Preparation method and device of optical fiber preform, preform and optical fiber

Abstract

The invention provides a preparation method and device of an optical fiber preform, the preform and an optical fiber. The device comprises a quartz accessory, a first heating furnace, a second heating furnace and a clamp, the first heating furnace can move relative to the quartz accessory so as to heat a sleeve of the quartz accessory, and the clamps are arranged at the upper end and the lower end of the quartz accessory; the quartz accessory comprises a core rod, a core rod tail handle connected with the core rod, a sleeve formed by a quartz tube containing a negative dopant and the core rod, and a sleeve tail handle connected with the sleeve; a medicament chamber is arranged in the core rod tail handle; an air outlet hole is formed in the medicament chamber; the second heating furnace is used for heating the medicament chamber. By means of the device, the doping and melting shrinkage process of alkali metal can be achieved at a time, the preparation efficiency is improved, an oxyhydrogen flame heating process is replaced with a melting process, and the core rod utilization rate is increased.

Description

Technical Field

[0001] This application relates to the field of optical fiber fabrication, and in particular to a method, apparatus, preform, and optical fiber for fabricating an optical fiber preform. Background Technology

[0002] In traditional single-mode fiber design, to achieve optical signal transmission within the core layer, positive dopants such as Ge and Al are added. These dopants create a relatively positive refractive index relative to pure silica glass while reducing core layer viscosity. However, Ge and Al increase Rayleigh scattering, so further reducing fiber transmission loss requires reducing or eliminating these dopants. A known alternative is to prepare a pure silica core layer combined with a fluorine-containing silica cladding to create a refractive index difference, while adding alkali metal elements such as K, Cs, and Na to the core layer. With the core containing these alkali metal elements, the core layer viscosity can be significantly reduced during preform stretching, resulting in a relaxed silica network structure and ultimately achieving the desired transmission loss reduction.

[0003] Patent JP2012271333 describes a method for preparing a core layer. A raw material tank is formed by bubbling a section of a tail tube, and alkali metal compound powder is placed in the tank. The raw material tank is heated to 860°C, and oxygen is introduced from the tail tube as a carrier gas. Part of the oxygen reacts with decomposed potassium bromide to generate potassium oxide, which is deposited on the surface of a glass tube. The outside of the glass tube is heated to 2000°C using a hydrogen-oxygen burner at a constant speed, causing the potassium oxide to distribute evenly and diffuse to the inner surface of the glass tube. After the diffusion process is complete, the negative pressure inside the tube is reduced, and the glass tube is heated again using the hydrogen-oxygen burner until the inner diameter is reduced to 4 mm. Then, hexafluoroethane is introduced to perform vapor-phase etching on the inner wall. After etching, the glass tube is heated to 1400°C using the hydrogen-oxygen burner, and the negative pressure is reduced to 100 kPa to solidify, resulting in an alkali metal-containing glass rod with an outer diameter of approximately 25 mm. The glass rod needs to be ground thoroughly on the outside by at least 30% to remove hydroxyl groups. Then, the ground core rod is embedded in another glass tube to form the final core layer. Finally, a fluorine-containing quartz tube is bonded to the outside of the core layer as the first cladding and the second cladding, which is then used as the preform for glass fiber optical fiber.

[0004] Patent WO2023096799 describes an alkali metal doping method using a tail tube with a ring-shaped memory. The method involves heating and pressing two points within the tail tube to form two annular necks, which serve as storage sites for the doping agent. The alkali metal compound used is one of potassium bromide or potassium iodide. The memory is heated, causing the alkali metal halide to evaporate and flow through a glass tube. Heating the glass tube allows the metal oxide or alkali metal to diffuse to its inner surface, reaching a depth of approximately 100 μm to 500 μm. After diffusion, a negative pressure is increased within the glass tube to cause partial collapse. The inner surface of the tube can be etched using a hydrofluoric acid aqueous solution to an etching depth of approximately 5% of the alkali metal diffusion depth. After etching, the glass tube is heated to form a solid glass rod. This glass rod requires separate etching of its outer surface to remove hydroxyl groups introduced by the external heat source during tube shrinkage. Inner and outer cladding layers are then deposited on this core rod to ultimately form a preform of the glass fiber optical fiber.

[0005] The alkali metal doping process involves three stages: doping, shrinkage, and solidification. The overall doping time is too long, and most of the preparation processes use an oxyhydrogen flame as a heat source, which introduces a large number of hydroxyl groups, resulting in excessive wear during subsequent mandrel polishing. Summary of the Invention

[0006] This application provides a method, apparatus, preform, and optical fiber for preparing optical fiber preforms to improve alkali metal doping.

[0007] In a first aspect, this application provides an apparatus for preparing optical fiber preforms, the apparatus comprising: a quartz accessory, a first heating furnace, a second heating furnace, and a fixture;

[0008] The quartz accessory includes a core rod, a core rod tail shank connecting the core rod, a sleeve consisting of a quartz tube containing a negative dopant and the core rod, and a sleeve tail shank connecting the sleeve.

[0009] The mandrel tail contains a medicine chamber, and the medicine chamber is provided with an air outlet;

[0010] The first heating furnace is configured to be movable relative to the quartz accessory in order to heat the sleeve in the quartz accessory, and the clamps are disposed at the upper and lower ends of the quartz accessory.

[0011] The second heating furnace is used to heat the pharmaceutical chamber.

[0012] Secondly, this application provides a method for preparing an optical fiber preform, applied to the preparation apparatus described in the first aspect, the preparation method comprising:

[0013] A mandrel containing a positive dopant is joined to a mandrel shank, and the mandrel is inserted into a quartz tube containing a negative dopant to form a sleeve. A dopant, comprising an alkali metal element, is then filled into the reagent chamber. The refractive index difference between the mandrel and the quartz tube containing the negative dopant is greater than 0.25%.

[0014] The second heating furnace and the first heating furnace are turned on; the heating temperature of the second heating furnace is lower than the melting point of the dopant, and the temperature of the first heating furnace is 900℃~2200℃; the first heating furnace moves back and forth to heat the sleeve;

[0015] After the first preset time, the second heating furnace is turned off, and gas with etching function is pumped in through the tail shank of the sleeve for etching treatment.

[0016] After the second preset time, the gas pumping is stopped, the quartz accessories are evacuated, and the preform is obtained by stretching through a fixture.

[0017] The preform precursor is clad with an outer layer to obtain an optical fiber preform.

