Graded-index optical fiber with negative correlation between pump absorption and core numerical aperture along the axial direction and preparation method thereof

Abstract

The present invention relates to the field of gain optical fibers, and in particular to a graded-index optical fiber in which pump absorption and core numerical aperture are negatively correlated along the axial direction, and a preparation method thereof. The graded-index optical fiber in which pump absorption and core numerical aperture are negatively correlated along the axial direction comprises a core layer, a step layer, a cladding layer, and a coating layer arranged in sequence from the inside to the outside in the radial direction, wherein the refractive index n1 of the core layer is greater than the refractive index n2 of the step layer, the refractive index n3 of the cladding layer is greater than the refractive index of the coating layer, the refractive index n1 of the core layer has a gradual trend along the axial direction, the refractive index n2 of the step layer has a gradual trend along the axial direction, and the rate of change of n2-n3 along the axial direction is greater than the rate of change of n1-n3 along the axial direction. The present invention achieves a gradual increase in pump absorption along the axial direction and a gradual decrease in core numerical aperture along the axial direction by controlling the rate of change of the relative refractive index (n2-n3) of the console layer along the axial direction to be greater than the rate of change of the relative refractive index (n1-n3) of the core layer along the axial direction.

Description

Technical Field

[0001] The present invention relates to the field of gain optical fibers, and in particular to a gradient optical fiber in which pump absorption and core numerical aperture are negatively correlated along an axial direction and a preparation method thereof. Background Art

[0002] Fiber lasers and amplifiers have seen widespread application and rapid development in recent years in industries such as industry, military, medicine, and scientific research due to their high conversion efficiency, compact size, excellent beam quality, ease of thermal management, and high stability. The gain medium of a fiber laser is primarily a gain fiber, which absorbs the cladding pump light and converts it into signal laser light. To ensure that the injected pump light is fully absorbed by the gain fiber, a specific length of gain fiber is typically selected based on the gain fiber's pump absorption coefficient. Currently, the pump absorption and core numerical aperture of a given length of gain fiber are uniform along the fiber axis. This simple fiber structure has led to widespread application in the fiber laser field.

[0003] In fiber lasers, whether forward pumping, reverse pumping or bidirectional pumping is used, it is difficult to maintain a consistent degree of population inversion in a gain fiber with uniform axial absorption. Usually, the gain fiber close to the pump end is highly pumped and has a high degree of population inversion, while the gain fiber far from the pump end is less pumped and has a correspondingly low degree of population inversion. Therefore, there must be differences in the degree of population inversion along the axis of the gain fiber, which in turn causes uneven heat distribution in the axial direction of the gain fiber. The accumulation of heat can lead to a decrease in laser performance at best, and in severe cases, it can cause the gain fiber in the laser to burn out, thereby limiting the further increase in the output power of high-power fiber lasers.

[0004] In addition, in order to maintain the advantage of the fiber laser's small size and achieve high-quality laser beam output through bending mode selection, the selected gain fiber of a certain length is usually coiled in a certain shape and fixed on a certain water-cooled plate. Usually, from the inner circle to the outer circle, the coiling diameter gradually increases and the bending loss gradually decreases. If a certain length of gain fiber is coiled on a flat plate, the inner circle loss is large and the outer circle loss is small. On the one hand, it will cause the temperature of the inner circle to rise, and it is also not conducive to the uniform bending mode selection of the optical fiber, thereby affecting the beam quality. Summary of the Invention

[0005] The purpose of the present invention is to provide a graded-index optical fiber in which pump absorption and core numerical aperture are negatively correlated along the axial direction and a preparation method thereof, wherein the gain optical fiber can realize that the pump absorption gradually increases along the axial direction of the optical fiber while the core numerical aperture gradually decreases along the axial direction of the optical fiber.

[0006] The present invention provides a graded-index optical fiber in which pump absorption and core numerical aperture are negatively correlated along the axial direction, comprising a core layer, a step layer, a cladding layer and a coating layer arranged in sequence from the inside to the outside along the radial direction.

[0007] Among them, the refractive index of the core layer n1> the refractive index of the step layer n2> the refractive index of the cladding layer n3> the refractive index of the coating layer,

[0008] The refractive index n1 of the core layer changes gradually along the axial direction, and the refractive index n2 of the step layer changes gradually along the axial direction, and the changing rate of n2-n3 along the axial direction is greater than the changing rate of n1-n3 along the axial direction.

