High-brightness optical fiber beam combining device based on multimode optical fiber and cascading beam combining method of high-brightness optical fiber beam combining device

Abstract

The invention discloses a high-brightness optical fiber beam combining device based on multimode optical fibers and a cascading method of the high-brightness optical fiber beam combining device. The device adopts a modular framework and comprises an input module, a beam combining module, a light beam optimizing module, a mode matching module and an output module which are connected in sequence. The core of the device is that the light beam optimization module is composed of one or more cascaded multimode optical fibers with parabolic or approximate parabolic refractive index distribution, a high-order mode can be effectively inhibited and a low-order mode can be enhanced through controlled mode coupling and energy redistribution after beam combination, so that synchronous improvement of output power and light beam brightness is realized. The device and the method disclosed by the invention are compatible with single-stage and multi-stage cascade application, are compact in structure and stable in performance, can effectively inhibit a nonlinear effect, and are suitable for meeting the requirements on high-brightness laser in the fields of industrial processing, optical communication, scientific research and the like.

Description

Technical Field

[0001] This invention relates to the field of high-power fiber laser technology, specifically to a fiber combining technology for improving laser brightness, and particularly to a high-brightness fiber combining device and its cascaded combining method based on multimode fiber with a specific refractive index distribution for beam optimization. Background Technology

[0002] High-brightness lasers play an irreplaceable role in advanced manufacturing, scientific research, and national defense. As the mainstream high-power laser source, fiber lasers are physically limited by factors such as nonlinear effects and thermal damage in their single-fiber output capability. Therefore, combining the energy of multiple fiber laser beams has become a key technological approach to overcome the single-fiber power bottleneck and obtain higher output power.

[0003] Among various beam combining techniques, coherent beam combining, while achieving beam quality close to the diffraction limit, suffers from extreme system complexity, high environmental sensitivity, and high cost, making it difficult to meet the reliability and cost-effectiveness requirements of industrial high-power applications. Spectral beam combining, limited by wavelength management and combination efficiency, is also complex and struggles to achieve extremely high power densities. In contrast, incoherent power beam combining technology, due to its simple structure, high stability, and strong power scalability, is considered the most promising engineering solution for achieving kilowatt- to megawatt-level industrial laser output.

[0004] However, while incoherent power combining achieves high power, it also presents fundamental technical challenges: its output beam is a simple superposition of multiple multimode beams, leading to severe mode aliasing, significant degradation of beam quality (M² factor), and a decrease in far-field brightness instead of an increase. This is the core bottleneck restricting its application in high-precision manufacturing, long-distance transmission, and other scenarios requiring high-brightness light sources.

[0005] To address this bottleneck, existing technologies have proposed various improvement schemes, but the problems of mode optimization and brightness enhancement have not yet been systematically solved. For example, in existing technologies, CN119395815A uses multi-core fiber coherent combining to achieve high-efficiency combining through phase and polarization modulation. Although it can achieve high brightness output, the system is complex and difficult to extend to high-power scenarios. CN111562651B proposes a multi-channel fiber power combining scheme with good channel expansion, but the output beam is a multimode superposition, resulting in limited brightness and beam quality. CN105759358B and CN105182482B propose mechanical limiting and beam modulation methods to improve combining stability, respectively, but they have not systematically solved the problems of mode optimization and high-power transmission capability. CN105026971B and CN105026969B use multi-channel power superposition and wavelength combining schemes, which can achieve a certain power improvement, but the output beam quality is poor and the multi-wavelength combination control is complex. In summary, existing incoherent power beam combining schemes and their improvements generally suffer from problems such as complex structures, insufficient mode controllability, and limited output brightness, failing to fundamentally achieve a balance between high power and high beam quality (high brightness). Therefore, there is an urgent need in this field for an innovative technical solution that can effectively improve the output beam brightness while maintaining the original advantages of incoherent beam combining technology (simple structure, high stability, and easy scalability). Summary of the Invention

[0006] To address the shortcomings of incoherent power combining in the background art regarding brightness and beam quality, this invention proposes a high-brightness fiber combining device based on a multimode fiber beam optimization section. The core of this invention lies in introducing a multimode fiber with a parabolic or near-parabolic refractive index distribution after the combining point in a traditional incoherent power combining optical path, forming a beam optimization module. This module utilizes its graded-index waveguide characteristics to induce controlled mode coupling and energy redistribution during laser transmission, effectively suppressing the proportion of higher-order modes, enhanced fundamental modes, and lower-order modes. This significantly improves the spatial energy distribution of the output beam while increasing the combining power, achieving a fundamental improvement in beam self-shaping and far-field brightness. Based on this core concept, the specific technical solution of the device of the present invention is as follows: A high-brightness fiber combining device based on multimode fiber, comprising at least one stage combining unit arranged sequentially along the optical path; each stage of the combining unit includes at least an input module, a combining module, and a beam optimization module connected sequentially; the beam optimization module is specifically implemented by one or more cascaded multimode fibers with a parabolic or approximately parabolic refractive index distribution; the device further includes at least one mode matching module disposed between or at the end of the combining units, and an output module for guiding the laser out of the device. The advantage of this structure is that it is suitable for single-stage combining systems, and can also be used to construct multi-stage high-brightness laser output systems with progressively amplified power and synchronously optimized beam quality through modular cascading.

