High-peak power nanosecond pulse fiber laser and use method thereof

Abstract

The invention relates to a high-peak power nanosecond pulse fiber laser and a use method, and belongs to the technical field of fiber lasers. The nanosecond pulse optical fiber laser sequentially comprises a nanosecond pulse seed, a seed isolation filter arranged at the output end of the nanosecond seed, a first-stage amplification structure, a second-stage amplification structure arranged behind the first-stage amplification structure, a passive homogenization optical fiber, a third-stage amplification structure and an anti-resonance hollow-core optical fiber. And before entering the three-stage amplification, a passive homogenizing optical fiber is adopted to degrade the beam quality of the signal light, a high-order mode is further excited, energy distribution is homogenized, the highest energy density borne by the end face of the optical fiber is reduced, various nonlinear effects are further inhibited, and nanosecond pulse laser output with higher peak power is realized.

Description

Technical Field

[0001] The present invention relates to the technical field of optical fiber lasers, and more specifically, to a high peak power nanosecond pulse optical fiber laser and a use method thereof. Background Art

[0002] Fiber lasers are widely used in industrial processing, military defense and other fields due to their excellent beam quality, high conversion efficiency and high compactness. In recent years, as the manufacturing industry has increasingly stringent requirements on environmental protection and precision in the production process, the demand for high-power pulse fiber laser systems that have both high pulse energy and high peak power while maintaining the inherent advantages of fiber laser systems has greatly increased. However, the power amplification of high-peak power nanosecond pulse fiber lasers is still limited by nonlinear effects such as stimulated Raman scattering (SRS) and self-phase modulation (SPM). In response to this problem, various research institutions have proposed many solutions.

[0003] The Chinese patent with publication number CN118970603A discloses a high-power fiber laser with low nonlinear effect, including: one or more high-power fiber laser modules with low nonlinear effect, a laser beam combiner, a mode stripper and a third output head, wherein the high-power fiber laser module with low nonlinear effect, the laser beam combiner, the mode stripper and the third output head are sequentially connected by fiber fusion; the high-power fiber laser module with low nonlinear effect includes an active fiber, two output gratings respectively connected to the two ends of the active fiber, at least one pump source and a pump signal combiner corresponding to the pump source, two cladding light strippers for filtering residual pump light and high-order modes in the signal light, and two output heads respectively arranged at the output ends of the cladding light strippers, wherein the output grating has a reflectivity of <5% and a transmittance of >95%. The patent directly reduces the energy density of the signal light by reducing the reflectivity of the output grating, and has a good inhibitory effect on the nonlinear effect to a certain extent. However, this method will also result in a lower seed laser power output by the oscillator cavity, which may cause problems such as spontaneous amplification radiation (ASE) and reduced light-to-light conversion efficiency in the final amplification process.

[0004] In addition, the publication number CN117673874A discloses a high-power fiber laser and its use method, which combines double-clad gain chiral helical fiber, single-clad passive chiral helical fiber and anti-resonant hollow fiber to ensure coupling efficiency while reducing the nonlinear effect of the laser system during transmission to increase output power. This laser improves the threshold of the system's nonlinear effect by improving the back-end energy transmission fiber, but does not improve the nonlinear effect of the amplifier stage. Summary of the invention

[0005] In view of the above defects and improvement requirements of the prior art, the present invention provides a high peak power nanosecond pulsed fiber laser. The nanosecond signal output by the seed source is amplified in two stages to increase the output power of the signal light. Before the signal light enters the third-stage amplification structure for amplification, a section of passive homogenizing fiber is used to convert the energy distribution of the signal light from Gaussian distribution to super-Gaussian distribution, reducing the energy density while keeping the output power of the signal light unchanged and suppressing the nonlinear effect. In addition, an anti-resonant hollow fiber is adopted at the output end of the third-stage amplification structure to further reduce the nonlinear effect of the laser output by the third amplification stage, realizing the output of nanosecond pulsed laser with higher peak power. The laser designed by the present invention can reduce the energy density of the signal light by homogenizing its energy, so as to suppress the nonlinear effect, and at the same time can maintain the power of the finally output signal light unchanged, thereby solving the technical problem in the prior art that the output power of the signal light is reduced or weakened due to reducing the output energy to reduce the energy density.

