A method for the chlorine-doped production of an optical fiber preform rod

Abstract

The application relates to a method for preparing a chlorine-doped optical fiber preform rod, which adopts a tube-in-gas deposition process, a glass lining tube is clamped on a deposition lathe, one end of the glass lining tube is connected with a raw material gas, the raw material gas continuously reacts in the lining tube inner wall under the action of a high-temperature heat source to continuously generate a doped silica deposition layer, the raw material gas comprises silicon tetrachloride and oxygen, during the deposition process, the oxygen flow and the ratio of the oxygen partial pressure to the silicon tetrachloride partial pressure are adjusted, and the heat source temperature of the deposition area is adjusted, so that the deposition layer has different chlorine-doped concentrations. The application can realize regional chlorine-doping control with different concentrations, especially the chlorine doping of the core layer can accurately control the refractive index profile, and the core layer chlorine doping can form a better glass lattice structure, and the attenuation of the optical fiber is reduced.

Description

Technical Field

[0001] This invention relates to a method for preparing chlorine-doped optical fiber preform core rods, belonging to the field of optical fiber manufacturing technology. Background Technology

[0002] Optical fiber transmission boasts advantages such as large transmission capacity, long transmission distance, and high transmission speed, making it widely used in optical communication systems such as long-distance trunk lines, metropolitan area networks, and access networks. In recent years, data traffic has increased dramatically, making broadband an indispensable infrastructure. Network operators are implementing network upgrade plans and deploying new equipment to meet this demand. In the 100G and beyond era, nonlinear effects and fiber attenuation have become the main factors restricting the improvement of system transmission performance. Therefore, large effective area optical fibers and low-attenuation optical fibers have become current research and application hotspots.

[0003] In order for optical signals to be transmitted smoothly in optical fibers, the fiber core needs to have a high refractive index, while the cladding needs to have a low refractive index to form total internal reflection. Usually, germanium is added to the fiber core to increase the core refractive index, while the cladding uses pure silicon or fluorine doping to reduce the refractive index. In order to obtain lower attenuation, the core can also be doped with chlorine-containing compounds and the outer cladding can be deeply doped with fluorine to reduce the refractive index and obtain a suitable refractive index difference.

[0004] In the doping of optical fibers to improve viscosity, adding an appropriate amount of chloride to the core layer can not only increase the refractive index of the fiber but also ultimately improve its attenuation. However, chloride doping, especially high-concentration chloride doping, requires relatively strict conditions. Most existing technologies employ external deposition methods using silica powder for chloride doping. Because this doping is integral, it is not only difficult to control the distribution of chloride in different parts of the core layer but also challenging to achieve high-concentration doping and concentration control. Summary of the Invention

[0005] The following are definitions and explanations of some terms used in this invention:

[0006] ppm: parts per million by weight.

[0007] Starting from the axis at the very center of the optical fiber, the layer closest to the axis is defined as the core layer, based on the change in refractive index, while the outermost layer of the optical fiber is defined as the cladding.

[0008] The relative refractive index difference Δni between the layers of an optical fiber is defined by the following equation:

[0009]

[0010] Where ni is the refractive index of the fiber core, and nc is the refractive index of pure silicon dioxide.

[0011] The contribution of the relative refractive index difference ΔGe to the Ge doping in the fiber core is defined by the following equation.

[0012]

[0013] Where n Ge Let n be the amount of change in the refractive index of silica glass caused by the Ge dopant in the fiber core when it is doped into pure silica without other dopants. c is the refractive index of pure silicon dioxide.

[0014] The relative refractive index contribution ΔF of F doping in the fiber core and inner cladding i Defined by the following equation,

[0015]

[0016] Where n F Let n be the change in refractive index of a dopant F, assuming it is located in the core or inner cladding layer, when incorporated into pure silica glass without other dopants. c is the refractive index of pure silicon dioxide.

[0017] The effective area Aeff of the optical fiber

[0018]

[0019] Where E is the electric field related to propagation, and R is the distance between the axis and the point where the electric field is distributed.

[0020] The technical problem to be solved by the present invention is to provide a method for preparing optical fiber preform core rods by chlorine doping, which not only can dope high concentrations of chlorine, but also can achieve control of chlorine doping concentrations in different regions.

