A self-bleaching composite grating device, a laser system and a working method thereof

Abstract

The application discloses a self-bleaching composite grating device, a laser system and a working method thereof, and relates to the field of high-power fiber lasers. The device comprises a fiber base, an outer reflection and heat management structure, and an up-conversion self-bleaching area, a visible light recoupling area and a residual cladding light stripping area arranged along the axial direction of the fiber base in sequence. The outer reflection and heat management structure can be wrapped outside the up-conversion self-bleaching area and the visible light recoupling area. The visible light recoupling area is provided with a visible light recoupling grating. The residual cladding light stripping area is used for stripping and heat dissipating residual near-infrared cladding non-effective light which is not used by the up-conversion bleaching function layer. The outer reflection and heat management structure is used for reflecting the visible bleaching light radiated outward back to the inside of the fiber base to improve the utilization rate of the bleaching light, and simultaneously leads out the heat generated in the process of operation of the device. The self-bleaching composite grating device, the laser system and the working method thereof can break the traditional optical isolation and utilize residual pump light in situ for targeted self-bleaching.

Description

Technical Field

[0001] This invention relates to the field of high-power fiber laser technology, and in particular to a self-bleaching composite grating device, a laser system, and its operating method. Background Technology

[0002] In recent years, high-power continuous-wave ytterbium-doped double-clad fiber lasers have been widely used due to their high electro-optical conversion efficiency, excellent beam quality, and good heat dissipation characteristics. Currently, the output power of single-fiber lasers has exceeded 20kW. However, with the rapid increase in optical power density within the fiber core, a series of degradation problems caused by extreme thermal loads inside the fiber have become increasingly prominent. Among them, photodarkening (PD) and transverse mode instability (TMI) have become the core physical bottlenecks restricting further increases in output power and maintaining long-term stable beam quality. In current high-power fiber laser systems such as all-fiber oscillators and master oscillator power amplifiers, the high power carrying capacity of core optical components faces severe challenges from background defect absorption and thermal loads. Their failure mechanisms mainly stem from inherent microscopic physical defects within the gain medium and passive grating components, specifically manifested as two similar thermal degradation mechanisms.

[0003] The first category concerns the accumulation of photo-induced color center defects in ytterbium-doped gain fibers and its premature induction of transverse mode instability. Under a high inversion particle number state maintained by long-term high-frequency pumping, ytterbium ions inside the ytterbium-doped gain fiber are prone to charge transfer processes, and some Yb... 3+ Restored to Yb 2+ This process is accompanied by the generation of oxygen vacancies, leading to a high concentration of photoinduced color center defects. Spectroscopic analysis shows that these color centers exhibit strong broadband background absorption in the 1 μm band, with loss coefficients reaching several dB / m under extreme conditions. When the high-quality laser transmitted by the system is absorbed by these color centers, it is directly converted into localized thermal energy through a non-radiative multiphonon relaxation pathway. This intense nonlinear heating not only creates a severe thermal lensing effect but also accelerates the formation of thermally induced long-period refractive index gratings within the fiber core. This results in strong dynamic energy coupling between the fundamental mode and higher-order modes, significantly reducing the system's TMI threshold and severely degrading the output beam quality.

[0004] The second category concerns the absorption and thermal runaway issues related to background defects in ultraviolet-written grating components. At the tail end of a laser system, whether it's an open-circuit grating (OC-FBG) used for oscillator resonant feedback or a tilted-bracing grating (CTFBG) used to filter stimulated Raman scattering, the fabrication process introduces significant microstructural damage. These gratings are typically fabricated using heavily germanium-doped or hydrogen-loaded silica fiber as a substrate, exposed and written with high-energy ultraviolet lasers at wavelengths such as 248 nm and 193 nm. While inducing refractive index modulation, high-energy ultraviolet photons inevitably break Ge-Si or Si-Si bonds in the silica network, generating numerous germanium-oxygen deficient centers and hydroxyl-related defects. When a high-power laser penetrates this physical region, these deep-level or shallow-level defects exhibit weak but persistent linear or nonlinear absorption of infrared lasing light; under extremely high local photon flux densities, even with extremely low absorption coefficients, considerable thermal load accumulation occurs within a very short physical interval. Such extreme local thermal loads can cause a linear thermal drift of about 10 pm / ℃ at the center wavelength of the grating, resulting in spectral distortion. Under extreme conditions, it may also directly exceed the thermal control limit of the device package, inducing carbonization of the polymer coating or catastrophic optical damage to the quartz matrix.

[0005] To address the aforementioned photodarkening and the resulting thermal degradation problems, the current common approach in the field is to employ active photobleaching using short-wavelength visible light (such as 405nm, 450nm, and 532nm). This aims to reverse the photodarkening process by inducing the recombination of trapped electrons in the color centers through high-energy photon excitation. However, this approach still has significant technical limitations in practical engineering applications: First, the intrinsic transmission loss in space is high, and the bleaching effect at the tail end is limited. Existing active injection technology mainly relies on wavelength division multiplexing devices to introduce bleaching light from the laser front end through beam combining. However, ytterbium-doped silica fiber cores and cladding materials exhibit strong intrinsic absorption and Rayleigh scattering of short-wavelength visible light. For example, the transmission loss of 532nm green light in conventional ytterbium-doped fiber cores is typically as high as tens of dB / m. When hundreds of milliwatts or even watts of bleaching light are injected from the front end and travel through tens of meters of gain fiber to reach the tail end of the system, where the thermal load is heaviest and the light darkening is most severe, the residual optical power has already been significantly attenuated. The effective bleaching photon flux is difficult to reach the nonlinear excitation threshold required to suppress deep defects, and it cannot effectively offset the defect regrowth rate under high-power conditions.

[0006] Secondly, the system has high redundancy, resulting in significant additional losses and costs. Implementing an active bleaching scheme requires adding an independent short-wavelength semiconductor laser source and temperature control drive equipment to the existing system architecture. It also requires passive components such as wavelength division multiplexing and special beam combiners in series. This not only significantly increases the hardware cost and control complexity of the system, but the additional passive components also become new infrared loss points and heat accumulation points, raising the overall thermal load of the system, which contradicts the development trend of compactness and high efficiency of high-power fiber lasers.

[0007] On the other hand, in the operation of conventional double-clad fiber laser systems, due to the limitations of fiber cladding geometry and the theoretical limits of pump light absorption efficiency, approximately 10% of the pump light (typical wavelength 976nm) cannot be completely absorbed by the gain medium and propagates along the inner cladding to the end of the system. In high-power fiber lasers with tens of thousands of watts, the absolute power of this residual pump light can even reach 1kW to 2kW. To prevent high-power waste light from entering and damaging subsequent passive devices, existing technologies typically place a cladding power stripper (CPS) at the end of the system. This CPS uses chemical etching or laser micromachining techniques to destroy the total internal reflection boundary of the fiber, extracting hundreds of watts of optical energy in the form of diffuse scattering, which is then dissipated as heat by an external water-cooled metal heat sink.

[0008] This approach objectively results in a significant waste of high-quality optical energy. Simultaneously, the instantaneous conversion of kilowatt-level light energy into heat makes the cladding power stripper one of the most concentrated areas of heat flux in the entire system. Its local heat flux density often exceeds the thermal control limit of tens of watts per square centimeter, making thermal management extremely difficult. Current industrial and academic approaches to handling residual pump light are almost entirely limited to passive defense strategies that improve photothermal conversion and dissipation efficiency. There are few reports on how to utilize this high-brightness, high-energy-density residual infrared light through secondary nonlinear optics, particularly transforming it into an effective light source to suppress internal system defects.

[0009] In summary, existing high-power fiber laser technology suffers from a significant contradiction in spatial energy allocation: on the one hand, the tail region, where thermal decay is most severe, lacks short-wavelength high-energy bleaching photons capable of effectively repairing color center defects; on the other hand, in similar physical locations, the system directly strips and converts hundreds of watts of residual near-infrared pump light into waste heat, requiring an additional cooling system to provide extra energy for processing. Existing technologies treat grating / fiber defect suppression and residual pump light processing as two isolated and costly technical steps, failing to address the underlying physical mechanisms of material energy level transitions and optical mode spatial coupling to construct a synergistic strategy for closed-loop optical energy conversion and adaptive cascade utilization within the system. Therefore, there is an urgent need in this field to design a technical solution that can break through traditional optical isolation and utilize residual pump light in situ for targeted self-bleaching. Summary of the Invention

[0010] The purpose of this invention is to provide a self-bleaching composite grating device, a laser system, and its operating method to solve the problems existing in the prior art, and to break through traditional optical isolation and perform targeted self-bleaching in situ using residual pump light.

