Bend-resistant quartz optical fiber preform based on high deposition efficiency and method of making same

Abstract

The application discloses a bending-resistant quartz optical fiber preform based on high deposition efficiency and a preparation method thereof, and belongs to the technical field of feed processing. The preparation method comprises the following steps: preparation of a SiO2-GeO2-Al2O3 ternary system core layer, preparation of a B2O3 and F collaborative gradient-doped transition layer, preparation of a porous quartz layer, preparation of a dense quartz layer, vacuum stepwise temperature sintering and hydroxyl removal, and precise external circle grinding. Through the double-bending-resistant structure of the gradient transition layer and the porous buffer layer, in cooperation with the double-closed-loop gradient regulation and the all-gas-phase porous layer preparation process, the problems of easy loss of transparency of B doping, difficult consideration of bending resistance and transmission performance, and low deposition efficiency in the traditional technology are solved, the unification of high deposition efficiency of the preform and high bending resistance and low loss of the optical fiber is realized, and the application is suitable for multiple fields.

Description

Technical Field

[0001] This invention relates to the field of quartz optical fiber preform preparation technology, and particularly to a bend-resistant quartz optical fiber preform with high deposition efficiency and its preparation method. Background Technology

[0002] Optical fiber preforms are the core raw material for optical fiber production, and their structure and performance directly determine the quality of the optical fiber. With the widespread application of optical fibers in communication networks, intelligent sensing, medical equipment, laser processing and other fields, the requirements for the bending resistance of optical fibers are increasing, especially in scenarios such as wiring in confined spaces, flexible equipment integration, and laser transmission. Bending-resistant optical fibers can effectively reduce signal loss.

[0003] Traditional methods for fabricating quartz optical fiber preforms mainly include external vapor deposition (OVD), axial vapor deposition (VAD), and modified chemical vapor deposition (MCVD). OVD has high deposition efficiency, but the preform core-cladding concentricity is poor, resulting in poor bending resistance of the drawn fiber. VAD produces preforms with uniform structures, but its deposition rate is slow, making it difficult to meet the demands of large-scale production. MCVD offers high refractive index control precision, but its deposition efficiency is low, and the cladding fabrication process is complex.

[0004] In existing technologies, methods such as optimizing the fiber refractive index profile or reducing the core diameter are commonly used to improve the bending resistance of optical fibers. However, these methods often come at the cost of reduced deposition efficiency or sacrificed transmission performance. Furthermore, in transition layer design, the introduction of boron (B) can effectively regulate the refractive index. However, studies have shown that when the mass fraction of boron doping exceeds 2.0%, devitrification of the quartz glass is prone to occur, leading to damage to the integrity of the preform structure, deterioration of optical performance, and severely affecting the quality of subsequent fiber drawing. Meanwhile, traditional boron / fiber doping often employs uniform doping or simple layered doping methods, making it difficult to achieve continuous gradient control of the refractive index. Moreover, existing processes lack systematic solutions for boron doping devitrification. Porous cladding preparation often relies on solid-state deposition processes (such as PSOD), which can easily cause process conflicts with the underlying structure, making it difficult to balance porosity and bonding strength.

[0005] Under the strategic backdrop of "carbon peaking and carbon neutrality," green manufacturing has become an industry trend. Traditional OVD (Optical Vapor Deposition) processes suffer from problems such as highly corrosive raw materials and high waste gas treatment costs. While quartz sand rod fabrication is environmentally friendly, it has limitations in optimizing bending resistance. Furthermore, the demand for rare-earth-doped quartz optical fibers in the laser field is growing, but rare-earth ions tend to form clusters in quartz glass, requiring co-dopersants such as Al2O3 to improve dispersion. This places higher demands on the design of the preform core layer system. Therefore, developing a quartz optical fiber preform and its preparation method that combines high deposition efficiency, excellent bending resistance, no devitrification risk, process compatibility, and environmental friendliness has significant practical application value. Summary of the Invention

[0006] To address the aforementioned technical problems, this invention provides a bend-resistant silica optical fiber preform with high deposition efficiency and its preparation method.

