Method and apparatus for deposition of optical fiber preform in an atmospheric pressure plasma torch

Abstract

The application discloses a method and device for depositing and preparing an optical fiber preform in an atmospheric pressure plasma flame tube, comprising the following steps: inserting a quartz liner into a resonant cavity; transmitting atmospheric pressure microwaves to the quartz liner through a waveguide system and the resonant cavity in sequence; introducing a reaction gas into the quartz liner under a strong electromagnetic field environment, so that the reaction gas is excited by the atmospheric pressure microwaves to generate a first plasma flame in the quartz liner; rotating the quartz liner at a first speed and moving the resonant cavity at a second speed, so that the reaction gas is pyrolyzed in the quartz liner and a loose body is deposited on the inner surface of the quartz liner; obtaining the quartz liner with the deposited loose body; forming a hollow preform by the quartz liner with the deposited loose body; sintering the hollow preform to obtain a vitrified hollow preform; and fusing and shrinking the vitrified hollow preform to obtain an optical fiber preform. The loose body prepared by the application has more uniform dispersity, high deposition efficiency, and is more conducive to the preparation of a large-size preform.

Description

Technical Field

[0001] This invention relates to the field of optical fiber preform preparation technology, specifically to a method and apparatus for preparing optical fiber preforms by deposition in an atmospheric pressure plasma flame tube. Background Technology

[0002] The continuous development of downstream applications of specialty optical fibers has placed increasingly higher performance demands on optical fibers. As the core raw material for optical fiber manufacturing, the fiber preform's preparation process directly determines the fiber's performance. Traditional optical fiber preform preparation technologies mainly include external deposition and internal deposition methods. Among them, MCVD (Modified Chemical Vapor Deposition) internal deposition is primarily used for high-quality core rod preparation. The core principle of MCVD internal deposition is to introduce high-purity gas (such as...) into the interior of a rotating quartz glass tube... , , , , Methods include high-temperature oxidation to deposit doped silica porous bodies (soot) on the tube wall or direct high-temperature vitrification. MCVD (Mechanical Vapor Deposition) for preparing porous bodies, combined with solution immersion or vapor-phase doping, is primarily used for rare-earth-doped optical fiber preforms. When using soot deposition, the structure and doping characteristics of the porous body are typically adjusted by regulating the deposition temperature and gas flow rate. As a substrate material for liquid-phase immersion doping, the porous body effectively captures and immobilizes rare-earth dopants, ensuring their stable presence in the core region of the optical fiber.

[0003] In the fabrication of optical fiber preforms, the core diameter and doping concentration are key performance indicators. Larger liner sizes result in larger core diameters and higher deposition efficiency. However, MCVD typically uses an oxyhydrogen flame for heating outside the liner, with heat transferred through the liner to the chemical reactants. Therefore, to maintain the internal temperature, the liner diameter for MCVD is usually less than 30 mm. Excessively large liner sizes lead to uneven heating, affecting deposition rate and uniformity. Secondly, the external heating method also results in lower chemical reactivity of the reactants, leading to low concentrations and uneven distribution of some dopants, thus affecting core quality. Furthermore, prolonged and repeated heating of the reaction liner under high-temperature flames can easily cause deformation. Summary of the Invention

[0004] The method and apparatus for preparing optical fiber preforms by atmospheric pressure plasma flame tube deposition provided by the present invention have a more uniform dispersion of loose material prepared in quartz liner, high deposition efficiency, and the quartz liner is less prone to deformation, which is more conducive to the production of large-size preforms.

[0005] According to a first aspect of this application, a method for preparing optical fiber preforms by deposition in an atmospheric pressure plasma flame tube is provided, comprising the following steps:

[0006] Step 1: Insert the quartz liner into the resonant cavity, and transmit the ambient pressure microwave to the quartz liner in sequence through the waveguide system and the resonant cavity;

[0007] Step 2: Under a strong electromagnetic field, the reactant gas is introduced into the quartz liner tube so that the reactant gas is excited by atmospheric pressure microwaves, generating a first plasma flame in the quartz liner tube. The quartz liner tube is controlled to rotate at a first speed in the resonant cavity, and the resonant cavity is controlled to move at a second speed so that the reactant gas is pyrolyzed in the quartz liner tube and a porous body is deposited on the inner surface of the quartz liner tube, thus obtaining a quartz liner tube with a porous body deposited.

[0008] Step 3: Repeat step 2 multiple times until the porous body is stacked to the preset deposition wall thickness. The quartz liner with deposited porous body forms a hollow preform. The hollow preform is sintered to obtain a vitrified hollow preform.

[0009] Step 4: Melt and shrink the vitrified hollow preform to obtain the optical fiber preform.

[0010] In some embodiments of this application, based on the foregoing scheme, the method further includes the following before step 2:

[0011] Polishing gas is introduced into a quartz liner tube so that the polishing gas is excited by atmospheric pressure microwaves, generating a second plasma flame in the quartz liner tube, so that the polishing gas is pyrolyzed inside the quartz liner tube and polishes the inner surface of the quartz liner tube.

[0012] In some embodiments of this application, based on the foregoing scheme, step 2 further includes:

[0013] An inlet end is fitted onto one end of the quartz liner tube, and the reaction gas is introduced into the quartz liner tube through the inlet end.

[0014] A first vacuum pump is placed at the other end of the quartz liner tube, and the quartz liner tube is evacuated and ignited by the first vacuum pump.

