High-beam-quality fiber laser amplifier without nonlinear effect limitation
By employing the multi-ring structure and geometric asymmetry design of hollow anti-resonant fiber, the nonlinearity problem of high-power fiber amplifiers is solved, achieving stable output with high beam quality and high output power, suitable for high-power narrow-linewidth fiber lasers.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-11-28
- Publication Date
- 2026-04-14
AI Technical Summary
Existing methods for limiting nonlinear effects in high-power, narrow-linewidth fiber lasers have limited effectiveness, leading to deteriorated beam quality and mode instability, and failing to fundamentally solve the nonlinearity problem of high-power fiber amplifiers.
By employing hollow anti-resonant fiber and utilizing a multi-ring structure and geometric asymmetry design, optical field energy is transmitted in the central air core. Combined with the design of the anti-resonant ring, higher-order modes are suppressed, resulting in high beam quality output and low nonlinearity.
Significantly reducing the nonlinear coefficient and increasing the thresholds for stimulated Brillouin scattering and stimulated Raman scattering, this enables stable single-mode or near-single-mode output of high-power narrow-linewidth fiber laser amplifiers, thereby improving beam quality and output power.
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Figure CN121863172A_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of fiber laser technology, and more specifically, relates to a high beam quality fiber laser amplifier without nonlinear effect limitations. Background Technology
[0002] With the increasing output power of high-power narrow-linewidth fiber lasers, nonlinear effects in the fiber (stimulated Brillouin, stimulated Raman, etc.) have become a major constraint on their further development. Current methods to suppress nonlinear effects generally involve increasing absorption, shortening the fiber length (e.g., Optical Materials Express, 2019, 10(1):36-45), or reducing the acousto-optic overlap area of the fiber (e.g., Optics Express, 2023, 2(31):1888-1900). However, the former increases heat generation per unit length, leading to a deterioration in beam quality and a decrease in the mode instability threshold, while the latter requires precise design of the fiber's structure and composition, increasing the difficulty of fiber fabrication. Nevertheless, the above methods have limited effectiveness in improving nonlinear effects and cannot fundamentally solve the high nonlinearity problem faced by high-power fiber amplifiers.
[0003] Therefore, there is an urgent need in this field to propose a high beam quality fiber laser amplifier without nonlinear effect limitations. Based on the unique light guiding principle of hollow anti-resonant fiber, more than 99% of the energy is confined to the air core for transmission, achieving an order-of-magnitude reduction in the nonlinear coefficient. This fundamentally solves the nonlinear problem in the fiber laser amplification process. At the same time, based on the high beam quality characteristics of hollow anti-resonant fiber, the high beam quality output of the laser amplifier is achieved. Summary of the Invention
[0004] In view of the above-mentioned defects or improvement needs of the existing technology, the present invention provides a high beam quality fiber laser amplifier without nonlinear effect limitations. It utilizes the high beam quality output and low nonlinearity characteristics of hollow anti-resonant fiber to fundamentally solve the nonlinearity problem faced by high-power laser amplifiers while maintaining the high beam quality output of the laser amplifier.
[0005] To achieve the above objectives, this invention proposes a high beam quality fiber laser amplifier without nonlinear effect limitations, comprising: Seed light source, used to generate seed signal light with a preset center wavelength; The pre-amplification stage is used to perform the first-stage power amplification of the seed signal light; The main power amplifier stage is used to perform a second-stage high-power amplification of the pre-amplified signal light. The output and transmission stage is used for outputting and long-distance transmission of the signal light after main power amplification; among which, The main power amplifier stage includes an active hollow anti-resonant fiber and a pump coupler for injecting pump light into the active hollow anti-resonant fiber. The active hollow anti-resonant fiber is a multi-ring hollow anti-resonant fiber with a multi-ring structure inside.
[0006] As a further preferred embodiment, the multi-ring hollow anti-resonant optical fiber comprises: The central air core is used to transmit high-beam-quality signal light at the target wavelength. A multi-ring structure, the multi-ring structure comprising: The first anti-resonant ring is arranged around the central air core and is composed of multiple first capillaries arranged in an array along the circumferential direction. The second anti-resonant ring is arranged outside the first anti-resonant ring and is composed of multiple first capillaries arranged in an array along the circumferential direction.