[0011] To achieve the above objectives, the present invention provides the following solution: This invention provides a self-bleaching composite grating device, comprising an optical fiber substrate, an external reflection and thermal management structure, and an upconversion self-bleaching region, a visible light backcoupling region, and a remaining cladding light stripping region sequentially arranged along the axial direction of the optical fiber substrate. The external reflection and thermal management structure can wrap around the outside of the upconversion self-bleaching region and the visible light backcoupling region. The upconversion self-bleaching region includes a cladding pump control grating and an upconversion bleaching functional layer. The cladding pump control grating is used to control the propagation state of near-infrared cladding ineffective light transmitted in the optical fiber cladding, so that some of the near-infrared cladding ineffective light is converted from the cladding guided mode state into leakage light, scattered light, or higher-order cladding mode light that can interact with the upconversion bleaching functional layer. The upconversion bleaching functional layer is used to absorb the near-infrared cladding ineffective light and generate visible light through a nonlinear upconversion process. The visible bleaching light is used to act on the target defect region upstream of the device to repair photo-induced color center defects and grating-written defects. A visible light backcoupling grating is provided in the visible light backcoupling region. The reflection band of the visible light backcoupling grating matches the emission band of the visible bleaching light, and is used to reflect the visible bleaching light transmitted downstream back to the upstream target defect region. The visible light backcoupling grating does not effectively reflect ineffective near-infrared cladding light or signal laser transmitted in the fiber core. The remaining cladding light stripping region is used to strip and thermally dissipate the remaining ineffective near-infrared cladding light that is not utilized by the upconversion bleaching functional layer. The external reflection and thermal management structure is used to reflect the outwardly radiated visible bleaching light back into the fiber matrix to improve the utilization rate of the bleaching light, while simultaneously dissipating the heat generated during device operation.

[0012] In one embodiment, the cladding pump control grating is a surface microstructure grating, which is a periodic, quasi-periodic, or axially gradient distributed microstructure formed on the outer surface of the fiber cladding. The microstructure includes microgrooves, micropits, annular etching bands, spiral grooves, coarsening stripes, or micro / nano scattering units. The upconversion bleaching functional layer covers the outside of the surface microstructure grating or fills the interior of the microstructure.

[0013] In one embodiment, the cladding pump control grating is a femtosecond-written cladding perturbation grating, which is a periodic, quasi-periodic, spiral, or multi-point array refractive index perturbation structure written by a femtosecond laser in the outer region inside the fiber cladding.

[0014] In one embodiment, the cladding pump control grating is a high-refractive-index segmented coupling grating, comprising high-refractive-index coupling units distributed along the fiber axis. The high-refractive-index coupling units are segmented strips, annular strips, spiral strips, lattice units, or quasi-periodic coating units. The upconversion bleaching functional layer is disposed outside the high-refractive-index coupling units, or the high-refractive-index coupling units themselves are doped with upconversion luminescent components, thereby simultaneously realizing cladding optical coupling and upconversion luminescence functions.

[0015] In one embodiment, the cladding pump control grating is a long-period grating and / or a tilted grating; the long-period grating is used to achieve coupling between cladding modes, converting low-leakage cladding modes into higher-order cladding modes or leakage modes; the tilted grating is used to couple and deflect near-infrared cladding ineffective light to the outside of the cladding.

[0016] In one embodiment, the upconversion bleaching functional layer is prepared using rare earth-doped upconversion materials, including fluorides, oxides, silicates, tungstates, molybdates, glass ceramics, nanocrystals, or polymer composites of Yb / Er, Yb / Tm, and Yb / Ho co-doped systems; the wavelength range of the visible bleaching light is 400 nm to 700 nm.

[0017] In one embodiment, the visible light backcoupling grating is a cladding Bragg reflection grating etched by a femtosecond laser; the visible light backcoupling grating is a chirped grating, a multi-segment cascaded grating, or a broadband visible light backcoupling structure to match an upconversion bleaching functional layer with multiple emission peaks.

[0018] In one embodiment, the remaining cladding optical stripping region is integrated with the self-bleaching composite grating device and adopts an axial gradient stripping heat dissipation structure; or, the remaining cladding optical stripping region is a cladding power stripper independently disposed downstream of the visible light backcoupling region.

[0019] In one embodiment, the external reflection and thermal management structure includes an inner visible light high reflectivity layer, an intermediate thermally conductive insulating layer, and an outer cooling structure; the visible light high reflectivity layer is a metal reflectivity layer or a multilayer dielectric film reflectivity layer, and the outer cooling structure is a water-cooled shell, a metal heat sink, or an air-cooled heat dissipation structure.

[0020] The present invention also provides a high-power fiber laser system, including the self-bleaching composite grating device; the self-bleaching composite grating device is disposed on the ineffective light transmission path of the near-infrared cladding of the system and is located downstream of the target defect region; the target defect region includes a low-reflection Bragg grating region, a ytterbium-doped active fiber tail, an amplifier gain fiber output end, a fiber fusion splice region, or an output cap front end region.

[0021] The present invention also provides a method for operating the high-power fiber laser system, comprising the following steps: S1. When the high-power fiber laser system is operating normally, the near-infrared signal laser is transmitted in the fiber core. The residual pump light, cladding leakage signal light, and cladding ASE light that are not completely absorbed by the gain medium together constitute the near-infrared cladding ineffective light, which is transmitted along the fiber cladding to the self-bleaching composite grating device. S2. The near-infrared cladding ineffective light enters the upconversion self-bleaching region, and the propagation mode is changed by the cladding pump control grating. Some of the near-infrared cladding ineffective light is coupled to the upconversion bleaching functional layer. S3. The upconversion bleaching functional layer utilizes the absorbed near-infrared cladding ineffective light to undergo nonlinear upconversion and generate visible bleached light. The visible bleached light radiated outward is reflected back into the optical fiber by external reflection and thermal management structure. Part of it propagates directly upstream to the target defect region to repair the photoinduced defect, and the other part propagates downstream along the optical fiber. S4. Visible bleaching light propagating downstream enters the visible light backcoupling region and is selectively reflected by the visible light backcoupling grating, and is recoupled back to the upstream target defect region to continuously bleach and repair color center defects, oxygen vacancy defects, and grating writing-induced defects. S5. The remaining ineffective light from the near-infrared cladding layer that is not utilized by the upconversion bleaching functional layer continues to be transmitted to the remaining cladding light stripping area, where it is stripped and converted into heat energy, which is then dissipated by the external reflection and thermal management structure.

[0022] The present invention achieves the following technical effects compared to the prior art: This invention partially converts residual pump laser, cladding leakage signal light, and cladding ASE light—near-infrared cladding ineffective light that would otherwise need to be stripped and dissipated in high-power fiber laser systems—into visible bleached light, thus realizing the functional utilization of this ineffective cladding light. By enhancing the coupling between near-infrared light and the upconversion bleaching functional layer through a cladding pump modulation grating, and increasing the proportion of visible bleached light acting on the target defect region through a visible light backcoupling grating, near-in-situ bleaching can be performed on low-reflection gratings, active fiber tails, amplifier outputs, and near fusion splices for color center defects, photodarkening centers, and grating-written induced defects without introducing external bleaching light sources, additional combiners, or complex control circuits. This reduces defect-related absorption and localized thermal deposition, improving the long-term stability of grating devices and adjacent areas of the active fiber. Simultaneously, the remaining cladding light not utilized by upconversion can still be safely handled by the downstream stripping and heat dissipation structure, balancing self-bleaching functionality and high-power thermal safety. This scheme also features passive adaptive characteristics. The visible bleaching light of the self-bleaching composite grating device originates from the near-infrared cladding ineffective light generated by the system itself, especially the residual pump laser. Therefore, when the system operating power increases, or the power of the residual pump laser or other near-infrared cladding ineffective light increases, the excitation energy obtained by the upconversion bleaching functional layer also increases accordingly. Within a certain operating range, the visible bleaching light output will increase with the increase of the near-infrared cladding ineffective light power, thus forming a passive adaptive bleaching mechanism. The higher the laser operating power, the stronger the residual pump laser, and the stronger the generated visible bleaching light, resulting in a stronger response to the bleaching demand in the high-power region. This adaptive characteristic allows the scheme to automatically adjust its bleaching capability under different power levels and operating conditions without external control or adjustment, exhibiting good applicability and robustness. Attached Figure Description

