WO2021037248A1 - Optical fiber preform, preparation method therefor, and plasma deposition device - Google Patents

Abstract

Disclosed are an optical fiber preform, a preparation method therefor, and a plasma deposition device. According to the method, by means of fluorine-doped sintering, a fluorine-doped distribution with an inner cladding layer gradient is formed, and the preparation process difficulty of viscosity matching between a core layer, an inner cladding layer and an optical cladding layer, especially viscosity matching on the boundary of the core layer and the optical cladding layer, can be solved; meanwhile, a constrained diffusion distribution of each layer of fluoride in a powder rod is realized, and the refractive index requirement of each layer is ensured; and the stress problem of a large-thickness fluorine-doped layer can be effectively solved by means of the synchronous implementation of fluorine-doped deposition and stress relief, and the phenomenon whereby the thicker the fluorine-doped layer, the more prone same is to cracking is avoided.

Description

Optical fiber preform, preparation method thereof, and plasma deposition equipment Technical field

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

Background technique

In the future 400G and above transmission systems, reducing fiber loss and obtaining a large effective area is one of the important issues in the field of fiber manufacturing. For silica fiber, the attenuation between 600nm and 1600nm mainly comes from Rayleigh scattering, and 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/μm4), and B is the corresponding constant.

In order to reduce fiber loss, the most important process is to reduce the core layer germanium-doped 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 fiber is not only affected by the doping concentration Rc, but also by the density fluctuation Rd. Its expression is R=Rc+Rd. The pure silicon core design used in the traditional process is likely to cause the viscosity mismatch between the core layer and the cladding layer to cause density fluctuations. It is necessary to reduce the doping of germanium in the core layer and improve the viscosity matching between the core layer and the cladding layer to reduce it. Fiber loss.

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

Summary of the invention

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

The technical solution provided by the present invention is: a method for preparing an optical fiber preform, including the following steps:

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 further includes germanium dioxide generated by the reaction;

The powder rod is processed in three stages of dehydroxylation, sintering, and vitrification in order to form a glass rod in which fluoride is constrainedly diffused in the core layer, inner cladding layer and optical cladding layer, wherein fluorine is introduced into the sintering stage. The flow rate of fluoride gas increases linearly, and then enters the vitrification stage, and the flow rate of fluoride gas gradually decreases until it becomes zero when the vitrification stage is completed;

Extending the glass rod to a target radius, and depositing a fluorine-doped layer on its surface layer by a plasma deposition process and a stress relief process to obtain a fluorine-doped glass rod;

An outer covering 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 subjecting the powder rod to three stages of dehydroxylation, sintering, and vitrification to form a glass rod in which fluoride is constrainedly diffused in the core layer, the inner cladding layer and the optical cladding layer, The content of fluoride in the glass rod is the least in the core layer, and is the most and uniformly distributed in the optical cladding layer. The content of fluoride in the inner cladding layer is gradually from the outer layer of the core layer. Increase the fluoride content up to 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 , and C ₂ F ₂ Cl ₂ Two combinations.

Furthermore, the temperature in the dehydroxylation stage is controlled at 1200 to 1250°C; when entering the sintering stage, the temperature in the dehydroxylation stage is used as the starting temperature, and the temperature rises to 1320°C to 1450°C at a temperature rise rate of 0.5 to 5°C/min. The gas increases linearly at a flow rate of 5-25cc/min until the end of the sintering stage; entering the vitrification stage, maintaining the temperature at the end of the sintering stage, and the constant temperature for 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 further includes germanium dioxide generated by the reaction, the core layer is formed The reaction gas includes oxygen, hydrogen, silicon tetrachloride, germanium tetrachloride, and Ar gas, and 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 silicon dioxide to obtain a powder rod, wherein the core layer further 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 silicon dioxide powder produced 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 radius of the core layer.

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 further includes germanium dioxide generated by the reaction, forming an optical cladding layer 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 produced by the reaction is controlled at 0.2～0.6g/ cm ³ , the total thickness of the optical cladding layer and the inner cladding layer is 0.5 to 5.0 times the radius of the core layer.

Further, in the step of extending the glass rod to a target radius, and depositing a fluorine-doped layer on its surface layer by a plasma deposition process and a stress-relieving process to obtain a fluorine-doped glass rod, the plasma deposition process is performed by a POD torch The fluorine-containing gas is sprayed back and forth on the surface of the glass rod 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 at least two combinations; the stress relief process is that when spraying fluorine-containing gas, the side of the fluorine-containing gas is sprayed with a mixture of oxygen and nitrogen in the same direction to eliminate Glass stress.

Furthermore, the variable △V of the translation speed of the POD torch is -0.1～-0.3m/min; the variable △C of the deposition thickness 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 covering on the outer layer of the fluorine-doped glass rod to obtain a transparent optical fiber preform includes depositing an outer covering on the outer layer of the fluorine-doped glass rod by a vapor deposition process, and then sintering , Obtain a transparent optical fiber preform.