[0007] As a further preferred embodiment, the second anti-resonant ring is further provided with a rare earth doped region, wherein the rare earth doped region is doped with any one or more rare earth elements selected from erbium, ytterbium, thulium, holmium, and neodymium.
[0008] As a further preferred embodiment, the multi-ring structure is a geometrically asymmetrical structure, specifically: in the first anti-resonant ring and / or the second anti-resonant ring, the inner diameter and / or wall thickness of at least one capillary is different from that of other capillary tubes in the same ring, and / or the center-to-center distance between at least one pair of adjacent capillary tubes is different from that of other capillary tubes in the same ring, so as to form asymmetry in the orientation direction. The active hollow-core anti-resonant fiber is configured to maintain low-loss transmission of the fundamental mode in the central air core at the target signal wavelength, and to introduce additional losses to higher-order modes that are greater than those of the fundamental mode through the multi-ring geometric asymmetric structure, thereby achieving single-mode or near-single-mode output with higher-order mode suppression under high-power amplification conditions.
[0009] As a further preferred embodiment, the wall thickness of the first anti-resonant ring in the active hollow anti-resonant optical fiber... Relative to the target signal wavelength The following equation satisfies the anti-resonance relationship: in, It is a non-negative integer. The refractive index of the glass material is such that the first anti-resonant ring generates anti-resonant reflection of the light field in the central air core at the target signal wavelength, thereby realizing low-loss light guiding of the fundamental mode in the central air core.
[0010] As a further preferred embodiment, the wall thickness of the second anti-resonant ring in the active hollow anti-resonant optical fiber... Relative to pump wavelength Designed to approximately satisfy the anti-resonance condition: In the formula, It is a non-negative integer. The second anti-resonant ring is designed to be close to the anti-resonant state near the pump wavelength and slightly deviate from the anti-resonant state near the target signal wavelength. This improves the transmission efficiency of the pump light in the hollow region and enhances the ability of higher-order modes to leak to the outer layer.
[0011] As a further preferred embodiment, the active hollow antiresonant fiber is configured at the target signal wavelength to minimize the transmission loss per unit length of at least one higher-order mode. Transmission loss per unit length with the fundamental mode The ratio of the two modes must be at least 10 to ensure that the loss of higher-order modes is at least one order of magnitude higher than that of the fundamental mode, thereby suppressing the effective accumulation of higher-order modes during the main power amplification process.
[0012] As a further preferred embodiment, the fundamental mode acousto-optic overlap factor Γ of the active hollow antiresonant optical fiber at the target signal wavelength satisfies: in, Let represent the amplitude of the transverse electric field distribution of the fundamental mode. The numerator integration region is the glass material region in the fiber cross-section, and the denominator integration region is the entire fiber cross-section. As a further preferred embodiment, the effective mode field area of the fundamental mode of the active hollow antiresonant fiber at the target signal wavelength is... The ratio of the acoustic-optical overlap factor Γ satisfies: in, In the formula, The amplitude of the transverse electric field distribution of the fundamental mode is given, and the integration region is the entire cross-section of the optical fiber. The preset threshold is used to characterize the presence of small acousto-optic overlap while having a large effective mode field area, so that the main power amplifier stage has a high stimulated Brillouin scattering threshold and a high nonlinear threshold.
[0013] In summary, compared with the prior art, the above-described technical solutions conceived by this invention mainly possess the following technical advantages: 1. This invention uses hollow anti-resonant fiber to replace traditional solid fiber. By utilizing the high beam quality output and low nonlinearity characteristics of hollow anti-resonant fiber, the nonlinearity problem faced by high-power laser amplifiers is fundamentally solved while maintaining the high beam quality output of the laser amplifier.