[0023] To more clearly illustrate the technical solutions in the embodiments of the present invention 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 the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0024] Figure 1 This is an axial schematic diagram of a self-bleaching composite grating device in one or more embodiments of the present invention; Figure 2 For the present invention Figure 1 A schematic diagram of the transverse cross-section of the upconversion from the bleaching region, the visible light recoupling region, and the remaining cladding light stripping region; Figure 3 This is a schematic diagram illustrating the upconversion principle from the bleaching zone in one or more embodiments of the present invention; Figure 4 This is an axial schematic diagram of a surface microstructure grating in one or more embodiments of the present invention; Figure 5 This is an axial schematic diagram of the femtosecond writing cladding perturbation grating in one or more embodiments of the present invention; Figure 6 This is an axial schematic diagram of a high-refractive-index segmented coupled grating in one or more embodiments of the present invention; Figure 7 This is an axial schematic diagram of a long-period grating / tilted grating in one or more embodiments of the present invention; Figure 8 This is an axial schematic diagram of the present invention using the independent peel-off heat dissipation method; Figure 9 This is an axial schematic diagram of the external reflection and thermal management structure in one or more embodiments of the present invention; Figure 10 This is a schematic diagram illustrating the working principle of the self-bleaching composite grating device in one or more embodiments of the present invention; Figure 11 This is a schematic diagram of the optical structure of the high-power all-fiber oscillator in Example 1; Figure 12 This is a schematic diagram of the optical structure of the all-fiber amplifier in Example 2.

[0025] In the diagram: 101-Fiber substrate; 102-Fiber cladding; 111-Cladding pump control grating; 112-Upconversion bleaching functional layer; 121-Visible light backcoupling grating; 131-Remaining cladding light stripping region; 141-External reflection and thermal management structure; 201-Microstructure grating; 301-Femtosecond writing cladding perturbation grating; 401-High refractive index segmented coupling grating; 501-Long period grating; 502-Tilted grating; 601-Visible bleached light; 602-Near-infrared cladding ineffective light; 603-High reflectivity layer for visible light in the external reflection and thermal management structure; 604-Cooling medium flowing inside the external reflection and thermal management structure; 605-High absorption layer for near-infrared light in the external reflection and thermal management structure; 606-Near-infrared cladding ineffective light in the external reflection and thermal management structure. The structure contains the following components: 801 - Oscillator first pump source; 802 - Oscillator first pump signal combiner; 803 - High reflectivity fiber grating; 804 - Oscillator gain fiber; 805 - Low reflectivity Bragg grating; 806 - Oscillator second pump signal combiner; 807 - Oscillator second pump source; 808 - Self-bleached composite grating device; 809 - Oscillator output cap; 901 - Seed source; 902 - Preamplifier; 903 - Preamplifier cladding power stripper; 904 - Mode field adapter; 905 - Amplifier first pump source; 906 - Amplifier first pump signal combiner; 907 - Amplifier gain fiber; 908 - Amplifier second pump signal combiner; 909 - Amplifier second pump source; 910 - Amplifier output cap. Detailed Implementation

[0026] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0027] The purpose of this invention is to provide a self-bleaching composite grating device, a laser system, and its operating method to solve the problems existing in the prior art, and to break through traditional optical isolation and perform targeted self-bleaching in situ using residual pump light.

[0028] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0029] In this invention, some common terms are first explained. To simplify the explanation and process, typical values ​​of their characteristic parameters are used as representative examples. However, it should be emphasized that the example parameters here represent typical parameters only, not their unique values. Designs with parameters different from those in the examples of this application but similar schemes are still considered variations of this scheme.

[0030] Self-bleaching composite grating device: The core composite device of this invention is mainly composed of a cladding pump control grating, an upconversion bleaching functional layer, a visible light backcoupling grating, and an external reflection and thermal management structure.

[0031] Signal laser: The laser generated by a fiber laser and used for target output or amplification, typically located in the near-infrared band. For ytterbium-doped fiber lasers, the signal laser is generally located in the range of 1030 nm to 1100 nm, with typical wavelengths including 1064 nm, 1070 nm, or 1080 nm. The signal laser usually propagates forward along the output direction, primarily within the core of the double-clad fiber.

[0032] Residual pump laser: Pump light that is not completely absorbed by the gain medium after being injected into a fiber laser and continues to propagate along the fiber. Residual pump lasers are typically located in the near-infrared band, such as around 915nm, 940nm, 976nm, or 1018nm, with 976nm serving as a typical example in this scheme. In double-clad fibers, residual pump lasers usually propagate primarily along the cladding and can have forward, reverse, or bidirectional propagation components, serving as the main near-infrared excitation energy source for self-bleached composite grating devices.

[0033] Cladding leakage signal light: This refers to signal wavelength light that is the same as or close to the signal laser, but leaks from the fiber core into the cladding due to reasons such as weld defects, bending disturbances, mode coupling, transverse mode degradation, core-cladding coupling, end cap scattering, or passive device scattering. Cladding leakage signal light is part of the near-infrared cladding ineffective light, and its power is usually lower than that of the residual pump laser. It is generally not the main energy source for self-bleached composite grating devices, but it can be used as auxiliary excitation light.

[0034] ASE light: Amplified spontaneous emission light, is the non-target laser component formed after stimulated amplification of spontaneous emission in the gain medium. ASE light is typically located in the near-infrared band associated with the gain medium and can propagate along the fiber core or cladding. When ASE light leaks into the cladding and does not participate in the target laser output or amplification process, it can be classified as ineffective near-infrared cladding light, which can be synchronously controlled, utilized, or dissipated by self-bleaching composite grating devices.

[0035] Near-infrared cladding ineffective light: Non-target laser propagating in the cladding of a double-clad fiber, mainly including residual pump laser, cladding leakage signal light, cladding ASE light, and other near-infrared stray light. Among these, the residual pump laser is usually the main component of near-infrared cladding ineffective light and is also the main excitation energy source for self-bleached composite grating devices; cladding leakage signal light, cladding ASE light, and other near-infrared stray light are usually secondary components and can be used as auxiliary excitation. In the description of the working principle and typical structure, the residual pump laser is mainly used as the representative of near-infrared cladding ineffective light, and the residual pump power is used as a typical parameter for the power of near-infrared cladding ineffective light.

[0036] Visible bleaching light: Short-wavelength light generated by the upconversion bleaching functional layer under near-infrared cladding ineffective light excitation, typically located in the visible light band, such as the range of 400nm to 700nm, with a typical example being green light around 532nm. The single-photon energy of visible bleaching light is higher than that of near-infrared cladding ineffective light, and it can be used to excite, de-depress, recombine, or alter color center defects, oxygen vacancy-related defects, photodarkening centers, or grating-written defects in the target defect region. Visible bleaching light can radiate or propagate in multiple directions, and a portion of it can be reflected, scattered, guided, or coupled to the target defect region by external reflection and thermal management structures, visible light backcoupling gratings, or other backcoupling structures.

[0037] Target defect region: The fiber region that needs to be exposed to visible bleaching light, including writing-induced defects in the fiber grating region, defects related to hydrogen loading or annealing processes, color centers or oxygen vacancy-related defects in the quartz matrix, and color center defects in the core of ytterbium-doped active fibers caused by photodarkening. In this paper, "photodarkening center" generally refers to a color center defect formed or participating in absorption during the photodarkening process, which can be considered a specific manifestation of the color center defect under a specific formation mechanism. The target defect region can be located in low-reflection gratings, the end of active fibers, the output end of amplifier gain fibers, near fusion splices, or other high-power near-infrared light exposure areas. The target defect region is the target of visible bleaching light, but is not part of the self-bleaching composite grating device itself.

