阅读说明：本技术 一种纯二氧化硅芯光纤预制棒的制备方法 (Preparation method of pure silica core optical fiber preform ) 是由 冯高锋 沈杰 于 2020-06-28 设计创作，主要内容包括：本发明提供了一种纯二氧化硅芯光纤预制棒的制备方法,包括以下步骤：a)采用气相轴向沉积法制作芯棒松散体；芯棒松散体由芯层松散体和包覆所述芯层松散体的内包层松散体组成；所述芯层松散体的密度为1.0～1.25g/cm<Sup>3</Sup>,内包层松散体的密度为0.1～0.6g/cm<Sup>3</Sup>；b)将步骤a)得到的芯棒松散体依次进行脱水、预烧结、掺氟和玻璃化,得到芯棒；芯棒由芯层和包覆所述芯层的内包层组成；所述芯层的密度大于等于1.3g/cm<Sup>3</Sup>,内包层的密度小于等于0.8g/cm<Sup>3</Sup>；c)将步骤b)得到的芯棒包覆外包层,得到纯二氧化硅芯光纤预制棒。该制备方法通过控制芯层不同阶段的密度,使脱水时氯气能够进入芯层与羟基反应,后续掺氟时氟无法进入芯层,从而解决纯二氧化硅芯光纤预制棒羟基含量偏高的问题。(The invention provides a preparation method of a pure silica core optical fiber preform, which comprises the following steps: a) preparing a loose core rod body by adopting a vapor axial deposition method; the core rod loose body consists of a core layer loose body and an inner cladding loose body coating the core layer loose body; the density of the loose body of the core layer is 1.0-1.25 g/cm 3 The density of the inner cladding loose body is 0.1-0.6 g/cm 3 (ii) a b) Sequentially dehydrating, pre-sintering, fluorine doping and vitrifying the loose core rod body obtained in the step a) to obtain a core rod; the core rod consists of a core layer and an inner cladding layer covering the core layer; the density of the core layer is more than or equal to 1.3g/cm 3 The density of the inner cladding is less than or equal to 0.8g/cm 3 (ii) a c) And c) coating the outer cladding layer on the core rod obtained in the step b) to obtain the pure silica core optical fiber prefabricated rod. According to the preparation method, the densities of different stages of the core layer are controlled, so that chlorine can enter the core layer to react with hydroxyl during dehydration, and fluorine cannot enter the core layer during subsequent fluorine doping, so that the problem that the hydroxyl content of a pure silica core optical fiber preform is high is solved.)

1. A preparation method of a pure silica core optical fiber preform comprises the following steps:

a) preparing a loose core rod body by adopting a vapor axial deposition method; the core rod loose body consists of a core layer loose body and an inner cladding loose body coating the core layer loose body; the density of the core layer loose body is 1.0g/cm³～1.25g/cm³The density of the inner cladding loose body is 0.1g/cm³～0.6g/cm³；

b) Sequentially dehydrating, pre-sintering, fluorine doping and vitrifying the loose core rod body obtained in the step a) to obtain a core rod; the core rod consists of a core layer and an inner cladding layer covering the core layer; the density of the core layer is more than or equal to 1.3g/cm³The density of the inner cladding is less than or equal to 0.8g/cm³；

c) And c) coating the outer cladding layer on the core rod obtained in the step b) to obtain the pure silica core optical fiber prefabricated rod.

2. The method according to claim 1, wherein the vapor axial deposition process in step a) is specifically:

adopting VAD device, firstly, vertically fixing the seed rod on a clamp; then, the raw material gas SiCl is introduced₄Delivering to oxyhydrogen flame blowtorch for combustion, and performing hydrolysis reaction at high temperature of oxyhydrogen flame to generate a large amount of SiO₂Depositing particles onto the seed rods; and spraying flame to the central part of the seed rod by using a core layer blowtorch to form a core layer loose body at the tail end of the seed rod, spraying flame to the peripheral part of the seed rod by using a cladding blowtorch to form an inner cladding loose body on the core layer, and simultaneously, continuously lifting and rotating the seed rod upwards to obtain a core rod loose body.

