Macroenergy pulse all-fiber laser
By combining a laser seed source, an ultra-large mode field active fiber amplifier, and a passive graded-index multimode fiber, the beam quality and stability issues of all-fiber lasers at high energy output have been solved, achieving high-energy, high-brightness laser output suitable for materials processing and national defense security.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- SHANGHAI INST OF OPTICS & FINE MECHANICS CHINESE ACAD OF SCI
- Filing Date
- 2025-12-26
- Publication Date
- 2026-04-21
AI Technical Summary
Existing all-fiber lasers struggle to simultaneously achieve beam quality, system stability, and compact integrated design when outputting high energy. Traditional amplification links lack sufficient flexibility in energy control and have limited mode control and adaptation capabilities, making it difficult to meet the demands of high-energy, high-brightness applications.
By employing a combination of a laser seed source, an ultra-large mode field active fiber amplifier, and a passive graded-index multimode fiber, high-energy pulsed laser output is achieved through multi-stage amplification and beam shaping techniques.
It achieves high-energy pulsed light output at the millijoules level, maintaining excellent beam quality and high stability, and is suitable for fields such as materials processing and national defense security.
Smart Images
Figure CN121906211A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the fields of fiber optics and laser technology, and in particular to a high-energy pulsed all-fiber laser. Background Technology
[0002] In the field of pulsed lasers, high-energy output faces multiple challenges. With increasing single-pulse energy, the cumulative effect of nonlinear phase shift in the fiber is significantly enhanced, easily causing pulse waveform distortion and spectral broadening, thus affecting the pulse beam quality. Simultaneously, high-energy transmission makes the fiber endface and core susceptible to damage, limiting further energy increases. Furthermore, there is an inherent contradiction between high beam quality and significant energy increases: while increasing the core diameter and employing multimode transmission can help improve single-pulse energy, they can easily lead to mode degradation, resulting in a decrease in beam quality. Currently, fiber pulsed lasers have achieved single-pulse energies in the hundreds of millijoules range, but a severe decline in beam quality persists.
[0003] Various schemes for increasing pulse energy have been proposed in the prior art. For example, patent CN106207726 discloses a high-energy nanosecond pulse all-fiber laser based on incoherent beam combining. This scheme combines the outputs of multiple nanosecond pulse fiber lasers to obtain an output with an average power in the kilowatt range and a single pulse energy in the hundreds of millijoules range.
[0004] Patent CN221305232 discloses a high-energy miniaturized pulsed fiber laser. It constructs a laser seed source consisting of a high-reflectivity fiber grating, a resonant cavity gain fiber, a resonant cavity combiner, a resonant cavity pump source, an acousto-optic modulator, and a low-reflectivity fiber grating. An amplifier is formed by an amplification stage pump source, an amplification stage combiner, an amplification stage gain fiber, and an isolator. The amplification stage employs a double-clad gain fiber with a large core diameter and a small cladding diameter. This double-clad fiber has a core diameter between 25µm and 35µm and a cladding diameter between 120µm and 170µm, reducing the fiber bending diameter and increasing the pulse energy of the pulsed fiber laser. Its high-energy, miniaturized size meets the requirements of portable laser marking machines, and its output energy is similar to that of pulsed fiber lasers used in industrial laser marking machines, thus expanding the application range of pulsed fiber lasers. This design outputs a single pulse energy of 1mJ to 2mJ, meeting the energy requirements of portable laser marking machines.
[0005] Overall, existing all-fiber solutions still struggle to simultaneously achieve beam quality, system stability, and compact integration when pushing the limits of high energy. Traditional amplification links mostly rely on a single pumping method, lacking sufficient flexibility in energy control and having limited mode control and adaptation capabilities, resulting in performance that fails to meet requirements in higher energy and higher brightness applications. Therefore, there is an urgent need for an innovative all-fiber architecture that integrates key technologies such as mode-adaptive transmission, switchable or multi-dimensional pumping mechanisms, and beam shaping. This architecture should significantly suppress nonlinear effects and mode instability while achieving high-brightness, high-energy, and high-reliability laser output to meet the pressing needs of high-end pulsed laser sources in fields such as materials processing and national defense. Summary of the Invention
[0006] In view of the shortcomings of the existing technology, the purpose of this invention is to provide a high-energy pulsed all-fiber laser.