[0007] The technical solutions provided by the embodiments of the present invention are as follows: The method for preparing a bend-resistant silica optical fiber preform with high deposition efficiency includes the following steps: S1. Core layer preparation: A vapor-phase axial deposition method was used, with SiCl4, GeCl4 and AlCl3 as precursors. At a deposition temperature of 1200-1400℃, the carrier gas flow rate was controlled, with He flow rate of 5-10 L / min and O2 flow rate of 8-15 L / min. A porous core preform was deposited on the end of a rotating quartz core rod. A dual-blowtorch structure was set in the deposition reaction zone, with a blowtorch spacing of 15-20 cm, a blowtorch scanning speed of 5-8 mm / s, and a deposition rate of 4-6 g / min. This process is the core stage of preparing the core layer of quartz optical fiber preforms using vapor axial deposition (VAD). It revolves around three main mechanisms: the oxidation reaction of the vapor precursor, the vapor transport and deposition of product particles, and the formation of the porous preform. Simultaneously, process parameters such as dual torches are used to control deposition efficiency and product uniformity. The specific mechanisms, material changes, product characteristics, and effects are as follows: The core of this process involves the gas-phase oxidation reaction of various halide precursors with oxygen at high temperatures, generating SiO2, the basic constituent phase of quartz glass, as well as GeO2 and Al2O3 doped phases. All reactions are spontaneous oxidation reactions at high temperatures, and the specific chemical reaction formulas are as follows: SiCl4(g) + 2O2(g) SiO2(s) + 2Cl2(g) GeCl4(g) + 2O2(g) GeO2(s) + 2Cl2(g) 4AlCl3(g) + 3O2(g) 2Al₂O₃(s) + 6Cl₂(g) After the reaction occurs, the form and distribution of matter in the system undergo phased changes: First, He, as an inert carrier gas, uniformly transports the gaseous SiCl4, GeCl4, and AlCl3 precursors to the high-temperature deposition reaction zone at 1200-1400℃, while O2, as a reactant gas, is fully mixed with the precursors. Under high temperature, the precursor reacts rapidly with O2 to undergo the above-mentioned oxidation reaction, transforming from gaseous halides into nano-sized SiO2, GeO2, and Al2O3 solid particles, while generating Cl2 gaseous byproducts. The byproducts are discharged from the reaction system with the exhaust gas. The generated solid oxide particles are directionally transported to the end of the rotating quartz core rod under the drive of the high-temperature gas flow, and gradually deposited and accumulated on the surface of the core rod. The process parameters in the deposition stage control the product's packing state and deposition efficiency from multiple dimensions: the rotation of the quartz mandrel allows oxide particles to be deposited uniformly along the circumference, avoiding local unevenness in thickness; the dual-torch structure increases the precursor concentration and high-temperature range in the reaction zone; the 15-20cm spacing between the torches ensures thorough mixing of the two precursors and reactants, avoiding incomplete local reactions; and the 5-8mm / s torch scanning speed allows particles to be deposited uniformly along the mandrel axis, ultimately achieving a high deposition rate of 4-6g / min; the flow ratio of carrier gas He to reactant gas O2 (5-10L / min: 8-15L / min) ensures stable transport of the precursors and provides sufficient O2 for the oxidation reaction. At the same time, the transport efficiency of solid particles is controlled by the airflow velocity, allowing the particles to form a preform in a loosely packed manner rather than a densely fused manner. After the above-mentioned gas phase reaction, particle transport and directional deposition process, a porous preform with SiO2 as the substrate and uniformly doped with GeO2 and Al2O3 is finally formed at the end of the quartz core rod. The preform has a three-dimensional network loose porous structure without a dense glassy structure. It has a large number of micropores inside, and the GeO2 and Al2O3 doped phases are uniformly dispersed in the SiO2 substrate in the form of nanoparticles without obvious agglomeration. This porous core preform serves as the core functional layer of the entire optical fiber preform, playing three key roles: First, SiO2 acts as a substrate, providing the basic framework of quartz glass for subsequent sintering and densification. It is the main component of the fiber core, determining its fundamental optical and mechanical properties. Second, GeO2, as a refractive index dopant, enhances the refractive index of the core through uniform doping, providing the refractive index difference for total internal reflection transmission, a crucial prerequisite for the fiber's light-guiding function. Third, if rare earth ions are introduced later, Al2O3 doping can effectively suppress clustering, while simultaneously improving the mechanical strength and thermal stability of the core, preventing structural cracking during subsequent sintering and fiber drawing, and optimizing the optical uniformity of the core. Furthermore, the porous structure of the preform provides a good adhesion interface for the subsequent vapor deposition of the transition layer and cladding, ensuring the bonding strength between the deposited layers and the core. Simultaneously, the porous structure can achieve densification through pore shrinkage during subsequent sintering, ultimately forming a uniform glassy core. S2. Transition layer preparation: An improved external vapor deposition method was adopted, using SiCl4, BCl3 and CF4 as precursors to deposit a gradient refractive index transition layer on the core surface. By real-time monitoring of the precursor flow rate calibration curve and the unreacted concentration of the tail gas, the flow rate was dynamically adjusted to achieve the gradient distribution of B2O3 and F. The temperature of the deposition area was stabilized at 800-1000℃, and the deposition rate was 3-5 g / min. This process is the core stage of the improved external vapor deposition (OVD) method for preparing the transition layer of optical fiber preforms. It revolves around three main mechanisms: the gas-phase reaction of the halide precursor, the directional deposition of product particles, and the precise control of the B / F doping gradient. Through the coordinated control of temperature and flow rate, a transition layer with a continuously varying refractive index is formed on the surface of the porous preform in the core layer. This achieves a seamless connection between the refractive index of the core and cladding layers while avoiding devitrification issues caused by high-concentration B doping. The specific mechanisms, material changes, product characteristics, and functions are as follows: The core of this process is the gas-phase oxidation / decomposition reaction of the gaseous precursor in the deposition region at 800-1000℃, generating the SiO2 substrate phase and B2O3 and F doped phases. The reaction proceeds spontaneously at high temperatures without the participation of any additional catalyst. The specific chemical reaction formula is as follows: SiCl4(g) + 2O2(g) SiO2(s) + 2Cl2(g) 4BCl3(g) + 3O2(g) 2B₂O₃ + 6Cl₂(g) CF3(g) C(s) + 2F₂(g) generates F₂ which undergoes a complexation reaction with SiO₂ to form an F-containing quartz glass phase (SiO₂-xF). x A small amount of elemental C is discharged with the exhaust gas or is oxidized and removed during subsequent sintering. The reaction and material changes follow a phased pattern of "gas phase transport - high temperature reaction - solid phase deposition": the inert carrier gas uniformly transports the gaseous SiCl4, BCl3, and CF4 precursors to the deposition area and mixes them thoroughly with the reactant gas O2. Under high temperatures of 800-1000℃, SiCl4 and BCl3 rapidly undergo oxidation reactions with O2, transforming from gaseous halides into nanoscale SiO2 and B2O3 solid particles. CF4 decomposes at high temperatures into gaseous F2 and solid C particles. F2 then complexes with the generated SiO2 particles, achieving the doping of F element in the SiO2 substrate. During the reaction, the generated Cl2, unreacted precursors, and a small amount of byproducts form exhaust gas with the airflow. The remaining solid products (SiO2, B2O3, and F-containing quartz particles) are directionally transported to the surface of the rotating porous core preform under the influence of the high-temperature airflow, gradually completing the deposition and accumulation. The core of the deposition process lies in the precise gradient control of B2O3 and F doping content. This control is achieved through a dual closed-loop logic of "precursor flow rate calibration curve and real-time monitoring of unreacted concentration in the tail gas". At the same time, the stable control of process parameters ensures the deposition effect: calibration curves are prepared in advance through laboratory pilot tests to clarify the correspondence between BCl3 and CF4 inlet flow rates and B2O3 and F content in the deposited layer; during formal deposition, detection equipment is installed at the tail gas emission port to monitor the concentration of unreacted BCl3 and CF4 in real time. This concentration directly reflects the reaction efficiency. If the concentration deviates from the calibration value, the control system will dynamically adjust the precursor inlet flow rate. For example, if the BCl3 concentration in the tail gas is too high, the inlet flow rate will be appropriately increased to compensate for reaction losses and ensure that the doping content is consistent with the design value. During the deposition process, the BCl3 flow rate was gradually adjusted from high to low, and the CF4 flow rate was gradually adjusted from low to high, so that the B2O3 content decreased from 1.8% on the core side to 0.5% on the cladding side, and the F content increased from 0.8% on the core side to 2.5% on the cladding side. At the same time, the stable deposition temperature of 800-1000℃ avoids over- or under-reaction, and the deposition rate of 3-5 g / min balances deposition efficiency and doping uniformity. The rotation of the core layer ensures that the transition layer is deposited uniformly along the circumference, without local thickness unevenness. After the above-mentioned gas-phase reaction, directional deposition and gradient control, a transition layer with SiO2 as the substrate and B2O3 and F continuously gradient doped is finally formed on the surface of the porous preform of the core layer. The transition layer is a loose porous particle stacking structure that is tightly combined with the porous structure of the core layer without obvious interface gaps. The doped phases B2O3 and F are uniformly dispersed in the SiO2 substrate at the nanoscale without local enrichment. Moreover, from the core layer side to the cladding side, the B2O3 content continuously decreases and the F content continuously increases, so that the refractive index of the transition layer changes smoothly and continuously from the high refractive index of the core layer side to the low refractive index of the cladding side without any abrupt refractive index nodes. This gradient refractive index transition layer, serving as a "connecting functional layer" between the core layer and the outer cladding, is crucial for achieving low-loss optical fiber guidance and improving the stability of the preform structure. Its core functions are threefold: First, it bridges the refractive index gradient. The core layer, doped with GeO2, has a high refractive index, while the cladding is low-refractive-index quartz glass. The transition layer, through gradient doping with B2O3 (to increase the refractive index) and F (to decrease the refractive index), achieves a continuous transition between high and low refractive indices, avoiding reflection loss during light transmission due to abrupt changes in refractive index and significantly improving the optical fiber's guiding efficiency. Second, it mitigates the risk of devitrification due to boron doping by strictly controlling the amount of boron doping. The O3 content in any micro-region does not exceed 1.8% and exhibits a gradient decreasing distribution, completely solving the devitrification problem of quartz glass caused by high-concentration boron doping and ensuring the optical uniformity of the transition layer; thirdly, the structure and performance transition is achieved by matching the porous structure of the transition layer with the porous structure of the core layer and subsequent cladding layer, realizing a tight bond between layers, improving the overall structural integrity of the preform, and avoiding interlayer cracking during subsequent sintering and wire drawing; at the same time, appropriate doping of B2O3 can reduce the softening point of quartz glass, and F doping can improve the chemical stability and radiation resistance of the glass, allowing the transition layer to have the dual functions of structural connection and performance optimization; S3. Cladding preparation: S301. Using external vapor deposition combined with low-temperature sintering, SiCl4 is deposited on the surface of the transition layer at 1000-1200℃ to form an incompletely densified loose quartz layer. By controlling the flame temperature to 800-900℃, the torch scanning speed to 10-15mm / s, the SiCl4 flow rate to 5-8L / min, the O2 flow rate to 10-12L / min, and the flame power to 1500-1800W during the low-temperature deposition stage, it is then transformed into a porous quartz layer through a sintering process. This process is the core stage of preparing the inner porous quartz layer using external vapor deposition (OVD) combined with low-temperature sintering. It revolves around three main mechanisms: low-temperature vapor-phase oxidation reaction, directional deposition of loose particles, and sintering to maintain porosity. By precisely controlling parameters such as deposition temperature, flame power, and gas flow rate, SiO2 particles are deposited in a loose state. Low-temperature sintering then achieves particle bonding and shaping, avoiding complete densification throughout the process. The final result is a quartz layer with controllable porosity. The specific mechanisms, material changes, methods for ensuring the porous structure, and product characteristics are as follows: Silicon tetrachloride undergoes a gas-phase oxidation reaction at high temperatures to generate SiO2, the basic constituent phase of quartz glass. The reaction proceeds spontaneously in a deposition environment of 1000-1200℃ without the participation of any additional doped phases. SiCl4(g) + 2O2(g) SiO2(s) + 2Cl2(g) The material form of the entire process follows a phased change: "gas-state precursor → nano-solid particles → loose aggregate → sintered and shaped porous body". Inert carrier gas stably delivers gaseous SiCl4 to the deposition area at a flow rate of 5-8 L / min, and mixes it thoroughly with O2 at a flow rate of 10-12 L / min. Under the synergistic effect of flame temperature of 800-900℃ and deposition area temperature of 1000-1200℃, SiCl4 and O2 undergo rapid oxidation reaction, transforming from gaseous halide into nano-sized SiO2 solid particles. The gaseous Cl2 byproduct generated by the reaction is discharged from the reaction system with the tail gas. The generated nano-SiO2 particles are directionally transported to the surface of the rotating transition layer under the drive of the high-temperature gas flow, and gradually complete the deposition and accumulation on the surface of the transition layer to form an incompletely densified loose quartz layer. Subsequently, this loose layer is treated by sintering process and finally transformed into a stable porous quartz layer. The coordinated control of process parameters during the deposition stage is the core foundation for forming an incompletely dense, porous quartz layer and a prerequisite for a porous structure. Each parameter, from different dimensions, prevents SiO2 particles from melting and densifying, achieving only simple physical stacking: the deposition region of 1000-1200℃ is a low-temperature deposition range, far below the temperature at which SiO2 particles completely melt and densify (above 1600℃), so the SiO2 nanoparticles remain solid and cannot undergo melting and fusion; the low flame power of 1500-1800W further controls local heat output, preventing high temperatures at the flame center from causing local particle melting, ensuring that the deposited SiO2 particles remain discrete solids; 10- The high-speed torch scanning speed of 15mm / s ensures that the torch stays on the surface of the transition layer for a very short time, preventing particles from continuously accumulating and melting in the same position due to heat accumulation. At the same time, it ensures that the particles are evenly distributed along the circumferential and axial directions. The flow ratio of SiCl4 to O2 ensures that the rate of oxidation reaction matches the amount of SiO2 particles generated. The particles are stacked at a moderate density, forming a loose structure with initial pores, rather than a dense glassy layer. The various parameters work together to ensure that the deposited quartz layer is a porous precursor of loosely stacked nano-SiO2 particles, laying the structural foundation for subsequent sintering to maintain porosity. The subsequent sintering process is crucial for the transformation of the loose layer into a stable porous quartz layer. The core principle is low-temperature sintering, slight bonding, and preservation of porosity, rather than high-temperature melting and densification. This is the key to ensuring the final porous structure: During sintering, the loose quartz layer does not withstand the high temperature that would allow SiO2 to completely melt. It is only heat-treated at a temperature below the softening point of quartz. At this time, only slight melting occurs on the surface of the SiO2 nanoparticles, and the contact points between the particles form a molten bonding zone. After cooling, this zone transforms into a stable chemical bond, allowing the loose particle stack to form an integral structure with a