[0015] The resonant cavity is controlled to move from the gas inlet end sleeve toward the first vacuum pump along the axial direction of the quartz liner tube, so that the reactant gas is pyrolyzed inside the quartz liner tube and deposited in a single reverse pass multiple times on the inner surface of the quartz liner tube to form a porous body.

[0016] In some embodiments of this application, based on the foregoing scheme, step 2 further includes:

[0017] Obtain the actual temperature of the quartz liner;

[0018] If the actual temperature is greater than or equal to the preset deposition temperature, the input power of the ambient pressure microwave is reduced to the first preset power.

[0019] If the actual temperature is lower than the preset deposition temperature, the input power of the ambient pressure microwave is increased to the second preset power.

[0020] In some embodiments of this application, based on the foregoing scheme, step 2 further includes:

[0021] Obtain the real-time deposition wall thickness of the quartz liner;

[0022] If the difference between the real-time deposited wall thickness and the preset deposited wall thickness is greater than or equal to the difference threshold, the input power of the ambient pressure microwave is increased to the third preset power.

[0023] If the difference between the real-time deposition wall thickness and the preset deposition wall thickness is less than the difference threshold, the input power of the ambient pressure microwave is reduced to the fourth preset power.

[0024] In some embodiments of this application, based on the foregoing scheme, step 2 further includes:

[0025] The temperature error and the rate of change of the actual temperature of the quartz liner relative to the preset deposition temperature are obtained, or the difference error and the rate of change of the difference error between the real-time deposition wall thickness of the quartz liner and the preset deposition wall thickness are obtained.

[0026] An improved self-tuning PID controller is established, using temperature error and its rate of change or differential error and its rate of change as input parameters. The adjustment of the PID parameters is used as the output parameter. The PID parameters are updated based on the adjustment, and the input power of the ambient pressure microwave is adjusted based on the PID parameters. The method for establishing the improved self-tuning PID controller is as follows:

[0027] Based on the identified model parameters, the initial PID parameters corresponding to the improved self-tuning PID controller are calculated using the Ziegler-Nichols rule.

[0028] Relating the actual temperature to the preset deposition temperature The temperature error and the rate of change of temperature error, or the difference error between the real-time deposition wall thickness and the preset deposition wall thickness and the rate of change of the difference error, and the adjustment amount of the PID parameters are all divided into multiple fuzzy sets and the membership function is set to Gaussian to establish a fuzzy rule base.

[0029] Set control cycle In each control cycle The center-of-gravity method is used to obtain the output parameters of the self-tuning PID controller, and the input power of the ambient pressure microwave is adjusted accordingly.

[0030] According to a second aspect of this application, an apparatus for preparing optical fiber preforms by deposition in an atmospheric pressure plasma flame tube is provided, the apparatus comprising:

[0031] Magnetrons are used to generate ambient pressure microwaves under strong electromagnetic fields.

[0032] A three-pin tuner, connected to a magnetron, is used to adjust the output power of ambient pressure microwaves.

[0033] A waveguide system, connected to a three-pin tuner, is used to generate and transmit ambient pressure microwaves.

[0034] The resonant cavity is connected to the waveguide system, and a quartz liner is inserted through the resonant cavity;

[0035] Two rotary sealing joints are fixed at both ends of the quartz liner tube, and one rotary sealing joint is driven to rotate by the first driving component;

[0036] The inlet sleeve is located on a rotary sealing joint and is used to input reaction gas or polishing gas;

[0037] The dust removal system, located on another rotary sealing joint, is used for dust removal from the quartz liner tube;

[0038] The first vacuum pump, located on another rotary sealing joint, is used to evacuate and ignite the quartz liner.

[0039] The resistance furnace is located below the quartz liner tube and is used to heat the quartz liner tube.

[0040] In some embodiments of this application, based on the foregoing scheme, a resistance furnace is also included. The resistance furnace includes a heating groove, and a quartz liner is placed in the heating groove and spaced a certain distance from the inner wall of the heating groove.

[0041] In some embodiments of this application, based on the foregoing scheme, the following further methods are also included:

[0042] An improved self-tuning PID controller is used to adjust the input power of atmospheric pressure microwaves so that the actual temperature of the quartz liner reaches the preset deposition temperature, and / or the real-time deposition wall thickness of the quartz liner reaches the preset deposition wall thickness.

[0043] Compared with the prior art, the advantages of the present invention are as follows:

[0044] 1. This invention differs from traditional MCVD deposition technology by employing a novel plasma flame deposition technology. Due to internal heating of the quartz liner, the chemical reactivity is enhanced compared to MCVD, thereby improving efficiency and raw material utilization.

[0045] 2. Compared to MCVD, this invention achieves higher and more uniform doping concentration. MCVD has low deposition efficiency and low doping concentration because its doping methods primarily involve high-temperature gas-phase doping or solution immersion. However, high-temperature gas-phase doping itself results in a lower doping concentration, and the glass inside the quartz liner becomes relatively dense during deposition, further reducing the doping concentration. In contrast, this invention employs a first plasma flame, which remains inside the quartz liner, enabling internal deposition. Furthermore, due to the slight negative pressure, the deposition is a soot deposition of dust, resulting in a porous body with larger pores, thus increasing the doping concentration.

[0046] 3. This invention employs atmospheric pressure microwave plasma flame chemical vapor deposition, which features high energy, high electron ion density, and high electron temperature. Through optimized design of the resonant cavity, it can effectively achieve uniform temperature control in the deposition experiment, thereby realizing the uniformity of deposition. Compared with the thermophoretic effect of MCVD, its deposition uniformity performance is effectively controllable.