[0021] The technical solution adopted by the present invention to solve the above-mentioned problems is as follows:

[0022] An in-tube vapor deposition process is employed, in which a glass liner is clamped onto a deposition lathe, and a raw material gas is introduced into one end of the glass liner. Under the action of a high-temperature heat source, the raw material gas continuously reacts along the axial direction of the glass liner, continuously generating a doped silica deposition layer on the inner wall of the liner. The key feature is that the raw material gas includes silicon tetrachloride and oxygen. During the deposition process, different chlorine doping concentrations in the deposition layer are achieved by adjusting the oxygen flow rate and the ratio of oxygen partial pressure to silicon tetrachloride partial pressure, as well as adjusting the heat source temperature in the deposition area.

[0023] According to the above scheme, when performing high-concentration Cl (chlorine) doping, the oxygen flow rate is reduced to decrease the oxygen partial pressure, ensuring that the oxygen partial pressure / silicon tetrachloride partial pressure ratio is ≤15, ideally ≤5. Simultaneously, the reaction temperature in this region is increased to allow more Cl to be released.- The reaction between Cl2 and silicon dioxide increases the Cl content in the region, resulting in a Cl element content of ≥3000ppm.

[0024] According to the above scheme, when performing low-concentration Cl (chlorine) doping, increasing the oxygen flow rate increases the oxygen partial pressure, while decreasing the silicon tetrachloride partial pressure, resulting in an oxygen partial pressure / silicon tetrachloride partial pressure ratio >15, ideally ≥20. This ensures a more complete reaction between silicon tetrachloride and oxygen, fewer by-products, and less Cl. - The reaction of Cl2 with silicon dioxide reduces the Cl content in the region to ≤1000ppm.

[0025] According to the above scheme, when performing low-concentration Cl (chlorine) doping, alkali metal compounds are added with a peak concentration ≤100ppm.

[0026] According to the above scheme, for the convex double-core structure, when the first core layer is doped with a high concentration of Cl, the oxygen flow rate is reduced to decrease the oxygen partial pressure; the oxygen partial pressure / silicon tetrachloride partial pressure is ≤15, preferably ≤5, while the reaction temperature at this location is increased to allow more Cl to be doped. - The reaction of Cl2 with silicon dioxide increases the Cl content in this part, reaching ≥3000ppm. During low-concentration Cl doping in the second core layer, increasing the oxygen flow rate and partial pressure (oxygen partial pressure / silicon tetrachloride partial pressure) results in a ratio >15, ideally ≥20, while reducing the partial pressure of silicon tetrachloride. This allows for a more complete reaction between silicon tetrachloride and oxygen, reducing byproducts and thus minimizing Cl content. - In addition, Cl2 reacts with silicon dioxide to reduce the Cl content in this part, with the Cl content ≤1000ppm.

[0027] According to the above scheme, for the concave double-core structure, when the first core layer is doped with a low concentration of Cl, the oxygen flow rate is increased to increase the oxygen partial pressure, making the oxygen partial pressure / silicon tetrachloride partial pressure >15, preferably ≥20, while reducing the partial pressure of silicon tetrachloride. This allows for a more complete reaction between silicon tetrachloride and oxygen, resulting in fewer by-products and less Cl. - The reaction of Cl2 with silicon dioxide reduces the Cl content in this area to ≤1000ppm. Simultaneously, alkali metal compounds are added to the first core layer, with a peak concentration ≤100ppm. When the second core layer requires high Cl doping, the oxygen flow rate is reduced to decrease the oxygen partial pressure, achieving an oxygen partial pressure / silicon tetrachloride partial pressure ≤15, ideally ≤5. Simultaneously, the reaction temperature at this location is increased to allow more Cl to react. - The reaction between Cl2 and silicon dioxide increases the Cl content in this part, resulting in a Cl element content of ≥3000ppm.

[0028] According to the above scheme, the in-tube vapor deposition method includes plasma chemical vapor deposition (PCVD), modified chemical vapor deposition (MCVD), and other methods for depositing and preparing optical fiber preforms in glass-lined tubes.

