Ring spot shaping system based on multimode fiber mode interference

Abstract

This application provides a ring beam shaping system based on multimode fiber mode interferometry, comprising an input unit, a mode conversion unit, and an output unit. The input unit is a single-mode fiber with a first fiber parameter, used to input a Gaussian beam. The mode conversion unit is a multimode fiber with a second fiber parameter, connected to the single-mode fiber via fusion splicing, used to excite and interfere with higher-order modes. The output unit is a ring-core fiber with a third fiber parameter, connected to the mode conversion unit via fusion splicing, used to maintain and output a ring beam distribution. The core diameters of the single-mode fiber, multimode fiber, and ring-core fiber increase progressively, as do the NA values ​​of the single-mode fiber, multimode fiber, and ring-core fiber. This system achieves high-quality ring beam output through a simple, flexible, and efficient all-fiber structure, significantly reducing cost and complexity, improving stability and reliability, and has broad application prospects.

Description

Technical Field

[0001] This application relates to the field of laser technology, and in particular to a ring spot shaping system based on multimode fiber mode interference. Background Technology

[0002] Ring spot shaping technology has significant application value in fields such as laser processing, fiber optic communication, and optical manipulation. Currently, the main technical solutions for achieving ring spots include ring spot generation schemes based on special optical fibers and combiners, shaping schemes based on diffractive optical elements (DOEs), shaping schemes based on refractive optical elements, and mode conversion schemes based on fiber offset fusion splicing.

[0003] However, ring spot generation schemes based on special optical fibers and combiners require complex fiber design and manufacturing processes, resulting in high equipment manufacturing and maintenance costs. Schemes based on diffractive optical elements require precise optical components and complex alignment systems, increasing system complexity and cost. Schemes based on special fiber lasers typically only achieve a limited power adjustment range, and the size and shape of the ring spot are difficult to adjust flexibly. Schemes based on refractive optical elements usually require replacing optical components to adjust the ring spot parameters, making operation inconvenient. Schemes based on diffractive optical elements often suffer from energy loss problems, with efficiency potentially falling below 80%. Schemes based on refractive optical elements have low energy utilization due to interface reflection and aberration issues. Mode conversion schemes based on fiber eccentric fusion splicing typically have poor stability, and schemes based on diffractive optical elements are sensitive to the quality of the incident beam and are easily affected by external interference. Summary of the Invention

[0004] In view of this, embodiments of this application provide a ring spot shaping system based on multimode fiber mode interference to solve the technical defects existing in the prior art.

[0005] According to a first aspect of the embodiments of this application, a ring spot shaping system based on multimode fiber mode interferometry is provided, comprising an input unit, a mode conversion unit, and an output unit, wherein... The input unit is a single-mode fiber with the first fiber parameter, used to input a Gaussian beam; The mode conversion unit is a multimode fiber with second fiber parameters, which is connected to the single-mode fiber by fusion splicing and is used to excite and interfere with higher-order modes. The output unit is a loop-core fiber with the third fiber parameter, which is connected to the mode conversion unit by fusion splicing to maintain and output a ring-shaped optical field distribution. The core diameters of the single-mode fiber, the multimode fiber, and the loop-core fiber increase progressively, and the NA of the single-mode fiber, the multimode fiber, and the loop-core fiber also increase progressively.

[0006] Optionally, after the Gaussian beam is input from the single-mode fiber, multiple guided modes are excited in the multimode fiber due to the mismatch between the fiber core size and the NA. Through the multimode fiber of a preset length, each mode generates destructive interference at the output end, forming a ring-shaped light spot with zero central light intensity. The ring-shaped light spot is coupled into the ring-core fiber to maintain the ring-shaped refractive index distribution and transmit the ring-shaped light field.

[0007] Optionally, the core diameter and NA in the second fiber parameters are determined by mode interference theory, and the length in the second fiber parameters is determined based on the core diameter and NA in the second fiber parameters.

[0008] Optionally, determining the core diameter and NA in the second fiber parameters using mode interference theory includes: Based on mode interference theory, by analyzing the core diameter, NA, and length of the multimode fiber, the influence of the power coupling coefficient, propagation constant difference, and laser transmittance in the fundamental and secondary modes of the multimode fiber is determined, thereby determining the core diameter and NA in the second fiber parameters.

[0009] Optionally, determining the length of the second fiber parameter based on the core diameter and NA in the second fiber parameter includes: Based on the core diameter and NA in the second fiber parameters, the propagation constants corresponding to the fundamental mode and the second mode of the multimode fiber are calculated, and the interference phase depletion beat length is determined based on the calculation results. The length in the second fiber parameter is determined based on the interference phase de-beat length.

[0010] Optionally, in the third fiber parameter of the ring-core fiber, the outer ring diameter is set to the core diameter of the multimode fiber, the inner ring diameter is set to the core diameter of the single-mode fiber, the inner ring and cladding are made of pure silica material, and the outer ring is doped with high-refractive-index material. The NA value in the third fiber parameter is determined by the laser transmittance of the overall fiber shaping structure and the uniformity of the ring spot.

[0011] Optionally, during the splicing process, core alignment and cladding alignment techniques are employed to ensure minimal splice loss and controllable mode excitation. The first splice core is aligned between the multimode fiber and the single-mode fiber, and the second splice cladding is aligned between the multimode fiber and the ring-core fiber.

[0012] Optionally, after fusion splicing, the cladding light energy introduced by coupling can be removed by roughening the surface of the ring-core fiber.