Further, the step of forming an outer coating 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 to form an optical fiber preform.

The present invention also provides an optical fiber preform, which is formed by using the method for preparing the optical fiber preform, and the optical fiber preform sequentially includes coaxially arranged from the inside to the outside:

The middle core layer has a radius of 4 to 6 μm, and the refractive index relative to silica is 0.15 to 0.25%;

The inner structure cladding has a radius of 4.5～7.5μm, and the refractive index of silica is gradual distribution;

The optical structure layer has a radius of 10-25μm, and the refractive index relative to silica is -0.05～-0.25%;

The fluorine-doped structural layer has a radius of 20-30μm, and the refractive index relative to silica is -0.4～-0.6%;

The outer cladding layer 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 torch group. The POD torch group includes a main torch and several stress-relieving torches arranged side by side. The torch is used to spray-deposit silicon tetrachloride, oxygen and fluoride introduced on the surface of the glass rod; the stress relief torch is used to introduce oxygen and nitrogen to remove the glass stress.

Further, the POD torch group includes two stress-relieving torches, the two stress-relieving torches are located on both sides of the main torch, the three torches are arranged side by side in the translation direction, and the outlets of the three torches are away from the glass The spacing of the bars is consistent.

Further, the distance from the outlet of the main torch or the stress relief torch to the surface of the glass rod is not greater than the height of the spray flame.

Further, the distance from the outlet of the main torch or the stress relief torch to the surface of the glass rod is half of the height of the spray flame.

Further, the axial distance between the main torch and the stress relief torch is less than or equal to half of the sum of the widths of the flames injected by the two torches.

Further, the main torch is provided with multiple pipes arranged side by side, and the pipes are used to pass silicon tetrachloride, oxygen, and fluoride to react to form and deposit fluorine-doped silica powder; the stress-relieving torch is equipped with Several pipelines are used to pass in oxygen and nitrogen.

Compared with the prior art, the fluorine-doped distribution of the inner cladding layer in this application can solve the difficulty of the preparation process of the viscosity matching between the core layer, the inner cladding layer and the optical cladding layer, especially the viscosity matching on 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 realized to ensure the refractive index of each layer; the simultaneous development of fluorine-doped deposition and stress relief can effectively solve the stress problem of the large-thick fluorine-doped layer and avoid the occurrence of fluorine-doped The thicker the layer, the easier it is to crack.

Description of the drawings

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

Fig. 1 is a flow chart of the preparation of an optical fiber preform in an 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 diagram of the end surface of the upper deposition chamber shown in FIG. 2.

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 in the sintering stage of the present invention.

Fig. 6 is a schematic diagram of controlling the amount of fluorine in the process of dehydroxylation, sintering and vitrification of the powder rod of the present invention.

Fig. 7 is a schematic diagram of the structure of the plasma deposition equipment of the present invention.

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

Fig. 9 is a schematic diagram of the refractive index profile of the optical fiber preform of the present invention.

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

Figure 11 is a test diagram of the attenuation performance of the optical fiber under different air intake modes.

Description of reference signs:

Powder rod deposition equipment 10

Core layer blowtorch 101

Inner cladding blowtorch 102

Optical cladding blowtorch 103

Deposition Room 104

Boom 105

Target rod 106

Powder stick 107

Upper deposition chamber 108

Inner layer of upper deposition chamber 108a

The outer layer of the upper deposition chamber 108b

Upper deposition chamber end cover 108c

Plasma deposition equipment 30

Glass rod 301

POD blowtorch group 303

Main blowtorch 3031

The first stress relief blowtorch 3032

The second stress relief blowtorch 3033

Optical fiber preform 50

Intermediate core layer 501

Inner structure cladding 503

Optical structure layer 505

Fluorine-doped structural layer 507

Outsourcing layer 509

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

detailed description

In order to be able to understand the above-mentioned objectives, features and advantages of the embodiments of the present invention more clearly, the present invention will be described in detail below with reference to the accompanying drawings and specific implementations. It should be noted that, in the case of no conflict, the features in the embodiments of the present application can be combined with each other.

In the following description, many specific details are set forth in order to fully understand the embodiments of the present invention. The described embodiments are only a part of the embodiments of the present invention, rather than all of them. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative work shall fall within the protection scope of the embodiments of the present invention.

In this article, "stress" refers to the stress caused by the micro-inhomogeneous zone formed by uneven composition, also known as structural stress or microscopic stress.

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

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

In this article, "OVD" refers to the external vapor deposition method, the full English name is Outside Vapour Deposition, or OVD for short.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the technical field belonging to the embodiments of the present invention. The terminology used in the specification of the present invention herein is only for the purpose of describing specific embodiments, and is not intended to limit the embodiments of the present invention.

Please refer to Fig. 1, which is a flow chart of the preparation of the optical fiber preform in a specific embodiment of the present invention, which includes the following steps:

Step S1: sequentially forming a core layer r01, an inner cladding layer r02, and an optical cladding layer r03 mainly composed of silicon dioxide to obtain a powder rod, wherein the core layer further includes germanium dioxide produced by the reaction.

