A gain fiber and a fiber laser

Abstract

The application discloses a gain optical fiber which is composed of small numerical aperture and high doping concentration fiber sections and large numerical aperture and low doping concentration fiber sections which are alternately distributed in sequence. The small numerical aperture and high doping concentration sections of the gain optical fiber are used to restrain the generation of high-order modes, and the high gain makes the fundamental mode laser amplification. The large numerical aperture and low doping concentration sections of the gain optical fiber amplify the laser power, but generate low gain to the high-order modes, so that the generation and amplification of the high-order modes can be effectively restrained, the high-quality laser output is realized, the mode instability threshold is improved, and a new technical means is provided for further realizing high-power fiber laser output.

Description

Technical Field

[0001] This invention relates to the field of fiber laser technology, and more specifically, to a gain fiber and a fiber laser. Background Technology

[0002] With the technological advancements in fiber lasers, their applications are expanding rapidly. Currently, high-power fiber lasers have achieved kilowatt-level engineering applications. In the future, whether in civilian manufacturing or specialized equipment, the miniaturization and high beam quality requirements of high-power lasers will inevitably be the future direction of development.

[0003] In the field of fiber laser technology, obtaining high beam quality laser output at high power has always been a research goal. To obtain higher power laser output and reduce the influence of stimulated Raman scattering, gain fibers with larger core diameters are often used. However, large core diameter gain fibers inevitably introduce more higher-order mode components, making them more prone to mode instability and other nonlinear effects. On the other hand, if gain fibers with smaller core diameters are used, they will inevitably introduce stronger stimulated Raman scattering effects. Summary of the Invention

[0004] In view of at least one defect or improvement requirement of the prior art mentioned in the background section above, the present invention provides a gain fiber to balance stimulated Raman scattering and mode instability effects and obtain laser with high mode instability threshold and high beam quality.

[0005] In a first aspect, the present invention provides a gain optical fiber, comprising: a linear fiber core and a cladding covering the fiber core;

[0006] The gain fiber is formed by alternating division of a first type of fiber segment and a second type of fiber segment from one end to the other, with one end of the gain fiber being the first type of fiber segment.

[0007] The first numerical aperture of the fiber core of the first type of optical fiber segment is smaller than the second numerical aperture of the fiber core of the second type of optical fiber segment.

[0008] The first doping concentration of the cladding of the first type of optical fiber segment is greater than the second doping concentration of the cladding of the second type of optical fiber segment.

[0009] Furthermore, the alternating division is a periodic alternating division of line length.

[0010] Furthermore, the first numerical aperture of the fiber core is less than 0.055, and the second numerical aperture of the fiber core is greater than 0.075;

[0011] The pump light cladding absorption coefficient characterized by the first doping concentration of the cladding is greater than 1.6 dB / m;

[0012] The pump light cladding absorption coefficient characterized by the second doping concentration of the cladding is less than 1 dB / m.

[0013] Furthermore, non-rare earth element ions are incorporated into the core and / or cladding to change the numerical pore size of the core.

[0014] Rare earth element ions are incorporated into the cladding to alter the magnitude of the pump light cladding absorption coefficient.

[0015] Furthermore, the other end of the gain fiber is also a fiber segment of the first type.

[0016] Furthermore, the length ratio of the second type of fiber segment to the first type of fiber segment in the middle portion of the non-end of the gain fiber is 3:2, and the length of the fiber segment with the length period is 5m to 8m.

[0017] The diameter of the fiber core is 14μm to 50μm, and the diameter of the cladding is 250μm to 1200μm.

[0018] Secondly, the present invention provides a fiber laser, which is an amplifier structure, comprising a laser seed source, a pump source module group, a pump coupler, and a gain fiber as described above, connected in sequence.

[0019] Furthermore, the center wavelength output by the laser seed source corresponds to the wavelength corresponding to the maximum gain of the gain fiber, and the output power range is 70W to 200W.

[0020] The pump source module group provides a pump wavelength corresponding to the wavelength of the maximum pump light cladding absorption coefficient of the gain fiber, and provides a pump power of not less than 3500W.

[0021] Furthermore, the fiber size at the pump output end of the pump coupler is not less than the fiber size of the first type of fiber segment at one end of the gain fiber.

[0022] Furthermore, the gain fiber is arranged by bending and winding.

