CN112441736B - Optical fiber preform, preparation method thereof and plasma deposition equipment - Google Patents

Abstract

The application provides an optical fiber preform, a preparation method thereof and plasma deposition equipment, wherein fluorine doping distribution of an inner cladding gradual change type is formed by fluorine doping sintering, so that the preparation process difficulty of viscosity matching among a core layer, the inner cladding and an optical cladding can be solved, and particularly the viscosity matching on the boundary between the core layer and the optical cladding is realized; meanwhile, the constraint diffusion distribution of each layer of fluoride in the powder rod is realized, and the refractive index requirement of each layer is ensured; the stress problem of the fluorine-doped layer with large thickness can be effectively solved by synchronously carrying out fluorine-doped deposition and stress relief, and the phenomenon that the thicker the fluorine-doped layer is, the more easily the fluorine-doped layer is cracked is avoided.

Description

Optical fiber preform and preparation method thereof, plasma deposition equipment

Technical field

The invention relates to the field of optical communication technology, and in particular, to an optical fiber preform, a preparation method thereof, and plasma deposition equipment.

Background technique

In future 400G and above transmission systems, reducing optical fiber loss and obtaining a large effective area is one of the important issues in the field of optical fiber manufacturing. For quartz optical fiber, the attenuation between 600nm and 1600nm mainly comes from Rayleigh scattering. The attenuation a _R caused by Rayleigh scattering can be calculated by the following formula: a _R =R/λ ⁴ +B. Among them, λ is the wavelength, R is the Rayleigh scattering coefficient (dB/km/μm ⁴ ), and B is the corresponding constant.

In order to reduce fiber loss, the most important process is to reduce the core layer doped with germanium or pure silicon core design. By reducing the fiber doping concentration, the Rayleigh scattering of the fiber can be effectively reduced. However, the Rayleigh scattering R of the optical fiber is not only affected by the doping concentration Rc, but also affected by the density fluctuation Rd. Its expression is R=Rc+Rd. The pure silicon core design used in the traditional process can easily cause a viscosity mismatch between the core layer and the cladding layer, causing density fluctuations. It is necessary to reduce the germanium doping of the core layer and improve the viscosity matching between the core layer and the cladding layer to possibly reduce the density. Fiber loss.

In order to obtain a large effective area, the main method is to reduce the refractive index of the core layer and increase the core diameter. However, although simply reducing the refractive index of the core layer and increasing the core diameter can increase the effective area of the optical fiber, it comes with the cutoff. The increase in wavelength and the deterioration of fiber attenuation and bending performance cause the fiber to exceed relevant indicators. Moreover, if a pure silicon core design is adopted, the refractive index of the core layer cannot be reduced.

Contents of the invention

In view of the above, it is necessary to provide an ultra-low loss large effective area optical fiber preform.

The technical solution provided by the invention is: a preparation method of optical fiber preform, including the following steps:

Form a core layer, an inner cladding layer, and an optical cladding layer mainly composed of silicon dioxide in sequence to obtain a powder rod, wherein the core layer also includes germanium dioxide generated by the reaction;

The powder rod is sequentially subjected to three stages of dehydroxylation, sintering and vitrification to form a glass rod in which fluoride is constrained to diffuse in the core layer, inner cladding and optical cladding. Since entering the sintering stage, fluorine is introduced. The flow rate of fluoride gas increases linearly, and then enters the vitrification stage. The flow rate of fluoride gas gradually decreases until it becomes zero when the vitrification stage is completed;

Extend the glass rod to a target radius, and use a plasma deposition process and a stress relief process to deposit a fluorine-doped layer on its surface to obtain a fluorine-doped glass rod;

An outer cladding is formed on the outer layer of the fluorine-doped glass rod to obtain a transparent optical fiber preform.

Further, in the step of sequentially performing three stages of dehydroxylation, sintering and vitrification on the powder rod to form a glass rod in which fluoride is constrained to diffuse in the core layer, inner cladding and optical cladding, The fluoride content in the glass rod is the least in the core layer, the most in the optical cladding layer and evenly distributed, and the fluoride content in the inner cladding layer gradually increases from the outer layer of the core layer. Increase the fluoride content until the inner layer of the optical cladding; the fluoride includes one or at least one of SiF ₄ , CF ₄ , SF ₆ , C ₂ F ₆ , SOF ₂ , C ₂ F ₂ Cl ₂ Two combinations.

Further, the temperature in the dehydroxylation stage is controlled at 1200~1250℃; entering the sintering stage, the dehydroxylation stage temperature is used as the starting temperature, and the temperature rises to 1320℃~1450℃ at a heating rate of 0.5~5℃/min, in which fluoride The gas flow rate increases linearly at 5 to 25 cc/min until the end of the sintering stage; it enters the vitrification stage, maintaining the temperature at the end of the sintering stage, and the constant temperature time is 1 to 3 hours.

Further, in the step of sequentially forming a core layer, an inner cladding layer, and an optical cladding layer mainly composed of silicon dioxide to obtain a powder rod, wherein the core layer also includes germanium dioxide generated by the reaction, the core layer is formed. The reaction gases include oxygen, hydrogen, silicon tetrachloride, germanium tetrachloride, and Ar gas, in which the flow rate of germanium tetrachloride is controlled at 50-200cc/min.

Further, in the step of sequentially forming a core layer, an inner cladding layer, and an optical cladding layer mainly composed of silica to obtain a powder rod, wherein the core layer also includes germanium dioxide generated by the reaction, the inner cladding layer is formed. The reaction gas includes oxygen, hydrogen, silicon tetrachloride, and Ar gas. The flow rate of silicon tetrachloride is controlled at 4g/min ~ 12g/min, and the density of the silica powder generated by the reaction is controlled at 0.5 ~ 1.5g/cm ^3. The thickness of the inner cladding is 1/2 to 1/8 of the core radius.