In a specific embodiment, the reaction gas forming the core layer includes oxygen, hydrogen, silicon tetrachloride, germanium tetrachloride, and Ar gas, and the flow rate of germanium tetrachloride is controlled at 50-200 cc/min. The reaction gases forming the inner cladding layer 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 silicon dioxide powder produced by the reaction is controlled at 0.5~ 1.5g/cm ³ , the thickness of the inner cladding layer is 1/2 to 1/8 of the radius of the core layer, that is, (r02-r01)/r01. The reaction gas for forming the optical cladding layer includes 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 silicon dioxide powder produced by the reaction is controlled at 0.2 ~0.6g/cm ³ , the total thickness of the optical cladding layer and the inner cladding layer is 0.5-5.0 times the radius of the core layer, that is, (r03-r01)/r01, preferably 1.5-3.0 times. In this step, the reaction gas can be introduced separately or mixed gas. The above-mentioned raw materials react in the flame at high temperature to produce silicon dioxide particles or germanium dioxide and silicon dioxide particles, such as oxygen, hydrogen, silicon tetrachloride, and tetrachloride. 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 layer and optical cladding layer are optimized to achieve the combination of powder layers distributed in different density regions. The composition of the layers is similar, and the core layer is doped as little as possible. The viscosity of each powder layer during the sintering process is adjusted to a high degree of matching. It is not suitable to cause density fluctuations. It also achieves a near-pure silicon core design or a low germanium-doped core layer design, thereby reducing Swiss It facilitates scattering and reduces the loss of the final optical fiber.

Step S2: The powder rod is processed in the three stages of dehydroxylation, sintering, and vitrification in sequence to form a glass rod in which fluoride is constrainedly diffused in the core layer, the inner cladding layer and the optical cladding layer. The fluoride gas is introduced and its flow rate increases linearly, and then enters the vitrification stage, and the flow rate of the fluoride gas gradually decreases until it becomes zero when the vitrification stage is completed. The fluoride gas refers to a gas including one or a combination of _{SiF 4} , CF ₄ , SF ₆ , C ₂ F ₆ , SOF ₂ , and C ₂ F ₂ Cl _2.

Please refer to FIG. 4 together. Based on the optimized design of thickness and density, combined with the linear sintering fluorine-doping process, the content of fluoride in the glass rod is the least in the core layer and the most in the optical cladding layer. The fluoride content in the inner cladding layer is gradually increased from the outer layer of the core layer to the fluoride content in the inner layer of the optical cladding layer, that is, the fluoride content in the core layer, The constrained diffusion in the inner cladding layer and the optical cladding layer; at the same time, it avoids the uncontrolled diffusion of fluoride to the core layer to reduce the refractive index of the core layer, which affects the refractive index requirements of the core layer and the optical cladding layer. The fluoride includes one or a combination of at least two of _{SiF 4} , CF ₄ , SF ₆ , C ₂ F ₆ , SOF ₂ , and C ₂ F ₂ Cl _2.

Please refer to Figure 5 and Figure 6 together, the temperature in the dehydroxylation stage is controlled at 1200～1250℃; in the sintering stage, the temperature in the dehydroxylation stage is used as the starting temperature, and the temperature rise rate is 0.5～5℃/min to 1320℃ ～1450℃, the fluoride gas increases linearly at a flow rate of 5～25cc/min until the end of the sintering stage; enter the vitrification stage, keep the temperature at the end of the sintering stage, and hold the temperature for 1～3h.

Through the above process, the requirement of fluorine doping in the optical cladding layer and the gradual distribution of fluoride in the inner cladding layer are realized, which has a good transitional effect between the core layer and the optical cladding layer, and the core layer and the outer layer at the center The viscosity between the optical cladding layers is effectively matched. The traditional ultra-low loss and large effective area optical fiber adopts the sinking auxiliary design method, and the energy distribution in the optical fiber is Gaussian distribution. The present invention can effectively increase the optical fiber mode field diameter and increase the effective area of the optical fiber by changing the core refractive index structure. There is no need to reduce the refractive index of the core layer and increase the diameter of the core layer blindly.

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

Please also refer to FIG. 7, in a specific embodiment, the plasma deposition process sprays fluorine-containing gas on the surface of the glass rod through a POD torch, and deposits 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 , and C ₂ F ₂ Cl _{2; the stress relief process includes spraying} In the case of fluorine gas, a pipe is provided on the side of the disk where the POD torch sprays fluorine-containing gas, and the outlet of the pipe is sprayed with a mixture of oxygen and nitrogen in the same direction to eliminate glass stress. POD blowtorch translation speed variable △V is -0.1～-0.3m/min; deposition thickness variable △C is 5～10mm, initial translation speed is 1m/min, initial rod diameter is 30mm, and minimum translation speed is not less than 0.1m/min. Through the non-constant rate deposition of POD and the online stress relief process, it is possible to eliminate the phenomenon of easy cracking caused by stress concentration of the large-thick fluorine-doped layer prepared by POD. The design of the deep fluorine-doped recessed layer is beneficial to improve the bending resistance of the optical fiber.