[0007] To achieve this objective, the present invention adopts the following technical solution: A high-energy pulsed all-fiber laser, characterized in that it comprises: a laser seed source, an ultra-large mode field active fiber amplifier, a passive graded-index multimode fiber, and a laser output head; The output end of the laser seed source is connected to the input end of the ultra-large mode field active fiber amplifier, the output end of the ultra-large mode field active fiber amplifier is connected to the input end of the passive graded-index multimode fiber, and the output end of the passive graded-index multimode fiber is connected to the laser output head.
[0008] Furthermore, the laser seed source is used to output a pulsed laser signal. The laser seed source can be any one of a Q-switched laser, a pulse-modulated continuous laser, or other lasers with pulse output capability. In addition, an optical isolator can be connected in series at the output end to protect the laser.
[0009] Furthermore, the ultra-large mode field active fiber amplifier is an amplifier based on ultra-large mode field active fiber, used to amplify the energy of pulsed laser signals; the ultra-large mode field active fiber can be ytterbium-doped, thulium-doped, erbium-doped or other rare earth element-doped fiber, or chromium-doped, titanium-doped or other transition metal element-doped fiber, or co-doped fiber of two or more rare earth elements, or rare earth and transition metal elements.
[0010] Furthermore, the active fiber used in the ultra-large mode field active fiber amplifier has a normalized frequency V parameter of not less than 5.3 at the operating wavelength, and can be optionally extended to 10 or higher, and is configured to support pulse pumping or continuous pumping.
[0011] Furthermore, the passive graded-index multimode fiber is used to shape the input beam, adjusting the energy distribution of the output laser beam to a Gaussian distribution. The refractive index distribution of the passive graded-index multimode fiber is formed by the radial distribution of refractive index modulation elements in the fiber core, possessing a parabolic or approximately parabolic structure.
[0012] Furthermore, the radial refractive index n(r) of the graded-index multimode fiber has or approximately has the following refractive index distribution characteristics. (1) In equation (1), n0 is the maximum refractive index at the center of the fiber core. R is the relative refractive index difference, r is the core radius, r is the radial coordinate, and α is the refractive index distribution coefficient. α is preferably between 1.8 and 2.2 to form a parabolic or approximately parabolic structure.
[0013] Meanwhile, the specific graded-index multimode fiber may have defects such as a central depression in its refractive index distribution curve due to the influence of the fabrication technology, which may affect the beam shaping results to some extent, but is still within the scope of protection of this invention patent. The special waveguide structure of the graded-index multimode fiber enables the phase velocity of different transmission modes to be synchronized, thereby exciting a nonlinear spatial self-cleaning effect. When a high-power laser is input, the nonlinear Kerr effect and mode coupling effect in the fiber work together to drive the energy transfer of higher-order modes to the fundamental mode, ultimately forming a near-Gaussian distributed steady-state beam at the output end.
[0014] The evolution of the pulse in GRIN-MMF satisfies the generalized multimode nonlinear Schrödinger equation (GMM-NLSE), the core equation of which can be expressed as the evolution equation of the time envelope of mode p with propagation distance z: (2) This equation describes the nonlinear effects (such as intermodal four-wave mixing) that occur between different modes of light propagating in a super-large mode field waveguide structure, causing energy transfer from higher-order modes to the fundamental mode. In equation (2), the first to third terms on the right-hand side represent the dispersion operator at the center frequency ω0, expanded using a Taylor series. The first two terms describe propagation constant mismatch, modal dispersion, or modal drift, respectively, while the third term represents higher-order dispersion effects. n2 is the nonlinear refractive index. For the Raman response of the medium, The fractional contribution to the Raman effect. Wrong and denoted as the nonlinear coupling coefficient between the Kerr and Raman effects.
[0015] The beam shaping mechanism of the passive graded-index multimode fiber can be analyzed by examining the distribution of laser energy among different transmission modes. Based on the differences in mode propagation constants, different modes have varying capacities for consuming input energy, with the fundamental mode having the lowest propagation constant. When the total input energy is within an appropriate range, the fundamental mode can capture the majority of the energy, thus achieving a beam shaping effect by concentrating the beam energy towards the fundamental mode.
