Gain optical fiber
By designing a rare-earth ion-doped gain fiber structure and adopting an axial single-trend variable diameter design, optimizing thermal management and pump absorption, the thermal management problem of existing gain fibers was solved, and laser output with higher output power and better beam quality was achieved.
Patent Information
- Application Number
- CN202511616893.7
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-11-06
- Publication Date
- 2025-12-19
AI Technical Summary
The output power of existing gain fibers is limited, and thermal management issues are becoming increasingly prominent, leading to thermal lensing effects and mode instability, which restricts further improvements in high-power laser output.
A gain fiber structure was designed, comprising a rare earth ion-doped core, a fused silica first inner cladding, a fluorine-containing silicon-based silica second inner cladding, and a coating layer. Through an axial single-trend variable diameter design, thermal management and pump absorption were optimized to achieve a more uniform temperature distribution and higher mode stability.
It significantly improves the output power and beam quality of fiber lasers, alleviates thermal lensing effects and mode instability, and supports higher power and more stable laser output.
Smart Images
Figure CN121172541A_ABST
Abstract
Description
Technical Field
[0001] This application belongs to the field of fiber laser technology, and particularly relates to a gain fiber. Background Technology
[0002] The demand for high-power fiber lasers is surging in industrial processing, scientific research, and other fields, especially with increasingly higher requirements for output power. The average power limit of high-power fiber laser systems is constrained by various factors, including nonlinear effects (such as stimulated Raman scattering, SRS, stimulated Brillouin scattering, SBS), thermally induced mode instability (TMI), thermal lensing, and pump brightness. To improve the output performance of fiber lasers, researchers have explored various technical approaches, including continuous optimization of laser structure design and pump parameters (such as pump wavelength and mode). Considering that the performance of the gain fiber is one of the core factors of fiber laser output power and beam quality, designing novel gain fibers has become a key research focus. Traditional large mode area (LMA) fibers reduce power density and suppress nonlinear effects by increasing the core diameter (typically >20 μm). However, their thermal management problems are becoming increasingly prominent: under high-power pumping, doped ions (such as Yb) within the gain fiber... 3+ The absorption of pump light generates localized heat, creating a temperature gradient that leads to thermal lensing and thermally induced mode instability (TMI), limiting further increases in output power.
[0003] The main classifications of existing gain fibers and their technical limitations are as follows: (1) Step-index fiber: mode field area is limited (usually <1000μm) 2 (1) ), weak bending resistance, easily causing mode field distortion due to bending; (2) Photonic crystal fiber (PCF): although it can realize ultra-large mode fields (such as rod fiber), it is difficult to coil, has a large volume and high cost, and low industrial applicability; (3) Tapered double-clad fiber (T-DCF): The University of Tampere in Finland uses the tapering method to design a tapered structure with a constant core-cladding ratio. The diameter of the fiber core and the inner cladding gradually change along the fiber length direction (axial direction) in proportion. Although it improves the mode field area and pump absorption efficiency, its size is large and the heat load density at the larger end is high (>500W / cm). 2 A composite heat dissipation design (such as a microchannel cladding) is required; otherwise, a thermal lensing effect will occur. Summary of the Invention
[0004] This application aims to solve the technical problem of limited output power improvement in existing gain fibers. To this end, this application provides a gain fiber with advantages such as weak thermal deposition effect, uniform axial temperature distribution and high mode instability threshold. Under the same parameter conditions, this fiber structure can achieve laser output performance with higher output power and better beam quality.
[0005] This application provides a gain optical fiber, which includes fibers with sequentially increasing outer diameters:
[0006] The fiber core is doped with rare earth ions as gain ions.
[0007] The first inner cladding layer covers the circumference of the fiber core, and its outer diameter is set to change unidirectionally along the axial direction of the fiber core. The first inner cladding layer is made of fused silica material.
[0008] The second inner cladding layer covers the circumference of the first inner cladding layer, and the second inner cladding layer is fluorine-containing silicon-based quartz.
[0009] A quartz layer is located circumferentially in the second inner cladding;
[0010] A coating layer that covers the circumference of the quartz layer.
[0011] In some embodiments, the concentration of rare earth ions ranges from 0.02% to 0.8% wt.