[0047] 4. Compared with the existing PCVD (Plasma-Activated Chemical Vapor Deposition) external deposition method, the present invention can be achieved under normal pressure and does not require a complex vacuum system and holding furnace, resulting in a simpler structure and lower cost.

[0048] 5. Unlike traditional internal deposition of small-diameter optical fiber preforms, this invention employs real-time temperature and power control, which improves deposition efficiency while ensuring deposition uniformity, enabling rapid fabrication of large-diameter optical fiber preforms.

[0049] 6. This invention scientifically and rationally designs the power control during the deposition process, adjusting the corresponding power according to different deposition wall thicknesses, to avoid the influence of changes in the discharge tube center field strength on the plasma as the tube wall thickness increases, thereby affecting the uniformity and quality of deposition. Attached Figure Description

[0050] The accompanying drawings, which are included to provide a further understanding of the invention and form part of this application, illustrate exemplary embodiments of the invention and are intended to explain the invention, but do not constitute an undue limitation thereof. In the drawings:

[0051] Figure 1 This is a flowchart of a method for preparing optical fiber preforms by deposition in an atmospheric pressure plasma flame tube according to the present invention.

[0052] Figure 2 This is a cross-sectional view of the electric field intensity distribution of the resonant cavity in a specific embodiment of the present invention.

[0053] Figure 3This is a longitudinal cross-sectional view of the electric field intensity distribution in a resonant cavity according to a specific embodiment of the present invention.

[0054] Figure 4 This is an electric field intensity diagram along the radial direction of the deposition liner tube, representing the electric field intensity distribution of the resonant cavity in a specific embodiment of the present invention.

[0055] Figure 5 This is a graph showing the relationship between the real-time deposition wall thickness of the quartz liner and the maximum field strength at the center of the discharge tube in a specific embodiment of the present invention.

[0056] Figure 6 This is a graph showing the relationship between the real-time deposition wall thickness of the quartz liner and the electric field intensity of the inner wall of the microwave feed section in a specific embodiment of the present invention.

[0057] Figure 7 This is a comparison diagram showing the relationship between the aperture and aperture area of ​​an optical fiber preform obtained in a specific embodiment of the present invention and an optical fiber preform obtained by the MCVD internal deposition method.

[0058] Figure 8 This is an electron microscope image of an MCVD sample from a specific embodiment of the present invention.

[0059] Figure 9 This is an electron micrograph of a plasma-borne porous body according to a specific embodiment of the present invention.

[0060] Figure 10 This is a schematic diagram of the apparatus for preparing optical fiber preforms by deposition in an atmospheric pressure plasma flame tube according to the present invention. Attached image description:

[0062] 1. Magnetron; 2. Resonant cavity; 201. Thermal imaging aperture; 3. Three-pin tuner; 4. Waveguide system; 5. Dust removal system; 6. Inlet sleeve; 7. Rotary sealing joint; 8. Resistance furnace; 9. Thermal imaging temperature sensor; 10. First vacuum pump; 11. Quartz liner. Detailed Implementation

[0063] Specific embodiments of the invention will now be described in detail with reference to the accompanying drawings, which illustrate examples of the invention. Although the invention will be described in conjunction with specific embodiments, it will be understood that it is not intended to limit the invention to the embodiments described herein. Rather, it is intended to cover variations, modifications, and equivalents included within the spirit and scope of the invention as defined by the appended claims. It should be noted that the method steps described herein can be implemented by any functional block or functional arrangement, and any functional block or functional arrangement can be implemented as a physical entity or a logical entity, or a combination of both.

[0064] To enable those skilled in the art to better understand the present invention, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0065] Note: The examples described below are merely specific examples and are not intended to limit the embodiments of the present invention to the specific steps, values, conditions, data, order, etc. Those skilled in the art can utilize the concepts of the present invention to construct more embodiments not mentioned herein by reading this specification.

[0066] To facilitate understanding of this application, a more complete description will be provided below with reference to the accompanying drawings, which illustrate embodiments of the present application. However, the present application can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided so that the disclosure of this application will be more thorough and complete.

[0067] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used herein in the specification of this application is for the purpose of describing particular embodiments only and is not intended to be limiting of this application.

[0068] It is understood that spatial relation terms such as "below," "under," "below," "below," "above," "over," etc., can be used here to describe the relationship between one element or feature shown in the figure and other elements or features. It should be understood that, in addition to the orientation shown in the figure, spatial relation terms also include different orientations of the device in use and operation. For example, if the device in the figure is flipped, the element or feature described as "below" or "under" or "below" of other elements or features will be oriented "over" of other elements or features. Therefore, the exemplary terms "below" and "under" can include both upper and lower orientations. Furthermore, the device may also include other orientations (e.g., rotated 90 degrees or other orientations), and the spatial descriptive terms used herein will be interpreted accordingly.

[0069] It should be noted that when one element is considered to be "connected" to another element, it can be directly connected to the other element or connected to the other element through an intermediary element. In the following embodiments, "connection" should be understood as "electrical connection," "communication connection," etc., if the connected circuits, modules, units, etc., have the transmission of electrical signals or data between them.

[0070] When used here, the singular forms of “a,” “an,” and “ / the” may also include the plural forms unless the context clearly indicates otherwise. It should also be understood that the terms “including / contains” or “having” specify the presence of the stated feature, whole, step, operation, component, part, or combination thereof, but do not preclude the possibility of the presence or addition of one or more other features, wholes, steps, operations, components, parts, or combinations thereof.