Further, in the step of sequentially forming a core layer, an inner cladding layer, and an optical cladding layer mainly composed of silicon dioxide to obtain a powder rod, wherein the core layer also includes germanium dioxide generated by the reaction, an optical cladding layer is formed. The reaction gases include oxygen, hydrogen, silicon tetrachloride, and Ar gas. The flow rate of silicon tetrachloride is controlled at 20g/min~40g/min, and the density of the silica powder generated by the reaction is controlled at 0.2~0.6g/min. cm ³ , the total thickness of the optical cladding and inner cladding is 0.5 to 5.0 times the core radius.

Further, in the step of extending the glass rod to the target radius, using a plasma deposition process and a stress relief process to deposit a fluorine-doped layer on its surface to obtain a fluorine-doped glass rod, the plasma deposition process is carried out through a POD blowtorch. The surface of the glass rod is sprayed back and forth with fluorine-containing gas and deposited layer by layer; the fluorine-containing gas includes silicon tetrachloride, oxygen and fluoride; the fluoride includes SiF ₄ , CF ₄ , SF ₆ and C ₂ F ₆ , SOF ₂ , C ₂ F ₂ Cl ₂ or one or at least two combinations; the stress relief process is that when spraying fluorine-containing gas, a mixed gas of oxygen and nitrogen is sprayed sideways of the fluorine-containing gas in the same direction to eliminate Glass stress.

Further, the POD blowtorch translation speed variable △V is -0.1~-0.3m/min; the deposition thickness variable △C is 5~10mm, the initial translation speed is 1m/min, the initial rod diameter is 30mm, and the minimum translation speed is not less than 0.1m /min.

Further, the step of forming an outer cladding on the outer layer of the fluorine-doped glass rod to obtain a transparent optical fiber preform includes using a vapor deposition process to deposit an outer cladding on the outer layer of the fluorine-doped glass rod, and then sintering , a transparent optical fiber preform is obtained.

Further, the step of forming an outer cladding on the outer layer of the fluorine-doped glass rod to obtain a transparent optical fiber preform includes directly loading the fluorine-doped glass rod into a silica sleeve and assembling it into an optical fiber preform.

The present invention also provides an optical fiber preform, which is formed by the preparation method of the optical fiber preform. The optical fiber preform includes coaxially arranged: from the inside to the outside:

The middle core layer has a radius of 4 to 6 μm and a refractive index of 0.15 to 0.25% relative to silicon dioxide;

The inner structural cladding has a radius of 4.5-7.5 μm and a gradient distribution relative to the refractive index of silicon dioxide;

Optical structure layer, radius 10~25μm, refractive index relative to silicon dioxide -0.05~-0.25%;

Fluorine-doped structural layer, radius 20~30μm, refractive index relative to silicon dioxide -0.4~-0.6%;

The outer cladding has a radius greater than or equal to 60 μm and a refractive index of 0.

The present invention also relates to a plasma deposition equipment for depositing a fluorine-doped layer on the surface of a glass rod. The equipment includes a POD blowtorch group. The POD blowtorch group includes a main blowtorch and several stress-relieving blowtorches arranged side by side. The main blowtorch The blowtorch is used to spray and deposit silicon tetrachloride, oxygen and fluoride on the surface of the glass rod; the stress-removing blowtorch is used to introduce oxygen and nitrogen to remove glass stress.

Further, the POD blowtorch group includes two stress-relieving blowtorches, the two stress-relief blowtorches are located on both sides of the main blowtorch, the three blowtorches are arranged side by side along the translation direction, and the exits of the three blowtorches are at a distance from the glass The spacing of the rods is consistent.

Further, the distance between the outlet of the main blowtorch or the stress relief blowtorch and the surface of the glass rod is no greater than the height of the jet flame.

Further, the distance between the outlet of the main blowtorch or the stress relief blowtorch and the surface of the glass rod is half of the height of the jet flame.

Further, the axial distance between the main blowtorch and the stress relief blowtorch is less than or equal to half of the sum of the widths of the two jet flames.

Further, the main blowtorch is equipped with multiple side-by-side pipes, and the pipes are used to respectively introduce silicon tetrachloride, oxygen and fluoride to react to form deposited fluorine-doped silica powder; the stress relief blowtorch is equipped with Several pipelines are used to introduce oxygen and nitrogen.

Compared with the existing technology, this application can solve the difficulty in the preparation process of viscosity matching between the core layer, the inner cladding layer, and the optical cladding through the gradient fluorine doping distribution of the inner cladding layer, especially the viscosity matching at the boundary between the core layer and the optical cladding layer. ; At the same time, the constrained diffusion distribution of fluoride in each layer of the powder rod is achieved to ensure the refractive index requirements of each layer; by synchronously carrying out fluorine-doped deposition and stress relief, the stress problem of large-thickness fluorine-doped layers can be effectively solved and fluorine-doped layers can be avoided. The thicker the layer, the easier it is to crack.

Description of the drawings

The present invention will be described in further detail below with reference to the accompanying drawings and specific embodiments.

Figure 1 is a flow chart of the preparation of optical fiber preform in one embodiment of the present invention.

Figure 2 is a schematic diagram of the powder rod deposition equipment used in the present invention.

FIG. 3 is a schematic end view of the upper deposition chamber shown in FIG. 2 .

Figure 4 is a cross-sectional view of the fluorine doping amount and refractive index of each layer of the glass rod of the present invention.

Figure 5 is a schematic diagram of furnace temperature control during the sintering stage of the present invention.

Figure 6 is a schematic diagram of the fluorine doping amount control during the dehydroxylation, sintering, and vitrification processes of the powder rod of the present invention.

Figure 7 is a schematic structural diagram of the plasma deposition equipment of the present invention.

Figure 8 is a schematic cross-sectional structural diagram of the optical fiber preform of the present invention.

Figure 9 is a schematic cross-sectional view of the refractive index of the optical fiber preform of the present invention.

Figure 10 is a schematic diagram of the fluctuation of the diameter of the powder rod under different air intake modes.

Figure 11 shows the attenuation performance test chart of optical fiber under different air intake methods.