[0023] In summary, compared with the prior art, the above-described technical solutions conceived by this invention can achieve the following beneficial effects:

[0024] (1) The gain fiber of the present invention adopts a small numerical aperture and high doping concentration design at its laser injection end, which suppresses and controls the high-order modes in the initial stage to a certain extent. At the same time, the high doping concentration gain ions amplify the fundamental mode laser and improve the beam quality of the output laser. On the other hand, in order to compensate for the fundamental mode laser loss introduced by the small numerical aperture and high doping concentration fiber section, an alternating distribution of small numerical aperture and high doping concentration fiber section and large numerical aperture and low doping concentration fiber section is specially adopted on the gain fiber. This compensates for part of the fundamental mode laser loss introduced by the small numerical aperture and high doping concentration fiber section. At the same time, the large numerical aperture and low doping concentration fiber section also has a "delay" suppression effect on the amplification of high-order modes. Its mode field diameter is larger, which can also suppress stimulated Raman scattering effect.

[0025] (2) The gain fiber of the present invention can control parameters such as the numerical aperture of the fiber core and the absorption coefficient of the pump light cladding by adding or removing doping ions in the fiber core and / or cladding. This allows the size and structure of the fiber core and cladding to remain unchanged, thereby making the manufacturing process of the gain fiber simpler and easier to process and produce.

[0026] (3) The fiber laser using the gain fiber of the present invention can have a high mode instability threshold under high power pumping and balance the stimulated Raman scattering effect and mode instability effect, and can maintain high beam quality under high laser power output. Attached Figure Description

[0027] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying 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.

[0028] Figure 1 A schematic diagram of a gain optical fiber provided in an embodiment of the present invention;

[0029] exist Figure 1 In the middle section, 41-gain fiber has a small numerical aperture and high doping concentration; 42-gain fiber has a large numerical aperture and low doping concentration.

[0030] Figure 2 A schematic diagram of the structure of a fiber laser provided in an embodiment of the present invention;

[0031] exist Figure 2 In the middle, 1-pump source module group; 11-16-each module of the pump source; 2-pump coupler; 3-laser seed source; 4-gain fiber; 5-fiber end cap;

[0032] Figure 3 Simulation results of conventional gain optical fiber provided by existing technology;

[0033] Figure 4 The simulation results of the novel gain fiber based on alternating small numerical aperture, high doping concentration fiber section and large numerical aperture, low doping concentration fiber section provided in the embodiments of the present invention are shown in the figure. Detailed Implementation

[0034] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0035] The terms "first," "second," or "third," etc., used in the specification, claims, or accompanying drawings of this application are used to distinguish different objects and not to describe a particular order. Furthermore, the terms "comprising" or "having," and any variations thereof, are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or apparatus that includes a series of steps or units is not limited to the listed steps or units, but may optionally include steps or units not listed, or may optionally include other steps or units inherent to such processes, methods, products, or apparatus.

[0036] Given the dilemma mentioned in the background section of the technology that it is difficult to balance stimulated Raman scattering and mode instability effects, how to obtain a gain fiber that can balance stimulated Raman scattering and mode instability effects and obtain lasers with high mode instability threshold and high beam quality has become an important technical problem that urgently needs to be solved.

[0037] The number of modes that can exist in a gain fiber and the stimulated Raman scattering threshold are closely related to the numerical aperture of the fiber core. A small numerical aperture can limit the number of modes in the gain fiber, thereby suppressing the generation and amplification of higher-order modes. At the same time, a small numerical aperture increases the mode field area, thereby increasing the stimulated Raman scattering threshold. However, if the gain fiber with too small a numerical aperture is used, it will cause a high fundamental mode power loss during the bending mode selection process. Therefore, it is necessary to introduce a gain fiber with a higher numerical aperture to compensate for it.

[0038] To achieve high beam quality, it is necessary to control the modes within the gain fiber, suppress higher-order modes, and amplify the gain of the fundamental mode. This invention employs a special design for the gain fiber. On one hand, a small numerical aperture and high doping concentration are used at the laser injection and / or output ends to suppress and control higher-order modes to a certain extent in the initial and / or output stages. Simultaneously, the high concentration of gain ions amplifies the fundamental mode laser, improving the beam quality of the output laser. On the other hand, to compensate for the fundamental mode laser loss introduced by the small numerical aperture, the gain fiber uses alternating distributions of fiber segments with small numerical aperture, high doping concentration / large numerical aperture, and low doping concentration. This compensates for some of the fundamental mode laser loss introduced by the small numerical aperture and high doping concentration. Furthermore, the low doping concentration also has a "delaying" suppression effect on the amplification of higher-order modes, and its larger mode field diameter also suppresses stimulated Raman scattering. The numerical aperture of the optical fiber can be controlled by doping the core and / or cladding with non-rare earth element ions (such as fluorine, boron, germanium, etc.), while the size and shape of the core and cladding remain unchanged. This invention suppresses the generation and amplification of higher-order modes along the entire gain fiber, improving the mode instability threshold under high-power pumping without altering the shape and size of the core and cladding. This addresses the problems of mode instability, nonlinear effects, and base film signal power loss, maintaining high beam quality even under high-power laser output.

