Long period grating assisted pump self-equalized side-pumped fiber and laser

Abstract

The application discloses a long-period grating assisted pump self-balancing side-pumped optical fiber and a laser, and relates to the field of fiber lasers.The long-period grating assisted pump self-balancing side-pumped optical fiber comprises an active signal optical fiber, a pump optical fiber, a temperature response low-refractive coupling layer and a long-period grating assisted coupling area.The pump optical fiber is arranged in parallel to the side of the active signal optical fiber and forms a distributed side coupling.The refractive index of the temperature response low-refractive coupling layer is lower than the refractive index of the adjacent pump transmission area and the cladding of the active signal optical fiber and has reversible thermoelasticity.The long-period grating assisted coupling area is arranged in the low absolute pump area or the residual pump area of the pump optical fiber along the pump direction.The side-pumped optical fiber can reduce the local thermal load peak value of the high pump power area, control the residual pump, realize pump absorption self-balancing, improve the thermal stability and the mode instability threshold.

Description

Technical Field

[0001] This application relates to the field of fiber laser technology, and in particular to a long-period grating-assisted pumped self-equalized side-pumped fiber and laser. Background Technology

[0002] Distributed side-pumped fibers enable the pump light to be gradually coupled laterally into the signal fiber cladding along the fiber length, providing an important technical path for high-power single-fiber laser output. As output power enters the 10 kW and even 20 kW levels and above, the contradiction between axial thermal load distribution and residual pump control under high-power operation becomes increasingly prominent: the high pump power region needs to reduce local coupling strength to suppress over-absorption and thermal peaks, while the low absolute pump region needs to maintain sufficient coupling capacity to avoid residual pump rise. Existing technologies struggle to meet both requirements simultaneously. Summary of the Invention

[0003] The purpose of this application is to provide a long-period grating-assisted pumped self-equalized side-pumped fiber and laser, which can solve the problem that existing distributed side-pumped fibers are difficult to control residual pumping while reducing the local thermal load peak in the high pump power region.

[0004] To achieve the above objectives, this application provides the following solution: In a first aspect, this application provides a long-period grating-assisted self-equalizing side-pumping fiber, the long-period grating-assisted self-equalizing side-pumping fiber comprising: An active signal fiber, wherein the core of the active signal fiber is doped with gain ions; A pump fiber is disposed parallel to the side of the active signal fiber, and the pump fiber and the active signal fiber form a distributed side coupling. A temperature-responsive low-refractive-index coupling layer is disposed between the active signal fiber and the pump fiber. The refractive index of the temperature-responsive low-refractive-index coupling layer is lower than that of the adjacent pump transmission region and the cladding of the active signal fiber. The temperature-responsive low-refractive-index coupling layer has reversible thermoelasticity. The temperature-responsive low-refractive-index coupling layer is used to deform due to temperature rise in the high pump power region to reduce the local natural lateral coupling strength. A long-period grating-assisted coupling region is provided in the pump fiber along the pump direction in the low absolute pump region or residual pump region. The long-period grating-assisted coupling region is used to perform mode perturbation on the residual multimode pump light. The temperature-responsive low-refractive-index coupling layer and the long-period grating-assisted coupling region form an axial division of labor along the pumping direction; the high pump power region and the low absolute pump region are axial functional regions divided according to local pump power and local temperature rise state.

[0005] Optionally, the temperature-responsive low-refractive-index coupling layer has an equivalent temperature response coefficient, an equivalent thermal expansion coefficient, an effective deformation thickness, and an initial effective coupling gap; the equivalent temperature response coefficient is determined based on the target operating power, axial thermal load distribution, and thermal stability requirements; the equivalent thermal expansion coefficient and the effective deformation thickness are determined based on the equivalent temperature response coefficient; the value of the initial effective coupling gap is located within the sensitive range of natural evanescent coupling to spacing changes, and within the sensitive range, a small change in the initial effective coupling gap can cause a significant change in the local natural lateral coupling strength.

[0006] Optionally, the temperature-responsive low-refractive-index coupling layer is continuously disposed along the axial direction of the pump fiber, or is disposed in segments along the axial direction of the pump fiber.

[0007] Optionally, the long-period grating auxiliary coupling region can be any one of a weakly modulated long-period grating structure, a chirped long-period grating structure, an apodized long-period grating structure, a quasi-period long-period grating structure, a segmented long-period grating structure, or multiple discrete long-period modulation segment structures; the equivalent auxiliary coupling strength of the long-period grating auxiliary coupling region is 0.04m. -1 up to 0.12m -1 .

[0008] Optionally, the long-period grating-assisted coupling region is formed in the pump fiber by any one of the following methods: femtosecond laser writing, ultraviolet exposure, carbon dioxide laser heating, periodic microbending, periodic pressure modulation, periodic geometric perturbation, or periodic material modulation.

[0009] Optionally, it also includes a limiting structure disposed around the active signal fiber and the pump fiber, the limiting structure being used to maintain the relative position and coupling gap between the active signal fiber and the pump fiber.

[0010] Optionally, it further includes an encapsulation structure and a heat dissipation structure; the encapsulation structure covers the outside of the active signal fiber and the pump fiber; the heat dissipation structure is disposed on the outer surface or inside of the encapsulation structure.

[0011] Optionally, the pump fiber is a single pump fiber, or multiple pump fibers symmetrically distributed circumferentially along the active signal fiber, or multiple pump fibers asymmetrically distributed circumferentially along the active signal fiber; the long-period grating-assisted self-balancing side pump fiber is a single-plus-one structure or an N-plus-one structure, wherein the single-plus-one structure refers to a side pump fiber structure composed of one active signal fiber and one pump fiber, and the N-plus-one structure refers to a side pump fiber structure composed of one active signal fiber and N pump fibers, where N is an integer greater than or equal to 2.

