Burner and fluorine-doped preform preparation method suitable for atmospheric plasma outside deposition

Abstract

The application discloses a burner suitable for atmospheric plasma external deposition, which comprises a flame spraying part, a main pipeline and an air inlet part; the flame spraying part is a U-shaped port; the main pipeline is connected with the flame spraying part and the air inlet part at two ends; the air inlet part is embedded in the inner wall of the main pipeline and comprises a raw material pipeline and a combustion-supporting pipeline; the raw material pipeline is used for spraying raw material gas; the combustion-supporting pipeline is a multi-path spiral air inlet structure and comprises a plurality of combustion-supporting sub-pipelines which are arranged around the raw material pipeline; each combustion-supporting sub-pipeline is arranged at the same relative inclination angle and relative position with the raw material pipeline. The U-shaped port design of the burner can constrain the plasma, so that the raw material is concentratedly sprayed onto a target rod, and the torch presents a cladding shape on the target rod, which can not only reduce the waste of the raw material, but also play a heat preservation role; the multi-path spiral air inlet structure of the combustion-supporting pipeline improves the air flow rotation speed, increases the stability of the plasma, and can constrain the raw material in the center, so that the ionization activity of fluorine and silicon raw materials is the strongest.

Description

Technical Field

[0001] This invention relates to the field of optical fiber preform manufacturing technology, and more specifically, to a burner adapted for atmospheric pressure plasma external deposition and a method for preparing fluorine-doped preforms. Background Technology

[0002] Large-core-diameter ultraviolet energy transmission optical fibers employing a high-hydroxyl-content pure silica core and highly fluorine-doped silica cladding structure exhibit low transmission loss, high damage threshold, and significantly improved coupling efficiency and mechanical strength, along with higher bending resistance and radiation resistance due to their high numerical aperture. However, conventional external chemical vapor deposition (OVD), axial chemical vapor deposition (VAD), and modified chemical vapor deposition (MCVD) processes for fabricating large-core-diameter, high-numerical-aperture, high-hydroxyl-content, highly fluorine-doped silica core and highly fluorine-doped silica cladding preforms are limited by thermal equilibrium conditions, with fluorine doping concentration limits of 1.5–2%, corresponding to a fiber numerical aperture below 0.17. Furthermore, VAD and OVD methods require secondary sintering and vitrification, where hydroxyl and fluorine elements readily diffuse and volatilize, making precise control of high doping concentration and radial refractive index distribution difficult. While PCVD (plasma deposition) offers high doping efficiency, as an in-tube method, the diameter of the deposited preform is limited by the cavity size, making it difficult to fabricate large-sized optical fiber preforms.

[0003] Chinese patent CN202310046925.9 describes the fabrication of optical fiber preforms using microwave plasma chemical vapor deposition. A high-energy and stable plasma torch is crucial for the one-step vitrification of large-diameter, high-hydroxyl-core, highly fluorine-doped quartz cladding preforms. However, as the input power increases, the utilization efficiency of the plasma torch for microwaves gradually decreases. Furthermore, under high-power conditions, excessively high local plasma energy can cause ablation of the quartz discharge tube. Highly reactive, high-energy ionized fluorine ions easily etch the torch tube wall, leading to thinning or even cracking, thus limiting the feed power and energy of the plasma torch. Therefore, the electromagnetic matching performance and discharge stability control of high-energy, ambient-pressure microwave plasma torches have become urgent technical problems to be solved.

[0004] Meanwhile, in the process of preparing fluorine-doped quartz cladding by atmospheric pressure microwave plasma chemical vapor deposition, the lower the concentration of ionized fluorine, the higher the surface temperature of the target rod, and the lower the fluorine doping concentration of the quartz; the higher the concentration of ionized fluorine, the stronger the ionized plasma etching, and the slower the quartz deposition rate; and when the content of ionized fluorine is too high, quartz glass cannot even be deposited. Therefore, there is an urgent need to develop new devices and methods suitable for preparing large-diameter, high-hydroxyl quartz core, highly fluorine-doped quartz cladding preforms. Summary of the Invention

[0005] To address at least one deficiency or improvement requirement of the prior art, the present invention provides a method for preparing a burner and a fluorine-doped preform adapted for atmospheric pressure plasma external deposition.

