CN103224325B - Optical fiber preform cladding fluorine doping method - Google Patents

Abstract

The invention discloses a method for doping fluorine in the cladding of an optical fiber prefabricated rod, which comprises the following steps: depositing a core layer loose body on a target rod; heating the outer surface of the core layer loose body to form a dense layer, so that the density of the dense layer is higher than The inner density of the core layer loose body; deposit the inner cladding loose body outside the dense layer to form a core rod loose body comprising the core layer loose body and the inner cladding loose body; extract the target rod to form a central hole in the center of the core rod loose body; The rod loose body is placed in a vitrification furnace and heated and dehydrated in a dehydration atmosphere, and dehydration gas is introduced into the central hole while heating; the mandrel loose body is heated in a fluoride atmosphere to selectively incorporate fluorine into the loose inner cladding A step-type refractive index distribution is formed in the body; the loose body of the vitrified core rod makes the central hole shrinkage cavity, the loose body of the core layer forms the core layer, and the loose body of the inner cladding layer forms the inner cladding layer to form the core rod. It can effectively reduce the OH ^- content in the core layer and reduce the attenuation of the optical fiber at 1383nm; the fluorine is evenly distributed in the radial direction of the cladding.

Description

A method for doping fluorine in the cladding of an optical fiber preform

technical field

The invention relates to a method for manufacturing an optical fiber preform, in particular to a method for doping the cladding of an optical fiber preform with fluorine.

Background technique

Optical fiber preform is the raw material for drawing optical fiber. Its basic structure includes a core layer and a cladding layer with a lower refractive index (the cladding layer can also include an inner cladding layer and an outer cladding layer). SiO ₂ is the main glass former used to make optical fiber preforms, and its refractive index can be changed by doping to form a waveguide structure. Generally, GeO ₂ is doped into the core layer, so that the refractive index of the core layer is higher than that of the pure silica glass of the cladding layer. The relative difference between the refractive index of the core layer and the cladding layer is expressed by the relative refractive index difference Δ, and the refractive indices of the core layer and the cladding layer are respectively and , the value of the relative refractive index difference Δ is given by equation (1):

…………(1)

Sometimes in order to increase the bending resistance of the fiber, it is necessary to increase the Δ value by increasing the _GeO2 content in the fiber core layer. But with the increase of GeO ₂ content, it will lead to the increase of fiber Rayleigh scattering. If the doped _GeO2 content is too high, it is easy to form GeO gas, and bubbles will be generated accordingly. This is detrimental to the transmission performance and strength of the final fiber. For the above reasons, the Δ value can be increased by lowering the refractive index of the cladding. Doping both _B2O3 and fluorine can lower _the refractive index of the cladding. However, B ₂ O ₃ has a large tailing absorption at 1.2 μm, which is not conducive to the reduction of loss. Therefore, it is best to use fluorine doping to reduce the refractive index of the cladding. In order to obtain the same relative refractive index difference Δ, by doping the cladding with fluorine, the content of GeO ₂ in the core layer can be reduced or even not doped with GeO ₂ , thereby further reducing the Rayleigh scattering caused by doping.

The current methods of manufacturing optical fiber preforms include in-tube methods (MCVD and PCVD) and out-of-tube methods (VAD and OVD). The extra-tube method is not limited by the size of the liner, has fast deposition speed and high production efficiency, and is suitable for large-scale production of optical fiber preforms. It is known that gaseous fluorine-containing compounds can be added to quartz glass during deposition by flame hydrolysis, but this method has the disadvantages of low deposition efficiency and low doping concentration. The reasons may be as follows: First, the fluorine-containing SiO ₂ particles are not formed immediately in the torch flame, but diffuse into the SiO ₂ particles as the fluorine migrates from the torch to the preform loose body. Diffusion takes a certain amount of time, and at the same time, the fluorine partial pressure around _SiO2 particles is very low due to the easy diffusion of fluorine entering the flame reaction into the surrounding environment. Secondly, a part of fluorine around SiO ₂ particles reacts with ^OH in the flame to generate HF, so only a small part of fluorine is incorporated into SiO ₂ particles. In addition, HF has a corrosive effect on glass particles and easily reacts with _SiO2 particles generated by the flame hydrolysis reaction:

SiO ₂ (s)+2HF (g)→SiOF ₂ (g)+H ₂ O (g)………(2)

SiO ₂ (s)+4HF (g)→SiF ₄ (g)+2H ₂ O (g)…………(3)

(s) and (g) in the formula represent solid state and gaseous state, respectively.

These reactions prevent the growth of _SiO2 particles while reducing the amount of _SiO2 particles deposited. Therefore, as the flow rate of the fluorine-containing compound increases, the deposition efficiency and deposition rate gradually decrease, and eventually no deposition will occur. On the other hand, due to the high diffusivity of fluorine, the deposited fluorine-containing loose body will lose 40%-50% of fluorine during the sintering process, which will destroy the refractive index profile structure and seriously affect the performance of the drawn optical fiber. .

In order to solve the above problems, U.S. Patent Application Publication No. US2003/0101771 A1 discloses a method for doping fluorine during the deposition of the loose body of the optical fiber preform: the loose body is deposited in advance by vapor deposition, and then the fluoride gas is passed through the flame of the blowtorch Combustion forms fluorine-containing gas and sprays it on the loose body to form a fluorine-containing atmosphere. At this time, no silicon-containing compound is passed into the torch to avoid the generation of _SiO2 particles. The fluorine-containing gas is doped by diffusion into the loose body. This method has the following problems: since the blowtorch sprays the fluorine-containing gas to a certain part of the loose body from one direction, it is difficult to ensure the axial and radial uniformity of fluorine doping. Although the patent proposes to use multiple torches and the method of opening the mask and heating to improve the uniformity of fluorine doping, there may still be fluctuations in the fluorine content in the axial and radial directions. In addition, fluorine will also diffuse and lose when the fluorine-containing loose body is sintered.