[0014] 2. This invention employs a structural design of "multi-ring anti-resonance and global geometric asymmetry" to achieve differentiated constraints on different modes in the radial and azimuth directions of the inner and outer anti-resonant rings: the first anti-resonant ring is precisely designed according to the signal wavelength to ensure low-loss light guiding of the fundamental mode in the central air core; the second anti-resonant ring achieves anti-resonance near the pump wavelength and deviates slightly from anti-resonance near the signal wavelength, while the azimuth geometric asymmetry makes it easier for higher-order modes to couple to the outer ring and cladding during propagation, resulting in rapid attenuation and significantly improving the ratio of higher-order mode loss to fundamental mode loss. The system can maintain stable single-mode or near-single-mode output even under high-power operating conditions, effectively avoiding beam quality degradation caused by mode instability.
[0015] 3. This invention confines the optical field energy primarily within the air core through a hollow multi-ring structure, significantly reducing the energy proportion of the optical field in the glass region. Combined with a larger effective mode area and a smaller acousto-optic overlap factor obtained through structural optimization, the equivalent nonlinear coefficient is significantly reduced, and the thresholds for nonlinear effects such as stimulated Brillouin scattering and stimulated Raman scattering are significantly improved. Under the same linewidth conditions, this invention can achieve higher output power and a longer effective operating length, making it particularly suitable for high-power narrow-linewidth fiber laser amplifiers and providing a structural basis for further increasing system power.
[0016] 4. This invention utilizes the functional division of multiple anti-resonant rings. The inner ring achieves efficient anti-resonant light guiding for the signal wavelength, while the outer ring is parameter-designed based on the pump wavelength and higher-order mode leakage requirements. Combined with local geometric asymmetry, this ensures low-loss transmission of the fundamental mode while improving the transmission and utilization efficiency of the pump light in the hollow region. Furthermore, the structure of this invention is easily integrated with conventional pre-amplification stages, pump combiners, passive transmission stages, and other modules, enabling the construction of a complete MOPA fiber laser amplifier system. This achieves comprehensive optimization of "high-efficiency pump absorption, high-power single-mode output, and long-distance low-loss transmission," thereby improving the overall output power, beam quality, and engineering applicability of the fiber laser amplifier. Attached Figure Description
[0017] Figure 1 This is a schematic diagram of the structure of a high beam quality fiber laser amplifier without nonlinear effect limitations according to an embodiment of the present invention; Figure 2 This is a schematic diagram of the structure of a high beam quality fiber laser amplifier without nonlinear effect limitations according to another embodiment of the present invention. Detailed Implementation
[0018] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.
[0019] This embodiment provides a high beam quality fiber laser amplifier without nonlinear effect limitations, comprising: Seed light source, used to generate seed signal light with a preset center wavelength; The pre-amplification stage is used to perform the first-stage power amplification of the seed signal light; The main power amplifier stage is used to perform a second-stage high-power amplification of the pre-amplified signal light. The output and transmission stage is used for outputting and long-distance transmission of the signal light after main power amplification; among which, The main power amplifier stage includes an active hollow anti-resonant fiber and a pump coupler for injecting pump light into the active hollow anti-resonant fiber. The active hollow anti-resonant fiber is a multi-ring hollow anti-resonant fiber with a multi-ring structure inside.
[0020] More specifically, such as Figure 2 As shown, in this embodiment, the seed light source and pre-amplification stage 11 are general-purpose devices for the main oscillation power amplifier. The pump source 121 of the pump coupler adopts a forward pumping method. The gain fiber 122 of the main power amplifier stage is an active hollow-core anti-resonant fiber, and this active hollow-core anti-resonant fiber is a single-ring anti-resonant hollow-core active fiber. Its magnified cross-section is shown in 123. The active hollow-core anti-resonant fiber is a multi-ring hollow-core anti-resonant fiber with a multi-ring structure inside. The multi-ring structure is a single-ring structure arranged in an array around the central air core. Generally, the single-ring structure is a glass capillary, and the doped region 1231 on the glass capillary is doped with rare earth ions. Alternatively, the rare earth ions are ytterbium ions. The output stage 131 is an anti-resonant hollow-core passive fiber of the same specification, and its cross-section is shown in 132. Except for the absence of a rare earth doped region, the other parameters are consistent with the gain fiber 122 of the main power amplifier stage. This embodiment is a high beam quality fiber laser amplifier without nonlinear effect limitations suitable for the 1μm band.
