A low-refractive-index doped mother tube, capillary, preparation method, and application

Abstract

This invention belongs to the field of optical fiber communication technology and discloses a low-refractive-index doped mother tube, a capillary, a preparation method, and applications. The invention first deposits a low-refractive-index doped silicon dioxide layer on the inner surface of a pure silicon dioxide liner to prepare a low-refractive-index doped mother tube. Then, the low-refractive-index doped mother tube is heated to its softening temperature and drawn to obtain a low-refractive-index doped capillary. The minimum relative refractive index difference of the low-refractive-index doped silicon dioxide layer in this invention is -1.8% to -2.6%. The low-refractive-index doped capillary designed in this invention can be used to prepare optical fiber power combiners and can improve the performance of the combiners.

Description

Technical Field

[0001] This invention belongs to the field of optical fiber communication technology, and more specifically, relates to a low refractive index doped mother tube, a capillary tube, a preparation method, and an application. Background Technology

[0002] Currently, the output power of single-mode fiber lasers has reached the kilowatt level, and there is still room for improvement. However, due to limitations such as thermal damage, nonlinear effects, fiber end-face damage, and thermal lensing effects, the output power of single-mode fiber lasers cannot be increased indefinitely. Since further increasing the output power of individual fiber lasers is becoming increasingly difficult, fiber laser combining technology is being considered to overcome the power bottleneck of single-mode fiber lasers. Among these technologies, fiber laser combining based on fiber power combiners can achieve all-fiber integration, offering advantages such as compact structure, high reliability, low cost, and good stability. It is currently widely used in high-power all-fiber laser systems. Based on the core components of high-power fiber lasers, overcoming the key technologies of fiber power combiners and developing high-efficiency, high-power, and high-beam-quality fiber power combiners has become a critical problem urgently needing to be solved in the field of fiber lasers.

[0003] In the fabrication of fiber optic power combiners, the tapering of the fused fiber bundle is a crucial step. Currently, the main methods for fabricating fused fiber bundles include the torsion method and the sleeve method. The torsion method involves twisting the input fiber bundle after it has been bundled, bringing the fibers together tightly, and then heating and tapering them to obtain the tapered fiber bundle. This bundle is then cut and fused with the output fiber to obtain the fiber optic power combiner. The sleeve method involves inserting all bare fiber areas and a portion of the coated area of ​​the input fiber into a glass tube with an inner diameter slightly larger than the equivalent diameter of the fiber bundle during the input fiber bundle assembly process. The glass tube and the fiber bundle inside are then tapered to obtain the tapered fiber bundle. This bundle is then cut and fused with the output fiber to obtain the fiber optic power combiner. The parameters of the sleeve used in the sleeve method have a significant impact on subsequent steps.

[0004] Existing research, such as CN111995231A, discloses a drawing device and method for deeply fluorine-doped capillary tubes used in bundle combiners. This patent only describes the drawing device and method for the deeply fluorine-doped capillary tubes, without imposing any restrictions on the refractive index of the capillary tubes. However, the refractive index of the capillary tubes directly affects the performance of the bundle combiner. How to design appropriate mother tubes and capillary tubes to improve the performance of the bundle combiner is a technical problem that needs to be solved in this field. Summary of the Invention

[0005] In view of the above-mentioned defects or improvement needs of the existing technology, the purpose of this invention is to provide a low refractive index doped mother tube, a capillary, a preparation method, and an application. The low refractive index doped mother tube and the low refractive index doped capillary designed by this invention both include a pure silicon dioxide layer as the outer layer and a low refractive index doped silicon dioxide layer as the inner layer. The low refractive index doped capillary can be used to prepare optical fiber power combiners and improve the performance of the combiners.

[0006] In a first aspect, the present invention provides a method for preparing a low-refractive-index doped mother tube, wherein a low-refractive-index doped silicon dioxide layer is deposited on the inner surface of a pure silicon dioxide liner using plasma vapor deposition to obtain a low-refractive-index doped mother tube; the cross-sections of the pure silicon dioxide liner and the low-refractive-index doped silicon dioxide layer in the low-refractive-index doped mother tube are both annular tubular structures; the minimum relative refractive index difference of the low-refractive-index doped silicon dioxide layer is -1.8% to -2.6%.