Explanation of reference signs:

Powder rod deposition equipment 10

Core blowtorch 101

Inner cladding blowtorch 102

Optical cladding blowtorch 103

Deposition chamber 104

Boom 105

target stick 106

Powder stick 107

Upper deposition chamber 108

Upper deposition chamber inner layer 108a

Upper deposition chamber outer layer 108b

Upper deposition chamber end cap 108c

Plasma deposition equipment 30

glass rod 301

POD blowtorch set 303

Main blowtorch 3031

The first stress relief blowtorch 3032

Second stress relief blowtorch 3033

Optical fiber preform 50

Middle core layer 501

Internal structural cladding 503

Optical structure layer 505

Fluorine-doped structural layer 507

Outer cladding 509

The following detailed description will further illustrate the embodiments of the present invention in conjunction with the above-mentioned drawings.

Detailed ways

In order to more clearly understand the above objects, features and advantages of the embodiments of the present invention, the present invention will be described in detail below in conjunction with the accompanying drawings and specific implementation modes. It should be noted that, as long as there is no conflict, the features in the embodiments of the present application can be combined with each other.

Many specific details are set forth in the following description to facilitate a full understanding of the embodiments of the present invention. The described embodiments are only some, but not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the scope of protection of the embodiments of the present invention.

"Stress" in this article refers to the stress caused by micro-inhomogeneous areas formed due to uneven composition, also called structural stress or micro-stress.

"POD" in this article refers to plasma outside deposition, the full English name is plasma outside deposition, or POD for short.

"VAD" in this article refers to axial vapor deposition, the full English name is: Vapor Axial Deposition, or VAD for short.

“OVD” in this article refers to the external vapor deposition method, the full English name is: Outside Vapor Deposition, or OVD for short.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which embodiments of the invention belong. The terms used herein in the description of the present invention are only for the purpose of describing specific embodiments and are not intended to limit the embodiments of the present invention.

Please refer to Figure 1, which is a flow chart for preparing an optical fiber preform in a specific embodiment of the present invention, which includes the following steps:

Step S1: Form a core layer r01 mainly composed of silicon dioxide, an inner cladding layer r02, and an optical cladding layer r03 in sequence to obtain a powder rod, wherein the core layer also includes germanium dioxide generated by the reaction.

In a specific embodiment, the reaction gas forming the core layer includes oxygen, hydrogen, silicon tetrachloride, germanium tetrachloride, and Ar gas, wherein the inflow flow rate of germanium tetrachloride is controlled at 50-200 cc/min. The reaction gases that form the inner cladding include oxygen, hydrogen, silicon tetrachloride, and Ar gas. The flow rate of silicon tetrachloride is controlled at 4g/min~12g/min, and the density of the silica powder generated by the reaction is controlled at 0.5~ 1.5g/cm ³ , the thickness of the inner cladding is 1/2 to 1/8 of the core radius, that is, (r02-r01)/r01. The reaction gases that form the optical cladding include oxygen, hydrogen, silicon tetrachloride, and Ar gas. The flow rate of silicon tetrachloride is controlled at 20g/min ~ 40g/min, and the density of the silica powder generated by the reaction is controlled at 0.2 ~0.6g/cm ³ , the total thickness of the optical cladding and inner cladding is 0.5 to 5.0 times the radius of the core layer, that is, (r03-r01)/r01, preferably 1.5-3.0 times. In this step, the reaction gases can be introduced separately or mixed gases are introduced, and the above raw materials react at high temperature in the flame to generate silica particles or germanium dioxide and silica particles, such as oxygen, hydrogen, silicon tetrachloride, tetrachlorine The flow ratio of germanium and Ar gas can be (1-3):(2-5):3:(0.15-0.3):(1-1.5), the flow ratio of oxygen, hydrogen, silicon tetrachloride and Ar gas It can be (1-3):3:3:(1-1.5); in this step, the thickness and density of the deposited inner cladding and optical cladding are optimized to achieve a combination of powder layers distributed in different density areas. The layer composition is similar, and the core layer is as little doped as possible. During the sintering process, the viscosity of each powder layer is adjusted to a high degree of matching, which is not suitable to cause density fluctuations. It also achieves a near-pure silicon core design or a low-doped germanium core layer design, thereby reducing the risk of It improves scattering and reduces the loss of the final optical fiber.

Step S2: The powder rod is sequentially subjected to three stages of dehydroxylation, sintering, and vitrification to form a glass rod in which fluoride is constrained to diffuse in the core layer, inner cladding and optical cladding. Since entering the sintering stage, Fluoride gas is introduced and its flow rate increases linearly, and then enters the vitrification stage. The flow rate of fluoride gas gradually decreases until it becomes zero when the vitrification stage is completed. The fluoride gas refers to a gas including one or at least two combinations of SiF ₄ , CF ₄ , SF ₆ , C ₂ F ₆ , SOF ₂ and C ₂ F ₂ Cl ₂ .

Please refer to Figure 4 together. Based on the optimized design of thickness and density, combined with the linear sintering fluorine doping process, the fluoride content in the glass rod is the least in the core layer and the most in the optical cladding. Evenly distributed in the inner cladding layer, the fluoride content in the outer layer of the core layer gradually increases until the fluoride content in the inner layer of the optical cladding layer, that is, the fluoride content in the core layer, Constrained diffusion within the inner cladding and optical cladding; at the same time, it prevents uncontrolled diffusion of fluoride into the core layer, resulting in a decrease in the refractive index of the core layer, which affects the refractive index requirements of the core layer and optical cladding. The fluoride includes one or at least two combinations of SiF ₄ , CF ₄ , SF ₆ , C ₂ F ₆ , SOF ₂ , C ₂ F ₂ Cl ₂ .

Please refer to Figure 5 and Figure 6 together. In the dehydroxylation stage, the temperature is controlled at 1200~1250℃; when entering the sintering stage, the temperature in the dehydroxylation stage is used as the starting temperature and rises to 1320℃ at a heating rate of 0.5~5℃/min. ~1450℃, in which the fluoride gas increases linearly at a flow rate of 5~25cc/min until the end of the sintering stage; enters the vitrification stage, maintains the temperature at the end of the sintering stage, and keeps the constant temperature for 1~3 hours.