[0021] In another embodiment of the present invention, a high beam quality fiber laser amplifier without nonlinear effect limitations includes: Seed light source, used to generate seed signal light with a preset center wavelength; The pre-amplification stage is used to perform the first-stage power amplification of the seed signal light; The main power amplifier stage is used to perform a second-stage high-power amplification of the pre-amplified signal light. The output and transmission stage is used for outputting and long-distance transmission of the signal light after main power amplification; among which, The main power amplifier stage includes an active hollow anti-resonant fiber and a pump coupler for injecting pump light into the active hollow anti-resonant fiber. The active hollow anti-resonant fiber is a multi-ring hollow anti-resonant fiber with a multi-ring structure inside.
[0022] More specifically, such as Figure 1 As shown, in this embodiment, the seed light source and pre-amplification stage 21 are general-purpose devices of the main oscillation power amplifier; the pumping method of the main power amplifier stage adopts bidirectional pumping, namely, a forward pump source 221 and a reverse pump source 224; the gain fiber 222 of the main power amplifier stage is a nested ring anti-resonant active fiber, and its magnified cross-section is shown in 223; the multi-ring hollow anti-resonant fiber includes: a central air core, used to transmit the fundamental mode signal light under the target wavelength signal; and a multi-ring structure, which includes: a first anti-resonant ring, arranged around the central air core, composed of multiple first capillaries arranged in an array along the circumferential direction; and a second anti-resonant ring, arranged outside the first anti-resonant ring, composed of multiple first capillaries arranged in an array along the circumferential direction. The second anti-resonant ring has a doped region 2231. Optionally, in this embodiment, the rare earth ions in the doped region 2231 are erbium ions. The output stage 231 is an anti-resonant passive fiber of the same specification, with a cross-section as shown in 232. Except for the absence of a rare earth doped region, its other parameters are consistent with those of the gain fiber 222 of the main power amplifier stage. This embodiment is a high beam quality fiber laser amplifier without nonlinear effect limitations suitable for the 1.55μm band.
[0023] In another embodiment of the present invention, a high beam quality fiber laser amplifier without nonlinear effect limitations is proposed, comprising: Seed light source, used to generate seed signal light with a preset center wavelength; The pre-amplification stage is used to perform the first-stage power amplification of the seed signal light; The main power amplifier stage is used to perform a second-stage high-power amplification of the pre-amplified signal light. The output and transmission stage is used for outputting and long-distance transmission of the signal light after main power amplification; among which, The main power amplifier stage includes an active hollow anti-resonant fiber and a pump coupler for injecting pump light into the active hollow anti-resonant fiber. The active hollow anti-resonant fiber is a multi-ring hollow anti-resonant fiber with a multi-ring structure inside.
[0024] Optionally, the multi-ring hollow anti-resonant optical fiber includes: The central air core is used to transmit high beam quality signal light, i.e., fundamental mode signal light, at the target wavelength. A multi-ring structure, the multi-ring structure comprising: The first anti-resonant ring is arranged around the central air core and is composed of multiple first capillaries arranged in an array along the circumferential direction. The second anti-resonant ring is arranged outside the first anti-resonant ring and is composed of multiple first capillaries arranged in an array along the circumferential direction.
[0025] Optionally, the second anti-resonant ring may also be provided with a rare earth doped region, wherein the rare earth doped region is doped with any one or more rare earth elements selected from erbium, ytterbium, thulium, holmium, and neodymium.
[0026] Optionally, the multi-ring structure is a geometrically asymmetric structure, specifically: in the first anti-resonant ring and / or the second anti-resonant ring, the inner diameter and / or wall thickness of at least one capillary is different from that of other capillary tubes in the same ring, and / or the center distance between at least one pair of adjacent capillary tubes is different from that of other capillary tubes in the same ring, so as to form asymmetry in the orientation direction. The active hollow-core anti-resonant fiber is configured to maintain low-loss transmission of the fundamental mode in the central air core at the target signal wavelength, and to introduce additional losses to higher-order modes that are greater than those of the fundamental mode through the multi-ring geometric asymmetric structure, thereby achieving single-mode or near-single-mode output with higher-order mode suppression under high-power amplification conditions.