[0007] Preferably, the refractive index profile of the low-refractive-index doped silicon dioxide layer is one of the following: a mixed multilayer doped structure composed of gradient doping and step doping, a single-layer uniform doped structure, a single-layer gradient doped structure, or a multilayer step doped structure.

[0008] Preferably, when depositing the low-refractive-index doped silicon dioxide layer, the relative refractive index of the low-refractive-index doped silicon dioxide layer is adjusted by regulating the flow rate of the raw material gas introduced into the pure silicon dioxide liner; the doping element of the low-refractive-index doped silicon dioxide layer is one or more of germanium, fluorine, phosphorus, boron, chlorine, and aluminum.

[0009] Preferably, after depositing the low-refractive-index doped silicon dioxide layer, the inner surface of the low-refractive-index doped silicon dioxide layer is further polished.

[0010] Secondly, the present invention provides a low-refractive-index doped mother tube, which is prepared by the above-mentioned low-refractive-index doped mother tube preparation method; the low-refractive-index doped mother tube includes, from the outside to the inside, a pure silicon dioxide liner and a low-refractive-index doped silicon dioxide layer.

[0011] Preferably, the outer diameter of the low-refractive-index doped mother tube is 25–70 mm.

[0012] Thirdly, the present invention provides a method for preparing a low-refractive-index doped capillary, comprising the following steps:

[0013] Step 1: Using plasma vapor deposition, a low-refractive-index doped silicon dioxide layer is deposited on the inner surface of a pure silicon dioxide liner to obtain a low-refractive-index doped mother tube.

[0014] Step 2: Heat the low-refractive-index doped mother tube to the softening temperature and draw it to obtain a low-refractive-index doped capillary.

[0015] The cross-sections of the pure silicon dioxide liner and the low-refractive-index doped silicon dioxide layer in the low-refractive-index doped mother tube are both annular tubular structures; the minimum relative refractive index difference of the low-refractive-index doped silicon dioxide layer is -1.8% to -2.6%.

[0016] Preferably, the refractive index profile of the low-refractive-index doped silicon dioxide layer is one of the following: a mixed multilayer doped structure composed of gradient doping and step doping, a single-layer uniform doped structure, a single-layer gradient doped structure, or a multilayer step doped structure.

[0017] Preferably, the softening temperature is 1800–2000℃; during the drawing process, the inner-outer diameter ratio of the low-refractive-index doped capillary is controlled to reach the target value by adjusting the gas pressure inside the low-refractive-index doped mother tube; by introducing gas into the low-refractive-index doped mother tube to maintain a slight positive pressure inside the low-refractive-index doped mother tube, the inner-outer diameter ratio of the low-refractive-index doped capillary can be controlled to remain consistent with or slightly increase compared to the inner-outer diameter ratio of the low-refractive-index doped mother tube; the value range of the slight positive pressure is [0, 2000 Pa]; by evacuating the low-refractive-index doped mother tube to maintain a slight negative pressure inside the low-refractive-index doped mother tube, the inner-outer diameter ratio of the low-refractive-index doped capillary can be controlled to decrease compared to the inner-outer diameter ratio of the low-refractive-index doped mother tube; the value range of the slight negative pressure is [-2000 Pa, 0].

[0018] Fourthly, the present invention provides a low-refractive-index doped capillary, which is prepared by the above-described method for preparing a low-refractive-index doped capillary; the low-refractive-index doped capillary comprises, from the outside to the inside, a pure silicon dioxide layer and a low-refractive-index doped silicon dioxide layer.

[0019] Preferably, the outer diameter of the low-refractive-index doped capillary is 0.2–4 mm, the ratio of the outer diameter of the low-refractive-index doped silicon dioxide layer to the outer diameter of the low-refractive-index doped capillary is 0.3–0.95, and the ratio of the inner diameter of the low-refractive-index doped silicon dioxide layer to the outer diameter of the low-refractive-index doped capillary is 0.2–0.8.