[0018] Further, the step of cladding the preform precursor to obtain the optical fiber preform includes:

[0019] The preform precursor is deposited with silicon dioxide by vapor deposition to form a powder.

[0020] The powder is heated in an atmosphere containing a negative dopant at a temperature of 500℃~1200℃ for 120min~600min, and then sintered at 1300℃~1800℃ after vacuuming to obtain an optical fiber preform.

[0021] And / or, the preform precursor is matched with a sleeve containing a negative dopant to obtain an optical fiber preform.

[0022] Furthermore, the metal element of the dopant includes one or more of potassium, calcium, cesium, or boron;

[0023] And / or, the positive refractive index of the mandrel is -0.05% to 0.15%;

[0024] And / or, the negative refractive index of the quartz tube containing the negative dopant is -0.45% to -0.2%;

[0025] And / or, before starting the second heating furnace and the first heating furnace, the preparation method further includes: acid washing and drying the sleeve.

[0026] Furthermore, the method for preparing a core containing a positive dopant includes:

[0027] Silicon dioxide was deposited on a quartz target rod by vapor deposition to obtain a powder core rod; the deposition process used oxygen as a carrier gas to deposit the positive dopant into the powder core rod.

[0028] The powder core rod is subjected to dehydroxylation and dechlorination treatment, and then densified to obtain a quartz rod;

[0029] The quartz rod is extended and polished to obtain the core rod containing the positive dopant.

[0030] Thirdly, this application provides a method for preparing an optical fiber preform, applied to the preparation apparatus described in the first aspect, the preparation method comprising:

[0031] A mandrel containing a positive dopant is joined to a mandrel shank, and the mandrel is inserted into a quartz tube containing a negative dopant to assemble a sleeve. The refractive index of the quartz tube containing the negative dopant is -0.2% to -0.40%, and the reagent chamber is free of dopant. The refractive index difference between the mandrel and the quartz tube containing the negative dopant is greater than 0.25%.

[0032] The first heating furnace is turned on, and the temperature of the first heating furnace is 900℃~2200℃; the first heating furnace moves back and forth to heat the sleeve;

[0033] The gas with etching function is pumped into the sleeve tail shank for etching treatment. After etching treatment, the quartz accessories are evacuated and stretched by the fixture to obtain the composite sleeve.

[0034] The composite sleeve is processed to form an internally hollow ring structure.

[0035] The hollow composite sleeve is connected to the sleeve tail shank, and a dopant is filled into the reagent chamber, the dopant including an alkali metal element.

[0036] The second heating furnace and the first heating furnace are turned on. The heating temperature of the second heating furnace is lower than the melting point of the dopant, and the temperature of the first heating furnace is 900℃~2200℃. The first heating furnace moves back and forth along the movable track to heat the hollow composite sleeve.

[0037] After the first preset time, the second heating furnace is turned off, and gas with etching function is pumped in through the tail shank of the sleeve for etching treatment.

[0038] After the second preset time, the gas pumping is stopped, the quartz accessories are evacuated, and the preform is obtained by stretching through a fixture.

[0039] The preform precursor is clad with an outer layer to obtain an optical fiber preform.

[0040] Further, the step of cladding the preform precursor to obtain the optical fiber preform includes:

[0041] The preform precursor is deposited with silicon dioxide by vapor deposition to form a powder.

[0042] The powder is heated in an atmosphere containing a negative dopant gas at a temperature of 500℃~1200℃ for 120min~600min, and then sintered at 1300℃~1800℃ after vacuuming to obtain an optical fiber preform.

[0043] And / or, the preform precursor is matched with a sleeve containing a negative dopant to obtain an optical fiber preform.

[0044] Furthermore, the metal element of the dopant includes one or more of potassium, calcium, and cesium;

[0045] And / or, the positive refractive index of the mandrel is -0.05% to 0.15%;

[0046] And / or, the negative refractive index of the quartz tube containing the negative dopant is -0.45% to -0.2%;

[0047] And / or, before starting the second heating furnace and the first heating furnace, the preparation method further includes: pickling and drying the sleeve;

[0048] And / or, the preparation method further includes:

[0049] Silicon dioxide was deposited on a quartz target rod by vapor deposition to obtain a powder core rod; the deposition process used oxygen as a carrier gas to deposit the positive dopant into the powder core rod.

[0050] The powder core rod is subjected to dehydroxylation and dechlorination treatment, and then densified to obtain a quartz rod;

[0051] The quartz rod is extended and polished to obtain the core rod containing the positive dopant.

[0052] Fourthly, this application provides an optical fiber preform, which is prepared by the preparation method described in any one of the second aspects, or by the preparation method described in any one of the third aspects.

[0053] Fifthly, this application provides an optical fiber, which is prepared from the optical fiber preform described in the fourth aspect above.

[0054] This application provides a method, apparatus, preform, and optical fiber for fabricating an optical fiber preform. The apparatus includes a quartz auxiliary component, a first heating furnace, a second heating furnace, and a fixture. The quartz auxiliary component includes a core rod, a core rod shank connecting the core rod, a sleeve consisting of a quartz tube containing a negative dopant and the core rod, and a sleeve shank connecting the sleeve. The core rod shank contains a reagent chamber with an vent. The first heating furnace is movable relative to the quartz auxiliary component to heat the sleeve within it. The fixture is located at both ends of the quartz auxiliary component. The second heating furnace is used to heat the reagent chamber. This apparatus allows for the simultaneous doping and melting of alkali metals, improving fabrication efficiency. The use of a melting process instead of an oxyhydrogen flame heating process increases the core rod utilization rate. Attached Figure Description

[0055] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0056] Figure 1 A schematic diagram of a device for preparing an optical fiber preform provided in this application;

[0057] Figure 2 A structural schematic diagram of a quartz accessory provided in this application;

[0058] Figure 3 This application provides a schematic flowchart of a method for preparing an optical fiber preform.

[0059] Figure 4 This application provides a schematic flowchart of a method for preparing an optical fiber preform.

[0060] Figure 5 A schematic diagram showing the connection between the hollow composite sleeve and the sleeve tail shank is provided for this application;

[0061] Figure 6 This is a schematic diagram of the structure of the optical fiber preform provided in this application;

[0062] Figure 7 A schematic diagram showing the refractive index of each part of the optical fiber preform provided in this application.

[0063] Explanation of reference numerals in the attached figures:

[0064] 1-Casing tail shank; 2-Mandrel; 3-Mandrel tail shank; 4-Reagent chamber; 5-Clamp; 6-First heating furnace;

[0065] 7-Movable track; 8-Second heating furnace; 9-Quartz tube containing negative dopant.