[0012] Secondly, this application provides a fiber laser, including a long-period grating-assisted self-equalizing side-pumping fiber as described in the first aspect above. The long-period grating-assisted self-equalizing side-pumping fiber is disposed as an active gain segment in the resonant cavity, constituting a fiber oscillator or fiber amplifier.

[0013] Optionally, the fiber laser further includes a pump source, a high-reflectivity fiber grating, a low-reflectivity output fiber grating, an isolator, an end cap, a mode filter, a pump stripping structure, and a cooling encapsulation structure; the pump source is connected to the input end of the pump fiber; for forward pumping, reverse pumping, or bidirectional pumping structures, the long-period grating auxiliary coupling region is located in a low absolute pumping region far from any high pumping input side, or in a region where residual pumping still exists after thermal feedback coupling.

[0014] According to the specific embodiments provided in this application, the following technical effects are disclosed: This application provides a long-period grating-assisted pump self-equalizing side-pumped fiber and laser. A temperature-responsive low-refractive-index coupling layer is placed between the active signal fiber and the pump fiber. This low-refractive-index coupling layer has a reversible thermoelasticity, lower than the refractive index of the adjacent pump transmission region and the active signal fiber cladding. In the high pump power region, it undergoes thermoelastic deformation due to local temperature rise, reducing the local natural lateral coupling strength. This suppresses excessive concentrated coupling and absorption of pump light, effectively reducing local linear thermal load peaks and temperature peaks, and weakening thermal lensing effects and thermally induced refractive index disturbances. By setting the long-period grating-assisted coupling region in the low absolute pump region or the residual pump region, mode perturbation and redistribution of the residual multimode pump light are performed, increasing the lateral coupling probability and enabling the residual pump to achieve bottom-line absorption, avoiding residual pump rise caused by decoupling in the high-temperature region. By defining the high pump power region and the low absolute pump region as axial functional regions divided according to local pump power and local temperature rise states, the criteria for determining the two regions are clarified, providing a basis for axial control of pump coupling distribution. By configuring the temperature-responsive low-refractive-index coupling layer as a continuous or segmented structure, the layer can be designed differently based on the local thermal load distribution, improving the flexibility and specificity of thermal feedback coupling. The temperature-responsive low-refractive-index coupling layer and the long-period grating-assisted coupling region form an axial functional division along the pump direction, working together to achieve pump absorption self-equilibrium and smoothing out the heat deposition distribution. Therefore, this application solves both the thermal peak suppression problem in the high pump power region and the residual pump control problem in the low absolute pump region without relying on external active control, significantly improving the thermal stability, mode instability threshold, and long-term reliability of distributed side-pumped fibers. Attached Figure Description

[0015] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the 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.

[0016] Figure 1 A schematic diagram of the overall structure of a long-period grating-assisted pump self-equalizing side-pumping fiber provided in an embodiment of this application; Figure 2 A schematic diagram of the thermal feedback peak clipping mechanism of a long-period grating-assisted pump self-equalizing side-pumped fiber provided in an embodiment of this application, wherein (a) is a schematic diagram of the gap in the cold state and (b) is a schematic diagram of the gap in the hot state. Figure 3A schematic diagram showing the relationship between the effective coupling gap variation and the natural lateral coupling coefficient of a long-period grating-assisted pumped self-equilibrium side-pumped fiber, provided as an embodiment of this application. Figure 4 A schematic diagram of the long-period grating catch-up mechanism of a long-period grating-assisted pump self-equalizing side-pumped fiber provided in an embodiment of this application; Figure 5 A schematic diagram of the joint parameter design process for a long-period grating-assisted pumped self-equalized side-pumped fiber provided in an embodiment of this application; Figure 6 A schematic diagram of the axial coupling coefficient distribution of a long-period grating-assisted pump self-equalizing side-pumping fiber provided in an embodiment of this application. Figure 7 A schematic diagram of the pump power distribution of a long-period grating-assisted pump self-equalizing side-pumped fiber provided in an embodiment of this application; Figure 8 A comparative schematic diagram of the quantum defect thermal load distribution of a long-period grating-assisted pumped self-equilibrium side-pumped fiber provided in an embodiment of this application; Figure 9 A comparative schematic diagram of temperature distribution of a long-period grating-assisted pumped self-equilibrium side-pumped fiber provided in an embodiment of this application; In the picture: 1. Active signal fiber; 2. Pump fiber; 3. Temperature-responsive low-refractive-index coupling layer; 4. Long-period grating auxiliary coupling region. Detailed Implementation

[0017] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0018] To make the above-mentioned objectives, features and advantages of this application more apparent and understandable, the application will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0019] In one exemplary embodiment, such as Figure 1 As shown, a long-period grating-assisted self-equalizing side-pumping fiber is provided. The long-period grating-assisted self-equalizing side-pumping fiber includes an active signal fiber 1, a pump fiber 2, a temperature-responsive low-refractive-index coupling layer 3, and a long-period grating (LPG)-assisted coupling region 4. Wherein: The core of the active signal fiber 1 is doped with gain ions. In this embodiment, the gain ions are ytterbium ions. The active signal fiber 1 is used to transmit and amplify signal light.

[0020] Pump fiber 2 is arranged parallel to the side of active signal fiber 1, and pump fiber 2 and active signal fiber 1 form distributed side coupling, so that pump light gradually enters the cladding of active signal fiber 1 along the axial direction. Distributed side coupling means that pump light is gradually and dispersedly coupled into the cladding of active signal fiber 1 along the fiber length direction, which is different from the traditional end-face concentrated injection method.