[0016] To describe this shaping condition, the following expression can be introduced as a design criterion: (3) Where, n eff λ is the effective refractive index of the fundamental mode, P is the laser wavelength, P is the total energy of the input pulse, P0 is the minimum energy when all power is concentrated in the fundamental mode, and V0 is the fiber normalized frequency-related parameter, which depends on the structural parameters of the graded fiber, such as the core radius, NA, operating wavelength, and refractive index distribution coefficient α.
[0017] When the shaping factor η≈1, a better beam shaping effect can be obtained; if η 1. Higher-order modes occupy more space, reducing the shaping effect; if η A value of 1 indicates that the input energy is insufficient to excite effective mode coupling, which is also detrimental to forming a shaped output. Therefore, in practical fiber design and selection, this formula criterion can be used to adjust the fiber parameters to make η close to 1, thereby achieving a good shaping effect.
[0018] Furthermore, the ratio of the output M² to the input M² of the passive graded-index multimode fiber is less than 0.5, which is used to achieve a beam shaping effect that enhances the proportion of low-order modes.
[0019] To achieve the aforementioned beam shaping characteristics, the passive graded-index multimode fiber should be structurally matched with the large-mode-field active fiber of the preceding stage. Its core diameter and NA should be moderately larger than the preceding amplifying fiber to ensure efficient fusion coupling and mode splicing in the all-fiber structure, improve laser injection efficiency, and support the effective occurrence of multimode energy redistribution. Furthermore, the refractive index distribution coefficient α of the graded-index fiber should be as close to 2 as possible, so that the core forms a parabolic or approximately parabolic refractive index profile, which helps improve inter-mode coupling efficiency, suppress higher-order modes, and enhance beam shaping capabilities. At the target wavelength, a suitable V value range should also be ensured so that the number of supported modes meets the shaping requirements without being excessive and raising the critical energy threshold required for beam shaping. Through the above structural and parameter optimization design, the energy proportion of the fundamental mode in the output laser can be effectively increased, ultimately achieving high-quality laser output with improved beam brightness and concentrated and stable lateral energy distribution.
[0020] Furthermore, the laser output head is used to achieve safe output and spatial divergence control of the shaped high-energy pulsed laser.
[0021] Compared with the prior art, the beneficial effects of the present invention are as follows: The laser achieves high-energy pulsed light output at the millijoules level by using active optical fiber in a multi-stage amplification structure; combined with passive graded-index multimode fiber for beam shaping, the output laser energy distribution is optimized to a Gaussian distribution; the key amplification and shaping parts of the laser are all-fiber structures, which combine high pulse energy, excellent beam quality and high stability. Attached Figure Description
[0022] Figure 1 This is a schematic diagram of a structure according to Embodiment 1 of the present invention.
[0023] The diagram is labeled as follows: 11. Laser seed source, 12. Large mode field active fiber amplifier, 13. Passive graded-index multimode fiber, 14. Laser output head.
[0024] Figure 2 This is a schematic diagram of a structure of an ultra-large mode field active fiber amplifier according to Embodiment 1 of the present invention.
[0025] The following components are labeled in the diagram: semiconductor pump laser 121, fiber pump signal combiner 122, large mode field double-clad mirror-doped fiber 123, pump stripper 124, fiber isolator 125, and mode adapter 126.
[0026] Figure 3 A, Figure 3 B represents the transverse cross-section of the high-power, high-energy beam provided in the embodiments of the present invention, and the transverse cross-section of the high-power, high-energy beam after passing through a graded refractive index fiber structure. Figure 4 The energy evolution diagram of the fundamental mode, intermediate mode and higher-order mode transmitted for 2m in a graded refractive index fiber structure is provided in the embodiments of the present invention.
[0027] Figure 5 This is a schematic diagram of a structure according to Embodiment 2 of the present invention.
[0028] The diagram is labeled as follows: 21. Seed source for laser; 22. Seed preamplifier; 23. Active fiber amplifier with large mode field; 24. Passive graded-index multimode fiber; 25. Laser output head.
[0029] Figure 6 This is a schematic diagram of the structure of a seed light preamplifier according to Embodiment 2 of the present invention.