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

In a specific embodiment, in this step S4, a vapor deposition process may be used to deposit an outer coating on the outer layer of the fluorine-doped glass rod, and then sintered to obtain a transparent optical fiber preform. In this step S4, the fluorine-doped glass rod can also be directly assembled into the silica sleeve to form an optical fiber preform.

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

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

The inner structure cladding layer 503 has a radius r2=4.5～7.5μm, and the refractive index △n2 of the silicon dioxide is gradual distribution;

The optical structure layer 505 has a radius r3 = 10-25 μm, and a refractive index Δn3 relative to silica is -0.05 to -0.25%;

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

For the outer cladding layer 509, the radius r5 is greater than or equal to 60 μm, and the refractive index Δn5 is 0.

The powder rod deposition device 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 torch 103, an inner cladding torch 102, a core torch 101, a boom 105 and an upper deposition chamber 108. Among them, the upper deposition chamber 104 is provided with an upper deposition chamber 108, the upper deposition chamber 108 is equipped with a suspension rod 105, the suspension rod 105 is provided with a hook, the suspension rod 105 is connected to the lifting mechanism, and the target rod 106 is suspended on the On the hook of the boom 105, an optical cladding torch 103, an inner cladding torch 102, and a core torch 101 are sequentially installed on the lower side of the deposition chamber 104. These torches (103, 102, 101) spray airflow toward the target rod 106, thereby The powder reacts layer by layer to form a powder attached to the target rod 106. In a specific embodiment, the upper deposition chamber 108 is divided into inner and outer chambers, namely the inner layer of the upper deposition chamber 108a and the outer layer of the upper deposition chamber 108b. The end of the upper deposition chamber should be provided with an upper deposition chamber end cover 108c (as shown in Figure 3). ). The outer layer 108b of the upper deposition chamber is mainly filled with external air, the inner layer 108a of the upper deposition chamber mainly contains the lifting space of the powder rod, and the end cap 108c of the upper deposition chamber is used to seal the powder rod holding space and prevent airflow from entering. In this way, the upper deposition chamber 108 is divided into inner and outer chambers, which effectively separates the powder rod containing space and the gas entering the chamber, and avoids that as the diameter of the powder rod increases, the space for gas injection in the upper deposition chamber decreases. The resulting cavity pressure fluctuations cause fluctuations in the diameter of the powder rod. The above-mentioned structural design can effectively improve the fluctuation of the diameter 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 burner 101, and the silicon dioxide is formed through high-temperature reaction, and the germanium dioxide is attached to the end surface of the target rod to form a certain Density of the loose core layer. A silicon dioxide layer with a certain thickness surrounding the surface of the core layer is an inner cladding layer, and oxygen, hydrogen, silicon tetrachloride, and Ar gas are passed into the inner cladding torch 102. The silicon dioxide layer with a certain thickness surrounding the inner cladding surface is the optical cladding. Oxygen, hydrogen, silicon tetrachloride, and Ar gases are introduced into the optical cladding torch 103, and the powder is deposited to a set length and the deposition is stopped. 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 of the inner cladding and optical cladding torches (102, 103), and the ratio of hydrogen to oxygen flow.

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

The device 30 is used to deposit and form a fluorine-doped layer on the surface of the glass rod 301, and includes a POD blowtorch 303 and a POD machine (not shown in the figure) carrying a glass rod. The POD machine adjusts the glass rod to rotate around the glass rod axis; The POD torch assembly 303 includes a main torch 3031 and a number of stress relief torches arranged side by side, and the main torch 3031 is used to spray-deposit silicon tetrachloride, oxygen, and fluoride on the surface of the glass rod, and It can be sprayed back and forth; the stress-relieving torch is used for introducing oxygen and nitrogen to remove the glass stress. The above different airflows can be set in the blowtorch with separate pipes or separate pipes, or multiple airflows can be passed through the same pipe or pipe. As shown in Figure 7, the glass rod is horizontally clamped in the plasma deposition equipment 30. The lower side of the equipment 30 is provided with 3 POD torches side by side, namely the first stress-relieving torch 3032, the main torch 3031 and the second stress-relieving torch 3033. , The three torches (3032, 3031, 3033) can be synchronized horizontally and the distance between the outlets of the three and the glass rod is the same. Generally, the distance between the outlet of the POD torch 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 of the position, the axial distance between the stress relief torch and the main torch is not more than half of the sum of the width of the two spray flames, that is, the spray flames of adjacent torches overlap. In other embodiments, the number of POD torches is not limited to three, and the number of stress-relieving torches can also be one or more; the distance between the outlets of multiple torches and the glass rod depends on the process parameters and is not limited to The same; the deposition equipment can also be installed vertically or clamped obliquely, as long as the torch can deposit powder on its surface, it is not limited here, and needs to be set according to actual process requirements and product performance requirements.