[0006] According to a first aspect of the present invention, there is provided a nanosecond pulsed fiber laser, comprising: a nanosecond pulsed seed, a seed filtering isolator provided at the output end of the nanosecond pulsed seed, a first-stage amplification structure provided at the output end of the seed filtering isolator, a second-stage amplification structure provided at the output end of the first-stage amplification structure, a passive homogenizing fiber provided at the output end of the second-stage amplification structure, and a third-stage amplification structure provided after the passive homogenizing fiber;

[0007] The first-stage amplification structure includes a first-stage wavelength division multiplexer provided at the output end of the seed isolation filter, a first-stage gain fiber provided at the output end of the first-stage wavelength division multiplexer, a first-stage pump source provided at the input end of the first-stage wavelength division multiplexer, and a first-stage isolation filter provided at the output end of the first-stage gain fiber;

[0008] The second-stage amplification structure includes a second-stage beam combiner provided at the output end of the first-stage isolation filter, a second-stage gain fiber provided at the output end of the second-stage beam combiner, a second-stage pump source provided at the input end of the second-stage beam combiner, and a second-stage isolation filter provided at the output end of the second-stage gain fiber;

[0009] The third-stage amplification structure includes a third-stage beam combiner provided at the output end of the passive homogenizing fiber, a third-stage gain fiber provided at the output end of the third-stage beam combiner, and a third-stage pump source provided at the input end of the third-stage beam combiner;

[0010] The nanosecond pulsed fiber laser further includes an anti-resonant hollow fiber at the final output end, and the anti-resonant hollow fiber is connected to the third-stage gain fiber.

[0011] Preferably, the first-stage gain fiber is a single-clad ytterbium-doped fiber with a core diameter of 6 μm and a cladding diameter of 125 μm; the second-stage gain fiber is a double-clad ytterbium-doped gain fiber with a core diameter of 10 - 20 μm and an inner cladding diameter of 125 - 130 μm; the third-stage gain fiber is a double-clad ytterbium-doped gain fiber with a core diameter of 20 - 1000 μm and an inner cladding diameter of 400 - 600 μm.

[0012] Preferably, the single-clad ytterbium-doped fiber is surrounded by a coating layer.

[0013] Preferably, the refractive index of the outer cladding of the second-stage gain fiber is lower than that of the inner cladding and the core, and the refractive index of the outer cladding of the third-stage gain fiber is lower than that of the inner cladding and the core.

[0014] Preferably, the first-stage pump source, the second-stage pump source, and the third-stage pump source are semiconductor pump lasers with wavelengths in the range of 915 nm - 1018 nm, respectively.

[0015] Preferably, the passive homogenizing fiber is a double-clad passive homogenizing fiber with a core of 50 - 100 μm, an inner cladding of 60 - 120 μm, and an outer cladding of 400 - 600 μm.

[0016] According to another aspect of the present invention, there is provided a method for using the nanosecond pulsed fiber laser, including the following steps:

[0017] S1: Input the signal light from the nanosecond pulsed seed injection end, and perform wavelength selection and backward light protection through the seed isolation filter;

[0018] S2: The first-stage pump source injects the pump light into the first-stage gain fiber through the first-stage wavelength division multiplexer to amplify the power of the signal light. After the first-stage isolation filter filters out the stray light components in the signal light, the signal light is injected into the second-stage gain fiber through the second-stage combiner. The second-stage pump source injects the pump light into the second-stage gain fiber through the second-stage combiner to amplify the power of the signal laser again. Then, after passing through the second-stage isolation filter, wavelength selection and stray light filtering are performed again;