[0071] Example 1

[0072] See Figure 1 The diagram shows a flowchart of a method for preparing optical fiber preforms by deposition within an atmospheric pressure plasma flame tube according to the present invention. (See also...) Figure 10 The diagram shown is a structural schematic of an apparatus for fabricating optical fiber preforms by deposition within an atmospheric pressure plasma flame tube according to the present invention. This embodiment provides a method for fabricating optical fiber preforms by deposition within an atmospheric pressure plasma flame tube, comprising the following steps:

[0073] Step 1: Insert the quartz liner 11 into the resonant cavity 2, and transmit the ambient pressure microwave to the quartz liner 11 through the waveguide system 4 and the resonant cavity 2 in sequence;

[0074] Step 2: Under a strong electromagnetic field, the reaction gas is introduced into the quartz liner tube 11 so that the reaction gas is excited by atmospheric pressure microwaves and a first plasma flame is generated in the quartz liner tube 11. The quartz liner tube 11 is controlled to rotate at a first speed in the resonant cavity 2, and the resonant cavity 2 is controlled to move at a second speed so that the reaction gas is pyrolyzed in the quartz liner tube 11 and a porous body is deposited on the inner surface of the quartz liner tube 11, thus obtaining a quartz liner tube 11 with a porous body deposited on it. The porous body is SiO2 porous body.

[0075] Step 3: Repeat step 2 multiple times until the porous body is stacked to the preset deposition wall thickness. The quartz liner 11 with deposited porous body forms a hollow preform. The hollow preform is sintered to obtain a vitrified hollow preform.

[0076] Step 4: Melt and shrink the vitrified hollow preform to obtain the optical fiber preform.

[0077] This embodiment provides a method for preparing optical fiber preforms by deposition within a vacuum plasma flame tube at atmospheric pressure. Under a strong electromagnetic field, the reactant gas is excited to a high-energy state (excited molecules or ions) to form a first plasma flame. These high-energy particles have higher chemical reactivity, promoting molecular decomposition. Furthermore, by using the first plasma flame, it is kept inside the quartz liner tube 11, creating a slightly negative pressure state for internal deposition. The deposition state is dust soot deposition, resulting in a porous material with larger pores, thus increasing the doping concentration. In addition, the first plasma flame provides a localized high-temperature environment, enhancing thermal reactions. During excitation, the first plasma flame releases a large amount of heat. This high-temperature environment promotes the pyrolysis of the reactant gas (e.g., SiCl4 and O2 decompose more easily to form SiO precursor SiO and other byproducts), accelerating the chemical reaction. Simultaneously, the vacuum microwave first plasma flame has high energy, high electron and ion density, and electron-carrying temperature, providing a high-energy chemical reaction deposition atmosphere and effectively improving the chemical reactivity of the reactants. In the preparation of rare earth-doped preforms, elements such as phosphorus (P) and phosphorus (F) are typically added during the preparation of the porous body to adjust the rare earth doping concentration and numerical pore size. This method utilizes microwave excitation to directly generate an atmospheric pressure plasma flame within the liner. The high-energy first plasma flame directly acts on the reactant gas. The excited-state first plasma flame can overcome the thermal equilibrium conditions of the gas-phase reaction, resulting in higher chemical reactivity of the raw materials and facilitating the preparation of highly P and F-doped SiO2 porous bodies. This, in turn, increases the rare earth doping concentration and reduces the numerical pore size of the core rod.

[0078] In some embodiments of this example, before step 2, the following is also included:

[0079] Polishing gas is introduced into the quartz liner 11 so that the polishing gas is excited by atmospheric pressure microwaves, generating a second plasma flame in the quartz liner 11, so that the polishing gas is pyrolyzed in the quartz liner 11 and polishes the inner surface of the quartz liner 11.

[0080] In some embodiments of this example, step 2 further includes:

[0081] The inlet sleeve 6 is located at one end of the quartz liner tube 11 and is used to input the reaction gas into the quartz liner tube 11.

[0082] The first vacuum pump 10 is located at the other end of the quartz liner tube 11 and is used to evacuate and ignite the quartz liner tube 11.

[0083] The control resonant cavity 2 moves from the gas inlet sleeve 6 to the first vacuum pump 10 along the axial direction of the quartz liner 11, so that the reaction gas is pyrolyzed in the quartz liner 11 and deposited into a loose body in a single reverse pass multiple times on the inner surface of the quartz liner 11.

[0084] This embodiment employs a reverse single-pass multiple deposition method. Reverse deposition facilitates a full reaction between the resonant cavity 2 and the reactive gas, resulting in more uniform deposition on the quartz liner 11 and a higher deposition thickness compared to forward deposition. Single-pass deposition ensures more uniform doping in each layer and is also beneficial for subsequent sintering. According to thermal field analysis, the quartz liner 11 at the gas inlet sleeve 6 has a cooler tube wall, which is conducive to dust settling and adsorption. However, the quartz liner 11 near the first vacuum pump 10 has a higher temperature due to the effect of the resonant cavity 2, which is not conducive to dust settling. This also allows more reactive gas to be utilized, and the small portion of the settled reactive gas is relatively loose, increasing the porosity of the deposition and facilitating doping.

