Method for manufacturing optical fiber preform

Abstract

This method for manufacturing an optical fiber preform includes: a preparation step for preparing a core target that contains SiO2 as a main component; a polishing step for flame polishing the surface of the core target; a deposition step for supplying a starting material to a burner and spraying and depositing generated glass fine particles on the core target so as to manufacture a soot preform; and a sintering step for dehydrating and vitrifying the soot preform so as to manufacture an optical fiber preform. This method for manufacturing an optical fiber preform is characterized in that the outer diameter of the core target is reduced by 150 µm or more by flame polishing the surface of the core target in the polishing step. With respect to the core target, when R is the outer diameter of an optical fiber preform to be manufactured and r is the outer diameter of the core target, CR represented by the formula CR = r / R is 0.22 or more. During the flame polishing, the rotation speed of the core target is in the range of 10 rpm to 60 rpm and the relative speed between the core target and oxyhydrogen flame is 50 mm / min or less.

Description

Method for manufacturing an optical fiber preform 【0001】 The present invention relates to a method for manufacturing an optical fiber preform from which an optical fiber with low transmission loss can be obtained. 【0002】 Conventionally, when manufacturing an optical fiber preform using the OVD method, before depositing soot, the core target is flame-polished to suppress peeling between the core target and the soot cladding deposited thereon. Note that an oxyhydrogen flame or the like is used for flame polishing. General disclosure 【0003】 One form of the method for manufacturing an optical fiber preform includes a preparation step of preparing a core target having a main component of SiO2, a polishing step of flame-polishing the surface of the core target, a deposition step of supplying raw materials to a burner and spraying and depositing the generated glass fine particles onto the core target to manufacture a soot preform, and a sintering step of dehydrating and vitrifying the soot preform to manufacture an optical fiber preform. In the polishing step, the surface of the core target is flame-polished to reduce its outer diameter by 150 μm or more. 【0004】 In the polishing step, the burner used for flame polishing has a plurality of oxygen supply nozzles arranged in a tube to which hydrogen is supplied, and the flow rate ratio of oxygen to hydrogen is preferably 3.0 or more and 3.4 or less. 【0005】 In the polishing step, the linear velocity of oxygen at the burner outlet is greater than the linear velocity of hydrogen, and the linear velocity of oxygen at the burner outlet is preferably 90 m / s or more. 【0006】 In the polishing step, when the core target is rotated for flame polishing, the rotation speed is preferably in the range of 10 rpm to 60 rpm, and the relative speed between the core target and the oxyhydrogen flame is preferably 50 mm / min or less. When the outer diameter of the optical fiber preform to be manufactured is R and the outer diameter of the core target is r, it is preferable that CR represented by the formula CR = r / R is 0.22 or more. 【0007】 In the polishing step, when the core target is rotated for flame polishing, the rotation speed is in the range of 10 rpm to 60 rpm, and the relative speed between the core target and the oxyhydrogen flame is preferably 50 mm / min or less. 【0008】This figure shows a schematic of the structure of the optical fiber preform. An example of an OVD apparatus 10 is schematically shown. A schematic cross-section of the burner 30 cut perpendicular to the longitudinal direction is schematically shown. This figure shows the relationship between the linear velocity of oxygen at the burner outlet and the transmission loss at a wavelength of 1383 nm. 【0009】 After thorough investigation, we conceived the idea that the conventional method of flame polishing the core target surface before depositing the soot, which suppressed delamination between the core target and the soot cladding deposited on it, might be doping the core target with OH groups during this process, negatively affecting the transmission loss characteristics of the resulting optical fiber. We then resolved this issue by examining the ratio of oxygen and hydrogen supplied during flame polishing of the core target, the linear velocity of oxygen at the burner outlet, and the grinding allowance during flame polishing. 【0010】 In other words, the method for manufacturing an optical fiber preform of this embodiment comprises a preparation step of preparing a core target whose main component is SiO2, a polishing step of flame polishing the surface of the core target, a deposition step of supplying raw materials to a burner and spraying and depositing the generated glass fine particles onto the core target to manufacture a soot preform, and a sintering step of dehydrating and vitrifying the soot preform to manufacture an optical fiber preform, characterized in that in the polishing step, the surface of the core target is flame polished to reduce its outer diameter by 150 μm or more. 【0011】 In this embodiment, first, a core target whose main component is SiO2 is prepared in the preparation step. The core target used in this step is prepared by depositing glass soot by the VAD method, and the deposited soot body is dehydrated and vitrified to produce a transparent core ingot. Next, this core ingot is stretched and processed to a predetermined diameter. 【0012】 Figure 1 shows a cross-section of the optical fiber matrix. The core-to-target ratio CR is calculated from the outer diameter R of the optical fiber matrix and the outer diameter r of the core target using the formula CR = r / R. It is preferable to set the outer diameter r of the core target such that the CR is 0.22 or higher, as this further reduces transmission loss. 【0013】This is important to ensure that the interface between the core target and the soot cladding is properly positioned away from the pure core through which light propagates, thereby preventing any impact on the transmission loss of the manufactured optical fiber. 【0014】 Figure 2 schematically shows an example of an OVD apparatus 10 used in the polishing process following the preparation process. The OVD apparatus 10 comprises a base 12, a pair of support parts 14 and 16 provided on the left and right sides of the base 12, and a burner 30 that can move left and right on the base 12. Each of the support parts 14 and 16 is provided with gripping parts 20 and 22 that grip both ends of the core target 50. The support part 14 is further provided with a motor 18 that rotates the gripping part 20 about the central axis in the plane of the paper in the figure. The gripping part 22 of the support part 16 rotates along with the rotation of the gripping part 20 via the core target 50. 【0015】 Figure 3 schematically shows a cross-section of the burner 30 cut perpendicular to its longitudinal direction. The burner 30 has an outer tube 32 to which hydrogen is supplied, and a plurality of nozzles 34, 36, and 38, three in the figure, arranged inside the tube 32. Hydrogen is supplied to the tube 32. Oxygen is supplied to the nozzles 34, 36, and 38. 