[0019] S3: The signal laser output from the tail fiber of the second-stage isolation filter in the second-stage amplification system enters the passive homogenizing fiber for laser energy homogenization, reducing the hot spots in the signal light and making the light field distribution more uniform;

[0020] S4: The signal laser output from the passive homogenizing fiber is injected into the third-stage gain fiber, and the third-stage pump source injects the pump light into the system through the third-stage combiner to amplify the power of the signal laser;

[0021] S5: The signal light output from the third-stage gain fiber enters the anti-resonant hollow fiber to suppress the nonlinear effects brought by high-power lasers.

[0022] Generally speaking, compared with the prior art, the above technical solution conceived by the present invention mainly has the following technical advantages:

[0023] (1) The present invention provides a high-peak-power nanosecond pulsed fiber laser system, which suppresses the nonlinear effect through system design and realizes the output of high-peak-power nanosecond pulsed laser. The present invention provides a fiber laser, including a nanosecond pulse seed, a first-stage amplification system, a second-stage amplification system, a homogenizing fiber, and a third-stage amplification system. The nanosecond pulse seed is successively amplified in signal power through the first-stage amplification system and the second-stage amplification system. After passing through the homogenizing fiber, the energy distribution of the signal light spot changes from Gaussian distribution to super-Gaussian distribution, and while the laser power of the signal light remains unchanged, the energy distribution of the signal light on the fiber end face is homogenized. When the signal light is further amplified in the third amplification system, the nonlinear threshold is greatly improved. In addition, when the final laser output is performed, an anti-resonant hollow fiber is used for laser output. There is no solid transmission medium in the hollow fiber, further reducing the nonlinear effect in the system. This laser structure can provide fiber laser output with higher peak power, better beam quality, and better signal-to-noise ratio.

[0024] (2) The threshold of the nonlinear effect in a fiber laser is often related to the energy density. It is manifested that the higher the energy density of the signal light in the laser, the lower the nonlinear effect threshold and the stronger the nonlinear stray light. Therefore, a homogenizing fiber is used before the main amplification stage. Based on its special multi-core structure or micro-structure design, the signal light energy is homogenized, reducing the hot spots (i.e., energy concentration regions) in the optical field, making the optical field distribution more uniform, further increasing the nonlinear effect threshold in the main amplification stage (the third-stage amplification system), and reducing the proportion of nonlinear stray light. The present invention does not directly reduce the signal light power, ensuring the continuous growth of the laser power while suppressing the nonlinear effect. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is a connection schematic diagram of the high-peak-power nanosecond fiber laser provided by the embodiment of the present invention.

[0026] Figure 2 It is a comparison diagram of the output peak power between the nanosecond pulsed laser of the present invention and a conventional nanosecond pulsed laser.

[0027] Figure 3 It is a laser spectrogram when the output peak power of the present invention is 200 kW.

[0028] Figure 4 It is a mechanism diagram of the fiber laser of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0029] In order to make the objectives, technical solutions and advantages of the present invention clearer and more understandable, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0030] The present invention provides a high peak power nanosecond pulsed fiber laser, comprising:

[0031] A nanosecond pulsed seed source, including a nanosecond pulsed seed and a seed isolation filter disposed after the nanosecond pulsed seed;

[0032] A first-stage amplifier disposed at the output end of the nanosecond pulsed seed source, including a first-stage wavelength division multiplexer, a first-stage gain fiber, and a first-stage isolation filter; a wavelength division multiplexer is further connected between the first-stage gain fiber and the output end of the seed source, and a first-stage pump source is also connected to the input end of this wavelength division multiplexer;

[0033] A second-stage amplifier disposed after the first-stage amplifier, including a second-stage beam combiner, a second-stage gain fiber, a second-stage isolation filter, and a second-stage pump source;

[0034] A passive beam homogenizing fiber disposed after the second-stage isolation filter; used to homogenize the beam energy and suppress the nonlinear effect.