[0020] Fifthly, the present invention provides an application of a low-refractive-index doped capillary, which is used to fabricate an optical fiber power combiner.

[0021] One or more technical solutions provided in this invention have at least the following technical effects or advantages:

[0022] This invention first deposits a low-refractive-index doped silica layer on the inner surface of a pure silica liner to prepare a low-refractive-index doped mother tube. Then, the low-refractive-index doped mother tube is heated to a softening temperature and drawn to obtain a low-refractive-index doped capillary. The minimum relative refractive index difference of the low-refractive-index doped silica layer is -1.8% to -2.6%. The low-refractive-index doped capillary designed in this invention can be used to prepare optical fiber power combiners. The low-refractive-index doped silica layer can form a low-refractive-index cladding for preparing the combiner. The total internal reflection principle is applied to improve the beam-gathering capability of the combiner and reduce the loss in the optical fiber power transmission process. The pure silica layer as the outer layer can protect the low-refractive-index layer, improve the strength of the capillary, and reduce defects and impurities in the capillary. Attached Figure Description

[0023] Figure 1 A schematic diagram of the end face structure of a low-refractive-index doped mother tube provided by the present invention;

[0024] Figure 2 A schematic diagram of the drawing process for drawing a low-refractive-index doped mother tube into a low-refractive-index doped capillary tube, provided by the present invention;

[0025] Figure 3 A schematic diagram of the end face structure of a low refractive index doped capillary provided by the present invention;

[0026] Figure 4 This is a schematic diagram of the relative refractive index distribution of a low-refractive-index doped capillary provided in Embodiment 1 of the present invention;

[0027] Figure 5 This is a schematic diagram of the relative refractive index distribution of a low-refractive-index doped capillary provided in Embodiment 2 of the present invention;

[0028] Figure 6 This is a schematic diagram of the relative refractive index distribution of a low-refractive-index doped capillary provided in Embodiment 3 of the present invention. Detailed Implementation

[0029] This invention comprises five aspects, which are described below.

[0030] (1) Preparation method of low refractive index doped mother tube.

[0031] A low-refractive-index doped silicon dioxide layer 2 was deposited on the inner surface of a pure silicon dioxide liner 1 using plasma vapor deposition, resulting in a low-refractive-index doped mother tube 101. (See [link to documentation]). Figure 1 .

[0032] This involves fabricating a mother tube with an outer layer of pure silicon dioxide and an inner layer of low-refractive-index doped silicon dioxide. Both the pure silicon dioxide liner 1 and the low-refractive-index doped silicon dioxide layer 2 within the low-refractive-index doped mother tube 101 have annular tubular cross-sections. The minimum relative refractive index difference of the low-refractive-index doped silicon dioxide layer 2 is -1.8% to -2.6%. The refractive index profile of the low-refractive-index doped silicon dioxide layer 2 can be a mixed multilayer doped structure composed of gradient doping and step doping, a single-layer uniform doped structure, a single-layer gradient doped structure, or a multilayer step doped structure, etc.

[0033] During the deposition of the low-refractive-index doped silicon dioxide layer 2, the relative refractive index of the low-refractive-index doped silicon dioxide layer 2 is adjusted by regulating the flow rate of the raw material gas introduced into the pure silicon dioxide liner 1. The doping element of the low-refractive-index doped silicon dioxide layer 2 is one or more of germanium, fluorine, phosphorus, boron, chlorine, and aluminum. After the deposition of the low-refractive-index doped silicon dioxide layer 2, the inner surface of the low-refractive-index doped silicon dioxide layer 2 can also be polished.

[0034] (2) Low refractive index doped mother tube.

[0035] The low-refractive-index doped mother tube 101 includes, from the outside to the inside, a pure silicon dioxide liner 1 and a low-refractive-index doped silicon dioxide layer 2.

[0036] The outer diameter D1 of the low-refractive-index doped mother tube 101 is 25–70 mm. (See [link]) Figure 1 .