[0013] This application provides a ring beam shaping system based on multimode fiber mode interference, comprising an input unit, a mode conversion unit, and an output unit. The input unit is a single-mode fiber with a first fiber parameter, used for inputting a Gaussian beam. The mode conversion unit is a multimode fiber with a second fiber parameter, connected to the single-mode fiber via fusion splicing, used for exciting and interfering higher-order modes. The output unit is a ring-core fiber with a third fiber parameter, connected to the mode conversion unit via fusion splicing, used for maintaining and outputting a ring beam distribution. The core diameters of the single-mode fiber, multimode fiber, and ring-core fiber increase progressively, as do the NA values ​​of the single-mode fiber, multimode fiber, and ring-core fiber. Employing an all-fiber structure, the three fiber segments are directly connected via fusion splicing, eliminating the need for additional optical components and mechanical support structures. This significantly simplifies the system structure, reduces system size, and meets the requirements for miniaturization and integration, making it widely applicable to portable optical devices, micro-laser systems, and other scenarios. Meanwhile, the all-fiber structure avoids the influence of air gaps between components, reduces the interference of external environmental factors such as temperature and vibration on the optical path, and improves the stability and reliability of the system. Furthermore, the three fiber segments used are all standardized fiber types, resulting in lower procurement costs. The fusion splicing process is mature and simple to operate, requiring no complicated mechanical adjustments and calibration steps, which significantly reduces the system's production costs and assembly difficulty, and improves the practicality and scalability of the solution. Attached Figure Description

[0014] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0015] Figure 1 This is a schematic diagram of a ring spot shaping system based on multimode fiber mode interference provided in an embodiment of this application; Figure 2 This is a schematic diagram of a ring spot generation scheme based on a special optical fiber and a beam combiner provided in an embodiment of this application; Figure 3 This is a schematic diagram of a shaping scheme based on a diffractive optical element provided in an embodiment of this application; Figure 4 This is a schematic diagram of a shaping scheme based on a refractive optical element provided in an embodiment of this application; Figure 5 This is a schematic diagram of a mode conversion scheme based on fiber offset fusion splicing provided in an embodiment of this application; Figure 6This is a schematic diagram of the input and output distribution of a ring spot shaping system based on multimode fiber mode interference provided in an embodiment of this application; Figure 7 This is a schematic diagram of the refractive index distribution of the core fiber in a ring spot shaping system based on multimode fiber mode interference, provided in one embodiment of this application. Detailed Implementation

[0016] Many specific details are set forth in the following description to provide a full understanding of this application. However, this application can be implemented in many other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of this application; therefore, this application is not limited to the specific embodiments disclosed below.

[0017] The terminology used in one or more embodiments of this application is for the purpose of describing particular embodiments only and is not intended to limit the scope of one or more embodiments of this application. The singular forms “a,” “the,” and “the” used in one or more embodiments of this application and in the appended claims are also intended to include the plural forms unless the context clearly indicates otherwise. It should also be understood that the term “and / or” used in one or more embodiments of this application refers to and includes any or all possible combinations of one or more associated listed items.

[0018] It should be understood that although the terms first, second, etc., may be used to describe various information in one or more embodiments of this application, such information should not be limited to these terms. These terms are only used to distinguish information of the same type from one another. For example, first may also be referred to as second without departing from the scope of one or more embodiments of this application, and similarly, second may also be referred to as first.

[0019] This application provides a ring spot shaping system based on multimode fiber mode interference, which will be described in detail in the following embodiments.

[0020] Figure 1 This diagram illustrates a ring spot shaping system based on multimode fiber mode interferometry according to an embodiment of this application, specifically including an input unit, a mode conversion unit, and an output unit, wherein... The input unit is a single-mode fiber with the first fiber parameter, used to input a Gaussian beam; The mode conversion unit is a multimode fiber with second fiber parameters, which is connected to the single-mode fiber by fusion splicing and is used to excite and interfere with higher-order modes. The output unit is a loop-core fiber with the third fiber parameter, which is connected to the mode conversion unit by fusion splicing to maintain and output a ring-shaped optical field distribution. The core diameters of the single-mode fiber, the multimode fiber, and the loop-core fiber increase progressively, and the NA of the single-mode fiber, the multimode fiber, and the loop-core fiber also increase progressively.

[0021] First, it should be noted that in existing methods for implementing ring-shaped light spots, such as... Figure 2 A schematic diagram of a ring spot generation scheme based on special optical fibers and combiners is provided. A multi-module fiber laser independently controls the power output of the central core and the ring core through a ring spot combiner, forming a combined distribution of the central and ring spots. This scheme typically requires complex fiber combiner design and manufacturing processes, resulting in low throughput and a limited power adjustment range. Figure 3 A schematic diagram of a beam shaping scheme based on diffractive optical elements is provided. This scheme uses a spiral phase plate or binary diffractive element to convert a Gaussian beam into a vortex beam, generating a ring-shaped spot. This method typically requires precise optical elements and a complex alignment system, and is sensitive to the quality of the incident beam. Figure 4 A schematic diagram of a shaping scheme based on refractive optical elements is provided. Using optical elements such as conical mirrors or axonal prisms, a Gaussian beam is converted into a ring distribution through the principles of refraction or reflection. This scheme has a simple structure, but typically suffers from problems such as high fabrication difficulty and low energy utilization. Figure 5 A schematic diagram of a mode conversion scheme based on fiber offset fusion splicing is provided. The ring spot beam is shaped by flicting a single-mode fiber off-core to a high-refractive-index ring band of a ring-core fiber. This scheme has a compact structure, but it usually requires precise control of fiber parameters and fusion splicing process. It is difficult to manufacture, has a low yield, and the uniformity and stability of the ring spot are poor. If the offset is not well controlled, it can easily introduce additional loss.