[0009] According to the graded-index optical fiber with negative correlation between pump absorption and core numerical aperture along the axial direction provided by the present invention, the starting value of n2-n3 is between 0.0005 and 0.016, and the rate of change of n2-n3 along the axial direction is greater than or equal to 1*10 -6 / m;

[0010] And / or, the starting value of n1-n3 is between 0.0010 and 0.02, and is greater than the starting value of n2-n3, and the rate of change of n1-n3 along the axial direction is greater than or equal to 0.5*10 -6 / m, and is less than the rate of change of n2-n3 along the axial direction.

[0011] According to the graded-index optical fiber provided by the present invention, in which pump absorption and core numerical aperture are negatively correlated along the axial direction, the core layer is mainly composed of a matrix SiO2 and one or more rare earth oxides Yb2O3, Er2O3, Tm2O3, and Ho2O3; the gradient trend of n1 along the axial direction is achieved by controlling the doping concentration of the rare earth oxide in the core layer.

[0012] According to the gradient optical fiber provided by the present invention in which pump absorption and core numerical aperture are negatively correlated along the axial direction, the step layer is mainly composed of a matrix SiO2 and one or more dopants selected from Ge2O3 and P2O5; the gradient trend of n2 along the axial direction is achieved by controlling the concentration of the dopants in the step layer.

[0013] According to the graded-index optical fiber with a negative correlation between pump absorption and core numerical aperture along the axial direction provided by the present invention, the cladding is composed of a matrix SiO2.

[0014] According to the graded-index optical fiber provided by the present invention, in which pump absorption and core numerical aperture are negatively correlated along the axial direction, the coating layer consists of an inner low-refractive-index coating layer and an outer high-refractive-index coating layer, the refractive index of the low-refractive-index coating layer is n4, and the refractive index of the high-refractive-index coating layer is n5, where n4<n3 and n5>n1.

[0015] According to the graded-index optical fiber with a negative correlation between pump absorption and core numerical aperture along the axial direction provided by the present invention, the diameters of the core layer, step layer, cladding layer and coating layer are respectively between 10-100um, 20-300um, 130-800um and 250-1050um.

[0016] The present invention also provides a method for preparing the aforementioned graded-index optical fiber in which pump absorption and core numerical aperture are negatively correlated along the axial direction, comprising the following steps:

[0017] (1) Using a high-purity quartz tube as a base tube, and etching its inner wall;

[0018] (2) performing step layer deposition on the inner wall of the etched substrate;

[0019] (3) After the step layer is deposited, a loose layer is deposited inside the tube. The raw material selected is SiCl4. After the loose layer is deposited, the base tube with the loose layer is removed and immersed in a rare earth ion solution;

[0020] (4) oxidizing and vitrifying the solution-doped loose layer in sequence; repeating the processes (3) and (4) until the set core layer diameter requirement is reached;

[0021] (5) The reaction tube after the step layer and the core layer are deposited is melted and shrunk at high temperature to form an optical fiber preform.

[0022] In step (3), during the deposition process, the SiCl4 flow rate is 100-600 sccm, the deposition temperature is set to 1200-1400°C, and the deposition temperature gradually decreases as the axial position increases.

[0023] Furthermore, the preparation method further comprises:

[0024] (6) After the optical fiber preform rod has been tested, it is sheathed with a high-purity quartz tube according to a fixed core-to-shell ratio to form a sheathed rod;

[0025] (7) processing the cladding shape of the casing rod according to a certain core-cladding ratio to form a regular octagonal preform rod with a certain core-cladding ratio;

[0026] (8) Drawing, coating, and testing a regular octagonal preform rod to form an optical fiber in which pump absorption gradually increases along the axial direction of the optical fiber and the core numerical aperture gradually decreases along the axial direction of the optical fiber.

