Method for manufacturing optical fiber preform

Abstract

To provide a method for manufacturing an optical fiber porous glass preform having characteristic variation suppressed in a longitudinal direction.SOLUTION: A method for manufacturing an optical fiber preform by a VAD method, capable of obtaining the optical fiber glass preform, comprises: detecting a tip position of a porous glass preform during deposition to control a pulling speed so that the tip position is fixed; gradually changing and depositing the pulling speed to a predetermined time by setting a target pulling speed for each deposition time in the early stage of the deposition to control and correct a raw material gas flow rate to a burner for each predetermined correction interval so as to be the target pulling speed; depositing the pulling speed so as to be fixed in a stationary state to form the porous glass preform; and dehydrating and vitrifying the porous glass preform in a heating furnace.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing an optical fiber preform by the VAD method, and more particularly to a method for manufacturing an optical fiber preform that can stabilize the characteristic fluctuations in the longitudinal direction of the optical fiber preform. [Background technology]

[0002] In the VAD method, a combustible gas and a combustion-supporting gas are fed into a burner to generate an oxyhydrogen flame, and raw material gases such as silicon tetrachloride and germanium tetrachloride are introduced into the flame to undergo a hydrolysis reaction, producing glass particles. The glass particles are then deposited axially on a starting member attached to a rotating and ascending shaft, growing a cylindrical glass particle deposit and forming a porous glass preform. The resulting porous glass preform is then dehydrated and vitrified in a heating furnace to produce a glass preform for optical fiber.

[0003] Methods disclosed for suppressing longitudinal property variations in optical fiber glass preforms manufactured by the VAD method include changing the gas flow rate introduced into the burner depending on the pulling speed (see Patent Document 1) and changing the burner position and flow rate based on the deposition shape (see Patent Document 2). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2000-034131 [Patent Document 2] Japanese Patent Application Publication No. 09-227147 Summary of the Invention [Problem to be solved by the invention]

[0005] In this manufacturing method, it is desirable to keep the core soot tip position constant during deposition using the VAD method in order to stabilize the optical properties of the optical fiber preform. The optical properties referred to here refer to optical properties such as cutoff wavelength, mode field diameter, and dispersion characteristics. The core soot tip position is typically detected using a CCD camera or similar device, and the pulling speed or the flow rate of the raw material gas to the burner is adjusted sequentially to keep the position constant. Large fluctuations in the pulling speed or the flow rate of the raw material gas change the deposition state, resulting in significant fluctuations in the properties of the optical fiber preform.

[0006] The present invention has been made in view of the above problems, and an object of the present invention is to provide a method for manufacturing a porous glass preform for optical fiber in which fluctuations in properties in the longitudinal direction are suppressed. [Means for solving the problem]

[0007] In order to solve the problems, the present invention provides a method for manufacturing an optical fiber preform using the VAD method, which is characterized by detecting the tip position of the porous glass preform during deposition and adjusting the pulling speed so that the tip position remains constant, setting a target pulling speed for each deposition time in the early stages of deposition, and sequentially adjusting and correcting the flow rate of the raw material gas to the burner to achieve the target pulling speed, thereby gradually changing the pulling speed while depositing until a predetermined time has elapsed, and in the steady state, depositing is performed so that the pulling speed remains constant, and after generating the porous glass preform, the porous glass preform is dehydrated and vitrified in a heating furnace to obtain an optical fiber glass preform.

[0008] In the method for producing a porous glass base material according to the present invention, the predetermined time period for gradually changing the pulling speed is preferably 300 to 500 minutes from the start of deposition.

[0009] In the method for producing a porous glass base material according to the present invention, the correction interval of the raw material gas flow rate to the burner in the initial stage of deposition may be set to 15 to 300 seconds.

[0010] In the method for producing a porous glass base material according to the present invention, the correction interval of the raw material gas flow rate to the burner in the initial stage of deposition may be set to 30 to 90 seconds.