[0027] Optionally, the wall thickness of the first anti-resonant ring in the active hollow anti-resonant fiber... Relative to the target signal wavelength The following equation satisfies the anti-resonance relationship: in, It is a non-negative integer. The refractive index of the glass material is such that the first anti-resonant ring generates anti-resonant reflection of the light field in the central air core at the target signal wavelength, thereby realizing low-loss light guiding of the fundamental mode in the central air core.
[0028] Optionally, the wall thickness of the second anti-resonant ring in the active hollow anti-resonant fiber... Relative to pump wavelength Designed to approximately satisfy the anti-resonance condition: In the formula, It is a non-negative integer. The second anti-resonant ring is designed to be close to the anti-resonant state near the pump wavelength and slightly deviate from the anti-resonant state near the target signal wavelength. This improves the transmission efficiency of the pump light in the hollow region and enhances the ability of higher-order modes to leak to the outer layer.
[0029] Of course, as a preferred embodiment, the wall thickness of the second anti-resonant ring in the active hollow anti-resonant optical fiber is... Relative to pump wavelength Designed to satisfy the anti-resonance condition: In the formula, It is a non-negative integer. The second anti-resonant ring is designed to be close to the anti-resonant state near the pump wavelength and slightly deviate from the anti-resonant state near the target signal wavelength. This improves the transmission efficiency of the pump light in the hollow region and enhances the ability of higher-order modes to leak to the outer layer.
[0030] Optionally, the active hollow antiresonant fiber is configured at the target signal wavelength to reduce the transmission loss per unit length of at least one higher-order mode. Transmission loss per unit length with the fundamental mode The ratio of the two modes must be at least 10 to ensure that the loss of higher-order modes is at least one order of magnitude higher than that of the fundamental mode, thereby suppressing the effective accumulation of higher-order modes during the main power amplification process.
[0031] Optionally, the fundamental mode acousto-optic overlap factor Γ of the active hollow-core antiresonant fiber at the target signal wavelength satisfies: in, Let represent the amplitude of the transverse electric field distribution of the fundamental mode. The numerator integration region is the glass material region in the fiber cross-section, and the denominator integration region is the entire fiber cross-section. Optionally, the effective mode field area of the fundamental mode of the active hollow antiresonant fiber at the target signal wavelength... The ratio of the acoustic-optical overlap factor Γ satisfies: in, In the formula, The amplitude of the transverse electric field distribution of the fundamental mode is given, and the integration region is the entire cross-section of the optical fiber. The preset threshold is used to characterize the presence of small acousto-optic overlap while having a large effective mode field area, so that the main power amplifier stage has a high stimulated Brillouin scattering threshold and a high nonlinear threshold.
[0032] More specifically, based on any of the above embodiments, in this embodiment, the first anti-resonant ring is arranged around the central air core and consists of N1 = 8 first capillaries arranged in a circular array. Each first capillary has an inner radius of r1 and a wall thickness of t1. The first capillaries are connected to the central air core through multiple small support ribs to minimize disturbance to the optical field while ensuring mechanical strength. The second anti-resonant ring is arranged radially outside the first anti-resonant ring and consists of N2 = 8 second capillaries. Each second capillary has an inner radius of r2 and a wall thickness of t2. Preferably, r2 > r1. The second anti-resonant ring is mainly responsible for providing anti-resonance characteristics near the pump wavelength and forming a leakage channel for higher-order modes.
[0033] The capillaries in both the first and second anti-resonant rings are made of quartz glass with a refractive index of approximately 1.45 ns. The wall thickness t1 of the first anti-resonant ring is designed relative to the target signal wavelength to satisfy the anti-resonance condition. In this embodiment, m1=0, then t1 is approximately 0.18 μm. Through the above design, the first anti-resonant ring generates anti-resonant reflection of the light field in the central air core at a wavelength of 1064 nm, thereby achieving low-loss light guiding of the fundamental mode in the air core.