[0030] The following are labeled in the figure: semiconductor pump laser 221, fiber pump signal combiner 222, double-clad mirror-doped fiber 223, pump stripper 224, fiber isolator 225.
[0031] Figure 7 This is a schematic diagram of a structure according to Embodiment 3 of the present invention.
[0032] The diagram is labeled as follows: laser seed source 31, fiber multistage amplifier 32, ultra-large mode field active fiber amplifier 33, passive graded-index multimode fiber 34, and laser output head 35.
[0033] Figure 8 This is a schematic diagram of a fiber optic multistage amplifier according to Embodiment 3 of the present invention.
[0034] The following are labeled in the diagram: Semiconductor-pumped laser 320, fiber pump signal combiner 321, double-clad mirror-doped fiber 322, pump stripper 323, fiber isolator 324, semiconductor-pumped laser 325, fiber pump signal combiner 326, double-clad mirror-doped fiber 327, pump stripper 328, and fiber isolator 329.
[0035] Figure 9 Different refractive index distribution coefficients in Embodiment 4 of the present invention A schematic diagram of the refractive index distribution of a time-gradient refractive index optical fiber. Detailed Implementation
[0036] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings, so as to make the further applicable scope of the present invention clearer.
[0037] Example 1: like Figure 1 As shown, this embodiment of the invention provides a high-power, high-energy pulsed all-fiber laser for optoelectronic countermeasures, comprising: a laser seed source 11, an ultra-large mode field active fiber amplifier 12, a passive graded-index multimode fiber 13, and a laser output head 14. The output end of the laser seed source 11 is connected to the input end of the ultra-large mode field active fiber amplifier 12. The output end of the ultra-large mode field active fiber amplifier 12 is connected to the input end of the passive graded refractive index multimode fiber 13 through a mode adapter. The output end of the passive graded refractive index multimode fiber 13 is connected to the laser output head 14.
[0038] The laser seed source 11 is used to output pulsed laser signals. The laser seed source 11 can be any one of a Q-switched laser, a pulse-modulated continuous laser, or other lasers with pulse output capability. Furthermore, depending on the need to protect the laser, an optical isolator can be connected in series at its output end. The laser seed source 11 can output pulsed laser signals of 10 μJ or higher.
[0039] The ultra-large mode field active fiber amplifier 12 is an amplifier based on ultra-large mode field active fiber, used to amplify the energy of pulsed laser signals; the ultra-large mode field active fiber can be ytterbium-doped, thulium-doped, erbium-doped or other rare earth element-doped fiber, or chromium-doped, titanium-doped or other transition metal element-doped fiber, or co-doped fiber of two or more rare earth elements, or rare earth and transition metal elements.
[0040] One structure of the ultra-large mode field active fiber amplifier 12 of this invention is as follows: Figure 2 As shown, the ultra-large mode field active fiber amplifier 12 includes a first semiconductor pump laser 121, a first fiber pump signal combiner 122, a large mode field double-clad ytterbium-doped fiber 123, a first pump stripper 124, a first fiber isolator 125, and a mode adapter 126. The pump end of the first fiber pump signal combiner 122 is connected to the first semiconductor pump laser 121, and its output end is connected to the input end of the large mode field double-clad ytterbium-doped fiber 123. The output end of the large mode field double-clad ytterbium-doped fiber 123 is connected to the input end of the first fiber isolator 125 through the first pump stripper 124. The output end of the first fiber isolator 125 is connected to the output end of the passive graded-index multimode fiber 13 through the mode adapter 126.
[0041] Specifically, the first semiconductor-pumped laser 121 outputs 976nm laser light with an energy of 20mJ. The first fiber pump signal combiner 122 adopts a (6+1)×1 structure. The normalized frequency V parameter of the large-mode-field double-clad ytterbium-doped fiber 123 at the operating wavelength is not less than 5.3 and can be selectively extended to 10 or higher. The absorption coefficient at 976nm is approximately 0.5dB / m, balancing high power transmission capability and mode control characteristics. The first semiconductor-pumped laser 121 is configured as a pulsed pump source or a continuous pump source synchronized with the laser signal output from the laser seed source 1, to achieve efficient extraction of energy stored in the large-mode-field double-clad ytterbium-doped fiber. The mode adapter 126 is used to couple the output beam of the ultra-large-mode-field active fiber amplifier 12 to the passive graded-index multimode fiber 13. The mode adapter 126 can be a fusion-type mode field converter, whose core function is to control the beam divergence angle and suppress higher-order mode excitation, improve coupling efficiency, and provide a high-quality input optical field for subsequent beam shaping.