[0011] In the present invention, the optical fiber preform is measured using a refractive index distribution measuring device, the cutoff wavelength in the longitudinal direction is calculated and estimated, and the deposition conditions are adjusted based on the results obtained so that the target pulling speed at the beginning of deposition is reduced when it is desired to increase the cutoff wavelength at the beginning of deposition, and the target pulling speed at the beginning of deposition is increased when it is desired to decrease the cutoff wavelength at the beginning of deposition. [Effects of the Invention]

[0012] According to the present invention, it is possible to produce a porous glass preform for optical fiber in which fluctuations in properties in the longitudinal direction are suppressed. [Brief explanation of the drawings]

[0013] [Figure 1] 1 shows an example of a core soot manufacturing apparatus according to this embodiment. [Figure 2] 1 is a graph showing the transition of the pulling speed in the initial stage of deposition in Examples 1 to 3 and Comparative Example 1. [Figure 3] 1 shows a longitudinal graph of the cutoff wavelength of the core base material in Example 1. [Figure 4] 10 shows a longitudinal graph of the cutoff wavelength of the core preform in Example 2. [Figure 5] 10 shows a longitudinal graph of the cutoff wavelength of the core preform in Example 3. [Figure 6] 1 shows a longitudinal graph of the cutoff wavelength of the core preform in Comparative Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0014] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. In the following description and drawings, the same components are denoted by the same reference numerals, and the description of components that have already been described will be omitted or will be limited to the extent necessary.

[0015] FIG. 1 shows an example of a core soot manufacturing apparatus according to this embodiment. This manufacturing apparatus is an apparatus for manufacturing an optical fiber preform using the so-called VAD method. As shown in FIG. 1, this manufacturing apparatus includes a chamber 2 in which core soot (porous glass preform) 1 is formed, a shaft 3 suspended from above into the chamber 2, a core-forming burner 4 located at the bottom inside the chamber 2, cladding-forming burners 5 and 6, and a CCD camera 7 located outside the chamber 2 for photographing the lower end of the core soot 1. The apparatus also includes an elevator / rotator 8 for raising and lowering and rotating the shaft 3, a mass flow controller (MFC) 9 for controlling the flow rate of the source gas, an image processor 10 for processing the image signal of the camera image of the lower end of the core soot 1, and a controller 11 for controlling the operation of each component. During the manufacturing of the core soot 1, the pulling speed is adjusted under the control of the controller 11 so that the position of the lower end of the core soot 1 photographed by the CCD camera 7 remains constant. Furthermore, the control device 11 is equipped with a mechanism for calculating the average value of the pulling speed at every preset time, a mechanism for comparing the calculated average value of the pulling speed with a preset pulling speed to calculate the difference, and a mechanism for correcting the flow rate of the raw material gas supplied to the core formation burner 4 according to the calculated difference.

[0016] In the steady state of deposition, it is desirable to pull up at a set, constant pulling speed; however, in the early stages of deposition, deposition is not stable, and differences in the pulling speed often occur between devices and between lots due to factors such as the gas flow rate to burner 4, burner 4 settings, individual differences in burner 4 and equipment, and changes in burner 4 over time.

[0017] In the present invention, in order to suppress variations in the optical properties of the optical fiber preform in the longitudinal direction, particularly at the initial stage of deposition, a target pulling speed is set for each deposition time, and the pulling speed is gradually changed by successively adjusting and correcting the flow rate of the raw material gas to the burner 4 to achieve the target pulling speed, and deposition is performed. This stabilizes the pulling speed at the initial stage of deposition, suppresses variations between lots, and reduces lot-to-lot variations in property fluctuations. Furthermore, the target pulling speed at the initial stage of deposition can be adjusted for each device or lot, and adjusting the target pulling speed at the initial stage of deposition can suppress variations in the properties of the glass preform in the longitudinal direction. The target pulling speed may be set for each deposition time or according to the pulling length of the core soot.

[0018] A target pulling speed for each deposition time is set, and the flow rate of the raw material gas to the burner 4 is successively adjusted and corrected to gradually change the pulling speed to the target pulling speed. The deposition time is preferably from the start of deposition to 500 minutes, and more preferably from the start of deposition to 300 minutes. The time for gradually changing the pulling speed to reach the target pulling speed is not good for characteristic fluctuations if it is too long or too short, and the above time was found to be optimal.