[0007] Furthermore, the beam optimization module, composed of one or more cascaded multimode fibers with parabolic or near-parabolic refractive index distributions, is crucial for enhancing the brightness of the entire device. Its core diameter and numerical aperture (NA) are optimized to achieve good mode field coupling efficiency with the preceding beam combining module. After the combined beam enters this module, guided by its parabolic or near-parabolic refractive index distribution, controlled mode coupling and energy redistribution occur, effectively suppressing higher-order modes and enhancing the proportion of lower-order modes. The length of the multimode fiber can be set according to its mode field periodicity, preferably truncated at an integer multiple of the mode refocusing period to obtain the best beam shaping effect. Precise control of the fiber length can suppress higher-order modes while avoiding strong nonlinear effects, achieving stable beam optimization output at high power. This module can be constructed in a single-segment structure, suitable for low-to-medium power beam combining optimization scenarios, and features simple structure and low loss. Alternatively, it can be constructed by cascading two or more segments of multimode fibers with different characteristics, refractive index distributions, geometric parameters, or doping properties. Each fiber segment performs a specific function, such as the first segment being used for mode energy redistribution and the second segment being used for beam smoothing and spatial shaping, to achieve more refined functions such as mode redistribution, beam spatial shaping, beam optimization, and brightness control, thus meeting the requirements of high-power output.

[0008] Furthermore, the core diameter and numerical aperture (NA) of the multimode fiber in the beam optimization module are optimized to ensure good matching with the fiber parameters of its preceding beam combining module and its subsequent mode matching module, thereby guaranteeing efficient mode field coupling and energy transmission.

[0009] Furthermore, the input module includes multiple fiber lasers and corresponding optical isolators to provide multiple stable, high-beam-quality laser signals. The fiber lasers are preferably single-mode or quasi-single-mode rare-earth-doped fiber lasers capable of outputting continuous or pulsed signals.

[0010] Furthermore, the beam combining module is an optical fiber beam combiner, used to superimpose the power of multiple input lasers in an incoherent manner.

[0011] Furthermore, the mode matching module is used to match the spot size and numerical aperture (NA) between different fiber segments to reduce transmission loss. The mode matching module is preferably located at the input and / or output of the beam optimization module. Specifically, it can be implemented as a mode adapter, for example, using a tapered fiber transition section or a graded-index fiber connector.

[0012] Furthermore, the output module is used to safely and efficiently output the final optimized high-brightness laser, which can be specifically implemented as a laser output head or an optical isolator with an output pigtail.

[0013] Furthermore, the device described in this invention possesses inherent cascading scalability. It can be used as a standalone single-stage beam combiner, or it can be used to construct a multi-stage system by using the output of the preceding beam optimization module as the input of the following beam combiner module to overcome power bottlenecks. In a single-stage system, the beam combiner module and the beam optimization module work directly together; after connecting the beam optimization module at the output end, a significantly brighter output beam can be directly obtained. In a multi-stage system, each beam combiner unit can be cascaded, with the output of the preceding stage serving as the input of the following stage, achieving a progressive accumulation and enhancement of power. By introducing the beam optimization module at the final output end, beam self-shaping can be achieved while maintaining high power transmission, resulting in a system output that combines high power, high brightness, and high stability. Crucially, the beam optimization module can not only be used at the final output end of power beam combining but can also be flexibly embedded in the intermediate stages of a cascaded beam combiner link, dynamically adjusting the beam distribution while transmitting energy, thereby achieving continuous suppression and online optimization of mode degradation effects throughout the entire system link. Thanks to this, and the beam quality assurance provided by the integrated beam optimization modules at each stage, the entire system can simultaneously optimize and improve both power and beam quality (brightness) while increasing power. This all-fiber cascading and optimization integrated design allows the system to avoid contamination, collimation errors, and additional losses caused by complex free-space optical paths, thereby significantly improving the system's reliability, engineering feasibility, and overall performance ceiling.

[0014] Furthermore, to suppress stimulated Raman scattering (SRS) and thermal mode instability (TMI) that may occur during high-power transmission, this invention can incorporate a thermal control module and a spectral filtering unit in the bundled link to maintain the multimode fiber operating under optimal mode field distribution conditions. By adjusting the pump power and core temperature distribution, a synergistic balance between mode stability and nonlinear suppression is achieved.

[0015] Accordingly, this invention also proposes a high-brightness fiber-level beam combining method based on multimode fiber. This method alternates between multi-stage incoherent beam combining and mode optimization steps based on graded-index multimode fiber, achieving exponential power expansion and simultaneous improvement in beam quality. The core of the method lies in: immediately after each stage of beam combining, using multimode fiber with a specific refractive index distribution to perform "beam self-cleaning" optimization on the synthesized beam, suppressing higher-order modes, and then sending the optimized beam to the next stage as a high-quality input source. This process is iterated until high-power, high-brightness laser output is finally obtained. The specific steps are as follows:

[0016] A high-brightness fiber-level combined bundle method based on multimode fiber includes the following steps:

[0017] S1: Provides multiple input laser signals;

[0018] S2: Perform incoherent power combining on the multiple input laser signals;

[0019] S3: Pass the combined beam through a multimode optical fiber with a parabolic or near-parabolic refractive index distribution.

[0020] Beam optimization is performed to suppress higher-order modes and enhance lower-order modes;

[0021] S4: Based on the optimized beam, obtain high-brightness laser output; S5: Selectively perform at least one subsequent amplification operation; wherein, each subsequent amplification operation includes:

[0022] The optimized beam obtained in the previous stage is used as at least one input signal to perform the incoherent power combining and subsequent beam optimization again, so as to obtain a new optimized beam for output or for the next stage of amplification.

[0023] Furthermore, prior to the beam combining step S2, each of the input laser signals is passed through an optical isolator.

[0024] Furthermore, after each stage of beam optimization, an optical isolation step is included.

[0025] Furthermore, a pattern matching step is set before or after the beam optimization operation at each stage.

[0026] Furthermore, in step S5, after any level of beam combining operation, the combined beam at that level is subjected to optical...

[0027] Before beam optimization, a step is set up for signal transmission via the output optical fiber.

[0028] Beneficial technical effects of the present invention:

[0029] First, within a pure fiber optic structure, by introducing a multimode fiber beam optimization segment with a specific refractive index distribution at the bundle output end, a significant improvement in beam quality can be achieved without introducing a complex free-space optical path.