[0029] According to the above scheme, in the PCVD deposition process, the raw material gases SiCl4 and oxygen enter the glass liner in the reaction zone through a mass flow controller. There is a microwave resonant cavity outside the glass liner, and a heat preservation furnace outside the resonant cavity. The reaction raw materials are in the glass liner, and the microwave resonant cavity provides a high-temperature heat source by reciprocating movement, so that the reactants are vitrified and deposited in the reaction zone. The heat preservation furnace provides a heat preservation temperature of about 900 to 1200°C during the deposition process.

[0030] According to the above scheme, two mass flow controllers are connected in parallel in the oxygen supply pipeline, including a large-range mass flow controller and a small-range mass flow controller. When a large oxygen flow is required, the large-range mass flow controller is switched on, or both the large-range and small-range mass flow controllers are turned on simultaneously to adjust the oxygen partial pressure range to a larger range. When a small oxygen flow is required, the large-range mass flow controller is turned off, and the small-range mass flow controller is switched on to adjust the oxygen partial pressure range to a smaller range.

[0031] According to the above scheme, the alkali metal raw material is an alkali metal halide, that is, a compound composed of alkali metal elements and halogens. The alkali metal elements include Li, Na, K, Rb, and Cs; the halogens include F, Cl, Br, I, and At; and the alkali metal halide is any combination of alkali metal elements and halogens, such as doping with KCl, NaCl, KBr, NaBr, etc.

[0032] According to the above scheme, the core layer deposited by PCVD or MCVD is pagoda-shaped, parabolic, or concave, and is wholly or partially doped with high concentration of Cl, with a Cl content ≥3000ppm and a higher Cl content ≥5000ppm.

[0033] The beneficial effects of this invention are as follows: 1. By employing in-pipe deposition technology, especially PCVD or MCVD, high-concentration chlorine doping of the core layer can be achieved by controlling the partial pressure and flow rate ratio of oxygen to silicon tetrachloride, and the chlorine concentration can be precisely controlled to achieve chlorine doping control at different concentrations in different regions. In particular, the refractive index profile can be precisely controlled for Cl doping of the core layer, and the process is simple; 2. Chlorine doping of the core layer can enable the optical fiber core layer to form a better glass lattice structure, reducing optical fiber attenuation; 3. Selective addition of alkali metals to different regions of the core layer, utilizing the viscosity-reducing properties of alkali metals, further reduces the Rayleigh scattering coefficient of the optical fiber, reducing optical fiber attenuation; 4. Connecting two mass flow controllers in parallel in the oxygen supply pipeline allows for better control of the oxygen flow rate. Attached Figure Description

[0034] Figure 1 This is a schematic diagram of the PCVD deposition process in one embodiment of the present invention.

[0035] Figure 2 This is a schematic diagram of the MCVD deposition process in another embodiment of the present invention.

[0036] Figure 3 This is a schematic diagram of the radial structure of an optical fiber in one embodiment of the present invention.

[0037] Figure 4 This is a schematic diagram of the refractive index profile of an optical fiber in one embodiment of the present invention. Detailed Implementation

[0038] The present invention will be further described below with reference to the embodiments and accompanying drawings.

[0039] First embodiment: as Figure 1 As shown, the PCVD process is employed, with a convex double-core structure. The raw material gases SiCl4 and oxygen enter the glass liner 11 in the reaction zone via a mass flow controller. A microwave resonant cavity 12 is located outside the glass liner, and a heat preservation furnace 13 is located outside the resonant cavity. The first core layer has a Cl content of 6000 ppm. The oxygen flow rate is reduced to less than 800 sccm to decrease the oxygen partial pressure, achieving an oxygen partial pressure / silicon tetrachloride partial pressure ratio of 1. Simultaneously, the temperature of the heat preservation furnace is increased to 1200℃ to release more Cl... - The reaction of Cl2 with silicon dioxide increases the Cl content in this section. In the second layer, the oxygen partial pressure / silicon tetrachloride partial pressure is set to 10, the oxygen flow rate is increased to greater than 2000 sccm, and the silicon tetrachloride partial pressure is reduced, resulting in a more complete reaction between silicon tetrachloride and oxygen, fewer byproducts, and thus less Cl. - The Cl2 reacts with silicon dioxide to reduce the Cl content in this part, and the holding furnace temperature is 1000℃. The deposited glass liner is then melted and shrunk into a core rod. The first core layer of this optical fiber has a radius of 1.5 μm (R1) and a relative refractive index difference Δn1 of 0.06-0.08%. The second core layer has a radius of 6 μm (R2) and a relative refractive index difference Δn2 of 0-0.02%. The cladding of the optical fiber includes a recessed inner cladding and a recessed outer cladding. The radius of the recessed inner cladding is 30 μm (R3) and a relative refractive index difference Δn3 of -0.28%. The radius of the recessed outer cladding is 62.5 μm (R4) and a relative refractive index difference Δn4 of -0.22%. Finally, the optical fiber preform is drawn into an optical fiber using a drawing tower. The attenuation of the optical fiber at 1550 nm is 0.167 dB / km.