[0022] Therefore, a three-level architecture of input unit, mode conversion unit, and output unit connected by fusion splicing is proposed, with the whole being an all-fiber integrated structure. Specifically, the input unit uses single-mode fiber with the first fiber parameter to ensure the stability and monochromaticity of the input Gaussian beam, providing a standard initial optical field for subsequent mode conversion; the mode conversion unit uses multimode fiber with the second fiber parameter, tightly connected to the single-mode fiber by fusion splicing, utilizing the parameter difference between the two to achieve excitation and interference of higher-order modes; the output unit uses ring-core fiber with the third fiber parameter, connected to the mode conversion unit by fusion splicing, responsible for maintaining and outputting the ring optical field. Due to the gradient matching of the fiber parameters, i.e., the core diameters of the single-mode fiber, multimode fiber, and ring-core fiber increase progressively, it adapts to the transmission process of the optical field from concentration to diffusion to constraint, avoiding reflection and loss during optical field coupling; at the same time, the NA of the three gradually increases, respectively adapting to the requirements of fundamental mode constraint, multimode excitation, and ring optical field constraint, ensuring the efficiency and stability of optical field transmission at each stage. This three-stage all-fiber structure design eliminates the need for additional spatial optical components, enabling system miniaturization and integration. Simultaneously, parameter gradient matching ensures the orderly execution of mode excitation, interference shaping, and ring light field transmission, providing structural and parameter support for the output of high-quality ring light spots.

[0023] Multimode fiber can simultaneously carry and transmit multiple guided wave modes, including the fundamental mode and higher-order modes. Different modes, due to differences in propagation constants, generate a continuous phase accumulation difference during transmission. When these modes meet at the fiber output, they interfere according to the principle of wave superposition, resulting in constructive interference and destructive interference. This is the core physical mechanism for achieving beam spot shaping and is widely used in fiber optic sensing, laser processing, and other fields. Its interference effect directly determines the shape, intensity distribution, and stability of the beam spot. The input unit specifically refers to a single-mode fiber with the first fiber parameter. The core characteristic of a single-mode fiber is that it can only transmit the fundamental mode (LP). 01 The single-mode fiber, without higher-order mode transmission, can effectively ensure the monochromaticity, directionality, and stability of the input beam. Its function is to serve as the input carrier of the Gaussian beam, providing a standard and stable initial optical field for subsequent mode conversion and beam shaping, and avoiding interference from stray modes in the initial optical field on the shaping effect. It should be noted that the first fiber parameters include the core diameter, numerical aperture NA, cladding diameter, and refractive index distribution corresponding to the single-mode fiber.

[0024] Furthermore, the mode conversion unit is a multimode fiber with second fiber parameters. The core difference between multimode fiber and single-mode fiber is that multimode fiber has a larger core size and higher NA, and can transmit multiple guided wave modes simultaneously. It achieves mode excitation by fusion splicing with single-mode fiber and utilizing the parameter mismatch between the two. At the same time, it acts as a carrier for mode interference, and completes the initial shaping of the Gaussian beam into a ring light field by adjusting the phase difference between modes through its own length. It is the core link of mode control and interference in the entire system. The output unit is a ring-core fiber with third fiber parameters. Its structural feature is that the core is distributed in a ring shape, with a ring light guiding region and a central low refractive index region. Its core function is to receive the ring light field output by multimode fiber, and constrain the light field transmission through its own ring refractive index distribution to avoid the ring light field from spreading and deforming during transmission, ensuring the stability of the spatial distribution characteristics of the ring light field, and finally achieving the output of a high-quality ring light spot.

[0025] Based on this, the core diameters of single-mode fiber, multimode fiber, and ring-core fiber are increased sequentially, i.e., single-mode fiber core diameter < multimode fiber core diameter < ring-core fiber core diameter. This adapts to the transmission requirements of optical fields from concentrated Gaussian light to diffused multimode interference light, and then to ring-constrained ring light. It avoids excessive light field reflection and loss caused by abrupt changes in core size, ensuring efficient coupling and smooth transition of the optical field between units. The NA is the core parameter characterizing the light-gathering capability and mode constraint capability of the optical fiber. Its value is determined by the refractive index difference between the fiber core and cladding. The larger the NA, the larger the light-gathering angle of the fiber and the stronger the mode constraint capability. The NA increases progressively for single-mode fiber, multimode fiber, and ring-core fiber to adapt to the transmission characteristics of the optical field at different stages. That is, single-mode fiber needs strong constraint of the fundamental mode, so the NA is relatively small; multimode fiber needs to excite multiple higher-order modes, so the NA needs to be moderate; and ring-core fiber needs to stably constrain the ring optical field and avoid light field leakage, so a larger NA is required to ensure the optical transmission efficiency and spot stability of the entire system.

[0026] For example, such as Figure 1 A schematic diagram of a ring beam shaping system based on multimode fiber mode interference is provided. The input unit uses 10 / 200 single-mode fiber with a core diameter of 10 μm, an NA of 0.08, and a cladding diameter of 200 μm, for inputting a Gaussian beam. The mode conversion unit is a 30 / 250 multimode fiber of a specific length with a core diameter of 30 μm, an NA of 0.12, and a cladding diameter of 250 μm, which is connected to the single-mode fiber by fusion splicing for exciting and interfering higher-order modes. The multimode fiber core diameter, NA, and coupling length are optimally selected through calculation. The output unit is a ring-core fiber with a customized refractive index, which is connected to the mode conversion unit by fusion splicing to maintain and output the ring beam distribution.

[0027] The overall input and output during operation, such as Figure 6A schematic diagram of the input and output distribution of a ring spot shaping system based on multimode fiber mode interference is provided, wherein... Figure 6 (a) is a schematic diagram of the input distribution of a ring spot shaping system based on multimode fiber mode interference. Figure 6 (b) is a schematic diagram of the output distribution of a ring beam shaping system based on multimode fiber mode interference. It achieves the shaping of a Gaussian beam into a ring beam by fusing a specific length of 30 / 250 multimode fiber between a 10 / 200 single-mode fiber and a ring-core fiber with a customized refractive index. The ring beam is then maintained and output through the ring-core fiber, utilizing the mode interference effect in the multimode fiber.

[0028] Furthermore, after the Gaussian beam is input from the single-mode fiber, due to the mismatch between the fiber core size and the NA, multiple guided modes are excited in the multimode fiber. Through the multimode fiber of a preset length, each mode generates destructive interference at the output end, forming a ring-shaped light spot with zero central light intensity. The ring-shaped light spot is coupled into the ring-core fiber to maintain the ring-shaped refractive index distribution and transmit the ring-shaped light field.