[0006] In a first aspect, the present invention provides a burner adapted for atmospheric pressure plasma external deposition, comprising: a flame spray section, a main pipe and an air inlet section;

[0007] The flame ejector is U-shaped, so that the plasma flame ejected from it can cover the target rod placed perpendicular to the opening direction of the flame ejector.

[0008] The main pipe connects the flame section and the air inlet section located at both ends thereon;

[0009] The air intake section is embedded in the inner wall of the main pipe and includes a raw material pipe and a combustion-supporting pipe.

[0010] The raw material pipeline is used to inject raw material gas;

[0011] The combustion-supporting pipe is a multi-path spiral air intake structure, including several combustion-supporting branch pipes arranged around the raw material pipe; each of the combustion-supporting branch pipes is arranged at the same relative angle and relative position as the raw material pipe, so that the combustion-supporting gas sprayed from each of the combustion-supporting branch pipes can form a spiral shape.

[0012] Furthermore, the diameter of the U-shaped opening of the flame jet is larger than the diameter of the main pipe to which it is connected.

[0013] Furthermore, the air intake also includes a cooling isolation pipe for injecting cooling isolation gas to cool and isolate the reaction products;

[0014] The cooling isolation pipe is located between the inner wall of the combustion-supporting pipe and the main pipe, and is formed by the space between the cooling isolation pipe wall of the combustion-supporting pipe and the inner wall of the main pipe that are completely surrounded by the outside.

[0015] The gap between the cooling isolation pipe wall and the inner pipe wall of the main pipe gradually decreases along the injection direction of the cooling isolation gas.

[0016] Furthermore, the combustion-supporting pipe is rotatably arranged with the central axis of the raw material pipe as its axis.

[0017] Furthermore, the feed gas includes silicon compounds and fluorine compounds;

[0018] The combustion-supporting agent is oxygen;

[0019] The cooling barrier gas is one or more of nitrogen or oxygen.

[0020] Furthermore, the silicon compound includes SiCl4;

[0021] The fluorine compounds include one or more of C2F6, C2F4, CFCl3, and CF2Cl2.

[0022] Furthermore, the combustion-supporting pipeline includes four combustion-supporting sub-pipelines.

[0023] Furthermore, the raw material pipeline is located at the central axis of the main pipeline;

[0024] Several combustion-supporting sub-pipes are evenly arranged around the raw material pipe.

[0025] Secondly, the present invention provides a method for preparing a fluorine-doped preform using the above-mentioned burner, comprising:

[0026] Place the target rod to be processed at the U-shaped inlet of the flame jet section;

[0027] The raw material pipeline is used to inject raw material gas, the combustion-supporting pipelines are used to inject combustion-supporting gas, and the cooling and isolation pipelines are used to inject cooling and isolation gas.

[0028] The target rod to be treated is deposited using an atmospheric pressure microwave plasma torch ejected from the burner.

[0029] Furthermore, the feed gas includes silicon compounds and fluorine compounds;

[0030] The silicon compound includes SiCl4;

[0031] The fluorine compound includes one or more of C2F6, C2F4, CFCl3, and CF2Cl2;

[0032] The combustion-supporting agent is oxygen;

[0033] The cooling barrier gas is one or more of nitrogen or oxygen.

[0034] In summary, compared with the prior art, the above-described technical solutions conceived by this invention can achieve the following beneficial effects:

[0035] (1) This invention designs a U-shaped opening at the port of the burner outlet resonant cavity (part of the main pipe 3 covered by microwave waveguide 5) to confine the plasma, allowing the raw materials to be concentrated and sprayed onto the target rod, with the torch covering the target rod. This U-shaped opening design not only reduces material waste but also provides heat insulation. The burner bottom air inlet of this invention adopts a multi-directional spiral air inlet, which improves the airflow vortex, increases the stability of the plasma, and confines the raw materials in the center, maximizing the ionization activity of fluorine and silicon raw materials.

[0036] (2) The outer diameter of the U-shaped opening of the present invention is larger than the diameter of the main pipe connected to it. This flared design can reduce the flow velocity after the plasma torch exits the waveguide surface, which is beneficial to the deposition of raw materials.