[0038] Existing high-power fiber laser technology suffers from a significant contradiction in spatial energy distribution: on the one hand, the tail region, where thermal decay is most severe, lacks short-wavelength, high-energy bleaching photons capable of effectively repairing color center defects; on the other hand, in similar physical locations, the system directly strips and converts hundreds of watts of residual near-infrared pump light into waste heat, requiring an additional cooling system to provide extra energy for processing. Current technologies separate grating / fiber defect suppression and residual pump light processing into two isolated and costly technical steps, failing to address the underlying physical mechanisms of material energy level transitions and optical mode spatial coupling to construct a synergistic strategy for closed-loop optical energy conversion and adaptive cascade utilization within the system. To address this issue, the first objective of this invention is to provide a self-bleaching composite grating device, referencing… Figures 1-12As shown, the device includes an optical fiber substrate 101, and an upconversion self-bleaching region, a visible light backcoupling region, and a remaining cladding light stripping region 131 arranged sequentially along the axial direction of the optical fiber substrate 101. It also includes an external reflection and thermal management structure 141 wrapped around the upconversion self-bleaching region and the visible light backcoupling region. The upconversion self-bleaching region includes a cladding pump control grating 111 and an upconversion bleaching functional layer 112. The cladding pump control grating 111 is used to control the propagation state of near-infrared cladding ineffective light 602 transmitted in the optical fiber cladding 102, causing some of the near-infrared cladding ineffective light 602 to transform from the cladding guided mode state into leakage light, scattered light, or higher-order cladding mode light that can interact with the upconversion bleaching functional layer 112. The upconversion bleaching functional layer 112 is used to absorb the near-infrared cladding ineffective light 602 and generate visible bleached light 601 through a nonlinear upconversion process. The visible bleached light 601 is used to act on the upstream of the device. The target defect area is repaired by photo-induced color center defects and grating-written defects. A visible light backcoupling grating 121 is provided in the visible light backcoupling area. The reflection band of the visible light backcoupling grating 121 is matched with the emission band of the visible bleached light 601. It is used to reflect the visible bleached light 601 transmitted in the downstream direction back to the upstream target defect area. The visible light backcoupling grating 121 does not produce effective reflection of the near-infrared cladding ineffective light 602 and the signal laser transmitted in the fiber core. The remaining cladding light stripping area 131 is used to strip and heat dissipate the remaining near-infrared cladding ineffective light 602 that is not utilized by the upconversion bleaching functional layer 112. The external reflection and thermal management structure 141 has a high reflectivity layer 603 for visible light. It is used to reflect the outwardly radiated visible bleached light 601 back into the fiber matrix 101 to improve the utilization rate of bleached light and at the same time to dissipate the heat generated during device operation. This invention can intercept residual near-infrared pump light fields that would otherwise need to be converted into waste heat for dissipation in situ, and convert them into high-energy visible photons that can target color center defects through a nonlinear optical upconversion mechanism. This allows for the construction of a self-bleaching closed-loop link for color center defects within the system without increasing the system's additional power consumption. It enables the in-situ interception of residual cladding light at the tail end without the need for an external independent short-wavelength light source, and achieves targeted intervention of defects in core optical components through cross-band light-to-light conversion. Ultimately, without increasing the system's additional power consumption, it improves the anti-darkening capability of gain fiber and passive grating and the overall TMI threshold of the system.

[0039] Specifically, the upconversion self-bleaching region is the core functional area of ​​the self-bleaching composite grating device, primarily responsible for converting near-infrared cladding ineffective light 602 to visible bleached light 601. This region typically includes three substructures: the fiber substrate 101, the cladding pump control grating 111, and the upconversion bleaching functional layer 112. The fiber substrate 101 is the basic carrier structure of the self-bleaching composite grating device, composed of a quartz core and cladding. The core transmits the required signal laser, while the cladding carries residual pump laser, cladding leakage signal light, cladding ASE light, and other near-infrared cladding ineffective light 602. The main functions of the fiber substrate 101 are threefold: First, it provides a low-loss fiber core transmission path for the signal laser; second, it provides a cladding transmission path for the near-infrared cladding ineffective light 602 to enter the device, enabling it to be modulated by the cladding pump control grating 111 and enter the upconversion bleaching functional layer 112; third, it provides a spatial carrier and positioning reference for the cladding pump control grating 111, the visible light backcoupling grating 121, the upconversion bleaching functional layer 112, and the external reflection and thermal management structure 141. Therefore, the fiber substrate 101 is not the source of the bleaching function in this scheme, but it is the structural basis for the composite grating device to achieve optical field modulation, energy conversion, visible light backcoupling, and residual light dissipation. Its design requirement is to ensure the normal transmission of the signal laser while enabling the effective modulation and utilization of the near-infrared cladding ineffective light 602. The cladding pump modulation grating 111 is responsible for altering the propagation characteristics of near-infrared cladding ineffective light 602 within the cladding. This causes residual pump lasers to partially transform from their original cladding guided mode state into higher-order cladding modes closer to the outer cladding upconversion bleaching functional layer 112, achieving the goal of leakage or scattering into the upconversion functional layer. The upconversion bleaching functional layer 112 absorbs a portion of the near-infrared cladding ineffective light 602 and generates visible bleaching light 601. This visible bleaching light 601 radiates outwards in random directions, with a portion entering the cladding, core, outer cladding, or other visible light propagation paths. The design focus in this region is not to immediately convert all of the near-infrared cladding ineffective light 602, but rather to functionalize it, converting a portion of its energy into visible light that can be used for defect bleaching.

[0040] The visible light backcoupling region is mainly used to couple the bleaching light to the desired propagation direction, improving the utilization efficiency of the visible bleaching light 601. The visible bleaching light 601 generated by the upconversion bleaching functional layer 112 typically has a random direction and does not propagate along a single intended direction. After being constrained by the outer reflection layer, the visible bleaching light 601 enters the optical fiber and propagates axially. A portion of it naturally propagates towards the upstream target defect region, while another portion may propagate downstream, away from the area requiring bleaching. The visible light backcoupling region located on the downstream side can selectively control this downstream propagating visible bleaching light 601 through the visible light backcoupling grating 121, causing it to at least partially reflect, scatter, or couple back to the target defect region. The grating targets the visible bleaching light 601 (e.g., 532nm), not the 976nm residual pump laser or the signal laser near 1064nm. Therefore, the design principle of the visible light backcoupling grating 121 is to effectively reflect the visible bleached light 601 while not effectively reflecting the ineffective near-infrared cladding light 602 and the signal laser, thus avoiding interference with the transmission channel of the infrared optical path and preventing adverse feedback to the operation of the high-power laser. In a typical design, the visible light backcoupling grating 121 can be designed for green light near 532nm. If the upconversion bleaching functional layer 112 has multiple visible emission peaks, a chirped grating, a multi-segment cascaded grating, or a broadband visible light backcoupling structure can also be used to synchronously backcouple visible light in a certain wavelength band.

[0041] The residual cladding light stripping region 131 is located downstream of the upconversion self-bleaching region and the visible light backcoupling region. It is used to process the remaining ineffective near-infrared cladding light 602 that is not effectively utilized by the upconversion bleaching functional layer 112. The upconversion bleaching functional layer 112 cannot convert all the ineffective near-infrared cladding light 602 into visible bleached light 601. Unutilized residual pump laser, cladding leakage signal light, cladding ASE light, and other stray light still need further stripping, absorption, and heat dissipation; otherwise, they may continue to propagate to the end cap or subsequent devices, causing end-cap overheating, feedback disturbances, or reliability issues. The main function of this region is to strip this portion of the ineffective near-infrared cladding light 602, converting it into heat energy and safely dissipating it through an external heat dissipation structure. In design and engineering implementation, the residual cladding light stripping region 131 can be integrated with the self-bleaching composite grating device, or it can be implemented by a separate cladding power stripper located downstream of it. In the general architecture, it is preferred to describe it as an integrated composite device, with a separate form as an alternative.