[0042] The passive graded-index multimode fiber 13 is used to shape the input beam, adjusting the energy distribution of the output laser beam to a Gaussian distribution. The refractive index distribution of the passive graded-index multimode fiber 13 is formed by the radial distribution of refractive index control elements in the fiber core, possessing a parabolic or approximately parabolic structure. The structural parameters of the passive graded-index multimode fiber 13 should be reasonably matched with the large-mode-field active fiber of the preceding stage. Its core diameter and NA should be appropriately larger than those of the preceding amplifying fiber to ensure efficient fusion coupling and mode splicing in the all-fiber structure, improve laser injection efficiency, and support the effective occurrence of the multimode energy redistribution process. Furthermore, the refractive index distribution coefficient α of the passive graded-index multimode fiber 13 should be as close to 2 as possible, so that the fiber core forms a parabolic or approximately parabolic refractive index profile. This helps to improve inter-mode coupling efficiency, suppress higher-order modes, and enhance shaping capabilities. At the target wavelength, a suitable V value range should also be ensured, so that the number of supported modes meets the shaping requirements without being excessive and raising the critical energy threshold required for shaping. Through the above structural and parameter optimization design, the energy proportion of the fundamental mode in the output laser can be effectively increased, ultimately achieving high-quality laser output with improved beam brightness, concentrated and stable lateral energy distribution.
[0043] Specifically, the ratio of the output M² to the input M² of the passive graded-index multimode fiber 13 is less than 0.5, which is used to achieve a beam shaping effect that enhances the proportion of low-order modes. The core diameter of the passive graded-index multimode fiber 13 can be selected in the range of 100μm to 500μm, or it can be a larger size (such as 600μm), and the cladding diameter can be 200μm to 800μm or 1000μm. The material of the passive graded-index multimode fiber 13 can be silica-based germanium-doped fiber. In order to ensure coupling efficiency and suppress mode energy leakage, the core diameter and NA of the graded-index fiber should generally be larger than those of the front-stage large-mode-field double-clad ytterbium-doped fiber. For example, when the core diameter of the large-mode-field double-clad ytterbium-doped fiber 123 is 100 μm and the NA is 0.06, the graded-index multimode fiber can be selected with a core diameter of about 150 μm and an NA of about 0.08. The two fiber segments can be efficiently coupled by fused taper or direct fusion splicing.
[0044] Figure 3 A, Figure 3 B and Figure 4 Simulation results of high-energy laser propagation in a graded-index fiber are presented. The propagation of a high-energy pulsed laser in a graded-index fiber is compared with... Figure 3 A and Figure 3 As can be seen from B, the transverse beam profile at the output end of the graded-index fiber is significantly improved compared to the input end, resulting in a marked improvement in beam quality. For example... Figure 4The energy evolution curve shown reveals the dynamic process of mode coupling. When the beam propagates for 1m in a graded-index fiber, due to nonlinear effects such as intermodal four-wave mixing, the beam undergoes an energy exchange process. The energy of the intermediate mode is transferred to the fundamental mode and higher-order modes, and then gradually saturates. The energy ratio of the fundamental mode increases from 20% at the input end to about 60% at the output end.
[0045] The laser output head 14 adopts an optical end cap structure with an inclined end face, and a broadband anti-reflection coating layer is deposited on its end face to achieve safe beam output and resonance suppression.
[0046] The high-energy pulsed all-fiber laser output can achieve hundreds of millijoules of pulsed laser output while maintaining excellent beam quality.
