A multi-stage spindling high-order mode filter and a fiber laser

Abstract

The application discloses a multi-stage spindle high-order mode filter and a fiber laser, wherein the multi-stage spindle high-order mode filter comprises an input interface, a cascaded spindle filter structure and an output interface which are sequentially connected; the cascaded spindle filter structure comprises a plurality of spindle units which are cascaded along an optical transmission direction; each spindle unit comprises a taper-in section, a taper-waist section and a taper-out section which are sequentially connected along the optical transmission direction; and the taper-waist section gradually shrinks from the taper-in section and the taper-out section to the middle. The application realizes the cumulative attenuation of high-order modes through the plurality of cascaded spindle units, improves the quality of a laser beam, and is suitable for a high-power narrow-line-width fiber laser system.

Description

Technical Field

[0001] This application relates to the field of fiber optic technology, and more specifically, to a multi-stage spindle high-order mode filter and a fiber laser. Background Technology

[0002] In high-power fiber lasers, the laser output from the pre-amplification link needs to be further boosted in power before entering the main amplification link. However, the pre-amplification link retains higher-order modes (such as LP modes). 11 LP 21 This will be continuously amplified during the main amplification, leading to a decrease in the mode instability (TMI) threshold and beam quality (M). 2 The value deteriorates, severely affecting the performance of fiber lasers.

[0003] Application No. 200810079746.0 discloses a tapered microstructure fiber high-order mode filter, which uses a tapered microstructure fiber high-order mode filter to filter out high-order modes. However, its drawback is that the high-order mode suppression of the tapered microstructure fiber high-order mode filter is incomplete and cannot meet the stringent requirements of the main amplification for low high-order mode residue.

[0004] Therefore, existing technologies need to be improved. Utility Model Content

[0005] The purpose of this application is to provide a multi-stage spindle high-order mode filter and a fiber laser, aiming to solve the technical problem in the prior art of how to further filter out high-order modes to improve the quality of the laser beam.

[0006] To achieve the above objectives, the technical solution adopted in this application is as follows:

[0007] This application provides a multi-stage spindle high-order mode filter, which includes: an input interface, a cascaded spindle filter structure and an output interface connected in sequence;

[0008] The cascaded spindle filter structure includes multiple spindle units cascaded along the optical transmission direction. Each spindle unit includes a cone entry section, a cone waist section, and a cone exit section connected sequentially along the optical transmission direction. The cone waist section gradually narrows from the cone entry section and the cone exit section toward the middle.

[0009] In one embodiment, the cascaded spindle filter structure is provided with 2-5 cascaded spindle units along the optical transmission direction.

[0010] In one embodiment, the waist diameter of the conical waist segment is 90–100 μm.

[0011] In one embodiment, the input interface includes an FC / APC polarization maintainer connector, which is connected to the cascaded spindle filter structure.

[0012] In one embodiment, the output interface includes an FC / APC polarization maintainer connector, which is connected to the cascaded spindle filter structure.

[0013] In one embodiment, the cascaded spindle filter structure is connected to the output of the pre-amplification link via the input interface, and the cascaded spindle filter structure is connected to the input of the main amplification link via the output interface.

[0014] In one implementation, it further includes:

[0015] A higher-order mode absorption layer is wrapped around the periphery of the cascaded spindle filter structure.

[0016] A heat dissipation encapsulation layer is disposed around the higher-order mold absorption layer.

[0017] In one embodiment, the higher-order modulus absorption layer includes a curing adhesive that wraps around the periphery of the cascaded spindle filter structure.

[0018] In one embodiment, the heat dissipation encapsulation layer includes:

[0019] A metal sleeve, which is sleeved around the periphery of the higher-order mode absorption layer;

[0020] Thermally conductive potting compound, which is wrapped around the outer periphery of the metal sleeve.

[0021] Furthermore, based on the aforementioned multi-stage spindle high-order mode filter, this application also provides a fiber laser, which includes the multi-stage spindle high-order mode filter described above.