[0042] The external reflection and thermal management structure 141 is disposed on the outside of the entire composite device, covering the upconversion bleaching functional layer 112 and the visible light backcoupling region. It may also cover the remaining cladding light stripping region 131 (this part can be independent). As the encapsulation structure of the device, it can be cooled externally by the cooling medium 604 flowing inside the external reflection and thermal management structure. The cooling medium flowing inside the external reflection and thermal management structure can be water or coolant, which serves to protect and dissipate heat. In a preferred embodiment, this structure is set as an external auxiliary functional unit of the composite device; in other embodiments, it can also be selectively set according to the power level, encapsulation conditions, and visible light utilization requirements. This structure mainly undertakes two functions. One is an optical function, that is, partially reflecting, scattering, or guiding the visible bleached light 601 radiated outward from the upconversion bleaching functional layer 112 back to the direction of the fiber matrix 101, increasing the proportion of visible bleached light 601 entering the cladding, fiber core, or target defect region. Secondly, it has a thermal management function, which involves dissipating the heat generated during the modulation and upconversion processes of near-infrared cladding ineffective light 602 and the stripping of the remaining cladding light, avoiding local heat accumulation and maintaining long-term stable operation of the system. The external reflection and thermal management structure 141 can be designed with metal or other dielectric films to target the visible bleached light 601, achieving a high reflectivity effect. Externally, the high absorption layer 605 of the external reflection and thermal management structure absorbs waste heat, which is the heat energy 606 converted from the near-infrared cladding ineffective light after absorption within the external reflection and thermal management structure. The high absorption layer 605 of the external reflection and thermal management structure for near-infrared light includes a metal water-cooled shell or a multi-layer composite encapsulation structure. For the upconversion self-bleaching region, this structure focuses on increasing the return ratio of visible bleached light 601 to the inward side and removing non-radiative heat from the upconversion material. For the remaining cladding light stripping region 131, this structure primarily undertakes the function of high-power heat dissipation. For the visible light backcoupling region, this structure can assist the visible light backcoupling grating 121 in improving the visible light utilization efficiency. Therefore, the degree of dependence on the structure can vary among the three regions along the axial direction.

[0043] In one embodiment, the upconversion self-bleaching region mainly consists of a cladding pump control grating 111 and an upconversion bleaching functional layer 112. Its basic function is to first change the propagation state of the near-infrared cladding ineffective light 602 through the cladding pump control grating 111, partially converting it from a cladding guided mode state into leakage light, scattered light, a higher-order cladding mode, or a near-field light field; then, the upconversion bleaching functional layer 112 absorbs a portion of the near-infrared light and generates visible bleached light 601 through the upconversion process. The upconversion process can be based on the multi-level transition of rare-earth ions. After near-infrared photons are absorbed by the upconversion material, electrons can gradually transition to higher excited states through excited-state absorption, energy transfer upconversion, and co-sensitization processes, subsequently radiating transitions in the form of visible light. For structures using a 976nm residual pump laser as the primary excitation source, upconversion materials can be co-doped systems such as Yb / Er, Yb / Tm, and Yb / Ho, or rare-earth-doped fluorides, oxides, silicates, tungstates, molybdates, glass ceramics, nanocrystals, or polymer composite upconversion materials. Material selection should consider near-infrared absorption, visible emission band, thermal stability, optical loss, and compatibility with fiber optic packaging structures. Typical visible emission can be in the green, blue, or red light bands, with green light near 532nm serving as a typical example. A typical upconversion principle is as follows: Figure 3 As shown.

[0044] There are various ways to implement upconversion from the bleaching region. The main differences lie in how the cladding light is modulated, how the near-infrared light enters the upconversion bleaching functional layer 112, how the upconversion material is combined with the fiber substrate 101, and how to balance mechanical strength and thermal stability under high power conditions. For example, the cladding pump control grating 111 can be a microstructure grating 201, i.e., a surface microstructure grating. The microstructure grating 201 forms periodic, quasi-periodic, or gradient-distributed microstructures on the outer surface of the fiber cladding 102, such as microgrooves, micropits, annular etching bands, spiral grooves, coarsening stripes, or micro / nano scattering units. These microstructures together constitute the cladding pump control grating 111. In double-clad fibers, the residual pump laser mainly propagates along the cladding and is limited by total internal reflection at the outer boundary of the cladding. Surface microstructures can alter the local morphology, incident angle distribution, and scattering conditions of the cladding boundary, causing a portion of the near-infrared cladding ineffective light 602 to transform from the cladding guided mode state into leakage light, scattered light, or a localized optical field near the outer surface of the cladding. An upconversion bleaching functional layer 112, positioned outside or filled within the microstructure, can absorb this portion of near-infrared light and generate visible bleached light 601. This structure can be categorized into two types: overlay and filler. In the overlay structure, after forming a microstructure grating 201 on the cladding surface, the upconversion bleaching functional layer 112 is completely overlaid on its outer side; the near-infrared cladding ineffective light 602 enters the outer upconversion layer after scattering or leakage in the microstructure region. In the filler structure, upconversion material is directly filled into microgrooves, micropits, or roughened structures, embedding the upconversion material into the cladding boundary disturbance region, thereby improving the optical field overlap between near-infrared light and the upconversion material. (See schematic diagram below.) Figure 4 As shown, the left side represents a covered structure, while the right side represents a filled structure. In the example on the right, we employed an axially progressive dense microstructure design, controlling the axial heat load distribution by manipulating the microstructure gradient. This approach is intuitive, has multiple process paths, and is easily integrated with traditional cladding light stripping technology. The design focus is on controlling the depth, density, period, and operational length of the microstructures to achieve distributed weak or moderate coupling, avoiding direct thermal heating of a large amount of cladding light over short distances. To reduce single-point heat load, the microstructure can be designed with a gradient along the axial direction; for example, the upstream structure is shallower and sparser, the middle section strengthens the coupling, and the downstream section gradually transitions to the remaining cladding light stripping heat dissipation area.

[0045] In another embodiment, the cladding pump modulation grating 111 is a femtosecond-written cladding perturbation grating 301. The femtosecond-written cladding perturbation grating 301 is located inside the fiber cladding 102, particularly near the outer cladding surface, and is created by writing periodic, quasi-periodic, spiral, or multi-point array refractive index perturbation structures using a femtosecond laser. These internal refractive index perturbation units collectively constitute the cladding pump modulation grating 111. The residual pump laser typically propagates in multimode within the cladding, with some modes having weak spatial overlap with the outer upconversion bleaching functional layer 112. The femtosecond-written cladding perturbation grating can alter the mode distribution of the cladding light through refractive index modulation, causing some cladding light to undergo mode coupling, scattering, or angular redistribution, thereby transforming it into higher-order cladding modes, leakage modes, or scattered light fields closer to the cladding boundary, increasing its interaction probability with the upconversion bleaching functional layer 112. The advantage of this structure is that the grating properties are well-defined, it does not require significant damage to the outer surface of the fiber, and it is beneficial for showcasing the structural characteristics of composite grating devices. The perturbation region is preferably located on the outer side of the cladding and as far away from the fiber core as possible to reduce additional losses to the signal laser. To accommodate high-power applications, the perturbation structure should adopt a distributed, weak perturbation, multi-point accumulation approach to avoid the formation of strong local scattering points and local heat concentrations.

[0046] In one embodiment, the cladding pump control grating 111 is a high-refractive-index segmented coupling grating 401. The high-refractive-index segmented coupling grating 401 consists of axially distributed high-refractive-index coupling units disposed on the outer surface of the fiber cladding 102 or at the interface adjacent to the cladding. These coupling units can be segmented strips, annular strips, spiral strips, lattice units, or quasi-periodic coated units, collectively constituting the equivalent cladding pump control grating 111. This structure weakens or disrupts the total internal reflection condition of the cladding light by locally altering the refractive index condition of the outer boundary of the cladding, causing a portion of the near-infrared cladding ineffective light 602 to leak into the outer functional layer region in a distributed manner. Its mechanism of action differs from that of the surface microstructure grating 201: surface microstructures mainly rely on morphological perturbation and scattering, while high-refractive-index segmented coupling mainly relies on local refractive index boundary modulation. This structure can be either a separate or integrated composite structure. In the split structure, the high-refractive-index coupling unit is responsible for modulating the cladding boundary, causing the non-effective near-infrared cladding light 602 to leak to the outer or adjacent regions; the upconversion bleaching functional layer 112 is located on its outer side or adjacent position to absorb the coupled near-infrared light. In the integrated composite structure, the high-refractive-index coupling material itself is doped with upconversion particles or upconversion luminescent components, giving it both cladding light coupling and upconversion luminescence functions. This structure is relatively simple to engineer and can be achieved through coating, dispensing, or composite material deposition. The coupling strength can be adjusted by the length, spacing, thickness, refractive index, and duty cycle of the coupling unit. The segmented design aims to allow the cladding light to leak gradually and gently to the upconversion bleaching functional layer 112, achieving distributed utilization. Under high-power conditions, the high-refractive-index material needs to possess good thermal stability, low absorption loss, and encapsulation reliability.

