Manufacturing method of optical fiber preform

Abstract

To provide a manufacturing method of an optical fiber preform capable of reducing a time required for dehydrating and transparentizing a soot body.SOLUTION: A manufacturing method of an optical fiber preform comprises: a preparation step of preparing a glass target containing SiO2 as the main component; a deposition step of depositing of glass fine particles on the glass target to manufacture a soot body; and a sintering step of dehydrating and transparentizing the soot body in a mixed gas atmosphere consisting of an inert gas selected from a group consisting of helium, argon and nitrogen or a combination thereof and a dehydration gas selected from a group consisting of chlorine and silicon tetrachloride so as to manufacturing an optical fiber preform. The sintering step further includes a preheating step of preheating a part or the whole of the soot body in a preheating furnace installed above a main heater for vitrification of the soot body at 800°C or over and 1200°C or under.SELECTED DRAWING: Figure 4

Description

Technical Field

[0001] The present invention relates to a method for manufacturing an optical fiber preform that shortens the time required for dehydration treatment and vitrification treatment of a porous optical fiber preform.

Background Art

[0002] To manufacture an optical fiber glass preform, first, a porous glass preform (hereinafter referred to as a soot body) is produced by various methods including the vapor axial deposition method (VAD) and the outside vapor deposition method (OVD). The soot bodies produced by these methods are formed of an aggregate of only glass fine particles or a transparent glass rod with glass fine particles deposited on the outer periphery thereof. Then, the soot body is subjected to a dehydration treatment by heating at 1000 to 1200°C in a chlorine gas atmosphere, and then further subjected to a step of heating at 1400 to 1600°C in a helium gas atmosphere to be vitrified into a transparent glass to obtain an optical fiber glass preform.

Summary of the Invention

Problems to be Solved by the Invention

[0003] In particular, in the production of an optical fiber preform in which a soot is deposited around a glass target to form a soot body, which is then sintered to obtain an optical fiber preform, it is desired to shorten the time required for the dehydration treatment and the transparent vitrification treatment of the soot body without degrading the various properties of the optical fiber obtained by spinning. Therefore, an object of the present invention is to provide a method for manufacturing an optical fiber preform that enables shortening of the time required for the dehydration treatment and the transparent vitrification treatment of a soot body formed by depositing a soot around a glass target.

Means for Solving the Problems

[0004] The method for manufacturing an optical fiber preform of the present invention comprises a preparation step of preparing a glass target mainly composed of SiO2, a deposition step of depositing glass fine particles on the glass target to manufacture a soot body, and a sintering step of dehydrating and vitrifying the soot body in a mixed gas atmosphere of an inert gas selected from the group consisting of helium, argon, and nitrogen or a combination thereof and a dehydrating gas selected from the group consisting of chlorine and silicon tetrachloride to manufacture an optical fiber preform. The sintering step further includes a preheating step of preheating part or all of the soot body to 800°C or higher and 1200°C or lower in a preheating furnace provided above a main heater for vitrifying the soot body.

[0005] The concentration of the dehydrating gas in the mixed gas atmosphere is 24 [Vol%] or higher and 28 [Vol%] or lower. It is preferable that the preheating furnace sends the soot body to a sintering furnace and stops heating after the soot body no longer exists in the preheating furnace. The ratio CR value of the diameter of the portion corresponding to the glass target to the diameter of the optical fiber preform obtained by dehydration and vitrification is 0.25 or higher and 0.32 or lower. The transmission loss of the optical fiber obtained by spinning the optical fiber preform obtained by dehydration and vitrification at a wavelength of 1383 nm is 0.31 [dB / km] or lower.

Advantages of the Invention

[0006] According to the method for manufacturing an optical fiber preform of the present invention, a preheating furnace is provided above the main heater for dehydrating and vitrifying the soot body. By preheating the soot body in the preheating furnace before performing the dehydration and sintering vitrification treatment, the dehydration treatment by the main heater of the sintering furnace can be promoted, and high-speed processing of the optical fiber preform becomes possible by performing the dehydration and sintering vitrification treatment simultaneously. In addition, by preheating the soot body at 800°C or higher and 1200°C or lower, OH groups can be sufficiently removed in the dehydration / vitrification furnace, and excellent effects such as being able to manufacture an optical fiber having low transmission loss can be achieved.

Brief Description of the Drawings

[0007]

Figure 1

Figure 2

Figure 3

Figure 4

Embodiments for Carrying Out the Invention

[0008] FIG. 1 is a diagram showing an outline of the sintering furnace of the present invention. A preheating furnace 3 is provided above a dehydration / vitrification furnace 2. In the sintering furnace 1, a furnace core tube 4 penetrates the centers of the dehydration / vitrification furnace 2 and the preheating furnace 3 and is integrally formed. The dehydration / vitrification furnace 2 and the preheating furnace 3 are provided with a mechanism for temperature monitoring by a thermocouple (not shown) and an arithmetic unit, and controlling power so as to reach a predetermined temperature. Inside the furnace core tube 4, there is a mechanism for supplying helium and chlorine into the furnace core tube 4 from a mass flow controller (not shown) capable of controlling the flow rate for performing dehydration / vitrification. The sintering furnace of the present invention having such a configuration can perform dehydration and sintering vitrification treatment simultaneously.