[0027] By controlling the axial rate of change of the relative refractive index of the control layer to be greater than that of the core layer, the present invention achieves a gradual increase in pump absorption and a gradual decrease in the core numerical aperture along the axial direction. In other words, the pump absorption and the core numerical aperture exhibit a negative correlation and gradual change along the axial direction. This facilitates achieving better population inversion and uniform bending mode selection in practice. The present invention's preparation method is simple, and the control of the gradual change parameters during the preparation process can be achieved through automated equipment, resulting in excellent stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 A schematic diagram of the radial cross-sectional structure of a graded-index optical fiber in which pump absorption and core numerical aperture are negatively correlated along the axial direction, provided by an embodiment of the present invention;

[0029] Figure 2 Schematic diagram of the change trend of the ratio of step dopant flow rate / SiCl4 flow rate with axial position;

[0030] Figure 3 Schematic diagram of the variation trend of the deposition temperature of the loose layer in the core layer with the axial position;

[0031] Figure 4 Schematic diagram of the variation trend of the relative refractive index of the core layer and pump absorption with the axial position;

[0032] Figure 5 Schematic diagram of the variation trend of the relative refractive index of the core layer and the relative refractive index of the step layer with the axial position;

[0033] Figure 6 This is a graph showing the change in step dopant POCl3 flow rate with axial position;

[0034] Figure 7 This is the axial refractive index distribution diagram of the graded-index fiber, in which the pump absorption is negatively correlated with the core numerical aperture along the axial direction. DETAILED DESCRIPTION

[0035] To make the purpose, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions in the embodiments of the present invention are clearly and completely described below. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, embodiments obtained by ordinary technicians in this field without making creative work, and all other embodiments obtained by ordinary technicians in this field without making creative work, are within the scope of protection of the present invention.

[0036] The embodiment of the present invention provides a graded-index optical fiber in which pump absorption and core numerical aperture are negatively correlated along the axial direction, and its structural schematic diagram is shown as follows: Figure 1As shown, it includes a core layer, a step layer, a cladding layer and a coating layer arranged in sequence from the inside to the outside along the radial direction, and the coating layer is composed of an inner low-refractive index coating layer and an outer high-refractive index coating layer;

[0037] Among them, the refractive index of the outer coating layer n5> the refractive index of the core layer n1> the refractive index of the step layer n2> the refractive index of the cladding layer n3> the refractive index of the inner coating layer n4,

[0038] The refractive index n1 of the core layer changes gradually along the axial direction, and the refractive index n2 of the step layer changes gradually along the axial direction, and the changing rate of n2-n3 along the axial direction is greater than the changing rate of n1-n3 along the axial direction.

[0039] According to some embodiments of the present invention, the starting value of n2-n3 is between 0.0005 and 0.016, and the rate of change of n2-n3 along the axial direction is greater than or equal to 1*10 -6 / m;

[0040] And / or, the starting value of n1-n3 is between 0.0010 and 0.02, and is greater than the starting value of n2-n3, and the rate of change of n1-n3 along the axial direction is greater than or equal to 0.5*10 -6 / m, and is less than the rate of change of n2-n3 along the axial direction.

[0041] According to some embodiments of the present invention, the core layer is mainly composed of a matrix SiO2 and one or more rare earth oxides selected from Yb2O3, Er2O3, Tm2O3, and Ho2O3. The doping concentration of the rare earth oxide in the core layer is controlled to achieve a gradual change in n1 along the axial direction. The core layer absorbs the pump light and converts it into laser light. The refractive index of the core layer is n1, and the refractive index of the step layer is n2. n1>n2. The core numerical aperture NA=(n1 2 -n2 2 ) 1 / 2 The optical waveguide structure formed between the two realizes the transmission of laser in the fiber core, and the gradual change of the pump absorption of the gain fiber along the axial direction is achieved by controlling the gradual change of the doping concentration of rare earth oxide in the fiber core along the axial direction of the optical fiber.

[0042] According to some embodiments of the present invention, the step layer is primarily composed of a matrix SiO2 and one or more dopants selected from Ge2O3 and P2O5; the concentration of the dopants in the step layer is controlled to achieve a gradual change in n2 along the axial direction. The step layer is primarily used to form an optical waveguide structure with the core layer to ensure the transmission of laser light in the core. The step layer is primarily used to control the axial refractive index of the gain fiber step layer relative to the axial refractive index of the core layer to achieve a gradual change in the numerical aperture of the gain fiber core along the axial direction of the fiber. The refractive index of the step layer is n2, and the refractive index of the cladding is n3, where n2>n3. The step layer can also achieve the transmission of pump light in the same way as the cladding.