[0047] In one embodiment, the cladding pump modulation grating 111 is a long-period grating 501 and / or a tilted grating 502. The long-period grating 501 and the tilted grating 502, through grating structures with specific periods or tilt directions, perform mode coupling or direction modulation on the near-infrared cladding ineffective light 602. The long-period grating mainly achieves coupling between cladding modes through periodic refractive index modulation. For residual pump laser propagating along the cladding, the long-period grating 501 can couple some low-leakage cladding modes to higher-order cladding modes, boundary enhancement modes, or leakage modes, thereby increasing its spatial overlap with the outer upconversion bleaching functional layer 112. This structure is not intended to reflect the 976nm residual pump laser, but rather to primarily achieve co-directional mode coupling and optical field distribution modulation. The tilted grating 502 mainly introduces radial or oblique coupling components into the near-infrared cladding ineffective light 602. Because the grating surface is tilted relative to the fiber axis, when the cladding light passes through the tilted periodic perturbation, it can generate coupling, deflection, or scattering components towards the outside of the cladding, thus making it easier to enter the upconversion bleaching functional layer 112. The tilted grating 502 can be achieved through tilted refractive index modulation inside the cladding, tilted microstructures on the cladding surface, or tilted high-refractive-index stripes. Long-period structures are more suitable for cladding mode redistribution, while tilted structures are more suitable for directional external coupling. They can be used individually or in combination; for example, first changing the cladding mode distribution through a long-period structure, and then enhancing the external coupling of the upconversion bleaching functional layer 112 through a tilted structure. This approach is suitable for implementation schemes that emphasize the grating physical mechanism and waveguide mode manipulation.

[0048] In one embodiment, the visible light generated by the upconversion bleaching functional layer 112 typically has multiple propagation directions, a portion of which propagates downstream and away from the target defect region. The visible light backcoupling region, located downstream of the upconversion bleaching region, redirects this portion of the visible bleached light 601 to the upstream target defect region through selective reflection, scattering, or coupling structures. The design focus of this region is to effectively affect only the visible bleached light 601, minimizing reflection of the 976nm residual pump laser and the signal laser near 1064nm, thus avoiding adverse feedback to the near-infrared high-power optical path. In some embodiments, a femtosecond laser can preferably be used to inscribe a cladding Bragg reflection grating in the cladding. By forming periodic refractive index modulation inside or near the outer region of the fiber cladding 102, its reflection band is matched to the emission band of the visible bleached light 601. For the visible bleached light 601 near 532nm, a corresponding Bragg reflection period can be designed according to the Bragg reflection condition, causing the visible bleached light 601 propagating downstream to be reflected in this region and propagate upstream towards the target defect region. The core of this structure is wavelength selectivity. The grating period is primarily designed for visible bleached light 601, therefore it does not satisfy the effective Bragg reflection condition for the 976nm residual pump laser and the signal laser near 1064nm. This improves the utilization rate of visible bleached light 601 while avoiding significant feedback to the near-infrared main optical path and the residual pump path. Current femtosecond laser technology is relatively mature and can achieve high-precision writing of cladding Bragg reflection gratings, making it suitable as a form of implementation for the visible light backcoupling region.

[0049] The remaining cladding light stripping region 131 is located downstream of the upconversion self-bleaching region and the visible light backcoupling region. It is used to process the near-infrared cladding ineffective light 602 that is not utilized by the upconversion bleaching functional layer 112. The function of this region is to prevent residual pump laser, cladding leakage signal light, and cladding ASE light from continuing to propagate to the end cap or subsequent devices, and to safely strip, absorb, and dissipate them. This region can be processed using conventional CPS fabrication methods, and a gradient stripping heat dissipation design is preferred. This design gradually enhances the cladding light stripping strength and heat dissipation capacity axially, resulting in low single-point thermal load and preventing concentrated heating of residual cladding light over short distances, achieving a smooth transition for safe dissipation. The implementation method here mainly refers to its integration method and structural design with the self-bleaching composite grating device. Depending on the degree of integration, there are two main implementation forms: integrated stripping heat dissipation and independent stripping heat dissipation.

[0050] In the integrated stripping and heat dissipation method, the stripping and heat dissipation area is integrated with the self-bleaching composite grating device within the same fiber device and packaging structure. The front end completes up-conversion self-bleaching and visible light recoupling, while the back end continues to perform strong stripping and heat dissipation on the remaining ineffective near-infrared cladding light 602. Its advantages include compact structure, continuous optical path, and unified packaging, making it suitable for constructing complete composite cladding optical processing devices. (See [link to relevant documentation]). Figure 1 The diagram shows a general structural design.

[0051] In the independent heat dissipation method, a CPS (Continuous Processing System) is added downstream of the self-bleached composite grating device as a post-processing unit for the remaining cladding light. This method is suitable for retrofitting existing high-power fiber laser systems, as it does not require integrating all functions into a single device. Its advantages include simple engineering implementation, low risk, and the ability to design heat dissipation independently. Its disadvantages include an increased number of devices, resulting in a corresponding increase in system length and packaging complexity. Figure 8 As shown.

[0052] An external reflection and thermal management structure 141 is disposed outside the upconversion bleaching functional layer 112, the visible light backcoupling region, or the remaining cladding light stripping region 131. This structure increases the proportion of visible bleached light 601 returning to the fiber substrate 101 and dissipates heat generated during device operation. This structure can continuously cover the entire self-bleaching composite grating device or be disposed only in localized areas with high upconversion light intensity, high cladding light stripping intensity, or high thermal load. Inside this structure, a high reflectivity design is provided for the visible bleached light 601, while a high absorption design is provided for the near-infrared cladding ineffective light 602, to achieve effective backcoupling of the visible bleached light 601 and safe dissipation of the near-infrared cladding ineffective light 602. For the external reflection and thermal management structure, the high-reflectivity layer 603 for visible light can be designed using metal or dielectric films. Aluminum, copper, silver, gold, or other high-reflectivity, high-thermal-conductivity materials can be used to form the outer reflection structure. The dielectric reflection layer can employ multi-layer dielectric films or alternating high and low refractive index structures to achieve high reflection in the visible bleached light 601 band, while minimizing significant feedback to the 976nm residual pump laser and signal lasers near 1064nm. The thermal management layer can use metal heat sinks, ceramic heat dissipation layers, or water-cooled structures. The thermally conductive encapsulation and cooling structure can employ thermally conductive adhesive, ceramic thermally conductive layers, metal heat sinks, water-cooled shells, or air-cooled structures. Its main function is to dissipate heat generated by the non-radiative loss of the upconversion material, cladding light modulation loss, and residual cladding light stripping, preventing localized temperature rises that could lead to decreased upconversion efficiency, material aging, or fiber encapsulation failure. A multi-layer composite encapsulation structure can also be used, combining the visible light reflection layer, thermally conductive layer, mechanical protection layer, and cooling shell together. For example, the inner layer is used to reflect visible bleached light 601, the middle layer is used for thermal conductivity and electrical insulation, and the outer layer is used for mechanical protection and cooling connections. This approach is suitable for high-power systems and helps to simultaneously meet the requirements of visible light utilization, thermal management, and long-term reliability.