[0021] A temperature-responsive low-refractive-index coupling layer 3 is disposed between the active signal fiber 1 and the pump fiber 2. The refractive index of the temperature-responsive low-refractive-index coupling layer 3 is lower than that of the adjacent pump transmission region and the cladding of the active signal fiber 1, specifically lower than the refractive index of quartz by approximately 1.45, to maintain the necessary lateral pump optical boundary. The refractive index of the temperature-responsive low-refractive-index coupling layer 3 is preferably less than or equal to 1.373, a typical industrial reference value for low-refractive-index polymer materials, achieving both effective optical confinement and good engineering feasibility. The temperature-responsive low-refractive-index coupling layer 3 exhibits reversible thermoelasticity, allowing it to deform in high-pump-power regions due to localized temperature rise, thereby reducing the local natural lateral coupling strength. Through this mechanism, the temperature-responsive low-refractive-index coupling layer 3 transforms from a common low-refractive-index cladding material into a functional layer participating in the regulation of pump coupling distribution.

[0022] The long-period grating-assisted coupling region 4 is set in the low absolute pump region or residual pump region along the pump direction in the pump fiber 2. The long-period grating-assisted coupling region 4 is used to perform mode perturbation on the residual multimode pump light.

[0023] The temperature-responsive low-refractive-index coupling layer 3 and the long-period grating-assisted coupling region 4 form an axial division of labor along the pumping direction: the temperature-responsive low-refractive-index coupling layer 3 reduces the local natural lateral coupling strength in the high pump power region, while the long-period grating-assisted coupling region 4 increases the lateral coupling probability in the low absolute pump region. In this embodiment, the high pump power region and the low absolute pump region are axial functional regions divided according to the local pump power and local temperature rise state, and do not necessarily correspond to the fixed fiber endface position.

[0024] In this implementation, the thermally induced deformation of the temperature-responsive low-refractive-index coupling layer 3 reduces the local coupling strength in the high-pump-power region, effectively suppressing over-absorption and thermal peaks. Simultaneously, the long-period grating-assisted coupling region 4 redistributes residual pumping modes, ensuring bottom-line absorption in the low-absolute-pump region. Together, these two mechanisms achieve pump absorption self-equilibrium, significantly improving heat load distribution and residual pump control. By defining the high-pump-power region and the low-absolute-pump region as axial functional regions based on local pump power and local temperature rise, the criteria for determining these two regions are clarified, providing a foundation for axial control of pump coupling distribution.

[0025] As an optional implementation, the temperature-responsive low-refractive-index coupling layer 3 has an equivalent temperature response coefficient. Equivalent thermal expansion coefficient Effective deformation thickness and initial effective coupling gap The equivalent temperature response coefficient The equivalent thermal expansion coefficient is determined based on the target operating power, axial heat load distribution, and thermal stability requirements. and the effective deformation thickness According to the equivalent temperature response coefficient Confirmed. In this embodiment of the application, Depend on and Joint decision, By properly matching these parameters, the effective gap change under operating temperature rise is made to fall within the sensitive range of natural evanescent coupling (the energy transfer that occurs naturally when two optical fibers are close together, caused by evanescent waves, and the coupling strength mainly depends on the distance between the two optical fibers, the refractive index distribution, and the wavelength of the light) to the gap change, thus achieving controllable decoupling.

[0026] The initial effective coupling gap The value of is located within the sensitive range of the natural ephemeris coupling distance change, and within this sensitive range, the initial effective coupling gap is... Even minute changes can cause significant changes in the local natural lateral coupling strength. The sensitive region is not limited to a fixed geometric size, but is determined by the fiber refractive index, pump mode distribution, temperature response, refractive index of the low-refractive-index coupling layer 3, fiber-to-fiber contact state, and structural boundaries.

[0027] In this embodiment, the temperature-responsive low-refractive-index coupling layer 3 preferably meets the following design requirements: the refractive index of the temperature-responsive low-refractive-index coupling layer 3 is lower than that of the cladding of the adjacent active signal fiber 1 and the pump fiber 2, i.e., lower than the refractive index of quartz by about 1.45, to maintain the side pump optical boundary; the refractive index of the temperature-responsive low-refractive-index coupling layer 3 is preferably less than or equal to 1.373, which is a typical industrial reference value for low-refractive-index polymer materials, reflecting the current material level and being compatible with future materials with even lower refractive indices; the temperature-responsive low-refractive-index coupling layer 3 has low light absorption and reversible thermoelastic response, avoiding the introduction of additional heat sources in the operating band; the equivalent thermal expansion coefficient of the temperature-responsive low-refractive-index coupling layer 3 is... Effective deformation thickness and initial effective coupling gap Through parametric design, the temperature-responsive low-refractive-index coupling layer 3 is made to produce an effective gap change of a predetermined magnitude within the operating temperature rise range; the initial effective coupling gap By positioning the temperature-responsive low-refractive-index coupling layer 3 within the sensitive range of spacing changes in the natural evanescent coupling, thermally induced deformation can be transformed into controllable coupling modulation. Through this design, the temperature-responsive low-refractive-index coupling layer 3 passively reduces the natural lateral coupling strength in the high pump power and high temperature regions, suppressing local overcoupling and overabsorption. In regions with lower temperature rise, the decoupling effect of the temperature-responsive low-refractive-index coupling layer 3 is weakened, and the pump light can still be transferred to the cladding of the active signal fiber 1 according to the preset coupling strength. This mechanism transforms the temperature-responsive low-refractive-index coupling layer 3 from a common low-refractive-index cladding material into a functional layer that participates in the regulation of pump coupling distribution.

[0028] As an optional implementation method, the effective coupling gap The relationship between temperature and the change is as follows: ; in, The local temperature is Effective coupling gap at time, The initial effective coupling gap, The equivalent temperature response coefficient is... This is due to a localized temperature rise.

[0029] The relationship between the local natural lateral coupling coefficient and temperature is as follows: ; in, The local temperature is The local natural lateral coupling coefficient at that time, The initial natural lateral coupling coefficient, This represents the change in effective gap caused by temperature changes. The equivalent penetration depth of the evanescent field.