[0043] According to some embodiments of the present invention, the cladding is composed of a matrix SiO2. For the transmission of pump light, the refractive index of the cladding is n3, and the refractive index of the inner coating is n4, where n3>n4. The optical waveguide structure formed by the refractive index difference between the cladding and the inner coating can realize the transmission of pump light in the cladding. The numerical aperture NA of the cladding is (n3 2 -n4 2 ) 1 / 2 .

[0044] According to some embodiments of the present invention, the diameters of the core layer, step layer, cladding layer, and coating layer are respectively 10-100 μm, 20-300 μm, 130-800 μm, and 250-1050 μm. It should be noted that the diameters are the outermost widths of each layer in a radial cross-section, which is a common measurement method in the art. For example, the diameter of the step layer includes the diameter of the core layer.

[0045] The embodiment of the present invention also provides a method for preparing the aforementioned graded-index optical fiber in which pump absorption and core numerical aperture are negatively correlated along the axial direction, comprising the following steps:

[0046] (1) Using a high-purity quartz tube as a base tube, and etching its inner wall;

[0047] (2) performing step layer deposition on the inner wall of the etched substrate;

[0048] (3) After the step layer is deposited, a loose layer is deposited inside the tube. The raw material selected is SiCl4. After the loose layer is deposited, the base tube with the loose layer is removed and immersed in a rare earth ion solution;

[0049] (4) The loose layer doped with the solution is oxidized and vitrified in sequence; the processes (3) and (4) are repeated until the set core layer diameter requirement is reached, and the number of layers is usually between 2 and 10;

[0050] (5) melting and shrinking the reaction tube after depositing the step layer and the core layer at high temperature to form an optical fiber preform;

[0051] (6) After the optical fiber preform rod has been tested, it is sheathed with a high-purity quartz tube according to a fixed core-to-shell ratio to form a sheathed rod;

[0052] (7) processing the cladding shape of the casing rod according to a certain core-cladding ratio to form a regular octagonal preform rod with a certain core-cladding ratio;

[0053] (8) Drawing, coating, and testing a regular octagonal preform rod to form an optical fiber in which pump absorption gradually increases along the axial direction of the optical fiber and the core numerical aperture gradually decreases along the axial direction of the optical fiber.

[0054] Specifically, in step (1), a high-purity quartz tube is used as a substrate tube. After the substrate tube is cleaned with HF acid and high-purity water, the inner wall of the substrate tube is etched with gas, and the etching gas is one or more of SF6, C2F6, and CF4.

[0055] In step (2), a step layer is deposited on the inner wall of the etched substrate tube. The raw materials used for the step layer deposition are the substrate SiCl4 and one or more of the dopants GeCl4 and POCl3. The substrate SiCl4 flow rate is set to 100-600sccm, and the dopant flow rate is set to 10-1000sccm; the deposition temperature is set to 1800-2100℃; during the deposition process, the dopant flow rate / SiCl4 flow rate ratio is set to gradually increase with the increase of the axial position, which can be referred to Figure 2 As shown, the refractive index of the step layer gradually increases with the increase of the axial position. Repeat (2) until the set number of layers is reached to meet the step layer diameter requirement. The number of layers is usually between 5 and 40 layers, and the step layer diameter / core layer diameter is usually required to be ≥2.

[0056] In step (3), after the step layer is deposited, the core layer is deposited. The core layer is prepared by MCVD deposition of a loose layer combined with a solution doping method. After the step layer is deposited, the loose layer is continued to be deposited in the tube. The raw material selected is the matrix SiCl4. During the deposition process, the SiCl4 flow rate is 100-600sccm; the deposition temperature is set to 1200-1400℃; after the deposition of the core loose layer, the base tube with the loose layer is removed and immersed in a rare earth ion solution; the rare earth solution is composed of one or more rare earth chlorides YbCl3, ErCl3, TmCl3, and HoCl3; since the deposition temperature affects the deposition density of the core loose layer, and thus affects the doping concentration of rare earth ions, the deposition temperature is set to gradually decrease as the axial position increases during the deposition process. For reference, Figure 3 As shown, the loose layer of the core becomes looser as the axial position increases. In the process of solution doping, the doping concentration of the loose layer of the core increases as the axial position increases, thereby making the pump absorption gradually increase as the axial position increases. The relative refractive index (n1-n3) of the core layer also gradually increases as the axial position increases. For reference, Figure 4 When the slope of the change of the relative refractive index of the core layer with the axial position is smaller than the slope of the change of the relative refractive index of the step layer with the axial position, you can refer to Figure 5 As shown, it is possible to achieve a gradual increase in pump absorption along the axial direction while a gradual decrease in the core numerical aperture along the axial direction.