[0053] The second objective of this invention is to provide a high-power fiber laser system comprising a self-bleaching composite grating device. The self-bleaching composite grating device is positioned on the near-infrared cladding ineffective light 602 transmission path of the system and is located downstream of a target defect region. The target defect region includes the low-reflection Bragg grating region, the ytterbium-doped active fiber tail, the amplifier gain fiber output end, the fiber fusion splice region, or the output cap front end region. In the overall architecture design, the self-bleaching composite grating device is arranged on the near-infrared cladding ineffective light 602 transmission path of the high-power fiber laser system. Based on the signal laser transmission direction, this device is typically located downstream of the low-reflection grating, at the active fiber tail, at the amplifier gain fiber output end, at the end cap front end, or near the location of a conventional cladding power stripper. This device targets not the signal laser normally transmitted in the fiber core, but rather the near-infrared cladding ineffective light 602 propagating along the cladding. Functionally, this self-bleaching composite grating device is a composite functional device integrating near-infrared cladding light modulation, upconversion emission, visible bleaching light 601 backcoupling, residual cladding light stripping, and thermal management. Its general architecture includes several core functional modules: fiber substrate 101, cladding pump modulation grating 111, upconversion bleaching functional layer 112, visible light backcoupling grating 121, residual cladding light stripping region 131, and external reflection and thermal management structure 141. The cladding pump control grating 111 and the visible light backcoupling grating 121 together constitute the features of the composite grating, mainly implemented using femtosecond grating technology; the upconversion bleaching functional layer 112 is the core of energy conversion; the remaining cladding light stripping region 131 is used to ensure that the unused near-infrared cladding ineffective light 602 is ultimately safely processed; the external reflection and thermal management structure 141 is used to reflect the randomly oriented visible bleached light 601 back into the optical fiber for transmission, and to remove the heat generated during the upconversion process and the remaining pump light stripping process through an external water-cooling structure, thereby improving the energy utilization efficiency and thermal stability of the entire device. Along the fiber axis, the self-bleaching composite grating device is designed as follows: Figure 1 As shown. Functionally, the self-bleaching composite grating device can be divided into three functional regions: an upconversion self-bleaching region, a visible light backcoupling region, and a residual cladding light stripping region, as well as an external reflection and thermal management region. The target defect region to be bleached is located upstream of the device.

[0054] The visible bleaching light 601 in the self-bleaching composite grating device originates from the near-infrared cladding ineffective light 602 generated by the system itself, especially the residual pump laser. Therefore, when the system operating power increases, or the power of the residual pump laser or other near-infrared cladding ineffective light 602 increases, the excitation energy obtained by the upconversion bleaching functional layer 112 also increases accordingly. Within a certain operating range, the output of the visible bleaching light 601 will increase with the increase of the near-infrared cladding ineffective light 602 power, thus forming a passive adaptive bleaching mechanism. The higher the laser operating power, the stronger the residual pump laser, and the stronger the generated visible bleaching light 601, the stronger the response to the bleaching demand in the high-power region. This adaptive characteristic allows the scheme to automatically adjust the bleaching capability under different power levels and operating conditions without external control or adjustment, exhibiting good applicability and robustness.

[0055] The present invention also provides a method for operating a high-power fiber laser system, comprising the following steps: During the operation of a high-power fiber laser system, near-infrared ineffective light propagates along the cladding and enters the self-bleaching composite grating device. The residual pump laser is typically the primary energy source, while other near-infrared cladding ineffective light 602 serves as an auxiliary energy source or is processed synchronously. After entering the device, the near-infrared cladding ineffective light 602 does not directly enter the stripping heat dissipation area; instead, it first passes through the cladding pump modulation grating 111. This grating, through mode coupling, boundary perturbation, surface scattering, or local refractive index modulation, transfers a portion of the near-infrared cladding ineffective light 602 from its original cladding guided mode state to a region closer to the upconversion bleaching functional layer 112. Therefore, the near-infrared energy entering the device is divided into two parts: one part is introduced into the upconversion bleaching functional layer 112 to participate in visible light generation, while the other part continues to propagate downstream, awaiting subsequent stripping and heat dissipation. The near-infrared energy entering the upconversion bleaching functional layer 112 is absorbed by the upconversion material and converted into visible bleached light 601 through excited-state absorption, energy transfer upconversion, or co-sensitization processes. The visible bleaching light 601 is typically located in the 400nm to 700nm wavelength range, with a typical example being green light around 532nm. This step completes the energy property transformation: the near-infrared ineffective light originally belonging to the cladding is partially converted into short-wavelength visible light with defect bleaching capabilities. The visible bleaching light 601 generated by the upconversion propagates in multiple directions. The portion propagating towards the target defect region can directly participate in bleaching; the portion propagating downstream is partially reflected, scattered, or coupled to the target defect region through the visible light backcoupling grating 121, the external reflection structure, or other backcoupling structures. After the visible bleaching light 601 acts on the target defect region, it is beneficial for the repair of color center defects, photodarkening centers, oxygen vacancy-related defects, or grating writing-induced defects, helping to restore the optical performance of the region, reduce optical loss and thermal load, and help restore the system power carrying capacity and TMI threshold. The remaining ineffective near-infrared cladding light 602, not utilized by the upconversion bleaching functional layer 112, continues to propagate downstream and enters the remaining cladding light stripping and heat dissipation area where it is stripped and thermally converted, preventing it from continuing to propagate to the end cap or subsequent devices. Therefore, this solution does not completely eliminate traditional cladding power stripping, but adds an energy functionalization step before stripping, such as... Figure 10 As shown.

[0056] The self-bleaching composite grating device is essentially a functional cladding light processing device embedded in a high-power fiber laser system. It does not replace the basic structure of the laser, but is positioned where near-infrared cladding ineffective light 602 is likely to occur and is close to the target defect region. Its system function is to first utilize a portion of the residual pump laser and other cladding ineffective light to generate visible bleaching light 601, and then strip and dissipate the unused remaining cladding light. Several typical scenarios are described below.

[0057] Example 1 The application scenario in this embodiment is a high-power all-fiber oscillator, such as... Figure 11 The diagram shows the optical structure of a high-power all-fiber oscillator, which includes a forward-facing first pump source 801 connected to a forward-facing first pump signal combiner 802. The first pump signal combiner 802 is sequentially connected to a high-reflectivity fiber grating 803, an oscillator gain fiber 804, a low-reflection Bragg grating 805, a reverse-facing second pump signal combiner 806, a self-bleaching composite grating device 808, and an oscillator output cap 809. The reverse-facing second pump signal combiner 806 is connected to a reverse-facing second pump source 807. In this implementation, the self-bleaching composite grating device 808 is located at the output end of the fiber optic oscillator cavity, replacing the original cladding power stripper (CPS) or located upstream and adjacent to it. When the oscillator is running, the 976nm pump light propagates along the cladding and is absorbed by the oscillator gain fiber 804. Part of it is converted into a signal laser near 1064nm and output along the fiber core. At the same time, color center defects or grating-related defects may be formed in the tail region of the YDF and near the low-reflection grating due to fabrication defects, long-term high-power irradiation and local heat accumulation.

[0058] Residual pump laser light and other near-infrared cladding ineffective light 602 propagating in the cladding continue to enter the self-bleaching composite grating device. The device converts a portion of the near-infrared energy into visible bleached light 601 through the cladding pump modulation grating 111 and the upconversion bleaching functional layer 112. This visible bleached light 601 then passes through the visible light backcoupling grating 121 and the external reflection and thermal management structure 141, preferentially acting on the low-reflection grating region, the YDF tail end, and adjacent high-defect-sensitive regions. Unused remaining cladding light is safely processed in the cladding power stripping heat dissipation structure downstream of the device. This allows for near-in-situ bleaching of the tail-end failure-prone region while preventing residual cladding light from continuing to enter the end cap.

[0059] The advantage of this arrangement is that the bleaching light originates from the residual cladding light that already exists at the end of the system, without the need for additional external power supply; at the same time, the higher the system output power, the stronger the residual near-infrared cladding ineffective light 602 at the end is usually, and the visible bleaching light 601 that can be excited is also enhanced accordingly, thus forming an adaptive response to the bleaching demand in the high-power region.

[0060] Example 2 The application scenario in this embodiment is an all-fiber amplifier, such as... Figure 12The diagram shows the optical structure of an all-fiber amplifier. In the fiber amplifier, the seed source 901 provides a stable output laser signal, typically located around 1064 nm. This signal passes through a pre-amplifier 902, a pre-amplifier cladding power stripper 903, and a mode field adapter 904 before entering the amplifier gain fiber 907 for power amplification, and is finally output through the amplifier output cap 910. One end of the amplifier gain fiber 907 has a forward-facing amplifier first pump source 905 and a forward-facing amplifier first pump signal combiner 906, while the other end has a reverse-facing amplifier second pump signal combiner 908 and a reverse-facing amplifier second pump source 909. Unlike oscillator scenarios, amplifiers typically do not have resonant cavity fiber gratings; therefore, the target defect areas are mainly concentrated at the output end of the gain fiber, the splice between the gain fiber and the passive fiber, the transmission fiber before the end cap, and locations with high cladding power. The preamplifier cladding power stripper here is mainly used to handle residual pump laser and ineffective cladding light in the preamplifier stage, protecting the subsequent gain fiber from excessive cladding light load; while the self-bleaching composite grating device is mainly used to bleach defects at the output end of the gain fiber and its adjacent area, while also handling residual pump laser and ineffective cladding light generated in the gain stage.