certain mechanical strength. Most of the voids inside and between the particles are not filled by molten SiO2 because the overall melting does not occur. The initial pore structure formed during the deposition stage is completely preserved. At the same time, the heating rate and holding time during the sintering process are precisely controlled to avoid thermal stress causing the pore structure to collapse. Ultimately, the transformation of the loose layer into a target porous quartz layer with a porosity of 15-25% and a pore size of 50-200nm is achieved, which not only ensures the structural strength of the layer but also fully preserves the porous characteristics. After the above-mentioned low-temperature vapor deposition and low-temperature sintering shaping, a uniform porous quartz layer with nano-SiO2 as the substrate is finally formed on the surface of the transition layer. This layer is tightly bonded to the transition layer without interface cracking or delamination. The interior is an interconnected three-dimensional network pore structure with uniform porosity and pore size distribution, without local densification or pore collapse. Moreover, the layer as a whole has good mechanical strength and structural stability, and can withstand the process environment of subsequent outer layer deposition and overall sintering without structural deformation. This porous quartz layer, serving as the inner functional layer of the optical fiber preform cladding, plays a crucial role in stress buffering. During bending, the optical fiber generates radial and circumferential mechanical stress. The three-dimensional mesh-like pores of the porous structure can absorb these stresses through minute compression and expansion, preventing stress concentration at the interface between the core layer and the transition layer. This prevents defects such as microcracks and delamination at the interface, significantly improving the bending resistance of the optical fiber. Simultaneously, the pore size (50-200nm) of this porous layer is much smaller than the wavelength of communication light (1310nm, 1550nm), thus avoiding light scattering loss and ensuring the optical fiber's guiding efficiency. This achieves a balance between bending resistance and optical transmission performance. S302. Using external vapor deposition, SiCl4 is used as a precursor to deposit a dense quartz layer at 1600-1800℃, with a deposition rate of 5-8 g / min. The material form of the entire process follows a phased change from "gaseous precursor → molten / glassy SiO2 → dense deposit", which is fundamentally different from the "solid particle accumulation" of porous layers: the inert carrier gas stably transports the gaseous SiCl4 to the deposition reaction zone, and after being fully mixed with sufficient O2, it undergoes a rapid oxidation reaction at a high temperature of 1600-1800℃. At this time, the generated SiO2 is no longer a nano-sized solid particle, but is directly transformed into molten liquid SiO2 or glassy SiO2. The gaseous Cl2 byproduct generated by the reaction is quickly discharged from the reaction system with the tail gas, while the molten / glassy SiO2 is directionally transported to the surface of the rotating inner porous quartz layer under the drive of the high temperature gas flow, and melts and spreads directly while completing the deposition, achieving preliminary densification without the need for additional subsequent treatment. A high-temperature deposition environment is the core prerequisite for achieving densification. The temperature range of 1600-1800℃ is much higher than the softening point of quartz glass (about 1600℃) and close to its melting temperature (about 1900℃). This temperature ensures that SiO2 is in a molten / semi-molten state throughout the entire process from generation to deposition, eliminating the generation of pores from the source. On the one hand, the product of the SiCl4 oxidation reaction is directly molten SiO2, without gaps formed by the accumulation of solid particles, thus avoiding the "inter-particle pores" in the porous layer deposition stage. On the other hand, when molten SiO2 is deposited on the surface of the preform, the high temperature causes the newly deposited SiO2 to fuse in situ with the SiO2 already deposited at the bottom layer. There is no obvious interface between the two, and no micropores are generated due to interlayer bonding, achieving overall continuous densification of the deposition layer. At the same time, the high-temperature environment causes the small number of possible micropores in the deposition layer to expand and escape rapidly, further eliminating internal pores and ensuring the density of the deposition layer. The coordinated control of various process parameters is key to ensuring the uniformity, density, and deposition efficiency of the dense layer, making the high-temperature densification effect more stable and achieving a high-efficiency deposition rate of 5-8 g / min. The high deposition rate allows the SiO2 melt to spread and stack rapidly on the surface of the preform, avoiding the formation of pores due to local cooling and crystallization caused by slow deposition. The high-speed rotation of the preform allows the molten SiO2 to spread evenly in the circumferential direction, eliminating local depressions or protrusions caused by uneven deposition, and ensuring the surface flatness and thickness uniformity of the dense layer. The sufficient supply of reactive gas O2 allows SiCl4 to be fully oxidized, avoiding the formation of impurities such as carbides and chlorides due to insufficient reaction, and preventing impurity particles from forming pore nuclei. At the same time, the precise matching of flame power and torch scanning speed during the deposition process ensures that the temperature of the deposition area is stable at 1600-1800℃, avoiding insufficient melting of SiO2 due to local low temperature, or damage to the porous structure of the bottom layer of the preform due to local high temperature, ensuring densification while taking into account the structural compatibility with the inner layer. After the above-mentioned high-temperature vapor-phase oxidation, melt deposition, and in-situ densification, a continuous, non-porous, dense quartz glass layer is finally formed on the surface of the inner porous quartz layer. This layer has a uniform glassy structure, with no interstitial gaps between particles, no internal micropores, and a hydroxyl content ≤5×10⁻⁶. -6 The density is close to the theoretical density of quartz glass; at the same time, the dense layer is tightly bonded to the bottom porous layer. The high temperature causes a small amount of SiO2 on the surface of the porous layer to melt and form chemical bonds with the molten SiO2 deposited on the outer layer. There is no risk of interlayer cracking or delamination. The overall thickness is uniform, the surface is smooth, and the deposition rate is maintained at 5-8 g / min, which balances densification effect and industrial mass production efficiency. This dense quartz layer, serving as the outer structure of the optical fiber preform cladding, plays a crucial role in mechanical protection, environmental isolation, and structural shaping. Its dense, glassy structure possesses excellent mechanical strength and rigidity, effectively resisting external mechanical impacts during subsequent processing, fiber drawing, and use, preventing deformation of the preform / fiber. The non-porous structure effectively blocks external contaminants such as moisture and dust, preventing them from entering the inner porous layer and causing structural degradation or optical loss, thus improving the preform's environmental adaptability and storage stability. Simultaneously, the dense layer's enclosure fixes the pore structure of the inner porous layer, preventing excessive shrinkage or pore collapse due to thermal stress during subsequent overall sintering, ensuring the overall stability of the preform structure and laying the foundation for the subsequent drawing and fabrication of high-quality, bend-resistant optical fibers. S4. Sintering treatment: The deposited composite preform was placed in a graphite furnace and sintered at 1700-1900℃ and vacuum degree ≤10Pa for 2-4 hours. During the sintering process, chlorine gas was introduced for dehydroxylation treatment at a flow rate of 0.5-1L / min. The sintering process adopted a stepped heating method with a heating rate of 5-10℃ / min, and the temperature was held at 1200℃ and 1500℃ for 30 minutes respectively. The core changes in this process are the high-temperature sintering densification of quartz glass and the melting and fusion of the interfaces of each layer. The core layer, transition layer and cladding of the composite preform are all loosely packed structures of nano-oxide particles with a large number of pores and weak bonding interfaces between the particles. Under the action of high temperature of 1700-1900℃, SiO2 and doped oxide particles undergo surface diffusion and volume diffusion, and the contact area between particles continues to increase. The originally loose physical packing is gradually transformed into tight chemical bonding. At the same time, the micropores between particles shrink and merge at high temperature, and most of the pores are eventually eliminated, and each layer is densified. The oxygen-free environment with a vacuum degree ≤10Pa can avoid the preform from undergoing secondary reaction with oxygen at high temperature to generate impurities. At the same time, it reduces the resistance of gas molecules to pore shrinkage, allowing the residual gas in the pores to escape quickly, greatly improving the densification efficiency and ensuring that the overall density of the preform is close to the theoretical value of quartz glass. Stepped heating combined with segmented heat preservation is the core mechanism for avoiding thermal stress defects and ensuring the structural integrity of the preform. It is also the key to achieving stable sintering. The composition and structure of each layer of the composite preform are different. The core layer contains GeO2 and Al2O3 dopants, the transition layer contains B2O3 and F dopants, and the cladding is a pure SiO2 porous / dense structure. The thermal expansion coefficient and softening point of different components are slightly different. If rapid heating is used, huge internal stress will be generated inside the billet due to the temperature gradient, which will cause cracking, interlayer separation, or even devitrification of the transition layer. By gradually increasing the temperature at a low rate of 5-10℃ / min, the preform can be heated uniformly from the inside out, with the temperature gradient controlled within a very small range. Holding the preform at two key temperature points, 1200℃ and 1500℃, for 30 minutes each allows for the phased release of thermal stress: 1200℃ is the initial sintering temperature of the oxide particles, and holding at this temperature allows the particles to complete initial bonding, eliminating macroscopic stress in the stacked state; 1500℃ is close to the softening initiation temperature of SiO2, and holding at this temperature allows the particles at the interfaces of each layer to undergo initial melting, alleviating the thermal expansion mismatch stress between the interfaces, laying the structural foundation for subsequent high-temperature densification, and ultimately achieving stress-free and defect-free high-temperature sintering. The gas-phase dehydroxylation reaction under a chlorine atmosphere is the core mechanism for reducing the hydroxyl content of the preform and improving the optical performance of fiber optics. Hydroxyl groups (-OH) in quartz glass cause infrared absorption loss during light transmission and are a key impurity affecting low-loss fiber optic transmission. They must be completely removed through the sintering process. At a high temperature of 1700-1900℃, the hydroxyl groups inside the preform escape from the glass structure and undergo a gas-phase dehydroxylation reaction with the introduced Cl2, generating volatile HCl gas. The reaction formula is as follows: 2-OH (s)+Cl2(g)→2Cl(s)+2H2O (g) H2O (g)+Cl2(g)→2HCl (g)+0.5O2(g) The generated HCl gas is rapidly extracted from the graphite furnace under vacuum, disrupting the chemical equilibrium of the dehydroxylation reaction. This promotes the continuous diffusion of hydroxyl groups from inside the preform to the surface, where they participate in the reaction, ultimately reducing the hydroxyl content of the preform to ≤5×10⁻⁶. -6 The low-loss standard of 0.5-1L / min chlorine flow rate ensures sufficient reactants for the dehydroxylation reaction while avoiding the corrosion of the graphite furnace by excessive chlorine at high temperatures, thus achieving a balance between dehydroxylation effect and equipment protection. The high-temperature vacuum sintering process simultaneously achieves the structural integration of each layer of the composite preform, transforming the loosely bound preform of the core layer, transition layer, inner porous layer, and outer dense layer into a chemically bonded integral quartz glass structure. At a high temperature of 1700-1900℃, the SiO2 particles at the interface of each layer undergo slight melting, and the gaps between the particles in the original layers are filled by the molten quartz glass. The SiO2 molecules in different layers diffuse into each other, forming a continuous glass phase without obvious interfaces. For the inner porous quartz layer, the sintering temperature is lower than its complete melting temperature, allowing only slight bonding and shaping of the SiO2 particles on the pore walls. While preserving the pore structure (porosity 15-25%), it achieves close integration with the transition layer and the outer dense layer, ensuring the stress buffering function of the porous layer and eliminating the risk of interlayer cracking. The B2O3 and F doped phases in the transition layer further diffuse uniformly at high temperature, making the refractive index gradient smoother. At the same time, the high-temperature vacuum environment avoids the local enrichment of B2O3, completely avoiding the devitrification problem. The entire sintering and dehydroxylation process lasts 2-4 hours. This duration ensures that high-temperature densification, gas-phase dehydroxylation, and interface fusion are fully carried out, while avoiding excessive shrinkage, deformation, or collapse of the porous layer caused by prolonged high temperature. The composite preform after this process becomes a dense quartz glass preform with high core-packet concentricity, low hydroxyl content, integrated structure of each layer, and no cracking or devitrification. Only subsequent precision processing is needed to meet the core requirements for optical fiber drawing. S5, Precision Machining: The sintered preform is subjected to external cylindrical grinding to ensure that the core-cladding concentricity and cladding non-circularity meet the requirements, thus obtaining a bend-resistant silica optical fiber preform. The core of this process is the precision mechanical grinding mechanism for removing hard and brittle materials like quartz glass. Quartz glass is hard and brittle. During grinding, the high-speed rotating diamond grinding wheel abrasive grains cut into the surface of the preform under the action of grinding force, causing micro-cracks to be generated, expanded, and merged in the glass material in the contact area, forming tiny glass fragments that are carried away by the grinding fluid, achieving precise removal of excess material from the surface. The grinding fluid also plays a role in cooling and rinsing away the fragments, preventing grinding heat from causing micro-cracks in the glass, preventing secondary scratches on the surface caused by the fragments, and ensuring the optical-grade smoothness of the preform surface. Precise alignment and correction of core-cladding concentricity is the core control point of this process. Before processing, laser alignment detection technology is used to accurately identify the geometric center of the core and cladding layers and calculate the concentricity deviation by utilizing the difference in refractive index between the core and cladding layers. During grinding, the CNC system uses the eccentric grinding correction method to remove material from the off-center area based on the deviation value, gradually reducing the deviation between the core layer and the outer circle center until the core-cladding concentricity meets the design requirements, thus avoiding optical transmission loss caused by fiber core misalignment during subsequent fiber drawing. The grinding process simultaneously achieves high-precision control of cladding roundness and outer diameter. The preform is coaxially clamped and rotated at a uniform speed by a high-precision fixture, while the diamond grinding wheel is fed uniformly along the axial direction. An online roundness detection sensor collects the outer circle contour data in real time and calculates the deviation by comparing it with a standard perfect circle. The CNC system makes micron-level dynamic fine adjustments to the grinding wheel feed based on the detection data, removing excess material from protruding parts and reducing material removal from recessed parts. Closed-loop control eliminates contour deviations caused by uneven sintering shrinkage. At the same time, the outer diameter of the preform is machined to a standardized size to ensure that the cladding roundness and outer diameter accuracy meet the standards, avoiding problems such as fiber diameter fluctuations and fiber breakage during fiber drawing. After this process, the sintered preform not only eliminates defects such as surface unevenness and particle adhesion, but also achieves high-precision control of core-cladding concentricity, cladding non-circularity, and outer diameter. The surface finish and geometric parameters match the requirements of the optical fiber drawing process, ultimately forming a bend-resistant silica fiber preform with all parameters meeting the standards. This lays a key geometric foundation for the subsequent drawing of low-loss, high-bending-resistance silica fibers.

