Optical fiber with negative correlation between pump absorption and core numerical aperture along the axial direction and monotonically gradient change and preparation method thereof

Abstract

The present invention relates to the field of gain optical fibers, and in particular to an optical fiber in which pump absorption and core numerical aperture are negatively correlated and monotonically gradient along the axial direction, and a preparation method thereof. The optical fiber of the present invention comprises a core layer, a cladding layer, and a coating layer arranged radially from the inside out; the refractive index of the core layer is greater than the refractive index of the cladding, and the refractive index of the cladding is greater than the refractive index of the coating layer; the core layer comprises a matrix SiO2, a refractive index-reducing component, and a rare earth oxide; the doping concentrations of the refractive index-reducing component and the rare earth oxide both show a gradient trend along the axial direction, and the rate of decrease of the refractive index along the axial direction contributed by the gradual change in the doping concentration of the refractive index-reducing component is greater than the rate of increase of the refractive index along the axial direction contributed by the gradual change in the doping concentration of the rare earth oxide. The present invention can achieve a monotonically gradient with a negative correlation between pump absorption and core numerical aperture along the axial direction through reasonable design, and the preparation process is simple and the stability is good.

Description

Technical Field

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

[0002] In the field of fiber lasers, gain fibers are a core component. Currently, gain fibers with uniform pump absorption and core numerical aperture are the most widely used due to their simple structure and ease of production. However, when coiled on a water-cooled plate, this type of gain fiber has a smaller inner coil radius, resulting in higher absorption and higher inner coil temperatures. This temperature increase can lead to mode instability. Furthermore, the larger bending losses associated with a smaller inner coil radius can cause fundamental mode loss, making it difficult to further increase laser power.

[0003] Later, some optical fibers with axial gradient designs were developed. However, pump absorption is generally positively correlated with core numerical aperture, making it impossible to achieve a gain fiber with low pump absorption and high core numerical aperture at one end and high pump absorption and low core numerical aperture at the other end. In other words, it is impossible to achieve a negatively correlated gradient of pump absorption and core numerical aperture along the axial direction. For optical fibers with a positive correlation between pump absorption and core numerical aperture, if the end with high pump absorption and large core numerical aperture is coiled on the inner ring during use, the small coiling radius of the inner ring will further increase the pump absorption and cause the inner ring temperature to rise. If the end with low pump absorption and small core numerical aperture is coiled on the inner ring, the small coiling radius of the inner ring will result in high bending loss, which may cause fundamental mode loss and make it difficult to further increase the laser power.

[0004] For example, CN 110028236 A discloses an optical fiber with longitudinally graded ion doping concentration and its preparation method. By controlling the different soaking times of different regions of the silica loose layer in solution, an optical fiber preform with longitudinally graded rare earth ion doping concentration is prepared, as well as a gain fiber in which the rare earth doping ion distribution in the core region is uniform in the radial direction, but the doping ion concentration varies gradually along the fiber length. However, the gain fiber prepared using this method cannot achieve an infinitesimal gradient due to the distance control between different soaking times, resulting in a gradient in the longitudinal doping ion concentration of the gain fiber. CN 113603352 A discloses an active optical fiber preform with axially graded doping concentration and its preparation method. By axially graded deposition temperature, a loose layer is deposited on the inner wall of a reaction tube to achieve an axial gradient in doping ion concentration. Because the core numerical aperture is positively correlated with the doping concentration under the same doping system, the core numerical aperture of the gain fiber prepared by the methods used in the above two technologies will positively gradient with the gradient of the doping ion concentration, and cannot achieve an inverse gradient between the doping ion concentration and the core numerical aperture. Summary of the Invention

[0005] The purpose of the present invention is to provide an optical fiber in which pump absorption and core numerical aperture are negatively correlated and monotonically gradient along the axial direction, and a preparation method thereof, wherein the 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 an optical fiber in which pump absorption and core numerical aperture are negatively correlated and monotonically changed along the axial direction, comprising a core layer, a cladding layer and a coating layer arranged in sequence from the inside to the outside along the radial direction;

[0007] The refractive index of the core layer is greater than that of the cladding layer, and the refractive index of the cladding layer is greater than that of the coating layer.