[0022] The beneficial effects of the multi-stage spindle high-order mode filter and fiber laser provided in this application are at least as follows:

[0023] This application discloses a multi-stage spindle high-order mode filter and a fiber laser. The multi-stage spindle high-order mode filter includes: an input interface, a cascaded spindle filter structure, and an output interface connected in sequence. The cascaded spindle filter structure includes multiple spindle units cascaded along the optical transmission direction. Each spindle unit includes a cone-entry section, a cone-waist section, and a cone-exit section connected in sequence along the optical transmission direction. The cone-waist section gradually narrows from the cone-entry section and the cone-exit section towards the center. This application achieves cumulative attenuation of high-order modes through multiple cascaded spindle units, improving laser beam quality and is suitable for high-power, narrow-linewidth fiber laser systems. Attached Figure Description

[0024] To more clearly illustrate the technical solutions in the embodiments of this application, the drawings used in the description of the embodiments or the prior art 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.

[0025] Figure 1 This is a schematic diagram of the structure of a multi-stage spindle high-order mode filter provided in an embodiment of this application;

[0026] Figure 2 This is a schematic diagram of the cascaded spindle filter structure provided in the embodiments of this application;

[0027] Figure 3 This is a schematic diagram of the structure of the spindle unit provided in the embodiments of this application;

[0028] Figure 4 A schematic diagram illustrating the fabrication of the cascaded spindle filter structure provided in the embodiments of this application;

[0029] Figure 5 A schematic diagram of a specific embodiment of the multi-stage spindle high-order mode filter provided in this application;

[0030] Figure 6 This is a schematic diagram illustrating the application scenario of the multi-stage spindle high-order mode filter provided in the embodiments of this application.

[0031] The following are the labeling elements in the figure:

[0032] 100. Cascaded spindle filter structure; 200. High-order mode absorption layer; 300. Heat dissipation encapsulation layer; 400. Input interface; 500. Output interface; 600. Pre-amplification link output terminal; 700. Main amplification link input terminal; 110. Fiber core; 120. Cladding; 130. Spindle unit; 131. Cone entry section; 132. Cone waist section; 133. Cone exit section. Detailed Implementation

[0033] To make the technical problems, technical solutions, and beneficial effects to be solved by this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and are not intended to limit the scope of this application.

[0034] It should be noted that when a component is referred to as "fixed to" or "set on" another component, it may be directly or indirectly located on that other component. When a component is referred to as "connected to" another component, it may be directly or indirectly connected to that other component. The terms "upper," "lower," "left," "right," "front," "rear," "vertical," "horizontal," "top," "bottom," "inner," and "outer," etc., indicate orientations or positions based on the accompanying drawings, and are for ease of description only, and should not be construed as limiting the technical solution. The terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features. "A plurality" means two or more, unless otherwise explicitly defined.

[0035] To facilitate understanding of the embodiments of the present invention, several elements that will be introduced in the description of the embodiments of the present invention will be introduced first.

[0036] Fundamental Mode: The lowest-order mode within the resonant cavity, exhibiting a single-peak Gaussian field distribution. Almost all energy is concentrated at the center of the beam spot, with no dark fringes and a beam quality factor M. 2 ≈1(ideal fundamental mode M) 2 =1), which means the beam can be focused to the smallest size, making it the preferred mode for high-precision applications (such as laser cutting and precision marking).

[0037] Higher-order modes: Modes with orders higher than the fundamental mode, exhibiting more complex field distributions, such as multi-peak, toroidal, or dark-fringe patterns, with energy dispersed over a larger area. Their M... 2 >1. Poor beam focusing ability may lead to unstable output power, and it is only useful in specific scenarios (such as large-area lighting).

[0038] Transverse mode instability (TMI) is a nonlinear phenomenon unique to high-power fiber lasers (especially double-clad fiber amplifiers). When the output power of the fiber laser is below a certain threshold (called the "TMI threshold"), the laser can stably output the fundamental mode (or lower-order modes), and the beam quality factor M... 2 ≈1.1~1.2; Once the power exceeds the TMI threshold, M 2 The value jumps from 1.2 to over 2 within milliseconds. The physical mechanism is: quantum defect thermal deposition → refractive index grating (period ≈ fiber diameter) → energy periodic oscillation between the fundamental mode and higher-order modes (thermodynamic coupling positive feedback).