[0034] The wall thickness t2 of the second anti-resonant ring satisfies the following relative to the pump wavelength of 976 nm: In this system, m2 is also set to 0, t2 is approximately 0.17 μm, and Δt2 is set to 0.02 μm. This ensures that the second anti-resonant ring approaches an anti-resonant state near the pump wavelength and deviates slightly from the anti-resonant state near the signal wavelength. This improves the transmission efficiency of the pump light in the hollow region and provides a leakage path for higher-order modes.
[0035] Since the field distribution of the fundamental mode in the central air core is approximately circularly symmetrical and mainly limited to a small radius, it is not sensitive to the aforementioned local azimuth asymmetry disturbances, so its transmission loss only increases slightly; however, the higher-order modes have multiple field strength peaks in the azimuth direction, making them more sensitive to these local asymmetries, and they are prone to coupling in the asymmetric sector, leaking to the second anti-resonant ring and the outer cladding, thereby significantly increasing the transmission loss of the higher-order modes.
[0036] Numerical simulations show that, under the above parameter conditions, the fundamental mode loss... It can be below 0.1 dB / m, while the loss of the first higher-order mode is... The loss can reach over 1 dB / m, resulting in a higher-order mode loss to fundamental mode loss ratio greater than 10. Meanwhile, the calculated fundamental mode acousto-optic overlap factor is less than 0.01, indicating that the optical field is mainly distributed in the central air core region, with very little energy in the glass region, which is beneficial for increasing the stimulated Brillouin scattering threshold.
[0037] The hollow anti-resonant fiber described in this embodiment can be used as an active fiber, achieving gain by setting a rare-earth-doped thin layer on the inner glass wall of the first anti-resonant ring. It can also be used as a passive transmission fiber for long-distance transmission of high-power beams and beam quality maintenance.
[0038] In the main power amplifier stage, the active multi-ring asymmetric hollow-core antiresonant fiber adopts the structure described in any of the above embodiments. Signal light is transmitted within the central air core, and a ring-shaped doped layer of rare-earth ions such as ytterbium is disposed on the glass wall near the core. Pump light is injected into the hollow-core antiresonant fiber through a pump coupler and propagates together with the signal light in the hollow core region. The doped layer absorbs the pump light and provides gain to the signal light, thus achieving main power amplification.
[0039] Because the multi-ring asymmetric hollow-core anti-resonant fiber structure has a large effective mode field area and a small acousto-optic overlap factor, and the higher-order modes have large losses due to global geometric asymmetry, during high-power amplification, the signal light mainly propagates in the form of the fundamental mode in the central air core, and the higher-order modes are difficult to be effectively excited and accumulated, thus significantly reducing the risk of mode instability and the impact of nonlinear effects.
[0040] The output and transmission stages employ passive multi-ring asymmetric hollow-core antiresonant fiber, whose structure can be similar to or slightly modified from active fiber to further reduce transmission loss and maintain high-order mode suppression. Through this transmission fiber, the amplified high-power laser can be stably transmitted to the terminal optical system or processing head while maintaining high beam quality.
[0041] In this embodiment, the output power, spectral distribution, and backscatter signal of the main power amplifier stage are monitored in real time by the monitoring and control module. When the backscatter power is detected to be close to the set threshold, the pump power distribution, seed source linewidth, or operating temperature can be automatically adjusted to further improve the nonlinear threshold and operational stability of the system.
[0042] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.
Claims
1. A high beam quality fiber laser amplifier without nonlinear effect limitations, characterized in that, include: Seed light source, used to generate seed signal light with a preset center wavelength; The pre-amplification stage is used to perform the first-stage power amplification of the seed signal light; The main power amplifier stage is used to perform a second-stage high-power amplification of the pre-amplified signal light. The output and transmission stage is used for outputting and long-distance transmission of the signal light after main power amplification; among which, The main power amplifier stage includes an active hollow anti-resonant fiber and a pump coupler for injecting pump light into the active hollow anti-resonant fiber. The active hollow anti-resonant fiber is a multi-ring hollow anti-resonant fiber with a multi-ring structure inside.
2. The high beam quality fiber laser amplifier without nonlinear effect limitations according to claim 1, characterized in that, The multi-ring hollow anti-resonant optical fiber includes: The central air core is used to transmit high-beam-quality signal light at the target wavelength. The multi-ring structure is arranged in an array around the central air core in a circumferential direction.