[0037] (3) The outermost layer of the burner at the bottom of the present invention is designed with a cooling isolation pipe, which can spray out cooling isolation gas to carry away excess heat in the discharge tube. At the same time, it can also carry away a small amount of raw material adhering to the outer layer, preventing the discharge tube energy from accumulating at the accumulation site due to the raw material adhering to the tube wall, thus absorbing energy and forming glass particles. Moreover, the "narrowing" design of the cooling isolation pipe allows the cooling isolation gas to be sprayed out at a faster speed. The high-velocity cooling isolation gas will form an air wall on the tube wall, thereby avoiding tube wall corrosion caused by fluorine contact with the discharge tube wall.

[0038] (4) The present invention rotatably sets the combustion-supporting pipe with the central axis of the raw material pipe as the axis, thereby further improving the airflow vortex, confining the raw material to the center, and further increasing the stability of the plasma.

[0039] (5) The inner raw material pipeline is located at the central axis of the main pipeline. The electric field strength is strongest at this location, and the ionization activity of fluorine and silicon raw materials is strongest here. The fluorine doping reaction is most likely to occur, and the fluorine doping concentration can reach the maximum.

[0040] (6) First, existing methods using oxyhydrogen flame for VAD, OVD, and MCVD are difficult to prepare preforms with high-hydroxyl quartz cores and deep fluorine-doped quartz cladding. This is because these three methods are limited by thermodynamic equilibrium conditions, resulting in low fluorine concentrations. Second, secondary high-temperature sintering or melting can lead to diffusion and volatilization of dopant elements. This invention addresses the problem of shallow fluorine-doped cladding depths in MCVD, OVD, and VAD by employing atmospheric pressure microwave plasma coupling to enhance reaction activity. High-energy, high-concentration plasma quenches on the target rod surface to generate a glassy, ​​deeply fluorine-doped cladding, eliminating the need for secondary sintering or melting and requiring no insulation facilities. This effectively avoids the diffusion and volatilization of dopant elements and is suitable for preparing large-diameter, high-hydroxyl, deeply fluorine-doped preforms. By designing a three-layer sleeve structure, the internal airflow is optimized, effectively reducing the problem of discharge tube blockage and achieving one-step glassization of the preform cladding. Attached Figure Description

[0041] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0042] Figure 1 A three-dimensional structural schematic diagram of a burner adapted for atmospheric pressure plasma external deposition is provided for an embodiment of the present invention;

[0043] Figure 2 A schematic cross-sectional structure diagram of a burner adapted for atmospheric pressure plasma external deposition is provided in an embodiment of the present invention;

[0044] Figure 3 A bottom view of a burner adapted for atmospheric pressure plasma external deposition, provided as an embodiment of the present invention;

[0045] Figure 4 The jet streamline diagram of the inner layer feed gas provided in the embodiment of the present invention;

[0046] Figure 5 The injection streamline diagram of the middle-layer combustion-supporting gas provided in the embodiment of the present invention;

[0047] Figure 6 The jet streamline diagram of the outer cooling barrier gas provided in the embodiment of the present invention;

[0048] Figure 7 A diagram showing the relationship between the axial measurement point position and the numerical aperture provided in an embodiment of the present invention;

[0049] Figure 8 A diagram showing the relationship between the radial measurement point position and the relative refractive index provided in an embodiment of the present invention;

[0050] Reference numerals: 1. Target rod to be processed; 2. Flame jet section; 3. Main pipe; 4. Air inlet; 5. Microwave waveguide; 6. Raw material pipe; 7. Combustion-supporting pipe; 8. Cooling and isolation pipe; 9. Cooling and isolation pipe wall; 10. Inner wall of the main pipe; 11. Raw material pipe inlet (air inlet); 12. Combustion-supporting branch pipe inlet (air inlet); 13. Jet flow line of the inner layer raw material gas; 14. Jet flow line of the middle layer combustion-supporting gas; 15. Jet flow line of the outer layer cooling and isolation gas. Detailed Implementation

[0051] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0052] The terms "first," "second," or "third," etc., used in the specification, claims, or accompanying drawings of this application are used to distinguish different objects, not to describe a particular order. Furthermore, the terms "comprising" or "having," and any variations thereof, are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or apparatus that includes a series of steps or units is not limited to the listed steps or units, but may optionally include steps or units not listed, or may optionally include other steps or units inherent to these processes, methods, products, or apparatuses.