[0061] Self-bleaching composite grating devices can be positioned after the output end of the gain fiber and before the end cap to handle residual pump laser, cladding leakage signal light, and cladding ASE light generated during amplification. Their design and operating mechanism are the same as in the oscillator scenario: first, a portion of the near-infrared cladding ineffective light 602 is converted into visible bleached light 601 that acts in the opposite direction on the target region; then, the unused remaining cladding light is guided into the cladding power stripping heat dissipation structure. Since amplifiers typically lack a low-reflection grating region, the bleaching target in this scenario is primarily the end of the gain fiber and its adjacent high-heat-load region. This arrangement is suitable for forward-pumped, reverse-pumped, and bidirectional pumping structures.

[0062] Furthermore, for high-power or long-gain fiber systems, there may be more than one target defect region. For example, defect accumulation and cladding ineffective light risks may exist at the YDF output end, near low-reflection gratings, amplifier output fusion splices, and the front end of end caps. Under specific pump structures, local defect distributions, or cladding light redistribution conditions, the area near the YDF input end can also be considered a target defect region. In this case, multiple self-bleaching composite grating devices can be set up in the system to form a distributed self-bleaching structure. With multiple points of arrangement, each device only needs to process the near-infrared cladding ineffective light 602 in its neighboring area and bleach the nearby target defect region. This can reduce the thermal load and conversion stress of individual devices and also help improve the reliability of long-link systems.

[0063] Specific examples have been used to illustrate the principles and implementation methods of this invention. The descriptions of the above embodiments are only for the purpose of helping to understand the method and core ideas of this invention. Furthermore, those skilled in the art will recognize that, based on the ideas of this invention, 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 invention.

Claims

1. A self-bleaching composite grating device, characterized in that, include: The optical fiber matrix, external reflection and thermal management structure, and an upconversion self-bleaching region, a visible light backcoupling region and a remaining cladding light stripping region arranged sequentially along the axial direction of the optical fiber matrix, wherein the external reflection and thermal management structure can be wrapped around the outside of the upconversion self-bleaching region and the visible light backcoupling region. The upconversion self-bleaching region includes a cladding pump control grating and an upconversion bleaching functional layer. The cladding pump control grating is used to control the propagation state of near-infrared cladding ineffective light transmitted in the fiber cladding, so that some of the near-infrared cladding ineffective light is converted from the cladding guided mode state into leakage light, scattered light, or higher-order cladding mode light that can interact with the upconversion bleaching functional layer. The upconversion bleaching functional layer is used to absorb the near-infrared cladding ineffective light and generate visible bleached light through a nonlinear upconversion process. The visible bleached light is used to act on the target defect region upstream of the device to repair photo-induced color center defects and grating writing-induced defects. A visible light backcoupling grating is provided in the visible light backcoupling region. The reflection band of the visible light backcoupling grating is matched with the emission band of the visible bleaching light. It is used to reflect the visible bleaching light transmitted in the downstream direction back to the upstream target defect area. The visible light backcoupling grating does not produce effective reflection of the ineffective light of the near-infrared cladding and the signal laser transmitted in the fiber core. The remaining cladding light stripping region is used to strip and thermally dissipate the remaining ineffective near-infrared cladding light that is not utilized by the upconversion bleaching functional layer. The external reflection and thermal management structure is used to reflect the outwardly radiated visible bleached light back into the fiber matrix to improve the utilization rate of bleached light, while dissipating the heat generated during device operation.

2. The self-bleaching composite grating device according to claim 1, characterized in that, The cladding pump control grating is a surface microstructure grating, which is a periodic, quasi-periodic, or axially gradient distributed microstructure formed on the outer surface of the fiber cladding. The microstructure includes microgrooves, micropits, annular etching bands, spiral grooves, coarsening stripes, or micro / nano scattering units. The upconversion bleaching functional layer covers the outside of the surface microstructure grating or fills the interior of the microstructure.

3. The self-bleaching composite grating device according to claim 1, characterized in that, The cladding pump control grating is a femtosecond-written cladding perturbation grating, which is a periodic, quasi-periodic, spiral, or multi-point array refractive index perturbation structure written by a femtosecond laser in the outer region inside the fiber cladding.

4. The self-bleaching composite grating device according to claim 1, characterized in that, The cladding pump control grating is a high-refractive-index segmented coupling grating, comprising high-refractive-index coupling units distributed along the fiber axis. The high-refractive-index coupling units are segmented strips, annular strips, spiral strips, lattice units, or quasi-periodic coating units. The upconversion bleaching functional layer is disposed outside the high-refractive-index coupling units, or the high-refractive-index coupling units themselves are doped with upconversion luminescent components, thereby simultaneously realizing cladding optical coupling and upconversion luminescence functions.

5. The self-bleaching composite grating device according to claim 1, characterized in that, The cladding pump control grating is a long-period grating and / or a tilted grating; the long-period grating is used to realize coupling between cladding modes, converting low-leakage cladding modes into high-order cladding modes or leakage modes; the tilted grating is used to couple and deflect near-infrared cladding ineffective light to the outside of the cladding.

6. The self-bleaching composite grating device according to claim 1, characterized in that, The visible light backcoupling grating is a cladding Bragg reflection grating written with a femtosecond laser; the visible light backcoupling grating is a chirped grating, a multi-segment cascaded grating, or a broadband visible light backcoupling structure to match the upconversion bleaching functional layer with multiple emission peaks.

7. The self-bleaching composite grating device according to claim 1, characterized in that, The remaining cladding optical stripping region is integrated with the self-bleaching composite grating device and adopts an axial gradient stripping heat dissipation structure; or, the remaining cladding optical stripping region is a cladding power stripper independently set downstream of the visible light backcoupling region.

8. The self-bleaching composite grating device according to claim 1, characterized in that, The external reflection and thermal management structure includes an inner visible light high reflectivity layer, an intermediate thermally conductive insulating layer, and an outer cooling structure; the visible light high reflectivity layer is a metal reflectivity layer or a multilayer dielectric film reflectivity layer, and the outer cooling structure is a water-cooled shell, a metal heat sink, or an air-cooled heat dissipation structure.

9. A high-power fiber laser system, characterized in that, The system includes a self-bleaching composite grating device as described in any one of claims 1-8; the self-bleaching composite grating device is disposed on the ineffective optical transmission path of the near-infrared cladding of the system and is located downstream of the target defect region; the target defect region includes a low-reflection Bragg grating region, a ytterbium-doped active fiber tail, an amplifier gain fiber output end, a fiber fusion splice region, or an output cap front end region.

10. A method for operating the high-power fiber laser system as described in claim 9, characterized in that, Includes the following steps: S1. When the high-power fiber laser system is operating normally, the near-infrared signal laser is transmitted in the fiber core. The residual pump light, cladding leakage signal light, and cladding ASE light that are not completely absorbed by the gain medium together constitute the near-infrared cladding ineffective light, which is transmitted along the fiber cladding to the self-bleaching composite grating device. S2. The near-infrared cladding ineffective light enters the upconversion self-bleaching region, and the propagation mode is changed by the cladding pump control grating. Some of the near-infrared cladding ineffective light is coupled to the upconversion bleaching functional layer. S3. The upconversion bleaching functional layer utilizes the absorbed near-infrared cladding ineffective light to undergo nonlinear upconversion and generate visible bleached light. The visible bleached light radiated outward is reflected back into the optical fiber by external reflection and thermal management structure. Part of it propagates directly upstream to the target defect region to repair the photoinduced defect, and the other part propagates downstream along the optical fiber. S4. Visible bleaching light propagating downstream enters the visible light backcoupling region and is selectively reflected by the visible light backcoupling grating, and is recoupled back to the upstream target defect region to continuously bleach and repair color center defects, oxygen vacancy defects, and grating writing-induced defects. S5. The remaining ineffective light from the near-infrared cladding layer that is not utilized by the upconversion bleaching functional layer continues to be transmitted to the remaining cladding light stripping area, where it is stripped and converted into heat energy, which is then dissipated by the external reflection and thermal management structure.

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