[0030] like Figure 2 As shown in (a) and (b), in the cold or low-temperature rising state, a narrow effective coupling gap is maintained between the active signal fiber 1 and the side pump fiber 2, resulting in strong natural evanescent coupling. The pump light can transfer to the cladding of the active signal fiber 1 according to a preset coupling strength. When the local pump power and thermal load increase (hot state), the temperature-responsive low-refractive-index coupling layer 3 undergoes reversible thermoelastic deformation, increasing the effective coupling gap between the fibers and thus reducing the natural lateral coupling strength in this region. This process forms a local negative feedback limiting coupling mechanism to suppress overcoupling and overabsorption in the high-pump region. Figure 3 As shown, when the initial effective coupling gap Within the sensitive region, even submicron-level changes in the effective gap can significantly reduce the coupling strength, regardless of the evanescent field's equivalent penetration depth. The optimal value for this parameter, along with the significant attenuation relationship between the effective gap change caused by the low-refractive-index coupling layer 3 and the natural lateral coupling coefficient, provides a basis for designing... , and This provides a theoretical basis for achieving the predetermined thermal feedback confinement through matching. The design goal of the temperature-responsive low-refractive-index coupling layer 3 is to enable... , and Mutual matching, so that the working temperature rise is Falling into the sensitive modulation range of natural lateral coupling, the local temperature rise is effectively converted into a controllable decrease in the natural lateral coupling strength, which provides a quantitative basis for parameter design.

[0031] This implementation method, by establishing a quantitative formula, directly links material parameters with thermal feedback effects, making the design calculable and optimizable, and improving the feasibility of engineering implementation.

[0032] As an optional implementation, the temperature-responsive low-refractive-index coupling layer 3 can be formed using a low-refractive-index polymer, a fluoropolymer, a siloxane elastomer, a low-refractive-index adhesive layer, an organic-inorganic composite material, or other low-absorption, low-refractive-index elastic medium. The refractive index of these materials is lower than that of quartz (approximately 1.45), preferably less than or equal to 1.373. The temperature-responsive low-refractive-index coupling layer 3 can be continuously disposed or segmented along the axial direction. The thickness, material parameters, or initial effective coupling gap of the segmented regions can be adjusted. Different designs can be made based on the local heat load distribution.

[0033] By implementing this method, a variety of materials and structural options are available, allowing the temperature-responsive low-refractive-index coupling layer 3 to be flexibly adjusted according to specific application scenarios and preparation conditions, thereby improving the engineering adaptability and feasibility of the solution.

[0034] In this embodiment, the effective deformation thickness of the temperature-responsive low-refractive-index coupling layer 3 is... The effective thermal expansion coefficient can range from 50 μm to 200 μm. It can be 10 -4 / K scale, equivalent temperature response coefficient The value can be from 0.002 μm / K to 0.02 μm / K, preferably from 0.005 μm / K to 0.01 μm / K. The above parameters are used to produce a reversible effective gap change on the order of submicrometers in the temperature response low-refractive-index coupling layer 3 within the operating temperature rise range of tens of K, thereby causing a controllable decrease in the natural lateral coupling strength in the high-temperature region.

[0035] As an optional implementation, the long-period grating auxiliary coupling region 4 can be any one of a weakly modulated long-period grating structure, a chirped long-period grating structure, an apodized long-period grating structure, a quasi-period long-period grating structure, a segmented long-period grating structure, or a structure of multiple discrete long-period modulation segments. The equivalent auxiliary coupling strength of the long-period grating auxiliary coupling region 4 is 0.04 m. -1 up to 0.12 m -1 .like Figure 4 As shown, the long-period grating catch-up mechanism is illustrated: In the distributed side-pumped fiber 2, the pump light typically propagates in the pump fiber 2 in the form of multiple transmission modes or mode groups. As the pump light gradually transfers axially towards the cladding of the active signal fiber 1, the mode composition and spatial distribution of the remaining pump changes, and some of the residual pump may be in a mode state with low lateral coupling efficiency. The role of the long-period grating auxiliary coupling region 4 is to moderately perturb this part of the residual multimode pump under low absolute pump conditions, making it easier for it to participate in subsequent lateral transfer and absorption.

[0036] In this embodiment, the long-period grating-assisted coupling region 4 does not require precise phase matching of a single mode for the highly multimode pump light, nor is it limited to a narrowband coupling structure with a single period, single mode, or single wavelength. For multimode pump light, the long-period grating-assisted coupling region 4 can adopt weak modulation, broadband, chirped, apodized, quasi-periodic, or segmented structures. The technical goal of the long-period grating-assisted coupling region 4 is to achieve redistribution of the residual pump mode group, rather than pursuing complete coupling of a single pump mode.

[0037] The long-period grating-assisted coupling region 4 preferably meets the following design requirements: The long-period grating-assisted coupling region 4 is set in the low absolute pump region, the residual pump transmission region, or the middle and rear section region along the pump propagation direction. The long-period grating-assisted coupling region 4 is set in a low absolute pump region far away from the high pump input side, or in a region where residual pumping still exists after thermal feedback limiting coupling, so as to avoid further enhancing local pump transfer in the high thermal load region. The long-period grating-assisted coupling region 4 aims to improve the redistribution of residual pump mode groups and the probability of lateral transfer, rather than to maximize the coupling efficiency across the entire segment. The effective length, period, and modulation depth of the long-period grating-assisted coupling region 4 are determined based on the residual pump distribution, axial absorption distribution, and thermal stability requirements. While introducing auxiliary coupling, the long-period grating auxiliary coupling region 4 avoids the formation of new local thermal load peaks near the long-period grating auxiliary coupling region 4.