Through the above process, the fluorine doping requirements in the optical cladding and the gradual distribution of fluoride in the inner cladding are achieved, which plays a good transitional role between the core layer and the optical cladding. Effective viscosity matching between optical claddings. Traditional ultra-low loss large effective area optical fibers all use sag-assisted design methods, and the energy distribution in the optical fiber is Gaussian distribution. The present invention can effectively increase the mode field diameter of the optical fiber and increase the effective area of the optical fiber by changing the refractive index structure of the core layer. There is no need to blindly reduce the core refractive index and increase the core diameter.

Step S3: Extend the glass rod to a target radius, and use a plasma deposition process and a stress relief process to deposit a fluorine-doped layer on its surface to obtain a fluorine-doped glass rod.

Please refer to Figure 7 together. In a specific embodiment, the plasma deposition process sprays fluorine-containing gas back and forth on the surface of the glass rod through a POD blowtorch, depositing layer by layer; the fluorine-containing gas includes silicon tetrachloride, oxygen and fluoride; the fluoride includes one or at least two combinations of SiF ₄ , CF ₄ , SF ₆ , C ₂ F ₆ , SOF ₂ , C ₂ F ₂ Cl ₂ ; the stress relief process is spraying containing When fluorine gas is used, a pipeline is provided on the side of the plate where the POD blowtorch sprays fluorine-containing gas. The outlet of the pipeline sprays a mixed gas of oxygen and nitrogen in the same direction to eliminate glass stress. The POD blowtorch translation speed variable ΔV is -0.1~-0.3m/min; the deposition thickness variable ΔC is 5~10mm, the initial translation speed is 1m/min, the initial rod diameter is 30mm, and the minimum translation speed is not less than 0.1m/min. Through POD non-constant speed deposition and online stress relief process, the phenomenon of easy cracking caused by stress concentration in large-thickness fluorine-doped layers prepared by POD can be eliminated. The design of the deeply fluorine-doped recessed layer is beneficial to improving the bending resistance of the optical fiber.

Step S4: Form an outer cladding on the outer layer of the fluorine-doped glass rod to obtain a transparent optical fiber preform.

In a specific implementation, step S4 may use a vapor deposition process to deposit an outer cladding on the outer layer of the fluorine-doped glass rod, and then sinter it to obtain a transparent optical fiber preform. In step S4, the fluorine-doped glass rod can also be directly put into a silica sleeve to assemble into an optical fiber preform.

The optical fiber preform 50 formed by the above preparation method is shown in Figures 8 and 9. The optical fiber preform includes coaxially arranged from the inside to the outside:

The middle core layer 501 has a radius r1 = 4 to 6 μm, and a refractive index Δn1 of silicon dioxide of 0.15 to 0.25%;

The inner structural cladding 503 has a radius r2 = 4.5-7.5 μm and a gradient distribution relative to the refractive index Δn2 of silicon dioxide;

The optical structure layer 505 has a radius r3 = 10 to 25 μm and a refractive index Δn3 of silicon dioxide of -0.05 to -0.25%;

The fluorine-doped structural layer 507 has a radius r4 = 20 to 30 μm, and the refractive index Δn4 of silicon dioxide is -0.4 to -0.6%;

The outer cladding 509 has a radius r5 greater than or equal to 60 μm and a refractive index Δn5 of 0.

The powder rod deposition equipment 10 used in step S1 of the present invention will be described in detail below with reference to FIG. 2 .

The equipment includes a target rod 106, a deposition chamber 104, an optical cladding burner 103, an inner cladding burner 102, a core burner 101, a boom 105 and an upper deposition chamber 108. Among them, an upper deposition chamber 108 is provided on the upper part of the deposition chamber 104, and a suspension rod 105 is installed in the upper deposition chamber 108. The suspension rod 105 is provided with a hook, the suspension rod 105 is connected to a lifting mechanism, and the target rod 106 is suspended on the lifting mechanism. On the hook of the suspension rod 105, an optical cladding blowtorch 103, an inner cladding blowtorch 102 and a core layer blowtorch 101 are installed in sequence on the lower side of the deposition chamber 104. These blowtorches (103, 102, 101) spray airflow toward the target rod 106, thereby The powder reacts layer by layer and adheres to the target rod 106 . In a specific implementation, the upper deposition chamber 108 is divided into two inner and outer chambers, namely the upper deposition chamber inner layer 108a and the upper deposition chamber outer layer 108b, and an upper deposition chamber end cover 108c is provided at its end (as shown in Figure 3 ). The outer layer 108b of the upper deposition chamber is mainly filled with external gas, the inner layer 108a of the upper deposition chamber is mainly used to accommodate the lifting space of the powder rod, and the end cover 108c of the upper deposition chamber is used to seal the powder rod accommodating space to prevent airflow from entering. In this way, the upper deposition chamber 108 is divided into two chambers, the inner and outer chambers, which effectively separates the powder rod holding space and the gas entry chamber, and avoids the decrease in the space for gas injection in the upper deposition chamber as the diameter of the powder rod increases. The cavity pressure fluctuation caused causes the rod diameter of the powder rod to fluctuate. The above structural design can effectively improve the rod diameter fluctuation of the powder rod.

The deposition process of the powder rod: oxygen, hydrogen, silicon tetrachloride, germanium tetrachloride, and Ar gas are introduced into the core layer blowtorch 101, and silicon dioxide is formed through a high-temperature reaction. The germanium dioxide adheres to the end surface of the target rod, forming a certain Density loose core. The silicon dioxide layer with a certain thickness surrounding the surface of the core layer is the inner cladding layer, and oxygen, hydrogen, silicon tetrachloride, and Ar gas are introduced into the inner cladding blowtorch 102. The silicon dioxide layer with a certain thickness surrounding the surface of the inner cladding is the optical cladding. Oxygen, hydrogen, silicon tetrachloride, and Ar gas are introduced into the optical cladding blowtorch 103, and the deposition stops after the powder is deposited to a set length. During the access process, the thickness and density of the inner cladding and optical cladding can be controlled by controlling the silicon tetrachloride flow rate, hydrogen and oxygen flow ratio, etc. of the inner cladding and optical cladding blowtorches (102, 103).

The plasma deposition equipment 30 used in step S3 of the present invention will be described in detail below with reference to FIG. 7 .