[0008] The core layer includes a matrix SiO2, a refractive index reducing component and a rare earth oxide, wherein the doping concentrations of the refractive index reducing component and the rare earth oxide both show a gradual trend along the axial direction, and the rate of decrease of the refractive index along the axial direction contributed by the gradual change of the doping concentration of the refractive index reducing component is greater than the rate of increase of the refractive index along the axial direction contributed by the gradual change of the doping concentration of the rare earth oxide.

[0009] According to the optical fiber provided by the present invention, the pump absorption and the core numerical aperture are negatively correlated along the axial direction and monotonically gradient, and the rate of decrease of the refractive index along the axial direction due to the gradient contribution of the doping concentration of the refractive index reducing component is greater than or equal to 1*10 -6 / m;

[0010] And / or, the rate of increase of the refractive index along the axial direction contributed by the gradual change of the rare earth oxide doping concentration is greater than or equal to 0.5*10 -6 / m, and is less than the rate of decrease of the refractive index along the axial direction due to the gradual change in the doping concentration of the refractive index-lowering component.

[0011] According to the optical fiber provided by the present invention, in which pump absorption and core numerical aperture are negatively correlated along the axial direction and monotonically varied, the refractive index reducing component is one or more of F and B2O3.

[0012] According to the optical fiber provided by the present invention, in which pump absorption and core numerical aperture are negatively correlated and monotonically gradient along the axial direction, the rare earth oxide is one or more of Yb2O3, Er2O3, Tm2O3, and Ho2O3.

[0013] According to the optical fiber provided by the present invention, in which pump absorption and core numerical aperture are negatively correlated and monotonically varied along the axial direction, the cladding is composed of a matrix SiO2.

[0014] According to the optical fiber provided by the present invention, in which pump absorption and core numerical aperture are negatively correlated along the axial direction and monotonically gradient, the coating layer is composed 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 lower than the refractive index of the cladding layer, and the refractive index of the high-refractive-index coating layer is higher than the refractive index of the core layer.

[0015] According to the optical fiber provided by the present invention, the pump absorption and the core numerical aperture are negatively correlated along the axial direction and monotonically change, and the diameters of the core layer, cladding layer and coating layer are 10-100um, 130-800um and 250-1050um respectively.

[0016] The present invention also provides a method for preparing the above-mentioned optical fiber in which the pump absorption and the core numerical aperture are negatively correlated and monotonically changed 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) Depositing the core layer on the inner wall of the etched substrate tube by using MCVD to deposit a loose layer combined with solution doping;

[0019] (3) oxidizing and vitrifying the solution-doped loose layer in sequence;

[0020] (4) Repeat the process of (2) and (3) until the set core layer diameter requirement is reached;

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

[0022] Furthermore, in step (2), the raw materials used for depositing the loose layer are SiCl4 and raw materials for reducing the refractive index component.

[0023] Furthermore, during the deposition process, the SiCl4 flow rate is 100-600 sccm, the raw material flow rate of the refractive index reducing component is 5-300 sccm and gradually increases with the increase of the axial position, and the deposition temperature is set to 1200-1400°C and gradually decreases with the increase of the axial position.

[0024] Furthermore, the preparation method further comprises:

[0025] (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;

[0026] (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;

[0027] (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.

[0028] The present invention achieves a gradual increase in pump absorption and a gradual decrease in core numerical aperture along the axial direction by controlling the refractive index increase rate contributed by the gradual change in the rare earth oxide doping concentration in the core layer along the axial direction of the optical fiber to be less than the refractive index decrease rate contributed by the gradual change in the doping concentration of the refractive index-reducing component along the axial direction of the optical fiber. In other words, the pump absorption and core numerical aperture gradually change in an axial direction with a negative correlation. This facilitates achieving better population inversion and uniform bending mode selection in application. The preparation method of the present invention is simple, and the control of the gradual change parameters during the preparation process can be achieved through automated equipment, resulting in good stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 A schematic diagram of the radial cross-sectional structure of an optical fiber provided by an embodiment of the present invention, wherein the pump absorption and the core numerical aperture are negatively correlated and monotonically varied along the axial direction;