[0038] For forward-pumping, reverse-pumping, or bidirectional-pumping structures, the position of the long-period grating-assisted coupling region 4 can be reconfigured according to the pump propagation direction and residual pump distribution. Preferably, the long-period grating-assisted coupling region 4 is located in a low absolute pump region far from any high pump input side, or in a region where residual pumping still exists after thermal feedback coupling, so that the function of the long-period grating-assisted coupling region 4 remains as a residual pump catch-up transfer, rather than a high pump power region coupling enhancement.

[0039] This implementation method can adapt to different residual pumping mode distributions, avoid introducing additional heat loads in high pumping power regions, and ensure effective absorption in low absolute pumping regions. As an optional implementation, the long-period grating-assisted coupling region 4 is formed in the pump fiber 2 through any one of the following methods: femtosecond laser writing, ultraviolet exposure, carbon dioxide laser heating, periodic microbending, periodic pressure modulation, periodic geometric perturbation, or periodic material modulation. In this embodiment, femtosecond laser writing is preferred. Femtosecond laser writing can form a weakly modulated structure in a designated area, exhibiting good spatial selectivity and low damage risk. Femtosecond laser writing forms a localized, designable long-period refractive index modulation or perturbation structure in the pump fiber 2, used to achieve redistribution of residual pump mode groups. The femtosecond writing region is preferably located in the low absolute pump or residual pump transmission region and employs a weak modulation method to reduce additional scattering, local loss, and the risk of high-power damage.

[0040] This implementation method allows for flexible selection of preparation methods based on materials and process conditions, which is beneficial for actual manufacturing.

[0041] As an optional implementation, the pump fiber 2 can be a single pump fiber 2, or multiple pump fibers 2 symmetrically distributed circumferentially along the active signal fiber 1, or multiple pump fibers 2 asymmetrically distributed circumferentially along the active signal fiber 1 (e.g., asymmetrical or partitioned arrangement based on pump power distribution, heat dissipation boundaries, and coupling requirements). The long-period grating-assisted self-equalizing side pump fiber can be a single-plus-one structure or an N-plus-one structure. The single-plus-one structure refers to a side pump fiber 2 structure consisting of one active signal fiber 1 and one pump fiber 2, while the N-plus-one structure refers to a side pump fiber 2 structure consisting of one active signal fiber 1 and N pump fibers 2, where N is an integer greater than or equal to 2. The multiple pump fibers 2 can be flexibly arranged according to power distribution and heat dissipation requirements.

[0042] This implementation method can be adapted to different power levels and application scenarios, thus improving the versatility of the solution.

[0043] As an optional implementation, the long-period grating-assisted pump self-equalizing side-pumped fiber also includes a limiting structure. The limiting structure is disposed around the active signal fiber 1 and the pump fiber 2, and is used to maintain the relative position and coupling gap between the active signal fiber 1 and the pump fiber 2, preventing coupling state drift due to thermal expansion or vibration during long-term operation. The limiting structure can be a glass sleeve, a ceramic support, a metal clamp, a low thermal expansion frame, or a composite support structure.

[0044] Implementing this method can improve the long-term stability and repeatability of the structure.

[0045] As an optional implementation, the long-period grating-assisted pump self-equalizing side-pumped fiber also includes an encapsulation structure and a heat dissipation structure. The encapsulation structure covers the exterior of the active signal fiber 1 and the pump fiber 2, providing mechanical protection and environmental isolation. The encapsulation structure can employ low-absorption encapsulation materials, high thermal conductivity encapsulation materials, elastic encapsulation layers, or composite encapsulation layers. The heat dissipation structure is located on the outer surface or inside the encapsulation structure, reducing overall temperature rise and stabilizing axial heat distribution. The heat dissipation structure can employ a metal heat sink, water-cooled plate, thermally conductive adhesive layer, forced air cooling structure, or other external cooling structures.

[0046] Implementing this method can improve overall thermal management and provide stable thermal boundary conditions for the temperature-responsive low-refractive-index coupling layer 3.

[0047] To avoid the temperature-responsive low-refractive-index coupling layer 3 and the long-period grating-assisted coupling region 4 becoming an arbitrary superposition structure, this application further introduces a joint parameter design method based on operating power and thermal load distribution. This method treats the parameters of the temperature-responsive low-refractive-index coupling layer 3 and the parameters of the long-period grating-assisted coupling region 4 as co-design objects, enabling them to jointly serve thermal load peaking, residual pump control, and thermal stability improvement in the high pump power region.

[0048] In one exemplary embodiment, such as Figure 5 As shown, a joint parameter design method is provided.

[0049] In this embodiment, firstly, a rate equation and thermal load model for the distributed side-pumped fiber 2 are established based on the target output power, pump power, pump direction, fiber length, doping parameters, and cooling conditions. The rate equation describes the distribution of pump power, signal power, and population inversion of the gain medium along the fiber axis. The thermal load model calculates the thermal load distribution along the fiber axis based on quantum defect and intrinsic absorption. Subsequently, the pump power distribution, signal power distribution, lateral coupling distribution, ytterbium-doped absorption distribution, and quantum defect thermal load distribution are calculated under a reference structure. The reference structure refers to a conventional distributed side-pumped fiber 2 structure without a temperature-responsive low-refractive-index coupling layer 3 and a long-period grating auxiliary coupling region 4. Then, the parameters of the temperature-responsive low-refractive-index coupling layer 3 and the long-period grating auxiliary coupling region 4 are determined based on the high thermal load region and the residual pump region, respectively. The high thermal load region refers to the axial region with a high quantum defect thermal load, corresponding to the high pump power region. The residual pump region refers to the axial region where the remaining pump light continues to propagate after front-end coupling and absorption. Finally, the peak heat load, residual pump ratio, and thermal stability index are jointly verified to ensure that the maximum linear heat load, peak temperature, residual pump ratio, and thermal stability index meet the predetermined requirements.