[0037] Specifically, the low-refractive-index doped mother tube 101 is prepared using (1) the low-refractive-index doped mother tube preparation method.

[0038] (3) Preparation method of low refractive index doped capillary.

[0039] The method for preparing the low-refractive-index doped capillary includes the following steps:

[0040] Step 1: Using plasma vapor deposition, a low-refractive-index doped silicon dioxide layer 2 is deposited on the inner surface of the pure silicon dioxide liner 1 to obtain the low-refractive-index doped mother tube 101. (See [link to documentation]). Figure 1 .

[0041] In the low-refractive-index doped mother tube 101, the cross-sections of the pure silicon dioxide liner 1 and the low-refractive-index doped silicon dioxide layer 2 are both annular tubular structures; the minimum relative refractive index difference of the low-refractive-index doped silicon dioxide layer 2 is -1.8% to -2.6%.

[0042] Specifically, the refractive index profile of the low-refractive-index doped silicon dioxide layer 2 can be a mixed multilayer doped structure composed of gradient doping and step doping, a single-layer uniform doped structure, a single-layer gradient doped structure, a multilayer step doped structure, etc.

[0043] Plasma-conductive vapor deposition (PCVD) is used to introduce a raw material gas into the pure silica liner 1. The raw material gas is ionized through a high-frequency resonant cavity, thereby depositing a low-refractive-index doped silica layer 2 on the inner surface of the pure silica liner 1. The doping element may include one or more of the following: germanium, fluorine, phosphorus, boron, chlorine, and aluminum. The relative refractive index of the low-refractive-index doped silica layer 2 is controlled by changing the flow rate of the raw material gas.

[0044] After the low-refractive-index doped mother tube 101 is deposited, the low-refractive-index doped silicon dioxide layer 2 can be polished by flame polishing or graphite furnace polishing. That is, the high temperature generated by the oxy-oxygen flame or graphite furnace is used to heat the inner surface of the deposited low-refractive-index doped silicon dioxide layer 2 to the melting temperature. At this time, the particles on the glass surface will move under the action of surface tension, thereby forming a transparent and smooth polished surface on the inner surface of the low-refractive-index doped silicon dioxide layer 2.

[0045] Step 2: Heat the low-refractive-index doped mother tube 101 to its softening temperature and draw it to obtain the low-refractive-index doped capillary 201. See [link to relevant documentation]. Figure 2 .

[0046] Specifically, the low-refractive-index doped mother tube 101 is heated to its softening temperature (1800-2000℃) in a high-temperature furnace and drawn to the desired target diameter. Because pure silicon dioxide is used as the outer layer of the capillary, the doping elements in the tube are less likely to diffuse out of the mother tube during the high-temperature drawing process. Diffused doping elements would corrode the drawing furnace, generating dust impurities. Therefore, this invention avoids defects on the capillary surface caused by dust impurities.

[0047] Furthermore, in order to control the inner-outer diameter ratio of the low-refractive-index doped capillary 201 to reach the target value, gas can be introduced into the low-refractive-index doped mother tube 101 during the drawing process to maintain a certain micro-positive pressure inside the low-refractive-index doped mother tube 101. The micro-positive pressure can keep the inner-outer diameter ratio of the low-refractive-index doped capillary 201 consistent with or slightly increased compared to the inner-outer diameter ratio of the low-refractive-index doped mother tube 101. The value range of the micro-positive pressure is [0, 2000 Pa]. During the drawing process, a vacuum system can also be connected to the low-refractive-index doped mother tube 101 to maintain a certain micro-negative pressure inside the low-refractive-index doped mother tube 101 by evacuating the vacuum. The micro-negative pressure can reduce the inner-outer diameter ratio of the low-refractive-index doped capillary 201 compared to the inner-outer diameter ratio of the low-refractive-index doped mother tube 101. The value range of the micro-negative pressure is [-2000 Pa, 0].

[0048] (4) Low refractive index doped capillary.