3. The method according to claim 2, characterized in thatCharacterized in that SiCl of the core layer blast lamp₄SiCl with flow rate of 10-20 g/min for cladding blast lamp₄The flow rate is 50 g/min-70 g/min;

the deposition temperature of the core layer blowtorch is 1250-1450 ℃.

4. The preparation method according to claim 1, wherein the dehydration in step b) is specifically performed by:

adopting a vitrification furnace, firstly, gradually reducing the core rod loose body at the rotating speed of 2 rpm-8 rpm, and entering a high-temperature area with the temperature of 1000-1300 ℃; then Cl was introduced₂And He to remove OH from the loose core rod^-。

5. The method of claim 4, wherein the Cl is₂The flow rate of the He is 0.5L/min-1.5L/min, and the flow rate of the He is 10L/min-30L/min;

the dehydration time is 8-12 h.

6. The method according to claim 1, wherein the pre-sintering temperature in step b) is 1150-1400 ℃ and the moving speed is 1-15 mm/min.

7. The preparation method according to claim 1, wherein the fluorine-doped gas introduced in step b) is fluorine-containing gas and He, wherein the flow rate of the fluorine-containing gas is 0.1L/min to 3L/min, and the flow rate of the He is 0.4L/min to 12L/min;

the temperature of the fluorine doping is 1100-1300 ℃, and the time is 0.5-5 h.

8. The method according to claim 1, wherein the vitrification temperature in the step b) is 1400 ℃ to 1600 ℃ and the time is 0.5h to 8 h.

9. The method according to claim 1, wherein the step c) of overcladding comprises:

firstly, OVD deposition lathe is adopted to deposit loose outer cladding layer on the periphery of the core rod, and then the core rod is placed into a vitrification furnace to be dehydrated, doped with fluorine and vitrified in sequence to obtain pure SiO as the core layer₂And the inner cladding and the outer cladding are fluorine-doped optical fiber prefabricated rods.

10. The preparation method according to claim 9, wherein the fluorine-doped gas is fluorine-containing gas and He, wherein the flow rate of the fluorine-containing gas is 0.075L/min to 2.25L/min, and the flow rate of the He is 0.425L/min to 12.75L/min.

Technical Field

The invention relates to the technical field of optical fiber preforms, in particular to a preparation method of a pure silica core optical fiber preform.

Background

A preform for a raw material optical fiber from which an optical fiber is drawn has a basic structure including a core layer and a clad layer having a lower refractive index. SiO 2₂Is a glass former used to manufacture optical fiber preform, whose refractive index can be changed by doping to form a waveguide structure. Generally, GeO is doped into the core layer₂The refractive index of the core layer is made higher than that of the cladding pure quartz glass. The relative difference between the refractive indexes of the core layer and the cladding layer is the relative refractive indexThe refractive indices of the core layer and the cladding layer are n, respectively₁And n₂The Δ value is given by equation (1):

sometimes to increase the bend resistance of an optical fiber, it is desirable to increase the bending resistance of the optical fiber by increasing the GeO in the core layer of the optical fiber₂The content increases the value of Delta, but with GeO₂The increase of the content can cause the increase of the Rayleigh scattering of the optical fiber; if doped GeO₂When the content is too high, GeO gas is easily formed, and bubbles are generated; this is detrimental to both the transmission performance and the strength of the final fiber. For the above reasons, the value of Δ can be increased by lowering the refractive index of the cladding layer, and B is doped₂O₃And fluorine can both lower the refractive index. However, B₂O₃At 1.2 μm there is a large tail absorption which is detrimental to loss reduction, and therefore it is preferable to use fluorine doping to reduce the refractive index of the cladding. In order to obtain the same relative refractive index difference delta, GeO of the core layer can be reduced by doping the cladding layer with fluorine₂Content even without doping GeO₂Thus further reducing rayleigh scattering due to doping.