[0008] Preferably, the improved external vapor deposition method in S2 adopts a multi-channel nozzle structure with a nozzle spacing of 8-12cm. By optimizing the airflow field of the nozzle array, the precursor is uniformly distributed in the deposition area, ensuring the continuity of gradient doping.

[0009] Preferably, during the preparation of the porous quartz layer, the powder deposition trajectory is further optimized by adjusting the flow rate of the auxiliary gas Ar to 3-5 L / min, ensuring uniform pore distribution and avoiding the formation of an anomalous structure that is dense inside and porous outside.

[0010] Compared with the prior art, the beneficial effects of the present invention are: 1. This invention employs a dual-bending structure design consisting of a gradient refractive index transition layer and a porous stress buffer layer, combined with a synergistic gradient doping mechanism of B2O3 and F, to solve the problems of complex fabrication and difficult mode leakage elimination in traditional bending-resistant optical fibers, achieving simultaneous optimization of bending loss and transmission performance. It constructs a dual-bending mechanism of optical gradient and mechanical buffering. Optically, it achieves a continuous and smooth refractive index transition to eliminate mode leakage; mechanically, the porous layer absorbs more than 50% of the radial stress to prevent stress cracking. This significantly reduces bending loss at small curvature radii without additional scattering loss, achieving a balance between high bending resistance and low transmission loss.

[0011] 2. This invention employs a dual closed-loop gradient control mechanism combining flow calibration curves and real-time monitoring of exhaust gas concentration. Combined with a dual control strategy of decreasing B2O3 gradient and a micro-area content ≤1.8%, it solves the industry problem of devitrification in quartz glass caused by high B doping, achieving controllable fabrication of a devitrification-free gradient transition layer. A four-in-one doping control system is established to macroscopically and microscopically control the B2O3 content within a safe range. Combined with stepped heating sintering to eliminate thermal stress, the devitrification defect rate of the transition layer is reduced to 0%, overcoming the bottleneck in the engineering application of high B doping gradient structures.