[0049] The low-refractive-index doped capillary 201 is divided into two layers, which, from the outside to the inside, include a pure silicon dioxide layer 3 and a low-refractive-index doped silicon dioxide layer 4.

[0050] Specifically, the low-refractive-index doped capillary 201 is prepared using the low-refractive-index doped capillary preparation method (3).

[0051] Wherein, the outer diameter d1 of the low-refractive-index doped capillary 201 is 0.2–4 mm, the ratio of the outer diameter d2 of the low-refractive-index doped silicon dioxide layer 4 to the outer diameter d1 of the low-refractive-index doped capillary 201 is 0.3–0.95, and the ratio of the inner diameter d3 of the low-refractive-index doped silicon dioxide layer 4 to the outer diameter d1 of the low-refractive-index doped capillary 201 is 0.2–0.8. See [reference needed]. Figure 3 .

[0052] (5) Application of low refractive index doped capillary tubes.

[0053] The low-refractive-index doped capillary is used to fabricate an optical fiber power combiner.

[0054] Specifically, the low-refractive-index doped silicon dioxide layer in the low-refractive-index doped capillary can form a low-refractive-index cladding for fabricating a beam combiner. The beam combiner's beam-bearing capability is improved by applying the principle of total internal reflection, and the loss in the optical fiber energy transmission process is reduced. The pure silicon dioxide layer in the low-refractive-index doped capillary can protect the low-refractive-index layer, improve the strength of the capillary, and reduce defects and impurities in the capillary.

[0055] To better understand the above technical solutions, the following will provide a detailed explanation of the technical solutions in conjunction with the accompanying drawings and specific implementation methods.

[0056] Example 1:

[0057] (1.1) Fabrication of low-refractive-index doped mother tube:

[0058] See Figure 1 The low-refractive-index doped mother tube 101 prepared in Example 1 consists of a pure silicon dioxide liner 1 and a low-refractive-index doped silicon dioxide layer 2 deposited on the inner wall of the liner. The liner is cylindrical with an outer diameter D1 of 49 mm and is made of pure silicon dioxide. The low-refractive-index layer is deposited inside the liner using PCVD technology, and the doping elements are germanium and boron, with a consistent deep doping depth. Then, the deposited low-refractive-index doped mother tube 101 is placed on an oxyhydrogen flame bed and polished by flame to improve the roughness of the inner surface of the low-refractive-index doped silicon dioxide layer 2.

[0059] (1.2) Fabrication of low-refractive-index doped capillaries:

[0060] The low-refractive-index doped mother tube 101 was placed in a high-temperature furnace and heated to 1830℃ to draw a low-refractive-index doped capillary tube 201 with an outer diameter of 900 μm. A schematic diagram of the drawing process is shown below. Figure 2 As shown. The cross-section of the pulled-out low-refractive-index-doped capillary 201 is as follows. Figure 3 As shown, the thickness of the pure silicon dioxide layer 3 in the low-refractive-index doped capillary 201 is 22 μm, and the thickness of the low-refractive-index doped silicon dioxide layer 4 is 91 μm. The minimum relative refractive index difference of the low-refractive-index doped silicon dioxide layer 4 is -1.8%, and the refractive index distribution of the low-refractive-index doped capillary 201 is as follows. Figure 4 As shown, the optical fiber bundle fabricated using the low-refractive-index doped capillary 201 has a light transmission efficiency of 98.8%.

[0061] Example 2:

[0062] (2.1) Fabrication of low-refractive-index doped mother tube:

[0063] See Figure 1 In Example 2, the low-refractive-index doped mother tube 101 consists of a pure silicon dioxide liner 1 and a low-refractive-index doped silicon dioxide layer 2 deposited on the inner wall of the liner. The liner is cylindrical with an outer diameter D1 of 55 mm and is made of pure silicon dioxide. The low-refractive-index layer is deposited inside the liner using PCVD technology, and the doping elements are germanium and fluorine. The low-refractive-index doped silicon dioxide layer 2 has a gradient refractive index structure. Then, the deposited low-refractive-index doped mother tube 101 is placed on an oxyhydrogen flame bed and polished by flame to improve the roughness of the inner surface of the low-refractive-index doped silicon dioxide layer 2.