[0012] In some embodiments, the rare earth ion is at least one of the following: Yb 3+ Er 3+ 、Nd 3+ Tm 3+ Ho 3+ Dy 3+ Pr 3 + 、Sm 3+ .
[0013] In some embodiments, the fiber core is further doped with co-doped ions for modulating the refractive index, the co-doped ions being at least one of the following: Al 3+ 、Ge 4+ P 5+ Bi 3+ Mo 6+ Al-P, Al-Ge.
[0014] In some embodiments, the refractive indices of the fiber core, the first inner cladding, the second inner cladding, the quartz layer, and the coating layer are n1, n2, n3, n4, and n5, respectively, satisfying: n1>n2>n3 and n1>n2>n4.
[0015] In some implementations, the numerical aperture NA of the fiber core satisfies:
[0016]
[0017] The refractive index of the fiber core is n1, and the refractive index of the first inner cladding is n2.
[0018] In some implementations, the single-trend diameter setting includes a linear gradient or a curved gradient.
[0019] In some embodiments, the cross-sectional geometry of the first inner cladding layer is polygonal, D-shaped, eccentric circular, star-shaped, plum blossom-shaped, circular, or elliptical.
[0020] In some embodiments, the refractive index of the second inner cladding is n3 = 1.41 ± 0.05.
[0021] In some implementations, the quartz layer is made of undoped quartz or fluorine-containing silicon-based quartz.
[0022] As can be seen from the above technical solution, the beneficial effects of this application are as follows:
[0023] This application utilizes rare-earth ion doping in the fiber core as a gain medium to provide a basis for laser amplification. The first inner cladding is made of fused silica with an axially unidirectional diameter variation design. The fiber core and the first inner cladding work synergistically to effectively improve thermal management, pump absorption, and power spread capabilities, thereby promoting axial temperature uniformity and reducing thermal deposition effects. The second inner cladding is made of fluorine-containing silicon-based silica, whose low refractive index characteristics improve optical waveguide confinement. Both the first and second inner claddings exhibit unidirectional diameter variation along the length of the fiber, allowing light to propagate along the direction of increasing outer diameter, thus improving the pump fill factor (Γp = a). 2 / b 2 The doping concentration gradually increases along the pump propagation direction. Combined with a constant doping concentration and absorption cross-section, the pump absorption coefficient (β = k0N0σ) is... ap Γ p The corresponding increase in pump absorption leads to more uniform absorption of the pump light along the propagation direction. This homogenization effectively smooths the thermal deposition distribution along the fiber, alleviating the temperature peak problem caused by local overheating at the pump injection end. This characteristic helps to significantly improve the mode instability (TMI) threshold, thereby supporting higher power laser output and providing a reliable technical path for realizing high-power, high-stability fiber lasers. This optimizes the overall thermal and optical performance of this application, enabling higher output power and better beam quality laser output performance under the same parameter conditions. Attached Figure Description
[0024] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced one by one below. Obviously, the accompanying drawings described below are some embodiments of this application. For those skilled in the art, other embodiments and drawings can be obtained based on these drawings without creative effort. Various schematic diagrams according to the embodiments of this application are shown in the accompanying drawings. These drawings are not necessarily drawn to scale. For the purpose of clarity, some details have been enlarged and some details may have been omitted.
[0025] Figure 1A schematic diagram of a longitudinal section of an embodiment of the gain fiber (linear graded) of the present invention is shown;
[0026] Figure 2 A schematic diagram of the longitudinal interface of an embodiment of the gain fiber (curved gradient) of the present invention is shown;
[0027] Figure 3 A schematic diagram of the cross-sections of the smaller end (left) and larger end (right) of the octagonal first inner cladding of an embodiment of the gain fiber of the present invention is shown.
[0028] Figure 4 A schematic diagram of the cross-sections of the smaller end (left) and larger end (right) of the hexagonal first inner cladding of an embodiment of the gain fiber of the present invention is shown.
[0029] Figure 5 A schematic diagram illustrating an embodiment of the refractive index distribution of the cross-section at the larger diameter end of the first and second inner cladding layers of the gain fiber of the present invention is shown.
[0030] Figure 6 This diagram illustrates an embodiment of the refractive index distribution at the smaller diameter end of the first and second inner cladding layers when the quartz layer of the gain fiber of the present invention is fluorine-containing silicon-based quartz.