[0012] 3. This invention employs an OVD (Optical Vapor Deposition) low-temperature deposition and low-temperature sintering process for preparing a fully vapor-phase porous layer. Combined with the coordinated control of multiple parameters such as flame temperature, scanning speed, and flame power, it solves the problems of conflict between traditional PSOD solid-phase processes and the underlying structure, as well as the difficulty in balancing porosity and bonding strength. This achieves precise controllability of porous layer parameters and integrated interlayer bonding. By abandoning solid-phase deposition, the fully vapor-phase process enables integrated cladding preparation. Multi-parameter control preserves the porous structure, achieving porosity and pore size accuracy within ±2%, balancing stress buffering function and interlayer bonding strength, and providing a highly reliable engineering implementation path. Attached Figure Description

[0013] Figure 1 This is a schematic diagram of the quartz optical fiber preform structure produced by the present invention. Detailed Implementation

[0014] The technical solutions of this invention are described below. It should also be noted that, to make the embodiments more detailed, the following embodiments are the best and preferred embodiments; those skilled in the art can also use other alternative methods to implement some well-known technologies.

[0015] Example 1: Preparation of quartz optical fiber preform: S1. Core layer preparation: A vapor-phase axial deposition method was used with SiCl4, GeCl4, and AlCl3 as precursors. At a deposition temperature of 1300℃, the He flow rate was adjusted to 7.5 L / min and the O2 flow rate to 11.5 L / min. A porous core preform was deposited at the end of a rotating quartz core rod. A dual-blowtorch structure was set in the deposition reaction zone with a blowtorch spacing of 17.5 cm, a blowtorch scanning speed of 6.5 mm / s, and a deposition rate of 5 g / min. S2. Transition layer preparation: An improved external vapor deposition method was adopted, using SiCl4, BCl3 and CF4 as precursors to deposit a gradient refractive index transition layer on the core surface. By using the precursor flow calibration curve and real-time monitoring of the unreacted concentration of the tail gas, the flow rate was dynamically adjusted to achieve the gradient distribution of B2O3 and F. The average mass fraction of B2O3 was 1.0% and the average mass fraction of F was 1.6%. The temperature of the deposition area was stabilized at 900℃ and the deposition rate was 4 g / min. S3. Cladding preparation: S301. Using external vapor deposition combined with low-temperature sintering, SiCl4 is deposited on the surface of the transition layer at 1100℃ to form an incompletely densified porous quartz layer. By controlling the flame temperature to 850℃, the torch scanning speed to 12.5mm / s, the SiCl4 flow rate to 6.5L / min, the O2 flow rate to 11L / min, and the flame power to 1650W during the low-temperature deposition stage, it is subsequently transformed into a porous quartz layer through a sintering process. S302. Using external vapor deposition, SiCl4 is used as a precursor to deposit a dense quartz layer at 1700℃ with a deposition rate of 6.5 g / min. S4. Sintering treatment: The deposited composite preform was placed in a graphite furnace and sintered at 1800℃ and vacuum degree ≤10Pa for 3 hours. During the sintering process, chlorine gas was introduced for dehydroxylation treatment at a flow rate of 0.75L / min. The sintering process adopted a stepped heating method with a heating rate of 7.5℃ / min. The temperature was held at 1200℃ and 1500℃ for 30 minutes respectively. S5, Precision Machining: The sintered preform is subjected to external cylindrical grinding to ensure that the core-cladding concentricity error is ≤0.3μm and the cladding non-circularity is ≤0.2%, thus obtaining a bend-resistant silica fiber preform.

[0016] Example 2: Preparation of quartz optical fiber preform: S1. Core layer preparation: A vapor-phase axial deposition method was used with SiCl4, GeCl4, and AlCl3 as precursors. At a deposition temperature of 1300℃, the He flow rate was adjusted to 7.5 L / min and the O2 flow rate to 11.5 L / min. A porous core preform was deposited at the end of a rotating quartz core rod. A dual-blowtorch structure was set in the deposition reaction zone with a blowtorch spacing of 17.5 cm, a blowtorch scanning speed of 6.5 mm / s, and a deposition rate of 5 g / min. S2. Transition layer preparation: An improved external vapor deposition method was used, with SiCl4, BCl3 and CF4 as precursors, to deposit a gradient refractive index transition layer on the core surface. By using the precursor flow rate calibration curve and real-time monitoring of the unreacted concentration of the tail gas, the flow rate was dynamically adjusted to achieve the gradient distribution of B2O3 and F. The average mass fraction of B2O3 was 1.15% and the average mass fraction of F was 1.6%. The temperature of the deposition area was stabilized at 900℃ and the deposition rate was 4 g / min. S3. Cladding preparation: S301. Using external vapor deposition combined with low-temperature sintering, SiCl4 is deposited on the surface of the transition layer at 1000℃ to form an incompletely densified porous quartz layer. By controlling the flame temperature to 850℃, the torch scanning speed to 12.5mm / s, the SiCl4 flow rate to 6.5L / min, the O2 flow rate to 11L / min, and the flame power to 1650W during the low-temperature deposition stage, it is subsequently transformed into a porous quartz layer through a sintering process. S302. Using external vapor deposition, SiCl4 is used as a precursor to deposit a dense quartz layer at 1700℃ with a deposition rate of 6.5 g / min. S4. Sintering treatment: The deposited composite preform was placed in a graphite furnace and sintered at 1800℃ and vacuum degree ≤10Pa for 3 hours. During the sintering process, chlorine gas was introduced for dehydroxylation treatment at a flow rate of 0.75L / min. The sintering process adopted a stepped heating method with a heating rate of 7.5℃ / min. The temperature was held at 1200℃ and 1500℃ for 30 minutes respectively. S5, Precision Machining: The sintered preform is subjected to external cylindrical grinding to ensure that the core-cladding concentricity error is ≤0.3μm and the cladding non-circularity is ≤0.2%, thus obtaining a bend-resistant silica fiber preform.

[0017] Example 3: Preparation of quartz optical fiber preform: S1. Core layer preparation: A vapor-phase axial deposition method was used with SiCl4, GeCl4, and AlCl3 as precursors. At a deposition temperature of 1300℃, the He flow rate was adjusted to 7.5 L / min and the O2 flow rate to 11.5 L / min. A porous core preform was deposited at the end of a rotating quartz core rod. A dual-blowtorch structure was set in the deposition reaction zone with a blowtorch spacing of 17.5 cm, a blowtorch scanning speed of 6.5 mm / s, and a deposition rate of 5 g / min. S2. Transition layer preparation: An improved external vapor deposition method was used, with SiCl4, BCl3 and CF4 as precursors, to deposit a gradient refractive index transition layer on the core surface. By using the precursor flow rate calibration curve and real-time monitoring of the unreacted concentration of the tail gas, the flow rate was dynamically adjusted to achieve the gradient distribution of B2O3 and F. The average mass fraction of B2O3 was 1.15% and the average mass fraction of F was 1.6%. The temperature of the deposition area was stabilized at 900℃ and the deposition rate was 4 g / min. S3. Cladding preparation: S301. Using external vapor deposition combined with low-temperature sintering, SiCl4 is deposited on the surface of the transition layer at 1100℃ to form an incompletely densified porous quartz layer. By controlling the flame temperature to 850℃, the torch scanning speed to 12.5mm / s, the SiCl4 flow rate to 6.5L / min, the O2 flow rate to 11L / min, and the flame power to 1650W during the low-temperature deposition stage, it is subsequently transformed into a porous quartz layer through a sintering process. S302. Using external vapor deposition, SiCl4 is used as a precursor to deposit a dense quartz layer at 1700℃ with a deposition rate of 6.5 g / min. S4. Sintering treatment: The deposited composite preform was placed in a graphite furnace and sintered at 1800℃ and vacuum degree ≤10Pa for 3 hours. During the sintering process, chlorine gas was introduced for dehydroxylation treatment at a flow rate of 0.75L / min. The sintering process adopted a stepped heating method with a heating rate of 7.5℃ / min. The temperature was held at 1200℃ and 1500℃ for 30 minutes respectively. S5, Precision Machining: The sintered preform is subjected to external cylindrical grinding to ensure that the core-cladding concentricity error is ≤0.3μm and the cladding non-circularity is ≤0.2%, thus obtaining a bend-resistant silica fiber preform.

[0018] Comparative Example 1: Compared with Example 3, the average mass fraction of B2O3 in Comparative Example 1 was 2.0%, while other conditions remained unchanged.

[0019] Comparative Example 2: Compared to Example 3, in Comparative Example 2, an incompletely densified loose quartz layer was deposited on the surface of the transition layer at 1400°C, while other conditions remained unchanged.