[0057] In a specific embodiment of the present invention, the method for preparing a graded-index optical fiber in which pump absorption and core numerical aperture are negatively correlated along the axial direction comprises the following steps:

[0058] (1) A high-purity quartz tube is used as a substrate tube. After the substrate tube is cleaned with HF acid and high-purity water in sequence, the inner wall of the substrate tube is etched with gas, and C2F6 is selected as the etching gas;

[0059] (2) Step layer deposition is performed on the inner wall of the etched substrate tube, and the deposition temperature is set to 1950°C; the raw materials used for step layer deposition are substrate SiCl4 and POCl3, where the SiCl4 flow rate is fixed at 500 sccm and the initial POCl3 flow rate is 100 sccm. During the deposition process, the POCl3 flow rate is set as follows: Figure 6 The refractive index of the step layer is gradually increased with the axial position. The process (2) is repeated to deposit 13 layers.

[0060] (3) After the step layer is deposited, the rare earth doped core layer is deposited. The rare earth doped core layer is prepared by MCVD deposition of loose layer combined with solution doping method. After the step layer is deposited, the loose layer is continued to be deposited in the tube. The raw material used for the deposition of the core loose layer is the matrix SiCl4. During the deposition process, the SiCl4 flow rate is 400sccm; the deposition temperature is initially set to 1300℃; after the deposition of the core loose layer, the base tube with the loose layer is removed and immersed in a rare earth ion solution; the rare earth solution is composed of rare earth chloride YbCl3; since the deposition temperature affects the deposition density of the core loose layer, and thus affects the doping concentration of rare earth ions, the deposition temperature is set to gradually decrease as the axial position increases during the deposition process, as shown in the following example. Figure 3 As shown, the loose layer of the core becomes looser as the axial position increases, and the doping concentration of the loose layer of the core increases as the axial position increases during the solution doping process, thereby gradually increasing the pump absorption as the axial position increases, and the relative refractive index of the core layer also gradually increases as the axial position increases. Figure 4 When the slope of the change of the relative refractive index of the core layer with the axial position is smaller than the slope of the change of the relative refractive index of the step layer with the axial position, as shown in Figure 5 As shown, it is possible to achieve a gradual increase in pump absorption along the axial direction while a gradual decrease in the core numerical aperture along the axial direction.

[0061] (4) The loose layer doped with the solution is oxidized and vitrified in sequence: the oxidation mainly oxidizes the doped rare earth chloride into rare earth oxide; the vitrification mainly vitrifies the loose layer doped with rare earth oxide; the processes (3) and (4) are repeated to deposit 3 layers;

[0062] (5) melting and shrinking the reaction tube after depositing the step layer and the rare earth doped core layer at high temperature to form an optical fiber preform core rod;

[0063] (6) After the optical fiber preform rod has been tested, it is sheathed with a high-purity quartz tube according to a fixed core-to-shell ratio to form a sheathed rod;

[0064] (7) processing the cladding shape of the casing rod according to a certain core-cladding ratio to form a regular octagonal preform rod with a certain core-cladding ratio;

[0065] (8) The regular octagonal preform rod is drawn, coated and tested to form an optical fiber in which the pump absorption gradually increases along the axial direction of the optical fiber and the core numerical aperture gradually decreases along the axial direction of the optical fiber. The axial refractive index distribution of the core layer, step layer and cladding of the optical fiber is as follows: Figure 7 shown.

[0066] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the various embodiments of the present invention.