The deposition process of the fluorine-doped layer: as shown in FIG. 7, the glass rod 301 is placed on the POD machine platform, and the POD blowtorch group 303 is sprayed back and forth on the surface of the rod and deposited layer by layer. The main torch 3031 of the blowtorch group 303 is filled with SiCl ₄ , O ₂ , and fluoride to form a fluorine-containing glass layer. According to the design flow rate of fluorine doping, deep fluorine-doped recessed layers with different depths are formed. The design of the deep fluorine-doped recessed layer is beneficial Improve the bending resistance of the optical fiber. The first stress-relief torch 3032 and the second stress-relief torch 3033 on both sides are respectively introduced into _{a mixed air flow of O 2} and N ₂ to remove the glass stress. POD blowtorch translation speed variable △V is -0.1～-0.3m/min; deposition thickness variable △C is 5～10mm, initial translation speed is 1m/min, initial rod diameter is 30mm, and minimum translation speed is not less than 0.1m/min. Through the non-constant rate deposition of POD and the online stress relief process, it is possible to eliminate the phenomenon of easy cracking caused by stress concentration of the large-thick fluorine-doped layer prepared by POD.

The process and performance of forming the optical fiber preform 50 by the method of the present invention will be compared and analyzed below in conjunction with specific embodiments and comparative examples.

Example 1:

First, the core layer, inner cladding layer and optical cladding layer are prepared by the VAD vapor deposition process. During the deposition process, GeCl ₄ gas is introduced 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, powder density Controlled at 1.3g/cm ³ ; the SiCl ₄ flow rate in the optical cladding was controlled at 20g/min, and the powder density was controlled at 0.6g/cm ³ .

The deposited powder rods undergo dehydroxylation, sintering, and vitrification in a sintering furnace. First of all, the dehydroxylation temperature T1 is controlled at 1200℃; after the dehydroxylation is completed, it is increased to 1320℃ (T2) at a heating rate of 1℃/min, and at the same time, CF ₄ gas is introduced linearly at a flow rate of 5cc/min until the sintering stage End; after the temperature is raised to T2, enter the vitrification constant temperature stage, the constant temperature time is 1h, the powder rod is further sintered into a transparent glass body, and the fluorine doping flow rate gradually decreases with time to zero.

After extending the glass rod prepared above to the target rod diameter, a plasma deposition process (POD) deposits a deep fluorine-doped recessed layer. Place the glass rod on the POD machine platform, and spray the POD torch on the surface of the glass rod back and forth, and deposit layer by layer. _{In the blowtorch group, SiCl 4} , O ₂ , and CF ₄ are introduced into the main torch to form a fluorine-containing glass fluorine-doped layer, and O ₂ and N ₂ are introduced into the stress-relieving torches on both sides to remove the glass stress. The initial speed is 1m/min, the initial glass rod diameter is 30mm, and when the rod diameter is 35mm, the translation speed of the blowtorch group is controlled to 0.9m/min. For every 5mm increase in thickness, the translation speed will decrease by 0.1m/min, the lowest translation speed Not less than 0.1m/min, and so on, until the diameter reaches the target rod diameter of 58mm.

The above-mentioned fluorine-doped glass rod adopts the OVD gas phase synthesis process, and the pure silicon outer layer is deposited layer by layer. After reaching the target weight or rod diameter, the deposition is completed, and then sintering is performed to prepare the powder rod into a transparent glass rod. The effective area optical fiber preform 50.

Refractive index profile characteristics: middle core layer 501: △n1=0.16%, r1=4.2μm; inner structure cladding 503 is a fluorine-doped transition zone, r2=5.6μm; optical structure layer 505: △n3=-0.20%, r3 =12μm; fluorine-doped structure layer (hereinafter also referred to as deep fluorine-doped recessed layer) 507: Δn4=-0.42%, r4=28μm; outer cladding layer 509 is a pure silicon dioxide layer. Since the outer cladding layer 509 can also be formed by the casing process, the deposited outer cladding layer before and after the unified sintering and the outer cladding layer of the set quartz tube are all the outer cladding layer 509.

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

Example 2:

First, the core layer, inner cladding layer and optical cladding layer are prepared by the VAD vapor deposition process. During the deposition process, GeCl ₄ gas is introduced 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 It is controlled at 0.9g/cm ³ _{; the flow rate of SiCl 4 in the} optical cladding is controlled at 30 g/min, and the powder density is controlled at 0.4 g/cm ³ .