[0030] Secondly, this fiber structure can automatically redistribute mode energy and shape the beam under high-power transmission conditions, effectively suppressing higher-order modes and increasing the proportion of lower-order modes, thereby fundamentally improving the spatial energy distribution and far-field brightness of the output beam.

[0031] Furthermore, the device and method provided by this invention are compatible with single-stage and multi-stage beam combining systems, and are not only compact and highly stable, but also have good power scalability and engineering feasibility.

[0032] Ultimately, through the aforementioned all-fiber architecture, this invention can stably achieve high-power, high-brightness laser output. Its system has high reliability and is very suitable for applications with stringent requirements for laser brightness, such as industrial processing, optical communication, national defense, and scientific research. Attached Figure Description

[0033] Figure 1 This is a schematic diagram of a multi-stage beam amplification device.

[0034] Figure 2 This is a schematic diagram of a multi-stage combined beam amplification device.

[0035] Reference numerals: 1. First fiber laser, 2. First optical isolator, 3. First-stage fiber combiner, 4. First beam-optimizing fiber, 5. First mode adapter, 6. Second isolator, 7. Second fiber combiner, 8. Output fiber, 9. Second mode adapter, 10. Second beam-optimizing fiber, 11. Third isolator, 12. Laser output head.

[0036] Figure 3 This is a diagram of a single-stage beam combining amplifier.

[0037] Reference numerals: 1. First fiber laser, 2. First isolator, 3. First fiber combiner, 4. First beam optimization fiber, 5. First mode adapter, 6. Second isolator.

[0038] Figure 4 This is a simulation diagram of the light spot change during the 2×1 beam combining process. Figure 4 (a) Initial input light spot, Figure 4 (b) The light spot after the initial beam convergence evolution. Figure 4 (c) is the light spot after the basic beam combining evolution is completed. Figure 4 (d) The final output light spot;

[0039] Figure 5 This is a simulation diagram showing the change in light spot size during a linearly distributed 3×1 beam combining process. Figure 5 (a) Initial input light spot, Figure 5 (b) The light spot after the initial beam convergence evolution. Figure 5 (c) is the light spot after the basic beam combining evolution is completed. Figure 5 (d) The final output light spot;

[0040] Figure 6 This is a simulation diagram showing the change in light spot size during the 3×1 ring-shaped beam combining process. Figure 6 (a) Initial input light spot, Figure 6 (b) The light spot after the initial beam convergence evolution. Figure 6 (c) is the light spot after the basic beam combining evolution is completed. Figure 6 (d) The final output light spot;

[0041] Figure 7 This is a simulation diagram showing the change in light spot size during the 4×1 ring-shaped beam combining process. Figure 7 (a) Initial input light spot, Figure 7 (b) The light spot after the initial beam convergence evolution. Figure 7 (c) is the light spot after the basic beam combining evolution is completed. Figure 7 (d) The final output light spot;

[0042] Figure 8 This is a simulation diagram showing the change in light spot size during the 5×1 ring-shaped beam combining process. Figure 8 (a) Initial input light spot, Figure 8 (b) The light spot after the initial beam convergence evolution. Figure 8 (c) is the light spot after the basic beam combining evolution is completed. Figure 8 (d) The final output light spot;

[0043] Figure 9 This is a simulation diagram showing the change in light spot size during the (4+1)×1 beam combining process. Figure 9 (a) Initial input light spot, Figure 9 (b) The light spot after the initial beam convergence evolution. Figure 9 (c) is the light spot after the basic beam combining evolution is completed. Figure 9 (d) The final output light spot;

[0044] Figure 10 This is a simulation diagram showing the change in light spot size during the 6×1 ring-shaped beam combining process. Figure 10 (a) Initial input light spot, Figure 10 (b) The light spot after the initial beam convergence evolution. Figure 10 (c) is the light spot after the basic beam combining evolution is completed. Figure 10 (d) The final output light spot;

[0045] Figure 11 This is a simulation diagram showing the change in light spot size during the (6+1)×1 beam combining process. Figure 11 (a) Initial input light spot, Figure 11 (b) The light spot after the initial beam convergence evolution. Figure 11 (c) is the light spot after the basic beam combining evolution is completed. Figure 11 (d) The final output light spot;

[0046] Figure 12 This is a simulation diagram showing the beam spot change during the beam optimization process after the output of a (6+1)×1 beam with a surrounding distribution. Figure 12 (a) Initial input light spot, Figure 12 (b) The light spot after the initial beam convergence evolution. Figure 12 (c) The final output light spot;

[0047] Figure 13 Thermal diagrams of parametric coupling efficiency for different structured combiners. Figure 13 (a) 2×1 beam coupling efficiency diagram, Figure 13 (b) Coupling efficiency diagram of a 4×1 ring bundle. Figure 13 (c) 5-ring × 1 bundle coupling efficiency diagram, Figure 13 (d) Coupling efficiency diagram of a 6×1 ring bundle. Figure 13 (e) Coupling efficiency diagram of (4+1)×1 bundled bundles in a surrounding distribution. Figure 13 (f) Coupling efficiency diagram of (5+1)×1 bundled distribution.

[0048] Figure 14 This is the optimal bundle structure distribution diagram.

[0049] Figure 15 A bar chart comparing the maximum coupling efficiency. Detailed Implementation

[0050] To make the objectives, technical solutions and advantages of the embodiments of this application clearer, the apparatus and methods in the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings.