[0050] In one implementation, see Figure 5 The joint parameter design method includes: 1) Determine the target operating conditions, including target output power, pump power, pump direction, fiber length, doping parameters, cooling conditions, and allowable residual pump range; 2) Establish the rate equation and thermal load model for distributed side-pumped fiber, and calculate the pump power distribution, signal power distribution, lateral coupling distribution, ytterbium-doped absorption distribution, and quantum defect thermal load distribution under the reference structure; 3) Identify high-pumping, high-heat-load regions based on maximum linear heat load, peak temperature, and heat load non-uniformity, and determine the required decoupling strength for these regions; 4) Determine the equivalent temperature response coefficient of the temperature-responsive low-refractive-index coupling layer 3 based on the required decoupling strength. Equivalent thermal expansion coefficient Effective deformation thickness and initial effective coupling gap ; 5) After adding the thermal feedback limiting layer 3 with temperature response low refractive index coupling layer 3, recalculate the residual pump distribution and axial thermal load distribution to determine the low absolute pump region or residual pump transmission region. 6) Determine the position and operating length of the long-period grating-assisted coupling region 4 based on the residual pump distribution. Periodic range and modulation depth, including the effective length It can be used as a primary adjustment variable to control the degree of redistribution of residual pumping mode groups; 7) The parameters of the temperature-responsive low-refractive-index coupling layer 3 and the parameters of the long-period grating-assisted coupling region 4 are jointly checked to ensure that the maximum linear thermal load, peak temperature, residual pump ratio and thermal stability index meet the predetermined requirements.

[0051] The key point of the above method is the equivalent thermal expansion coefficient of the temperature-responsive low-refractive-index coupling layer 3. Effective deformation thickness and initial effective coupling gap The position and operating length of the long-period grating-assisted coupling region 4 are not arbitrarily selected. It is not a fixed setting, but is determined based on the actual operating power, axial absorption distribution, and thermal stability requirements of the distributed side-pumped fiber 2. Therefore, this application forms a closed-loop design process including operating condition input, rate equation and thermal load calculation, parameter design of the temperature response low-refractive-index coupling layer 3, parameter design of the long-period grating auxiliary coupling region 4, and joint verification.

[0052] This closed-loop design process does not require external active control during operation. Parameter optimization mainly occurs during the design phase; in actual operation, the structure relies on the reversible thermoelastic response of the temperature-responsive low-refractive-index coupling layer 3 and the pre-set long-period grating-assisted coupling region 4 to achieve passive self-balancing. Therefore, this application can suppress local over-absorption and thermal peaks in the high pump power region, and maintain the necessary pump transfer capability in the residual pump region, thus balancing thermal load peak reduction and residual pump control.

[0053] This implementation method closely integrates structural design with operating conditions, ensuring optimal performance under different operating conditions and improving the scientific nature and repeatability of the design.

[0054] As a further explanation of the typical engineering implementation of this application, Table 1 provides a set of representative parameter ranges: Table 1 Representative parameters of typical projects in this application

[0055] In one exemplary embodiment, the pump absorption self-equalization process of this application is described. This process is achieved through axial functional division of labor between the temperature-responsive low-refractive-index coupling layer 3 and the long-period grating auxiliary coupling region 4, based on distributed lateral pumping of the distributed side-pumped fiber 2. The basic idea is to limit excessively strong local coupling and overly concentrated absorption in high pump power and high thermal load regions, while maintaining necessary auxiliary coupling capabilities in low absolute pump regions or residual pump regions, thus making pump absorption and thermal deposition more gradual along the fiber axis.

[0056] like Figure 6 As shown, the temperature-responsive low-refractive-index coupling layer 3 primarily affects the natural lateral coupling term. When the local temperature rises, the temperature-responsive low-refractive-index coupling layer 3 reduces the natural lateral coupling strength through changes in the effective gap, thus restricting pump transfer in the high-pump-power region. The long-period grating-assisted coupling region 4 mainly provides additional coupling terms in the low absolute pump region, used to improve the mode group distribution of residual pump and the back-end transfer capability. Together, they form a total equivalent coupling relationship distributed along the axial direction. ,in The total equivalent lateral coupling coefficient is... The natural lateral coupling coefficient is affected by local temperature and effective gap. This is an auxiliary coupling term for long-period gratings.

[0057] In low-power or initial operating conditions, the coupling between the active signal fiber 1 and the pump fiber 2 is mainly determined by the preset geometry, the low refractive index boundary, and the initial effective coupling gap. The pump light is determined to gradually enter the cladding of the active signal fiber 1 along the axial direction and be absorbed by the doped gain medium. At this time, the temperature-responsive low-refractive-index coupling layer 3 has not yet produced significant thermal decoupling, and the system is close to the designed cold-state coupling condition.

[0058] As the pump power increases, if a region experiences a high thermal load due to strong pump coupling and absorption, the low-refractive-index coupling layer 3 undergoes reversible thermoelastic deformation in response to temperature, altering the effective coupling state in that region. Consequently, local natural lateral coupling is restricted, the concentrated transfer and absorption of pump light in that region are suppressed, and some pump light continues to propagate along the pump fiber 2 or the pump channel, thereby reducing the local linear thermal load peak and temperature peak.

[0059] like Figure 7As shown, in the distributed side-pumped structure, the pump light does not immediately enter the cladding of the active signal fiber 1 and be absorbed at the injection end. Instead, there is a gradual transfer process from the pump fiber 2 to the cladding of the active signal fiber 1. Therefore, the local thermal deposition is determined by the pump channel power, lateral coupling transfer, and ytterbium-doped absorption, rather than solely by the total pump power in the pump fiber 2. This characteristic makes it possible to change the thermal deposition distribution by controlling the axial coupling distribution.

[0060] When residual pump light enters the low absolute pump or residual pump transmission region, the long-period grating-assisted coupling region 4 perturbs and redistributes the pump transmission mode, increasing the probability that a portion of the residual pump light will transfer to the cladding of the active signal fiber 1. The pump light entering the cladding of the active signal fiber 1 is then absorbed and utilized by the doped gain medium, thereby reducing the risk of continued transmission of residual pump light or end leakage.