The deposited powder rod is processed for dehydroxylation, sintering and vitrification in a sintering furnace. First, control the dehydroxylation temperature T1 at 1200°C; after the dehydroxylation is completed, it will increase to 1400°C (T2) at a heating rate of 3°C/min, and the SiF ₄ gas will increase linearly at a flow rate of 12cc/min until the end of the sintering stage; After the temperature rises to 1400°C, it enters the vitrification constant temperature stage. The constant temperature time is 2h, the powder rod is further sintered into a transparent glass body, and 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 deep fluorine-doped recessed layer) is deposited by a plasma deposition process (POD). Place the glass rod on the POD machine platform, and spray the POD torch on the surface of the glass rod back and forth, and deposit layer by layer. _{In the blowtorch group, SiCl 4} , O ₂ , and SiF ₄ are introduced into the main torch to form a fluorine-containing glass fluorine-doped layer, and O ₂ and N ₂ are introduced into the stress relief torches on both sides to remove the glass stress. The initial speed is 0.9m/min, the initial glass rod diameter is 35mm, and when the rod diameter is 43mm, the translation speed of the blowtorch is controlled to 0.7m/min, and the translation speed is reduced by 0.2m/min for every 8mm increase in thickness, the lowest 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 and large effective area optical fiber preform 50.

Refractive index profile characteristics: middle core layer 501: △n1=0.21%, r1=5.0μm; inner structure cladding 503 is a fluorine-doped transition zone, r2=6.2μm; optical structure cladding 505: △n3=-0.15%, r3=16μm; fluorine-doped structure layer 507: Δn4=-0.47%, r4=25μm; outer cladding layer 509: a pure silicon dioxide layer.

The diameter of the optical fiber preform can reach 125mm. After the fiber is drawn, the test results: the effective area of the fiber = ^{123μm 2} , the attenuation at 1550nm is 0.169dB/km, the bending radius R = 10mm*1, the bending loss at 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 by 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 It is controlled at 0.6g/cm ³ _{; the flow rate of SiCl 4 in the} optical cladding is controlled at 40g/min, and the powder density is controlled at 0.25g/cm ³ .

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

After extending the glass rod prepared above to the target rod diameter, a plasma deposition process (POD) deposits a deep fluorine-doped recessed layer. Place the glass rod on the POD machine platform, and spray the POD torch on the surface of the glass rod back and forth, and deposit layer by layer. _{SiCl 4} , O ₂ , and SF ₆ are introduced into the main burner of the blowtorch group to _{form a fluorine-containing glass layer, and O 2} and N ₂ are introduced into the stress relief burners on both sides to remove the glass stress. The initial speed is 0.8m/min, the initial glass rod diameter is 40mm, and when the rod diameter reaches 50mm, the translation speed of the blowtorch group is controlled to 0.5m/min. When the thickness is increased by 10mm, the translation speed is reduced by 0.3m/min, and the minimum translation The speed is not less than 0.1m/min, and so on, the diameter reaches the target rod diameter of 60mm.

The above-mentioned fluorine-doped glass rod adopts the OVD gas phase synthesis process, and the pure silicon cladding layer is deposited layer by layer. After reaching the target weight or rod diameter, the deposition is completed and then sintered to prepare a transparent glass rod, which completes a low-loss large effective area optical fiber Preform 50.

Refractive index profile characteristics: middle core layer 501: △n1=0.24%, r1=6.0μm; inner structure cladding 503 is a fluorine-doped transition zone, r2=7.3μm; optical structure layer 505: △n3=-0.08%, r3 =20μm; deep fluorine-doped recess 507: Δn4=-0.54%, r4=30μm; outer cladding layer 509 is a pure silicon dioxide layer.

The diameter of the optical fiber preform can reach 135mm. After the fiber is drawn, the test results: the effective area of the fiber=114μm ² , the 1550nm attenuation is 0.171dB/km, the bending radius R=10mm*1, the 1550nm and 1625nm bending losses 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, except that: in step S1, conventional VAD vapor deposition chamber deposition is used, that is, the air flow enters the lower cavity from the upper powder chamber, and is located in the same place as the powder body. room. During the deposition process, the core layer was filled with GeCl ₄ gas, and the flow rate was controlled at 150 cc/min; the SiCl ₄ flow rate in the inner cladding was controlled at 12 g/min, and the powder density was controlled at 0.58 g/cm ³ ; the SiCl ₄ flow rate in the optical cladding was controlled at 40 g /min, the powder density is controlled at 0.26g/cm ³ .

For powder rods formed by different air intake methods, we used the process conditions of Example 3 and Comparative Example 1 to prepare multiple sets of samples, and tested the thickness of the optical cladding and inner cladding in each sample and the respective core layer rods. The magnification of the diameter, in which the rod diameters of all core layers are the same, as shown in Figure 10. It can be seen in 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 22 sets of samples are maintained between 2.3-2.4 times, and the fluctuation is small; while using the conventional air intake method of Comparative Example 1, 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, and it is easier to realize the design and control of the thickness and density.

Refractive index profile characteristics: middle core layer △n1'=0.24%, r1'=6.1μm; inner structure cladding is fluorine-doped transition zone, r2'=7.3μm; optical structure cladding △n3'=-0.08%, r3 '=20μm; deep fluorine-doped recessed layer △n4'=-0.54%, r4'=22μm; the outer cladding layer is a pure silicon dioxide layer.