The equipment 30 is used to deposit a fluorine-doped layer on the surface of the glass rod 301, and includes a POD blowtorch group 303 and a POD machine (not shown) carrying the glass rod. The POD machine regulates the rotation of the glass rod around the axis of the glass rod; The POD blowtorch group 303 includes a main blowtorch 3031 arranged side by side and several stress relief blowtorches. The main blowtorch 3031 is used to spray and deposit the introduced silicon tetrachloride, oxygen and fluoride on the surface of the glass rod, and It can be sprayed back and forth; the stress-removing blowtorch is used to introduce oxygen and nitrogen to remove glass stress. The above different air flows can be provided with separate pipes or separate pipelines inside the blowtorch, or multiple air flows can be introduced through the same pipe or pipeline. As shown in Figure 7, the glass rod is clamped horizontally in the plasma deposition equipment 30. Three POD blowtorches are arranged side by side on the lower side of the equipment 30, namely the first stress-relieving blowtorch 3032, the main blowtorch 3031 and the second stress-relief blowtorch 3033. , the three blowtorches (3032, 3031, 3033) can move horizontally synchronously, and the distance between the exits of the three blowtorches and the glass rod is the same. Usually, the distance between the exit of the POD blowtorch group and the surface of the glass rod is not greater than the height of the jet flame, preferably the height of the jet flame. At half position, the axial distance between the stress relief blowtorch and the main blowtorch shall not be greater than half of the sum of the widths of the two jet flames, that is, the jet flames of adjacent blowtorches overlap. In other embodiments, the number of POD blowtorches is not limited to 3, and the number of stress-relieving blowtorches can also be 1 or more than 2; the distance between the exits of the multiple blowtorches and the glass rods depends on the process parameters and is not limited to Same; the deposition equipment can also be set up vertically or tilted, as long as the blowtorch can deposit powder on its surface. There is no limit here, and it needs to be set according to the actual process requirements and product performance needs.

The deposition process of the fluorine-doped layer: As shown in Figure 7, the glass rod 301 is placed on the POD machine table, and the POD blowtorch group 303 sprays back and forth on the surface of the rod to deposit layer by layer. SiCl ₄ , O ₂ , and fluoride are introduced into the main blowtorch 3031 of the blowtorch group 303 to form a fluorine-containing glass layer. According to the fluorine-doped design flow rate, deep fluorine-doped concave layers of different depths are formed. The design of the deeply fluorine-doped concave layer is beneficial to Improve the bending resistance of optical fiber. Mixed gas flows of O ₂ and N ₂ are introduced into the first stress relief burner 3032 and the second stress relief burner 3033 on both sides respectively to remove glass stress. The POD blowtorch translation speed variable ΔV is -0.1~-0.3m/min; the deposition thickness variable ΔC is 5~10mm, the initial translation speed is 1m/min, the initial rod diameter is 30mm, and the minimum translation speed is not less than 0.1m/min. Through POD non-constant speed deposition and online stress relief process, the phenomenon of easy cracking caused by stress concentration in large-thickness fluorine-doped layers prepared by POD can be eliminated.

The following is a comparative analysis of the process of forming the optical fiber preform 50 and its performance using the method of the present invention with reference to specific embodiments and comparative examples.

Example 1:

First, the core layer, inner cladding layer and optical cladding layer are prepared using the VAD vapor deposition process. During the deposition process, GeCl ₄ gas is passed into the core layer, and the flow rate is controlled at 50cc/min; the SiCl ₄ flow rate in the inner cladding layer is controlled at 4g/min, and the powder density Control it at 1.3g/cm ³ ; the SiCl ₄ flow rate in the optical cladding is controlled at 20g/min, and the powder density is controlled at 0.6g/cm ³ .

The powder rod after deposition is dehydroxylated, sintered, and vitrified in a sintering furnace. First, the dehydroxylation temperature T1 is controlled at 1200°C; after the dehydroxylation is completed, it is raised to 1320°C (T2) at a heating rate of 1°C/min, and CF ₄ gas is introduced at a linear increase rate of 5cc/min until the sintering stage. End; after rising to T2 temperature, it enters the vitrification constant temperature stage. The constant temperature time is 1 hour. The powder rod is further sintered into a transparent glass body. The fluorine doping flow rate gradually decreases to zero over time.

After extending the glass rod prepared above to the target rod diameter, a deeply fluorine-doped recessed layer is deposited using a plasma deposition process (POD). Place the glass rod on the POD machine, and spray the POD blowtorch back and forth on the surface of the glass rod, depositing it layer by layer. SiCl ₄ , O ₂ , and CF ₄ are introduced into the main blowtorch of the blowtorch group to form a fluorine-containing glass fluorine-doped layer. O ₂ and N ₂ are introduced into the stress-relieving blowtorches on both sides to remove glass stress. The starting speed is 1m/min, and the initial glass rod diameter is 30mm. When the rod diameter is deposited to 35mm, the translation speed of the blowtorch group is controlled to 0.9m/min. For every 5mm increase in thickness, the translation speed is reduced by 0.1m/min, and the minimum translation speed is Not less than 0.1m/min, and so on, until the diameter reaches the target rod diameter of 58mm.

Use the OVD vapor phase synthesis process to deposit the pure silicon outer cladding on the above-mentioned fluorine-doped glass rod layer by layer. After reaching the target weight or rod diameter, the deposition is completed, and then sintering is performed. The powder rod is prepared into a transparent glass rod, that is, low loss and high efficiency are achieved. The effective area of the optical fiber preform is 50.

Refractive index profile characteristics: middle core layer 501: △n1=0.16%, r1=4.2μm; inner structural cladding layer 503 is a fluorine-doped transition zone, r2=5.6μm; optical structure layer 505: Δn3=-0.20%, r3 =12 μm; fluorine-doped structural layer (hereinafter also referred to as deeply fluorine-doped recessed layer) 507: Δn4=-0.42%, r4=28 μm; the outer cladding 509 is a pure silicon dioxide layer. Since the outer cladding 509 can also be formed by the casing process, the outer cladding deposited before and after sintering and the outer cladding of the encased quartz tube are both the outer cladding 509.