[0061] Therefore, this application establishes an axial adjustment process that combines high-temperature region confinement coupling with residual pump region auxiliary coupling. The temperature-responsive low-refractive-index coupling layer 3 is responsible for suppressing local overcoupling and overabsorption in the high pump power region, while the long-period grating auxiliary coupling region 4 is responsible for improving residual pump transfer in the low absolute pump region. The two are not simply superimposed in the same region, but act on different axial functional regions respectively, jointly achieving passive self-balancing of pump coupling and absorption distribution.

[0062] It should be noted that the high pump power region, low absolute pump region, and residual pump transmission region in this application are functional regions defined based on local pump power, temperature rise status, and residual pump distribution, and do not necessarily correspond to fixed fiber endface positions. For unidirectional pumping structures such as forward or reverse pumping structures, the above regions can be determined along the pump propagation direction. For bidirectional pumping structures, the long-period grating auxiliary coupling region 4 should be conditionally set according to the pump distribution at both ends and the residual pump transmission path, preferably avoiding the high pump power region on either pump input side, and aiming at residual pump control rather than high pump power region coupling enhancement.

[0063] This implementation method achieves a unified approach to heat load shaving and residual pump control through the synergistic effect of high-temperature zone limiting coupling and low-pump zone backstop, without requiring external active control, making the system simple and reliable.

[0064] In an exemplary embodiment, a 50 / 300 μm active signal fiber 1 and a 310 μm pump fiber 2 are used, with an initial effective coupling gap. Approximately 0.3 μm, with an initial natural lateral coupling coefficient of approximately 0.2 m. -1 Under operating conditions of approximately 30 kW reverse pump injection, such as Figure 8As shown, compared with the baseline structure, the peak value of quantum defect thermal load in the high-pump region is significantly reduced after adopting the temperature-responsive low-refractive-index coupling layer 3. The combined structure retains the thermal feedback peak-shaving effect while using the long-period grating-assisted coupling region 4 to perform back-end transfer of residual pumping, thus achieving both thermal load peak-shaving and residual pump control. Figure 9 As shown, the combined structure can reduce the local temperature peak in the high-pump region, making the axial temperature distribution more gradual. After the temperature peak is reduced, the thermal lensing effect, thermal refractive index disturbance, and thermal stress concentration in the optical fiber can be alleviated, which is beneficial to improving the thermal stability and long-term reliability of the distributed side-pumped fiber 2 under high-power operating conditions.

[0065] Furthermore, the thermal load state is closely related to the mode instability threshold; a smoother thermal load distribution and a lower temperature rise peak are beneficial for improving the mode instability threshold. When the thermal distribution is smoothed using the method described in this application, the local thermal load peak and temperature peak are reduced, and the thermally induced mode coupling strength in the high-power region is correspondingly weakened, thereby improving the mode instability suppression capability. It should be noted that mode instability cannot be completely eliminated. This application improves the mode instability threshold by reducing the concentration of local thermal load and improving the axial thermal distribution, thus suppressing thermally induced mode coupling.

[0066] In particular, by implementing this method, the significant effects of this application in thermal management and residual pump control were verified through quantitative comparison.

[0067] In an exemplary embodiment, this application also provides a fiber laser, which includes the aforementioned long-period grating-assisted self-equalizing side-pumping fiber. The long-period grating-assisted self-equalizing side-pumping fiber is disposed as an active gain section within the resonant cavity, constituting a fiber oscillator or fiber amplifier. The long-period grating-assisted self-equalizing side-pumping fiber is used as a pump-coupled gain fiber in high-power fiber lasers. Its common characteristics are: pump light enters the long-period grating-assisted self-equalizing side-pumping fiber through the pump fiber 2 and is gradually coupled into the cladding of the active signal fiber 1 along the axial direction; a temperature-responsive low-refractive-index coupling layer 3 is used for thermal feedback limiting coupling in the high-pump power region; and a long-period grating-assisted coupling region 4 is used for auxiliary coupling in the low absolute pump region or residual pump transmission region.

[0068] In the fiber optic oscillator implementation, the resonant cavity can be composed of a high-reflectivity fiber grating and a low-reflectivity output fiber grating; in the fiber optic amplifier implementation, the seed light is injected after passing through an isolator, and the pump light is input through the pump fiber 2. The long-period grating auxiliary coupling region 4 only functions in the pump band and does not serve as a signal grating. The fiber laser can adopt a single-ended or double-ended output structure, suitable for forward pumping, reverse pumping, or bidirectional pumping, and can be used with conventional devices such as pump strippers, mode filters, end caps, heat sinks, and water-cooling structures. The above-mentioned additional devices are used to realize system-level optical path connection, residual pump processing, output end protection, and heat dissipation management. For forward pumping, reverse pumping, and bidirectional pumping structures, the positions of the temperature-responsive low-refractive-index coupling layer 3 and the long-period grating auxiliary coupling region 4 can be reconfigured according to the actual pump distribution.

[0069] In this implementation method, the side-pumped fiber 2 of this application is applied to an actual laser system, which can effectively improve the thermal stability and mode stability during high-power operation, and is conducive to achieving higher power laser output.

[0070] As an optional implementation, the fiber laser further includes a pump source, a high-reflectivity fiber grating, a low-reflectivity output fiber grating, an isolator, an end cap, a mode filter, a pump stripping structure, and a cooling encapsulation structure. The pump source is connected to the input end of the pump fiber 2. For forward pumping, reverse pumping, or bidirectional pumping structures, the long-period grating auxiliary coupling region 4 is located in a low absolute pump region far from any high pump input side, or in a region where residual pumping still exists after thermal feedback coupling, to ensure that the function of the long-period grating auxiliary coupling region 4 is always to provide a backup for residual pumping rather than to enhance the main coupling.