[0066] The accompanying drawings illustrate specific embodiments of this application, which will be described in more detail below. These drawings and descriptions are not intended to limit the scope of the concept in any way, but rather to illustrate the concept of this application to those skilled in the art through reference to particular embodiments. Detailed Implementation

[0067] Exemplary embodiments will now be described in detail, examples of which are illustrated in the accompanying drawings. When the following description relates to the drawings, unless otherwise indicated, the same numbers in different drawings denote the same or similar elements. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with this application. Rather, they are merely examples of apparatuses and methods consistent with some aspects of this application as detailed in the appended claims.

[0068] First, let me explain the terms used in this application:

[0069] Optical fiber preform: This refers to the core intermediate product in the manufacture of optical fibers. It is a cylindrical glass rod with the same refractive index distribution as the target optical fiber. Optical fiber preforms typically consist of a high-refractive-index core and a low-refractive-index cladding. The preform is then subjected to a high-temperature drawing process, heated to over 2000°C, and stretched on a drawing tower into optical fibers with diameters down to the micrometer level.

[0070] External vapor deposition (OVD) is a process that involves mixing raw materials such as SiCl4, GeCl4, and POC3 with oxygen, and then subjecting the mixture to a hydrolysis reaction in a high-temperature oxyhydrogen flame to generate fine powder particles such as SiO2 and GeO2. These particles are then deposited onto the outer surface of a rotating substrate to form a porous body. Subsequently, through steps such as rod removal, sintering, and densification, the porous body is transformed into a transparent quartz glass preform.

[0071] Unlike the lateral deposition in OVD (Optical Vapor Deposition), the axial vapor deposition (VAD) process involves gradual growth along the axial direction of a preform. Reactive gases (such as SiCl4, GeCl4, and O2) hydrolyze in a flame to generate oxide particles, which are deposited at the top of the preform. As the seed preform slowly rises, a porous structure extending axially is formed. Different torches can be used to deposit the core layer and cladding layer separately, achieving precise refractive index control. Subsequently, through dehydroxylation, dechlorination, and high-temperature densification, a high-purity, low-OH-content transparent quartz glass rod is obtained.

[0072] In the fabrication of traditional glass fiber optical preforms, to achieve efficient transmission of optical signals in the core layer, a refractive index difference is formed by combining a pure quartz core layer with a fluorine-containing quartz cladding, replacing traditional doping elements. However, alkali metals (such as K, Na, and Cs) need to be introduced into the core layer to reduce its viscosity and facilitate the stretching and shaping of the preform. Alkali metal doping typically involves three stages: the doping stage, in which alkali metal compounds are evaporated at high temperatures and diffused to the inner surface of the quartz tube; the shrinkage stage, in which the inner diameter of the quartz tube is reduced and solidified using an oxyhydrogen flame or induction heating; and the post-processing stage, in which additional grinding is required to remove hydroxyl contamination and to connect with subsequent processes such as cladding deposition.

[0073] Doping, melting, and grinding are steps that need to be carried out in separate steps, resulting in a long production cycle and easy fluctuations in parameter transfer between each step. In addition, heating with an oxyhydrogen flame introduces a large number of hydroxyl groups, requiring grinding to remove more than 30% of the core rod material, which significantly reduces the core rod utilization rate (usually below 90%). After melting, the core rod diameter is small, making it difficult to meet the processing requirements of long-sized optical fiber preforms.

[0074] In view of this, this application integrates the alkali metal doping, melting and stretching processes into a continuous process through integrated process design, and combines it with the composite sleeve structure design to achieve synergistic optimization of doping uniformity, hydroxyl contamination control and production efficiency in the preparation of optical fiber preforms.

[0075] The technical solution of this application and how the technical solution of this application solves the above-mentioned technical problems are described in detail below with specific embodiments. These specific embodiments can be combined with each other, and the same or similar concepts or processes may not be described again in some embodiments. The embodiments of this application will now be described with reference to the accompanying drawings.

[0076] Figure 1 This application provides a schematic diagram of the structure of an optical fiber preform fabrication apparatus, as shown below. Figure 1 As shown, the device includes: a quartz accessory, a first heating furnace 6, a second heating furnace 8, and a clamp 5.

[0077] The clamps 5 are set at the upper and lower ends of the quartz accessory.

[0078] The first heating furnace 6 is configured to be movable relative to the quartz accessory in order to heat the sleeve in the quartz accessory.

[0079] In some embodiments, the first heating furnace 6 is fixed on the movable track 7, and the sleeve is reciprocated for heating as the movable track 7 moves up and down.

[0080] The structure of quartz accessories is as follows Figure 2 As shown, Figure 2 A structural schematic diagram of a quartz accessory provided in this application is shown below. Figure 2As shown, the quartz accessories include a core rod 2, a core rod tail 3 connecting the core rod, a sleeve composed of a quartz tube 9 containing a negative dopant and the core rod 2, and a sleeve tail 1 connecting the sleeve.

[0081] The core rod tail stalk 3 contains a reagent chamber 4, which is provided with an vent hole to facilitate the evaporation of the dopant after heating;

[0082] The second heating furnace 8 is used to heat the medicine chamber 4.

[0083] exist Figure 2 In the diagram, the densely dotted lines in the center of the sleeve represent the core rod, while the sparsely dotted lines around it represent the quartz tube containing negative dopants.

[0084] Understandably, external gas can flow through the sleeve tailstock into the gap between the quartz tube containing the negative dopant and the mandrel. A sealing device can be connected to the outside of the sleeve tailstock to seal the inside of the quartz accessory.

[0085] The above-mentioned device can realize the doping, melting and stretching steps of the preform in a single device, avoiding parameter fluctuations caused by multiple transfers of the mandrel in the step-by-step process; the synergistic heating of two furnaces combined with fluorine gas etching avoids the use of oxyhydrogen flame, reduces the introduction of hydroxyl groups, reduces grinding, and improves the utilization rate of the mandrel.

[0086] The second heating furnace can be a resistance furnace with a temperature control range of 400℃~1200℃.

[0087] The first heating furnace can be a graphite furnace with a temperature control range of 900℃~2200℃.

[0088] The various devices, in conjunction with quartz accessories, enable element doping of quartz tubes in a vertical state. The energy driving the element doping comes from a graphite furnace. After doping, direct melting and stretching can be achieved, avoiding losses and contamination caused by subsequent transfers.