[0029] In this context, core size and NA mismatch refers to the mismatch between the core diameter and NA of two adjacent fiber segments, specifically single-mode and multimode fibers. This means there is a significant difference in their core size and NA, causing the optical field coupling from the single-mode fiber to the multimode fiber to not fully adapt to the multimode fiber's waveguide conditions. This mismatch disrupts the single transmission state of the fundamental mode, causing some of the input fundamental mode light energy to couple into higher-order modes of the multimode fiber. This provides the preconditions for subsequent mode interference and is a key triggering mechanism for mode conversion. The degree of mismatch directly affects the type and proportion of higher-order modes being excited. Guided modes, also known as waveguided modes, are optical field modes that satisfy the fiber waveguide conditions—that is, light undergoes total internal reflection at the core-cladding interface and is phase-matched—and can be effectively constrained by the fiber core and stably propagated along the fiber axis. They are distinct from unconstrained cladding light propagating within the cladding. The type of guided mode is determined by the fiber's structural parameters, namely core diameter, NA, and refractive index distribution. Multimode fibers contain multiple guided modes, including the fundamental mode LP. 01 Module and second-order module LP 02 Different guided modes have different propagation constants and light intensity distributions, and are the core participants in mode interference.

[0030] Furthermore, destructive interference is a type of wave interference phenomenon. When two or more light waves with the same frequency and vibration direction are superimposed, if their phase difference is an odd multiple of π (i.e., opposite phases), the light field energies cancel each other out, forming a region of extremely low light intensity. Therefore, through destructive interference, the phase difference generated by the difference in propagation constants of different guided modes excited in a multimode fiber satisfies the destructive condition when transmitted to the output end, causing the light field energies in the central region of the spot to cancel each other out, forming a central dark region. This is the core physical process for achieving ring spot shaping, and the effect of destructive interference directly determines the ring spot shape. The depth of the dark area at the center of the spot and the clarity of the bright area at the edge; and a special light field distribution is obtained through destructive interference. Its core feature is that the light intensity in the central region of the spot approaches zero, and the light energy is concentrated in the ring area around the central dark area, forming a structure of dark center and bright ring, that is, a ring spot with zero central light intensity. This spot is not formed by simple light intensity attenuation, but is achieved by precise control through mode destructive interference. It has the characteristics of uniform light intensity distribution, high edge sharpness and good stability. It can meet the special requirements of laser processing, medical and other scenarios for the spot and avoid damage to the target by the strong central light.

[0031] Finally, the ring-core fiber, through its unique refractive index profile design—a high-refractive-index region in the ring core and low-refractive-index regions in the center and cladding—effectively confines the input ring-shaped optical field, preventing it from diffusing into the central region or leaking into the cladding. This ensures that the ring-shaped optical field maintains its spatial distribution characteristics of a dark center and a bright ring throughout transmission, achieving the maintenance of the ring-shaped refractive index distribution. This maintenance is not a passive constraint but rather a light trap effect formed by the refractive index difference, which stably confines the optical field within the ring core for transmission. This ensures that the shape and intensity distribution of the ring-shaped light spot do not deform, which is key to achieving stable output of the ring-shaped light spot.

[0032] Therefore, by controlling the excitation of higher-order modes through fiber core size and NA mismatch, the problem of chaotic mode excitation is avoided, ensuring that the types and proportions of modes participating in the interference meet the requirements, laying the foundation for the formation of a high-quality annular spot. Subsequently, by precisely controlling the destructive interference of modes through preset multimode fiber length, an annular spot with a central light intensity close to zero can be achieved. The central dark area of ​​the annular spot has sufficient depth, the edge bright area has high sharpness, and the light intensity distribution is uniform, fully meeting the special requirements of annular spots in laser processing, medical and other scenarios, and significantly improving the spot shaping quality. Finally, the annular refractive index distribution of the ring-core fiber is used to maintain the transmission of the annular light field, effectively avoiding the diffusion, deformation and energy leakage of the annular light field during transmission, ensuring that the spatial distribution characteristics of the annular spot remain stable, and maintaining good spot quality even after long-distance transmission. Furthermore, the entire optical field modulation process is completed inside the optical fiber, eliminating the need for additional spatial optical components. This further enhances the system's integration and stability, reduces interference from external environmental factors, minimizes optical transmission loss, and improves the system's laser transmittance. As a result, the system can be adapted to more complex real-world application scenarios, promoting the practical development of ring spot shaping technology.

[0033] Furthermore, using mode interference theory, the core diameter and NA in the second fiber parameters are determined, and based on the core diameter and NA in the second fiber parameters, the length in the second fiber parameters is determined.

[0034] Furthermore, the process of determining the core diameter and NA in the second fiber parameters using mode interference theory is specifically implemented as follows in this embodiment: Based on mode interference theory, by analyzing the core diameter, NA, and length of the multimode fiber, the influence of the power coupling coefficient, propagation constant difference, and laser transmittance in the fundamental and secondary modes of the multimode fiber is determined, thereby determining the core diameter and NA in the second fiber parameters.

[0035] Furthermore, the process of determining the length of the second optical fiber based on the core diameter and NA in the second optical fiber parameters is specifically implemented as follows in this embodiment: Based on the core diameter and NA in the second fiber parameters, the propagation constants corresponding to the fundamental mode and the secondary mode of the multimode fiber are calculated, and the interference phase de-beat length is determined based on the calculation results; based on the interference phase de-beat length, the length in the second fiber parameters is determined.

