical regions, the quality of the injected laser beam will not deteriorate during the transmission of the laser beam through the spindle fiber, and it has the effect of maintaining the quality of the laser beam transmission.

[0032] Preferably, both the first and second conical regions meet the adiabatic tapering condition. Under this condition, the mode field matches during the transmission of the laser throughout the entire conical region, thereby reducing insertion loss. This entire spindle-shaped fiber is obtained by controlling the different drawing speeds during the preform drawing process. The injected laser is bundled using a spindle-shaped annular fiber combiner. Through electronic control design, the laser power of the central spot (≥3kW) and the annular cladding laser power (≥12kW) can be individually adjusted for different spot laser outputs.

[0033] The cladding stripper 4 is preferably located in the third non-conical region to effectively remove the cladding light and ensure the quality of the output laser beam. For its manufacturing process, preferably, a CO2 laser can be used to etch a 200μm cladding diameter with an etching depth greater than or equal to 25μm and a roughened region length greater than or equal to 5cm, creating a regular groove structure on the cladding surface.

[0034] Output device 5 is located at the output end of the third non-conical region. Preferably, by increasing the effective cross-sectional area of ​​the laser output end face, the power density of the output end face is effectively reduced. A CO2 fusion splicer is used to fuse the output optical fiber to the quartz end cap to increase the upper limit of the output power.

[0035] The technical principles of the present invention have been described above with reference to specific embodiments. These descriptions are merely for explaining the principles of the invention and should not be construed as limiting the scope of protection of the invention in any way. Based on this explanation, those skilled in the art can readily conceive of other specific embodiments of the invention without inventive effort, and these embodiments will all fall within the scope of protection of the present invention.

Claims

1. A tunable spot laser based on a spindle-shaped ring fiber, comprising a central region laser module, a ring region laser module, and a ring beam laser combiner, characterized in that: The output fiber of the ring beam laser combiner is a spindle-shaped optical fiber. The spindle-shaped optical fiber includes a first non-conical region, a first conical region, a second non-conical region, a second conical region, and a third non-conical region connected sequentially from the incident end towards the output end. The diameter of the first conical region gradually increases along the light output direction, and the diameter of the second conical region gradually decreases along the light output direction. The center of the spindle-shaped optical fiber is a central core. A first cladding is placed immediately outside the central core, covering the central core. A second cladding is placed immediately outside the first cladding, covering the first cladding. A third cladding is placed immediately outside the second cladding, covering the second cladding. A fourth cladding is placed immediately outside the third cladding, covering the third cladding. A coating layer is placed immediately outside the fourth cladding, covering the fourth cladding. The ring beam laser combiner uses an (N+1)*1 combiner to combine the laser beams output from the central region laser module and the ring region laser module and output them through the spindle-shaped optical fiber. N is a positive integer greater than 1.

2. The tunable spot laser based on a spindle-shaped ring fiber according to claim 1, characterized in that: The input end of the ring beam laser combiner includes a central fiber and N side fibers. The ring region laser module includes N ring region laser units, which respectively input lasers into the N side fibers. The core of the central fiber is fused with the central core of the first non-conical region of the spindle ring fiber, and the central laser is injected into the central core of the spindle ring fiber. The cores of the N side fibers are fused with the annular cladding of the first non-conical region of the spindle ring fiber, and the annular laser is injected into the annular cladding of the spindle ring fiber. N is greater than 3.

3. The tunable spot laser based on a spindle-shaped ring fiber according to claim 2, characterized in that: In a spindle-shaped optical fiber, the refractive index of the first cladding is less than that of the central core, and the refractive index of the first cladding is less than that of the second cladding; the refractive index of the third cladding is less than that of the second cladding; the refractive index of the third cladding is less than that of the fourth cladding; the central fiber core is fused to the central core of the first non-conical region of the spindle-shaped optical fiber, and the N side fiber cores are fused to the second cladding of the first non-conical region of the spindle-shaped optical fiber.

4. The tunable spot laser based on a spindle-shaped ring fiber according to claim 3, characterized in that: The central core, second cladding, and fourth cladding are composed of pure quartz material, while the first and third cladding are composed of fluorine-doped quartz material with low refractive index.

5. The tunable spot laser based on a spindle-shaped ring fiber according to claim 3, characterized in that: Both the first and second cone regions are uniformly tapered. The first and second cone regions have the same cone length and tapering ratio, but opposite tapering directions. The first and second cone regions have symmetrical cone structures. The mode field diameter of the second non-cone region is larger than the mode field diameter of the first and second cone regions. The second non-cone region is a large mode field transmission region.

6. The tunable spot laser based on a spindle-shaped ring fiber according to claim 2, characterized in that: Both the first and second conical regions meet the thermal tapering conditions, with the length of the first conical region being greater than 3m and the length of the second conical region being greater than 3m; the length of the optical fiber in the second non-conical region is greater than or equal to 10m.

7. The tunable spot laser based on a spindle-shaped ring fiber according to claim 2, characterized in that: The output power of the central region laser module is greater than 3kW, the output power of a single ring region laser unit is greater than 2kW, and N is greater than or equal to 6.

8. The tunable spot laser based on a spindle-shaped ring fiber according to claim 2, characterized in that: A cladding stripper is installed on the third non-conical region to strip the cladding light. A CO2 laser is used to etch the cladding diameter with an etching depth greater than or equal to 25 μm and a roughened region length greater than or equal to 5 cm, generating a regular groove structure on the cladding surface. The output device is located at the output end of the third non-conical region, and a CO2 fusion splicer is used to fusion splice the output fiber to the quartz end cap.

9. The tunable spot laser based on a spindle-shaped ring fiber according to claim 2, characterized in that: The fiber parameters are the same in the first and third non-conical regions. In both regions, the fiber core diameter is 10-40 μm; the first cladding diameter is 15-50 μm, the second cladding diameter is 50-130 μm, the third cladding diameter is 60-150 μm, the fourth cladding diameter is 150-280 μm, and the coating diameter is 170-300 μm. The numerical aperture of the core is 0.06-0.22, and the numerical aperture of the second cladding is 0.1-0.

24. The fiber length in either the first or third non-conical region is 0.3 m-10 m.

10. The tunable spot laser based on a spindle-shaped ring fiber according to claim 9, characterized in that: In the second non-conical region, the diameter of the central core is 20-80 μm, the diameter of the first cladding is 30-100 μm, the diameter of the second cladding is 100-260 μm, the diameter of the third cladding is 120-300 μm, the diameter of the fourth cladding is 300-560 μm, the diameter of the coating layer is 340-600 μm, the numerical aperture of the central core is 0.06-0.22, the numerical aperture of the second cladding is 0.1-0.24, and the length of the second non-conical region is ≥10 m.

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