The rod diameter (diameter) of the optical fiber preform can reach 122mm (2*r5). After the optical fiber is drawn, the test results: the effective area of the optical fiber = 128 μm ² , the attenuation of 1550nm is 0.173dB/km, the bending radius R = 10mm*1 circle, the bending losses of 1550nm and 1625nm are 0.08dB and 0.18dB respectively, and the cable wavelength is 1470nm.

Example 2:

First, the core layer, inner cladding layer and optical cladding layer are prepared using the VAD vapor deposition process. During the deposition process, GeCl ₄ gas is passed into the core layer, and the flow rate is controlled at 100cc/min; the SiCl ₄ flow rate in the inner cladding layer is controlled at 8g/min, and the powder density Control it at 0.9g/cm ³ ; the SiCl ₄ flow rate in the optical cladding is controlled at 30g/min, and the powder density is controlled at 0.4g/cm ³ .

The powder rod after deposition is dehydroxylated, sintered and vitrified in a sintering furnace. First, the dehydroxylation temperature T1 is controlled at 1200°C; after the dehydroxylation is completed, the temperature rises to 1400°C (T2) at a heating rate of 3°C/min, while the SiF ₄ gas linearly increases at a flow rate of 12cc/min until the end of the sintering stage; After rising to 1400°C, it enters the vitrification constant temperature stage. The constant temperature time is 2 hours. The powder rod is further sintered into a transparent glass body. The fluorine doping flow rate gradually decreases to zero over time.

After the glass rod prepared above is extended to the target rod diameter, a fluorine-doped layer (that is, a deeply fluorine-doped recessed layer) is deposited using a plasma deposition process (POD). Place the glass rod on the POD machine, and spray the POD blowtorch back and forth on the surface of the glass rod, depositing it layer by layer. SiCl ₄ , O ₂ , and SiF ₄ are introduced into the main blowtorch in the blowtorch group to form a fluorine-containing glass fluorine-doped layer. O ₂ and N ₂ are introduced into the stress-relieving blowtorches on both sides to remove glass stress. The starting speed is 0.9m/min, and the initial glass rod diameter is 35mm. When the rod diameter is deposited to 43mm, the translational speed of the blowtorch group is controlled to 0.7m/min. For every 8mm increase in thickness, the translational speed is reduced by 0.2m/min. The minimum The translation speed is not less than 0.1m/min, and so on, the diameter reaches the target rod diameter of 55mm.

The above-mentioned fluorine-doped glass rod is extended to the target rod diameter and assembled with a pure silica quartz glass sleeve to form a low-loss, large-effective-area optical fiber preform 50 .

Refractive index profile characteristics: middle core layer 501: △n1=0.21%, r1=5.0μm; inner structural cladding 503 is a fluorine-doped transition zone, r2=6.2μm; optical structure cladding 505: Δn3=-0.15%, r3=16μm; fluorine-doped structural layer 507: Δn4=-0.47%, r4=25μm; outer cladding 509: pure silicon dioxide layer.

The diameter of the optical fiber preform can reach 125mm. After the optical fiber is drawn, the test results: the effective area of the optical fiber = 123 μm ² , the attenuation of 1550nm is 0.169dB/km, the bending radius R = 10mm*1 circle, the bending losses of 1550nm and 1625nm are 0.06dB and 0.15dB respectively, and the cable wavelength is 1510nm.

Example 3:

First, the core layer, inner cladding layer and optical cladding layer are prepared using the VAD vapor deposition process. During the deposition process, GeCl ₄ gas is passed into the core layer, and the flow rate is controlled at 150cc/min; the SiCl ₄ flow rate in the inner cladding layer is controlled at 12g/min, and the powder density Control it at 0.6g/cm ³ ; the SiCl ₄ flow rate in the optical cladding is controlled at 40g/min, and the powder density is controlled at 0.25g/cm ³ .

The powder rod after deposition is dehydroxylated, vitrified and sintered in a sintering furnace. First, the dehydroxylation temperature T1 is controlled at 1250°C; after the dehydroxylation is completed, it is raised to 1450°C (T2) at a heating rate of 5°C/min, and at the same time, the flow rate of SF ₆ gas is increased linearly at 20cc/min until the end of the sintering stage; After rising to 1450°C, the sintering ends and enters the vitrification constant temperature stage. The constant temperature time is 3 hours. The powder rod is further sintered into a transparent glass body. The fluorine doping flow rate gradually decreases to zero over time.

After extending the glass rod prepared above to the target rod diameter, a deeply fluorine-doped recessed layer is deposited using a plasma deposition process (POD). Place the glass rod on the POD machine, and spray the POD blowtorch back and forth on the surface of the glass rod, depositing it layer by layer. SiCl ₄ , O ₂ , and SF ₆ are introduced into the main blowtorch in the blowtorch group to form a fluorine-containing glass layer. O ₂ and N ₂ are introduced into the stress-relieving blowtorches on both sides to remove glass stress. The starting speed is 0.8m/min, and the initial glass rod diameter is 40mm. When the rod diameter reaches 50mm, the translation speed of the blowtorch group is controlled to 0.5m/min. For every 10mm increase in thickness, the translation speed is reduced by 0.3m/min, and the minimum translation is The speed is not less than 0.1m/min, and so on, the diameter reaches the target rod diameter of 60mm.

Use the OVD vapor phase synthesis process to deposit the pure silicon outer cladding on the above-mentioned fluorine-doped glass rod layer by layer. After reaching the target weight or rod diameter, the deposition is completed and then sintered to prepare a transparent glass rod, that is, a low-loss large effective area optical fiber is completed. Preform 50.

Refractive index profile characteristics: middle core layer 501: △n1=0.24%, r1=6.0μm; inner structural cladding 503 is a fluorine-doped transition zone, r2=7.3μm; optical structure layer 505: Δn3=-0.08%, r3 =20μm; deeply fluorine-doped recess 507: △n4=-0.54%, r4=30μm; the outer cladding 509 is a pure silicon dioxide layer.