[0039] Trigger threshold:

[0040]

[0041] Where κ: thermal conductivity, n²: nonlinear coefficient, dn / dT: thermo-optic coefficient, α: absorption. When power > threshold, TMI increases exponentially, and fundamental mode dominance collapses. M 2 The value rapidly deteriorated from 1.1 to >2, and the laser focusing ability plummeted by 90%.

[0042] Stimulated Brillouin Scattering (SBS) is a stimulated scattering process of light-phonon-light. When a high-intensity laser (called the "pump light") propagates in a medium (such as the silica core of an optical fiber), it excites acoustic vibrations within the medium (forming "acoustic phonons") through the electrostriction effect (an electric field causes periodic density changes in the medium). These acoustic phonons then scatter the pump light in turn, producing a beam of scattered light with a lower frequency and a propagation direction usually opposite to that of the pump light (called "Stokes light"). Its physical mechanism involves the backscattering of narrow-linewidth light by acoustophotons (the narrower the linewidth, the higher the SBS gain). The SBS threshold is inversely proportional to the mode area. To suppress SBS, large-mode-area (LMA) fibers are used, but this leads to the TMI trap. The contradiction lies in the fact that increasing power requires increasing Aeff (reducing power density), but increasing Aeff leads to an increase in V-parameters, increasing the risk of high-order mode surge.

[0043] The FC / APC polarization-maintaining connector is a fiber optic connector that combines the FC interface structure with the APC end-face polishing process. The FC / APC polarization-maintaining connector includes an FC interface, an APC end-face, and polarization-maintaining fiber. The FC interface (Ferrule Connector) uses a circular thread locking structure, achieving a physical connection by rotating a nut. It offers advantages such as a stable connection, strong shock resistance, and suitability for outdoor or vibrating environments, such as communication base stations and fiber optic sensor networks. The APC end-face, short for "Angled Physical Contact," is polished at an 8-degree bevel, deflecting reflected light away from the incident path and significantly reducing Fresnel reflection to prevent reflected light from interfering with signal transmission. The polarization-maintaining fiber, through special design (such as panda fiber), maintains the stable polarization direction of the optical signal, with a polarization dependent loss (PDL) ≤0.1dB and a polarization axis alignment accuracy ≤0.5°.

[0044] DC10 / 130 double-clad polarization-maintaining fiber, where DC stands for double cladding, and the numbers following it represent the core diameter (x, in μm) and the inner cladding diameter (y, in μm), respectively. The core diameter is 10 μm and the inner cladding diameter is 130 μm. Polarization-maintaining means that the fiber has the ability to maintain the polarization state.

[0045] DC10 / 130 double-clad polarization-maintaining fiber has a core diameter of 20μm and an inner cladding diameter of 400μm. Its large diameter allows for coupling of higher-power pump light, making it suitable for the main amplification stage and increasing the output power to the W level or above.

[0046] Mode purity management is a crucial research area in high-power, narrow-linewidth lasers. For kilowatt-level high-power, narrow-linewidth fiber lasers, rigorous optimization of mode purity is essential to suppress higher-order modes while preserving the fundamental mode. The goal is to achieve near-diffraction-limited beam quality to facilitate coherent combining and increase the energy transfer distance.

[0047] In existing technologies, the conventional method of combining double-clad single-mode fiber with bending and winding still cannot completely eliminate higher-order modes. New technical approaches need to be considered. Managing mode purity in high-power, narrow-linewidth lasers requires attention to two key areas. The first is the pre-amplification stage, where DC 10 / 130 double-clad polarization-maintaining fiber is used, and the average power at this stage before injection into the main amplification is typically in the tens of watts range. The second is the main amplification stage, where DC 20 / 400 double-clad polarization-maintaining fiber is typically used, achieving output power exceeding 4000W while maintaining an extremely narrow linewidth. However, after the main amplification stage, due to the increased fiber core size, beam quality is affected by the TMI effect and stimulated Brillouin scattering at high power, making effective control difficult. Therefore, this application addresses the management of mode purity at the first key area to suppress higher-order modes while retaining the fundamental mode.

[0048] Example 1:

[0049] Please see Figure 1 This embodiment provides a multi-stage spindle high-order mode filter, wherein the multi-stage spindle high-order mode filter includes: an input interface 400, a cascaded spindle filter structure 100 and an output interface 500 connected in sequence.