【0016】 During the polishing process, both ends of the core target 50 are gripped by the gripping parts 20 and 22. In this state, the surface of the core target 50 is flame-polished from one end to the other while rotating the core target 50 with an oxyhydrogen flame generated from hydrogen and oxygen supplied to the burner 30. 【0017】 In this polishing process, the outer diameter of the core target 50 is flame polished to a depth of 150 μm or more. The surface of the core target created in the preparation process may contain a high concentration of OH groups, but by removing the surface layer of the core target 50 by flame polishing to a depth of 150 μm or more, the average OH concentration of the core target 50 can be effectively suppressed, which is preferable. Note that if the depth is less than 150 μm, the effect of suppressing the average OH concentration is limited. 【0018】Furthermore, for the burner 30, the flow rate ratio of oxygen to hydrogen should be 3.0 or more and 3.4 or less, and the linear velocity of oxygen at the burner outlet should be greater than the linear velocity of hydrogen, so that the linear velocity of oxygen at the burner outlet is 90 m / s or more. In addition, when performing flame polishing, it is preferable to rotate the core target in the range of 10 rpm to 60 rpm, and the relative velocity between the core target and the oxyhydrogen flame should be 50 mm / min or less. 【0019】 These measures allow for flame polishing while suppressing the doping of OH groups into the core target during the polishing process. 【0020】 In the deposition process, glass microparticle raw material is supplied to the main burner and the glass microparticles generated by hydrolysis in an oxyhydrogen flame are blown onto the flame-polished core target and deposited. By moving the main burner relatively from one end of the core target to the other, the glass microparticles are deposited onto the core target to a predetermined weight, thereby obtaining the soot base material. 【0021】 Furthermore, in the sintering process, the soot base material is suspended above the carbon heater of a sintering furnace equipped with a furnace tube made of synthetic silica and a carbon heater. The soot base material suspended inside the furnace tube is then heated to 1250°C in the center of the tube by the carbon heater after the atmosphere inside the furnace tube has been replaced from helium and chlorine with a dehydrating gas atmosphere. In the heated area, the soot base material is pulled down from one end to the other at a maximum pulling speed of 1.5 mm / min while rotating the soot base material at a speed of 7 rpm or less, thereby performing chlorine dehydration treatment on the soot base material. 【0022】 Next, the chlorine-dehydrated soot base material is pulled up again to the top of the carbon heater, and the inside of the reactor tube is replaced with helium. The temperature inside the reactor tube, now replaced with a helium atmosphere, is raised to 1600°C using the carbon heater to make the soot base material transparent glass. The soot base material is then pulled down from one end to the other at a maximum speed of 2.4 mm / min from the top of the carbon heater, rotating the soot base material at a speed of 7 rpm or less, in order to obtain the optical fiber base material. 【0023】By drawing this optical fiber matrix, a low-loss optical fiber with a transmission loss of 0.31 dB / km or less at a wavelength of 1383 nm can be obtained. 【0024】 [Example 1] A core target mainly composed of SiO2 was used, and its surface was flame polished. In this polishing process, a burner with multiple oxygen supply nozzles arranged in a hydrogen supply pipe was used, and the flow rate ratio of oxygen to hydrogen was set to 3.0 or more and 3.4 or less to make the linear velocity of oxygen greater than that of hydrogen, and the linear velocity of oxygen at the burner outlet was set to 93 m / s for flame polishing. At that time, the relative movement speed between the core target and the burner was set to 37 mm / min, and the rotation speed of the core target was set to 10 rpm, and flame polishing was performed from one end of the core target to the other, reducing the average outer diameter by 150 μm. 【0025】 The average OH concentration of the target obtained during the polishing process was measured by FTIR. At any point within the polished area of ​​the core target, a wavelength of 3800 cm was measured from the side. -1 From 3400cm -1 An absorption spectrum was obtained by transmitting infrared light in the radial direction within the specified range. From the peak height of Si-OH in the obtained absorption spectrum, the average Si-OH concentration of the radially transmitted light passing through the center of the core target was 1.0 ppm. 【0026】 Next, in the process of depositing the soot matrix onto the flame-polished core target, glass microparticles generated by supplying raw materials to a burner were sprayed onto the core target and deposited onto the core target so that the CR (Critical Resistance) was 0.23, thereby producing the soot matrix. The obtained soot matrix was then subjected to a sintering process to dehydrate and vitrify it, thereby producing the optical fiber matrix. 【0027】 The transmission loss of the optical fiber obtained by drawing the optical fiber matrix at a wavelength of 1383 nm was favorably less than 0.31 dB / km, as shown in Figure 4, when the linear velocity of oxygen at the burner outlet was set to 93 m / s. 【0028】[Comparative Example 1] The core target was flame polished using the burner used in Example 1. In this polishing process, the linear velocity of the oxygen supplied to the burner outlet was set to 80 mm / s. The relative movement speed between the core target and the burner was set to 37 mm / min, and the rotation speed of the core target was set to 10 rpm. The core target was flame polished from one end to the other, reducing the average outer diameter by 90 μm. 【0029】 The average OH concentration of the target obtained during the polishing process was measured by FTIR. At any point within the polished area of ​​the core target, a wavelength of 3800 cm was measured from the side. -1 From 3400cm -1 An absorption spectrum was obtained by transmitting infrared light in the radial direction within the specified range. From the peak height of Si-OH in the obtained absorption spectrum, the average Si-OH concentration of the radially transmitted light passing through the center of the core target was 1.3 ppm. 【0030】 Using a core target polished in the polishing process, a soot base material was deposited to achieve a CR of 0.23, and then dehydrated and sintered to produce an optical fiber base material. The transmission loss of the optical fiber obtained by drawing this base material at a wavelength of 1383 nm was significantly higher than in the example, exceeding 0.31 dB / km, as shown in Figure 4, when the linear velocity of oxygen at the burner outlet was set to 80 m / s.