In view of the above problems, U.S. Patent No. US4629485 discloses a method for vitrification and fluorine doping of fiber preform loose body: put the vapor-deposited loose body containing pores into a vitrification furnace for heating, and introduce fluoride gas Flow through the surface of the loose body and make it fully diffuse into the loose body, and finally vitrify the loose body into transparent fluorine-doped glass. The specific steps are: first deposit pure SiO ₂ loose body by OVD method, then put the loose body into a vitrification furnace for dehydration vitrification, then extend the vitrified mandrel, and then use OVD method to deposit the pure SiO 2 after extension. The outer cladding is deposited on the silicon oxide core rod, and the deposited loose body is dehydrated in a vitrification furnace, then fluoride gas is introduced, and then vitrified to obtain a cladding fluorine-doped prefabricated rod. This method has fast fluorine doping speed and high doping concentration, but the hydroxyl content of the core layer of the optical fiber preform manufactured by this method is relatively high, and the transmission loss of the drawn optical fiber at 1383nm wavelength reaches more than 2dB/km. The reasons are: (1) The surface of the vitrified pure silica mandrel is polluted by OH ^- during the extension process, and OH ^- diffuses into the mandrel at high temperature. (2) The cladding is deposited on the extended mandrel by flame hydrolysis reaction, and the ^OH- generated by the hydrolysis reaction diffuses into the core layer under the subsequent high temperature environment (vitrification and wire drawing). In order to reduce OH ^- pollution, not only the core layer but also part of the cladding (inner cladding) are manufactured when producing the mandrel. And when the low water peak optical fiber preform is manufactured outsourced by the flame hydrolysis method, the core cladding ratio (the ratio of the cladding diameter of the core rod to the diameter of the core layer) must be controlled above 4.0. The problem caused by this is: since the core layer and cladding layer of the deposited mandrel are both loose bodies containing a large number of pores, when vitrification is doped with fluorine, fluorine not only diffuses into the cladding layer, but also enters the core layer, resulting in It is difficult for fluorine to be selectively incorporated into the cladding layer to form a refractive index profile structure.

For this reason, US Patent No. US4620861, US Patent Application Publication No. US2002/0073740 A1, and Chinese Patent Application No. 00805475.4 proposed methods for controlling the refractive index profile of the mandrel during vitrification doped with fluorine. The method of the U.S. Patent No. US4620861 is that the core layer and the cladding layer are doped with different concentrations of dopants during the core rod deposition process to make the softening temperature lower than that of pure quartz glass. After heating, the core layer and the uncoated layer are doped Doping the cladding (or different concentrations of dopants in the core and cladding) forms different densities and porosities to achieve selective vitrification of the core rod, and then passes through fluoride gas, and fluorine can be selectively incorporated into the cladding layer. But this method is not suitable for the manufacture of pure silica core optical fiber preforms. Although the softening temperature of the core layer of the core rod decreases after doping, the density increases and the porosity decreases after heating, so that when vitrification is doped with fluorine, fluorine cannot effectively diffuse into the core layer. However, when the concentration of dopant in the core layer and the cladding layer is not much different, the core layer and the cladding layer cannot form an effective interface, so that fluorine will inevitably diffuse into the core layer. The U.S. Patent Application Publication No. US2002/0073740 A1 discloses a method for controlling the refractive index profile of the mandrel according to the relationship between the fluoride reaction rate and the diffusion rate: when the loose body is vitrified and doped with fluorine, the fluoride gas and the loose body Both the reaction speed and the diffusion speed of fluoride gas in the loose body are functions of temperature. As the temperature rises, the reaction speed of fluoride gas and loose body is faster than the diffusion speed of fluoride gas in the loose body. Therefore, by controlling the temperature and time of fluorine doping, the parameter Ф≥1 (Ф is defined as ,in is the radius of the loose body, is the diffusion coefficient of fluoride gas in the loose body, is the reaction rate constant between fluoride gas and loose body), thus controlling the radial doping depth of fluorine in loose body. However, this method is obviously difficult to form a step-type refractive index profile structure. The method disclosed in the Chinese patent application whose application number is 00805475.4 is: after the loose body obtained by vapor deposition is dehydrated and dried in a vitrification furnace, CF ₄ flows through the outer surface of the loose body, and the doping time and temperature are sufficient to make The fluorine doping concentration on the surface of the loose body in contact with CF ₄ is the lowest, and gradually increases towards the center. Apparently, this method is as difficult to form a step-type refractive index profile structure as the method disclosed in US Patent Application Publication No. US2002/0073740 A1.

Contents of the invention

The technical problem to be solved and the technical task proposed by the present invention are to overcome the defects in the prior art and provide a method for doping the cladding of an optical fiber preform rod with fluorine.

For this reason, the present invention adopts following technical scheme:

A method for doping fluorine in the cladding of an optical fiber preform is characterized in that it comprises the following steps:

Depositing the core layer loose body on the target rod;

heating the outer surface of the loose body of the core layer to form a dense layer, so that the density of the dense layer is higher than the density inside the loose body of the core layer;

Depositing an inner cladding loose body outside the dense layer to form a core rod loose body comprising a core layer loose body and an inner cladding loose body;

Extracting the target rod to form a central hole in the center of the mandrel loose body;

Put the mandrel loose body into a vitrification furnace and heat and dehydrate in a dehydration atmosphere, and pass dehydration gas into the central hole while heating;

heating the core rod loose body in a fluoride atmosphere, so that fluorine is selectively incorporated into the inner cladding loose body;

Vitrifying the core rod loose body, making the central hole shrinkage cavity, the core layer loose body form a core layer, and the inner cladding loose body form an inner cladding layer to form a core rod.