[0036] Mode interference theory, also known as mode interference theory, is one of the core theories in the field of fiber optics. It mainly studies the excitation, coupling, transmission, and interference superposition of different guided modes in multimode fibers, revealing the intrinsic relationship between parameters such as the propagation constant of the guided modes, mode field distribution, and power coupling, and fiber structural parameters such as core diameter, NA, and length. This theory aims to achieve the desired interference effect by controlling fiber parameters. The second fiber parameter specifically refers to the core structural parameters of the multimode fiber in the mode conversion unit. It is not a single parameter but a collection of multiple key parameters, including core diameter, NA, fiber length, and refractive index distribution. Core diameter, NA, and length are the core parameters affecting mode excitation and interference effects. Core diameter determines the types and number of guided modes that the multimode fiber can carry; NA determines the light-gathering capability and mode confinement capability of the multimode fiber; and length determines the phase difference and interference effect between guided modes. These three parameters are interrelated and mutually influential, jointly determining the mode control performance of the multimode fiber. The rationality of its parameter design directly determines the beam shaping quality of the entire system.

[0037] Furthermore, the fundamental mode is the lowest-order guided wave mode in a multimode fiber, denoted as LP. 01 The fundamental mode, with the largest propagation constant and the most concentrated mode field distribution (approximately Gaussian distribution), possesses the strongest confinement ability. It is the initial mode with the highest energy proportion after the optical field is coupled from a single-mode fiber to a multimode fiber. The mode field distribution and propagation constant of the fundamental mode are mainly determined by the core diameter and NA of the multimode fiber. Its power coupling and phase difference with higher-order modes are the core foundation for achieving mode interference. The energy coupling efficiency of the fundamental mode directly affects the excitation intensity and interference effect of higher-order modes. The second-order mode refers to the lower-order higher-order mode in the multimode fiber, denoted as LP. 02The second-order mode, with a propagation constant smaller than the fundamental mode and a more dispersed mode field distribution, is the core high-order mode participating in destructive interference. The excitation intensity and mode field distribution of the second-order mode are closely related to the core diameter and NA of the multimode fiber. Its power coupling coefficient and propagation constant difference with the fundamental mode directly determine the destructive interference effect, thus affecting the quality of the ring spot. It is a key participant in mode interference in this system. The power coupling coefficient characterizes the physical quantity of optical power transfer and distribution between different guided modes. Its value ranges from 0 to 1. The larger the coefficient, the higher the coupling efficiency of optical power between the two modes. In this embodiment, it mainly refers to the power coupling coefficient between the fundamental mode and the second-order mode. Its magnitude is determined by the core diameter and NA of the multimode fiber, directly affecting the proportion of energy transferred from the fundamental mode to the second-order mode, thus affecting the intensity ratio of the two modes, and ultimately determining the edge intensity and central dark area depth of the ring spot after destructive interference. The propagation constant difference Δβ refers to the difference between different guided modes. The example specifically refers to the difference in phase change per unit length between the fundamental mode and the secondary mode when they propagate in a multimode fiber. Its magnitude is determined by the characteristics of the guided mode itself and the fiber parameters. The propagation constant difference is a core prerequisite for mode interference. Different propagation constant differences cause the phase difference between guided modes to change with the transmission length, thus affecting the interference effect. Only when the propagation constant difference is within a reasonable range can ideal destructive interference be achieved by controlling the fiber length. Laser transmittance, also known as optical transmission efficiency, refers to the ratio of laser power at the system output to laser power at the input. It is a core indicator for measuring the optical transmission performance of the system; a higher value indicates lower optical loss. Laser transmittance is mainly affected by factors such as fiber parameter matching, fusion splice loss, and mode coupling efficiency. In this example, the core diameter and NA of the multimode fiber directly affect the coupling efficiency and optical transmission loss between the fundamental and secondary modes, thus affecting the laser transmittance of the entire system. It is one of the key indicators to consider in parameter design.

[0038] The propagation constant, denoted by β, is a physical quantity describing the phase change per unit length of a guided mode propagating in an optical fiber. Its magnitude is closely related to the order of the guided mode, the structural parameters of the optical fiber, and the wavelength of the incident light. The propagation constant is a core parameter characterizing the transmission characteristics of guided modes. Different guided modes have different propagation constants, and this difference is the fundamental reason for mode interference. That is, during transmission, guided modes will generate a phase difference based on the difference in propagation constants, leading to interference superposition. Accurate calculation of the propagation constant is the basis for controlling the interference effect. The destructive beat length, also known as the destructive interference period length, refers to the multimode light... The fiber contains two guided modes, specifically the fundamental mode and the secondary mode in this embodiment. These modes are transmitted from the same phase to the opposite phase, or from the opposite phase to the same phase, completing the alternation of one destructive interference and one constructive interference. The length of the destructive interference beat length is inversely proportional to the difference in propagation constants between the two guided modes. The larger the difference in propagation constants, the shorter the destructive interference beat length. Its size directly determines the minimum effective length of the multimode fiber and is the core basis for determining the length of the multimode fiber. Only when the length of the multimode fiber is an odd multiple of the destructive interference beat length can ideal destructive interference be achieved at the output end. Specifically, the destructive interference beat length L_π = π / |Δβ|. The length in the second fiber parameter specifically refers to the physical length of the multimode fiber and is one of the core parameters of the mode conversion unit. It is different from structural parameters such as fiber core diameter and NA. Its length directly determines the phase difference between the fundamental mode and the secondary mode transmitted in the multimode fiber, and thus determines the interference effect. This length is not arbitrarily set, but is taken as an integer multiple of the beat length (usually the first-order beat length) to achieve the optimal ring spot output.

[0039] For example, after determining the 10 / 200 single-mode fiber, based on mode interference theory, the effects of multimode fiber core diameter, NA, and length on LP were analyzed. 01 and LP 02 The effects of power coupling coefficient, propagation constant difference Δβ, and laser transmittance on the mode were investigated. It was determined that the optimal performance was achieved with a core diameter of 30 micrometers and NA of 0.12. Subsequently, the LP under this condition was calculated. 01 and LP 02 The propagation constant of the mode determines the interference phase depletion beat length, and the length L of the multimode fiber is taken as an integer multiple of the beat length.