Claims

1. A graded-index optical fiber in which pump absorption and core numerical aperture are negatively correlated along the axial direction, characterized in that: It includes a core layer, a step layer, a cladding layer and a coating layer arranged in sequence from the inside to the outside along the radial direction. Among them, the refractive index of the core layer n1> the refractive index of the step layer n2> the refractive index of the cladding layer n3> the refractive index of the coating layer, the core numerical aperture NA = (n1 2 -n2 2 ) 1 / 2 ; The refractive index n1 of the core layer changes gradually along the axial direction, and the refractive index n2 of the step layer changes gradually along the axial direction, and the rate of change of n2-n3 along the axial direction is greater than the rate of change of n1-n3 along the axial direction; The starting value of n2-n3 is between 0.0005 and 0.016, and the rate of change of n2-n3 along the axial direction is greater than or equal to 1×10 -6 / m; And / or, the starting value of n1-n3 is between 0.0010 and 0.02, and is greater than the starting value of n2-n3, and the rate of change of n1-n3 along the axial direction is greater than or equal to 0.5×10 -6 / m, and is less than the rate of change of n2-n3 along the axial direction.

2. The graded-index optical fiber with a negative correlation between pump absorption and core numerical aperture along the axial direction according to claim 1, characterized in that: The core layer is mainly composed of a matrix SiO2 and one or more rare earth oxides Yb2O3, Er2O3, Tm2O3, and Ho2O3; and a gradual change trend of n1-n3 along the axial direction is achieved by controlling the doping concentration of the rare earth oxides in the core layer.

3. The graded-index optical fiber with a negative correlation between pump absorption and core numerical aperture along the axial direction according to claim 1, characterized in that: The step layer is mainly composed of a matrix SiO2 and one or more dopants selected from Ge2O3 and P2O5. The n2-n3 gradient along the axial direction is achieved by controlling the concentration of the dopants in the step layer.

4. The graded-index optical fiber with a negative correlation between pump absorption and core numerical aperture along the axial direction according to claim 1, characterized in that: The cladding layer is composed of a matrix SiO2.

5. The graded-index optical fiber with a negative correlation between pump absorption and core numerical aperture along the axial direction according to claim 1, characterized in that: The coating layer consists of an inner low-refractive-index coating layer and an outer high-refractive-index coating layer. The refractive index of the low-refractive-index coating layer is n4, and the refractive index of the high-refractive-index coating layer is n5, where n4<n3 and n5>n1.

6. The graded-index optical fiber with a negative correlation between pump absorption and core numerical aperture along the axial direction according to claim 1, characterized in that: The diameters of the core layer, step layer, cladding layer and coating layer are respectively between 10-100um, 20-300um, 130-800um and 250-1050um.

7. The method for preparing a graded-index optical fiber with a negative correlation between pump absorption and core numerical aperture along the axial direction according to any one of claims 1 to 6, characterized in that: The following steps are involved: (1) Using a high-purity quartz tube as a base tube, and etching its inner wall; (2) performing step layer deposition on the inner wall of the etched substrate; (3) After the step layer is deposited, a loose layer is deposited inside the tube. The raw material selected is SiCl4. After the loose layer is deposited, the base tube with the loose layer is removed and immersed in a rare earth ion solution; (4) oxidizing and vitrifying the solution-doped loose layer in sequence; repeating the processes (3) and (4) until the set core layer diameter requirement is reached; (5) The reaction tube where the step layer and the core layer are deposited is melted and shrunk at high temperature to form an optical fiber preform.

8. The method for preparing a graded-index optical fiber with a negative correlation between pump absorption and core numerical aperture along the axial direction according to claim 7, characterized in that: In step (3), during the deposition process, the SiCl4 flow rate is 100-600 sccm, the deposition temperature is set to 1200-1400°C, and the deposition temperature gradually decreases as the axial position increases.

9. The method for preparing a graded-index optical fiber with a negative correlation between pump absorption and core numerical aperture along the axial direction according to claim 7, characterized in that: The preparation method further comprises: (6) After the optical fiber preform rod has been tested, it is sheathed with a high-purity quartz tube according to a fixed core-to-shell ratio to form a sheathed rod; (7) processing the cladding shape of the casing rod according to a certain core-cladding ratio to form a regular octagonal preform rod with a certain core-cladding ratio; (8) Drawing, coating, and testing a regular octagonal preform rod to form an optical fiber in which pump absorption gradually increases along the axial direction of the optical fiber and the core numerical aperture gradually decreases along the axial direction of the optical fiber.

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