[0053] Existing technologies typically employ cylindrical burners for preform fabrication, resulting in high-speed thermal jets. This causes some glass raw material particles for the outer cladding to be carried away by the gas flow, failing to deposit onto the target rod and leading to material waste. Furthermore, the heat utilization rate of microwave plasma torches is low; most of the heat is directly exchanged with the air after being ejected by the plasma thermal jet, making it ineffective for preform heating. Conventional in-tube methods also limit the diameter of the deposited preform due to cavity size, making it difficult to fabricate large-sized optical fiber preforms.

[0054] In view of this, refer to Figures 1 to 3 One embodiment of the present invention provides a burner adapted for atmospheric pressure plasma external deposition, comprising: a flame section 2, a main pipe 3, and an air inlet 4. Raw materials, carrier gas, and / or combustion-supporting gas enter from the bottom air inlet 4, pass through the main pipe 3, and are excited into plasma by the electric field of the microwave waveguide 5 (where microwaves are transmitted within the microwave waveguide 5 to form a periodic electric field distribution, with the region of strongest electric field coinciding with the burner's central axis). The plasma is then ejected as a plasma torch from the upper burner flame section 2.

[0055] The flame jet section 2 is U-shaped, so that the plasma flame ejected from it can cover the target rod placed perpendicular to the opening direction of the flame jet section. For example... Figure 1 As shown, the plasma flame passing through the microwave waveguide 5 region inside the main pipe 3 can wrap around the target rod 1 with an outer diameter of at least 40 mm at the U-shaped nozzle 2, which can effectively confine the plasma flame. The design of the U-shaped nozzle 2 not only has a heat preservation effect, but also reduces material waste and improves the collection rate.

[0056] Preferably, the diameter of the flame jet 2 is larger than the diameter of the connected main pipe 3, that is, the flame jet 2 adopts a flared U-shaped design, as shown in the reference. Figure 1 and Figure 2 This flared design reduces the flow velocity of the plasma torch after exiting the pipe area covered by the microwave waveguide 5, allowing as few glass raw material particles as possible to be carried away by the gas flow and as many as possible to be deposited on the target rod, thus avoiding a large waste of raw materials and improving the utilization rate of raw materials.

[0057] This invention features a U-shaped flame jet at the burner outlet of the resonant cavity. This U-shaped structure confines the plasma, concentrating the raw material onto the target rod, allowing the torch to envelop the target rod. This U-shaped design not only reduces material waste but also provides insulation. The outer diameter of the U-shaped outlet is larger than the diameter of the connected main pipe 3. This flared design reduces the flow velocity of the plasma torch after exiting the waveguide surface, which is beneficial for material deposition.

[0058] The main pipeline 3 connects the flame section 2 and the air inlet section 4 located at both ends of it. It is a tubular structure and serves as a channel for transmitting various raw material gases, combustion-supporting gases, and / or carrier gases.

[0059] The air intake 4 is embedded in the inner wall 10 of the main pipe, including the raw material pipe 6 and the combustion-supporting pipe 7.

[0060] The raw material pipeline 6 is used for injecting raw material gas and has a slender tubular structure. The raw material gas includes silicon compounds and fluorine compounds; preferably, the fluorine compound is one or more of C2F6, C2F4, CFCl3, and CF2Cl2, and the silicon compound is SiCl4 (liquid at room temperature). Considering engineering design and minimizing corrosion of the inner wall 10 of the main pipeline, the raw material pipeline 6 is preferably located at the central axis of the main pipeline 3, with reference to... Figure 2 The raw material pipeline 6 is located in the center and Figure 3 The raw material pipeline inlet 11 is centrally located. Meanwhile, when the inner raw material pipeline 6 is located at the central axis of the main pipeline 3, the electric field strength at that location is the strongest, the ionization activity of fluorine and silicon raw materials is the strongest, the fluorine doping reaction is most likely to occur, and the fluorine doping concentration can reach the maximum.