[0071] This implementation method allows for flexible adaptation to various pumping schemes, ensuring that the long-period grating-assisted coupling region 4 always functions in the low absolute pumping region, thus avoiding the introduction of additional thermal peaks.

[0072] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0073] This document uses specific examples to illustrate the principles and implementation methods of this application. The descriptions of the above embodiments are only for the purpose of helping to understand the methods and core ideas of this application. Furthermore, those skilled in the art will recognize that, based on the ideas of this application, there will be changes in the specific implementation methods and application scope. Therefore, the content of this specification should not be construed as a limitation of this application.

Claims

1. A long-period grating-assisted pumped self-equalized side-pumped fiber, characterized in that, The long-period grating-assisted self-equalizing side-pumping fiber includes: An active signal fiber, wherein the core of the active signal fiber is doped with gain ions; A pump fiber is disposed parallel to the side of the active signal fiber, and the pump fiber and the active signal fiber form a distributed side coupling. A temperature-responsive low-refractive-index coupling layer is disposed between the active signal fiber and the pump fiber. The refractive index of the temperature-responsive low-refractive-index coupling layer is lower than that of the adjacent pump transmission region and the cladding of the active signal fiber. The temperature-responsive low-refractive-index coupling layer has reversible thermoelasticity. The temperature-responsive low-refractive-index coupling layer is used to deform due to temperature rise in the high pump power region to reduce the local natural lateral coupling strength. A long-period grating-assisted coupling region is provided in the pump fiber along the pump direction in the low absolute pump region or residual pump region. The long-period grating-assisted coupling region is used to perform mode perturbation on the residual multimode pump light. The temperature-responsive low-refractive-index coupling layer and the long-period grating-assisted coupling region form an axial division of labor along the pumping direction; the high pump power region and the low absolute pump region are axial functional regions divided according to local pump power and local temperature rise state.

2. The long-period grating-assisted pumped self-equalized side-pumped fiber according to claim 1, characterized in that, The temperature-responsive low-refractive-index coupling layer has an equivalent temperature response coefficient, an equivalent thermal expansion coefficient, an effective deformation thickness, and an initial effective coupling gap. The equivalent temperature response coefficient is determined based on the target operating power, axial thermal load distribution, and thermal stability requirements. The equivalent thermal expansion coefficient and the effective deformation thickness are determined based on the equivalent temperature response coefficient. The value of the initial effective coupling gap is located within the sensitive range of natural evanescent coupling to spacing changes. Within this sensitive range, a small change in the initial effective coupling gap can cause a significant change in the local natural lateral coupling strength.

3. The long-period grating-assisted pumped self-equalized side-pumped fiber according to claim 1, characterized in that, The temperature-responsive low-refractive-index coupling layer is continuously disposed along the axial direction of the pump fiber, or is disposed in segments along the axial direction of the pump fiber.

4. The long-period grating-assisted pumped self-equalized side-pumped fiber according to claim 1, characterized in that, The long-period grating auxiliary coupling region can be any one of the following: a weakly modulated long-period grating structure, a chirped long-period grating structure, an apodized long-period grating structure, a quasi-period long-period grating structure, a segmented long-period grating structure, or multiple discrete long-period modulation segment structures; the equivalent auxiliary coupling strength of the long-period grating auxiliary coupling region is 0.04m. -1 up to 0.12m -1 .

5. The long-period grating-assisted pumped self-equalized side-pumped fiber according to claim 1, characterized in that, The long-period grating-assisted coupling region is formed in the pump fiber by any one of the following methods: femtosecond laser writing, ultraviolet exposure, carbon dioxide laser heating, periodic microbending, periodic pressure modulation, periodic geometric perturbation, or periodic material modulation.

6. The long-period grating-assisted pumped self-equalized side-pumped fiber according to claim 1, characterized in that, It also includes a limiting structure, which is disposed around the active signal fiber and the pump fiber, and the limiting structure is used to maintain the relative position and coupling gap between the active signal fiber and the pump fiber.

7. The long-period grating-assisted pumped self-equalized side-pumped fiber according to claim 1, characterized in that, It also includes an encapsulation structure and a heat dissipation structure; the encapsulation structure covers the outside of the active signal fiber and the pump fiber; the heat dissipation structure is disposed on the outer surface or inside of the encapsulation structure.

8. The long-period grating-assisted pumped self-equalized side-pumped fiber according to claim 1, characterized in that, The pump fiber is a single pump fiber, or multiple pump fibers symmetrically distributed circumferentially along the active signal fiber, or multiple pump fibers asymmetrically distributed circumferentially along the active signal fiber; the long-period grating-assisted self-balancing side pump fiber is a single-plus-one structure or an N-plus-one structure, wherein the single-plus-one structure refers to a side pump fiber structure composed of one active signal fiber and one pump fiber, and the N-plus-one structure refers to a side pump fiber structure composed of one active signal fiber and N pump fibers, where N is an integer greater than or equal to 2.

9. A fiber laser, characterized in that, The invention includes a long-period grating-assisted pump self-balancing side-pumping fiber as described in any one of claims 1 to 8, wherein the long-period grating-assisted pump self-balancing side-pumping fiber is disposed as an active gain segment in the resonant cavity to form an optical fiber oscillator or an optical fiber amplifier.

10. The fiber laser according to claim 9, characterized in that, The fiber laser also includes a pump source, a high-reflectivity fiber grating, a low-reflectivity output fiber grating, an isolator, an end cap, a mode filter, a pump stripping structure, and a cooling encapsulation structure; the pump source is connected to the input end of the pump fiber; for forward pumping, reverse pumping, or bidirectional pumping structures, the long-period grating auxiliary coupling region is located in a low absolute pumping region far from any high pumping input side, or in a region where residual pumping still exists after thermal feedback coupling.

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