[0051] To make the cascaded beam assembly device and its cascaded method described in this invention more intuitive, firstly through... Figure 1 A schematic diagram of a demonstration-grade joint beam amplifier is shown. For the specific location and connection method of the mode matching module, please refer to [reference needed]. Figure 2 and Figure 3 The specific embodiments shown are as follows. Figure 1 This paper provides a general description of the basic components and signal flow of the cascaded beam amplification device described in this invention, embodying the core design concept of modularity and cascadability. The device includes at least one beam combining unit, each of which includes at least an input module, a beam combining module, and a beam optimization module. The device also includes a mode matching module and an output module. In this invention, the input module in the beam combining unit can be specifically implemented as multiple fiber lasers and corresponding optical isolators; the beam combining module can be specifically implemented as a fiber beam combiner; and the beam optimization module can be specifically implemented as a section of multimode fiber with a parabolic or approximately parabolic refractive index distribution. The mode matching module in the device can be specifically implemented as a tapered fiber transition section or a graded-index fiber connector; and the output module can be specifically implemented as a laser output head or an optical isolator with an output pigtail. Those skilled in the art will understand that the above specific implementations are merely examples and not limitations of the invention.

[0052] Example 1:

[0053] based on Figure 1 The overall architecture shown is as follows: Figure 2As shown in the figure, an embodiment of the present invention discloses a multi-stage combined beam amplification device. The system is constructed using an optical fiber structure. Its input stage includes multiple parallel fiber laser units. Each unit includes a first fiber laser 1 and a first optical isolator 2. The output end of the multiple units is connected to a first-stage fiber combiner 3, which is then sequentially connected to a first beam optimization fiber 4, a first mode adapter 5, a second isolator 6, a second fiber combiner 7, an output fiber 8, a second mode adapter 9, a second beam optimization fiber 10, a third isolator 11, and a laser output head 12.

[0054] The plurality of first fiber lasers 1 are used to output high-power laser signals, and their core function is to provide laser output with high stability, high efficiency, and high beam quality. Fiber lasers that meet these requirements are applicable, such as single-mode or quasi-single-mode fiber lasers, and rare-earth-doped fiber lasers such as ytterbium-doped, thulium-doped, or erbium-doped lasers.

[0055] The corresponding first optical isolator 2 is set at the output end of each first fiber laser to prevent the reflected light from the subsequent stage from returning to the laser and to avoid power fluctuations or cavity damage caused by feedback interference.

[0056] The first-stage fiber combiner 3 is used to incoherently superimpose the power of multiple fiber laser signals to form a high-power combined beam output. Its specific implementation is not limited; for example, it can be implemented using multi-fiber fused taper, porous quartz sleeves (i.e., signal combiners), or fiber bundle fusion splicing. The number of input channels and topology can be flexibly configured according to the system's power expansion requirements.

[0057] The first beam optimization fiber 4 is located at the output end of the first-stage fiber combiner and is used to redistribute mode energy and shape the combined beam. This fiber is a section of multimode fiber with a parabolic or near-parabolic refractive index distribution (i.e., graded-index multimode fiber). During beam transmission, it achieves high-order mode suppression and low-order mode enhancement through mode coupling and phase velocity matching, thereby improving the energy distribution of the output beam.

[0058] The first mode adapter 5 is located at the output end of the first beam-optimized fiber 4 and is used to match the spot size and numerical aperture (NA) between the first beam-optimized fiber 4 and the subsequent optical path. Its function is to make the spot size and numerical aperture (NA) of the front and rear fibers transition smoothly through tapered transition or graded refractive index design, thereby significantly reducing the splice loss of the fusion splice or connection point and improving the mode coupling efficiency, so as to achieve efficient energy transmission and continuous mode transition.

[0059] The second isolator 6 is located after the first mode adapter 5 and is used to block reflected light from the subsequent optical path from returning to the previous system, thus ensuring the stable operation of the cascaded structure.

[0060] The second fiber combiner 7 is structurally similar to the first-stage fiber combiner 3. At least one of its input channels is connected to the output of the second isolator 6 to receive the beam processed in the first stage. This combiner can also accept other laser signals to achieve second-stage power combining and obtain higher power output. The number of input channels of this combiner can be flexibly configured according to the target power of the system.

[0061] Output fiber 8 is used to transmit the high-power beam after the second-stage power combining. Its core diameter and NA parameters need to be reasonably matched with the pre-stage beam optimization fiber to ensure efficient energy transmission and continuous mode transition, and maintain good mode distribution.

[0062] The second mode adapter 9 is located at the output end of the output fiber 8 and is used to achieve mode matching and smooth transition of the optical field between the output fiber 8 and the subsequent optical path.

[0063] The second beam-optimizing fiber 10 is positioned after the second mode adapter 9 to optimize and shape the final combined output beam, making the output beam approach a Gaussian distribution and improving far-field brightness and focusing performance. This fiber also has parabolic or near-parabolic refractive index distribution characteristics (i.e., graded refractive index distribution), which can induce mode coupling and redistribution in multimode energy transmission, achieving low-order mode energy concentration.

[0064] The third isolator 11 is located after the second beam optimization fiber to prevent reflected light from the end face from entering the system, thereby improving operational safety and stability.

[0065] The laser output head 12 is positioned after the third isolator 11 to achieve safe output and spatial divergence control of the shaped high-brightness laser. It may include an end cap structure or a beam expander and collimator unit to improve the end face damage threshold and maintain the uniformity of the output beam.

[0066] Through the above design, this invention achieves high-power progressive expansion of multi-stage fiber combining, and introduces beam-optimized multimode fiber segments at the output ends of each stage to improve beam quality. This structure effectively suppresses higher-order mode components while ensuring power enhancement, obtaining a near-Gaussian distributed output beam spot, significantly improving system brightness and stability. Compared with traditional incoherent combining devices, the multi-stage combined beam device of this invention achieves coordinated control of power expansion and beam optimization within an optical fiber structure, offering advantages such as compact structure, strong scalability, and high engineering practicality.