[0047] Example 2: This embodiment is a variation of Embodiment 1, and its principle and structure are similar to Embodiment 1. The difference lies in that, when the energy of the input pulsed light seed source is relatively low, the seed light preamplifier 22 can be added. Its structure is as follows: Figure 5 As shown, the output end of the laser seed source 21 is directly connected to the input end of the seed light preamplifier 22, the output end of the seed light preamplifier 22 is connected to the input end of the ultra-large mode field active fiber amplifier 23, the output end of the ultra-large mode field active fiber amplifier 23 is connected to the input end of the passive graded refractive index multimode fiber 24 through a mode adapter, and the output end of the passive graded refractive index multimode fiber 4 is connected to the laser output head 25.
[0048] One structure of the seed light preamplifier 22 is as follows: Figure 6 As shown, the system includes a second fiber pump signal combiner 222, a first active fiber 223, a second pump stripper 224, a second fiber isolator 225, and a second semiconductor pump laser 221 connected in series at the rear end of the laser seed source 21. The first active fiber 223 can be a double-clad ytterbium-doped fiber with a core diameter of 20–50 μm and an inner cladding diameter of 100–200 μm, which can be adjusted as needed. The second semiconductor pump laser 221 can be a continuous pump source, a quasi-continuous pump source, or a pulsed pump source synchronized with the seed laser pulse signal, with an operating wavelength of 976 nm. The second fiber pump signal combiner 222 adopts a (1+1)×1 structure, and the second pump stripper 224 is used to remove residual pump light.
[0049] After adding the seed light preamplifier 22, the system output pulse energy can be further increased. Correspondingly, the large-mode-field active fiber used in the ultra-large-mode-field active fiber amplifier 23 has a core diameter of 100 μm and an NA of 0.06, which can be adjusted as needed; the passive graded-index multimode fiber 24 has a core diameter of 150 μm and an NA of 0.08, which can also be adjusted as needed. Through the matching of these dimensional parameters, efficient coupling and energy transfer in the all-fiber structure can be guaranteed, thereby further enhancing the shaping output effect.
[0050] Example 3: This embodiment is a variation of Embodiment 1, and its principle and structure are similar to Embodiment 1. The difference lies in that, when higher output pulse energy is required, the fiber optic multistage amplifier 32 can be added, and its structure is as follows: Figure 7 As shown, the output end of the laser seed source 31 is directly connected to the input end of the fiber multistage amplifier 32, the output end of the fiber multistage amplifier 32 is connected to the input end of the ultra-large mode field active fiber amplifier 33, the output end of the ultra-large mode field active fiber amplifier 33 is connected to the input end of the passive graded refractive index multimode fiber 34 through a mode adapter, and the output end of the passive graded refractive index multimode fiber 34 is connected to the laser output head 35.
[0051] The fiber multistage amplifier 32 is composed of at least two fiber amplification stages connected in series. The fiber multistage amplifier 32 is not limited to two fiber amplification stages; additional fiber amplification stages can be added to achieve higher-energy pulsed light. Each stage can employ a forward or backward pumping structure to progressively increase the pulse energy. Each fiber amplification stage includes, in series with the laser seed source 31, a third fiber pump signal combiner, a second active fiber, a third pump stripper, a third fiber isolator, and a third semiconductor pump laser connected to the third fiber pump signal combiner. The third semiconductor pump laser can be a continuous pump source, a quasi-continuous pump source, or a pulsed pump source synchronized with the laser signal output from the laser seed source 31. Adjacent fiber amplification stages are cascaded by connecting the output of the preceding third fiber isolator to the signal end of the following third fiber pump signal combiner; the signal end of the first fiber amplification stage is connected to the output of the seed light preamplifier, and the output of the last fiber amplification stage is connected to the input of the ultra-large mode field active fiber amplifier 32.