[0050] Specifically, the input interface 400 is connected to the input end of the cascaded spindle filter structure 100, and the output interface 500 is connected to the output end of the cascaded spindle filter structure 100. The cascaded spindle filter structure 100 serves as a filtering unit to achieve cumulative attenuation of higher-order modes and improve the quality of the laser beam.

[0051] Please see Figure 2 , Figure 2 A schematic diagram of a cascaded spindle filter structure 100 is shown, which includes multiple spindle units 130 cascaded along the optical transmission direction.

[0052] Please see Figure 3 , Figure 3A schematic diagram of the structure of the spindle unit is shown. Each spindle unit 130 includes a cone entry section 131, a cone waist section 132 and a cone exit section 133 connected sequentially along the light transmission direction. The cone waist section 132 gradually tapers from the cone entry section 131 and the cone exit section 133 toward the middle.

[0053] In this embodiment, the cascaded spindle filter structure 100 has multiple cascaded spindle units 130. The spindle units 130 gradually taper from both ends to the middle. That is, the spindle unit 130 includes a cone entry section 131, a cone waist section 132, and a cone exit section 133. The cone entry section 131 and the cone exit section 133 are located at both ends of the cone waist section 132, and the cone waist section 132 tapers from the cone entry section 131 and the cone exit section 133 towards the middle.

[0054] Please see Figure 4 , Figure 4 A schematic diagram of the fabrication of a cascaded spindle filter structure is shown. The optical fiber includes a core 110 and a cladding 120 covering the core 110. Because the fundamental mode's field distribution is concentrated at the center of the core 110, it is somewhat restricted on the spindle unit 130, but still maintains a certain level of confinement and throughput. Higher-order modes, due to their more dispersed field distribution and closer proximity to the cladding 120, experience significantly greater scattering and radiation losses within the reduced core 110.

[0055] For example, a Vytran fusion splicer can be used to process DC10 / 130 double-clad polarization-maintaining fiber to form multiple spindle units 130. If a DC10 / 130 double-clad polarization-maintaining fiber with a core diameter of 10 μm and an inner cladding diameter of 130 μm is selected, a Vytran fusion splicer can be used to achieve a variable diameter structure of the spindle unit 130 through three steps: heating and softening, precise stretching, and conformal cooling. The processing of DC10 / 130 double-clad polarization-maintaining fiber using a Vytran fusion splicer can be considered as existing technology, and its specific steps will not be elaborated here.

[0056] In this embodiment, the laser (including the fundamental mode and residual higher-order modes) enters the cascaded spindle filter structure 100 through the input interface 400. The cone entry section 131 of each spindle unit 130 smoothly transitions the laser field to the cone waist section 132. The contraction structure of the cone waist section 132 causes the distribution of higher-order mode fields to exceed the constraint range of the fiber core 110. Because the field distribution of the fundamental mode is concentrated, it can be stably transmitted in the cone waist section 132, and then smoothly restored to the transmission state through the cone exit section 133. After the cumulative filtering of multiple spindle units 130, the higher-order modes are efficiently suppressed, and the purified fundamental mode is output through the output interface 500.

[0057] Therefore, this embodiment uses multiple cascaded spindle units 130, each of which further attenuates the residual higher-order mode energy, ultimately accumulating to achieve an extremely high higher-order mode suppression ratio, thereby significantly improving the output beam quality and reducing M. 2 factor.

[0058] Optionally, please refer to Figure 2 The cascaded spindle filter structure is equipped with 2-5 cascaded spindle units along the optical transmission direction.

[0059] For example, if the number of spindle units 130 in the cascaded spindle filter structure 100 is set to 2, the cumulative effect of the two cascaded spindle units 130 can be simulated using the finite element method (FEM) and beam propagation method (BPM) to ensure that the total suppression ratio is greater than a preset threshold, for example, a total suppression ratio > 30 dB. Alternatively, if the number of spindle units 130 in the cascaded spindle filter structure 100 is set to 3, the cumulative effect of the three cascaded spindle units 130 can be simulated using the finite element method (FEM) and beam propagation method (BPM) to ensure that the total suppression ratio is greater than a preset threshold. Or, if the number of spindle units 130 in the cascaded spindle filter structure 100 is set to 4, the cumulative effect of the four cascaded spindle units 130 can be simulated using the finite element method (FEM) and beam propagation method (BPM) to ensure that the total suppression ratio is greater than a preset threshold. Alternatively, the number of spindle units 130 in the cascaded spindle filter structure 100 can be set to 5. The cumulative effect of the cascaded 5 spindle units 130 can be simulated using the finite element method (FEM) and beam propagation method (BPM) to ensure that the total suppression ratio is greater than the preset threshold.