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

For fibers formed by different air intake methods, we also compared and tested the attenuation at 1550nm, as shown in Figure 11. The results in the figure show that the attenuation of the 22 sets of samples in Example 3 is basically between 0.165-0.175dB/km, and more precisely between 0.168-0.172dB/km. Among the 22 groups of Comparative Example 1, the attenuation was as high as 0.185 dB/km or more, and the lowest was about 0.168 dB/km. The average value was greater than that of Example 3. The reproducibility of the samples was poor and the pass rate was 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 air flow, a constant ratio between the core layer, the inner cladding layer and the optical cladding layer 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 process of Examples 2 and 3, compare the effects of the stress relief torch off and the constant speed of the main torch on the cracking of the glass rod in the plasma deposition process (POD). The results are as follows:

It can be seen from the above table that when the POD is deeply doped with fluorine for deposition, the stress-relief torch is turned off, and when the non-constant speed deposition is used simultaneously, the thicker the rod diameter is, the more cracks; if the stress-relief torch is turned off and the constant-rate deposition is used, almost all of them appear Cracking phenomenon. In the present invention, a non-constant speed process is adopted and an online stress-relief torch 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} , preferably up to 128 ^{μm 2} , loss and attenuation. Both are relatively low and are the preferred material for G.654E optical fiber. The advantages of this method are: (1) After the thickness and density of the inner cladding layer and the optical cladding layer are optimized in VAD deposition, the combination of powder layers in different density regions is realized, and the linear sintering fluorine-doped process is combined to realize the fluoride in the core. Confinement diffusion in the inner cladding layer, inner cladding layer and optical cladding layer; at the same time, to avoid the uncontrolled diffusion of fluoride to the core layer, resulting in a decrease in the refractive index of the core layer, which will affect the refractive index requirements of the core layer and optical cladding layer; (2) ) Through the above process, the fluorine-doped requirement in the optical cladding layer and the gradual distribution of fluoride in the inner cladding layer are realized, which play a good transitional role between the core layer and the optical cladding layer, and the core layer at the center and the outer layer The viscosity between the optical cladding layers of the layer is effectively matched; (3) Through the non-constant speed deposition of POD and the on-line stress relief process, it can eliminate the phenomenon of easy cracking caused by the stress concentration of the large-thick fluorine-doped layer prepared by the POD, and the deep fluorine-doped depression The layer design is beneficial to improve the bending resistance of the optical fiber. (4) The outermost layer adopts a pure silica design structure, which reduces the proportion of doped glass in the optical fiber, which is conducive to the preparation of large-size optical fiber preforms. (5) The upper deposition chamber is divided into inner and outer chambers, which effectively separates the powder rod holding space and the gas entering the chamber, avoiding the cavity caused by the increase of the powder rods and the reduction of the space for gas injection in the upper deposition chamber Body pressure fluctuates, this structure effectively improves the rod diameter fluctuation of the powder rod.

The above embodiments are only used to illustrate the technical solutions of the embodiments of the present invention and not to limit them. 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 technical solutions of the embodiments of the present invention can be compared. Modifications or equivalent replacements of the solutions should not depart from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims (13)

1. A method for preparing an optical fiber preform is characterized in that it comprises the following steps:

   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 further includes germanium dioxide generated by the reaction;

   The powder rod is processed in three stages of dehydroxylation, sintering, and vitrification in order to form a glass rod in which fluoride is constrainedly diffused in the core layer, inner cladding layer and optical cladding layer, wherein fluorine is introduced into the sintering stage. The flow rate of fluoride gas increases linearly, and then enters the vitrification stage, and the flow rate of fluoride gas gradually decreases until it becomes zero when the vitrification stage is completed;

   Extending the glass rod to a target radius, and depositing a fluorine-doped layer on its surface layer by a plasma deposition process and a stress relief process to obtain a fluorine-doped glass rod;