[0030] Figure 2 is a graph showing the change in the flow rate of the refractive index reducing component material in the core layer with the axial position;

[0031] Figure 3 This is a graph showing the variation of deposition temperature of the loose layer in the core layer with the axial position;

[0032] Figure 4 The refractive index contributed by the rare earth oxide and the refractive index-lowering component (the slope is originally negative, but it is placed on the positive Y-axis for easier comparison) varies with axial position;

[0033] Figure 5 The axial refractive index distribution diagram (stereoscopic diagram) of a monotonically graded-index optical fiber in which pump absorption is negatively correlated with the core numerical aperture along the axial direction;

[0034] Figure 6 This is the axial refractive index distribution diagram (plan view) of a monotonically graded-index optical fiber in which pump absorption is negatively correlated with the core numerical aperture along the axial direction. DETAILED DESCRIPTION

[0035] To make the objectives, 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 only part of the embodiments of the present invention, not all of them. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0036] The embodiment of the present invention provides an optical fiber in which pump absorption and core numerical aperture are negatively correlated and monotonically changed along the axial direction, and a schematic diagram of the radial cross-sectional structure thereof is shown as follows: Figure 1 As shown, it includes a core layer, a cladding layer and a coating layer arranged in sequence from the inside to the outside along the radial direction;

[0037] The core layer includes a SiO2 matrix, a refractive index-lowering component, and a rare earth oxide. The doping concentrations of the refractive index-lowering component and the rare earth oxide both exhibit a gradual change along the axial direction, and the rate of decrease in refractive index along the axial direction due to the gradual change in the doping concentration of the refractive index-lowering component is greater than the rate of increase in refractive index along the axial direction due to the gradual change in the doping concentration of the rare earth oxide. It should be noted that the rate of decrease only considers the magnitude, not the positive or negative value.

[0038] The coating layer consists of an inner low-refractive-index coating layer and an outer high-refractive-index coating layer; the refractive index relationship of each layer is:

[0039] The refractive index n4 of the outer coating layer is greater than the refractive index n1 of the core layer, and the refractive index n2 of the cladding layer is greater than the refractive index n3 of the inner coating layer.

[0040] According to some embodiments of the present invention, the rate of decrease of the refractive index along the axial direction contributed by the gradual change in the doping concentration of the refractive index reducing component is greater than or equal to 1*10 -6 / m;

[0041] The rate of increase of the refractive index along the axial direction contributed by the gradual change of the rare earth oxide doping concentration is greater than or equal to 0.5*10 -6 / m, and is less than the rate of decrease of the refractive index along the axial direction due to the gradual change in the doping concentration of the refractive index-lowering component.

[0042] According to some embodiments of the present invention, the refractive index reducing component is one or more of F and B2O3.

[0043] According to some embodiments of the present invention, the rare earth oxide is one or more of Yb2O3, Er2O3, Tm2O3, and Ho2O3.

[0044] 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 cladding is n2. n1>n2, and the numerical aperture of the core NA=(n1 2 -n2 2 ) 1 / 2The optical waveguide structure formed between the two realizes the transmission of laser light in the fiber core. The gradual change (increase) of the rare earth oxide doping concentration in the core layer along the axial direction of the optical fiber realizes the gradual change (increase) of the pump absorption of the gain fiber along the axial direction, and at the same time brings about an increase in n1. The gradual change (increase) of the doping concentration of the refractive index reducing component in the core layer along the axial direction of the optical fiber brings about a decrease in n1. When the decrease rate of n1 is greater than the increase rate of n1, the refractive index of the entire core layer gradually decreases along the axial direction, realizing the gradual change (decreasing) of the core numerical aperture of the gain fiber along the axial direction, that is, realizing a monotonic gradient with a negative correlation between the pump absorption and the core numerical aperture along the axial direction.

[0045] 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 n2, and the refractive index of the inner coating is n3, n2>n3. 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 (n2 2 -n3 2 ) 1 / 2 .

[0046] According to some embodiments of the present invention, the diameters of the core layer, cladding layer, and coating layer are 10-100 μm, 130-800 μm, and 250-1050 μm, respectively. 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 cladding layer includes the diameter of the core layer.