[0040] Second embodiment: as Figure 2 , 3As shown in Figure 4, the MCVD process is adopted, and the core layer has a concave double-core structure. The raw material gases SiCl4 and oxygen enter the glass liner 11 in the reaction zone through a mass flow controller. A high-temperature oxyhydrogen flame 12 is placed outside the glass liner. The first core layer has a Cl doping content of 400 ppm. Increasing the oxygen flow rate to make it greater than 2000 sccm increases the oxygen partial pressure, making the oxygen partial pressure / silicon tetrachloride partial pressure equal to 30. Decreasing the silicon tetrachloride partial pressure makes the reaction between silicon tetrachloride and oxygen more complete, with fewer by-products and less Cl. - Simultaneously with the reaction of Cl2 and silicon dioxide, alkali metal compounds are added to the first core layer 1-1 at a peak concentration of 100 ppm. The second core layer 1-2 requires a Cl doping concentration of 6000 ppm. The oxygen flow rate is reduced to less than 800 sccm to decrease the oxygen partial pressure; this brings the oxygen partial pressure / silicon tetrachloride partial pressure to 1, allowing more Cl2 to be added. - The reaction of Cl2 with silicon dioxide increases the Cl content in this part, reaching 6000 ppm. The deposited glass liner is melted and shrunk into a core rod, which is then used to create an optical fiber preform. This preform is then drawn into an optical fiber using a drawing tower. The first core layer, R1, is 1.5 μm thick and doped with alkali metals, with a relative refractive index difference Δn1 of 0-0.02%. The second core layer, R2, is 6 μm thick, with a relative refractive index difference Δn2 of 0.06-0.08%. The cladding of the optical fiber includes a recessed inner cladding 2 and a recessed outer cladding 3. The recessed inner cladding has a radius of 30 μm and a relative refractive index difference Δn3 of -0.28%, while the recessed outer cladding has a radius of 62.5 μm and a relative refractive index difference Δn4 of -0.22%. The attenuation of the optical fiber at 1550 nm is 0.163 dB / km.

[0041] In the comparative example, ordinary optical fibers with the same core waveguide structure contain less than or equal to 1500 ppm of Cl, and the attenuation at 1550 nm is 0.185-0.20 dB / km.

Claims

1. A method for preparing chlorine-doped optical fiber preform core rods, employing an in-tube vapor deposition process, wherein a glass liner is clamped on a deposition lathe, a raw material gas is introduced into one end of the glass liner, and under the action of a high-temperature heat source, the raw material gas continuously reacts along the axial direction of the glass liner, continuously generating a doped silica deposition layer on the inner wall of the liner, characterized in that... The raw material gases include silicon tetrachloride and oxygen. During the deposition process, different chlorine doping concentrations in the deposited layer are achieved by adjusting the oxygen flow rate and the ratio of oxygen partial pressure to silicon tetrachloride partial pressure, as well as adjusting the heat source temperature of the deposition zone. When performing high-concentration Cl doping, the oxygen flow rate is reduced to decrease the oxygen partial pressure, ensuring that the oxygen partial pressure / silicon tetrachloride partial pressure is ≤15. Simultaneously, the reaction temperature of the deposition zone is increased to allow more Cl to be deposited. - The reaction of Cl2 with silicon dioxide increases the Cl content in the deposited layer, reaching ≥3000 ppm. When performing low-concentration Cl doping, increasing the oxygen flow rate increases the oxygen partial pressure and decreases the silicon tetrachloride partial pressure, resulting in an oxygen partial pressure / silicon tetrachloride partial pressure >15. This ensures a more complete reaction between silicon tetrachloride and oxygen, reduces byproducts, and minimizes Cl content. - In addition, Cl2 reacts with silica to reduce the Cl content of the deposition layer in the deposition area, with the Cl content ≤1000ppm.