The rod diameter of optical fiber preform can reach 135mm. After the optical fiber is drawn, the test results: the effective area of the optical fiber = 114μm ² , the attenuation of 1550nm is 0.171dB/km, the bending radius R = 10mm*1 circle, the bending losses of 1550nm and 1625nm are 0.04dB and 0.09dB respectively, and the cable wavelength is 1525nm.

Comparative example 1:

The preparation process and parameter settings of this example are basically the same as those in Example 3. The difference is that in step S1, a conventional VAD vapor deposition chamber is used for deposition, that is, the airflow enters the lower chamber from the upper powder chamber and is in the same place as the powder. room. During the deposition process, GeCl ₄ gas is passed into the core layer, and the flow rate is controlled at 150cc/min; the SiCl ₄ flow rate in the inner cladding layer is controlled at 12g/min, and the powder density is controlled at 0.58g/cm ³ ; the SiCl ₄ flow rate in the optical cladding layer is controlled at 40g /min, and the powder density is controlled at 0.26g/cm ³ .

For powder rods formed with different air intake methods, we prepared multiple sets of samples using the process conditions of Example 3 and Comparative Example 1, and tested the thickness of the optical cladding and inner cladding in each sample and the respective core layer rods. The magnification of diameter, in which the rod diameters of all core layers are consistent, as shown in Figure 10. It can be seen from the figure that the rod diameters of the powder rods using the air intake method of the present invention are basically the same, and the results of the 22 groups of samples remain between 2.3-2.4 times, with very small fluctuations; while using the conventional air intake method of Comparative Example 1, the diameter of the powder rods The rod diameter fluctuates greatly, ranging from ^2.5+ times to ^2.1+ times. 15 samples are lower than 2.3 times, and 7 samples are higher than 2.3 times, which is very unstable. Therefore, the results show that the air intake method of the present invention helps to reduce the fluctuation of the diameter of the powder rod, making it easier to realize the design and control of thickness and density.

Refractive index profile characteristics: middle core layer △n1'=0.24%, r1'=6.1μm; inner structural cladding is fluorine-doped transition zone, r2'=7.3μm; optical structure cladding Δn3'=-0.08%, r3 '=20μm; deeply fluorine-doped recessed layer △n4'=-0.54%, r4'=22μm; the outer cladding is a pure silicon dioxide layer.

The rod diameter of optical fiber preform can reach 135mm. After the optical fiber is drawn, the test results: the effective area of the optical fiber = 114 μm ² , the attenuation of 1550nm is 0.172dB/km, the bending radius R = 10mm*1 circle, the bending losses of 1550nm and 1625nm are 0.05dB and 0.105dB respectively, and the cable wavelength is 1520nm.

We also compared and tested the attenuation at 1550nm for optical fibers formed with different air intake methods, as shown in Figure 11. The results in the figure show that the attenuation of the 22 groups of samples in Example 3 is basically between 0.165-0.175dB/km, more precisely between 0.168-0.172dB/km. Among the 22 groups of Comparative Example 1, the highest attenuation is more than 0.185dB/km, and the lowest value is about 0.168dB/km. The average value is greater than that of Example 3. The reproducibility of the sample is poor and the pass rate is difficult to control. It can be seen that the air intake method also has an impact on the attenuation of the optical fiber. Through the control of the powder rod deposition airflow, a constant ratio between the core layer, the inner cladding and the optical cladding in the core rod can be achieved, thereby avoiding the longitudinal attenuation caused by the longitudinal fluctuation of the core rod. instability.

Comparative example 2:

Based on the preparation processes of Examples 2 and 3, the effects of closing the stress relief burner and the constant speed of the main burner on the cracking of the glass rod in the plasma deposition process (POD) were compared. The results are as follows:

As can be seen from the above table, when POD is deeply doped with fluorine, the stress relief burner is turned off and non-constant speed deposition is used simultaneously. The thicker the rod diameter is, the more cracks will occur. If the stress relief blowtorch is turned off and constant speed deposition is used at the same time, almost all cracks will occur. Cracking phenomenon. In the present invention, a non-constant speed process is adopted and an online stress-relieving blowtorch is added, which can ensure normal deposition and avoid cracking.

In summary, the preparation method of the optical fiber preform of the present invention is simple, can effectively control the quantitative production of optical fibers with ultra-low loss and large effective area, and has excellent performance. The effective area can reach more than 114 μm ² , and the best can reach 128 μm ² , and the loss and attenuation are Both are low and are the preferred materials for G.654E optical fiber. The benefits of this method are: (1) After the thickness and density of the inner cladding and optical cladding are optimized and designed in VAD deposition, powder layer combinations distributed in different density areas can be realized, combined with the linear sintering fluorine doping process, to achieve fluoride in the core Constrained diffusion within the core layer, inner cladding and optical cladding; at the same time, avoid uncontrolled large-scale diffusion of fluoride into the core layer, resulting in a decrease in the refractive index of the core layer, which affects the refractive index requirements of the core layer and optical cladding; (2 ) Through the above process, the fluorine doping requirements in the optical cladding and the gradual distribution of fluoride in the inner cladding are achieved, which plays a good transitional role between the core layer and the optical cladding, connecting the core layer in the center and the outer cladding. The viscosity between the optical cladding layers is effectively matched; (3) Through POD non-constant speed deposition and online stress relief process, the phenomenon of easy cracking caused by stress concentration of large-thickness fluorine-doped layers prepared by POD can be eliminated, and deep fluorine-doped depressions can be eliminated The layer design is beneficial to improving the bending resistance of the optical fiber. (4) The outermost layer adopts a pure silica design structure to reduce the proportion of doped glass in the optical fiber, which is beneficial to the preparation of large-size optical fiber preforms. (5) The upper deposition chamber is divided into two chambers, the inner and outer chambers, which effectively separates the powder rod holding space and the gas entering the chamber to avoid cavity problems caused by the decrease in the space for gas injection in the upper deposition chamber as the number of powder rods increases. This structure effectively improves the fluctuation of the rod diameter of the powder rod due to the fluctuation of body pressure.