[0035] A third-stage amplifier disposed after the passive beam homogenizing fiber, including a third-stage beam combiner, a third-stage gain fiber, and a third-stage pump source;

[0036] An anti-resonant hollow fiber disposed after the third-stage isolation filter; used for system output and further alleviating the nonlinear effect. To achieve high peak power nanosecond pulsed laser output. The anti-resonant hollow fiber (12) serves as the final system output fiber.

[0037] Among them, each stage of isolation filter blocks the return light, prevents it from damaging the previous stage of amplification system, and maintains the system stability. The pump source is pumped multiple times in the laser through the wavelength division multiplexer and the beam combiner to increase the output power of the signal light, and finally realizes the high peak power nanosecond pulsed laser output.

[0038] Further, the first-stage gain fiber is a single-clad ytterbium-doped fiber with a core diameter of 6 μm and a cladding diameter of 125 μm, and its absorption efficiency is 250 dB / m @ 976 nm.

[0039] Further, the second-stage gain fiber is a double-clad ytterbium-doped fiber with a core diameter of 10 - 20 μm and an inner cladding diameter of 125 - 130 μm, and its absorption efficiency is 4.3 dB / m @ 976 nm.

[0040] Further, the third-stage gain fiber is a double-clad ytterbium-doped fiber with a core diameter of 20 - 1000 μm, an inner cladding diameter of 400 - 600 μm, and an absorption efficiency of 3.6 dB / m @ 976 nm.

[0041] Further, the first-stage gain fiber further includes: a coating layer coated on the periphery of the single-clad ytterbium-doped fiber.

[0042] Further, the second-stage gain fiber and the third-stage gain fiber further include a low-refractive-index outer cladding coated on the periphery of the inner cladding, that is, the refractive index of the outer cladding of the second-stage gain fiber and the third-stage gain fiber is lower than that of the inner cladding and the core.

[0043] Further, the second-stage gain fiber and the third-stage gain fiber further include a coating layer coated on the periphery of the outer cladding.

[0044] Further, the passive homogenizing fiber is a double-clad passive homogenizing fiber with a core of 50 - 100 μm, an inner cladding of 60 - 120 μm, and an outer cladding of 400 - 600 μm.

[0045] Further, the core medium of the anti-resonant hollow-core fiber is air, without a solid transmission medium.

[0046] The following are specific embodiments

[0047] Embodiment 1

[0048] Please refer to Figure 1 , the present invention provides a nanosecond fiber laser, including a nanosecond seed, a first-stage amplification system, a second-stage amplification system, a passive homogenizing fiber, a third-stage amplification system, an anti-resonant hollow-core fiber, and a pump module. Among them, the first-stage amplification system and the second-stage amplification system are used to amplify the nanosecond seed laser, the passive homogenizing fiber is used to homogenize the laser energy, the third-stage amplification system is the main amplification stage for the final amplification of the signal laser, and the anti-resonant hollow-core fiber is used for laser energy transmission and output. The pump module provides pump laser for each amplification system.

[0049] Among them, the first-stage amplification system uses the first-stage gain fiber 3 for optical amplification. The first-stage gain fiber is a single-clad quartz ytterbium-doped fiber with a core diameter of 6 μm and a cladding diameter of 125 μm. The second-stage amplification system uses the second-stage gain fiber 6 for optical amplification. The second-stage gain fiber is a double-clad quartz ytterbium-doped fiber with a core diameter of 20 μm and an inner cladding diameter of 125 μm. The third-stage gain fiber uses the third-stage gain fiber 10 for optical amplification. The third-stage gain fiber is a double-clad quartz ytterbium-doped fiber with a core diameter of 50 μm and an inner cladding diameter of 400 μm.