[0085] In some embodiments of this example, the deposition temperature of the transport body can be adjusted by setting a preset deposition temperature, a first preset power, and a second preset power, enabling real-time temperature and power control. This ensures that deposition uniformity is met while improving deposition efficiency, thus achieving rapid fabrication of large-diameter optical fiber preforms. Specifically, step 2 further includes:

[0086] Obtain the actual temperature of the quartz liner 11;

[0087] If the actual temperature is greater than or equal to the preset deposition temperature, the input power of the ambient pressure microwave is reduced to the first preset power.

[0088] If the actual temperature is lower than the preset deposition temperature, the input power of the ambient pressure microwave is increased to the second preset power.

[0089] In some embodiments of this example, step 2 specifically further includes:

[0090] Obtain the real-time deposition wall thickness of the quartz liner 11;

[0091] If the difference between the real-time deposited wall thickness and the preset deposited wall thickness is greater than or equal to the difference threshold, the input power of the ambient pressure microwave is increased to the third preset power.

[0092] If the difference between the real-time deposition wall thickness and the preset deposition wall thickness is less than the difference threshold, the input power of the ambient pressure microwave is reduced to the fourth preset power.

[0093] In some embodiments of this example, the input power of the ambient pressure microwave can be adjusted by a self-tuning PID controller to make the actual temperature reach the preset deposition temperature, or to make the real-time deposition wall thickness reach the preset deposition wall thickness. Step 2 further includes:

[0094] Obtain the temperature error and the rate of change of the actual temperature of the quartz liner 11 relative to the preset deposition temperature, or obtain the difference error and the rate of change of the difference error between the real-time deposition wall thickness and the preset deposition wall thickness.

[0095] An improved self-tuning PID controller is established, using temperature error and its rate of change, or differential error and its rate of change, as input parameters. The adjustment amount of the PID parameters is... , , As the output parameter of the improved self-tuning PID controller, the PID parameter is updated based on the adjustment amount of the PID parameter, and the input power of the atmospheric pressure microwave is adjusted based on the PID parameter.

[0096] The method for establishing an improved self-tuning PID controller is as follows:

[0097] Based on the identified model parameters The initial PID parameters were calculated using the Ziegler-Nichols rule. , , The calculation formula is:

[0098] , , ,in , ;

[0099] The temperature error and rate of change of the actual temperature relative to the preset deposition temperature, or the difference error and rate of change of the difference error between the real-time deposition wall thickness and the preset deposition wall thickness, and the adjustment amount of the PID parameters are used. , , Each set is divided into multiple fuzzy sets and the membership function is set to Gaussian to establish a fuzzy rule base;

[0100] Set control cycle In each control cycle The center of gravity (COG) method is used to obtain the output parameters of the self-tuning PID controller, i.e., the adjustment amount of the output PID parameters. , , Adjustment amount of PID parameters , , For initial PID parameters , , Adjustments are made to obtain the adjusted PID parameters, and the input power of the ambient pressure microwave is adjusted based on the adjusted PID parameters.

[0101] This embodiment employs a first plasma in-flame deposition method. The internal temperature of the reaction gas is precisely measured by infrared thermal imaging, and the input power of the corresponding atmospheric pressure microwave is adjusted in real time by a computer and power supply PID. This achieves precise control of the internal temperature of the reaction gas, improves energy utilization, and also meets the requirements for deposition uniformity.

[0102] In addition, this embodiment adjusts the input power of the ambient pressure microwave for different deposition wall thicknesses to avoid the influence of changes in the field strength at the center of the discharge tube on the plasma as the tube wall thickness increases, thereby affecting the uniformity and quality of deposition.

[0103] In one specific embodiment, a cylindrical resonant cavity 2 is connected to the system. This cavity has an axially oriented Φ50mm through-hole through which a high-purity quartz reaction liner extends to the outside of the cavity. Its front end integrates multiple inlet pipes via a rotary sealing clamp, supporting precise injection of process gases such as Ar / O2 / N2. The rear end connects to a first vacuum pump 10 and an exhaust gas treatment system via a similar rotary sealing clamp, maintaining a 10-stage vacuum cycle. 3 Pa-level working vacuum. Resonant cavity 2 is equipped with a linear servo mechanism, enabling reciprocating motion of 50-200 mm / s. When 2.45 GHz microwave energy enters resonant cavity 2 through the waveguide, a high-intensity electromagnetic field is formed in TM010 standing wave mode. This ionizes the process gas inside the quartz tube, generating a high-density non-equilibrium plasma flame with an electron temperature of 3-5 eV. The spatial distribution of the plasma flame is modulated by the cavity motion, ultimately achieving coordinated control of the microwave field, gas flow field, and temperature field, providing key technological support for large-area uniform deposition. Please refer to [link to relevant documentation]. Figures 2-6 As shown, the deposition of quartz liner 11 under a strong electromagnetic field in resonant cavity 2 is illustrated. Figure 2 This is a cross-sectional view of the electric field intensity distribution of the resonant cavity 2 in a specific embodiment of the present invention. Figure 3 This is a longitudinal cross-sectional view of the electric field intensity distribution of the resonant cavity 2 in a specific embodiment of the present invention. Figure 4 This is an electric field intensity diagram along the radial direction of the deposition liner tube, showing the electric field intensity distribution of the resonant cavity 2 in a specific embodiment of the present invention. Figure 5 This is a graph showing the relationship between the real-time deposition wall thickness of the quartz liner 11 and the maximum field strength at the center of the discharge tube in a specific embodiment of the present invention. Figure 6 This is a graph showing the relationship between the real-time deposition wall thickness of the quartz liner 11 and the electric field intensity of the inner wall of the microwave feed section in a specific embodiment of the present invention.