As the preferred technical means:

Depositing an outer cladding loose body outside the inner cladding;

Put the loose outer cladding body together with the mandrel into a vitrification furnace and heat and dehydrate in a dehydration atmosphere;

heating the loose body of the outer cladding in a fluoride atmosphere, so that fluorine is incorporated into the loose body of the outer cladding;

The loose body of the outer cladding is vitrified so that the loose body of the outer cladding forms an outer cladding.

As a preferred technical means: the ratio of the diameter of the inner cladding layer to the diameter of the core layer is controlled above 4.0; the density of the dense layer is greater than 1.2 g/cm ³ .

As a preferred technical means: the dehydration atmosphere and dehydration gas contain Cl ₂ gas and He gas; the heating temperature in the vitrification furnace is 1100°C-1400°C.

As a preferred technical means: the dehydration gas flows through the central hole from top to bottom.

As a preferred technical means: the fluoride atmosphere contains SiF ₄ , CF ₄ , SF ₆ , C ₂ F ₆ , C ₂ F ₂ Cl ₂ , F ₂ , C ₃ F ₈ , NF ₃ , SOF ₂ , SO ₂ ClF One or a combination of two or more of them.

As a preferred technical means: the fluoride atmosphere contains He.

As a preferred technical means: the loose body of the core layer is deposited after the target rod is inserted into the tubular handle, and the tubular handle is located at one end of the loose body of the deposited core layer. Further, after the target rod is pulled out, a tubular extension handle is welded to the tubular handle, and then the tubular extension handle is installed on the fixture above the vitrification furnace, and the mandrel loose body is slowly lowered into the vitrification furnace for vitrification .

The beneficial effect of the present invention is: in the mandrel deposition process of the present invention method, the interface of core layer loose body and inner cladding loose body forms dense layer (that is the dense layer that heats core layer loose outer surface to form), and core layer loose body and The inner cladding loose body is still porous and loose, and the dehydration gas is passed through the central hole during the dehydration process, which effectively reduces the OH ^- content in the core layer, thereby greatly reducing the attenuation of the optical fiber at 1383nm. Secondly, due to the existence of a dense layer at the interface between the core soot and the inner cladding soot, the fluoride gas selectively diffuses into the inner cladding soot when vitrified and doped with fluorine, but cannot enter the core soot, thus forming a step-type refraction rate distribution. Compared with the prior art, due to the doping and vitrification in the fluoride atmosphere, the fluorine is evenly distributed in the radial direction of the cladding, and the refractive index profile will not be destroyed due to the loss of fluorine. Finally, the method of the present invention has high fluorine doping speed and high doping concentration, and is suitable for large-scale production of optical fiber preform rods.

Description of drawings

Figure 1 is a schematic cross-sectional view of an optical fiber preform;

Fig. 2 is the schematic diagram of depositing loose body according to the method of the present invention;

Fig. 3 is a schematic cross-sectional view when depositing loose body according to the method of the present invention;

Fig. 4 is the schematic diagram when implementing vitrification according to the method of the present invention;

Figure 5 is a cross-sectional view of the refractive index of the preform according to Embodiment 1 of the present invention;

6 is a cross-sectional view of the refractive index of the preform rod according to Embodiment 2 of the present invention;

Figure 7 is a cross-sectional view of the refractive index of the preform according to Embodiment 3 of the present invention;

In Figure 5-7, the horizontal axis represents the radial distance of the preform, and the vertical axis represents the refractive index of the preform (the zero point is the refractive index of pure SiO ₂ );

Explanation of symbols in the figure:

1-mandrel loose body 1, 2-core layer loose body, 3-inner cladding loose body, 4-outer cladding loose body, 5-target rod, 6-tubular handle, 7-blower, 8-dense layer, 9- Tubular extension handle, 10-central hole, 11-capillary, 12-capillary hole, 13-pure quartz furnace core tube, 14-heating body, 15-gas supply port, 16-exhaust port;

1'-mandrel, 2'-core, 3'-inner cladding, 4'-outer cladding;

a-core diameter, t-inner cladding diameter, D-outer cladding diameter.

Detailed ways

The present invention will be further described below in conjunction with the accompanying drawings of the description.

The method for doping the cladding of an optical fiber preform rod with fluorine of the present invention comprises the following steps:

Depositing the core layer loose body on the target rod;

heating the outer surface of the loose body of the core layer to form a dense layer, so that the density of the dense layer is higher than the density inside the loose body of the core layer;

Depositing an inner cladding loose body outside the dense layer to form a core rod loose body comprising a core layer loose body and an inner cladding loose body;

Extracting the target rod to form a central hole in the center of the mandrel loose body;

Put the mandrel loose body into a vitrification furnace and heat and dehydrate in a dehydration atmosphere, and pass dehydration gas into the central hole while heating;

heating the core rod loose body in a fluoride atmosphere, so that fluorine is selectively incorporated into the inner cladding loose body;

Vitrifying the core rod loose body, making the central hole shrinkage cavity, the core layer loose body form a core layer, and the inner cladding loose body form an inner cladding layer to form a core rod.