[0067] Example 2:

[0068] like Figure 3As shown, this embodiment of the invention provides a single-stage beam combining and amplification device. The device is constructed entirely of optical fiber, and its input stage includes multiple parallel fiber laser units. Each unit includes a first fiber laser 1 and a first isolator 2. The output of each multiple unit is connected to a first fiber combiner 3, which is then sequentially connected to a first beam optimization fiber 4, a first mode adapter 5, and a second isolator 6.

[0069] The plurality of first fiber lasers 1 are used to output high-power laser signals, and their core function is to provide laser output with high stability, high efficiency, and high beam quality. Fiber lasers that meet these requirements are applicable, such as single-mode or quasi-single-mode fiber lasers, and rare-earth-doped fiber lasers such as ytterbium-doped, thulium-doped, or erbium-doped lasers.

[0070] The corresponding first isolator 2 is set after each first fiber laser 1 to block the reflected light from the subsequent stage from returning to the laser, prevent laser power fluctuations or device damage caused by feedback, and ensure long-term stable operation of the system.

[0071] The first fiber optic combiner 3 is used to incoherently combine multiple input fiber optic signals and output a high-power beam. The first fiber optic combiner can be implemented using a porous quartz sleeve or a fused biconical tapered structure, and its number of input channels can be configured according to system design requirements. This combining structure features low transmission loss, high thermal stability, and high integration.

[0072] The first beam-optimizing fiber 4 is located at the output end of the first fiber combiner 3 and is used for mode redistribution and spatial shaping of the combined high-power beam. This fiber is a multimode fiber with parabolic or near-parabolic refractive index distribution characteristics (i.e., graded-index multimode fiber), which can induce mode coupling and energy redistribution during beam transmission, thereby suppressing higher-order mode components, enhancing the proportion of lower-order modes, and achieving improved beam quality and brightness. By appropriately selecting the fiber length, a near-Gaussian distribution beam spot can be obtained at the output end, significantly reducing the output M² value.

[0073] The first mode adapter 5 is located at the output end of the first beam-optimizing fiber 4 and is used to match the spot size and numerical aperture (NA) between the first beam-optimizing fiber 4 and the subsequent optical path, thereby improving energy transmission efficiency and reducing splice loss. The first mode adapter 5 can be implemented using a tapered transition fiber structure or a graded-index connector, etc.

[0074] The second isolator 6 is located at the system output end to prevent reflected light from returning to the beam optimization fiber, thus avoiding parasitic oscillations or mode instability and ensuring long-term stability of the output beam.

[0075] Through the above design, this embodiment integrates single-stage incoherent fiber combining and beam optimization functions. The combined high-power beam automatically redistributes energy in the beam-optimized fiber via mode coupling, thereby achieving high-brightness output while maintaining high-power transmission. Compared with traditional combining devices, the single-stage combining device described in this invention has a simpler structure and superior beam quality, enabling high-stability, high-brightness laser output in fiber optic systems. It is suitable for high-power laser processing, optical communication, and scientific research applications.

[0076] Example 3: Simulation of beam combining process and analysis of beam evolution

[0077] To optimize beam combining, simulation analysis was conducted on beam combining structures with different numbers of input channels (including 2-in-1, 3-in-1, 4-in-1, 5-in-1, 6-in-1, and 7-in-1), revealing that the fiber arrangement significantly affects the spatial energy distribution of the output beam. When a ring-symmetric arrangement (such as 5-in-1 or 7-in-1) is used, the output beam exhibits approximately circular symmetry in the lateral direction, resulting in higher beam center intensity, gentler edge attenuation, and a smaller far-field divergence angle. This energy distribution characteristic highly matches the low-order mode field shape of the beam-optimized fiber, facilitating efficient mode coupling and spontaneous beam shaping after entering the beam-optimized fiber, thereby achieving better beam quality and higher output brightness. In contrast, linear or asymmetrical beam combining structures (such as 3-in-1 and 4-in-1) exhibit uneven energy distribution at the output end, easily exciting higher-order mode components, leading to a decrease in shaping efficiency after entering the beam-optimized fiber.

[0078] This embodiment is based on Figures 4 to 12 The simulation results shown illustrate the evolution process and optimization effect of the beam under different beam combining structures, providing a theoretical basis for the design of this invention.

[0079] like Figure 4 The image shows a simulation of the beam evolution in the most basic 2×1 beam combining process. The beam starts from the initial input state ( Figure 4 (a)), initially evolved through the combiner ( Figure 4 (b)) until the basic completion of the bundle ( Figure 4 (c) ultimately forms a uniform, near-Gaussian distributed light spot at the output end of the beam-optimized fiber. Figure 4 (d)). This process visually demonstrates the core mechanism of the invention: through the synergistic effect of beam combining and subsequent beam optimization fibers, even from separate multi-beam inputs, a high-quality single-beam output can ultimately be evolved.

[0080] To explore better beam combining performance, this invention further simulated various high-order beam combining structures. For example... Figure 5 The linear distribution 3×1 bundle shown Figure 6 The ring-shaped 3×1 bundle shown, and Figure 7 , Figure 8 The diagram shows 4×1 and 5×1 ring-shaped beam combining structures. Simulation results show that, compared to linear arrangements, the ring-shaped arrangement enables more symmetrical mode interference and coupling of the input beam in the beam combining region, thereby allowing the beam spot to evolve more smoothly (from...). Figure 6 (a) to Figure 6 (d) The energy converges more quickly towards the center, resulting in a more concentrated energy distribution, which is beneficial for improving the beam combining efficiency.