[0104] Experimental operation: 1) Deposition process: Under the condition of high purity O2 purging, a clean quartz liner 11 with tail tubes fused at both ends is installed on the inner deposition machine tool, the translation and rotation mechanism of the inner deposition machine tool is turned on, and the microwave plasma flame power output is turned on.

[0105] 2) Before the deposition begins, hexafluoroethane is introduced to polish the inner wall of the liner.

[0106] 3) Based on the preset parameters of the optical fiber preform, oxygen 1-6 SLM, silicon 1.5 SLM, phosphorus oxychloride 70 sccm are introduced, the moving speed is 3-48 mm / s, and the rotation speed is 30 rpm. The power and temperature are controlled by PID through computer to ensure that the deposition temperature is accurately controlled at about 1200℃. Multiple rounds of deposition are carried out to deposit a loose body of about 1 mm.

[0107] 5) The loose material is dried by passing chlorine gas through it. Under the heating of the plasma flame, the residual hydroxide ions or other solvents are volatilized through chemical reaction, which avoids the introduction of hydroxide ions and can effectively reduce the fiber substrate loss caused by water peak absorption.

[0108] 6) The dried hollow preforms are subjected to multiple processes 3 to 5.

[0109] 7) The hollow preforms after completing step 6 are sintered and vitrified at 1800-2000℃.

[0110] 8) Place the sintered hollow preform on a melting and shrinking machine that has completed the preparatory conditions for melting and shrinking, and start the translation and rotation mechanism of the melting and shrinking machine. Purge the hollow preform with 800 sccm of high-purity O2, set the tail pressure to 5 Pa, the rotation speed of the translation and rotation mechanism of the melting and shrinking machine to 30 rpm, the moving speed to 100~500 mm / min, and the temperature to 2190℃, and then melt and shrink to obtain optical fiber preform D-27.

[0111] After thinning, the end face was cut for electron probe microanalysis (EPMA) concentration testing. The test results showed that the P2O5 doping concentration at the highest point reached 11.6 mol%.

[0112] In this specific embodiment, see Figure 7 As shown, the black curve represents the optical fiber preform D-27 obtained in this specific embodiment, and the red curve represents the optical fiber preform obtained by the MCVD internal deposition method. By comparing the optical fiber preform D-27 obtained in this specific embodiment with the optical fiber preform obtained by the MCVD internal deposition method, it can be found that, under the same aperture, the optical fiber preform D-27 obtained in this specific embodiment has a smaller aperture area and the sample is more compact.

[0113] In this specific embodiment, referring to Table 1, the optical fiber preform D-27 obtained is compared with the optical fiber preform obtained by the MCVD internal deposition method. It can be found that the specific surface area and total pore volume of the optical fiber preform D-27 obtained in this specific embodiment are smaller than those of the optical fiber preform obtained by the MCVD internal deposition method. However, the average pore size of the optical fiber preform D-27 obtained in this specific embodiment is larger than that of the optical fiber preform obtained by the MCVD internal deposition method, indicating that the optical fiber preform D-27 obtained in this specific embodiment has fewer pores and the sample is more compact.

[0114] Table 1 Test results of optical fiber preforms

[0115]

[0116] In this specific embodiment, compared with MCVD, the optical fiber preform obtained in this specific embodiment has a higher porosity and presents a more ideal mesh structure, which is beneficial for subsequent gas phase doping or solution immersion doping.

[0117] Example 2

[0118] This embodiment provides an apparatus for fabricating optical fiber preforms by deposition within a normal-pressure plasma flame tube, such as... Figure 10 As shown, the device includes:

[0119] Magnetron 1 is used to generate ambient pressure microwaves under strong electromagnetic fields.

[0120] Three-pin tuner 3 is connected to magnetron 1 to adjust the output power of ambient pressure microwave;

[0121] Waveguide system 4, connected to three-pin tuner 3, is used to generate and transmit ambient pressure microwaves;

[0122] The resonant cavity 2 is connected to the waveguide system 4, and the quartz liner tube 11 passes through the resonant cavity 2;

[0123] Two rotary sealing joints 7 are fixed at both ends of the quartz liner tube 11, and the rotary sealing joints 7 are driven to rotate by the first driving component.

[0124] The inlet sleeve 6 is located on a rotary sealing joint 7 and is used to input reaction gas or polishing gas;

[0125] Dust removal system 5 is installed on another rotary sealing joint 7 and is used to remove dust from quartz liner tube 11;

[0126] The first vacuum pump 10 is located on another rotary sealing joint 7 and is used to evacuate and ignite the quartz liner tube 11.

[0127] The resistance furnace 8 is located below the quartz liner tube 11 and is used to heat the quartz liner tube 11.

[0128] The apparatus for preparing optical fiber preforms by in-tube deposition of atmospheric pressure plasma provided in this embodiment, during use...

[0129] The first driving component includes a chuck connector and a first driving motor. The chuck connector is fixed on the rotary sealing joint 7. The output shaft of the first driving motor is fixedly connected to the chuck connector. The first driving motor drives the chuck connector to rotate, which in turn drives the rotary sealing joint 7 to rotate, causing the quartz liner 11 to rotate. This causes the reaction gas to pyrolyze inside the quartz liner 11 and deposit a porous body on the inner surface of the quartz liner 11, or causes the polishing gas to uniformly polish the inner surface of the quartz liner 11.