[0047] The embodiment of the present invention also provides a method for preparing the optical fiber in which the pump absorption and the core numerical aperture are negatively correlated and monotonically gradient along the axial direction, comprising the following steps:

[0048] (1) Using a high-purity quartz tube as a base tube, and etching its inner wall;

[0049] (2) Depositing the core layer on the inner wall of the etched substrate tube by using MCVD to deposit a loose layer combined with solution doping;

[0050] (3) oxidizing and vitrifying the solution-doped loose layer in sequence; wherein the oxidation mainly oxidizes the doped rare earth chloride into rare earth oxide; and the vitrification mainly vitrifies the loose layer doped with rare earth oxide and the refractive index reducing component;

[0051] (4) Repeat the process of (2) and (3) until the required core diameter is reached; the number of layers is usually between 2 and 10;

[0052] (5) melting and shrinking the reaction tube after depositing the core layer at high temperature to form an optical fiber preform;

[0053] (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;

[0054] (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;

[0055] (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.

[0056] 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.

[0057] In step (2), a core layer is deposited on the inner wall of the etched substrate tube. The core layer is prepared by MCVD deposition of a loose layer combined with a solution doping method. The raw materials used for the loose layer deposition are one or more of the raw materials SiCl4 for the substrate and SF6, C2F6, SiF4, CF4, C2F2Cl2, SOF2, Si2F6, BCl3, and BF3 for reducing the refractive index. During the deposition process, the flow rate of SiCl4 is 100-600 sccm, and the flow rate of the raw material for reducing the refractive index is 5-300 sccm. During the deposition process, the flow rate of the raw material for reducing the refractive index is set to gradually increase with the increase of the axial position. Figure 2 As shown (the starting point and slope of the straight line can be adjusted within the allowable range); the deposition temperature is set to 1200-1400℃; after the deposition of the loose layer is completed, the substrate 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 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, which can be referred to Figure 3 As shown (the starting point and slope of the straight line can be adjusted within the allowable range), the loose layer becomes looser as the axial position increases, and the doping concentration of the loose layer increases as the axial position increases during the solution doping process, thereby gradually increasing the pump absorption as the axial position increases. When the slope of the refractive index of the component with reduced refractive index decreases with axial position is greater than the slope of the refractive index of the rare earth oxide with axial position, you can refer to Figure 4 As shown (the starting point and slope of the straight line can be adjusted within the allowable range), it is possible to achieve a gradual increase in absorption along the axial direction while a gradual decrease in the numerical aperture of the fiber core along the axial direction.

[0058] In a specific embodiment of the present invention, a method for preparing an optical fiber in which pump absorption and core numerical aperture are negatively correlated and monotonically gradient along the axial direction comprises the following steps:

[0059] (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 CF4 is selected as the etching gas;

[0060] (2) The core layer is deposited on the inner wall of the etched substrate. The core layer is prepared by MCVD deposition of a loose layer combined with a solution doping method. The raw materials used for loose layer deposition are SiCl4, a substrate raw material, and C2F6, a raw material for reducing the refractive index component. During the deposition process, the SiCl4 flow rate is 400 sccm, and the initial flow rate of C2F6 is 20 sccm. The C2F6 flow rate is set as follows during the deposition process: Figure 2 The deposition temperature is initially set to 1300°C. After the deposition of the loose layer is completed, the substrate 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 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. Specifically, Figure 3 As shown, the loose layer becomes looser as the axial position increases, and the doping concentration of the loose layer increases as the axial position increases during the solution doping process, thereby gradually increasing the pump absorption as the axial position increases. When the slope of the refractive index of the component with reduced refractive index decreases with the axial position is greater than the slope of the refractive index of the rare earth oxide with the axial position, as shown in FIG. Figure 4 As shown, it is possible to achieve a gradual increase in absorption along the axial direction while a gradual decrease in the numerical aperture of the fiber core along the axial direction.