[0089] The following describes the usage of the above-mentioned device using a specific preparation process.

[0090] Figure 3 This application provides a schematic flowchart of a method for fabricating an optical fiber preform, as shown below. Figure 3 As shown, the preparation method includes the following steps:

[0091] S1. Connect the mandrel containing positive dopant to the mandrel tail shank, assemble it into a sleeve with a quartz tube containing negative dopant, and fill the reagent chamber with dopant, which includes alkali metal elements. The refractive index difference between the mandrel and the quartz tube containing negative dopant is greater than 0.25%.

[0092] Positive dopants can effectively adjust the refractive index of optical fibers to meet the needs of different fibers. This application does not limit the specific doping amount or method. A quartz tube containing negative dopants serves as the inner cladding for the subsequent core rod. A refractive index difference greater than 0.25% between the two ensures sufficient refractive index contrast between the core rod and the cladding, thereby achieving complete light confinement, preventing light leakage, and improving the transmission efficiency of the optical fiber.

[0093] S2. Turn on the second heating furnace and the first heating furnace; the heating temperature of the second heating furnace is lower than the melting point of the dopant, and the temperature of the first heating furnace is 900℃~2200℃; the first heating furnace moves back and forth to heat the sleeve.

[0094] In this step, the heating temperature of the second heating furnace is controlled below the melting point of the dopant to ensure that the dopant does not evaporate too quickly due to excessively high temperature during heating, thus preventing crystallization and ensuring the stability of the dopant. Specifically, the heating temperature of the second heating furnace is 10°C, 20°C, or 30°C below the melting point of the dopant, or other temperature values.

[0095] The temperature range of the first heating furnace is 900℃~2200℃. The heating temperature is high enough to fully melt the silicon glass, and the reciprocating track ensures uniform heating of the sleeve, avoiding local overheating or uneven temperature.

[0096] S3. After the first preset time, the second heating furnace is turned off, and gas with etching function is pumped in through the sleeve tail shank for etching treatment.

[0097] The first preset time can be 5 minutes, 10 minutes, 20 minutes, or other times; this application does not impose specific limitations. After the first preset time, the second heating furnace is turned off, and the dopant no longer volatilizes. The doped optical fiber is then subjected to gas etching to remove any possible surface impurities and bubbles. The etching gas can remove minute defects and uneven areas on the surface of the quartz tube, ensuring more uniform optical performance inside and outside the optical fiber.

[0098] In one specific implementation, the gas with etching function is hexafluoroethane or sulfur hexafluoride.

[0099] In one specific implementation, the etching depth is no less than 200 μm.

[0100] S4. After the second preset time, stop the gas pumping, evacuate the quartz accessories, and stretch them through the fixture to obtain the preform rod precursor.

[0101] The second preset time can be 5 minutes, 20 minutes, 50 minutes, or other times. Stop the gas pumping and begin evacuating a vacuum through the top of the sleeve tail handle, while the first heating furnace continues reciprocating heating. Understandably, it is necessary to seal both ends of the sleeve tail handle during vacuuming.

[0102] In one specific implementation, a vacuum is drawn to achieve a negative pressure of over 0.09 MPa, the lower clamp is stretched downwards at a uniform speed, while the upper clamp remains stationary, thus completing the melting and stretching process.

[0103] Vacuuming can further remove moisture, impurities, and air bubbles from the gas, thus avoiding affecting the mechanical strength and optical performance of the optical fiber.

[0104] The prepared preform includes a core and an inner cladding. The purpose of the inner cladding is to serve as a non-light-guiding protective layer to prevent hydroxyl corrosion by the oxyhydrogen flame during the subsequent preparation of the outer cladding. Using a composite sleeve can reduce the preparation of the core and inner cladding to a single step, thus reducing the overall turnaround time.

[0105] S5. The preform precursor is clad with an outer layer to obtain the optical fiber preform.

[0106] The cladding effectively protects the core of the optical fiber, preventing damage from external environmental factors such as moisture and contaminants. The cladding also increases the fiber's mechanical strength, enabling it to withstand external pressure and tension, thus ensuring its smooth operation during subsequent fiber drawing and use.

[0107] There are various methods for cladding, such as OVD (Optical Vapor Deposition), chemical vapor deposition, plasma chemical deposition, tubular coating, and melt coating. Any of these methods can be chosen.

[0108] In one specific implementation, the preform precursor is deposited with silicon dioxide by external vapor deposition (OVD) to form a powder. The powder is then heated in a fluorine-containing atmosphere at a temperature of approximately 500°C to 1200°C for 120 min to 600 min. After vacuuming, the temperature is raised to 1300°C to 1800°C for sintering to obtain the optical fiber preform.

[0109] This application provides a method for preparing optical fiber preforms. During the preparation of the inner cladding, a heating furnace is used with a melting and shrinking process, replacing the traditional vapor deposition process which uses an oxyhydrogen flame as a heat source. This avoids the excessive use of hydroxyl groups and reduces core rod polishing losses. The integrated process also reduces production steps, shortens the production cycle, and improves production efficiency.

[0110] The diameter of the preform precursor prepared by this method can reach 35mm~40mm, which exceeds the diameter of the preform precursor prepared by the traditional in-tube vapor deposition process (less than 20mm).

[0111] In some embodiments, the dopant metal element includes one or more of potassium, calcium, and cesium.

[0112] In some embodiments, the positive refractive index of the mandrel is -0.05% to 0.15%. For example, -0.05%, 0%, 0.05%, 0.10%, 0.15%, or any combination thereof.

[0113] In some embodiments, the negative refractive index of the quartz tube containing the negative dopant is -0.45% to -0.2%. For example, -0.45%, -0.35%, -0.30%, -0.25%, -0.2%, or any combination thereof.

[0114] In some embodiments, the quartz tube containing the negative dopant is an F-containing quartz tube.

[0115] In some embodiments, before turning on the second and first heating furnaces, the preparation method further includes: acid washing and drying the sleeve. Acid washing can be performed using a mixed solution of hydrofluoric acid and nitric acid. Drying can be carried out by turning on the first heating furnace, or by pumping in oxygen and helium to dry the component.