[0130] In this embodiment, the quartz liner 11 is inserted into the resonant cavity 2. Atmospheric pressure microwaves are transmitted to the quartz liner 11 through the waveguide system 4 and the resonant cavity 2. The reactant gas introduced into the quartz liner 11 is directly excited by the atmospheric pressure microwaves to generate a first plasma flame. Under the thermophoretic effect of the first plasma flame, a soot porous body is deposited downstream of the first plasma flame. Because the first plasma flame directly acts on the reactant gas, compared to MCVD technology, the size of the quartz liner 11 is only constrained by the resonant cavity 2, thus significantly increasing the size of the quartz liner 11, allowing for larger preforms and higher fabrication efficiency. Secondly, the high-energy first plasma flame directly acts on the reactant gas raw material. The excited state of the first plasma flame can overcome the thermal equilibrium conditions of the gas-phase reaction, resulting in higher chemical reactivity of the raw material and making it easier to prepare soot porous bodies with high doping concentration and more uniform dispersion. Furthermore, the internal heating method keeps the liner temperature lower than the reactant temperature, effectively avoiding deformation and energy loss of the quartz liner 11 caused by high-temperature external heating.

[0131] In this embodiment, the resonant cavity 2 is a circular slit-excited resonant cavity 2. The circular slit-excited resonant cavity 2 ensures that the strong electromagnetic field is symmetrical, thereby guaranteeing the symmetry and uniformity of the first plasma flame electric field intensity. This results in a uniform strong electromagnetic field being generated on the inner surface of the quartz liner tube 11, improving the deposition uniformity. Simultaneously, the deposition wall thickness of the quartz tube is considered to affect the maximum field strength at the center of the discharge tube.

[0132] In some embodiments of this example, a fuzzy self-tuning PID controller is also included. This improved self-tuning PID controller adjusts the input power of the ambient pressure microwave to ensure that the actual temperature of the quartz liner 11 reaches the preset deposition temperature, and / or the real-time deposition wall thickness of the quartz liner 11 reaches the preset deposition wall thickness. By regulating the input power of the ambient pressure microwave, the fuzzy self-tuning PID controller reduces the impact of changes in the intensity of the strong electromagnetic field on the first plasma flame during the deposition process.

[0133] In this embodiment, a thermal imaging hole 201 is provided on the resonant cavity 2, and a thermal imaging temperature sensor 9 is provided on the outside of the thermal imaging hole 201. The actual temperature of the quartz liner tube 11 is obtained by using the thermal imaging temperature sensor 9.

[0134] In this embodiment, a wall thickness sensor is provided on the outside of the resonant cavity 2 to obtain the real-time deposited wall thickness of the quartz liner 11.

[0135] The temperature error and the rate of change of the actual temperature of the quartz liner 11 relative to the preset deposition temperature are obtained, or the difference error and the rate of change of the difference error between the real-time deposition wall thickness of the quartz liner 11 and the preset deposition wall thickness are obtained.

[0136] An improved self-tuning PID controller is established. Temperature error and its rate of change or differential error and its rate of change are used as input parameters of the self-tuning PID controller. The adjustment amount of the PID parameters is used as the output parameter of the improved self-tuning PID controller. The PID parameters are adjusted based on the adjustment amount to obtain the adjusted PID parameters. The input power of the atmospheric pressure microwave is then adjusted based on the adjusted PID parameters.

[0137] In some embodiments of this example, a resistance furnace 8 is also included. The resistance furnace 8 includes a heating groove, and a quartz liner 11 is placed in the heating groove and spaced a certain distance from the inner wall of the heating groove. The heating groove is used to heat the quartz liner 11 to prevent the reaction gas or polishing gas from condensing in the quartz liner 11.

[0138] Furthermore, dust will only deposit on the inner wall of the quartz liner 11 when its temperature is higher than the temperature of the inner wall of the quartz liner 11. By using the resistance furnace 8 to locally heat the front end of the feed, the deposition begins at the heating point of the resistance furnace 8, effectively improving the utilization rate of the quartz liner 11 and increasing the effective deposition length; and by precisely controlling the temperature, effective and uniform deposition can be achieved in large-diameter tubes.

[0139] In summary, this invention proposes a method for preparing optical fiber preforms by in-tube deposition using a normal-pressure microwave plasma flame. In this technique, a quartz liner 11 is placed inside a resonant cavity 2. Microwave energy is transmitted to the quartz liner 11 through a waveguide and the resonant cavity 2. The reactant gas introduced into the liner is directly excited by the microwave energy to generate a first plasma flame. Under the thermophoretic effect of the first plasma flame, a porous material is deposited downstream of the first plasma flame. Because the plasma flame directly acts on the reactant gas, compared to MCVD technology, the liner size is only constrained by the resonant cavity 2, thus significantly increasing the liner size and allowing for larger preforms with higher preparation efficiency. Secondly, the high-energy plasma flame directly acts on the reactants, and the excited-state first plasma flame can overcome the thermal equilibrium conditions of the gas-phase reaction, resulting in higher chemical reactivity of the reactants and making it easier to prepare porous materials with high doping concentration and more uniform dispersion. Furthermore, the internal heating method keeps the liner temperature lower than the reactants, effectively avoiding liner deformation and energy loss caused by high-temperature external heating.

[0140] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the claims of this invention and their equivalents, this invention also intends to include these modifications and variations.