[0039] In one embodiment, a gain fiber is a double-clad doped fiber, which is sequentially wrapped with a core, inner cladding, outer cladding, and coating. This gain fiber is formed by alternating sections from one end to the other: a small numerical aperture, high doping concentration fiber section 41 and a large numerical aperture, low doping concentration fiber section 42. Preferably, the alternation is a periodic alternation of line length, such as... Figure 1 As shown. The core diameter is controlled between 14 and 50 μm, and the cladding diameter is controlled between 250 and 1200 μm. The shape of the cladding can be circular, octagonal, D-shaped, panda-shaped, or other irregular shapes. The doping ions of the cladding are rare earth element ions, and the gain fiber is doped with a single rare earth element ion or a mixture of rare earth element ions.

[0040] For fiber sections with small numerical aperture and high doping concentration, the core numerical aperture is less than 0.055, and the pump light cladding absorption coefficient (corresponding to the wavelength of maximum pump absorption) is greater than 1.6 dB / m. Conversely, for fiber sections with large numerical aperture and low doping concentration, the core numerical aperture is greater than 0.075, and the pump light cladding absorption coefficient (corresponding to the wavelength of maximum pump absorption) is less than 1 dB / m. The pump light cladding absorption coefficient can be controlled by adjusting the amount of rare earth element dopant ions incorporated into the cladding.

[0041] Preferably, both the pump light incident end and the signal light output end of the gain fiber are configured as small numerical aperture, high doping concentration fiber sections, with the fiber length at both ends ranging from 1 to 2 meters. This configuration allows for a certain degree of suppression and control of higher-order modes in both the initial and output stages. Simultaneously, the high concentration of gain ions amplifies the fundamental mode laser, thereby improving the beam quality of the output laser.

[0042] Preferably, the middle section of the gain fiber has a variation period of 5 to 8 meters in length. Within one length period, the ratio of the length of the fiber section with large numerical aperture and low doping concentration to that of the fiber section with small numerical aperture and high doping concentration is preferably 3:2.

[0043] Gain fibers have no melting point, and the core and cladding dimensions, shape, and position remain unchanged along the signal light transmission direction. By adding or removing doping ions in the core and / or cladding, parameters such as the numerical aperture of the core and the pump light cladding absorption coefficient can be controlled, keeping the core and cladding dimensions and structure constant. This simplifies the fabrication process of gain fibers, making them easier to process and produce.

[0044] For more specific embodiments, please refer to Figure 1 A gain fiber is described, consisting of a core, inner cladding, outer cladding, and coating layer sequentially wrapped together. The core is single-doped with ytterbium ions from the rare-earth element family, with a core size of 20 μm. The cladding size is 400 μm, and the coating layer size is 550 μm. Both ends of the gain fiber have 1.5 m long sections with small numerical aperture and high doping concentration. The middle section features a periodic alternation of lengths between 3 m sections with large numerical aperture and low doping concentration and 2 m sections with small numerical aperture and high doping concentration, resulting in a total length of 18 m. Specifically, the core numerical aperture of the small numerical aperture and high doping concentration fiber section is 0.051, and the pump light cladding absorption coefficient (corresponding to the wavelength of maximum pump absorption) is 1.7 dB / m; the core numerical aperture of the large numerical aperture and low doping concentration fiber section is 0.076, and the pump light cladding absorption coefficient (corresponding to the wavelength of maximum pump absorption) is 0.9 dB / m.

[0045] like Figure 2 As shown, in another embodiment, a high-power fiber laser has an amplifier structure, mainly composed of a laser seed source 3, a pump source module group 1, a pump coupler 2, a gain fiber 4, and a fiber end cap 5 connected in sequence. The gain fiber 4 is the gain fiber of the aforementioned embodiment. Various components such as the laser seed source, pump source module group, pump coupler, high-reflection fiber grating, gain fiber, and low-reflection fiber grating can be fused onto a single fiber using a fiber optic fusion splicer. This allows the entire fiber laser system to exist within the structural range of a single fiber without any additional protruding spatial structural components.

[0046] The center wavelength of the laser seed source 3 output corresponds to the maximum gain wavelength of the gain fiber 4, preferably with an output power range of 70 to 200W.

[0047] The pump wavelength provided by pump source module group 1 corresponds to the wavelength of the maximum pump absorption coefficient of gain fiber 4, and preferably, the pump power provided is not less than 3500W.

[0048] Preferably, the core and numerical aperture of the pump output fiber of the pump coupler 2 are not smaller than the core and numerical aperture of the gain fiber 4 and the high-doped fiber portion. More preferably, the fiber size at the pump output end of the pump coupler 2 is the same as the fiber size of the low-doped fiber 4 and the high-doped fiber portion.