[0020] Comparative Example 3: Compared with Example 3, the porous stress buffer layer was removed in Comparative Example 3, and the cladding was a pure dense quartz layer, while other conditions remained unchanged.

[0021] Comparative Example 4: Compared with Example 3, in Comparative Example 4, the transition layer B2O3 content was 1.15% and the F content was 1.6%, with uniform doping and no gradient change, while other conditions remained unchanged.

[0022] Comparative Example 5: Compared with Example 3, the porous layer in Comparative Example 5 was replaced with a PSOD solid phase deposition process, while other conditions remained unchanged.

[0023] Comparative Example 6: Compared with Example 3, in Comparative Example 6, sintering was performed by direct heating (20°C / min) without holding at 1200 / 1500°C, while other conditions remained unchanged.

[0024] Performance testing: 1. For the quartz optical fiber preforms prepared in the above embodiments and comparative examples, the core-cladding concentricity test of the preform was completed according to GB / T 15972.41-2021 "Fiber Optic Test Methods Specification Part 41: General Rules for Fiber Optic Characteristic Measurement Methods and Test Procedures"; the non-roundness and outer diameter deviation of the preform were completed according to GB / T 15972.45-2021 "Fiber Optic Test Methods Specification Part 45: Geometric Characteristics of Fiber Optic Characteristic Measurement Methods and Test Procedures"; the length and effective core diameter of the preform were completed according to GB / T 30556.2-2014 "Fiber Optic Preforms Part 2: Dimensional Parameter Test Methods"; and the hydroxyl content (OH) of the preform was completed according to YD / T 2152-2021 "Quartz Glass Optical Fiber Preforms for Communication". - ), refractive index profile distribution test; execute GB / T 26193-2010 "Determination of Trace Elements in Quartz Glass - Inductively Coupled Plasma Atomic Emission Spectrometry" to complete the test of the content of doped elements such as Ge, Al, B, and F in the preform.

[0025] 2. Performance testing of the drawn optical fiber is a core verification indicator for the preform. Specifically, it follows GB / T 15972.31-2021 "Fiber Optic Test Methods Specification Part 31: Mechanical Properties - Tensile Strength" to complete the fiber tensile strength test; GB / T 15972.21-2021 "Fiber Optic Test Methods Specification Part 21: Optical Characteristics - Attenuation" to complete the fiber transmission attenuation test in the 1310nm and 1550nm bands; GB / T 15972.22-2021 "Fiber Optic Test Methods Specification Part 22: Optical Characteristics - Bandwidth" to complete the fiber mode field diameter and cutoff wavelength test; GB / T 15972.32-2021 "Fiber Optic Test Methods Specification Part 32: Mechanical Properties - Bending Loss" to complete the fiber macrobending loss (static and dynamic) test; YD / T 1272.3-2022 "Low Water Peak Single-Mode Fiber for Communication - Part 3: Bending Loss Insensitive Single-Mode Fiber" to complete the fiber microbending loss test; and GB / T 7962.1-2010 "Test Methods for Colorless Optical Glass - Part 1: Refractive Index and Dispersion", completed the test of refractive index distribution of optical fiber.

[0026] Table 1 Test data of quartz optical fiber preform body Data Analysis: Figure 1 This is a schematic diagram of the quartz fiber preform structure produced by this invention. The quartz fiber preform has a coaxial multi-layer cylindrical structure, consisting of a core layer, a transition layer, and a cladding layer from the inside out. The core layer is the core of the optical fiber guiding system, using a SiO2-GeO2-Al2O3 system. GeO2 is used to increase the refractive index to achieve total internal reflection. The transition layer is a loosely packed layer of B / F synergistic gradient doping, achieving a continuous and smooth transition of refractive index and avoiding B-doping devitrification. The cladding layer contains an inner porous quartz layer and an outer dense quartz layer. The porous layer absorbs bending stress, while the dense layer provides mechanical protection and environmental barrier. The three-layer structure works synergistically to achieve both low transmission loss and excellent bending resistance.

[0027] Observing Table 1 above, the quartz optical fiber preforms prepared in Example 3 and Comparative Example 3 exhibit superior body performance. Example 3 benefits from the precise matching and synergistic effect of various preparation process parameters. In the core layer preparation stage, the dual-torch structure and reasonable carrier gas flow ratio ensured the uniform dispersion of dopant elements in the core layer, laying a good structural foundation for subsequent layer deposition. The transition layer preparation adopted a dual closed-loop gradient control mechanism to strictly control the gradient distribution and micro-area content of dopant elements, effectively avoiding the risk of devitrification. In the cladding preparation, the synergistic application of low-temperature deposition and low-temperature sintering processes achieved precise control of porous layer structural parameters, balancing porosity and interlayer bonding strength. The stepped heating method in the sintering stage effectively released the thermal stress inside the preform, and the chlorine dehydroxylation treatment significantly reduced the hydroxyl content. The final precision external cylindrical grinding ensured that geometric parameters such as core-cladding concentricity and cladding non-circularity met the design requirements. The overall synergy of the processes resulted in the preform having optimal comprehensive performance, with no devitrification, no structural defects, and a high deposition efficiency.

[0028] The core difference between Comparative Example 3 and Example 3 is the removal of the porous stress buffer layer; the cladding uses a pure dense quartz layer structure, while the remaining process parameters are the same as in Example 3. From a mechanistic perspective, without the porous layer, the cladding of the preform only provides mechanical protection and loses its stress buffering function. Furthermore, due to the change in cladding structure, although the refractive index connection between the core layer and the cladding is not significantly affected, the interlayer bonding synergy is reduced, leading to slight deviations in some geometric parameters. In addition, the absence of the porous layer does not affect the distribution of doped elements or the dehydroxylation effect; therefore, no devitrification occurs, and the hydroxyl content, deposition efficiency, and other indicators are basically the same as in Example 3. However, this structure can no longer achieve the expected bending resistance of this invention, highlighting the core value of the porous stress buffer layer in the preform structure design. Compared to Example 3, Example 1 exhibits slight fluctuations in various core performance indicators, and is generally slightly inferior to Example 3. Mechanistically, this fluctuation mainly stems from subtle differences in the gradient distribution of doped elements in the transition layer and the porosity of the porous layer. In Example 1, the gradient difference of doped elements in the transition layer differs slightly from that in Example 3, while the porosity of the porous layer is slightly higher than in Example 3, leading to slight deviations in indicators such as core-packet concentricity and hydroxyl content. However, since it still strictly follows the core process route of this invention, without deviating from key technologies such as dual-loop control and low-temperature deposition, no devitrification phenomenon occurs, and the deposition efficiency is basically the same as in Example 3. The overall performance remains at a superior level, only slightly lower than Example 3 in parameter matching. Compared to Example 3, some performance indicators of Example 2 showed slight deterioration, mainly in three aspects: hydroxyl content, porous layer porosity, and core-packet concentricity. The underlying mechanism was that the low-temperature deposition temperature during the cladding preparation stage differed from that of Example 3. Additionally, slight adjustments to the torch scanning speed resulted in a lower porosity in the porous layer compared to Example 3, and a slight decrease in the uniformity of the pore structure. Furthermore, the average content of doped elements in the transition layer deviated slightly from that in Example 3. Although this did not exceed the safe range or cause devitrification, it still had a certain impact on the core-packet concentricity and hydroxyl content, ultimately leading to a slightly lower overall performance than Example 3. However, it still met the design requirements of this invention and showed no obvious structural defects. Compared to Example 3, Comparative Example 1 showed a significant performance degradation. The core issue was the presence of obvious devitrification, with multiple performance indicators deviating from the standard range. Mechanistic analysis revealed that this phenomenon stemmed from the failure to strictly adhere to the dual-control strategy of this invention during the transition layer preparation process. This resulted in the maximum micro-area content of dopants exceeding the safety threshold, surpassing the devitrification critical value of the quartz glass, and consequently causing structural degradation and devitrification defects. The devitrification not only disrupted the structural integrity of the preform but also affected the uniform distribution of dopants, indirectly leading to deviations in indicators such as hydroxyl content and core-packet concentricity. Although the deposition efficiency was essentially the same as in Example 3, the overall performance failed to meet design requirements, fully demonstrating the necessity of the micro-area content control strategy for dopants in this invention. Compared to Example 3, the most significant change in Comparative Example 2 is the substantial decrease in the porosity of the porous layer. While other performance indicators show no obvious abnormalities, they are still slightly inferior to those of Example 3. The mechanism mainly lies in the cladding preparation stage, where the deposition temperature of the porous layer is set too high, far exceeding the low-temperature deposition range specified in this invention. This causes excessive melting of the SiO2 particles generated during deposition, resulting in the molten agglomeration of the particles that should have been loosely packed, leading to a significant reduction in the porosity of the porous layer. This decrease in porosity directly weakens the stress buffering function of the porous layer. Although it does not cause serious defects such as devitrification or delamination, it deviates from the original design intent of this invention, verifying the crucial role of the low-temperature deposition process in the formation of the porous layer structure. Compared to Example 3, Comparative Example 4 shows a slight deterioration in performance indicators, mainly reflected in the indirect impact on core-cladding concentricity and related optical properties. There is no devitrification, but the overall performance is lower than that of the examples. The mechanism lies in the transition layer preparation stage, where the gradient doping process of this invention was not used; instead, a uniform doping method was employed, eliminating the dual-closed-loop gradient control mechanism. Uniform doping cannot achieve a continuous and smooth transition of refractive index between the core and cladding layers, resulting in a decrease in refractive index matching at the core-cladding interface, which in turn affects the control accuracy of core-cladding concentricity. At the same time, uniform doping results in insufficient rationality of dopant element distribution. Although it does not exceed the devitrification threshold, it still has an adverse effect on the overall structural uniformity of the preform, verifying the important significance of gradient doping process for optimizing preform performance. Compared with Example 3, Comparative Example 5 showed significant deterioration in all performance indicators, with larger deviations in core-packet concentricity and cladding non-circularity, increased hydroxyl content, a substantial decrease in deposition efficiency, and interfacial bonding problems. The underlying mechanism is that the porous layer was prepared using the traditional PSOD solid-phase deposition process, replacing the OVD full-phase deposition process of this invention. The PSOD solid-phase process has poor compatibility with the OVD deposition process for the transition layer, easily leading to interfacial contamination during deposition, resulting in loose interlayer bonding and consequently causing deviations in geometric parameters such as core-packet concentricity and cladding non-circularity. Furthermore, the deposition efficiency of solid-phase deposition is much lower than that of vapor-phase deposition, and the process control is more difficult, making it impossible to precisely control the porous layer structure. This also leads to increased hydroxyl content and disrupts the overall process synergy, fully demonstrating the advantages of the full-phase deposition process of this invention. Compared to Example 3, Comparative Example 6 exhibited the most significant performance degradation, showing localized devitrification, significant deviations in core-cladding concentricity and cladding roundness, and a substantial increase in hydroxyl content, indicating obvious structural defects. The mechanism primarily lies in the sintering stage, where the stepped heating method of this invention was not employed; instead, direct rapid heating was used without proper insulation at critical temperature points. Rapid heating resulted in an excessively large internal temperature gradient within the preform, causing significant internal stress due to differences in thermal expansion coefficients between layers, leading to microcracks and localized devitrification. Simultaneously, rapid heating resulted in insufficient dehydroxylation reaction, preventing the effective removal of hydroxyl groups and increasing hydroxyl content. Internal stress also affected the control of core-cladding concentricity and cladding roundness, compromising the structural integrity of the preform. This verifies the necessity of the stepped heating sintering process for ensuring the performance of the preform.