As the preferred technical means:

Depositing an outer cladding loose body outside the inner cladding;

Put the loose outer cladding body together with the mandrel into a vitrification furnace and heat and dehydrate in a dehydration atmosphere;

heating the loose body of the outer cladding in a fluoride atmosphere, so that fluorine is incorporated into the loose body of the outer cladding;

The loose body of the outer cladding is vitrified so that the loose body of the outer cladding forms an outer cladding.

As a preferred technical means: the ratio of the diameter of the inner cladding layer to the diameter of the core layer is controlled above 4.0; the density of the dense layer is greater than 1.2 g/cm ³ .

As a preferred technical means: the dehydration atmosphere and dehydration gas contain Cl ₂ gas and He gas; the heating temperature in the vitrification furnace is 1100°C-1400°C.

As a preferred technical means: the dehydration gas flows through the central hole from top to bottom.

As a preferred technical means: the fluoride atmosphere contains SiF ₄ , CF ₄ , SF ₆ , C ₂ F ₆ , C ₂ F ₂ Cl ₂ , F ₂ , C ₃ F ₈ , NF ₃ , SOF ₂ , SO ₂ ClF One or a combination of two or more of them.

As a preferred technical means: the fluoride atmosphere contains He.

As a preferred technical means: the loose body of the core layer is deposited after the target rod is inserted into the tubular handle, and the tubular handle is located at one end of the loose body of the deposited core layer. Further, after the target rod is pulled out, a tubular extension handle is welded to the tubular handle, and then the tubular extension handle is installed on the fixture above the vitrification furnace, and the mandrel loose body is slowly lowered into the vitrification furnace for vitrification .

Preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

FIG. 1 is a schematic cross-sectional view of an optical fiber preform. Optical fiber preforms are formed by vitrification of deposited loose bodies. The optical fiber preform consists of a core rod 1' and an outer cladding 4', wherein the core rod 1' includes a core 2' and an inner cladding 3'. The core layer 2' has a higher refractive index than the inner cladding layer 3' and the outer cladding layer 4', thereby forming a waveguide structure. In order to ensure that the optical fiber has excellent optical performance, when producing the core rod 1', not only the core layer 2' but also a part of the cladding (inner cladding 3') must be fabricated.

Fig. 2 is a schematic diagram of the OVD deposition process (equipment) used to manufacture loose core rods. The target rod 5 is inserted into the tubular handle 6 and mounted on a lathe (not shown). While the target rod 5 is rotating, it also moves back and forth in a straight line relative to at least one torch 7 (it can also be performed by moving the torch 7 relative to the target rod 5). The reaction of gaseous halide raw materials ( _SiCl4 , etc.) with oxyhydrogen flame or methane flame produces a large number of _SiO2 particles, the particle size of which varies from a few nanometers to hundreds of nanometers, and the particles grow gradually with the agglomeration, and then the particles Along with the air flow and thermophoresis, it is brought to the target rod 5 for deposition. As the rod body moves back and forth, _SiO2 particles are deposited layer by layer on the outer surface of the mandrel loose body 1 . Both ends of the loose body need to be heated with two taillights (not shown in the figure) to prevent the loose body from cracking during deposition. In order to obtain the refractive index difference, other substances need to be mixed into the raw materials during the process of depositing the core layer loose body 2 . The most common is doping GeO ₂ to increase the refractive index of the core loose body 2 . After the deposition of the core layer loose body 2 is completed, the supply of the gas halide raw material is stopped in the blowtorch 7, and the outer surface of the core layer loose body 2 is heated with an oxyhydrogen flame or a methane flame to vitrify it to form a dense layer 8. The density is higher than that inside the loose body of the core layer 2, preferably greater than 1.2 g/cm ³ , more preferably greater than 1.8 g/cm ³ . When heating, it is necessary to ensure that the outer surface of the entire core loose body 2 and the invalid parts at both ends are vitrified to form a dense layer, thereby preventing fluorine from diffusing into the core loose body 2 when the vitrified cladding is doped with fluorine. For the dense layer 8, furnace heating, laser heating and other methods that can make the loose layer dense can also be used. The thickness of the dense layer 8 is controlled at 0.01mm-0.5mm. Next, the inner cladding loose body 3 is continuously deposited on the outer surface of the dense layer 8, and at this time, only SiCl ₄ gas raw material is passed into the oxyhydrogen flame or methane flame torch. 3 is a schematic cross-sectional view of the mandrel loose body 1, and the dense layer 8 is located on the interface between the core loose body 2 and the inner cladding loose body 3 to isolate the core layer loose body 2 from the inner cladding loose body 3. In order to prevent the OH ^- introduced in the manufacturing process of the outer cladding loose body 4 from diffusing into the core layer 2, it is necessary to calculate the ratio of the diameter of the inner cladding loose body 3 to the diameter of the core layer loose body 2 (as shown in Figure 5, Figure 6 and Figure 7) It is controlled above 4.0, preferably above 4.5, more preferably above 5.0.

The mandrel loose body 1 after deposition is in a loose and porous state, with a density of 0.1-1.2 g/cm ³ , preferably a density of 0.2-0.8 g/cm ³ , more preferably a density of 0.4-0.6 g /cm ³ . Next, the target rod 5 is pulled out, the tubular extension handle 9 is welded to the tubular handle 6, and then the mandrel loose body 1 is suspended by the tubular extension handle 9 and placed in a vitrification furnace for vitrification.