[0061] The combustion-supporting pipe 7 is a multi-path spiral air intake structure, including several combustion-supporting branch pipes arranged around the raw material pipe 6. Each combustion-supporting branch pipe is arranged at the same relative angle and position as the raw material pipe 6, so that the combustion-supporting gas ejected from each combustion-supporting branch pipe can form a spiral shape. The combustion-supporting gas is oxygen. (Reference) Figure 3 Preferably, several (two, three, four, five, etc.) combustion-supporting sub-pipes (including combustion-supporting sub-pipe inlets 12) are evenly arranged around the raw material pipe inlet 11. Each combustion-supporting sub-pipe is equidistant from the straight line of the raw material pipe 6, and each combustion-supporting sub-pipe makes an equal angle with the plane perpendicular to the raw material pipe 6. The jet direction of each combustion-supporting sub-pipe tends to the central raw material pipe 6, i.e., an inclined design is adopted.

[0062] Preferably, four combustion-supporting sub-pipes are evenly distributed around the raw material pipe inlet 11. This design best maintains the stability of the airflow. Compared with the technical defects of the traditional direct oxygen intake method, which has insufficient uniformity in the pipe distribution, this spiral gas has better uniformity, increases the airflow vortex, and can better confine the raw material to the center, which is more conducive to increasing the stability of the flare. At the same time, the longer spiral motion trajectory allows O2 to stay in the resonant cavity for a longer time, increasing the degree of oxygen ionization and increasing the flare temperature. Actual measurements show that the flare temperature can be increased by more than 20°C.

[0063] In a further preferred embodiment, the combustion-supporting pipe 7 is rotatably arranged with the central axis of the raw material pipe 6 as its axis. That is, several combustion-supporting sub-pipes evenly distributed around the raw material pipe inlet 11 can rotate with the central axis of the raw material pipe 6 as their axis. This rotating jetting structure design further increases the uniformity of the spiral gas and further increases the stability of the flare.

[0064] Plasma-activated ionized fluorine easily etches the tube wall, causing it to thin or even crack, thus preventing cladding deposition. Therefore, this embodiment further includes a cooling isolation pipe 8 in the air inlet 4 for injecting cooling isolation gas to cool and isolate the reactants. The cooling isolation pipe 8 is located between the combustion-supporting pipe 7 and the inner wall 10 of the main pipe, and is formed by an external cooling isolation pipe wall 9 that surrounds the combustion-supporting pipe 7 and the space between the inner wall 10 of the main pipe. The gap between the cooling isolation pipe wall 9 and the inner wall 10 of the main pipe is as follows... Figure 2 The vertically upward spray direction of the cooling barrier gas (which can be carrier gas N2, O2, or a mixture of N2 and O2; the following explanation uses N2 as the preferred cooling barrier gas) gradually decreases. The outermost layer of the cooling barrier gas N2 in the inlet 4 carries away excess heat from the discharge tube. Simultaneously, it carries away a small amount of raw material adhering to the outer layer, preventing it from accumulating on the inner wall 10 of the main pipe, thus avoiding energy concentration at the accumulation point and the formation of glass particles. Furthermore, the outer layer of the inlet 4 is diameter-reduced, with the inlet width greater than the outlet width. This constriction design increases the N2 airflow velocity, forming a high-velocity air wall on the surface of the inner wall 10 of the main pipe. This design not only removes heat from the discharge tube wall but also minimizes the risk of corrosion caused by fluorine gas in the raw material contacting the tube wall.

[0065] The outermost layer of the burner's bottom tube is designed with a cooling and insulating channel, which can spray out cooling and insulating gas to carry away excess heat from the discharge tube. Simultaneously, it can also remove a small amount of material adhering to the outer layer, preventing energy accumulation on the tube wall and the formation of glass particles due to energy absorption. Furthermore, the "narrowing" design of the cooling and insulating channel allows for accelerated spraying of the cooling and insulating gas. This high-velocity gas forms an air wall on the tube wall, preventing fluorine from contacting the discharge tube wall and causing etching.

[0066] Figure 4 The jet streamline diagram of the inner layer feed gas provided in the embodiments of the present invention is as follows: Figure 4 The jet stream 13 of the inner layer of raw material gas shows that the raw material is well confined in the center of the main pipe 3, effectively reducing corrosion caused by the contact of the raw material fluorine with the pipe wall. The raw material at the flare outlet is also more concentrated, increasing collection efficiency and reducing raw material waste.