[0009] FIG. 2 is a flowchart showing the manufacturing process of the optical fiber preform according to an embodiment of the present invention. The flowchart 11 shows a manufacturing process including a preparation process 20, a deposition process 21, a sintering process 22, and a measurement process 23. The sintering process 22 includes a hanging-down process 22-1 for hanging down the soot body manufactured in the deposition process 21 into the sintering furnace, a preheating process 22-2 for replacing the atmosphere with a mixed gas of helium and chlorine and heating the soot body during the process, a dehydration / vitrification process 22-3 for dehydrating / vitrifying the soot body, and a taking-out process 22-4 for taking out the optical fiber preform from the sintering furnace 1.

[0010] In the sintering process 22, first, in the suspension process 22-1, the soot body manufactured in the deposition process 21 is suspended and set in the sintering furnace 1. Next, in the preheating process 22-2, the inside of the furnace core tube 4 is replaced with a dehydrated gas atmosphere to preheat the soot body. For example, while supplying a carrier gas of helium 5.4 L / min and chlorine 1.9 L / min and a dehydrated gas as a mixed gas, the preheating furnace 3 is heated to 800°C and the dehydration / vitrification furnace 2 is heated to 1175°C to preheat the soot body, and the soot body is held in the preheating furnace 3 for 1 hour. Note that the dehydrated gas concentration is preferably 24 [Vol%] or more and 28 [Vol%] or less. This is because if it is less than 24 [Vol%], a sufficient dehydration effect cannot be obtained. Also, the CR value indicating the ratio of the diameter of the portion corresponding to the glass target to the diameter of the optical fiber preform obtained by dehydration and transparent vitrification is preferably 0.25 or more and 0.32 or less. By setting it to 0.25 or more, the thickness of the porous soot layer deposited on the glass target becomes relatively thin, and the processing time can be effectively shortened. However, if it exceeds 0.32, a glass target with a large diameter needs to be prepared, resulting in a decrease in the productivity of the preparation process.

[0011] In the dehydration / vitrification process 22-3, the soot body preheated to 800°C or higher in the preheating process 22-2 is inserted into the dehydration / vitrification furnace 2 with a part of the soot body inserted. With the inside of the dehydration / vitrification furnace 2 set as the vitrification temperature, the temperature is raised to 1500°C, held for 1 hour after the temperature rise, and then, under the high-speed dehydration / vitrification sintering conditions shown in Fig. 3-2, the soot body is drawn into the dehydration / vitrification furnace 2 at a maximum lowering speed of 2.2 mm / min, and heat treatment is performed from one end of the soot body to achieve transparent vitrification.

[0012] In the take-out process 22-4, after replacing the atmosphere inside the furnace core tube with nitrogen, the transparently vitrified optical fiber preform is taken out of the furnace core tube 4. The taken-out optical fiber preform is cooled, and its optical properties are measured in the measurement process 23 to confirm that it is within the specifications.

[0013] The optical fiber preform manufactured through the above steps of the present invention, despite being dehydrated and vitrified at a higher speed than conventional ones, has optical characteristics that meet the standards. The transmission loss at a wavelength of 1383 nm is 0.31 dB / km or less at any position along the longitudinal direction of the optical fiber preform, showing good characteristics. In the present invention, the inert gas can be replaced with a gas selected from the group consisting of helium, argon, and nitrogen or a combination thereof, and the dehydration gas can be replaced with silicon tetrachloride.

Example

[0014] Example 1; Manufacture a suit body according to Flowchart 11 and set the suit body at the preheating position of sintering furnace 1 in the suspension step 22-1. In the preheating step 22-2, supply dehydration gas of helium at 5.4 l / min and chlorine at 1.9 l / min into the furnace core tube to replace the inside of the furnace core tube 4 with a dehydration gas atmosphere. Set the temperature of preheating furnace 3 at 800°C and the temperature of dehydration and vitrification furnace 2 at 1175°C, and hold for 1 hour. In the dehydration and vitrification step 22-3, raise the temperature of dehydration and vitrification furnace 2 to 1500°C, move the preheated suit body at 800°C or higher to the dehydration and vitrification start position, pull it down at a maximum speed of 2.2 mm / min, and supply the dehydration gas at a flow rate of helium at 2.0 l / min and chlorine at 0.7 l / min to sinter the suit body from one end to the other end in sintering furnace 1. The processing time in the sintering step 22 is 26.5 hours when using the high-speed dehydration and vitrification sintering conditions shown in Figure 3-2, and it can be recognized that the pulling-down speed is extremely fast compared with Comparative Example 1 below. The relationship between the pulling-down distance of the suit body and the high-speed pulling-down speed at this time is shown in Figure 3-2.