[0029] Table 2. Fiber Optic and Bending Performance Test Data Data Analysis: Based on the data in Tables 1 and 2, Example 3 achieved optimal overall performance. From the perspective of the preform's bulk performance, its core-cladding concentricity and cladding non-circularity are precisely controlled, with extremely low hydroxyl content, reasonable and uniform porosity distribution in the porous layer, no devitrification in the transition layer, a smooth doping gradient, and tight bonding between layers. These excellent bulk properties provide a solid foundation for fiber drawing. Reflected in fiber performance, transmission attenuation remains at a low level, all types of bending losses (macro-bending and micro-bending) meet design requirements, interlayer bonding is excellent, and there are no structural defects. The core mechanism lies in the synergistic effect of the various preform fabrication processes, ensuring a synergistic match between the structure and performance of the fiber core, transition layer, and cladding. The reasonable porous layer structure effectively buffers bending stress, the smooth refractive index gradient avoids mode leakage during optical transmission, and the low hydroxyl content reduces infrared absorption loss, ultimately achieving a balance between high bending resistance and low transmission loss in the fiber.

[0030] Compared to Example 3, Example 1 exhibits slight fluctuations in various optical and bending performance indicators of the optical fiber, generally performing slightly worse than Example 3. This change is directly related to subtle deviations in the preform's properties. Based on the preform data, it can be seen that the porosity of the porous layer and the doping gradient of the transition layer in Example 1 differ slightly from those in Example 3, resulting in slight deficiencies in stress buffering capacity and refractive index matching of the drawn optical fiber. Specifically, the slightly higher porosity of the porous layer slightly reduces the uniformity of stress buffering during fiber bending, leading to a slight increase in macro-bending and micro-bending losses. The slight deviation in the doping gradient of the transition layer results in imperfect refractive index bonding at the core-cladding interface, causing a small amount of optical signal leakage and indirectly leading to slight fluctuations in transmission attenuation. However, since the preform in Example 1 does not exhibit serious defects such as devitrification or poor interlayer bonding, the optical fiber performance remains at a relatively high level, only slightly lower than Example 3 in parameter stability.

[0031] Compared to Example 3, the optical fiber and bending resistance of Example 2 showed a slight deterioration, mainly reflected in a slight increase in bending loss and transmission attenuation. This is closely related to the performance deviation of the preform body. Based on the preform body test data, the porosity of the porous layer in Example 2 was lower than that in Example 3, and the hydroxyl content was slightly higher. Simultaneously, there was a slight deviation in core-cladding concentricity. These factors collectively led to the deterioration of fiber performance. Mechanistically, the reduced porosity of the porous layer weakens the stress buffering capacity of the fiber during bending, making it unable to effectively absorb the radial and circumferential stress generated by bending, thus leading to increased macro-bending and micro-bending losses. The increased hydroxyl content induces additional infrared absorption loss during optical transmission, slightly increasing fiber transmission attenuation. The slight deviation in core-cladding concentricity affects the total internal reflection transmission path of the optical signal, further exacerbating the fluctuations in transmission attenuation. Ultimately, the overall fiber performance was slightly lower than that of Example 3, but still met the design requirements.

[0032] Compared to Example 3, the fiber performance of Comparative Example 1 showed a significant deterioration, with a substantial increase in transmission attenuation, a surge in various bending losses, and the appearance of localized interlayer cracking. This directly corresponds to the severe defects in the preform body. Based on the preform body data, Comparative Example 1 exhibited significant devitrification, and the maximum micro-area content of dopants exceeded the safety threshold. This inherent defect was directly transmitted to the drawn fiber. The core mechanism is that the devitrification of the preform transition layer disrupts the homogeneity of the quartz glass, forming light scattering centers, leading to a significant increase in fiber transmission attenuation. Devitrification also disrupts the integrity of interlayer bonding, making interlayer cracking more likely during fiber drawing and bending, further exacerbating bending losses. Simultaneously, the abnormal enrichment of dopants leads to uneven refractive index distribution, resulting in severe mode leakage during optical signal transmission, further deteriorating the optical and bending properties of the fiber, ultimately causing the fiber performance to fail to meet application requirements.

[0033] Compared to Example 3, the most significant degradation in fiber performance in Comparative Example 2 was a substantial increase in bending loss. While transmission attenuation fluctuated, the change was minimal, which is directly related to the performance defects of the porous layer in the preform body. Based on the preform body data, the porosity of the porous layer in Comparative Example 2 was significantly lower than that in Example 3, which is the core reason for the deterioration of the fiber's bending resistance. From a mechanistic perspective, the core function of the porous layer is to buffer the stress generated during fiber bending. However, in Comparative Example 2, the deposition temperature of the porous layer was too high, leading to excessive melting of particles, filling of pores, and damage to the porous structure, resulting in almost complete loss of stress buffering capacity. When the fiber bends, the stress cannot be effectively absorbed and concentrates at the interface between the core and transition layers, leading to increased optical signal mode leakage and a significant increase in macro-bending and micro-bending losses. Since the preform did not exhibit devitrification or abnormal hydroxyl content, the transmission attenuation did not change significantly, but the bending resistance had seriously deviated from the original design, fully verifying the crucial role of a reasonable porosity in the fiber's bending resistance.

[0034] The main difference in fiber performance between Comparative Example 3 and Example 3 lies in the significant increase in bending loss, especially macro-bending loss at small radii of curvature. Transmission attenuation, however, remains essentially the same as in Example 3. This is directly related to the absence of a porous stress buffer layer in the preform body's structural design. Based on the preform body data, Comparative Example 3 lacks a porous layer, employing only a pure dense cladding. This structural deficiency directly impacts the fiber's bending resistance. Mechanistically, the dense cladding lacks stress buffering capabilities, preventing the effective dispersion of mechanical stress generated during fiber bending. Instead, stress concentrates at the core-cladding interface, leading to micro-stress concentration and optical signal leakage, resulting in a significant increase in bending loss. In contrast, the preform's dopant distribution, hydroxyl content, and core-cladding concentricity are essentially identical to Example 3, not affecting normal optical signal transmission. Therefore, transmission attenuation remains unchanged, but the fiber no longer achieves the bending resistance expected in this invention, highlighting the core value of a porous stress buffer layer in fiber bending resistance design.