[0052] Specifically, one structure of the fiber optic multistage amplifier 32 in this embodiment of the invention is as follows: Figure 8As shown, the fiber optic multistage amplifier 32 consists of two fiber optic amplification stages. The first fiber optic amplification stage includes: a third semiconductor pump laser 320, a third fiber optic pump signal combiner 321, a second active fiber 322, a third pump stripper 323, and a third fiber optic isolator 324. The second fiber optic amplification stage includes: a third semiconductor pump laser 325, a third fiber optic pump signal combiner 326, a second active fiber 327, a third pump stripper 328, and a third fiber optic isolator 329. In the first fiber amplification stage of the fiber multistage amplifier 32, the third semiconductor pump laser 320 emits a laser with a working wavelength of 976nm and a pulse energy of 0.5mJ. The third fiber pump signal combiner 321 adopts a (2+1)×1 structure. The core diameter / cladding of the second active fiber 322 can be 20 / 130μm, core NA 0.08, and 976nm absorption coefficient of approximately 10dB / km, which can be increased or decreased as needed. In the second fiber amplification stage of amplifier 32, the third semiconductor pump laser 325 emits a laser with a working wavelength of 976nm and a pulse energy of 5mJ. The third fiber pump signal combiner 326 adopts a (2+1)×1 structure. The core diameter of the second active fiber 327 can be 50–100μm, the inner cladding diameter can be 200–400μm, and the NA is about 0.06–0.08. It can be adjusted as needed to balance the matching of energy enhancement and beam transmission.
[0053] After adding the fiber multistage amplifier 32, the overall output pulse energy of the system is significantly improved. To ensure efficient energy transfer and avoid mode mismatch, the large-mode-field double-clad ytterbium-doped fiber selected in the ultra-large-mode-field active fiber amplifier 33 has a core diameter of 200 μm and an NA of 0.1, which can be adjusted appropriately according to application requirements. Correspondingly, the core diameter of the passive graded-index multimode fiber 34 can be designed to be 270 μm and the NA to be 0.15, which can be optimized as needed. Through the above parameter matching, not only can stable amplification and transmission of high-energy pulses be achieved in the all-fiber link, but the proportion of the beam in low-order modes can also be further increased, thereby obtaining higher beam quality and output brightness.
[0054] Example 4 This embodiment is a variation of Embodiment 1, and its principle and structure are similar to Embodiment 1. The difference is that both the final stage active amplification fiber and the connected graded-index multimode fiber adopt a large core diameter design to meet the needs of higher energy pulse transmission. At the same time, the refractive index distribution curve of the graded-index multimode fiber is no longer strictly an ideal parabola, and the refractive index distribution index α is not necessarily strictly equal to 2.
[0055] In this embodiment, when the refractive index difference Δn is fixed, the refractive index distribution of the graded-index multimode fiber with different α values is as follows: Figure 9 As shown. Figure 9 The vertical axis represents the refractive index *n*, and the horizontal axis represents the radial axis *r*, where *r0* represents the cladding radius of the graded-index multimode fiber. As can be seen from the figure, when α=1.5, the refractive index curve is relatively flat, indicating weaker constraint on lower-order modes and a tendency for higher-order modes to occupy a larger proportion. When α=2.2, the refractive index distribution is close to an ideal parabola, resulting in higher energy coupling efficiency between modes and relatively better shaping effect. When α=3 and α=4, the refractive index curve gradually becomes steeper, inter-mode coupling weakens, the increase in the fundamental mode's occupancy is limited, and the overall shaping effect is lower than when α≈2.
[0056] Therefore, this embodiment illustrates the direct impact of the refractive index distribution coefficient α of a graded-index fiber on beam shaping when it deviates from the ideal value. In practical applications, an appropriate α value can be selected in conjunction with parameters such as core diameter and NA, based on the output beam quality of the final-stage active fiber and the required degree of shaping, to achieve a balance between high-energy transmission stability and shaping quality.
[0057] This invention provides a high-energy pulsed all-fiber laser. A multi-stage amplification architecture using fiber multi-stage amplifiers and large-mode-field fiber amplifiers achieves a stepwise increase in energy. Simultaneously, graded-index multimode fiber is used for beam shaping to overcome the traditional contradiction between high energy and high beam quality, achieving high-beam-quality laser output of high-energy pulsed light at the 100-millijoule level. This invention possesses significant technical advantages and application prospects.
[0058] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., 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-energy pulsed all-fiber laser, characterized in that, The optical path features an all-fiber fusion splicing integrated structure, including: A pulse seed source unit is used to generate an initial pulse laser. The ultra-large mode field active fiber amplifier unit has its signal input end connected to the signal output end of the pulse seed source unit through a first fiber fusion splice, and is used to receive the initial pulse laser and amplify it to the level of hundreds of millijoules; wherein, the ultra-large mode field main amplifier unit adopts a large mode field double-clad active fiber with a normalized frequency V parameter of not less than 5.