[0015] Measure the refractive index characteristics of the high-speed dehydrated and vitrified optical fiber preform in the measurement step in Flowchart 11 to obtain the refractive index difference of the cladding layer with respect to the synthetic quartz cell of the measuring instrument. In the high-speed treatment with preheating at 800°C in the preheating furnace, as shown in Figure 4, relatively lower refractive index difference characteristics were obtained compared with those vitrified in a conventional sintering furnace. In FIG. 4, the horizontal axis represents the longitudinal relative position between the optical fiber preform subjected to the 800°C preheating treatment and the optical fiber preform processed in the conventional sintering furnace, and the vertical axis represents the measured clad layer refractive index difference. In the figure, the plot ▲ represents the one processed at 800°C in the preheating furnace, and the plot 〇 represents the one processed in the conventional sintering furnace.

[0016] As a result of measuring the characteristics of the dehydrated and vitrified optical fiber preform in the measurement process in Flowchart 11, the transmission loss at a wavelength of 1383 nm was extremely good, being 0.27 dB / km at the longitudinal relative position of 0, 0.28 dB / km at the relative position of 0.5, and 0.28 dB / km at the relative position of 1.0. Most of the plots ▲ according to the examples processed at 800°C in the preheating furnace are located below the plots 〇 processed by the conventional method, and favorable results were obtained.

[0017] Comparative Example 1; Using Flowchart 11, the manufacturing method of the optical fiber preform according to the prior art will be described. As the conventional manufacturing method, in the suspension step 22-1, the soot body manufactured in the deposition step 21 is set at the dehydration and vitrification position of the sintering furnace. In the conventional manufacturing method, preheating of the soot body by the preheating step 22-2 is not performed. In the next dehydration and vitrification step 22-3, in order to replace the inside of the furnace core tube where the soot body is set with a dehydrated gas atmosphere, helium at 7.9 l / min and chlorine at 2.8 l / min are supplied, the temperature is raised to 1500°C and held for 1 hour, and the soot body is sintered and dehydrated and vitrified from one end to the other end by pulling it down at a maximum speed of 1.6 mm / min. This conventional manufacturing method required 29.9 hours to manufacture the optical fiber preform. The relationship between the pulling-down distance and the pulling-down speed of the soot body at this time is shown in FIG. 3-1.

[0018] As a result of measuring the characteristics of the dehydrated and vitrified optical fiber preform in the measurement process of Flow Chart 11, the transmission loss at a wavelength of 1383 nm is 0.27 dB / km at a longitudinal relative position of 0, 0.28 dB / km at a relative position of 0.5, and 0.28 dB / km at a relative position of 1.0. The clad layer refractive index difference is distributed at a relatively higher position than in Example 1.

Explanation of Signs

[0019] 1. Sintering furnace 2. Dehydration and vitrification furnace 3. Preheating furnace 4. Core tube

Claims

Claim 1 SiO 2 A method for manufacturing an optical fiber preform, comprising: a preparation step of preparing a glass target containing SiO as a main component; a deposition step of depositing glass fine particles on the glass target to manufacture a soot body; and a sintering step of dehydrating and vitrifying the soot body in a mixed gas atmosphere of an inert gas selected from the group consisting of helium, argon, and nitrogen or a combination thereof and a dehydrating gas selected from the group consisting of chlorine and silicon tetrachloride to manufacture an optical fiber preform. The sintering step further includes a preheating step of preheating part or all of the soot body to 800°C or higher and 1200°C or lower in a preheating furnace provided above a main heater for vitrifying the soot body. Claim 2 The method for manufacturing an optical fiber preform according to claim 1, wherein the concentration of the dehydrated gas in the mixed gas atmosphere is 24 [Vol%] or more and 28 [Vol%] or less. Claim 3 The method for manufacturing an optical fiber preform according to claim 1 or 2, wherein the preheating furnace feeds out the soot body and stops heating after the soot body no longer exists in the preheating furnace. Claim 4 The method for manufacturing an optical fiber preform according to claim 1, wherein the ratio CR value of the diameter of the portion corresponding to the glass target to the diameter of the optical fiber preform obtained by dehydration and vitrification is 0.25 or more and 0.32 or less. Claim 5 The method for manufacturing an optical fiber preform according to claim 1, wherein the transmission loss at a wavelength of 1383 nm of the optical fiber obtained by spinning the optical fiber preform obtained by dehydration and vitrification is 0.31 [dB / km] or less.

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