[0050] Specifically, a ytterbium-doped silica fiber is used as the gain medium. When light is incident on the fiber, due to the absorption of the medium, the energy carried by the incident light excites the electrons in the medium to a higher energy level. Through the relaxation phenomenon, the electrons transition from the high energy level to the ground state and release energy, emitting photons. In the example of the present invention, the nanosecond pulse seed 1 is used to output a signal light in the 1064 nm band with a repetition rate of 10 Hz and a pulse width of 10 ns. The input end of the first-stage isolation filter 2 is connected to the output end of the nanosecond seed module, and is used to prevent the backward reflected light in the first-stage amplification system from damaging the seed source.

[0051] Specifically, the output wavelength of the seed signal light is in the range of 1050 - 1070 nm. The seed isolation filter 2 contains a band-pass filter with a central wavelength of 1064 nm and an output bandwidth of 8 nm. After passing through the isolation filter, the output wavelength of the seed signal light is 1060 nm - 1068 nm.

[0052] Specifically, the length of the first-stage gain fiber is 1.5 m, the length of the second-stage gain fiber is 3.5 m, and the length of the third-stage gain fiber is 4 m.

[0053] Specifically, the seed isolation filter 2 also includes an isolator. An isolator is a passive optical isolator device that allows unidirectional light to pass through. Its working principle is based on the non-reciprocity of Faraday rotation. The seed isolation filter 2 is a passive device that allows light to pass through in one direction and blocks it from passing through in the opposite direction. Its function is to limit the direction of light, enabling light to only be transmitted in one direction. The light reflected by the fiber echo can be well isolated by the first-stage isolation filter 5, improving the optical wave transmission efficiency.

[0054] Specifically, in the example of the present invention, the working principles of the first-stage isolation filter 5 and the second-stage isolation filter 8 are similar to that of the seed isolation filter 2. The input end of the first-stage isolation filter 5 is connected to the first-stage gain fiber 3, and the corresponding passive fiber of the first-stage isolation filter 5 is matched with the first-stage gain fiber 3. The input end of the second-stage isolation filter 8 is connected to the second-stage gain fiber 6, the output end of the second-stage isolation filter 8 is connected to the passive homogenizing fiber 9, and the corresponding passive fiber of the second-stage isolation filter 8 is matched with the second-stage gain fiber 6.

[0055] Specifically, the first-stage wavelength division multiplexer 4, the second-stage beam combiner 7, and the third-stage beam combiner 11 are optical devices that realize the distribution or combination of optical signal power among different fibers. They are formed by utilizing the interaction of the guided wave energy in the adjacent fiber core regions of different fiber surfaces.

[0056] Specifically, in the embodiment of the present invention, the input end of the first-level wavelength division multiplexer 4 is connected to the output end of the seed isolation filter 2, and the output end of the first-level wavelength division multiplexer 4 is connected to the first-level gain optical fiber 3. The input end of the second-level beam combiner 7 is connected to the output end of the first-level isolation filter 5, and the output end of the second-level wavelength division multiplexer 7 is connected to the second-level gain optical fiber 6. The input end of the third-level beam combiner 11 is connected to the passive homogenizing optical fiber 9, and the output end of the third-level wavelength division multiplexer 11 is connected to the third-level gain optical fiber 10.

[0057] Specifically, the anti-resonant hollow-core optical fiber is connected to the third-level gain optical fiber and is used for the final output of the laser system.

[0058] Specifically, the first-level pump source 13, the second-level pump source 14, and the third-level pump source 15 are respectively connected to the pump arms of the first-level wavelength division multiplexer 4, the second-level beam combiner 7, and the third-level beam combiner 11 to provide pump energy for the amplification system.

[0059] As Figure 2 shown, in the prior art, the peak power of a 10 ns, 10 Hz narrow pulse width and low repetition rate laser signal based on the output of a 50 / 400 active optical fiber is generally on the order of 100 kW. A high-power fiber laser provided by an embodiment of the present invention has an output power of up to more than 200 kW, and at the same time, the Raman suppression ratio characterizing its non-linear optical intensity reaches 14 dB, as Figure 3 shown.