[0081] To further improve power capacity and beam quality, this invention further simulates a "surround distribution" beam combining structure. For example... Figure 9 The (4+1)×1 structure shown (i.e., 1 central path and 4 outer ring paths) and Figure 11 The (6+1)×1 structure shown has an initial input light spot ( Figure 9 (a) Figure 11 (a) exhibits a clear core-surround layout. Simulations show that this structure effectively utilizes the central channel to stabilize the beam core, while the outer ring channel injects high power. Through optimized fiber mode field matching and mode coupling, the final output beam spot ( Figure 9 (d) Figure 11 (d) Not only does it have high power, but it also boasts outstanding center brightness and superior mode distribution. In particular, Figure 12 This demonstrates the further optimization process of the beam in the second beam optimization fiber 10 after (6+1)×1 beam combining output, where the beam spot changes from the initial annular mixed state ( Figure 12 (a) The evolution into a final output state with highly concentrated and uniform energy distribution proves the effectiveness of the multi-level optimization design.

[0082] Example 4: Parametric Analysis and Optimization of Bundle Structure

[0083] This embodiment combines Figures 13 to 15 The present invention performs parametric analysis on the bundle combiner structure, quantitatively demonstrates the performance advantages of the present invention, and provides clear guidance for the selection of the optimal structure.

[0084] Figure 13 The parametric coupling efficiency heatmaps for different bundle combining structures are shown. Figure 13 (a) is a 2×1 bundle. Figure 13 (b) is a 4×1 ring bundle. Figure 13 (c) is a 5×1 ring bundle. Figure 13 (d) is a 6×1 ring bundle. Figure 13 (e) is a (4+1)×1 bundle with a surrounding distribution. Figure 13(f) shows a (5+1)×1 bundle with a surrounding distribution. The heatmaps use key structural parameters such as fiber spacing and core size as variables to intuitively reflect the coupling efficiency under different configurations. A comprehensive analysis of these heatmaps reveals that the surrounding distribution structure (especially the (6+1)×1, whose heatmap is not shown but is...) Figure 11 , 12 (Performance verified in China) It can maintain high coupling efficiency (typically >95%) over a wider range of process parameters, exhibiting stronger process tolerance and stability.

[0085] Based on the above analysis Figure 14 This paper summarizes the various beam combining structures explored in this invention and clearly identifies the optimal beam combining structure distribution. This optimal structure combines the advantages of high channel number, annular symmetry, and core-surround layout, enabling maximum power expansion while ensuring the highest beam quality and beam combining efficiency.

[0086] Figure 15 The bar chart comparing the maximum coupling efficiency provides a quantitative comparison for the above conclusions. The chart clearly compares the theoretical maximum coupling efficiencies of several representative structures, including 2×1, ring-shaped 4×1, ring-shaped 6×1, (4+1)×1, and (6+1)×1. The data clearly show that the peak coupling efficiency of the surrounding distribution structure (especially (6+1)×1) is significantly higher than that of a simple ring or linear distribution structure. This theoretically verifies that designing the first fiber combiner 3 or the second fiber combiner 7 in this invention as a surrounding distribution structure is the optimal technical path to achieve high-power, high-brightness laser output.

[0087] Based on the above simulation results and parametric analysis, the beam combiner structure design of this invention follows the following optimization criteria:

[0088] Symmetry priority: For the same number of channels, a ring-shaped symmetrical bundle structure (such as...) Figure 6 , 7 Compared to linear or asymmetric arrangements (as shown in Figure 8), Figure 5 As shown in the figure, it can produce a more circular and symmetrical interference spot with more concentrated energy, which is beneficial for exciting efficient mode coupling in the beam optimization fiber, thereby obtaining better output beam quality.

[0089] The core-surround architecture has significant advantages: a surrounding distributed structure (i.e., one central path and multiple peripheral ring paths, such as...) Figure 9 , 11The (4+1)×1 and (6+1)×1 structures shown are preferred embodiments of the present invention. This structure combines high power capacity with excellent beam shaping potential: the central channel helps stabilize and define the core mode of the beam, while the outer ring channel injects high power; the two evolve synergistically under the mode field matching and controlled mode coupling of the beam-optimized fiber, ultimately achieving the best balance of power, brightness and beam quality (M² value) at the output end.

[0090] Quantitative verification: Figure 15 The comparative data on maximum coupling efficiency quantitatively confirms the above conclusions. The theoretical maximum coupling efficiency of the (6+1)×1 surrounding distributed structure is significantly higher than that of other comparative structures, indicating that this design can achieve maximum power expansion while ensuring the highest energy transfer efficiency and process tolerance.

[0091] Therefore, in a preferred embodiment of the present invention, it is recommended that the fiber combiners (such as the first fiber combiner 3 and the second fiber combiner 7) be designed as a surrounding distribution structure, particularly a (6+1)×1 layout, in order to achieve optimal performance of high power and high brightness laser output.

[0092] Example 5: Cascade Method Flow

[0093] Combination Figure 1 The overview architecture shown and Figure 2 The specific structure of the multi-stage combined beam amplification device shown is illustrated, and the cascading method described in this invention is specifically implemented as follows, with steps corresponding to those in claim 7:

[0094] S1: Provide multiple input laser signals, specifically, activate multiple fiber lasers 1 to generate the multiple high-power laser signals;

[0095] S2: Perform incoherent power combining on the multiple input laser signals, specifically, input the multiple signals to the first-stage fiber combiner 3 for incoherent combining;

[0096] S3: The combined beam is passed through a multimode fiber with a parabolic or near-parabolic refractive index distribution for beam optimization, in order to suppress higher-order modes and enhance lower-order modes. Specifically, the combined beam is fed into the first beam optimization fiber 4, and the first beam optimization is completed by controlled mode coupling using its refractive index distribution.

[0097] S4: Use the optimized beam as the optimized output;

[0098] S5: Selectively execute at least one level of follow-up operations, each level of follow-up operations including:

[0099] Beam combining: The optimized output from the previous stage (i.e., the beam processed by the first stage) is used as an input signal and input to the second fiber beam combiner 7 for incoherent power combining (other laser signals can also be connected during this process).