[0116] In some embodiments, the core rod containing a positive dopant comprises: preparing a powder core rod by vapor deposition, using oxygen as the carrier gas in the deposition process, and depositing the positive dopant into the powder core rod; subjecting the powder core rod to dehydroxylation and dechlorination treatment, followed by densification to obtain a quartz rod; and extending and polishing the quartz rod to obtain the core rod containing the positive dopant. In this embodiment, the vapor deposition method uses two or three longitudinally arranged torches. The first torch is responsible for core deposition, controlling the positive refractive index of the core rod; the second or third torch is responsible for depositing a protective layer on the core, the purpose of which is to isolate the metal impurities or hydroxyl groups from erosion during subsequent processing. The quartz rod is extended and polished to remove the protective layer deposited by the second or third torch.

[0117] The above method embodiments describe one way of using the above device. The following is an embodiment describing another way of using the above device.

[0118] Figure 4 This application provides a schematic flowchart of a method for fabricating an optical fiber preform, as shown below. Figure 4 As shown, the preparation method includes the following steps:

[0119] D1. Connect the mandrel containing positive dopant to the mandrel tail and assemble it with the quartz tube containing negative dopant to form a sleeve. The negative refractive index of the quartz tube containing negative dopant is -0.2% to -0.4%, and the reagent chamber does not contain dopant. The refractive index difference between the mandrel and the quartz tube containing negative dopant is greater than 0.25%.

[0120] The difference from step S1 is that no dopant is added to the reagent chamber in this step.

[0121] D2. Turn on the first heating furnace. The temperature of the first heating furnace is 900℃~2200℃. The first heating furnace moves back and forth to heat the sleeve.

[0122] D3. Etching is performed by pumping an etching gas into the sleeve tail shank. After etching, the quartz accessories are evacuated and stretched using a fixture to obtain the composite sleeve.

[0123] In steps D2 and D3 above, the sleeves are fused together by heating in the first heating furnace and gas etching, and then a seamless composite sleeve is obtained by stretching and fusion locking.

[0124] D4. Process the composite sleeve to make it into a hollow ring structure.

[0125] In this step, the inner diameter of the finished hollow composite sleeve is smaller than the diameter of the mandrel.

[0126] In one implementation, the inner diameter of the finished hollow composite sleeve is less than 1 / 5 of the outer diameter.

[0127] D5. Connect the hollow composite sleeve to the sleeve tail shank, and fill the reagent chamber with dopant, which includes alkali metal elements.

[0128] The method of connecting the hollow composite sleeve to the sleeve tail shank is as follows: Figure 5 As shown, Figure 5 A schematic diagram of the connection between the hollow composite sleeve and the sleeve tail shank is provided for this application. Figure 5 The sleeve is hollow in the center. The dense dashed lines in the annular part represent the remaining core rod part, and the sparse dashed lines represent the quartz tube part containing negative dopants.

[0129] D6. Turn on the second heating furnace and the first heating furnace. The heating temperature of the second heating furnace is lower than the melting point of the dopant, and the temperature of the first heating furnace is 900℃~2200℃. The first heating furnace moves back and forth to heat the hollow composite sleeve.

[0130] The selection of dopants, etching gases, and subsequent settings of the first and second preset times in this scheme are the same as those in Method 1, and will not be repeated here.

[0131] D7. After the first preset time, turn off the second heating furnace and pump in gas with etching function through the sleeve tail shank for etching treatment.

[0132] D8. After the second preset time, stop the gas pumping, evacuate the quartz accessories, and stretch them through the fixture to obtain the preform rod precursor.

[0133] D9. The preform precursor is clad with an outer layer to obtain the optical fiber preform.

[0134] This step is the same as S5, and will not be repeated here.

[0135] The difference between Example 2 of this method and Example 1 of the aforementioned method lies in the preparation of the preform precursor. First, the core rod and a quartz tube containing a negative dopant are assembled and then melt-stretched. After hollowing, the dopant is introduced through the central aperture, and finally, a dense preform precursor is formed through melt stretching. This process is also an integrated preparation, realizing the integration of doping and melting processes, and improving the utilization rate of the core rod.

[0136] The selection of dopants, the refractive index requirements of the core rod, the refractive index requirements of the quartz tube containing negative dopants, the acid washing and drying methods, and the preparation methods of the core rod containing positive dopants are all similar to those in Method Example 1.

[0137] This application also provides an optical fiber preform, which is prepared by any of the above-described preparation methods.

[0138] Figure 6 This is a schematic diagram of the structure of the optical fiber preform provided in this application, as shown below. Figure 6 As shown, it includes a core, an inner cladding layer, and an outer cladding layer. The core represents the core rod portion, the inner cladding layer is the quartz tube portion containing negative dopant, and the outer cladding layer is the portion formed by cladding the preformed rod precursor.

[0139] Figure 7 A schematic diagram showing the refractive index of each part of the optical fiber preform provided in this application, as shown below. Figure 7 As shown, the refractive index of the fiber core is -0.05% to 0.15%, which is greater than that of the inner cladding. The negative refractive index of the inner cladding is -0.45% to -0.2%, and the refractive index of the outer cladding is -0.1% to -0.2%.

[0140] This application also provides an optical fiber, which is an optical fiber glass fiber prepared from the above-mentioned optical fiber preform.

[0141] The following is a specific example to verify the data. Specific Implementation Example 1

[0143] S1: Powder is deposited on a quartz target rod (40mm) using the VAD process. The deposition process involves two torches. The first torch is responsible for the core deposition, using oxygen as the carrier gas to dope Ge into the powder. This step requires control of the Ge carrier gas flow rate, aiming to control the positive refractive index of the core rod within 0.05%~0.1%. The second torch is responsible for the deposition of the core protective layer (10mm thick), which aims to isolate the powder from the corrosion of metal impurities or hydroxyl groups during subsequent processing.

[0144] S2: The powder is placed in a sintering furnace and chlorine gas is introduced successively for dehydroxylation and oxygen gas is introduced for dechlorination. Then it is densified to form a transparent quartz rod. The quartz rod produced in this step is required to have a chlorine content of less than 10 ppm and a hydroxyl content of less than 20 ppb.

[0145] S3: Extend and polish the quartz rod sequentially. The target diameter after polishing is about 17mm. The polishing amount is required to be greater than 2mm. The surface roughness after polishing is less than 0.02μm.

[0146] S4: Connect the mandrel to the mandrel tailstock with the reagent chamber, and then assemble it with the fluorine-containing quartz tube (20mm thick). The negative refractive index of the fluorine-containing quartz tube should be between -0.2% and -0.25%. After assembly, acid washing (hydrofluoric acid and nitric acid in a 1:1 ratio) and drying are required. After drying, fill the reagent chamber of the tailstock with potassium bromide, and then insert the mandrel loaded with dopant into the sleeve. Connect the two sides of the tailstock to the sealing device, and then fix it vertically to the equipment fixture.