   An outer covering is formed on the outer layer of the fluorine-doped glass rod to obtain a transparent optical fiber preform.
2. The method for preparing an optical fiber preform according to claim 1, wherein the powder rod is processed in the three stages of dehydroxylation, sintering, and vitrification in order to form fluoride in the core layer and the inner cladding layer. In the step of constrained diffusion of the glass rod in the optical cladding, the content of fluoride in the glass rod is the least in the core layer, and is the most and uniformly distributed in the optical cladding, and in the inner cladding The fluoride content in the outer layer of the core layer gradually increases to the fluoride content in the inner layer of the optical cladding layer; the fluoride includes SiF ₄ , CF ₄ , SF ₆ , and C ₂ F ₆ , SOF ₂ , C ₂ F ₂ Cl ₂ or a combination of at least two.
3. The method for preparing an optical fiber preform according to claim 1, wherein the temperature in the dehydroxylation stage is controlled at 1200-1250°C; in the sintering stage, the temperature in the dehydroxylation stage is taken as the starting temperature, and 0.5-5°C/ The heating rate of min rises to 1320℃～1450℃, and the fluoride gas increases linearly at a flow rate of 5～25cc/min until the end of the sintering stage; enters the vitrification stage, maintaining the temperature at the end of the sintering stage, and the constant temperature time is 1～3h.
4. The method for preparing an optical fiber preform according to claim 1, wherein the core layer, the inner cladding layer, and the optical cladding layer mainly composed of silica are sequentially formed to obtain a powder rod, wherein the core layer is also In the step of including germanium dioxide produced by the reaction, the reaction gas for forming the core layer includes oxygen, hydrogen, silicon tetrachloride, germanium tetrachloride, and Ar gas, and the flow rate of germanium tetrachloride is controlled at 50-200cc/ min.
5. The method for preparing an optical fiber preform according to claim 1, wherein the core layer, the inner cladding layer, and the optical cladding layer mainly composed of silica are sequentially formed to obtain a powder rod, wherein the core layer is also In the step including the germanium dioxide produced by the reaction, the reaction gas for forming the inner cladding layer includes oxygen, hydrogen, silicon tetrachloride, and Ar gas, and the flow rate of silicon tetrachloride is controlled at 4g/min~12g/min. The density of the produced silica powder is controlled within 0.5～1.5g/cm ³ , and the thickness of the inner cladding layer is 1/2～1/8 of the radius of the core layer.
6. The method for preparing an optical fiber preform according to claim 1, wherein the core layer, the inner cladding layer, and the optical cladding layer mainly composed of silica are sequentially formed to obtain a powder rod, wherein the core layer is also In the step including the germanium dioxide produced by the reaction, the reaction gas for forming the optical cladding includes oxygen, hydrogen, silicon tetrachloride, and Ar gas, and the flow rate of silicon tetrachloride is controlled at 20g/min-40g/min, The density of the silica powder produced by the reaction is controlled within 0.2-0.6g/cm ³ , and the total thickness of the optical cladding layer and the inner cladding layer is 0.5-5.0 times the radius of the core layer.
7. The method for preparing an optical fiber preform according to claim 1, wherein the glass rod is extended to a target radius, and a plasma deposition process and a stress relief process are used to deposit a fluorine-doped layer on the surface of the glass rod to obtain a fluorine-doped layer. In the glass rod step, the plasma deposition process sprays fluorine-containing gas on the surface of the glass rod through a POD torch, and deposits layer by layer; the fluorine-containing gas includes silicon tetrachloride, oxygen, and fluoride; the fluorine The compound includes SiF ₄ , CF ₄ , SF ₆ , C ₂ F ₆ , SOF ₂ , C ₂ F ₂ Cl ₂ or a combination of at least two; the stress relief process is that when spraying fluorine-containing gas, the fluorine-containing gas The side of the gas is sprayed with a mixture of oxygen and nitrogen in the same direction to eliminate glass stress.
8. The method for preparing an optical fiber preform according to claim 7, characterized in that: the variable ΔV of the translational velocity of the POD torch is -0.1～-0.3m/min; the variable ΔC of the deposition thickness is 5-10mm, and the initial translational speed is 1m/min. min, the initial rod diameter is 30mm, and the minimum translation speed is not less than 0.1m/min.
9. The method for preparing an optical fiber preform according to claim 1, wherein the step of forming an outer coating on the outer layer of the fluorine-doped glass rod to obtain a transparent optical fiber preform includes using a vapor deposition process in the An outer coating is deposited on the outer layer of the fluorine-doped glass rod, and then sintered to obtain a transparent optical fiber preform.
10. The method for preparing an optical fiber preform according to claim 1, wherein the step of forming an outer layer on the outer layer of the fluorine-doped glass rod to obtain a transparent optical fiber preform comprises combining the fluorine-doped glass rod The rod is directly inserted into the silica sleeve and assembled into an optical fiber preform.
11. An optical fiber preform, characterized in that it is formed by the method for preparing an optical fiber preform according to any one of claims 1-10, and the optical fiber preform sequentially includes coaxially arranged from the inside to the outside:

    The middle core layer has a radius of 4 to 6 μm, and the refractive index relative to silica is 0.15 to 0.25%;

    The inner structure cladding has a radius of 4.5～7.5μm, and the refractive index of silica is gradual distribution;

    The optical structure layer has a radius of 10-25μm, and the refractive index relative to silica is -0.05～-0.25%;

    The fluorine-doped structural layer has a radius of 20-30μm, and the refractive index relative to silica is -0.4～-0.6%;

    The outer cladding layer has a radius greater than or equal to 60 μm and a refractive index of 0.
12. A plasma deposition equipment is used to deposit a fluorine-doped layer on the surface of a glass rod. The equipment includes a POD torch group. The torch is used to spray-deposit silicon tetrachloride, oxygen and fluoride introduced on the surface of the glass rod; the stress relief torch is used to introduce oxygen and nitrogen to remove the glass stress.
13. The plasma deposition equipment according to claim 12, wherein the POD torch group includes two stress-relieving torches, the two stress-relieving torches are located on both sides of the main torch, and the three torches are along the The translation direction is arranged side by side and the distance between the outlets of the three torches and the glass rod is the same.

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