[0064] (2.2) Preparation of low-refractive-index capillaries:

[0065] The low-refractive-index doped mother tube 101 was placed in a high-temperature furnace and heated to 1830℃ to draw a low-refractive-index doped capillary tube 201 with an outer diameter of 1440 μm. A schematic diagram of the drawing process is shown below. Figure 2 As shown. The cross-section of the pulled-out low-refractive-index-doped capillary 201 is as follows. Figure 3 As shown, the thickness of the pure silicon dioxide layer 3 in the low-refractive-index doped capillary 201 is 42 μm, the thickness of the graded refractive-index low-refractive-index doped silicon dioxide layer 4 is 141 μm, the minimum relative refractive index difference of the low-refractive-index doped silicon dioxide layer 4 is -2.2%, and the refractive index distribution of the low-refractive-index doped capillary 201 is as follows. Figure 5 As shown, the optical fiber bundle fabricated using this low-refractive-index doped capillary 201 has a light transmission efficiency of 99.4%.

[0066] Example 3:

[0067] (3.1) Fabrication of low-refractive-index doped mother tube:

[0068] See Figure 1 In Example 3, the low-refractive-index doped mother tube 101 consists of a pure silicon dioxide liner 1 and a low-refractive-index doped silicon dioxide layer 2 deposited on the inner wall of the liner. The liner is cylindrical with an outer diameter D1 of 42 mm and is made of pure silicon dioxide. The low-refractive-index layer is deposited inside the liner using PCVD technology, and the doping elements are fluorine and boron. The low-refractive-index doped silicon dioxide layer 2 has a gradient refractive index + horizontal refractive index structure. Then, the deposited low-refractive-index doped mother tube 101 is placed on an oxyhydrogen flame bed and polished by flame to improve the roughness of the inner surface of the low-refractive-index doped silicon dioxide layer 2.

[0069] (3.2) Preparation of low-refractive-index capillaries:

[0070] The low-refractive-index doped mother tube 101 was placed in a high-temperature furnace and heated to 1840℃ to draw a low-refractive-index doped capillary tube 201 with an outer diameter of 1200 μm. A schematic diagram of the drawing process is shown below. Figure 2 As shown. The cross-section of the pulled-out low-refractive-index-doped capillary 201 is as follows. Figure 3 As shown, the thickness of the pure silicon dioxide layer 3 in the low-refractive-index doped capillary 201 is 33 μm, the thickness of the low-refractive-index doped silicon dioxide layer in the graded refractive index section is 16 μm, the thickness of the low-refractive-index doped silicon dioxide layer in the horizontal refractive index section is 108 μm, the minimum relative refractive index difference of the low-refractive-index doped silicon dioxide layer 4 is -2.6%, and the refractive index distribution of the low-refractive-index doped capillary 201 is as follows. Figure 6 As shown, the optical fiber bundle fabricated using the low-refractive-index doped capillary 201 has a light transmission efficiency of 99.6%.

[0071] Finally, it should be noted that the above specific embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to examples, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for preparing a low-refractive-index doped mother tube, characterized in that, A low-refractive-index doped silicon dioxide layer is deposited on the inner surface of a pure silicon dioxide liner using plasma vapor deposition to obtain a low-refractive-index doped mother tube. The cross-sections of both the pure silicon dioxide liner and the low-refractive-index doped silicon dioxide layer in the low-refractive-index doped mother tube are annular tubular structures. The minimum relative refractive index difference of the low-refractive-index doped silicon dioxide layer is -2.6% to -2.2%.

2. The method for preparing a low-refractive-index doped mother tube according to claim 1, characterized in that, The refractive index profile of the low-refractive-index doped silicon dioxide layer is one of the following: a mixed multilayer doped structure composed of gradient doping and step doping, a single-layer uniform doped structure, a single-layer gradient doped structure, or a multilayer step doped structure.