[0100] Optimization: For the beam after the second-stage beam combining, perform the beam optimization by passing it through the second beam optimization fiber 10;

[0101] S6: Outputs the final optimized high-brightness laser, i.e., the final laser with significantly improved brightness output from the second beam optimization fiber 10.

[0102] It should be noted that step S5 can be repeated multiple times according to power expansion requirements to achieve cascading of more stages.

[0103] Example 6: Parameter Design and Length Calculation of Beam Optimization Fiber

[0104] This embodiment provides the specific design basis and length calculation method for the beam-optimized multimode fiber.

[0105] 1) Refractive index distribution

[0106] The radial refractive index n(r) of the multimode fiber in the beam optimization section has or approximately has the following refractive index distribution characteristics:

[0107]

[0108] 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. α is preferably between 1.8 and 2.2 to form a parabolic or approximately parabolic structure.

[0109] Furthermore, in the actual fabrication process of a specific multimode fiber, due to limitations in process conditions and the uniformity of doping distribution, its refractive index distribution curve may exhibit slight structural defects such as central depressions or edge deviations. While such imperfections may have some impact on beam optimization, they still fall within the scope of this invention. The multimode fiber, through a special graded-index waveguide design, synchronizes the group velocities between different transmission modes, thereby forming a stable linear mode coupling mechanism during energy transmission. Through this mechanism, the energy of higher-order modes gradually transfers to lower-order modes during transmission, ultimately forming an energy distribution dominated by the fundamental mode at the output end, achieving a near-Gaussian steady-state beam output. This process primarily relies on the mode-matching effect guided by the fiber's refractive index distribution and geometry, rather than strong nonlinear effects. Therefore, it can maintain stable beam quality and suppress the accumulation of nonlinear effects under high-power transmission conditions.

[0110] 2) Calculation of self-cleaning length

[0111] The beam-optimized multimode fiber has a beam self-cleaning length that can be calculated and determined using the following method. In this multimode fiber, beam optimization mainly originates from the energy coupling and self-imaging effect between multimodes, which gradually increases the proportion of low-order mode energy, thereby achieving spatial shaping of the output beam.

[0112] Let the fiber core radius be The relative refractive index difference between the core and the cladding is Then the self-imaging period of the optical fiber Represented as

[0113]

[0114] Let the operating wavelength of the optical fiber be... The material's nonlinear refractive index is The input peak power is The effective area of ​​the model is Then the nonlinear characteristic length of the optical fiber is:

[0115]

[0116] Define the effective overlap integral of the combination of the target low-order mode f and the participating background mode as:

[0117]

[0118] Target low-order model Effective nonlinear overlap integrals with all participating higher-order mode combinations.

[0119] : Summation index, representing other modes involved in nonlinear coupling.

[0120] Weighting coefficients, determined by the incident light in the mode The initial energy distribution (modal weights) on the surface is determined.

[0121] : Involves target pattern and pattern The specific nonlinear overlap integral value.

[0122] Where the coefficient Determined by the incident mode weights. This gives the mode overlap factor (dimensionless).

[0123]

[0124] : Pattern overlap factor (dimensionless).

[0125] Effective nonlinear overlap integral The modulus.

[0126] : Effective mode area of ​​optical fiber.

[0127] Its value range is approximately 0 < η overlap ≲1. η overlap The larger the value, the more efficient the lateral coupling.

[0128] For those containing phase factors The coupling term has an effective accumulation amount within a single self-imaging cycle of .

[0129]

[0130] Phase matching efficiency factor (dimensionless).

[0131] : Self-imaging period length of GRIN multimode fiber.

[0132] : The mismatch between the propagation constants of the various modes involved in the nonlinear interaction. .

[0133] :Singer function, .

[0134] in .when When the value is close to an integer multiple of 2π, S≈1 (quasi-phase matching); otherwise, S decreases significantly. The range of S is 0. <S≤1。

[0135] Define pattern overlap factor Characterizes the lateral coupling efficiency between low-order and high-order modes, and defines the phase-matching efficiency factor. Characterizing the degree of phase matching along the propagation direction, the characteristic length required for beam self-cleaning (low-order mode energy concentration) is:

[0136]

[0137] : Beam self-cleaning feature length.

[0138] : Non-linear length.

[0139] : Pattern overlap factor.

[0140] Phase matching efficiency factor.

[0141] Target low-order model The initial energy percentage.

[0142] Target low-order model The desired percentage of energy.

[0143] 3) Applications under different light source conditions

[0144] The computational method for beam-optimized multimode fiber is applicable to continuous light, nanosecond pulses, and sub-nanosecond (picosecond, femtosecond) pulse incidence conditions. At different timescales, the nonlinear accumulation process and the triggering mechanism for beam self-cleaning have different dominant factors, and their calculation rules are as follows:

[0145] (1) Continuous light (CW) case: When the incident light is a steady-state continuous wave and the thermal effect and average power dominate the nonlinear response in the optical fiber, the effective power is taken. That is, the average input power; the nonlinear length and self-cleaning length are calculated by the following formula:

[0146]

[0147] : Average power of continuous light (CW) input.

[0148] Other parameters ( (The definition is the same as before.)

[0149] (2) Nanosecond-level pulse case: When the pulse duration is on the order of nanoseconds ( s) When the internal dispersion effect of the pulse is weak, the light intensity within the pulse can be considered basically stable, and its nonlinear accumulation is mainly driven by the peak power. The effective power is taken as the peak power. :

[0150]

[0151] in, For single-pulse energy, The pulse duration. If the average power is known... With repetition frequency ,but Such pulses typically satisfy the dispersion length... Much greater than the self-cleaning length The peak power can be directly substituted into the calculation.

[0152] Formula parameter explanation (supplementary):

[0153] : Average power of the optical pulse train.