[0031] Figure 7 A schematic diagram of an embodiment showing the refractive index distribution of the cross-section at the smaller diameter end of the first inner cladding and the second inner cladding when the quartz layer of the gain fiber of the present invention is undoped quartz is shown.
[0032] Figure 8 The temperature distribution diagram of the gain fiber of the present invention and the Nufern-LMA-25 / 400-M fiber under the same conditions is shown.
[0033] Reference numerals: 1. Fiber core; 2. First inner cladding; 3. Second inner cladding; 4. Quartz layer; 5. Coating layer. Detailed Implementation
[0034] The technical solutions in the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. The detailed description of the embodiments of this application provided in the drawings is not intended to limit the scope of the claimed application. The described embodiments are only a part of the embodiments of this application, not all of them. Based on the embodiments in this application, they can be arranged and designed in various different configurations. All other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.
[0035] This application is described below with reference to the accompanying drawings and specific embodiments:
[0036] Please refer to Figure 1 and Figure 2 Embodiments of the present application provide a gain fiber, which includes, from the inside to the outside, a core 1, a first inner cladding 2, a second inner cladding 3, a quartz layer 4, and a coating layer 5, whose outer diameters increase in sequence. The radii of each layer are a (core 1), b (first inner cladding 2), c (second inner cladding 3), d (quartz layer 4), and e (coating layer 5), respectively, and satisfy the size relationship of a < b < c < d < e. The core 1 is doped with rare earth ions as gain ions, the radius a of the core 1 does not change along the fiber axis, and the core 1 is made of quartz material; the first inner cladding 2 is circumferentially coated on the core 1, and the minimum outer diameter of the first inner cladding 2 is greater than the outer diameter of the core 1 to ensure that the core 1 is always completely coated. The outer diameter of the first inner cladding 2 is set to change in a single trend along the axis of the core 1. For example, along the direction from one end to the other end of the core 1, the outer diameter of the first inner cladding 2 gradually increases or gradually decreases, such as showing a conical change. The first inner cladding 2 is made of fused quartz material and is not doped with rare earth ions; the second inner cladding 3 is arranged in a coating manner by referring to the above first inner cladding 2. The second inner cladding 3 is circumferentially coated on the first inner cladding 2, and the second inner cladding 3 is a fluorine-containing silicon-based quartz; the quartz layer 4 is circumferentially coated on the second inner cladding 3. In fact, the quartz layer 4 is located between the second inner cladding 3 and the coating layer 5; the coating layer 5 is circumferentially coated on the quartz layer 4 to protect the internal structure of the fiber.
[0037] In some embodiments, the core 1, the first inner cladding 2, the second inner cladding 3, the quartz layer 4, and the coating layer 5 can adopt different structural combination methods, such as Figure 3 , for the first inner cladding 2 fiber with an octagonal shape, the cross-sections of the smaller end and the larger end of the first inner cladding 2, the radius of the core 1 does not change at both ends of the fiber, the outer diameter dimensions of the tapered first cladding surrounding the core 1 and the fluorine-containing silicon-based quartz second cladding change equally at both ends of the fiber, and the outer diameters of the filled quartz layer 4 and the coating layer 5 do not change at both ends of the fiber; it can also adopt, for example, Figure 4 the cross-sections at both ends of the fiber with a hexagonal first inner cladding 2 as shown in
[0038] In some embodiments, the concentration range of the rare earth ions is 0.02% - 0.8% wt. The rare earth ions are the gain working substance of the laser amplifier. Based on the above-mentioned low-concentration doped rare earth ions, the fiber structure of the present application can increase the distance between gain ions, reduce the probability of cross relaxation and quantum deficit probability (low quantum deficit) between ions, and reduce heat deposition.
[0039] In some embodiments, the rare earth ions are at least one of the following: Yb 3+ , Er 3+ , Nd 3+ , Tm 3+ , Ho 3+ , Dy 3+ , Pr 3+ 、Sm 3+ In some embodiments, the fiber core 1 is further doped with co-doped ions for modulating the refractive index, the co-doped ions being at least one of the following: Al 3+ 、Ge 4+ P 5+ Bi 3+ Mo 6+ Al-P complex ions and Al-Ge complex ions.