[0040] Therefore, guided by mode interference theory, this study ensures that the designed fiber core diameter and NA can excite suitable higher-order modes, guaranteeing power coupling efficiency and avoiding mode chaos. Furthermore, the designed parameters enable efficient coupling between the fundamental and secondary modes, allowing for the rational transfer of fundamental mode energy to the secondary mode. This provides a sufficiently strong interference field for destructive mode interference, avoiding insufficient mode excitation or mode chaos, and providing parameter guarantees for the formation of a high-quality ring spot. The multimode fiber length is determined based on the core diameter and NA, ensuring that ideal destructive interference can be achieved by controlling the length, forming a ring spot with zero intensity at the center and uniform intensity at the edges. This precise matching of the three parameters ensures that the phase difference between the guided modes precisely meets the destructive interference condition, improving the stability and reliability of the spot shaping, and significantly enhancing the spot quality and consistency. Through theoretical design, the parameters of multimode fiber are well matched with those of single-mode fiber and ring-core fiber, effectively reducing coupling loss between units and improving the overall laser transmittance of the system. This also shortens the parameter design cycle, reduces design costs and R&D difficulty, and facilitates mass production and widespread application. Furthermore, by determining the destructive beat length of the interference based on the propagation constant difference, a fundamental reference value for the multimode fiber length is established. This value can be optimized according to actual application requirements, ensuring that the multimode fiber length achieves ideal destructive interference. This results in sufficient depth of the central dark area and uniform edge intensity distribution in the output ring spot, significantly improving spot quality. The system can adapt to different application requirements; by adjusting the multimode fiber parameters, different specifications of ring spots can be output, expanding the system's application range and enhancing its practicality and flexibility. Furthermore, in the third fiber parameter of the ring-core fiber, the outer ring diameter is set to the core diameter of the multimode fiber, the inner ring diameter is set to the core diameter of the single-mode fiber, the inner ring and cladding are made of pure silica material, and the outer ring is doped with high-refractive-index material. The NA value in the third fiber parameter is determined by the laser transmittance of the overall fiber shaping structure and the uniformity of the ring spot.

[0041] The outer ring diameter of the ring-core fiber refers to the outer edge diameter of the ring core, i.e., the high-refractive-index light-guiding region. It is one of the core structural parameters of the ring-core fiber, and its size directly determines the outer diameter of the ring optical field. It also affects the confinement effect and coupling efficiency of the ring optical field. The design of the outer ring diameter must match the core diameter of the multimode fiber to ensure that the ring optical field output from the multimode fiber can be efficiently coupled into the ring core of the ring-core fiber, avoiding optical field leakage or excessive coupling loss. It is a fundamental parameter for achieving stable transmission of the ring optical field. The inner ring diameter of the ring-core fiber refers to the diameter of the low-refractive-index region in the center of the ring-core fiber, i.e., the inner edge diameter of the ring core. Its size determines the inner diameter of the ring optical field and the size of the central dark area. The design of the inner ring diameter must match the core diameter of the single-mode fiber. On the one hand, it can avoid optical field interference in the central region and ensure the stability of the central dark area of ​​the ring optical field. On the other hand, it can adapt to the distribution characteristics of the ring optical field, improve the optical field confinement effect, and prevent the ring optical field from spreading towards the center.

[0042] Furthermore, since pure silica is a high-purity, low-loss, and low-refractive-index optical material with a refractive index of approximately 1.458, the inner ring and cladding of a ring-core fiber are made of pure silica primarily to form a low-refractive-index region. This creates a refractive index difference with the high-refractive-index material of the outer ring, thereby confining the ring-shaped optical field through total internal reflection. Simultaneously, the low-loss characteristic of pure silica reduces energy loss during optical transmission, improving the system's laser transmittance. High-refractive-index doping refers to the doping of high-valence elements such as germanium and phosphorus into the outer ring region of the ring-core fiber, altering the refractive index of the fiber material. This makes the refractive index of the outer ring higher than that of the pure silica material in the inner ring and cladding, forming a ring-shaped high-refractive-index light-guiding region. This doping process is called ring-core fiber doping. The core fiber is the core of the technology for confining the ring-shaped optical field. It uses the refractive index difference to create an optical trap effect, which stably confines the ring-shaped optical field within the core for transmission, preventing optical field leakage and ensuring the stability of the shape and intensity distribution of the ring-shaped optical field. The uniformity of the ring spot refers to the smoothness of the intensity distribution in the bright area at the edge of the ring spot, that is, the magnitude of the intensity difference between points in the ring area. The smaller the difference, the better the uniformity. The uniformity of the ring spot is one of the core indicators for measuring the quality of the spot and directly affects the effect of laser processing, medical and other applications. For example, in laser cutting, poor uniformity will lead to inconsistent cut widths. In medical treatment, poor uniformity will lead to uneven energy distribution in the treatment area, affecting the treatment effect. Its uniformity is mainly determined by the parameter design of the ring-shaped fiber.

[0043] Specifically, such as Figure 7 A schematic diagram of the refractive index distribution of the ring core fiber in a ring spot shaping system based on multimode fiber mode interference is provided. After optimization by simulation design software, the NA in the third fiber parameter of the ring core fiber is designed to be 0.22.

[0044] Furthermore, during the splicing process, core alignment and cladding alignment techniques are employed to ensure minimal splice loss and controllable mode excitation. The first splice point between the multimode fiber and the single-mode fiber is core aligned, and the second splice point between the multimode fiber and the ring-core fiber is cladding aligned.