[0019] Regarding the correction interval for the source gas flow rate to the burner 4 in the early stage of deposition, if the correction interval is too long, the averaging time required to determine the correction flow rate will be long, resulting in poor tracking of the target pulling speed. In a steady state where the pulling speed is constant and deposition is stable, it is not problematic to set the correction interval relatively long, but it is not desirable to set the correction interval long in the early stage of deposition when the pulling speed is gradually changed. Furthermore, if the correction interval is too short, the source gas flow rate will be corrected frequently, making it more likely that small fluctuations in the pulling speed will occur. Therefore, in the early stage of deposition, the correction interval for the source gas flow rate is preferably 15 to 300 seconds, and more preferably 30 to 90 seconds.

[0020] Cleaning or replacing the burner, or changing the burner settings, can change the deposition conditions, which can result in changes in the longitudinal variation of the cutoff wavelength even if the same pull-up speed is used at the beginning of deposition. In such cases, the optical fiber preform can be measured using a refractive index distribution measurement device to calculate and estimate the longitudinal cut-off wavelength, and the target pull-up speed at the beginning of deposition can be adjusted based on the results. By setting the deposition start side of the VAD method as the product start side of the core preform, if you want to increase the cut-off wavelength at the product start side, you can reduce the target pull-up speed at the beginning of deposition, and if you want to decrease the cut-off wavelength at the product start side, you can increase the target pull-up speed at the beginning of deposition. This can reduce the variation in the cut-off wavelength by reflecting this in the deposition conditions. In particular, when it is desired to adjust the cutoff wavelength in the range of 0 mm to 100 mm at the product start end, it is preferable to change the target pulling speed in the range of 0 to 250 mm of the pulling length using the VAD method. When it is desired to adjust the cutoff wavelength in the range of 100 mm to 200 mm at the product start end, it is preferable to change the target pulling speed in the range of 250 to 500 mm of the pulling length using the VAD method.

[0021] Examples 1 to 3 and Comparative Example 1, which were carried out to confirm the effects of the present invention, will be described below. [Example]

[0022] [Example 1] In the deposition using the VAD method, silicon tetrachloride and germanium tetrachloride were supplied as glass raw materials to core-forming burner 4, and silicon tetrachloride was supplied as a glass raw material to cladding-forming burners 5 and 6. The target pulling speed was set to 0.98 mm / min 90 minutes after the start of deposition (pull length 85 mm), 0.94 mm / min 120 minutes after the start of deposition (pull length 115 mm), 0.92 mm / min 180 minutes after the start of deposition (pull length 170 mm), 0.90 mm / min 240 minutes after the start of deposition (pull length 225 mm), and 0.91 mm / min in the steady state after 300 minutes after the start of deposition (pull length 280 mm). The pulling speed was gradually changed by sequentially adjusting and correcting the flow rate of the raw material gas to core-forming burner 4 so that the target pulling speed was achieved at each time. The correction interval for the raw material gas flow rate to the core-forming burner 4 was set to 30 seconds during deposition. The thick solid line in Figure 2 shows the transition of the pulling speed during the initial stage of deposition in Example 1. The final pulling length of the core soot 1 was 1,450 mm. The core soot 1 deposited using the VAD method was then heated in an electric furnace to dehydrate and vitrify it into a transparent glass core preform. The product length of the core preform was 580 mm. The refractive index distribution of the core preform in the longitudinal direction was measured using a refractive index distribution measurement device, and optical properties such as the cutoff wavelength and mode field diameter were calculated and estimated. The refractive index distribution of the core preform was measured by irradiating a laser beam along a cross section perpendicular to the axis of the preform and measuring the change in the refraction angle within the plane. The finite element method was used to estimate the optical properties. Figure 3 shows a longitudinal graph of the cutoff wavelength of the core preform in Example 1.

[0023] The deposition start side of the VAD method was defined as the product start side of the core base material, and the average value of the cutoff wavelength in the range of 200 to 400 mm from the product start end of the core base material was calculated.The difference between the cutoff wavelength at each measurement point 70 to 200 mm from the product start end and the previously calculated average value was calculated at each point, and the total value was divided by the number of measurement points.This value was defined as the cutoff wavelength variation.

[0024] The amount of cutoff wavelength variation was calculated for the core preform manufactured under the conditions of Example 1, and was found to be 3.4 nm, which was extremely small for a variation in cutoff wavelength.