[0060] Optionally, please refer to Figure 3 The waist diameter of the conical waist segment 132 is 90–100 μm.

[0061] In this embodiment, the waist diameter of the conical waist segment 132 is 90–100 μm to achieve a single-stage higher-order mode loss >8 dB and a fundamental mode loss <0.2 dB. For example, the waist diameter of the conical waist segment 132 can be 90 μm, or 91 μm, or 99 μm, or 100 μm.

[0062] Please see Figure 5 Specifically, the multi-stage spindle high-order mode filter further includes a high-order mode absorption layer 200 and a heat dissipation encapsulation layer 300. The high-order mode absorption layer 200 is wrapped around the periphery of the cascaded spindle filter structure 100, and the heat dissipation encapsulation layer 300 is disposed around the high-order mode absorption layer 200.

[0063] In this embodiment, the higher-order mode absorption layer 200 is wrapped around the cascaded spindle filter structure 100 to absorb the dissipated higher-order mode energy and prevent the dissipated energy from being reflected back to the fiber core 110 or causing a temperature rise. The heat dissipation encapsulation layer 300 is disposed around the higher-order mode absorption layer 200. Due to the reduced core diameter of the tapered waist section 132 of the spindle unit 130, this optical fiber becomes extremely fragile and easily affected by external stress or vibration, leading to changes in its mechanical properties or even breakage. The heat dissipation encapsulation layer 300 enhances the mechanical strength of the cascaded spindle filter structure 100 on the one hand to provide protection, and on the other hand to enhance heat conduction and improve the heat dissipation of the cascaded spindle filter structure 100.

[0064] Specifically, the higher-order mode absorption layer 200 includes a curable adhesive, which is wrapped around the periphery of the cascaded spindle filter structure. The curable adhesive is used to absorb dissipated higher-order mode energy. For example, the curable adhesive is wrapped around the periphery of the cone-entry section 131, the cone-waist section 132, and the cone-exit section 133. The curable adhesive can be a high-refractive-index UV-curable adhesive with a refractive index of 1.55 to 1.60 and a thickness of 50 to 100 μm.

[0065] The heat dissipation encapsulation layer 300 includes a metal sleeve and a thermally conductive potting compound. The metal sleeve is fitted around the periphery of the higher-order mold absorption layer 200, and the thermally conductive potting compound is wrapped around the periphery of the metal sleeve. For example, the metal sleeve is fitted around the cured adhesive and is in close contact with the cured adhesive. The thermally conductive potting compound can fill the gap between the metal sleeve and the cured adhesive to enhance heat conduction.

[0066] For details, please refer to Figure 6 The cascaded spindle filter structure 100 is connected to the pre-amplification link output terminal 600 through the input interface 400, and the cascaded spindle filter structure 100 is connected to the main amplification link input terminal 700 through the output interface 500, forming a cascaded link of "pre-amplification → filter → main amplification".

[0067] Input interface 400 includes an FC / APC polarization-maintaining connector, and cascaded spindle filter structure 100 is connected to the pre-amplification link output 600 via the FC / APC polarization-maintaining connector. Output interface 500 includes an FC / APC polarization-maintaining connector, and cascaded spindle filter structure 100 is connected to the main amplification link input 700 via the FC / APC polarization-maintaining connector. Pre-amplification link output 600 includes: DC10 / 130 double-clad polarization-maintaining fiber with a core diameter of 10μm and an inner cladding of 130μm. Main amplification link input 700 includes: DC20 / 400 double-clad polarization-maintaining fiber with a core diameter of 20μm and an inner cladding of 400μm. Both input interface 400 and output interface 500 use FC / APC polarization-maintaining connectors, resulting in low coupling loss with the pre-amplification and main amplification links. Furthermore, the high-precision alignment characteristics prevent the excitation of new higher-order modes during coupling, ensuring the overall mode purity of the link.