[0040] In some embodiments, the refractive indices of the fiber core 1, the first inner cladding layer 2, the second inner cladding layer 3, the quartz layer 4, and the coating layer 5 are n1, n2, n3, n4, and n5, respectively, satisfying: n1>n2>n3 and n1>n2>n4. In this application, the refractive index n2 of the first inner cladding layer 2 is approximately 1.4584; and the refractive index difference between the fiber core 1 and the first inner cladding layer 2 satisfies Δ1=(n1-n2) / n1 and Δ1>0; the refractive index n3 of the second inner cladding layer 3 is approximately 1.384, slightly lower than the refractive index n2 of the first inner cladding layer 2, and the refractive index difference between the two satisfies the numerical aperture of the first inner cladding layer 2. Furthermore, NA' = 0.22 ± 0.1 to form a stable waveguide structure. The quartz layer 4 can be selected from fluorine-containing silicon-based quartz material or undoped quartz material according to engineering requirements and process conditions. If fluorine-containing silicon-based quartz material is selected, the refractive index n4' is consistent with that of the fluorine-containing silicon-based quartz layer 4 (second inner cladding 3) (n4' = n3); if undoped quartz material is selected, the refractive index is consistent with that of the tapered first cladding 2 (n4'' = n2); the refractive index difference range of the coating layer 5 is 1.4 ≤ n5 ≤ 1.8. In other words, the optical fiber structure according to this embodiment has a step-index refractive index distribution or a second cladding recessed structure refractive index distribution.
[0041] like Figure 5-7 As shown, the refractive index distribution of each layer of the optical fiber structure is not limited to a step-type distribution, but can also adopt various smoothly curved distribution forms, including the refractive index distribution of the second cladding recessed structure (i.e., the refractive index of the second inner cladding 3 is lower than that of the first inner cladding 2 and the quartz layer 4).
[0042] In some implementations, the numerical aperture NA of fiber core 1 satisfies:
[0043]
[0044] The refractive index of fiber core 1 is n1, and the refractive index of the first inner cladding 2 is n2. This NA value is consistent with the NA value of Nufern's LMA-YDF-25 / 400-M fiber, i.e., NA = 0.065 ± 0.005.
[0045] In some embodiments, the outer diameter of the first inner cladding 2 is configured with a single-trend diameter variation along the optical fiber axis. This single-trend diameter variation includes a linear gradient or a curved gradient (such as a curve with varying slope or a curve approximating a sine function), and the direction of the diameter variation is adapted to the optical transmission direction: the optical signal is injected from the smaller end of the outer diameter of the first inner cladding 2 and output from the larger end. Pump light is injected from the larger end of the outer diameter of the first inner cladding 2, causing the first inner cladding 2 to gradually narrow along the pump light propagation direction.
[0046] In some implementations, the cross-sectional geometry of the first inner cladding 2 can be customized according to application requirements. The cross-sectional geometry of the first inner cladding 2 can be polygonal, D-shaped, eccentric circular, star-shaped, plum blossom-shaped, circular, or elliptical, such as octagonal, hexagonal, decagonal, etc. The minimum thickness of the second inner cladding 3 is not less than 15μm, and its outer diameter can vary along the optical fiber axis. The variation pattern is adapted to the diameter variation mode of the first inner cladding 2 (such as linear gradient or curved gradient), and the second inner cladding 3 completely covers the first inner cladding 2.
[0047] In some embodiments, the refractive index n3 of the second inner cladding is 1.41 ± 0.05, such as n3 = 1.384, which is lower than the refractive index n2 of the first inner cladding 2, thus forming a weak waveguide structure. This structure is based on the critical angle θ. c =sin -1 The (n3 / n2) selective filtering mechanism can effectively filter out propagation angles θ>θ. c The advanced pumping mode (which has lower absorption efficiency due to its distance from the core region) suppresses stray heat deposition on the outside of the core. This design significantly improves the thermal uniformity of core 1.