2. The method for preparing the optical fiber preform core rod by chlorine doping according to claim 1, characterized in that... When performing low-concentration Cl doping, alkali metal compounds are added, with a peak concentration ≤100ppm.

3. The method for preparing the optical fiber preform core rod by chlorine doping according to claim 1, characterized in that... For the convex double-core structure, when the first core layer is doped with a high concentration of Cl, the oxygen flow rate is reduced to decrease the oxygen partial pressure; the oxygen partial pressure / silicon tetrachloride partial pressure is kept ≤15. Simultaneously, the reaction temperature at this location is increased to allow more Cl to be released. - The reaction of Cl2 with silicon dioxide increases the Cl content in this part, reaching ≥3000ppm. During low-concentration Cl doping in the second core layer, increasing the oxygen flow rate and partial pressure (oxygen partial pressure / silicon tetrachloride partial pressure >15) reduces the partial pressure of silicon tetrachloride, leading to a more complete reaction between silicon tetrachloride and oxygen, fewer byproducts, and thus less Cl. - In addition, Cl2 reacts with silicon dioxide to reduce the Cl content in this part, with the Cl content ≤1000ppm.

4. The method for preparing the optical fiber preform core rod by chlorine doping according to claim 1, characterized in that... For a concave double-core structure, when the first core layer is doped with a low concentration of Cl, increasing the oxygen flow rate increases the oxygen partial pressure, making the oxygen partial pressure / silicon tetrachloride partial pressure > 15. This reduces the partial pressure of silicon tetrachloride, allowing for a more complete reaction between silicon tetrachloride and oxygen, resulting in fewer by-products and less Cl. - The reaction of Cl2 with silicon dioxide reduces the Cl content in this area to ≤1000ppm. Simultaneously, alkali metal compounds are added to the first core layer, with a peak concentration ≤100ppm. When the second core layer requires high Cl doping, the oxygen flow rate is reduced to decrease the oxygen partial pressure, ensuring the oxygen partial pressure / silicon tetrachloride partial pressure ratio is ≤15. At the same time, the reaction temperature at this location is increased to allow more Cl to be added. - The reaction between Cl2 and silicon dioxide increases the Cl content in this part, resulting in a Cl element content of ≥3000ppm.

5. The method for preparing the optical fiber preform core rod by chlorine doping according to claim 1, characterized in that... The in-tube vapor deposition method includes plasma chemical vapor deposition and modified chemical vapor deposition.

6. The method for preparing the optical fiber preform core rod by chlorine doping according to claim 5, characterized in that... In the plasma chemical vapor deposition process, the raw material gases SiCl4 and oxygen enter the glass liner in the reaction zone through a mass flow controller. There is a microwave resonant cavity outside the glass liner, and a heat preservation furnace outside the resonant cavity. The reactants are in the glass liner, and the microwave resonant cavity provides a high-temperature heat source by reciprocating movement, so that the reactants are vitrified and deposited in the reaction zone. The heat preservation furnace provides a heat preservation temperature of 900~1200℃ during the deposition process.

7. The method for preparing the optical fiber preform core rod by chlorine doping according to claim 5, characterized in that... Two mass flow controllers are connected in parallel in the oxygen supply pipeline, including a large-range mass flow controller and a small-range mass flow controller. When a large oxygen flow is required, the large-range mass flow controller is switched on, or both the large and small-range mass flow controllers are turned on simultaneously to adjust the oxygen partial pressure range to a larger range. When a small oxygen flow is required, the large-range mass flow controller is turned off, and the small-range mass flow controller is switched on to adjust the oxygen partial pressure range to a smaller range.

8. The method for preparing the optical fiber preform core rod by chlorine doping according to claim 5, characterized in that... The core layer deposited by the plasma chemical vapor deposition method or the improved chemical vapor deposition method is pagoda-shaped, parabolic, or concave, and is wholly or partially doped with high concentration of Cl, with a Cl content ≥3000ppm.

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