[0035] The fiber performance of Comparative Example 4 showed a slight degradation compared to Example 3, mainly manifested as increased bending loss and slight fluctuations in transmission attenuation, with no interlayer defects. This is closely related to the unreasonable doping method of the transition layer in the preform body. Based on the preform body data, the transition layer of Comparative Example 4 used uniform doping instead of gradient doping. This difference in process led to problems with the fiber refractive index connection. Specifically, uniform doping cannot achieve a continuous and smooth transition of refractive index between the core and cladding layers. There is a sudden change in refractive index at the core-cladding interface, which easily causes mode leakage during optical signal transmission, resulting in a slight increase in transmission attenuation and increased bending loss. At the same time, uniform doping makes the refractive index distribution of the transition layer unreasonable, and it cannot work with the porous layer to achieve the optimal stress buffering and optical transmission synergy effect. Therefore, although the fiber performance is not severely degraded, it is lower than that of the other examples, verifying the important significance of gradient doping process for optimizing fiber optics and bending resistance.

[0036] Compared to Example 3, the fiber performance of Comparative Example 5 showed a significant deterioration, with increased transmission attenuation, a slight increase in bending loss, and interlayer interface separation. This is directly related to the process defects and structural problems of the preform body. Based on the preform body data, Comparative Example 5 used a PSOD solid-state deposition process to prepare a porous layer, resulting in large deviations in core-cladding concentricity and cladding non-circularity, increased hydroxyl content, and poor interlayer bonding. These defects directly led to the deterioration of fiber performance. Mechanistic analysis revealed poor compatibility between the PSOD solid-state process and the transition layer OVD process, resulting in loose interlayer bonding. The drawn fiber was prone to interface separation, which in turn led to microcracks at the interface during bending, exacerbating bending loss. Deviations in core-cladding concentricity and cladding non-circularity affected the total internal reflection path of the optical signal, leading to increased transmission attenuation. The increased hydroxyl content increased infrared absorption loss during optical transmission, further deteriorating optical performance. At the same time, the insufficient uniformity of the porous layer structure in the solid-state deposition resulted in poor stress buffering effect, ultimately leading to a significant deterioration in the overall fiber performance.

[0037] Comparative Example 6 exhibited the most severe performance degradation, with a significant increase in transmission attenuation and bending loss, along with interlayer cracking. This directly corresponds to the severe structural defects in the preform body. Based on the preform body data, Comparative Example 6 employed rapid direct heating sintering without critical temperature holding, resulting in localized devitrification, significant deviations in core-cladding concentricity and cladding non-circularity, a substantial increase in hydroxyl content, and obvious structural defects. The mechanism lies in the excessive thermal stress within the preform caused by rapid heating, leading to microcracks and localized devitrification. These defects are retained during fiber drawing, forming light scattering centers and structural weak points, resulting in a significant increase in transmission attenuation. The microcracks generated by thermal stress in the interlayer further propagate during fiber bending, triggering interlayer cracking and exacerbating bending loss. Simultaneously, rapid heating leads to insufficient hydroxyl removal, resulting in excessively high hydroxyl content and additional infrared absorption loss. The superposition of these multiple defects severely degrades both the optical and bending properties of the fiber, rendering it unsuitable for use. This fully validates the necessity of a stepped heating sintering process for ensuring fiber performance.

[0038] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A bendable silica optical fiber preform based on high deposition efficiency, characterized in that, It includes a core layer, a transition layer, and a cladding layer arranged sequentially from the inside out; The core layer is a ternary system of SiO2-GeO2-Al2O3, wherein the mass fraction of GeO2 is 3-8%, the mass fraction of Al2O3 is 1-3%, the balance is SiO2, and the core layer diameter is 8-15mm. The transition layer is a gradient refractive index structure prepared by an improved external vapor deposition method, and the material is SiO2. B2O3 F-composite quartz glass, with a thickness of 3-6mm, has a refractive index that continuously decreases from the core layer side to the cladding side, with a refractive index difference of 0.01-0.03; The cladding layer comprises an inner porous quartz layer and an outer dense quartz layer. The inner porous quartz layer is prepared by external vapor deposition combined with a low-temperature sintering process, with a porosity of 15-25%, a pore size distribution of 50-200 nm, and a thickness of 5-10 mm. The outer dense quartz layer is prepared by external vapor deposition, with a thickness of 8-12 mm and a hydroxyl content not exceeding 5 × 10⁻⁶. -6 The total diameter of the cladding is 30-50 mm. The core-cladding concentricity error of the preform does not exceed 0.3 μm, and the cladding non-circularity does not exceed 0.2%.

2. The bend-resistant silica optical fiber preform based on high deposition efficiency according to claim 1, characterized in that, The overall B2O3 mass fraction in the transition layer is controlled between 0.5% and 1.8%, with a gradient from 1.8% on the core side to 0.5% on the cladding side. The average B2O3 mass fraction is 1.0% to 1.3%, and the B2O3 mass fraction in any micro-region does not exceed 1.8%, avoiding excessively high local B content that could lead to devitrification. The F mass fraction increases from 0.8% on the core side to 2.5% on the cladding side. By synergistic gradient doping of B2O3 and F, a continuous and smooth transition of refractive index is achieved from 1.466 on the core side to 1.449 on the cladding side.

3. A method for preparing a bend-resistant silica optical fiber preform based on high deposition efficiency as described in any one of claims 1-2, characterized in that, Includes the following steps: S1. Core layer preparation: A porous core preform was formed by depositing SiCl4, GeCl4 and AlCl3 as precursors at a deposition temperature of 1200-1400℃ on the end of a rotating quartz core rod by controlling the carrier gas flow rate. S2. Transition layer preparation: An improved external vapor deposition method was adopted, using SiCl4, BCl3 and CF4 as precursors to deposit a gradient refractive index transition layer on the core surface. By real-time monitoring of the precursor flow rate calibration curve and the unreacted concentration of the tail gas, the flow rate was dynamically adjusted to achieve the gradient distribution of B2O3 and F. The temperature of the deposition area was stabilized at 800-1000℃, and the deposition rate was 3-5 g / min. S3. Cladding preparation: S301. Using external vapor deposition combined with low-temperature sintering, SiCl4 is used as a precursor to deposit an incompletely densified loose quartz layer on the surface of the transition layer at 1000-1200℃. The flame power during the low-temperature deposition stage is controlled by adjusting the flame temperature, torch scanning speed and reactant gas concentration. The sintering process is then used to transform it into a porous quartz layer. S302. Using external vapor deposition, SiCl4 is used as a precursor to deposit a dense quartz layer at 1600-1800℃, with a deposition rate of 5-8 g / min. S4. Sintering treatment: The deposited composite preform was placed in a graphite furnace and sintered at 1700-1900℃ and a vacuum degree ≤10Pa for 2-4 hours. During the sintering process, chlorine gas was introduced for dehydroxylation treatment at a flow rate of 0.5-1L / min. S5, Precision Machining: The sintered preform is subjected to external cylindrical grinding to ensure that the core-cladding concentricity and cladding non-circularity meet the requirements, thus obtaining a bend-resistant silica optical fiber preform.

4. The method for preparing a bend-resistant silica optical fiber preform based on high deposition efficiency according to claim 3, characterized in that, The carrier gas in S1 is He and O2, with a He flow rate of 5-10 L / min and an O2 flow rate of 8-15 L / min. The deposition reaction zone is equipped with a dual-blowtorch structure with a blowtorch spacing of 15-20 cm, a blowtorch scanning speed of 5-8 mm / s, and a deposition rate of 4-6 g / min.

5. The method for preparing a bend-resistant silica optical fiber preform based on high deposition efficiency according to claim 3, characterized in that, The improved external vapor deposition method in S2 adopts a multi-channel nozzle structure with a nozzle spacing of 8-12cm. By optimizing the airflow field of the nozzle array, the precursor is uniformly distributed in the deposition area, ensuring the continuity of gradient doping.

6. The method for preparing a bend-resistant silica optical fiber preform based on high deposition efficiency according to claim 3, characterized in that, In the S301, the flame temperature is 800-900℃, the torch scanning speed is 10-15mm / s, the SiCl4 flow rate is 5-8L / min, the O2 flow rate is 10-12L / min, and the flame power during the low-temperature deposition stage is controlled at 1500-1800W. During the preparation of the porous quartz layer, the flow rate of the auxiliary gas Ar was adjusted to 3-5 L / min to further optimize the powder deposition trajectory, ensure uniform pore distribution, and avoid the formation of an anomalous structure with a dense interior and a porous exterior.

7. The method for preparing a bend-resistant silica optical fiber preform based on high deposition efficiency according to claim 3, characterized in that, The sintering process in S4 adopts a stepped heating method with a heating rate of 5-10℃ / min. It is held at 1200℃ and 1500℃ for 30 minutes respectively to avoid stress concentration and devitrification of the transition layer caused by sudden temperature changes.

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