[0048] In some embodiments, the quartz layer 4 is made of undoped quartz or fluorine-containing silicon-based quartz to ensure that the optical fiber preform has a uniform outer diameter before drawing. The quartz layer 4 is disposed circumferentially in the second inner cladding 3, and its function is to ensure that the optical fiber preform has a uniform outer diameter before drawing, and that the outer diameter of the quartz layer 4 does not change along the optical fiber axis. The material of the quartz layer 4 can be selected from two types: one is undoped quartz, with a thermal conductivity of approximately 1.38–1.5 W / (m·K), which has better heat dissipation performance and is suitable for heat dissipation... The first option is a fluorine-containing silicon-based quartz (the same material as the second inner cladding layer 3), which has a thermal conductivity of about 1.3 W / (m·K). The preparation process is simpler and suitable for scenarios where ease of preparation and economy are the priority. When undoped quartz is used for the quartz layer 4, its refractive index n4 is the same as the refractive index n2 of the first inner cladding layer 2 (n4=n2). When fluorine-containing silicon-based quartz is used, its refractive index n4 is the same as the refractive index n3 of the second inner cladding layer 3 (n4=n3).
[0049] In some embodiments, the coating layer 5 covers the circumferential direction of the quartz layer 4, and this coating layer 5 can significantly improve the tensile strength of the optical fiber (typical value >100kN / mm).2 It possesses excellent bending resistance (minimum bending radius <5mm) and impact resistance, while effectively blocking moisture through airtight packaging (permeability <0.01g·mm / m). 2 The coating layer 5 is designed to prevent the infiltration of chemicals and other substances, thus preventing increased transmission loss (Δα < 0.01 dB / km) in the fiber core 1 due to environmental factors. The inner and outer diameters of the coating layer 5 do not change along the fiber axis. The material of the coating layer 5 can be selected according to requirements: for general applications, a UV-cured acrylate system (elastic modulus 1-3 GPa) is used; for applications with special temperature resistance requirements, special high-temperature resistant polymer materials are used, including but not limited to silicone rubber (temperature resistance 300℃), polyimide (temperature resistance 400℃), polytetrafluoroethylene (temperature resistance 260℃), and polyamide (temperature resistance 280℃).
[0050] The structural design described in this application allows the pump light absorption coefficient to vary along the fiber length (axial direction), maintaining both high pump efficiency and a more uniform pump light absorption distribution. This results in a more uniform axial temperature distribution within the fiber, effectively solving the problem of excessively high thermal deposition temperature at the pump injection end. This is because the pump fill factor Γp is obtained through the formula Γp = a 2 / b 2 Calculate (a is the radius of fiber core 1, b is the radius of the first inner cladding 2). Because the first inner cladding 2 gradually shrinks along the pump light propagation direction, the radius b of the first inner cladding 2 gradually decreases along the pump propagation direction, causing Γp to gradually increase along the pump propagation direction; the pump absorption coefficient β is calculated using the formula β=k0N0σ. ap Γ p Calculate k0 = 4.343 (k0 = 4.343, N0 is the gain ion doping concentration of fiber core 1, σ ap (where N0 and σ1 are the absorption cross sections of fiber core 1 for the pump light), due to N0 and σ1 ap With a constant Γp along the pump light propagation direction, an increase in Γp leads to a corresponding increase in β, resulting in more uniform absorption of the pump light along the propagation direction. This homogenization of pump absorption smooths the thermal deposition distribution along the fiber, alleviates the temperature peak problem caused by local overheating at the pump injection end, significantly improves the mode instability (TMI) threshold, suppresses thermally induced mode instability and thermal lensing effects, thereby supporting higher power laser output, improving laser conversion efficiency, and providing a reliable technical path for high-power, high-stability fiber lasers, enabling continuous output of high-power, high-beam output.
[0051] Please refer to Figure 8The curved change of the fiber structure (octagonal first inner cladding 2) in this application is similar to the axial temperature distribution of the fiber core 1 when the Nufern-LMA-YDF-25 / 400-M gain fiber absorbs the same pump power. In this embodiment, the diameters of the fiber core 1 and the second inner cladding 3 at the larger end are the same as those of conventional fibers, which are 25μm and 400μm respectively. The diameter of the second inner cladding 3 at the smaller end is 123μm. The cladding size changes in an approximately sinusoidal form along the radial direction of the fiber. As can be seen from the figure, the temperature distribution of the fiber structure in this application is more uniform.