[0154] : The repetition frequency of a light pulse (the number of pulses per unit time).

[0155] Dispersion length.

[0156] : Group velocity dispersion parameter (second-order dispersion coefficient) of optical fiber.

[0157] (3) Sub-nanosecond pulses (picosecond to femtosecond): When the pulse duration is less than 1 ns, especially in the picosecond range... s) or femtosecond ( At the order of s), instantaneous effects such as group velocity dispersion and self-phase modulation are significant, and beam optimization and pulse time-domain broadening can occur simultaneously. In this case, the formula form should remain unchanged, but... It needs to be corrected by considering the time evolution of the pulse, and the applicable conditions are:

[0158]

[0159] Dispersion length.

[0160] The duration of the light pulse.

[0161] Fiber group velocity dispersion parameters The absolute value of.

[0162] : Beam self-cleaning feature length.

[0163] in, The fiber group velocity dispersion parameter is given. When this condition is not met, numerical calculations should be performed using the multimode nonlinear Schrödinger equation (GMMNLSE) to obtain the equivalent self-cleaning length.

[0164] Under picosecond and femtosecond laser incidence, beam optimization is usually completed within a length range of a few centimeters to tens of centimeters. Afterwards, due to the extremely high peak power and strong nonlinear interaction, processes such as supercontinuum spectral broadening, optical soliton formation, and intermodal four-wave mixing may be triggered. Therefore, the calculation results described in this paragraph should be used as an upper limit reference for the beam optimization range.

[0165] In addition, to ensure project controllability and process tolerance, a range coefficient κ can be introduced, which can be taken from 0.5 to 10, to obtain the design length range:

[0166]

[0167] : The length of optical fiber selected in actual engineering design.

[0168] : The lower and upper limits of the range coefficient.

[0169] : Beam self-cleaning feature length.

[0170] in and Determined based on the type of light source and the pulse duration.

[0171] Through the detailed description of the above embodiments, the present invention fully discloses the comprehensive content of high-brightness fiber bundling and cascading innovation based on multimode fiber:

[0172] Device Entities: Embodiments 1 and 2 respectively disclose the multi-level and single-level hardware device structures of the technology;

[0173] Principle and Verification: Through simulation and parametric analysis, Examples 3 and 4 reveal the working principle of beam evolution, verify the technical effect, and provide design criteria for the optimal structure.

[0174] Method Flow: Example 5 further illustrates the cascade operation method of the device, clarifying its feasibility as a process invention.

[0175] Parameter design guidance: Example 6 provides the refractive index distribution, key performance parameter calculation methods, and length design criteria of the beam-optimized fiber, providing theoretical basis and engineering guidance for specific implementation.

[0176] In summary, this invention provides a detailed description of the high-brightness fiber combining device and its cascade method based on multimode fiber, covering aspects such as device structure, working principle, effect verification, parameter design, optimization criteria, and operation methods. Those skilled in the art can implement this invention based on the above description and obtain the beneficial effect of achieving both high power and high beam quality output. Furthermore, the above description is merely a specific embodiment of this invention, but the scope of protection of this invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in this invention should be included within the scope of protection of this invention.

Claims

1. A high-brightness fiber combining device based on multimode fiber, characterized in that, The device includes at least one beam combining unit arranged sequentially along the optical path; each beam combining unit includes at least an input module, a beam combining module, and a beam optimization module connected sequentially; the beam optimization module is composed of one or more cascaded multimode optical fibers with parabolic or near-parabolic refractive index distributions; the device also includes at least one mode matching module disposed between or at the end of the beam combining units, and an output module for guiding the laser out of the device.

2. The apparatus according to claim 1, characterized in that, The input module includes multiple fiber lasers and an optical isolator disposed at the output end of each fiber laser.

3. The apparatus according to claim 2, characterized in that, The fiber laser is used to output continuous or pulsed laser signals, and the fiber laser is a single-mode or quasi-single-mode rare-earth-doped fiber laser.

4. The apparatus according to claim 1, characterized in that, The beam combining module is an optical fiber beam combiner.

5. The apparatus according to claim 1, characterized in that, The mode matching module is a mode adapter, which adopts a tapered fiber transition section or a refractive index graded fiber connection section, and the mode matching module is set at the input end and / or output end of the beam optimization module.

6. The apparatus according to claim 1, characterized in that, The output module includes a laser output head or an optical isolator with an output pigtail.

7. The apparatus according to claim 1, characterized in that, The device is a cascaded system, wherein the output of at least one of the beam optimization modules serves as at least one input signal for another beam combining module.

8. A high-brightness fiber-level combined bundle method based on multimode fiber, characterized in that, Includes the following steps: S1: Provides multiple input laser signals; S2: Perform incoherent power combining on the multiple input laser signals; S3: Pass the combined beam through a multimode optical fiber with a parabolic or near-parabolic refractive index distribution. Beam optimization is performed to suppress higher-order modes and enhance lower-order modes; S4: Based on the optimized beam, a high-brightness laser output is obtained; S5: Selectively perform at least one subsequent amplification operation; wherein each subsequent amplification operation includes: The optimized beam obtained in the previous stage is used as at least one input signal to perform the incoherent power combining and subsequent beam optimization again, so as to obtain a new optimized beam for output or for the next stage of amplification.

9. The method according to claim 8, characterized in that, Before the beam combining step S2, each of the input laser signals is passed through an optical isolator.

10. The method according to claim 8 or 9, characterized in that, After each stage of beam optimization, an optical isolation step is included.

11. The method according to claim 8, characterized in that, Before or after performing the beam optimization operation at each stage, a pattern matching step is set.

12. The method according to claim 8, characterized in that, In step S5, after any level of beam combining operation and before beam optimization of the combined beam at that level, a step is set to transmit signals through the output optical fiber.

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