At present, methods for manufacturing optical fiber preforms include in-tube methods (MCVD and PCVD) and out-of-tube methods (VAD and OVD), wherein the out-of-tube methods are not limited by the size of a liner tube, have high deposition speed and high production efficiency, and are suitable for large-scale production of large-size 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 disadvantage of low deposition efficiency and low doping concentration.

In order to solve the above problems, U.S. Pat. No. US4629485 discloses a method for vitrifying a loose body of an optical fiber preform and doping fluorine, which comprises the steps of putting the loose body with pores, which is deposited in a vapor phase, into a vitrification furnace for heating, introducing fluoride gas to flow through the surface of the loose body and fully diffuse the fluoride gas into the loose body, and finally vitrifying the loose body into transparent fluorine-doped glass, wherein the method comprises the following specific steps: first depositing pure SiO₂Loosening the mass, and then placing the loose mass into glassDehydrating and vitrifying in a furnace, extending the vitrified core rod, depositing an outer cladding layer on the extended pure silica core rod by adopting an OVD method, dehydrating the deposited loose body in the vitrification furnace, introducing fluoride gas, and vitrifying to obtain a fluorine-doped preform of the cladding layer. The method has high fluorine doping speed and high doping concentration, however, the hydroxyl content of the core layer of the optical fiber preform rod manufactured by the method is higher, and the transmission loss of the 1383nm wavelength of the drawn optical fiber reaches more than 2 dB/km. The reason for this is that: (1) the surface of the vitrified pure silica core rod is OH-coated during the extension process^-Contamination, OH^-Diffusing at high temperature into the core rod; (2) the extended core rod is subjected to flame hydrolysis reaction to deposit a cladding, and OH generated by the hydrolysis reaction^-And diffused into the core layer under subsequent high temperature conditions (vitrification and drawing). In order to reduce OH^-Not only the core layer but also a part of the cladding layer is manufactured when the core rod is produced; and when the low-water-peak optical fiber preform is manufactured by adopting the flame hydrolysis method outer cladding, the core-cladding ratio (the ratio of the cladding diameter of the core rod to the core diameter) must be controlled to be more than 4.0. However, the problems that arise from this are: because the core layer and the cladding layer of the deposited core rod are loose bodies containing a large number of pores, fluorine is diffused into the cladding layer and also into the core layer when being vitrified and doped with fluorine, so that fluorine is difficult to be selectively doped into the cladding layer to form a refractive index profile structure.

For this reason, US patent No. US4610709 proposes increasing the density at the core-core interface of the core rod during VAD deposition, and during vitrification fluorine doping, fluorine diffuses only into the inner cladding of the core rod and not into the core layer. However, although the method avoids fluorine from diffusing into the core layer, chlorine can not enter the core layer during dehydration, so that the hydroxyl content of the core layer is increased rapidly, and the 1383nm water peak absorption energy of the drawn optical fiber is up to more than 100 dB/km.

Disclosure of Invention

In view of the above, the present invention provides a method for preparing a pure silica core optical fiber preform, which includes controlling an initial density of a core layer to enable chlorine gas to enter the core layer to react with hydroxyl groups during dehydration, and then increasing the density of the core layer through a pre-sintering step to enable fluorine to not enter the core layer during subsequent fluorine doping, so as to form a pure silica core structure, thereby finally solving the problem of high hydroxyl group content of the pure silica core optical fiber preform.