3. The high beam quality fiber laser amplifier without nonlinear effect limitations according to claim 2, characterized in that, The multi-ring structure includes: The first anti-resonant ring is arranged around the central air core and is composed of multiple first capillaries arranged in an array along the circumferential direction. The second anti-resonant ring is arranged outside the first anti-resonant ring and is composed of multiple first capillaries arranged in an array along the circumferential direction.
4. The high beam quality fiber laser amplifier without nonlinear effect limitations according to claim 3, characterized in that, The second anti-resonant ring is further provided with a rare earth doped region, wherein the rare earth doped region is doped with any one or more rare earth elements selected from erbium, ytterbium, thulium, holmium, and neodymium.
5. A high beam quality fiber laser amplifier without nonlinear effect limitations according to any one of claims 1-4, characterized in that, The multi-ring structure is a geometrically asymmetric structure, specifically: In the first anti-resonant ring and / or the second anti-resonant ring, the inner diameter and / or wall thickness of at least one capillary is different from that of other capillary tubes in the same ring, and / or the center-to-center distance between at least one pair of adjacent capillary tubes is different from that of other capillary tubes in the same ring, so as to form asymmetry in the orientation direction. The active hollow-core anti-resonant fiber is configured to maintain low-loss transmission of the fundamental mode in the central air core at the target signal wavelength, and to introduce additional losses to higher-order modes that are greater than those of the fundamental mode through the multi-ring geometric asymmetric structure, thereby achieving single-mode or near-single-mode output with higher-order mode suppression under high-power amplification conditions.
6. A high beam quality fiber laser amplifier without nonlinear effect limitations according to claim 5, characterized in that, The wall thickness of the first anti-resonant ring in the active hollow anti-resonant fiber Relative to the target signal wavelength The anti-resonance relationship is satisfied by the following equation: ， in, It is a non-negative integer. The refractive index of the glass material is such that the first anti-resonant ring generates anti-resonant reflection of the light field in the central air core at the target signal wavelength, thereby realizing low-loss light guiding of the fundamental mode in the central air core.
7. A high beam quality fiber laser amplifier without nonlinear effect limitations according to claim 6, characterized in that, The wall thickness of the second anti-resonant ring in the active hollow anti-resonant fiber Relative to pump wavelength Satisfying the anti-resonance condition: ， In the formula, It is a non-negative integer. The second anti-resonant ring is designed to be close to the anti-resonant state near the pump wavelength and slightly deviate from the anti-resonant state near the target signal wavelength. This improves the transmission efficiency of the pump light in the hollow region and enhances the ability of higher-order modes to leak to the outer layer.
8. A high beam quality fiber laser amplifier without nonlinear effect limitations according to claim 7, characterized in that, The active hollow antiresonant fiber is configured at the target signal wavelength to minimize the transmission loss per unit length of at least one higher-order mode. Transmission loss per unit length with the fundamental mode The ratio of the two modes must be at least 10 to ensure that the loss of higher-order modes is at least one order of magnitude higher than that of the fundamental mode, thereby suppressing the effective accumulation of higher-order modes during the main power amplification process.
9. A high beam quality fiber laser amplifier without nonlinear effect limitations according to claim 8, characterized in that, The fundamental mode acousto-optic overlap factor Γ of the active hollow antiresonant fiber at the target signal wavelength satisfies: ， in, The amplitude of the transverse electric field distribution of the fundamental mode is given. The numerator integration region is the glass material region in the fiber cross-section, and the denominator integration region is the entire fiber cross-section.
10. A high beam quality fiber laser amplifier without nonlinear effect limitations according to claim 9, characterized in that, The effective mode field area of the fundamental mode of the active hollow antiresonant fiber at the target signal wavelength. The ratio of the acoustic-optical overlap factor Γ satisfies: ， in, ， In the formula, The amplitude of the transverse electric field distribution of the fundamental mode is given, and the integration region is the entire cross-section of the optical fiber. The preset threshold is used to characterize the presence of small acousto-optic overlap while having a large effective mode field area, so that the main power amplifier stage has a high stimulated Brillouin scattering threshold and a high nonlinear threshold.