[0068] In this embodiment, the pre-amplified laser output (including the fundamental mode and residual higher-order modes) enters the cascaded spindle filter structure 100 via the input interface 400. The cone-entry section 131 of each spindle unit 130 smoothly transitions the laser field to the cone-waist section 132. The contraction structure of the cone-waist section 132 causes the higher-order mode field distribution to exceed the constraint range of the fiber core 110, and the higher-order mode absorption layer 200 on the periphery is absorbed. Due to the concentrated field distribution, the fundamental mode can be stably transmitted in the cone-waist section, and then smoothly restored to the transmission state through the cone-exit section. After cumulative filtering by 2-5 spindle units 130, the higher-order modes are efficiently suppressed, and the purified fundamental mode is connected to the main amplification link via the output interface 500. At the same time, the energy absorbed by the higher-order mode absorption layer 200 is quickly discharged through the heat dissipation encapsulation layer 300 to ensure the thermal stability of the entire filter.

[0069] Example 2:

[0070] Based on the above-described multi-stage spindle high-order mode filter, this application also provides a fiber laser, wherein the fiber laser includes the multi-stage spindle high-order mode filter as described in the above embodiment.

[0071] In summary, this application discloses a multi-stage spindle high-order mode filter and a fiber laser. The multi-stage spindle high-order mode filter includes: an input interface, a cascaded spindle filter structure, and an output interface connected in sequence. The cascaded spindle filter structure includes multiple spindle units cascaded along the optical transmission direction. Each spindle unit includes a cone-entry section, a cone-waist section, and a cone-exit section connected in sequence along the optical transmission direction, with the cone-waist section gradually narrowing towards the center from the cone-entry and cone-exit sections. This application achieves cumulative attenuation of high-order modes through multiple cascaded spindle units, improving laser beam quality and is suitable for high-power, narrow-linewidth fiber laser systems.

[0072] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A multi-stage spindle high-order mode filter, characterized in that, include: The input interface, the cascaded spindle filter structure, and the output interface are connected in sequence. The cascaded spindle filter structure includes multiple spindle units cascaded along the optical transmission direction. Each spindle unit includes a cone entry section, a cone waist section, and a cone exit section connected sequentially along the optical transmission direction. The cone waist section gradually narrows from the cone entry section and the cone exit section toward the middle.

2. The multi-stage spindle high-order mode filter as described in claim 1, characterized in that, The cascaded spindle filter structure is provided with 2-5 cascaded spindle units along the optical transmission direction.

3. The multi-stage spindle high-order mode filter as described in claim 1, characterized in that, The waist diameter of the conical waist segment is 90~100µm.

4. The multi-stage spindle high-order mode filter as described in claim 1, characterized in that, The input interface includes an FC / APC polarization maintainer connector, which is connected to the cascaded spindle filter structure.

5. The multi-stage spindle high-order mode filter as described in claim 1, characterized in that, The output interface includes an FC / APC polarization maintainer connector, which is connected to the cascaded spindle filter structure.

6. The multi-stage spindle high-order mode filter as described in claim 1, characterized in that, The cascaded spindle filter structure is connected to the output of the pre-amplification link through the input interface, and the cascaded spindle filter structure is connected to the input of the main amplification link through the output interface.

7. The multi-stage spindle high-order mode filter as described in claim 1, characterized in that, Also includes: A higher-order mode absorption layer is wrapped around the periphery of the cascaded spindle filter structure. A heat dissipation encapsulation layer is disposed around the higher-order mold absorption layer.

8. The multi-stage spindle high-order mode filter as described in claim 7, characterized in that, The higher-order modulus absorption layer includes a curing adhesive, which is wrapped around the periphery of the cascaded spindle filter structure.

9. The multi-stage spindle high-order mode filter as described in claim 7, characterized in that, The heat dissipation encapsulation layer includes: A metal sleeve, which is sleeved around the periphery of the higher-order mode absorption layer; Thermally conductive potting compound, which is wrapped around the outer periphery of the metal sleeve.

10. A fiber laser, characterized in that, Including the multi-stage spindle high-order mode filter as described in any one of claims 1-9.

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