[0060] In summary, different from the existing technical solutions, the present invention provides a nanosecond pulse fiber laser, including a nanosecond seed, a first-level amplification system, a second-level amplification system, a passive homogenizing optical fiber, a third-level amplification system, an anti-resonant hollow-core optical fiber, and a pump module. Among them, the first-level amplification system and the second-level amplification system are used to amplify the nanosecond seed laser, the passive homogenizing optical fiber is used to homogenize the laser energy, the third-level amplification system is the main amplification stage and is used for the final amplification of the signal laser, and the anti-resonant hollow-core optical fiber is used for laser energy transmission and output. The pump module provides pump laser for each amplification system. The nanosecond pulse fiber laser provided by the present invention inserts a section of passive homogenizing optical fiber at the front end of the third-level amplification system to homogenize the signal light laser energy, disperse its energy density to further suppress the non-linear effect, and can achieve a laser output with a higher peak power. The structure of the nanosecond pulse fiber laser of the present invention is simple, has a low cost, and is easy to integrate into the laser system.

[0061] Embodiment 2

[0062] As Figure 4 shown, the present invention provides a method for using a high-peak-power nanosecond pulse fiber laser, including the following steps:

[0063] S1: Input the signal light from the nanosecond pulse seed injection end, and perform wavelength selection and backlight protection through the seed isolation filter.

[0064] S2: The laser output from the seed source enters the first-stage gain fiber 3. The first-stage pump source 13 injects pump light into the first-stage gain fiber 3 through the first-stage wavelength division multiplexer 4 to amplify the power of the signal light. After the first-stage isolation filter 5 filters out other stray light components in the signal light, the signal light is injected into the second-stage gain fiber 6 through the second-stage beam combiner. The second-stage pump source 14 injects pump light into the second-stage gain fiber 6 through the second-stage beam combiner 7 to amplify the power of the signal laser again. Then, wavelength selection and stray light filtering are performed again through the second-stage isolation filter 8.

[0065] S3: The signal laser output from the tail fiber of the second-stage isolation filter 8 in the second-stage amplification system enters the passive homogenizing fiber 9 for laser energy homogenization, reducing the hot spots in the signal light and making the light field distribution more uniform.

[0066] S4: The signal laser output from the passive homogenizing fiber 9 is injected into the third-stage active gain fiber 10. The third-stage pump source 15 injects pump light into the system through the third-stage beam combiner 11 to amplify the power of the signal laser.

[0067] S5: The signal light output from the third-stage active gain fiber 10 enters the anti-resonant hollow fiber 12 to suppress the nonlinear effects brought by high-power lasers.

[0068] In the above steps, the pump light wavelengths output by the first-stage pump source 13, the second-stage pump source 14, and the third-stage pump source 15 are any combination between 915 nm and 1018 nm, and the pump structure can adopt unidirectional pumping or double-end pumping.