The invention provides a preparation method of a pure silica core optical fiber preform, which comprises the following steps:

a) preparing a loose core rod body by adopting a vapor axial deposition method; the core rod loose body consists of a core layer loose body and an inner cladding loose body coating the core layer loose body; the density of the core layer loose body is 1.0g/cm³～1.25g/cm³The density of the inner cladding loose body is 0.1g/cm³～0.6g/cm³；

b) Sequentially dehydrating, pre-sintering, fluorine doping and vitrifying the loose core rod body obtained in the step a) to obtain a core rod; the core rod consists of a core layer and an inner cladding layer covering the core layer; the density of the core layer is more than or equal to 1.3g/cm³The density of the inner cladding is less than or equal to 0.8g/cm³；

c) And c) coating the outer cladding layer on the core rod obtained in the step b) to obtain the pure silica core optical fiber prefabricated rod.

Preferably, the vapor axial deposition method in step a) specifically comprises the following steps:

adopting VAD device, firstly, vertically fixing the seed rod on a clamp; then, the raw material gas SiCl is introduced₄Delivering to oxyhydrogen flame blowtorch for combustion, and performing hydrolysis reaction at high temperature of oxyhydrogen flame to generate a large amount of SiO₂Depositing particles onto the seed rods; and spraying flame to the central part of the seed rod by using a core layer blowtorch to form a core layer loose body at the tail end of the seed rod, spraying flame to the peripheral part of the seed rod by using a cladding blowtorch to form an inner cladding loose body on the core layer, and simultaneously, continuously lifting and rotating the seed rod upwards to obtain a core rod loose body.

Preferably, SiCl of the core burner₄SiCl with flow rate of 10-20 g/min for cladding blast lamp₄The flow rate is 50 g/min-70 g/min;

the deposition temperature of the core layer blowtorch is 1250-1450 ℃.

Preferably, the dehydration process in step b) is specifically:

adopting a vitrification furnace, firstly, gradually reducing the core rod loose body at the rotating speed of 2 rpm-8 rpm, and entering a high-temperature area with the temperature of 1000-1300 ℃; then Cl was introduced₂And He to remove OH from the loose core rod^-。

Preferably, the Cl is₂The flow rate of the He is 0.5L/min-1.5L/min, and the flow rate of the He is 10L/min-30L/min;

the dehydration time is 8-12 h.

Preferably, the temperature of the pre-sintering in the step b) is 1150-1400 ℃, and the moving speed is 1-15 mm/min.

Preferably, the fluorine-doped gas introduced in the step b) is fluorine-containing gas and He, wherein the flow rate of the fluorine-containing gas is 0.1L/min-3L/min, and the flow rate of the He is 0.4L/min-12L/min;

the temperature of the fluorine doping is 1100-1300 ℃, and the time is 0.5-5 h.

Preferably, the vitrification temperature in the step b) is 1400-1600 ℃ and the time is 0.5-8 h.

Preferably, the process of covering the outer cladding in step c) specifically comprises:

firstly, OVD deposition lathe is adopted to deposit loose outer cladding layer on the periphery of the core rod, and then the core rod is placed into a vitrification furnace to be dehydrated, doped with fluorine and vitrified in sequence to obtain pure SiO as the core layer₂And the inner cladding and the outer cladding are fluorine-doped optical fiber prefabricated rods.

Preferably, the fluorine-doped introduced gas is fluorine-containing gas and He, wherein the flow rate of the fluorine-containing gas is 0.075L/min-2.25L/min, and the flow rate of the He is 0.425L/min-12.75L/min.