[0045] Core alignment is a precise alignment method in the fiber optic fusion splicing process. It involves using the fiber core as a reference and precise adjustment equipment to ensure the centers of the two fiber cores are completely aligned when splicing two fiber segments. This ensures efficient coupling of the fiber core light and reduces optical loss at the splice. Core alignment is mainly suitable for fiber connections with small core sizes and where the core is primarily used for light guiding. In this embodiment, it specifically refers to the connection between single-mode and multimode fibers. Its alignment accuracy directly affects splicing loss and the controllability of mode excitation. Excessive alignment deviation can lead to core light reflection and scattering, increasing loss and causing mode excitation chaos. Cladding alignment is another precise alignment method in the fiber optic fusion splicing process. It involves using the cladding of the fiber as a reference to ensure the centers of the cladding of the two fiber segments are completely aligned when splicing two fiber segments. Cladding alignment is mainly suitable for fiber connections with larger core sizes or special core structures. Its advantage lies in its ability to adapt to the alignment requirements of ring cores, avoiding splicing deviations caused by the difficulty of aligning ring cores and ensuring efficient coupling of the optical field.

[0046] Furthermore, splice loss refers to the transmission loss of laser at the fiber splice interface. It is mainly caused by factors such as core alignment deviation, fiber parameter mismatch, and improper splicing temperature. The smaller the value, the better the splice quality and the higher the optical transmission efficiency. Splice loss is an important factor affecting the overall laser transmittance of the system, especially in all-fiber systems, where the superposition of losses from multiple splices will significantly reduce system performance. Controllable mode excitation refers to the ability to excite specific types and proportions of guided modes as expected when the optical field is coupled from one fiber to another during the fiber splicing process by controlling the alignment method and splicing parameters, thus avoiding mode excitation chaos. Controllable mode excitation is the key to achieving high-quality beam shaping, because the type and proportion of mode excitation directly affect the effect of subsequent mode interference. Only by achieving controllable mode excitation can the stability of destructive interference and the quality of the ring beam be ensured.

[0047] Following the previous example, core alignment and cladding alignment techniques are used to ensure minimal splice loss and controllable mode excitation. The first splice, 10 / 200→30 / 250, needs to be precisely aligned to excite higher-order modes; the second splice, 30 / 250→ring core fiber, needs cladding alignment to match the ring optical field.

[0048] In practical applications, numerical simulations, such as the BPM beam propagation method, are used to simulate the transmission of light in optical fibers to determine the optimal fiber coupling length. Subsequently, an infrared camera or beam profiler is used to monitor the output spot shape in real time. The length of the 30 / 250 multimode fiber is gradually fine-tuned in the experiment until the optimal ring spot is observed, thus correcting the error between practice and theory and ensuring the shaping effect.

[0049] Furthermore, after the fusion splicing is completed, the surface of the ring-core fiber is roughened to remove the cladding light energy introduced by the coupling.

[0050] Surface roughening treatment refers to the micro-processing of the surface of the ring-core fiber after fusion splicing using physical or chemical methods. This creates tiny rough structures on the fiber surface, such as micro-scratches or uneven surfaces, which disrupt the transmission conditions of cladding light. Commonly used methods include mechanical polishing, chemical etching, and laser etching. The treatment is mainly concentrated near the output end of the ring-core fiber, with the core purpose of stripping cladding light and improving the purity of the output light spot. The processing precision must be controlled within a range that does not affect the light guiding of the ring core. Cladding light energy refers to stray light energy that is not effectively constrained by the fiber core during fiber transmission and enters the cladding for transmission. The main causes of its generation include fiber splicing alignment errors, excessive mode excitation, and fiber parameter mismatch. Cladding light is not part of the effective transmission light field and cannot participate in the formation of the ring light spot. Instead, it interferes with the main light field, reduces the purity and contrast of the light spot, and causes the fiber to heat up, increasing optical transmission loss.

[0051] Following the previous example, after the fusion splicing is completed, the surface of the ring-core fiber is roughened using laser etching or stripping grease to remove the cladding light energy introduced by coupling, thereby improving beam quality and performance stability. Then, the bare fiber is cleaned with hydrofluoric acid for a preset time, such as 30 seconds. After rinsing it clean with deionized water, the all-fiber ring spot shaping device is suspended, straightened, centered, and fixed in the glass tube using UV-cured adhesive.

[0052] In summary, by introducing 30 / 250 fiber as an intermediate transition section and precisely controlling its length, flexible control of the beam shaping process is achieved. Since the core diameter and numerical aperture of 30 / 250 fiber are between those of 10 / 200 fiber and ring-core fiber, its length affects the evolution of the transmission modes within it. By changing the length of the 30 / 250 fiber, the mode coupling and energy distribution changes of the Gaussian beam during transmission can be adjusted, thereby achieving precise control over parameters such as the inner and outer diameters, width, and uniformity of the final annular spot. This solves the problem of existing schemes based on a single special fiber, such as the off-core fusion coupling scheme, which can only rely on the parameters of the ring-core fiber for shaping and cannot be dynamically adjusted. For example, when it is necessary to increase the width of the annular spot, the length of the 30 / 250 fiber can be appropriately increased to allow for more sufficient mode expansion of the beam; when it is necessary to improve the uniformity of the spot, the length of the 30 / 250 fiber can be optimized to reduce mode crosstalk and ensure uniform energy distribution without replacing the ring-core fiber, making the operation flexible and cost-effective.

[0053] Furthermore, the parameter relationship between 10 / 200 fiber and 30 / 250 fiber, and between 30 / 250 fiber and ring-core fiber, shows a gradual increase in core diameter and numerical aperture (NA). Through a reasonable fusion splicing process, the coupling loss between different fiber segments can be significantly reduced. This solves the problem of high coupling loss in existing single-specialty fiber solutions where Gaussian beams are directly coupled from ordinary fiber into ring-core fiber due to significant differences in structure and parameters, making offset control difficult. For example, the core diameter of 10 / 200 fiber (10 micrometers) is relatively similar to that of 30 / 250 fiber (30 micrometers), resulting in less energy loss when the beam couples from 10 / 200 fiber into 30 / 250 fiber. The numerical aperture (NA) of 30 / 250 fiber (0.12) matches the NA of ring-core fiber well, allowing for perfect coupling at specific lengths and improving the system's energy utilization rate (>98%).