[0147] S5: Open the first heating furnace and bring it to 700°C (do not exceed the softening point of the current quartz tube). Move the furnace body up and down twice, while simultaneously pumping oxygen and helium into the tube to dry the components. Then open the second heating furnace to liquefy the reagent for 5 minutes, maintaining the previous gas supply. The purpose of this step is to remove impurities from the reagent. Lower the temperature of the second heating furnace to 10°C below the melting point of the dopant, while simultaneously raising the temperature of the main heating furnace to 1400°C. Move the heating element up and down at a speed of 50 mm / min for a total of 15 rounds, then turn off the heating furnace. Next, add hexafluoroethane to the pumped gas, ensuring an etching depth of at least 200 μm. Stop the gas pumping and start drawing a vacuum from above to make the negative pressure reach above 0.09 MPa. At the same time, move the first heating furnace from bottom to top at a speed of 5 mm / min. The lower clamp is stretched downward at a uniform speed, while the upper clamp remains stationary. This completes the melting and stretching process. The diameter of the stretched mandrel is about 40 mm, and the inner cladding diameter is about 2 times the core diameter.

[0148] S6: The fused and stretched quartz rod is subjected to OVD deposition to form a powder. Specifically, the quartz rod is placed horizontally, and three torches perpendicular to the mandrel are used to reciprocate left and right to deposit a uniform and loose layer of silica powder on the mandrel. During this process, hydrogen, oxygen, and silicon tetrachloride vapor are introduced into the torches. The silicon tetrachloride undergoes a hydrolysis reaction through combustion in an oxyhydrogen flame to generate silica. The diameter of the sintered powder is approximately 3.5 times that of the quartz rod.

[0149] S7: The powder is heated in a hexafluoroethane atmosphere at 850°C for about 200 minutes, and then sintered at 1400°C after vacuuming.

[0150] Comparative Example 1

[0151] S1: Powder is deposited on a quartz target rod (40mm) using the VAD process. The deposition process involves two torches. The first torch is responsible for the core deposition, using oxygen as the carrier gas to dope Ge into the powder. This step requires control of the Ge carrier gas flow rate, aiming to control the positive refractive index of the core rod within 0.05%~0.1%. The second torch is responsible for the deposition of the core protective layer (15mm thick), which aims to isolate the powder from the corrosion of metal impurities or hydroxyl groups during subsequent processing.

[0152] S2: The powder is placed in a sintering furnace and chlorine gas is introduced successively for dehydroxylation and oxygen gas is introduced for dechlorination. Then it is densified to form a transparent quartz rod. The quartz rod produced in this step is required to have a chlorine content of less than 10 ppm, a hydroxyl content of less than 20 ppb, and a diameter of 50 mm.

[0153] S3: Extend and polish the quartz rod sequentially. The target diameter after polishing is 40mm. The polishing amount should be greater than 2mm, and the surface roughness after polishing should be less than 0.02μm.

[0154] S4: Deposit the mandrel using the OVD process to make it into a powder body for sintering. The diameter of the sintered body should be about 60mm.

[0155] S5: The above powder rods are vitrified and sintered. Before sintering, the powder rods are immersed in carbon tetrafluoride gas for 2 hours. The test refractive index of the powder layer is required to reach -0.25% to -0.30%.

[0156] S6: Stretch the quartz rod to a diameter of 60 mm, and then perform OVD deposition again (the same deposition process as step S6 in Example 1) to make a powder for sintering. The diameter of the sintered powder is required to be about 2.5 times that of the quartz rod.

[0157] S7: The above powder rods are vitrified and sintered. Before sintering, the powder rods are immersed in F-containing gas for 2 hours. The test refractive index of the powder layer is required to reach -0.15% to -0.10%.

[0158] Test Example 1

[0159] The fiber preforms from the above embodiments and comparative examples were drawn into optical fibers (125 μm in diameter). Their structures were identical to the fiber preforms, consisting of a core, inner cladding, and outer cladding, and their refractive indices were the same as the preforms. The attenuation coefficients of the optical fibers at wavelengths of 1550 nm and 1383 nm were measured, and the mode field diameter was tested using the far-field scanning method. The test results are detailed in Table 1.

[0160] Table 1

[0161]

[0162] Compared to Comparative Example 1, Example 1 shortened the production cycle by one day. However, Example 1 replaced the OVD process of Comparative Example S4 with a melt-shrink process, reducing the attenuation coefficient (i.e., water peak test value) at 1383nm to below 0.54dB / km. Furthermore, the introduction of alkali metal dopants during the melt-shrink process effectively reduced the core layer viscosity, lowering the attenuation coefficient at 1550nm to below 0.165dB / km. Specific Implementation Example 2:

[0164] S1: Powder is deposited on a quartz rod with a diameter of 30 mm using the OVD process, and positive dopant Ge is doped into the powder using oxygen as a carrier gas.

[0165] S2: The powder is placed in a sintering furnace and chlorine gas is introduced successively for dehydroxylation, followed by oxygen introduction for dechlorination. Then, it is densified to form a transparent quartz rod. The quartz rod deposited in this step must have a chlorine content of less than 10 ppm and a hydroxyl content of less than 20 ppb. The ratio of the outer cladding layer to the initial quartz rod is approximately 3:1.

[0166] S3: Extend the quartz rod to 30mm and then polish it to a target outer diameter of 24mm. The surface roughness after polishing should be less than 0.1μm.

[0167] S4: Assemble the polished quartz rod with an F-containing sleeve having a relative refractive index difference of 0.35%, without adding any dopants. Turn on the first heating furnace and raise it to 500°C. Move the furnace body up and down twice, simultaneously pumping oxygen and helium into the tube to dry the component. Then, raise the temperature of furnace 1 to 1400°C and move the heating element 1 up and down at a speed of 40 mm / min downwards and 200 mm / min upwards. Simultaneously, add hexafluoroethane to the pumped gas for etching, requiring an etching depth of at least 200 μm. Stop the gas pumping and begin evacuating from above, raising the negative pressure to above 0.09 MPa. Simultaneously, move the first heating furnace upwards at a speed of 5 mm / min, with the lower clamp pulling downwards at a uniform speed while the upper clamp remains stationary, completing the melting and stretching process. The diameter of the stretched mandrel is approximately 60 mm.