The vitrification furnace is heated by a heating body 14 (resistance furnace or other heating device) to heat the pure quartz furnace core tube 13, the length of the heating zone is 400mm when the temperature fluctuates ±25°C, the rotation speed of the mandrel loose body 1 is 3-10rpm, and Gradually descend into the heating zone. Since a large amount of OH ^- will be introduced during the process of hydrolysis reaction deposition of loose body, and prefabricated rods, especially mandrel loose body 1, have particularly high requirements for OH ^- , in order to obtain low OH ^- content mandrel loose body 1, it is necessary to go through The process of dehydration. During the dehydration process, Cl ₂ , O ₂ and He enter the vitrification furnace from the gas supply port 15 , flow over the surface of the mandrel loose body 1 from bottom to top (arrow 17 ), and then flow out from the exhaust port 16 of the vitrification furnace. After the core rod loose body 1 is deposited, pulling away the target rod 5 will form a central hole 10 in the middle. Therefore, in the dehydration process, in addition to letting the gas flow through the outer surface of the core rod loose body 1, Cl ₂ , O ₂ and He pass into the central hole 10 (arrow 18) from the upper part of the tubular extension handle 9 to ensure sufficient dehydration of the core loose body 2; the role of Cl ₂ is a dehydrating agent, and its reaction equation is:

2Cl ₂ + 2H ₂ O = O ₂ + 4HCl……………………(4)

2Si-OH + 2Cl ₂ = 2Si-Cl + O ₂ + 2HCl... (5)

The main principle of dehydration is essentially the halogenation reaction process of replacing isolated OH ^- with Cl ^- , and the reaction result leads to the generation of Si-Cl bond. Since the fundamental frequency vibration absorption peak of this bond is located near 25 μm, it is far away from the current working wavelength of optical communication. area, so it has no significant impact on the absorption attenuation during optical fiber transmission; the main function of He is to assist dehydration, because He has the characteristics of small atomic volume, high diffusivity, and stable performance, so it can easily pass through the mandrel loose body 1 The pores in the core penetrate into the interior and will not affect the performance of the preform. On the one hand, He brings in Cl ₂ and O ₂ to make it fully contact with the mandrel loose body 1; brought out; the function of O ₂ is to prevent dopant elements (such as Ge) from being volatilized by the halogenation reaction with Cl ₂ , resulting in the loss of Ge. The dehydration drying step is carried out at a temperature lower than the glass transition temperature of the mandrel loose body 1, and the dehydration temperature is 1100°C-1400°C. If the temperature is too high, it is easy to cause the pores of the mandrel loose body 1 to close, and Cl ₂ cannot effectively enter the inside of the mandrel loose body 1 for dehydration. If the temperature is too low, the required time will be too long, which is not suitable for industrialized production. The drying time is 0.5-5 hours. After dehydration and drying, the core rod loose body 1 gradually descends into the heating zone again, and at the same time, fluoride gas is introduced from the gas supply port 15 to flow over the surface of the core rod loose body 1 (arrow 17) to do the inner cladding loose body 3 with fluorine. Preferably, the vitrification furnace preferably contains a diluent gas such as helium. Since the dense layer 8 exists at the interface between the core layer loose body 2 and the inner cladding loose body 3 of the mandrel loose body 1 , the fluoride gas diffused into the inner cladding loose body 3 cannot enter the core layer loose body 2 . The lower end of the core rod loose body 1 contains a capillary 11 to prevent the gas in the vitrification furnace from entering the central hole 10 of the core rod loose body 1 from the bottom of the core rod loose body 1 (at this time, the upper part of the tubular extension handle 9 can pass into Inert gas such as He). Fluoride gas is at least one or at least two of SiF ₄ , CF ₄ , SF ₆ , C ₂ F ₆ , C ₂ F ₂ Cl ₂ , F ₂ , C ₃ F ₈ , NF ₃ , SOF ₂ , SO ₂ ClF combination of species. The fluorine doping temperature can be the same as the dehydration temperature, or can be carried out at a temperature different from the dehydration temperature. Fluoride gas accounts for 1-30% of the total gas flow. The time for doping with fluorine is enough to make the fluorine fully diffuse into and evenly distribute in the inner cladding loose body 3 . The time is controlled at 0.5-5 hours. Then pass through He, _O2 to carry out vitrification, and the temperature range of vitrification is 1400-1600 ℃. Due to the doping of fluorine in the loose body 3 of the inner cladding, the glass transition temperature can be lowered. According to the diameter, length, glass transition temperature and speed of the mandrel loose body, the vitrification time is 0.5-8 hours. After the vitrification is completed, because the capillary hole 12 is melted and closed at high temperature, the lower part of the mandrel loose body is sealed. Then the transparent mandrel 1' with the central hole 10 is lifted outside the vitrification furnace to cool. Then install the mandrel 1' with the central hole 10 on the longitudinal extension equipment, vacuumize the central hole 10 from one end of the tubular extension handle 9, heat the sealing end of the mandrel to above 1900°C with a furnace, and gradually make the central hole 10 closure. The obtained solid transparent mandrel 1' is mounted on an OVD lathe as a target rod for deposition of the outer cladding loose body 4. In the torch, the gas raw material SiCl ₄ is hydrolyzed to generate SiO ₂ particles, which are deposited on the outer surface of the target rod to form a loose body. Stop deposition after reaching the set outer diameter, and put it into the vitrification furnace again for vitrification. The vitrification of the outer cladding loose body 4 in the conventional production process does not need to be dehydrated. In order to prevent the fluoride gas from reacting with the OH in the vitrification furnace when fluorine is added ^to vitrification to generate HF to corrode the pure quartz furnace core tube 13, the outer cladding of the present invention The vitrification of the loose body 4 needs to completely remove the OH ^- inside the loose body, and then perform fluorine doping and vitrification of the outer cladding loose body 4 according to the above-mentioned steps of fluorine doping and vitrification, and finally obtain a cladding fluorine-doped transparent preform. The preform is drawn into a step-type single-mode optical fiber according to a conventional drawing process.