[0054] Finally, the 30 / 250 fiber and the ring-core fiber work synergistically. The 30 / 250 fiber first performs preliminary mode screening and adjustment on the Gaussian beam, reducing unnecessary higher-order modes and allowing the beam to enter the ring-core fiber in a more optimal mode. The ring-core fiber then further constrains the beam mode through its customized refractive index distribution, suppressing mode crosstalk. This two-stage mode control mechanism effectively improves the control capability of the beam mode, reduces stray light generation, improves the purity of the ring spot, ensures the stable and reliable quality of the output ring spot, and solves the problem of limited mode constraint capability and easy mode crosstalk of existing single ring-core fibers.

[0055] The entire system is achieved through the fusion splicing of only three fiber segments, eliminating the need for complex optical components or special fiber lasers, thus significantly reducing equipment manufacturing and maintenance costs. By adjusting the length of the 30 / 250 multimode fiber, the size and shape of the ring spot can be flexibly adjusted, resulting in high uniformity of the ring spot, high central extinction ratio, and mode conversion efficiency >98%. The all-fiber structure avoids interface reflection and aberration problems, has high energy utilization, is insensitive to environmental factors, and significantly improves stability, making it suitable for high-power laser transmission, precision machining, and optical manipulation.

[0056] The foregoing has described specific embodiments of this application. Other embodiments are within the scope of the appended claims. In some cases, the actions or steps recited in the claims may be performed in a different order than that shown in the embodiments and may still achieve the desired results. Furthermore, the processes depicted in the drawings do not necessarily require the specific or sequential order shown to achieve the desired results. In some embodiments, multitasking and parallel processing are also possible or may be advantageous.

[0057] It should be noted that, for the sake of simplicity, the foregoing method embodiments are all described as a series of actions. However, those skilled in the art should understand that this application is not limited to the described order of actions, as some steps may be performed in other orders or simultaneously according to this application. Furthermore, those skilled in the art should also understand that the embodiments described in the specification are preferred embodiments, and the actions and modules involved are not necessarily essential to this application.

[0058] In the above embodiments, the descriptions of each embodiment have different focuses. For parts not described in detail in a certain embodiment, please refer to the relevant descriptions of other embodiments.

[0059] The preferred embodiments disclosed above are merely illustrative of this application. The optional embodiments do not exhaustively describe all details, nor do they limit the invention to the specific implementations described. Clearly, many modifications and variations can be made based on the content of this application. These embodiments are selected and specifically described in this application to better explain the principles and practical applications of this application, thereby enabling those skilled in the art to better understand and utilize this application. This application is limited only by the claims and their full scope and equivalents.

Claims

1. A ring spot shaping system based on multimode fiber mode interference, characterized in that, It includes an input unit, a mode conversion unit, and an output unit, wherein, The input unit is a single-mode fiber with the first fiber parameter, used to input a Gaussian beam; The mode conversion unit is a multimode fiber with second fiber parameters, which is connected to the single-mode fiber by fusion splicing and is used to excite and interfere with higher-order modes. The output unit is a loop-core fiber with the third fiber parameter, which is connected to the mode conversion unit by fusion splicing to maintain and output a ring-shaped optical field distribution. The core diameters of the single-mode fiber, the multimode fiber, and the loop-core fiber increase progressively, and the NA of the single-mode fiber, the multimode fiber, and the loop-core fiber also increase progressively.

2. The ring spot shaping system based on multimode fiber mode interference according to claim 1, characterized in that, After the Gaussian beam is input into the single-mode fiber, due to the mismatch between the fiber core size and the NA, multiple guided modes are excited in the multimode fiber. Through the multimode fiber of a preset length, each mode generates destructive interference at the output end, forming a ring-shaped light spot with zero central light intensity. The annular light spot is coupled into the annular core fiber to maintain the annular refractive index distribution and transmit the annular light field.

3. The ring spot shaping system based on multimode fiber mode interference according to claim 1, characterized in that, Using mode interference theory, the core diameter and NA in the second fiber parameters are determined, and the length in the second fiber parameters is determined based on the core diameter and NA.

4. The ring spot shaping system based on multimode fiber mode interference according to claim 3, characterized in that, The determination of the core diameter and NA in the second fiber parameters using mode interference theory includes: Based on mode interference theory, by analyzing the core diameter, NA, and length of the multimode fiber, the influence of the power coupling coefficient, propagation constant difference, and laser transmittance in the fundamental and secondary modes of the multimode fiber is determined, thereby determining the core diameter and NA in the second fiber parameters.

5. The ring spot shaping system based on multimode fiber mode interference according to claim 3, characterized in that, The step of determining the length of the second optical fiber based on the core diameter and NA in the second optical fiber parameters includes: Based on the core diameter and NA in the second fiber parameters, the propagation constants corresponding to the fundamental mode and the second mode of the multimode fiber are calculated, and the interference phase depletion beat length is determined based on the calculation results. The length in the second fiber parameter is determined based on the interference phase de-beat length.

6. The ring spot shaping system based on multimode fiber mode interference according to claim 1, characterized in that, In the third fiber parameter of the ring-core fiber, the outer ring diameter is set to the core diameter of the multimode fiber, the inner ring diameter is set to the core diameter of the single-mode fiber, the inner ring and cladding are made of pure silica material, and the outer ring is doped with high-refractive-index material. The NA value in the third fiber parameter is determined by the laser transmittance of the overall fiber shaping structure and the uniformity of the ring spot.

7. The ring spot shaping system based on multimode fiber mode interference according to claim 1, characterized in that, During the splicing process, core alignment and cladding alignment techniques are employed to ensure minimal splice loss and controllable mode excitation. The first splice between the multimode fiber and the single-mode fiber is core aligned, and the second splice between the multimode fiber and the ring-core fiber is cladding aligned.

8. The ring spot shaping system based on multimode fiber mode interference according to claim 1, characterized in that, After fusion splicing, the surface of the ring-core fiber is roughened to remove the cladding light energy introduced by coupling.

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