3. A passive graded-index beam shaping unit, whose input end is connected to the signal output end of the ultra-large mode field active fiber amplifier unit via a second fiber fusion splice, is used to shape the amplified millijoules-level pulsed laser beam. The passive graded-index beam shaping unit is composed of a section of passive graded-index multimode fiber, which simultaneously meets the following three conditions: I. The fiber core has a parabolic or approximately parabolic gradient refractive index distribution, defined by the refractive index distribution coefficient α, and the value of α ranges from 1.8 to 2.2; II. The core diameter and numerical aperture are respectively larger than the core diameter and numerical aperture of the output fiber of the ultra-large mode field main amplifier unit directly fused to it; III. Under high-power pulsed laser injection, a spatial self-cleaning effect based on intermodal nonlinear coupling can be excited, making the ratio of its output beam quality factor to its input beam quality factor less than 0.5, thereby producing a high-energy laser with an output beam energy distribution that approaches a Gaussian distribution.
2. The high-energy pulsed all-fiber laser according to claim 1, characterized in that, The pulse seed source unit includes an optical pulse generator and a first fiber isolator connected in series with its output end; the optical pulse generator is a Q-switched fiber laser or a pulse-modulated continuous fiber laser.
3. The high-energy pulsed all-fiber laser according to claim 1, characterized in that, The ultra-large mode field main amplifier unit comprises the following components connected sequentially along the optical path: The pump combiner has its signal port connected to the preceding optical path. A large-mode-field double-clad active optical fiber, the input end of which is fused to the output end of the pump combiner, has a V parameter that satisfies 5.3 ≤ V ≤ 12; A pump stripper, the input of which is connected to the output of the large-mode-field double-clad active optical fiber; The second fiber optic isolator has its input end connected to the output end of the pump stripper. In addition, at least one pump source, the output of which is connected to the pump port of the pump combiner; the pump source is configured as a pulse pump source or a continuous pump source, and when it is a pulse pump source, its pulse timing is synchronized with the output pulse of the pulse seed source unit.
4. The high-energy pulsed all-fiber laser according to claim 3, characterized in that, The doping elements of the large-mode-field double-clad active optical fiber are selected from at least one rare earth element among ytterbium (Yb), thulium (Tm), and erbium (Er), or at least one transition metal element among chromium (Cr) and titanium (Ti), or a combination of the aforementioned elements.
5. A high-energy pulsed all-fiber laser according to claim 3 or 4, characterized in that, The normalized frequency V parameter of the large-mode-field double-clad active fiber at the operating wavelength is not less than 5.3, and can be optionally extended to 10 or higher.
6. A high-energy pulsed all-fiber laser according to claim 3, characterized in that, The pump laser is configured as a pulsed pump source or a continuous pump source.
7. A high-energy pulsed all-fiber laser according to claim 1, characterized in that, The core of the passive graded-index multimode fiber has a parabolic or approximately parabolic graded-index distribution, and its refractive index satisfies: (1) In the formula, n0 is the maximum refractive index at the center of the fiber core. R is the relative refractive index difference, R is the core radius, r is the radial coordinate, and α is the refractive index distribution coefficient, with the value of α ranging from 1.8 to 2.
2.
8. A high-energy pulsed all-fiber laser according to claim 1, characterized in that, A seed light preamplifier or a fiber multistage amplifier is also provided between the laser seed source and the ultra-large mode field active fiber amplifier; the seed light preamplifier or fiber multistage amplifier includes at least one fiber amplification stage, each stage including a pump laser, a pump combiner, an active fiber, a pump stripper and a fiber isolator, and the stages are connected in series by optical fibers.
9. A high-energy pulsed all-fiber laser according to claim 1, characterized in that, A mode adapter is also provided between the output end of the ultra-large mode field active fiber amplifier and the input end of the passive graded-index multimode fiber to achieve mode field matching and suppress higher-order mode excitation.
10. A high-energy pulsed all-fiber laser according to claim 1, characterized in that, The laser output head is an optical end cap structure with an inclined end face, and its end face is coated with a broadband anti-reflection film.