Claims

1. A method for making an optical fiber preform by deposition within an atmospheric pressure plasma torch, characterized in that, The method comprises the following steps: Step 1: a quartz liner is arranged in a resonant cavity, and normal pressure microwaves are transmitted to the quartz liner through a waveguide system and the resonant cavity in sequence; Step 2: under a strong electromagnetic field, a reaction gas is introduced into the quartz liner, so that the reaction gas is excited by the normal pressure microwaves to generate a first plasma flame in the quartz liner, the quartz liner is controlled to rotate at a first speed, and the resonant cavity is controlled to move at a second speed, so that the reaction gas is pyrolyzed in the quartz liner and deposits loose bodies on the inner surface of the quartz liner, thereby obtaining a quartz liner with deposited loose bodies, a gas inlet end is arranged at one end of the quartz liner, the reaction gas is input into the quartz liner through the gas inlet end, a first vacuum pump is arranged at the other end of the quartz liner, the quartz liner is ignited by vacuumizing through the first vacuum pump, and the resonant cavity is controlled to move along the axial direction of the quartz liner from the gas inlet end to the first vacuum pump, so that the reaction gas is pyrolyzed in the quartz liner and deposits loose bodies on the inner surface of the quartz liner in a reverse single-pass multiple times; Step 3: step 2 is repeated multiple times until the loose bodies are stacked to a preset deposition wall thickness, and a hollow preform is formed by the quartz liner with deposited loose bodies, the hollow preform is sintered to obtain a vitrified hollow preform; Step 4: the vitrified hollow preform is fused and shrunk to obtain an optical fiber preform.

2. The method of claim 1, wherein, Before the step 2, the method further comprises: A polishing gas is introduced into the quartz liner, so that the polishing gas is excited by the normal pressure microwaves to generate a second plasma flame in the quartz liner, so that the polishing gas is pyrolyzed in the quartz liner and polishes the inner surface of the quartz liner.

3. The method of claim 1, wherein, The step 2 further comprises: An actual temperature of the quartz liner is obtained; If the actual temperature is greater than or equal to a preset deposition temperature, the input power of the normal pressure microwaves is reduced to a first preset power; If the actual temperature is less than the preset deposition temperature, the input power of the normal pressure microwaves is increased to a second preset power.

4. The method of claim 1, wherein, The step 2 further comprises: A real-time deposition wall thickness of the quartz liner is obtained; If the difference between the real-time deposition wall thickness and the preset deposition wall thickness is greater than or equal to a difference threshold value, the input power of the normal pressure microwaves is increased to a third preset power; If the difference between the real-time deposition wall thickness and the preset deposition wall thickness is less than the difference threshold value, the input power of the normal pressure microwaves is reduced to a fourth preset power.

5. The method of claim 1, wherein, The step 2 further comprises: A temperature error and a temperature error change rate of the actual temperature of the quartz liner relative to the preset deposition temperature are obtained, or a difference error and a difference error change rate between the real-time deposition wall thickness of the quartz liner and the preset deposition wall thickness are obtained; The improved self-tuning PID controller is established by taking the temperature error and the temperature error change rate or the difference error and the difference error change rate as the input parameters of the self-tuning PID controller, taking the adjustment amount of the PID parameters as the output parameter of the improved self-tuning PID controller, updating the PID parameters based on the adjustment amount of the PID parameters, and adjusting the input power of the atmospheric pressure microwave based on the PID parameters, wherein the method for establishing the improved self-tuning PID controller is as follows: According to the identified model parameters, the initial PID parameters corresponding to the improved self-tuning PID controller are calculated by using the Ziegler-Nichols rule. The temperature error and the temperature error change rate of the actual temperature relative to the preset deposition temperature, or the difference error and the difference error change rate of the real-time deposition wall thickness relative to the preset deposition wall thickness, and the adjustment amount of the PID parameters are all divided into multiple fuzzy sets and the membership functions are set as Gaussian type, and a fuzzy rule base is established. Setting control period In each control period The output parameters of the self-tuning PID controller are obtained by using the barycenter method to adjust the input power of the atmospheric pressure microwave.

6. A device for producing an optical fiber preform by atmospheric pressure plasma torch deposition, applied to a method for producing an optical fiber preform by atmospheric pressure plasma torch deposition according to any one of claims 1 to 5, characterized in that, The device comprises: A magnetron for generating atmospheric pressure microwave under a strong electromagnetic field; A three-pin tuner connected to the magnetron for adjusting the output power of the atmospheric pressure microwave; A waveguide system connected to the three-pin tuner for generating transmission of the atmospheric pressure microwave; A resonant cavity connected to the waveguide system, with a quartz liner tube passing through the resonant cavity; Two rotary sealing joints respectively fixed at both ends of the quartz liner tube, one of the rotary sealing joints being driven to rotate by a first driving member; An air inlet sleeve provided on one of the rotary sealing joints for inputting reaction gas or polishing gas; A dust removal system provided on the other rotary sealing joint for dust removal of the quartz liner tube; A first vacuum pump provided on the other rotary sealing joint for vacuumizing and igniting the quartz liner tube; A resistance furnace provided below the quartz liner tube for heating the quartz liner tube.

7. The apparatus of claim 6, wherein, Further comprising a resistance furnace, wherein the resistance furnace comprises a heating groove, and the quartz liner tube is placed in the heating groove and spaced apart from the inner wall of the heating groove by a certain distance.

8. The apparatus of claim 6, wherein, Further comprising: An improved self-tuning PID controller for adjusting the input power of the atmospheric pressure microwave, so that the actual temperature of the quartz liner tube reaches the preset deposition temperature, and / or the real-time deposition wall thickness of the quartz liner tube reaches the preset deposition wall thickness.

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