The invention provides a preparation method of a pure silica core optical fiber preform, which comprises the following steps: a) preparing a loose core rod body by adopting a vapor axial deposition method; the core rod loose body consists of a core layer loose body and an inner cladding loose body coating the core layer loose body; the density of the core layer loose body is 1.0g/cm³～1.25g/cm³Of loose bodies of said inner claddingThe density was 0.1g/cm³～0.6g/cm³(ii) a b) Sequentially dehydrating, pre-sintering, fluorine doping and vitrifying the loose core rod body obtained in the step a) to obtain a core rod; the core rod consists of a core layer and an inner cladding layer covering the core layer; the density of the core layer is more than or equal to 1.3g/cm³The density of the inner cladding is less than or equal to 0.8g/cm³(ii) a c) And c) coating the outer cladding layer on the core rod obtained in the step b) to obtain the pure silica core optical fiber prefabricated rod. Compared with the prior art, the preparation method provided by the invention has the advantages that chlorine can enter the core layer to react with hydroxyl during dehydration by controlling the initial density of the core layer, and then the density of the core layer is increased by the pre-sintering step, so that fluorine cannot enter the core layer during subsequent fluorine doping, thereby forming a pure silica core structure and finally solving the problem of high hydroxyl content of the pure silica core optical fiber preform. Experimental results show that the pure silica core optical fiber preform prepared by the preparation method provided by the invention is drawn into an optical fiber, the attenuation of 1550nm is 0.175dB/km, the attenuation of 1310nm is 0.35dB/km, the attenuation of 1383nm is 0.51dB/km, and the absorption energy of 1383nm water peak of the optical fiber drawn by the prior art (US4610709) is up to more than 100 dB/km; besides, other properties of the optical fiber meet the requirements of practical application, the diameter of a cladding is 125.1 mu m, the concentricity error of a core/the cladding is 0.15 mu m, the out-of-roundness of the cladding is 0.07%, the diameter of a mode field (1550nm) is 12.3 mu m, the cut-off wavelength of the optical fiber is 1483nm, and the dispersion coefficient (1550nm) is 20.5 ps/(nm-km).

In addition, the preparation method provided by the invention has the advantages of easily controlled conditions of each process step, good overall stability and good application prospect.

Drawings

FIG. 1 is a schematic cross-sectional view of a pure silica core optical fiber preform prepared by a preparation method according to an embodiment of the present invention;

FIG. 2 is a schematic diagram of a VAD device according to the manufacturing method of the present invention;

FIG. 3 is a schematic view showing the structure of a vitrification furnace in the production method according to the embodiment of the present invention;

FIG. 4 is a schematic diagram showing the density change of the loose core layer and the loose inner cladding layer before and after pre-sintering in the preparation method provided by the embodiment of the invention;

FIG. 5 is a schematic sectional view showing the refractive index of a core rod obtained by the manufacturing method according to the embodiment of the present invention;

fig. 6 is a schematic refractive index profile of a pure silica core optical fiber preform obtained by the preparation method according to an embodiment of the present invention.

Detailed Description

The technical solutions of the present invention will be described clearly and completely with reference to the following embodiments of the present invention, and it should be understood that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

The invention provides a preparation method of a pure silica core optical fiber preform, which comprises the following steps:

a) preparing a loose core rod body by adopting a vapor axial deposition method; the core rod loose body consists of a core layer loose body and an inner cladding loose body coating the core layer loose body; the density of the core layer loose body is 1.0g/cm³～1.25g/cm³The density of the inner cladding loose body is 0.1g/cm³～0.6g/cm³；

b) Sequentially dehydrating, pre-sintering, fluorine doping and vitrifying the loose core rod body obtained in the step a) to obtain a core rod; the core rod consists of a core layer and an inner cladding layer covering the core layer; the density of the core layer is more than or equal to 1.3g/cm³The density of the inner cladding is less than or equal to 0.8g/cm³；

c) And c) coating the outer cladding layer on the core rod obtained in the step b) to obtain the pure silica core optical fiber prefabricated rod.

The invention firstly adopts a vapor axial deposition method to manufacture the loose core rod body. In the present invention, the vapor axial deposition method preferably comprises the following steps:

adopting VAD device, firstly, vertically fixing the seed rod on a clamp; then, the raw material gas SiCl is introduced₄Delivering to oxyhydrogen flame blowtorch for combustion at the height of oxyhydrogen flameHydrolysis reaction is carried out at the temperature to generate a large amount of SiO₂Depositing particles onto the seed rods; and spraying flame to the central part of the seed rod by using a core layer blowtorch to form a core layer loose body at the tail end of the seed rod, spraying flame to the peripheral part of the seed rod by using a cladding blowtorch to form an inner cladding loose body on the core layer, and simultaneously, continuously lifting and rotating the seed rod upwards to obtain a core rod loose body. The VAD apparatus is not particularly limited in the present invention, and may be any VAD apparatus known to those skilled in the art.