[0061] (3) oxidizing and vitrifying the solution-doped loose layer in sequence, wherein the oxidation mainly oxidizes the doped rare earth chloride into rare earth oxide; and the vitrification mainly vitrifies the loose layer doped with rare earth oxide and the refractive index reducing component;

[0062] (4) Repeat the process of (2) and (3) to deposit 3 layers;

[0063] (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;

[0064] (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;

[0065] (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;

[0066] (8) Drawing, coating and testing the regular octagonal preform rod 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 refractive index distribution of the rare earth oxide in the core layer of the optical fiber axial direction and the refractive index distribution of the refractive index reducing component are shown in the figure. Figure 5 and Figure 6 shown.

[0067] 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. An optical fiber in which pump absorption and core numerical aperture are negatively correlated and monotonically varied along the axial direction, characterized in that: It includes a core layer, a cladding layer and a coating layer arranged in sequence from the inside to the outside along the radial direction; The refractive index of the core layer is greater than that of the cladding layer, and the refractive index of the cladding layer is greater than that of the coating layer. The core layer includes a matrix SiO2, a refractive index reducing component, and a rare earth oxide, wherein the doping concentrations of the refractive index reducing component and the rare earth oxide both show a gradual trend along the axial direction, and the rate of decrease of the refractive index along the axial direction due to the gradual change of the doping concentration of the refractive index reducing component is greater than the rate of increase of the refractive index along the axial direction due to the gradual change of the doping concentration of the rare earth oxide; The rate of decrease of the refractive index along the axial direction due to the gradual change in the doping concentration of the component that reduces the refractive index is greater than or equal to 1*10 -6 / m; And / or, the rate of increase of the refractive index along the axial direction contributed by the gradual change of the rare earth oxide doping concentration is greater than or equal to 0.5*10 -6 / m, and is less than the rate of decrease of the refractive index along the axial direction due to the gradual change in the doping concentration of the refractive index-lowering component.

2. The optical fiber according to claim 1, wherein the pump absorption and the core numerical aperture are negatively correlated and monotonically varied along the axial direction, characterized in that: The refractive index reducing component is one or more of F and B2O3.

3. The optical fiber according to claim 1, wherein the pump absorption and the core numerical aperture are negatively correlated and monotonically varied along the axial direction, characterized in that: The rare earth oxide is one or more of Yb2O3, Er2O3, Tm2O3, and Ho2O3.

4. The optical fiber according to claim 1, wherein the pump absorption and the core numerical aperture are negatively correlated and monotonically varied along the axial direction, The cladding layer is composed of a matrix SiO2.

5. The optical fiber according to claim 1, wherein the pump absorption and the core numerical aperture are negatively correlated and monotonically varied 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 lower than that of the cladding layer, and the refractive index of the high-refractive-index coating layer is higher than that of the core layer.

6. The optical fiber according to claim 1, wherein the pump absorption and the core numerical aperture are negatively correlated and monotonically varied along the axial direction, characterized in that: The diameters of the core layer, cladding layer and coating layer are 10-100um, 130-800um and 250-1050um respectively.

7. The method for preparing an optical fiber with a negative correlation between pump absorption and core numerical aperture along the axial direction and a monotonically gradient change 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) Depositing the core layer on the inner wall of the etched substrate tube by using MCVD to deposit a loose layer combined with solution doping; (3) oxidizing and vitrifying the solution-doped loose layer in sequence; (4) Repeat the process of (2) and (3) until the set core layer diameter requirement is reached; (5) The reaction tube where the core layer is deposited is melted and shrunk at high temperature to form an optical fiber preform.

8. The method for preparing an optical fiber with a negative correlation between pump absorption and core numerical aperture along the axial direction and a monotonically gradient change according to claim 7, characterized in that: In step (2), the raw materials used for depositing the loose layer are SiCl4 and a raw material for reducing the refractive index component; And / or, during the deposition process, the SiCl4 flow rate is 100-600 sccm, the raw material flow rate of the refractive index reducing component is 5-300 sccm and gradually increases with the increase of the axial position, and the deposition temperature is set to 1200-1400°C and gradually decreases with the increase of the axial position.

9. The method for preparing an optical fiber with a negative correlation between pump absorption and core numerical aperture along the axial direction and a monotonically gradient change 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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