[0067] Figure 5 The injection streamline diagram of the middle-layer combustion-supporting gas provided in the embodiments of the present invention is as follows: Figure 5 As can be seen from the jet streamline 14 of the middle-layer combustion-supporting gas, the spiral airflow helps to make the airflow distribution inside the tube more uniform, and also increases the stability of the plasma.

[0068] Figure 6 The jet streamline diagram of the outer cooling barrier gas provided in the embodiment of the present invention is as follows: Figure 6 As can be seen from the jet stream 15 of the outer cooling barrier gas, the cooling barrier gas adheres closely to the inner wall 10 of the main pipe, forming an air wall, which can also carry away excess heat from the discharge tube.

[0069] A method for preparing a large-diameter, high-hydroxyl-content quartz core, highly fluorine-doped quartz cladding preform using the burner of the above embodiments includes the following steps:

[0070] Step 1: The target rod used for deposition is a high-hydroxyl pure quartz core rod with a diameter of 30mm. The magnetron is coupled to increase the torch power and avoid excessive local energy caused by a single high-power magnetron. The negative pressure inside the chamber is adjusted to remove reaction waste gas; the burner is turned on, and the flame is used to preheat the target rod for a predetermined preheating time, thereby eliminating target rod stress and maintaining a constant target rod temperature. Because atmospheric pressure plasma external deposition technology has higher reactant reactivity compared to other methods, the required reaction temperature is lower.

[0071] Step 2: Before deposition begins, one or more fluorine compounds from C2F6, C2F4, CFCl3, and CF2Cl2 are introduced to pretreat and polish the target rod surface, forming a smoother pre-doped layer.

[0072] Step 3: During the deposition process, place the target rod 1 to be treated at the U-shaped inlet of the flame section, referring to... Figure 1 The distance between the core rod surface and the waveguide outlet plane is 3-6 cm, with 4 cm being suitable. The central layer is purged with SiCl4 and C2F6 (C2F6 is used as an example for fluorine compounds), the second outer layer is purged with O2, and the third outer layer is purged with cooling gas N2 or O2. During the deposition process, the target rod thickens, and the plasma power is simultaneously increased to maintain a constant target rod temperature.

[0073] Step 4: After deposition to the target thickness, reduce the microwave power and anneal the target rod to make the residual powdery glass on the surface transparent. Then, turn off the torch and wait for it to cool down naturally.

[0074] Step 5: After the optical fiber preform cools to room temperature, use a PK2600 instrument to take 7 points along the longitudinal direction of the optical fiber preform, one point every 100 mm. Test the refractive index and calculate the core-cladding relative refractive index difference Δn and the numerical aperture NA. Figure 7 As shown. Figure 8 The radial distribution of the refractive index difference in the preform in this example has been experimentally verified. Figure 8 The refractive index diagram is consistent with that of typical fluorine-doped preforms. The preform in this embodiment achieves one-step vitrification preparation, with a numerical aperture of 0.19, exhibiting excellent light-receiving capabilities.

[0075] Currently, mainstream preform deposition methods are two-step processes. The first step involves depositing SiO2 dust on a mother rod, and the second step involves melting and condensing the dust into a transparent quartz rod. The drawback of this two-step method is that the condensation requires high temperatures, which can lead to dopant diffusion. Existing VAD, OVD, and MCVD methods using oxyhydrogen flames are difficult to prepare preforms with high-hydroxyl quartz cores and deep fluorine-doped quartz cladding because these three methods are limited by thermodynamic equilibrium conditions, resulting in low fluorine concentrations. Furthermore, secondary high-temperature sintering or melting and condensation can lead to dopant diffusion and volatilization. This invention addresses the problem of shallow fluorine doping depth in MCVD, OVD, and VAD preforms by employing atmospheric pressure microwave plasma coupling to improve reaction activity. High-energy, high-concentration plasma quenches on the target rod surface, generating a glassy, ​​deeply fluorine-doped cladding. This eliminates the need for secondary sintering or melting and condensation, and avoids dopant diffusion and volatilization, making it suitable for preparing large-diameter, high-hydroxyl, deeply fluorine-doped preforms. The design of a three-layer sleeve structure optimizes the internal airflow, effectively reducing the blockage problem of the discharge tube and achieving one-step glassization of the preform cladding.