In the present invention, SiCl of the core burner₄The flow rate is preferably 10g/min to 20g/min, and more preferably 12g/min to 15 g/min; SiCl of the cladding torch₄The flow rate is preferably 50g/min to 70g/min, more preferably 55g/min to 65 g/min.

In the present invention, the deposition temperature of the core layer torch is preferably 1250 to 1450 ℃, more preferably 1300 to 1400 ℃. By adopting the deposition temperature, the density of the loose body of the core layer can meet the control requirement.

In the present invention, the core rod loose body consists of a core layer loose body and an inner cladding loose body coating the core layer loose body; wherein the density of the loose core body is 1.0g/cm³～1.25g/cm³Preferably 1.2g/cm³(ii) a The density of the inner cladding loose body is 0.1g/cm³～0.6g/cm³Preferably 0.2g/cm³～0.4g/cm³. In the present invention, when the density of the loose body reaches 1.3g/cm³In the above case, chlorine gas and fluorine-containing gas cannot diffuse into the glass during dehydration and vitrification; in the deposition process, the density of the loose core body is higher than that of the loose core body of the inner cladding by increasing the deposition temperature of the loose core body, and the density of the loose core body is controlled to be 1.0g/cm³～1.25g/cm³(ii) a Therefore, when the loose core rod body is dehydrated, the initial density of the loose core layer body is 1.3g/cm³The chlorine and other dehydrating agents can enter the loose core layer to react with the hydroxyl; the lower the density of the loose body of the core layer is, the higher the dehydration efficiency is, the dehydration time can be shortened, and the production efficiency can be improved; however, the density of the core layer loose body is too low, and the core layer density difference is small after fluorine dopingThe concentration difference of fluorine diffused into the loose core layer and the loose inner cladding layer is reduced, so that the refractive index difference of the core cladding is too small to form a waveguide structure. In the invention, the initial density of the inner cladding loose body is controlled to be 0.1g/cm³～0.6g/cm³Preferably, it is controlled to 0.2g/cm³～0.4g/cm³This is advantageous for doping with a high concentration of fluorine; if the density of the inner cladding loose body is too high, the core cladding refractive index difference cannot be effectively formed; and after the density of the inner cladding loose body is low to a certain degree, the loose body is easy to crack.

After the core rod loose body is obtained, the obtained core rod loose body is subjected to dehydration, pre-sintering, fluorine doping and vitrification in sequence to obtain the core rod.

In the present invention, the dehydration process is preferably specifically:

adopting a vitrification furnace, firstly, gradually reducing the core rod loose body at the rotating speed of 2 rpm-8 rpm, and entering a high-temperature area with the temperature of 1000-1300 ℃; then Cl was introduced₂And He to remove OH from the loose core rod^-；

More preferably:

adopting a vitrification furnace, firstly, gradually reducing the core rod loose body at the rotating speed of 4 rpm-6 rpm, and entering a high-temperature area with the temperature of 1100-1200 ℃; then Cl was introduced₂And He to remove OH from the loose core rod^-. The vitrification furnace of the present invention is not particularly limited, and a vitrification furnace that can be used for dehydration, pre-sintering, fluorine doping, and vitrification, which is well known to those skilled in the art, may be used. In the present invention, the high temperature zone refers to a highest temperature zone formed by heating a heater of the vitrification furnace.

In the present invention, the dehydration is carried out by introducing Cl₂He flows through the surface of the loose body of the core rod from bottom to top; wherein the He is a diluent gas. In the present invention, the Cl is₂The flow rate of (A) is preferably 0.5L/min to 1.5L/min, more preferably 1L/min; the flow rate of He is preferably 10L/min to 30L/min, and more preferably 20L/min. In the present invention, the time for dehydration is preferably 8 to 12 hours, and more preferably 10 hours.