Claims

1. A method for manufacturing an optical fiber preform, comprising: a preparation step of preparing a core target whose main component is SiO2; a polishing step of flame polishing the surface of the core target; a deposition step of supplying raw materials to a burner and spraying and depositing the generated glass fine particles onto the core target to manufacture a soot preform; and a sintering step of dehydrating and vitrifying the soot preform to manufacture an optical fiber preform, wherein in the polishing step, the surface of the core target is flame polished to reduce its outer diameter by 150 μm or more.

2. The method for manufacturing an optical fiber preform according to claim 1, wherein, in the polishing step, the burner used for flame polishing has a plurality of nozzles for supplying oxygen arranged in a tube for supplying hydrogen, and the flow rate ratio of oxygen to hydrogen is 3.0 or more and 3.4 or less.

3. The method for manufacturing an optical fiber preform according to claim 2, wherein in the polishing step, the linear velocity of oxygen at the burner outlet is greater than the linear velocity of hydrogen, and the linear velocity of oxygen at the burner outlet is 90 m / s or more.

4. The method for manufacturing an optical fiber preform according to claim 1, wherein in the polishing step, the core target has a CR of 0.22 or more, given by the formula CR = r / R, where R is the outer diameter of the optical fiber preform to be manufactured and r is the outer diameter of the core target.

5. The method for manufacturing an optical fiber preform according to claim 1, wherein, in the polishing step, the rotation speed of the core target is in the range of 10 rpm to 60 rpm.

6. The method for manufacturing an optical fiber preform according to claim 1, wherein in the polishing step, the relative velocity between the core target and the oxyhydrogen flame is 50 mm / min or less.

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