[0076] Those skilled in the art will understand that the features described in the various embodiments and / or claims of this disclosure can be combined and / or combined in various ways, even if such combinations or combinations are not explicitly described in this disclosure. In particular, the features described in the various embodiments and / or claims of this disclosure can be combined and / or combined in various ways without departing from the spirit and teachings of this disclosure, and all such combinations and / or combinations fall within the scope of this disclosure.

[0077] Although this disclosure has been shown and described with reference to specific exemplary embodiments thereof, those skilled in the art will understand that various changes in form and detail may be made to this disclosure without departing from the spirit and scope of the disclosure as defined by the appended claims and their equivalents. Therefore, the scope of this disclosure should not be limited to the above embodiments, but should be defined not only by the appended claims, but also by their equivalents.

Claims

1. A burner adapted for atmospheric pressure plasma external deposition, characterized in that, include: Flame section, main duct, and air intake section; The flame ejector is U-shaped, so that the plasma flame ejected from it can cover the target rod placed perpendicular to the opening direction of the flame ejector. The main pipe connects the flame section and the air inlet section located at both ends thereon; The air intake section is embedded in the inner wall of the main pipe and includes a raw material pipe and a combustion-supporting pipe. The raw material pipeline is used to inject raw material gas; The combustion-supporting pipe is a multi-path spiral air intake structure, including several combustion-supporting branch pipes arranged around the raw material pipe; each of the combustion-supporting branch pipes is arranged with the same relative inclination angle and relative position as the raw material pipe, so that the combustion-supporting gas sprayed from each of the combustion-supporting branch pipes can form a spiral shape. The air intake section also includes a cooling isolation pipe for spraying cooling isolation gas to cool and isolate the reactants. The cooling isolation pipe is located between the inner wall of the combustion-supporting pipe and the main pipe, and is formed by the space between the cooling isolation pipe wall of the combustion-supporting pipe and the inner wall of the main pipe that are completely surrounded by the outside. The gap between the cooling isolation pipe wall and the inner pipe wall of the main pipe gradually decreases along the injection direction of the cooling isolation gas.

2. The burner adapted for atmospheric pressure plasma external deposition as described in claim 1, characterized in that, The diameter of the U-shaped opening of the flame jet is larger than the diameter of the main pipe to which it is connected.

3. The burner adapted for atmospheric pressure plasma external deposition as described in claim 1, characterized in that, The combustion-supporting pipe is rotatably arranged with the central axis of the raw material pipe as its axis.

4. The burner adapted for atmospheric pressure plasma external deposition as described in claim 1, characterized in that, The feed gas includes silicon compounds and fluorine compounds; The combustion-supporting agent is oxygen; The cooling barrier gas is one or more of nitrogen or oxygen.

5. The burner adapted for atmospheric pressure plasma external deposition as described in claim 4, characterized in that, The silicon compound includes SiCl4; The fluorine compounds include one or more of C2F6, C2F4, CFCl3, and CF2Cl2.

6. The burner adapted for atmospheric pressure plasma external deposition as described in claim 1, characterized in that, The combustion-supporting pipeline includes four combustion-supporting sub-pipes.

7. The burner adapted for atmospheric pressure plasma external deposition as described in claim 1, characterized in that, The raw material pipeline is located at the central axis of the main pipeline; Several combustion-supporting sub-pipes are evenly arranged around the raw material pipe.

8. A method for preparing a fluorine-doped preform using the burner of claim 1, characterized in that, include: Place the target rod to be processed at the U-shaped inlet of the flame jet section; The raw material pipeline is used to inject raw material gas, the combustion-supporting pipelines are used to inject combustion-supporting gas, and the cooling and isolation pipelines are used to inject cooling and isolation gas. The target rod to be treated is deposited using an atmospheric pressure microwave plasma torch ejected from the burner.

9. The method for preparing fluorine-doped preforms as described in claim 8, characterized in that, The feed gas includes silicon compounds and fluorine compounds; The silicon compound includes SiCl4; The fluorine compound includes one or more of C2F6, C2F4, CFCl3, and CF2Cl2; The combustion-supporting agent is oxygen; The cooling barrier gas is one or more of nitrogen or oxygen.

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