[0069] Those skilled in the art can easily understand that the above is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent replacements, and improvements made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A nanosecond pulse fiber laser, characterized in that: include: A nanosecond pulse seed (1), a seed filter isolator (2) provided at the output end of the nanosecond pulse seed (1), a primary amplification structure provided at the output end of the seed filter isolator (2), a secondary amplification structure provided at the output end of the primary amplification structure, a passive homogenization optical fiber (9) provided at the output end of the secondary amplification structure, and a tertiary amplification structure provided after the passive homogenization optical fiber (9); The primary amplification structure comprises a primary wavelength division multiplexer (4) arranged at the output end of the seed isolation filter (2), a primary gain optical fiber (3) arranged at the output end of the primary wavelength division multiplexer (4), a primary pump source (13) arranged at the input end of the primary wavelength division multiplexer (4), and a primary isolation filter (5) arranged at the output end of the primary gain optical fiber (3); The secondary amplification structure comprises a secondary beam combiner (7) arranged at the output end of the primary isolation filter (5), a secondary gain optical fiber (6) arranged at the output end of the secondary beam combiner (7), a secondary pump source (14) arranged at the input end of the secondary beam combiner (7), and a secondary isolation filter (8) arranged at the output end of the secondary gain optical fiber (6); The three-stage amplification structure comprises a three-stage beam combiner (11) arranged at the output end of the passive homogenization optical fiber (9), a three-stage gain optical fiber (10) arranged at the output end of the three-stage beam combiner (11), and a three-stage pump source (15) arranged at the input end of the three-stage beam combiner (11); The nanosecond pulse fiber laser also includes an anti-resonance hollow core fiber (12) at the final output end, and the anti-resonance hollow core fiber (12) is connected to the three-level gain fiber (10).

2. The nanosecond pulse fiber laser according to claim 1, characterized in that: The first-level gain fiber (3) is a single-clad ytterbium-doped fiber with a core diameter of 6 μm and a cladding diameter of 125 μm; the second-level gain fiber (6) is a double-clad ytterbium-doped gain fiber with a core diameter of 10-20 μm and an inner cladding diameter of 125-130 μm; and the third-level gain fiber (10) is a double-clad ytterbium-doped gain fiber with a core diameter of 20-1000 μm and an inner cladding diameter of 400-600 μm.

3. The nanosecond pulse fiber laser according to claim 2, characterized in that: The single-clad ytterbium-doped optical fiber has a coating layer on its periphery.

4. The nanosecond pulse fiber laser according to claim 2, characterized in that: The refractive index of the outer cladding of the secondary gain optical fiber (6) is lower than the refractive index of the inner cladding and the core, and the refractive index of the outer cladding of the tertiary gain optical fiber (10) is lower than the refractive index of the inner cladding and the core.

5. The nanosecond pulse fiber laser according to claim 1, characterized in that: The primary pump source (13), the secondary pump source (14), and the tertiary pump source (15) are semiconductor pump lasers with wavelengths within the range of 915 nm to 1018 nm.

6. The nanosecond pulse fiber laser according to claim 1, characterized in that: The passive homogenizing optical fiber (9) is a double-clad passive homogenizing optical fiber with a core of 50-100 μm, an inner cladding of 60-120 μm, and an outer cladding of 400-600 μm.

7. The method for using the nanosecond pulse fiber laser according to any one of claims 1 to 6, characterized in that: The following steps are involved: S1: The signal light is input from the nanosecond pulse seed (1) injection port and passes through the seed isolation filter (2) for wavelength selection and return light protection; S2: The primary pump source (13) injects the pump light into the primary gain fiber (3) through the primary wavelength division multiplexer (4), amplifies the power of the signal light, filters out the stray light components in the signal light through the primary isolation filter (5), and then injects the signal light into the secondary gain fiber (6) through the secondary beam combiner. The secondary pump source (14) injects the pump light into the secondary gain fiber (6) through the secondary beam combiner (7), amplifies the power of the signal laser again, and then passes through the secondary isolation filter (8) to select the wavelength and filter out the stray light again. S3: In the secondary amplification system, the signal laser outputted by the pigtail of the secondary isolation filter (8) enters the passive homogenization optical fiber (9) for laser energy homogenization, thereby reducing hot spots in the signal light and making the light field distribution more uniform; S4: The signal laser outputted by the passive homogenization optical fiber (9) is injected into the three-stage gain optical fiber (10), and the three-stage pump source (15) injects the pump light into the system through the three-stage beam combiner (11) to amplify the power of the signal laser; S5: The signal light output by the three-stage gain optical fiber (10) enters the anti-resonance hollow-core optical fiber (12) to suppress the nonlinear effect caused by the high-power laser.

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