The above embodiments are only used to illustrate the technical solutions of the embodiments of the present invention and are not limiting. Although the embodiments of the present invention are described in detail with reference to the above preferred embodiments, those of ordinary skill in the art should understand that the technology of the embodiments of the present invention can be Any modification or equivalent substitution of the solution shall not depart from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims (9)

1. A method for preparing optical fiber preform, characterized in that it includes the following steps: Form a core layer, an inner cladding layer, and an optical cladding layer mainly composed of silicon dioxide in sequence to obtain a powder rod, wherein the core layer also includes germanium dioxide generated by the reaction; The powder rod is sequentially subjected to three stages of dehydroxylation, sintering and vitrification to form a glass rod in which fluoride is constrained to diffuse in the core layer, inner cladding and optical cladding, wherein the fluoride in the glass rod The content is the least in the core layer, the most in the optical cladding layer and evenly distributed. In the inner cladding layer, the content of fluoride gradually increases from the outer layer of the core layer until the optical cladding layer. The content of fluoride in the inner layer of the layer; the fluoride includes one or at least two combinations of SiF ₄ , CF ₄ , SF ₆ , C ₂ F ₆ , SOF ₂ , C ₂ F ₂ Cl ₂ , where since entering In the sintering stage, the flow rate of fluoride gas increases linearly from 5 to 25cc/min until the end of the sintering stage. Then it enters the vitrification stage, and the flow rate of fluoride gas gradually decreases until it becomes zero when the vitrification stage is completed; Extend the glass rod to the target radius, and use a plasma deposition process and a stress relief process to deposit a fluorine-doped layer on its surface to obtain a fluorine-doped glass rod. The plasma deposition process uses a POD blowtorch to move back and forth on the surface of the glass rod. Spray fluorine-containing gas and deposit layer by layer; the fluorine-containing gas includes silicon tetrachloride, oxygen and fluoride; the fluoride includes SiF ₄ , CF ₄ , SF ₆ , C ₂ F ₆ , SOF ₂ , C ₂ F ₂ One or at least two combinations of Cl ₂ ; the stress relief process is when spraying fluorine-containing gas, spraying a mixed gas of oxygen and nitrogen on the side of the fluorine-containing gas in the same direction to eliminate glass stress; An outer cladding is formed on the outer layer of the fluorine-doped glass rod to obtain a transparent optical fiber preform. 2. The preparation method of optical fiber preform according to claim 1, characterized in that: in the dehydroxylation stage, the temperature is controlled at 1200-1250°C; when entering the sintering stage, the dehydroxylation stage temperature is used as the starting temperature, and the temperature is 0.5-5 The heating rate of ℃/min rises to 1320℃~1450℃ until the end of the sintering stage; it enters the vitrification stage, maintains the temperature at the end of the sintering stage, and keeps the temperature for 1 to 3 hours. 3. The method for preparing an optical fiber preform according to claim 1, characterized in that: forming a core layer, an inner cladding layer, and an optical cladding layer mainly composed of silica in sequence to obtain a powder rod, wherein the core layer In the step of the layer also containing germanium dioxide generated by the reaction, the reaction gases forming the core layer include oxygen, hydrogen, silicon tetrachloride, germanium tetrachloride, and Ar gas, wherein the inflow flow rate of germanium tetrachloride is controlled at 50- 200cc/min. 4. The method for preparing an optical fiber preform according to claim 1, wherein a core layer, an inner cladding layer, and an optical cladding layer mainly composed of silica are formed in sequence to obtain a powder rod, wherein the core layer In the step of the layer also containing germanium dioxide generated by the reaction, the reaction gases to form the inner cladding layer include oxygen, hydrogen, silicon tetrachloride, and Ar gas, in which the flow rate of silicon tetrachloride is controlled at 4g/min to 12g/min. , the density of the silica powder generated by the reaction is controlled at 0.5 to 1.5g/cm ³ , and the thickness of the inner cladding layer is 1/2 to 1/8 of the radius of the core layer. 5. The method for preparing an optical fiber preform according to claim 1, characterized in that: forming a core layer, an inner cladding layer, and an optical cladding layer mainly composed of silica in sequence to obtain a powder rod, wherein the core layer In the step of the layer also containing germanium dioxide generated by the reaction, the reaction gases to form the optical cladding include oxygen, hydrogen, silicon tetrachloride, and Ar gas, in which the inflow flow rate of silicon tetrachloride is controlled at 20g/min ~ 40g/min. min, the density of the silica powder generated by the reaction is controlled at 0.2~0.6g/ ^cm3 , and the total thickness of the optical cladding and inner cladding is 0.5~5.0 times the radius of the core layer. 6. The preparation method of optical fiber preform according to claim 1, characterized in that: the POD blowtorch translation speed variable ΔV is -0.1~-0.3m/min; the deposition thickness variable ΔC is 5~10mm, and the initial translation speed 1m/min, initial rod diameter 30mm, minimum translation speed not less than 0.1m/min. 7. The method for preparing an optical fiber preform according to claim 1, wherein the step of forming an outer cladding on the outer layer of the fluorine-doped glass rod to obtain a transparent optical fiber preform includes using a vapor deposition process. An outer cladding is deposited on the outer layer of the fluorine-doped glass rod and then sintered to obtain a transparent optical fiber preform. 8. The method for preparing an optical fiber preform according to claim 1, wherein the step of forming an outer cladding on the outer layer of the fluorine-doped glass rod to obtain a transparent optical fiber preform includes: The fluorine glass rod is directly put into the silica sleeve to assemble the optical fiber preform. 9. An optical fiber preform, characterized in that it is formed by the preparation method of the optical fiber preform according to any one of claims 1 to 8, and the optical fiber preform includes coaxially arranged : The middle core layer has a radius of 4 to 6 μm and a refractive index of 0.15 to 0.25% relative to silicon dioxide; The inner structural cladding has a radius of 4.5-7.5 μm and a gradient distribution relative to the refractive index of silicon dioxide; Optical structure layer, radius 10~25μm, refractive index relative to silicon dioxide -0.05~-0.25%; Fluorine-doped structural layer, radius 20~30μm, refractive index relative to silicon dioxide -0.4~-0.6%; The outer cladding has a radius greater than or equal to 60 μm and a refractive index of 0.