Example 1:

The mandrel loose body 1 is manufactured by OVD method. Insert the target rod 5 into the tubular handle 6 and install it on the lathe. The gaseous halide raw materials SiCl ₄ and GeCl ₄ are hydrolyzed in an oxyhydrogen flame to produce a large amount of SiO ₂ doped GeO ₂ particles deposited on the target rod 5 . As the target rod 5 moves back and forth, SiO ₂ -GeO ₂ particles are deposited layer by layer on the outer surface of the rotating mandrel loose body 1 . After the core layer loose body 2 is deposited, the supply of raw materials SiCl ₄ and GeCl ₄ in the torch 7 is stopped, and the outer surface of the core layer loose body 2 is heated with an oxyhydrogen flame to vitrify it to form a dense layer 8 . The flow rates of hydrogen and oxygen in the blowtorch 7 are 200 slpm and 90 slpm respectively. Next, feed the raw material SiCl ₄ into the torch 7 to deposit the inner cladding loose body 3 on the outer surface of the dense layer 8 . After the deposition of the inner cladding loose body 3 is completed, the ratio of the diameter of the inner cladding loose body 3 to the diameter of the core layer loose body 2 is 4.5 (the loose body shrinks in the same proportion after vitrification, and the ratio of the diameter of the inner cladding layer 3' to the diameter of the core layer 2' is t /a is still 4.5, as shown in Figure 5). The mandrel loose body 1 has an outer diameter of 120mm and a length of 1500mm.

Extract the target rod 5 from the core of the loose body, weld the tubular extension handle 9 to the tubular handle 6, then install the tubular extension handle 9 on the clamp (not shown) above the vitrification furnace, and slowly put the core The rod loose body 1 descends into the vitrification furnace for vitrification. The vitrification furnace is heated by the heating body 14 to heat the pure quartz furnace core tube 13, and the length of the heating zone is 400mm when the temperature fluctuates ±25°C. The mandrel loose body 1 rotates at a speed of 3 rpm, and gradually descends into the heating zone. Firstly, an inert gas nitrogen is purged, and then Cl ₂ , O ₂ and He are passed through for dehydration and drying, so as to remove OH ⁻ in the mandrel loose body 1 . The dehydration gas enters the vitrification furnace from the gas supply port 15 , flows over the surface of the mandrel loose body 1 from bottom to top (arrow 17 ), and flows out from the exhaust port 16 of the vitrification furnace. In order to remove OH ^- in the core loose body 2, Cl ₂ , O ₂ and He pass through the central hole 10 (arrow 18) from the upper part of the tubular extension handle 9, and flow out from the lower end of the mandrel loose body 1; the dehydration temperature is 1100°C . The dehydration time is 3 hours. After dehydration and drying, the core rod loose body 1 gradually descends into the heating zone again, and at the same time, SF ₆ gas is introduced from the gas supply port 15 and He flows through the surface of the core rod loose body 1 (arrow 17) to do the inner cladding loose body 3 with fluorine. Because there is a dense layer 8 at the interface between the core layer loose body 2 and the inner cladding loose body 3 of the mandrel loose body 1 and the lower end of the central hole 10 is provided with a capillary 11, the fluoride gas diffused into the inner cladding loose body 3 cannot enter the core layer loose body 2. Fluoride gas cannot enter the central hole 10 either. The upper part of the tubular extension handle 9 is fed with inert gas He and discharged through the capillary 11 at the lower end of the central hole 10, so as to prevent the gas in the vitrification furnace from entering the central hole 10 of the mandrel loose body 1 from the lower end of the central hole 10 (if not from the tubular The upper part of the extension handle 9 feeds the inert gas He and discharges it through the capillary 11 at the lower end of the central hole 10, so that the lower end of the central hole can be blocked to realize that the fluoride gas cannot enter the central hole 10). The temperature of fluorine doping is 1200°C. SF ₆ gas accounts for 15% of the total gas flow. The fluorine doping time is 2 hours, so that fluorine can fully diffuse into the inner cladding loose body 3 . Then pass He, _O2 to carry out vitrification, the temperature of vitrification is 1450°C, and the time of vitrification is 5 hours. After the vitrification is completed, the mandrel 1' passes through the central shrinkage cavity to reduce its outer diameter to 30 mm. The extended mandrel 1' is installed on the OVD lathe to deposit the SiO ₂ outer cladding loose body 4 as the target rod for the deposition of the outer cladding loose body 4, and then put into the vitrification furnace again for dehydration, fluorine doping and vitrification. Finally, a transparent optical fiber prefabricated rod in which the core layer 2' is doped with germanium, the inner cladding layer 3' and the outer cladding layer 4' is doped with fluorine is obtained. Tested by the PK2600 preform comprehensive tester, the refractive index profile of the optical fiber preform is shown in Figure 5, and the Δ value reaches 1.1%. An optical fiber with a numerical aperture of 0.22 was obtained by conventional drawing.

Example 2

The mandrel loose body 1 is manufactured by the OVD method as described in Example 1. The difference is that when the core layer loose body 2 is deposited, the raw material SiCl ₄ is passed through to carry out hydrolysis reaction in the hydrogen-oxygen flame, and the generated SiO ₂ particles are deposited on the target rod 5 to form the pure SiO ₂ core layer loose body 2 . After heating to form the dense layer 8 as described in Example 1, the inner cladding loose body 3 is continuously deposited. After the deposition is completed, the ratio of the diameter of the inner cladding loose body 2 to the diameter of the core layer loose body 2 is 5.0. The mandrel loose body 1 has an outer diameter of 100mm and a length of 1800mm.