In the invention, the pre-sintering process is also carried out in the vitrification furnace, and the high-temperature area of the vitrification furnace is heated to the pre-sintering temperature after dehydration; the pre-sintering temperature is preferably 1150-1400 ℃, and more preferably 1200-1250 ℃; the movement speed of the presintering (lowering of the loose core rod body into the high-temperature zone again) is preferably 1mm/min to 15mm/min, more preferably 2mm/min to 8 mm/min.

After dehydration, in order to prevent fluorine from diffusing into the loose core body during subsequent fluorine doping, the invention adds a pre-sintering step which has the function of increasing the density of the loose core body by heating and shrinking the loose core body to enable the density to reach 1.3g/cm³This ensures that fluorine only diffuses into the inner cladding bulk and not into the core bulk when fluorine is doped. In the present invention, when the pre-sintering temperature is low, such as 1150 ℃, in order to increase the density of the loose core, the moving speed needs to be reduced as much as possible, such as 1 mm/min; when the pre-sintering temperature is high, such as 1400 ℃, the moving speed needs to be increased, such as 15mm/min, to prevent the density of the inner cladding loose body from becoming too high, so that fluorine cannot diffuse into the inner cladding loose body or the fluorine diffusing into the inner cladding loose body is too low to reach the required core refractive index difference.

In the invention, the fluorine doping process is also carried out in the vitrification furnace by introducing gas to flow over the surface of the loose core rod body from bottom to top; the introduced gas doped with fluorine is preferably fluorine-containing gas and He; wherein the fluorine-containing gas is SiF₄、CF₄、SF₆、C₂F₆、C₂F₂Cl₂、F₂、C₃F₈、NF₃、SOF₂And SO₂One or more of ClF, more preferably CF₄(ii) a And the He is a diluent gas.

In the present invention, the flow rate of the fluorine-containing gas is preferably 0.1L/min to 3L/min, and more preferably 0.5L/min to 2L/min; the flow rate of the He is preferably 0.4L/min-12L/min, and more preferably 2L/min-8L/min; ensuring that the flow of the fluorine-containing gas accounts for 3-15% of the total flow of the gas.

In the invention, the temperature of the fluorine doping is preferably 1100-1300 ℃, and more preferably 1200 ℃; the time for doping fluorine is preferably 0.5h to 5h, and more preferably 2h to 4 h. In the invention, the fluorine-doped temperature is too low, the fluorine diffusion speed is slow, and the production efficiency is low; if the temperature of fluorine doping is too high, the loose body is further shrunk, the density is increased, the uniformity of fluorine diffusion is affected, and even fluorine cannot diffuse to reach the vicinity of a core package interface; the time for doping fluorine is such that the fluorine is sufficiently diffused into the inner cladding loose mass of the core rod loose mass and uniformly distributed in the inner cladding loose mass; the invention adopts the fluorine doping process to fully diffuse fluorine into the loose body of the inner cladding.

In the present invention, the vitrification temperature is preferably 1400 to 1600 ℃, more preferably 1500 ℃; the vitrification time is preferably 0.5 to 8 hours, and more preferably 2 to 6 hours. In the present invention, the glass transition temperature can be lowered by incorporating fluorine into the bulk.

And obtaining the solid and transparent core rod after vitrification, wherein t/a (the ratio of the diameter of the inner cladding to the diameter of the core layer) of the core rod is preferably 3.0-6.0, and more preferably 3.5-4.5. In the invention, the core rod consists of a core layer and an inner cladding layer covering the core layer.