[0168] S5: Perform sleeve processing on the above-mentioned fused mandrel, with a target outer diameter of 60mm and a target inner diameter of 10mm.

[0169] S6: Connect the two sides of the above-mentioned casing to the casing tail shank, and then place the bubbling shank containing potassium bromide at the interface between the upper tail shank and the casing. Connect the two sides of the casing tail shank to the sealing device, and then fix it vertically to the equipment clamp.

[0170] S7: Open the first heating furnace and bring it to 600°C. Move the furnace body up and down three times, while simultaneously pumping oxygen and helium into the tubing to dry the components. Then open the second heating furnace to liquefy the reagent for 5 minutes, maintaining the previous gas supply. The purpose of this step is to remove impurities from the reagent. Lower the temperature of the second heating furnace to 10°C below the melting point of potassium bromide, while simultaneously raising the temperature of the first heating furnace to 1500°C. Move the furnace up and down repeatedly, with a downward movement speed of 40 mm / min and an upward movement speed of 200 mm / min, for a total of 15 rounds. Then close the second heating furnace. Next, add hexafluoroethane to the pumped gas for etching, requiring an etching depth of at least 200 μm. Stop the gas pumping and begin evacuating from above, bringing the negative pressure to above 0.09 MPa. Simultaneously, move the first heating furnace from bottom to top at a speed of 5 mm / min, with the lower clamp pulling downwards at a uniform speed while the upper clamp remains stationary. This completes the melting and stretching process, resulting in a core rod diameter of approximately 35 mm after stretching.

[0171] S8: The fused and stretched quartz rod is subjected to OVD deposition to form a powder. Specifically, the quartz rod is placed horizontally, and three blowtorches perpendicular to the mandrel are used to reciprocate left and right to deposit a uniform and loose layer of silica powder on the mandrel. During this process, hydrogen, oxygen, and silicon tetrachloride vapor are introduced into the blowtorches. The silicon tetrachloride undergoes a hydrolysis reaction through combustion in an oxyhydrogen flame to generate silica. The diameter of the sintered powder is approximately 3.5 times that of the quartz rod.

[0172] S9: The powder is placed in a fluorine-containing atmosphere and heated to 800℃ for 200 minutes. Then, after vacuuming, the temperature is raised to 1400℃ to begin sintering.

[0173] Comparative Example 2:

[0174] S1: Powder is deposited on a quartz rod with a diameter of about 30 mm using the OVD process, and Ge is doped into the powder using oxygen as a carrier gas.

[0175] S2: The powder is placed in a sintering furnace and chlorine gas is introduced successively for dehydroxylation, followed by oxygen introduction for dechlorination. Then, it is densified to form a transparent quartz rod. The quartz rod deposited in this step must have a chlorine content of less than 10 ppm and a hydroxyl content of less than 20 ppb. The ratio of the outer cladding layer to the initial quartz rod is approximately 3:1.

[0176] S3: Process the quartz rod into a quartz tube with an outer diameter of 35mm and an inner diameter of 21mm, requiring the inner surface roughness to be less than 0.05μm.

[0177] S4: Connect the two sides of the quartz tube to the tail of the mandrel, wherein the tail contains a reagent chamber for the second dopant, and a heating furnace is located around the reagent chamber.

[0178] S5: Heat the furnace to the temperature at which the reagent liquefies, and simultaneously use an oxyhydrogen flame to heat the target quartz tube back and forth at a speed of 40 mm / min for a total of 30 rounds.

[0179] S6: Stop the supply of carrier gas, control the pressure inside the quartz tube to -30pa, gradually reduce the inner diameter of the quartz tube to within 5mm, then further reduce the negative pressure to -700pa, and finally shrink the quartz tube into a solid rod.

[0180] S7: Grind the solid rods as described above. The diameter should be reduced by at least 4 mm and the surface roughness should be less than 0.5 μm after grinding.

[0181] S8: Connect the polished mandrel to the mandrel tail with the reagent chamber, and then assemble it with the F-containing quartz tube. The negative refractive index of the F-containing quartz tube should be between -0.2% and -0.25%. After assembly, acid washing and drying are required. After drying, connect the two sides of the tail with the sealing device, and then fix it vertically on the equipment fixture.

[0182] S9: Open the first heating furnace and bring it to 500°C. Move the furnace body up and down twice, while simultaneously pumping oxygen and helium into the tubing to further dry the components. Then, raise the temperature of heating furnace 1 to 1400°C and move the heating element 1 up and down at a speed of 40 mm / min downwards and 200 mm / min upwards, for a total of 4 rounds. Simultaneously, add hexafluoroethane to the pumped gas for etching, requiring an etching depth of at least 200 μm. Stop the gas pumping and begin evacuating from above, bringing the negative pressure to above 0.09 MPa. Simultaneously, move the first heating furnace upwards at a speed of 5 mm / min, with the lower clamp pulling downwards at a uniform speed while the upper clamp remains stationary. This completes the melting and stretching process. The diameter of the stretched mandrel is approximately 40 mm, and the inner cladding diameter is approximately twice the core diameter.

[0183] S10: The fused and stretched quartz rod is subjected to OVD deposition to form a powder. Specifically, the quartz rod is placed horizontally, and three blowtorches perpendicular to the mandrel are used to reciprocate left and right to deposit a uniform and loose layer of silica powder on the mandrel. During this process, hydrogen, oxygen, and silicon tetrachloride vapor are introduced into the blowtorches. The silicon tetrachloride undergoes a hydrolysis reaction through combustion in an oxyhydrogen flame to generate silica. The diameter of the sintered powder is approximately 3.5 times that of the quartz rod.

[0184] S11: The powder is placed in a fluorine-containing atmosphere and heated to 800℃ for 200min. Then, after vacuuming, the temperature is raised to 1400℃ to begin sintering.

[0185] Test Example 2: The testing process for Test Example 2 is the same as that for Test Example 1.

[0186] Table 2

[0187]

[0188] Compared to Comparative Example 2, Example 2 shortened the production cycle by 3 days. Furthermore, Example 2 employed a non-flame contact doping method, resulting in a reduction of the attenuation coefficient (i.e., water peak test value) at 1383 nm from over 4.0 dB / km to around 0.5 dB / km. The attenuation coefficient at 1550 nm decreased to below 0.160 dB / km.