Pull out the target rod 5, weld the tubular extension handle 9 to the tubular handle 6, then install the tubular extension handle 9 on the fixture above the vitrification furnace (not shown in the figure), and slowly lower the mandrel loose body 1 Vitrification in a vitrification furnace. The mandrel loose body 1 rotates at a speed of 5 rpm, and gradually descends into the heating zone. Firstly, an inert gas nitrogen is purged, and then Cl ₂ and He are passed through for dehydration and drying, so as to remove OH ⁻ in the mandrel loose body 1 . The dehydration gas enters the vitrification furnace from the gas supply port 15 , flows over the surface of the mandrel loose body 1 from bottom to top (arrow 17 ), and then flows out from the exhaust port 16 of the vitrification furnace. In order to remove OH ^- in the core loose body 2, Cl ₂ and He pass through the central hole 10 (arrow 18) from the upper part of the tubular extension handle 9, and flow out from the lower end of the mandrel loose body 1; the dehydration temperature is 1300°C. The dehydration time is 7 hours. After dehydration and drying, the core rod loose body 1 gradually descends into the heating zone again, and at the same time, SiF ₄ gas is introduced from the gas supply port 15 and He flows through the surface of the core rod loose body 1 (arrow 17) to do the inner cladding loose body 3 with fluorine. Because there is a dense layer 8 at the interface between the core layer loose body 2 and the inner cladding loose body 3 of the mandrel loose body 1 and the lower end of the central hole 10 is provided with a capillary 11, the fluoride gas diffused into the inner cladding loose body 3 cannot enter the core layer loose body 2. Fluoride gas cannot enter the central hole 10 either. The upper part of the tubular extension handle 9 is fed with inert gas He and discharged through the capillary 11 at the lower end of the central hole 10, so as to prevent the gas in the vitrification furnace from entering the central hole 10 of the mandrel loose body 1 from the lower end of the central hole 10 (if not from the tubular The upper part of the extension handle 9 feeds the inert gas He and discharges it through the capillary 11 at the lower end of the central hole 10, so that the lower end of the central hole can be blocked to realize that the fluoride gas cannot enter the central hole 10). The temperature of fluorine doping is 1350°C. SiF ₄ gas accounts for 30% of the total gas flow. The fluorine doping time is 3 hours, so that fluorine can fully diffuse into the inner cladding loose body 3 . Then, He was introduced to carry out vitrification, the temperature of vitrification was 1500° C., and the time of vitrification was 8 hours. After the vitrification is completed, the mandrel 1' passes through the central shrinkage cavity to reduce its outer diameter to 30mm. The extended mandrel 1' is installed on the OVD lathe to deposit the SiO ₂ outer cladding loose body 4 as the target rod for the deposition of the outer cladding loose body 4, and then put into the vitrification furnace again for dehydration, fluorine doping and vitrification. Finally, a transparent optical fiber prefabricated rod in which the core layer 2' is pure SiO ₂ , the inner cladding layer 3' and the outer cladding layer 4' doped with fluorine is obtained. Tested by the PK2600 preform comprehensive tester, the refractive index profile of the optical fiber preform is shown in Figure 6, and the Δ value is 0.32%. The ultra-low loss optical fiber was drawn by conventional methods, the attenuation at 1550nm was 0.170dB/km, and the attenuation at 1383nm was 0.248dB/km.

Example 3

The mandrel loose body 1 is manufactured by the OVD method as described in Example 2. The dense layer ₈ of the core layer loose body 2 and the inner cladding loose body 3 interface is formed by heating in a heating furnace to obtain pure SiO Core rod loose body 1 of the core layer, the diameter of the inner cladding loose body 3 and the diameter of the core layer loose body 2 The ratio is 5.0. The mandrel loose body 1 has an outer diameter of 100mm and a length of 1200mm.