After the core rod is obtained, the obtained core rod is coated with an outer cladding layer to obtain a pure silica core optical fiber prefabricated rod. In the present invention, the process of covering the outer cladding layer is preferably as follows:

firstly, OVD deposition lathe is adopted to deposit loose outer cladding layer on the periphery of the core rod, and then the core rod is placed into a vitrification furnace to be dehydrated, doped with fluorine and vitrified in sequence to obtain pure SiO as the core layer₂And the inner cladding and the outer cladding are fluorine-doped optical fiber prefabricated rods. The OVD deposition lathe is not particularly limited in the present invention, and may be one mounted with a target rod capable of depositing the core rod as the outer cladding loose body, which is well known to those skilled in the art.

In the present invention, the specific processes of dehydration, fluorine doping and vitrification in the process of cladding are preferably performed in a vitrification furnace, which is substantially the same as the specific processes described in the above technical solutions, except that: the flow rate of fluorine-containing gas in the fluorine-doped introduced gas is preferably 0.075L/min-2.25L/min, and the flow rate of He is preferably 0.425L/min-12.75L/min; so that the flow of the fluorine-containing gas accounts for 2% -10% of the total flow of the gas and is slightly lower than the previous level, and the refractive index of the outer cladding is slightly higher than that of the inner cladding. In the present invention, other specific processes are not described herein again.

According to the preparation method provided by the invention, chlorine can enter the core layer to react with hydroxyl during dehydration by controlling the initial density of the core layer, and then the density of the core layer is increased by the pre-sintering step, so that fluorine cannot enter the core layer during subsequent fluorine doping, thereby forming a pure silica core structure and finally solving the problem of high hydroxyl content of a pure silica core optical fiber preform. In addition, the preparation method provided by the invention has the advantages of easily controlled conditions of each process step, good overall stability and good application prospect.

The invention provides a preparation method of a pure silica core optical fiber preform, which comprises the following steps: a) preparing a loose core rod body by adopting a vapor axial deposition method; the core rod loose body consists of a core layer loose body and an inner cladding loose body coating the core layer loose body; the density of the core layer loose body is 1.0g/cm³～1.25g/cm³The density of the inner cladding loose body is 0.1g/cm³～0.6g/cm³(ii) a b) Sequentially dehydrating, pre-sintering, fluorine doping and vitrifying the loose core rod body obtained in the step a) to obtain a core rod; the core rod consists of a core layer and an inner cladding layer covering the core layer; the density of the core layer is more than or equal to 1.3g/cm³The density of the inner cladding is less than or equal to 0.8g/cm³(ii) a c) And c) coating the outer cladding layer on the core rod obtained in the step b) to obtain the pure silica core optical fiber prefabricated rod. Compared with the prior art, the preparation method provided by the invention has the advantages that chlorine can enter the core layer to react with hydroxyl during dehydration by controlling the initial density of the core layer, and then the density of the core layer is increased by the pre-sintering step, so that fluorine cannot enter the core layer during subsequent fluorine doping, thereby forming a pure silica core structure and finally solving the problem of high hydroxyl content of the pure silica core optical fiber preform. Experiment ofThe result shows that the pure silica core optical fiber preform prepared by the preparation method provided by the invention is drawn into an optical fiber, the attenuation of 1550nm is 0.175dB/km, the attenuation of 1310nm is 0.35dB/km, the attenuation of 1383nm is 0.51dB/km, and the absorption energy of 1383nm water peak of the optical fiber drawn by the prior art (US4610709) is up to more than 100 dB/km; besides, other properties of the optical fiber meet the requirements of practical application, the diameter of a cladding is 125.1 mu m, the concentricity error of a core/the cladding is 0.15 mu m, the out-of-roundness of the cladding is 0.07%, the diameter of a mode field (1550nm) is 12.3 mu m, the cut-off wavelength of the optical fiber is 1483nm, and the dispersion coefficient (1550nm) is 20.5 ps/(nm-km).

In addition, the preparation method provided by the invention has the advantages of easily controlled conditions of each process step, good overall stability and good application prospect.

To further illustrate the present invention, the following examples are provided for illustration.