[0189] Finally, it should be noted that other embodiments of the invention will readily occur to those skilled in the art upon consideration of the specification and practice of the invention disclosed herein. This invention is intended to cover any variations, uses, or adaptations of the invention that follow the general principles of the invention and include common knowledge or customary techniques in the art not disclosed herein, and is not limited to the precise structures described above and shown in the accompanying drawings, and various modifications and changes can be made without departing from its scope. The scope of the invention is limited only by the appended claims.

Claims

1. An apparatus for preparing optical fiber preforms, characterized in that, The device includes: a quartz accessory, a first heating furnace, a second heating furnace, and a clamp; The quartz accessory includes a core rod, a core rod tail shank connecting the core rod, a sleeve consisting of a quartz tube containing a negative dopant and the core rod, and a sleeve tail shank connecting the sleeve. The mandrel tail contains a medicine chamber, and the medicine chamber is provided with an air outlet; The first heating furnace is configured to be movable relative to the quartz accessory in order to heat the sleeve in the quartz accessory; The second heating furnace is used to heat the pharmaceutical chamber; The clamps are located at the upper and lower ends of the quartz accessory.

2. A method for preparing an optical fiber preform, characterized in that, The preparation method, applied to the preparation apparatus of claim 1, comprises: A mandrel containing a positive dopant is joined to a mandrel shank, and the mandrel is inserted into a quartz tube containing a negative dopant to form a sleeve. A dopant, comprising an alkali metal element, is then filled into the reagent chamber. The refractive index difference between the mandrel and the quartz tube containing the negative dopant is greater than 0.25%. The second heating furnace and the first heating furnace are turned on; the heating temperature of the second heating furnace is lower than the melting point of the dopant, and the temperature of the first heating furnace is 900℃~2200℃; the first heating furnace moves back and forth to heat the sleeve; After the first preset time, the second heating furnace is turned off, and gas with etching function is pumped in through the tail shank of the sleeve for etching treatment. After the second preset time, the gas pumping is stopped, the quartz accessories are evacuated, and the preform is obtained by stretching through a fixture. The preform precursor is clad with an outer layer to obtain an optical fiber preform.

3. The preparation method according to claim 2, characterized in that, The step of cladding the preform precursor to obtain an optical fiber preform includes: The preform precursor is deposited with silicon dioxide by vapor deposition to form a powder. The powder is heated in an atmosphere containing a negative dopant at a temperature of 500℃~1200℃ for 120min~600min, and then sintered at 1300℃~1800℃ after vacuuming to obtain an optical fiber preform. And / or, the preform precursor is matched with a sleeve containing a negative dopant to obtain an optical fiber preform.

4. The preparation method according to claim 2 or 3, characterized in that, The metal element of the dopant includes one or more of potassium, calcium, cesium, or boron; And / or, the positive refractive index of the mandrel is -0.05% to 0.15%; And / or, the negative refractive index of the quartz tube containing the negative dopant is -0.45% to -0.2%; And / or, before starting the second heating furnace and the first heating furnace, the preparation method further includes: acid washing and drying the sleeve.

5. The preparation method according to claim 2 or 3, characterized in that, Methods for preparing a core containing a positive dopant include: Silicon dioxide was deposited on a quartz target rod by vapor deposition to obtain a powder core rod; the deposition process used oxygen as a carrier gas to deposit the positive dopant into the powder core rod. The powder core rod is subjected to dehydroxylation and dechlorination treatment, and then densified to obtain a quartz rod; The quartz rod is extended and polished to obtain the core rod containing the positive dopant.

6. A method for preparing an optical fiber preform, characterized in that, The preparation method, applied to the preparation apparatus of claim 1, comprises: A mandrel containing a positive dopant is joined to a mandrel shank, and the mandrel is inserted into a quartz tube containing a negative dopant to assemble a sleeve. The refractive index of the quartz tube containing the negative dopant is -0.2% to -0.40%, and the reagent chamber is free of dopant. The refractive index difference between the mandrel and the quartz tube containing the negative dopant is greater than 0.25%. The first heating furnace is turned on, and the temperature of the first heating furnace is 900℃~2200℃; the first heating furnace moves back and forth to heat the sleeve; The gas with etching function is pumped into the sleeve tail shank for etching treatment. After etching treatment, the quartz accessories are evacuated and stretched by the fixture to obtain the composite sleeve. The composite sleeve is processed to form an internally hollow ring structure. The hollow composite sleeve is connected to the sleeve tail shank, and a dopant is filled into the reagent chamber, the dopant including an alkali metal element. The second heating furnace and the first heating furnace are turned on. The heating temperature of the second heating furnace is lower than the melting point of the dopant, and the temperature of the first heating furnace is 900℃~2200℃. The first heating furnace moves back and forth to heat the hollow composite sleeve. After the first preset time, the second heating furnace is turned off, and gas with etching function is pumped in through the tail shank of the sleeve for etching treatment. After the second preset time, the gas pumping is stopped, the quartz accessories are evacuated, and the preform is obtained by stretching through a fixture. The preform precursor is clad with an outer layer to obtain an optical fiber preform.

7. The preparation method according to claim 6, characterized in that, The step of cladding the preform precursor to obtain an optical fiber preform includes: The preform precursor is deposited with silicon dioxide by vapor deposition to form a powder. The powder is heated in an atmosphere containing a negative dopant gas at a temperature of 500℃~1200℃ for 120min~600min, and then sintered at 1300℃~1800℃ after vacuuming to obtain an optical fiber preform. And / or, the preform precursor is matched with a sleeve containing a negative dopant to obtain an optical fiber preform.

8. The preparation method according to claim 6 or 7, characterized in that, The metal element of the dopant includes one or more of potassium, calcium, and cesium; And / or, the positive refractive index of the mandrel is -0.05% to 0.15%; And / or, the negative refractive index of the quartz tube containing the negative dopant is -0.45% to -0.2%; And / or, before starting the second heating furnace and the first heating furnace, the preparation method further includes: pickling and drying the sleeve; And / or, the preparation method further includes: Silicon dioxide was deposited on a quartz target rod by vapor deposition to obtain a powder core rod; the deposition process used oxygen as a carrier gas to deposit the positive dopant into the powder core rod. The powder core rod is subjected to dehydroxylation and dechlorination treatment, and then densified to obtain a quartz rod; The quartz rod is extended and polished to obtain the core rod containing the positive dopant.

9. An optical fiber preform, characterized in that, The optical fiber preform is prepared by the preparation method according to any one of claims 2-5, or by the preparation method according to any one of claims 6-8.

10. An optical fiber, characterized in that, The optical fiber is prepared from the optical fiber preform described in claim 9.

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