[0025] [Example 2] In the deposition by the VAD method, silicon tetrachloride and germanium tetrachloride were supplied as glass raw materials to the burner 4 for forming the core, and silicon tetrachloride was supplied as a glass raw material to the burners 5 and 6 for forming the cladding. The target pulling speed was set to 0.80 mm / min 90 minutes after the start of deposition (pull length 75 mm), 0.80 mm / min 120 minutes after the start of deposition (pull length 100 mm), 0.80 mm / min 180 minutes after the start of deposition (pull length 150 mm), 0.85 mm / min, 0.87 mm / min 240 minutes after the start of deposition (pull length 200 mm), 0.89 mm / min 300 minutes after the start of deposition (pull length 250 mm), and 0.91 mm / min at steady state after 540 minutes after the start of deposition (pull length 470 mm). The pulling speed was gradually changed by sequentially adjusting and correcting the flow rate of the source gas to core formation burner 4 to achieve the target pulling speed at each time. The deposition was performed with a 30-second correction interval for the flow rate of the source gas to core formation burner 4. Thereafter, a core preform was obtained in the same manner as in Example 1. The dotted line graph in Figure 2 shows the transition of the pulling rate at the initial stage of deposition in Example 2. Figure 4 shows a longitudinal graph of the cutoff wavelength of the core preform in Example 2.

[0026] The cutoff wavelength variation was calculated to be 14 nm for the core preform manufactured under the conditions of Example 2. In Example 2, the target pulling speed at the beginning of deposition was slower than in Example 1, resulting in an increase in the cutoff wavelength on the product start side.

[0027] [Example 3] In the deposition by the VAD method, silicon tetrachloride and germanium tetrachloride were supplied as glass raw materials to the burner 4 for forming the core, and silicon tetrachloride was supplied as a glass raw material to the burners 5 and 6 for forming the cladding. The target pulling speed was set to 1.00 mm / min 90 minutes after the start of deposition (pull length 85 mm), 1.00 mm / min 120 minutes after the start of deposition (pull length 115 mm), 0.97 mm / min 180 minutes after the start of deposition (pull length 175 mm), 0.96 mm / min 240 minutes after the start of deposition (pull length 230 mm), 0.93 mm / min 300 minutes after the start of deposition (pull length 290 mm), and 0.91 mm / min at steady state after 540 minutes after the start of deposition (pull length 510 mm). The pulling speed was gradually changed by sequentially adjusting and correcting the flow rate of the source gas to core formation burner 4 to achieve the target pulling speed at each time. The deposition was performed with a 30-second correction interval for the flow rate of the source gas to core formation burner 4. Thereafter, a core preform was obtained in the same manner as in Example 1. The dashed line graph in Figure 2 shows the transition of the pulling rate at the initial stage of deposition in Example 3. Figure 5 shows a longitudinal graph of the cutoff wavelength of the core preform in Example 3.

[0028] The cutoff wavelength variation was calculated to be 21.4 mm for the core preform manufactured under the conditions of Example 3. In Example 3, the target pulling speed at the beginning of deposition was faster than in Example 1, and as a result, the cutoff wavelength on the product start side decreased.

[0029] [Comparative Example 1] In deposition by the VAD method, silicon tetrachloride and germanium tetrachloride were supplied as glass raw materials to core-forming burner 4, and silicon tetrachloride was supplied as a glass raw material to cladding-forming burners 5 and 6. In Comparative Example 1, a target pulling speed at the beginning of deposition was not set, and the flow rate of the raw material gas to core-forming burner 4 was set for each deposition time. The source gas flow rate was set to 440 sccm 90 minutes after the start of deposition (pull-up length: 85 mm), 500 sccm 120 minutes after the start of deposition (pull-up length: 115 mm), 585 sccm 180 minutes after the start of deposition (pull-up length: 170 mm), 620 sccm 240 minutes after the start of deposition (pull-up length: 230 mm), and 620 sccm 300 minutes after the start of deposition (pull-up length: 280 mm). The source gas flow rate to the core-forming burner 4 was sequentially adjusted and corrected so that the pulling rate was 0.91 mm / min in the steady state after 500 minutes after the start of deposition (pull-up length: 550 mm). The correction interval for the source gas flow rate to the core-forming burner 4 was set to 1200 seconds, and deposition was performed. Thereafter, a core base material was obtained in the same manner as in Example 1. The thin solid line graph in FIG. 2 shows the progress of the pulling rate during the initial stage of deposition in Comparative Example 1. FIG. 6 shows a longitudinal graph of the cutoff wavelength of the core preform in Comparative Example 1.