Pull out the target rod 5, weld the tubular extension handle 9 to the tubular handle 6, then install the tubular extension handle 9 on the fixture above the vitrification furnace (not shown in the figure), and slowly lower the mandrel loose body 1 Vitrification in a vitrification furnace. The mandrel loose body 1 rotates at a speed of 10 rpm, and gradually descends into the heating zone. Firstly, an inert gas nitrogen is purged, and then Cl ₂ and He are passed through for dehydration and drying, so as to remove OH ⁻ in the mandrel loose body 1 . The dehydration gas enters the vitrification furnace from the gas supply port 15 , flows over the surface of the mandrel loose body 1 from bottom to top (arrow 17 ), and then flows out from the exhaust port 16 of the vitrification furnace. In order to remove the OH ^- in the core loose body 2, Cl ₂ and He pass through the central hole 10 from the upper part of the tubular extension handle 9, and flow out from the lower end of the mandrel loose body 1 (arrow 18); the dehydration temperature is 1400°C. The dehydration time is 2 hours. After dehydration and drying, the core rod loose body 1 gradually descends into the heating zone again, and at the same time, CF ₄ gas is introduced from the gas supply port 15 and He flows through the surface of the core rod loose body 1 (arrow 17) to do the inner cladding loose body 3 with fluorine. Because there is a dense layer 8 at the interface between the core layer loose body 2 and the inner cladding loose body 3 of the mandrel loose body 1 and the lower end of the central hole 10 is provided with a capillary 11, the fluoride gas diffused into the inner cladding loose body 3 cannot enter the core layer loose body 2. Fluoride gas cannot enter the central hole 10 either. The upper part of the tubular extension handle 9 is fed with inert gas He and discharged through the capillary 11 at the lower end of the central hole 10, so as to prevent the gas in the vitrification furnace from entering the central hole 10 of the mandrel loose body 1 from the lower end of the central hole 10 (if not from the tubular The upper part of the extension handle 9 feeds the inert gas He and discharges it through the capillary 11 at the lower end of the central hole 10, so that the lower end of the central hole can be blocked to realize that the fluoride gas cannot enter the central hole 10). The temperature of fluorine doping is 1300°C. CF ₄ gas accounts for 30% of the total gas flow. The fluorine doping time is 3 hours, so that fluorine can fully diffuse into the inner cladding loose body 3 . Then, He was introduced to carry out vitrification, the temperature of vitrification was 1550° C., and the time of vitrification was 5 hours. After the vitrification is completed, the mandrel 1' passes through the central shrinkage cavity to reduce its outer diameter to 30 mm. The extended mandrel 1' is installed on the OVD lathe to deposit the SiO ₂ outer cladding loose body 4 as the target rod for the deposition of the outer cladding loose body 4, and then put into the vitrification furnace again for dehydration, fluorine doping and vitrification. In this embodiment, the fluorine doping concentration of the outer cladding loose body 4 is lower than that of Example 2 (when the outer cladding loose body 4 is doped with fluorine, the fluorine doping concentration is reduced by reducing the flow rate of the CF gas in the vitrification furnace), and finally _the core layer 2 is obtained. 'is pure SiO ₂ , the inner cladding 3' and the outer cladding 4' are fluorine-doped transparent optical fiber prefabricated rods. Tested by the PK2600 preform comprehensive tester, the refractive index profile of the optical fiber preform is shown in Figure 7, and the Δ value is 0.33%. The ultra-low loss optical fiber was drawn by conventional methods, the attenuation of 1550nm was 0.175dB/km, and the attenuation of 1383nm was 0.253dB/km.

It should be noted that the manner of the above-mentioned embodiment is limited to the description of the embodiment, but the present invention is not limited to the above-mentioned manner, and those skilled in the art can conveniently modify it without departing from the scope of the present invention. Therefore, the present invention The scope should include the broadest range of the principles and novel features disclosed in the present invention.

Claims (8)

1. optical fiber prefabricating stick cladding mixes a method for fluorine, it is characterized in that comprising the following steps:

Target rod deposits sandwich layer loose media;

The outside surface heating described sandwich layer loose media forms tight zone, makes the density of described tight zone higher than the density of described sandwich layer loose media inside;

The mandrel loose body comprising sandwich layer loose media, inner cladding loose media is formed at described tight zone external sediment inner cladding loose media;

Detach described target rod and be formed centrally centre hole in mandrel loose body;

Described mandrel loose body is put into vitrifying stove and at dehydration atomosphere thermal dehydration, while heating, in described centre hole, passes into dehydrated air;

In fluorochemical atmosphere, heat described mandrel loose body, make fluorine optionally mix in inner cladding loose media;

Mandrel loose body described in vitrifying, the centre hole shrinkage cavity described in order, sandwich layer loose media form sandwich layer, inner cladding loose media forms inner cladding, form plug;

The Ratio control of the diameter of described inner cladding and the diameter of sandwich layer is more than 4.0; The density of described tight zone is greater than 1.2g/cm ³.

2. optical fiber prefabricating stick cladding according to claim 1 mixes the method for fluorine, it is characterized in that:

At described inner cladding external sediment surrounding layer loose media;

Described surrounding layer loose media is put into vitrifying stove and at dehydration atomosphere thermal dehydration in company with plug;

In fluorochemical atmosphere, heat described surrounding layer loose media, make fluorine mix in surrounding layer loose media;

Vitrifying surrounding layer loose media, makes surrounding layer loose media form surrounding layer.

3. optical fiber prefabricating stick cladding according to claim 1 and 2 mixes the method for fluorine, it is characterized in that: contain Cl in described dehydration atomosphere and dehydrated air ₂gas and He gas; Heating temperature in vitrifying stove is 1100 DEG C-1400 DEG C.

4. optical fiber prefabricating stick cladding according to claim 1 and 2 mixes the method for fluorine, it is characterized in that: described dehydrated air flows through described centre hole from top to bottom.

5. optical fiber prefabricating stick cladding according to claim 1 and 2 mixes the method for fluorine, it is characterized in that: described fluorochemical atmosphere contains SiF ₄, CF ₄, SF ₆, C ₂f ₆, C ₂f ₂cl ₂, F ₂, C ₃f ₈, NF ₃, SOF ₂, SO ₂clF one or more combination wherein.

6. optical fiber prefabricating stick cladding according to claim 1 and 2 mixes the method for fluorine, it is characterized in that: described fluorochemical atmosphere contains He.

7. optical fiber prefabricating stick cladding according to claim 1 and 2 mixes the method for fluorine, it is characterized in that: deposit sandwich layer loose media by after target rod tubular handle, and pipe handle is positioned at one end of post-depositional sandwich layer loose media.

8. optical fiber prefabricating stick cladding according to claim 7 mixes the method for fluorine, it is characterized in that: after detaching target rod, one tubular elongate handle is welded on pipe handle, then tubular elongate handle is installed on the fixture above vitrifying stove and slow mandrel loose body being dropped in the middle of vitrifying stove carries out vitrifying.