3. The method for preparing a low-refractive-index doped mother tube according to claim 1, characterized in that, When depositing the low-refractive-index doped silicon dioxide layer, the relative refractive index of the low-refractive-index doped silicon dioxide layer is adjusted by regulating the flow rate of the raw material gas introduced into the pure silicon dioxide liner; the doping element of the low-refractive-index doped silicon dioxide layer is one or more of germanium, fluorine, phosphorus, boron, chlorine, and aluminum.

4. The method for preparing a low-refractive-index doped mother tube according to claim 1, characterized in that, After depositing the low-refractive-index doped silicon dioxide layer, the inner surface of the low-refractive-index doped silicon dioxide layer is further polished.

5. A low-refractive-index doped mother tube, characterized in that, The low-refractive-index doped mother tube is prepared by the method for preparing a low-refractive-index doped mother tube as described in any one of claims 1-4; the low-refractive-index doped mother tube comprises, from the outside to the inside, a pure silicon dioxide liner and a low-refractive-index doped silicon dioxide layer.

6. The low-refractive-index doped mother tube according to claim 5, characterized in that, The outer diameter of the low-refractive-index doped mother tube is 25~70mm.

7. A method for preparing a low-refractive-index doped capillary, characterized in that, Includes the following steps: Step 1: Using plasma vapor deposition, a low-refractive-index doped silicon dioxide layer is deposited on the inner surface of a pure silicon dioxide liner to obtain a low-refractive-index doped mother tube. Step 2: Heat the low-refractive-index doped mother tube to the softening temperature and draw it to obtain a low-refractive-index doped capillary. The cross-sections of the pure silica liner and the low-refractive-index doped silica layer in the low-refractive-index doped mother tube are both annular tubular structures; the minimum relative refractive index difference of the low-refractive-index doped silica layer is -2.6% to -2.2%.

8. The method for preparing a low-refractive-index doped capillary according to claim 7, characterized in that, The refractive index profile of the low-refractive-index doped silicon dioxide layer is one of the following: a mixed multilayer doped structure composed of gradient doping and step doping, a single-layer uniform doped structure, a single-layer gradient doped structure, or a multilayer step doped structure.

9. The method for preparing a low-refractive-index doped capillary according to claim 7, characterized in that, The softening temperature is 1800~2000℃; during the drawing process, the gas pressure inside the low-refractive-index doped mother tube is adjusted to control the inner-outer diameter ratio of the low-refractive-index doped capillary to reach the target value; by introducing gas into the low-refractive-index doped mother tube to maintain a slight positive pressure, the inner-outer diameter ratio of the low-refractive-index doped capillary can be controlled to remain consistent with or slightly increase compared to the inner-outer diameter ratio of the low-refractive-index doped mother tube; the value range of the slight positive pressure is [0, 2000 Pa]; by evacuating the low-refractive-index doped mother tube to maintain a slight negative pressure, the inner-outer diameter ratio of the low-refractive-index doped capillary can be controlled to decrease compared to the inner-outer diameter ratio of the low-refractive-index doped mother tube; the value range of the slight negative pressure is [-2000 Pa, 0].

10. A low-refractive-index doped capillary, characterized in that, The low-refractive-index doped capillary is prepared by the preparation method of any one of claims 7-9; the low-refractive-index doped capillary comprises, from the outside to the inside, a pure silicon dioxide layer and a low-refractive-index doped silicon dioxide layer.

11. The low-refractive-index doped capillary according to claim 10, characterized in that, The outer diameter of the low-refractive-index doped capillary is 0.2~4 mm, the ratio of the outer diameter of the low-refractive-index doped silicon dioxide layer to the outer diameter of the low-refractive-index doped capillary is 0.3~0.95, and the ratio of the inner diameter of the low-refractive-index doped silicon dioxide layer to the outer diameter of the low-refractive-index doped capillary is 0.2~0.

8.

12. An application of the low-refractive-index doped capillary as described in claim 10, characterized in that, The low-refractive-index doped capillary is used to fabricate an optical fiber power combiner.

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