[0030] The amount of cutoff wavelength variation was calculated to be 21.8 mm for the core preform produced under the conditions of Comparative Example 1. In Comparative Example 1, a target pulling speed at the beginning of deposition was not set, and the flow rate of the raw material gas to the core formation burner 4 was set for each deposition time, and control was performed so that the pulling speed could be adjusted to achieve the set flow rate. As a result, the pulling speed at the beginning of deposition was not stable, and as a result, the variation in the cutoff wavelength at the start of production also increased.

[0031] As mentioned above, the longitudinal fluctuation of the cutoff wavelength at the beginning of deposition does not always show the same trend, and the longitudinal fluctuation may change due to burner cleaning or replacement, burner setting, aging, etc. In such cases, to check the trend of the longitudinal fluctuation of the cutoff wavelength, it is recommended to manufacture an optical fiber preform, measure the characteristics of the optical fiber preform using a refractive index distribution measurement device, calculate and estimate the longitudinal cutoff wavelength, and adjust the target pull-up speed at the beginning of deposition based on the obtained results. From the results of Example 2 above, when it is desired to increase the cutoff wavelength at the start of production (i.e., when the cutoff wavelength at the start of production tends to be lower than the cutoff wavelength thereafter), it is possible to increase the cutoff wavelength at the start of production by reducing the target pull-up speed at the beginning of deposition, thereby suppressing the fluctuation of the cutoff wavelength of the entire product. Furthermore, from the results of Example 3, when it is desired to lower the cutoff wavelength on the product start side (i.e., when there is a tendency for the cutoff wavelength on the product start side to be higher than the cutoff wavelength thereafter), the cutoff wavelength on the product start side can be lowered by increasing the target pulling speed at the beginning of deposition, thereby suppressing fluctuations in the cutoff wavelength of the entire product.

[0032] The present invention is not limited to the above-described embodiments. The above-described embodiments are merely examples, and anything that has substantially the same configuration as the technical idea described in the claims of the present invention and exhibits similar effects is included within the technical scope of the present invention. In other words, appropriate modifications are possible within the scope of the technical idea expressed in the present invention, and forms incorporating such modifications and improvements are also included within the technical scope of the present invention. [Explanation of symbols]

[0033] 1 Core Suit 2 chambers 3 shafts 4 Core forming burner 5. Burner for cladding 6 Burner for cladding 7 CCD cameras 8 Lifting and rotating device 9 Mass Flow Controller (MFC) 10 Image processing device 11 Control device

Claims

1. In a method for manufacturing an optical fiber preform by the VAD method, The tip position of the porous glass base material being deposited is detected, and the pulling speed is adjusted so that the tip position is constant. In the initial stage of deposition, a target pulling speed is set for each deposition time, and the flow rate of the raw material gas to the burner is adjusted and corrected at predetermined correction intervals so as to reach the target pulling speed, thereby gradually changing the pulling speed until the predetermined time is reached, and deposition is performed; In the steady state, the pulling speed is kept constant and the porous glass base material is formed. The method for producing an optical fiber preform is characterized in that the porous glass preform is dehydrated in a heating furnace to form a transparent glass, thereby obtaining an optical fiber glass preform.

2. 2. The method for manufacturing an optical fiber preform according to claim 1, wherein the predetermined time for gradually changing the pulling speed is 300 to 500 minutes from the start of deposition.

3. 3. The method for manufacturing an optical fiber preform according to claim 1, wherein the correction interval of the raw material gas flow rate to the burner in the initial stage of deposition is set to 15 to 300 seconds.

4. 3. The method for manufacturing an optical fiber preform according to claim 1, wherein the correction interval of the raw material gas flow rate to the burner in the initial stage of deposition is set to 30 to 90 seconds.

5. 3. The method for manufacturing an optical fiber preform according to claim 1, wherein the optical fiber preform is measured by a refractive index distribution measuring device, the cutoff wavelength in the longitudinal direction is calculated and estimated, and based on the obtained results, the deposition conditions are reflected so that the target pulling speed at the beginning of deposition is reduced when it is desired to increase the cutoff wavelength at the deposition start side, and the target pulling speed at the beginning of deposition is increased when it is desired to decrease the cutoff wavelength at the deposition start side.

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