[0049] The passive optical fiber connected to the fiber end cap 5 is fused to the gain optical fiber 4. The size of the passive optical fiber connected to the fiber end cap 5 is the same as the size of the small numerical aperture and high doping concentration fiber portion of the gain optical fiber 4. The connecting optical fiber of the fiber end cap 5 may contain a cladding optical filter, which can be etched on the passive optical fiber connected to the fiber end cap 5, corresponding to a pump light stripping power greater than 450W.

[0050] Preferably, the gain fiber 4 is arranged in a bent coil or bent winding manner, with a bending radius ranging from 5 to 12 cm, and is subjected to real-time cooling control.

[0051] For a more specific embodiment, refer to Figure 2 A high-power amplifier made entirely of optical fiber is disclosed. Its overall device structure and parameters are as follows: a laser seed source 3 with a center wavelength of 1080 nm, a bandwidth of 2 nm, and a power of 100 W; a pump source module group 1 with a center wavelength of 976 nm and a pump power of 3.5 kW; a pump coupler 2, a 6×1 pump coupler, with one arm of the input end connected to a red light LD; and a gain fiber 4, which is the gain fiber from the previous embodiment, with a bending radius of 6–10 cm. Using this embodiment, high beam quality, a high mode instability threshold, and a laser output with a center wavelength of 1080 nm can be obtained. Figure 3 and Figure 4 Simulation results were obtained by comparing conventional optical fiber with the gain optical fiber described in this application. The simulation results show that using the gain optical fiber described in this application can reduce the output of higher-order mode lasers.

[0052] The foregoing description is merely an exemplary embodiment of this disclosure and should not be construed as limiting the scope of this disclosure. Any equivalent changes and modifications made in accordance with the teachings of this disclosure shall still fall within the scope of this disclosure. Other embodiments of this disclosure will be readily apparent to those skilled in the art upon consideration of the specification and practice of the disclosure herein. This invention is intended to cover any variations, uses, or adaptations of this disclosure that follow the general principles of this disclosure and include common knowledge or customary techniques in the art not described herein. The specification and embodiments are to be considered exemplary only, and the scope and spirit of this disclosure are defined by the claims.

[0053] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0054] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A gain optical fiber, characterized in that, include: A linear fiber core and a cladding covering the fiber core; The gain fiber is formed by alternatingly dividing a first type of fiber segment and a second type of fiber segment from one end to the other. One end of the gain fiber is a first type of fiber segment, and the other end of the gain fiber is also a first type of fiber segment. The first numerical aperture of the fiber core of the first type of fiber segment is smaller than the second numerical aperture of the fiber core of the second type of fiber segment; the first numerical aperture of the fiber core is less than 0.055, and the second numerical aperture of the fiber core is greater than 0.

075. The first doping concentration of the cladding of the first type of optical fiber segment is greater than the second doping concentration of the cladding of the second type of optical fiber segment; the pump light cladding absorption coefficient characterized by the first doping concentration of the cladding is greater than 1.6 dB / m; and the pump light cladding absorption coefficient characterized by the second doping concentration of the cladding is less than 1 dB / m.

2. The gain optical fiber as described in claim 1, characterized in that, The alternating division is a periodic alternating division of line length.

3. The gain optical fiber as described in claim 1, characterized in that, Non-rare earth element ions are incorporated into the core and / or cladding to change the numerical pore size of the core. Rare earth element ions are incorporated into the cladding to alter the magnitude of the pump light cladding absorption coefficient.

4. The gain optical fiber as described in claim 2, characterized in that, The length ratio of the second type of fiber segment to the first type of fiber segment in the middle part of the non-end of the gain fiber is 3:2, and the length of the fiber segment with the length period is 5 m to 8 m. The diameter of the fiber core is 14 μm to 50 μm, and the diameter of the cladding is 250 μm to 1200 μm.

5. A fiber laser, characterized in that, It is an amplifier structure, comprising a laser seed source, a pump source module group, a pump coupler, and a gain fiber as described in any one of claims 1 to 4, connected in sequence.

6. The fiber laser as described in claim 5, characterized in that, The center wavelength of the laser seed source output corresponds to the wavelength corresponding to the maximum gain of the gain fiber, and the output power range is 70 W ~ 200 W. The pump source module group provides a pump wavelength corresponding to the wavelength of the maximum pump light cladding absorption coefficient of the gain fiber, and provides a pump power of not less than 3500 W.

7. The fiber laser as described in claim 5, characterized in that, The fiber size at the pump output end of the pump coupler is not less than the fiber size of the first type of fiber segment at one end of the gain fiber.

8. The fiber laser as described in claim 5, characterized in that, The gain fiber is arranged by bending and winding.

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