[0052] Regarding the specific implementation methods of this application, it should be noted that:
[0053] In the description of this application, unless otherwise expressly specified and limited, the terms "connection," "fixed," "connected," etc., should be interpreted broadly. For example, "fixed" can refer to a fixed connection, a detachable connection, or an integral molding; "connection" can refer to a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium; it can refer to the internal connection of two components or the interaction between two components, unless otherwise expressly limited; "connected" can refer to the internal connection of two parts and the connection between two parts, or the spatial connection between them, whereby the two parts are directly or indirectly connected through the part forming the space. The terms "set," "installed," "equipped with," "configured," etc., should also be interpreted broadly. For those skilled in the art, the specific meaning of the above terms in this application can be understood according to the specific circumstances.
[0054] In the description of this application, the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," and "counterclockwise," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the system or component referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this application. All directional indications are only used to explain the relative positional relationship and movement between components in a specific orientation. If the specific orientation changes, the directional indication will also change accordingly.
[0055] In the description of this application, the use of terms such as "some embodiments," "optional embodiments," "example," "specific example," "optional example," or "optional embodiment," etc., indicates that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of this application, but does not imply that these embodiments illustrate and describe all possible forms of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Furthermore, those skilled in the art can combine and integrate the different embodiments or examples described in this specification.
[0056] The present invention has been described in detail above with reference to specific embodiments and exemplary examples. The above description is exemplary and not exhaustive, and is not limited to the disclosed embodiments; the above description should not be construed as a limitation of the present invention. Technical solutions between various embodiments can be combined with each other, but must be based on the ability of those skilled in the art to implement them. When the combination of technical solutions is contradictory or cannot be implemented, it should be considered that such a combination of technical solutions does not exist and is not within the scope of protection claimed in this application. Although embodiments of the present application have been shown and described, various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present application. Those skilled in the art will understand that various other specific changes and combinations of embodiments based on the technical teachings disclosed in this application, without departing from the essence of the present application, are still within the scope of protection defined by the claims of the present invention and their equivalent technical solutions.
Claims
1. A gain optical fiber, characterized in that, Including those with progressively increasing outer diameters: The fiber core (1) is doped with rare earth ions as gain ions; The first inner cladding layer (2) covers the circumference of the fiber core (1), and its outer diameter is set to change in a single direction along the axial direction of the fiber core (1). The first inner cladding layer (2) is made of fused silica material. The second inner cladding layer (3) covers the circumferential direction of the first inner cladding layer (2), and the second inner cladding layer (3) is a fluorine-containing silicon-based quartz. Quartz layer (4) is disposed circumferentially in the second inner cladding layer (3); A coating layer (5) is applied around the quartz layer (4).
2. The gain optical fiber according to claim 1, characterized in that, The concentration range of the rare earth ions is 0.02%–0.8% wt.
3. The gain optical fiber according to claim 2, characterized in that, The rare earth ion is at least one of the following: Yb 3+ Er 3+ 、Nd 3+ Tm 3+ Ho 3+ Dy 3+ Pr 3+ 、Sm 3+ .
4. The gain optical fiber according to claim 1, characterized in that, The fiber core (1) is also doped with co-doped ions for modulating the refractive index, wherein the co-doped ions are at least one of the following: Al 3+ 、Ge 4+ P 5+ Bi 3+ Mo 6+ Al-P, Al-Ge.
5. The gain optical fiber according to claim 1, characterized in that, The refractive indices of the fiber core (1), the first inner cladding layer (2), the second inner cladding layer (3), the quartz layer (4), and the coating layer (5) are n1, n2, n3, n4, and n5, respectively, satisfying: n1>n2>n3 and n1>n2>n4.
6. The gain optical fiber according to claim 5, characterized in that, The numerical aperture NA of the fiber core (1) satisfies: The refractive index of the fiber core (1) is n1, and the refractive index of the first inner cladding layer (2) is n2.
7. The gain optical fiber according to claim 1, characterized in that, The single-trend diameter setting includes linear gradient or curved gradient.
8. The gain optical fiber according to claim 1, characterized in that, The cross-sectional geometry of the first inner cladding layer (2) is polygonal, D-shaped, eccentric circular, star-shaped, plum blossom-shaped, circular or elliptical.
9. The gain optical fiber according to claim 1, characterized in that, The refractive index of the second inner cladding (3) is n3 = 1.41 ± 0.
05.
10. The gain optical fiber according to claim 1, characterized in that, The quartz layer (4) is made of undoped quartz or fluorine-containing silicon-based quartz.