JP2006096608A - Method for producing glass preform - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a multi-mode optical fiber preform capable of accurately forming refractive index distribution required for a GI-type multi-mode optical fiber. <P>SOLUTION: This method for producing the multi-mode optical fiber preform comprises the steps of introducing silicon tetrachloride, additives for adjusting the refractive index and a combustion gas into a burner 5 to generate a flame while rotating a long mandrel 4 around its axis, producing glass particulates by the flame, relatively moving the mandrel 4 and the burner 5 a plurality of times in the longitudinal direction of the mandrel 4 to deposit the glass particulates on the outside of the mandrel 4, and heating the deposited glass particulates to be sintered. The production conditions are determined as follows: measuring refractive index of each layer formed by every relative movement of an aforetime produced optical fiber preform to the burner 5, and determining production conditions adapted to target refractive index for each layer in the target refractive index distribution from the measured refractive index for each layer. The addition amount of the additives for adjusting the refractive index is adjusted according to the production conditions. <P>COPYRIGHT: (C)2006,JPO&amp;NCIPI

Description

  The present invention repeats the operation of rotating the starting material around its axis and relatively moving the burner in the longitudinal direction of the starting material a plurality of times in the radial direction of the starting material, silicon tetrachloride and an additive for adjusting the refractive index. And a combustion gas are introduced into the burner to generate a flame to generate glass fine particles. After the glass fine particles are deposited in a plurality of layers on the outside of the starting material, the deposited glass fine particles are heated and sintered. The present invention relates to a glass base material manufacturing method for manufacturing a glass base material.

  As an example of a communication optical fiber, a multimode optical fiber in which a plurality of modes propagates is used. There are two main types of refractive index distribution of a multimode optical fiber: a step index (SI) and a graded index (GI). Among these, the GI type optical fiber has a refractive index distribution in which the central portion of the core has the highest refractive index and gradually decreases toward the outside in the radial direction, and the spread of the optical signal to be transmitted (Mode dispersion) is suppressed.

  For example, an OVD method is used as a method of manufacturing an optical fiber preform for obtaining such a GI type refractive index profile. In the OVD method, glass fine particles (referred to as soot) are deposited in a layer around a long starting material to form a glass fine particle deposit. The obtained glass fine particle deposit body becomes a transparent glass body by removing the starting material and sintering or solidifying. This glass body becomes an optical fiber preform as it is, or becomes an optical fiber preform in combination with other glass bodies.

  In order to form a GI-type refractive index distribution by this OVD method, a glass fine particle contains a refractive index adjusting additive (for example, germanium) that increases the refractive index of glass when glass fine particles are deposited on a starting material. However, the content is gradually reduced according to the formation of the deposit. That is, high refractive index glass particles are deposited by adding a large amount of refractive index adjusting additive at a location close to the starting material, and low refractive index glass fine particles are reduced at a distant location by reducing the content of the refractive index adjusting additive. To deposit. As a result, a refractive index distribution that gradually decreases toward the outside in the radial direction is formed. Methods for manufacturing such a GI type multimode optical fiber preform using the OVD method are disclosed in various documents (see, for example, Patent Document 1).

U.S. Pat. No. 4,125,388

  By the way, an ideal refractive index distribution for suppressing mode dispersion in a GI type multimode optical fiber has an α power distribution shape having a peak at the center of the core. Therefore, it is important to appropriately adjust the amount of the refractive index adjusting additive to be contained when depositing the glass fine particles in accordance with the α power distribution.

  However, as the glass fine particles are deposited and the diameter of the glass fine particle deposit is gradually increased, the production conditions such as the air flow state and temperature in the reaction vessel in which the glass fine particles are deposited change every moment. In addition, due to deterioration of the equipment that implements the OVD method, the manufacturing conditions for each lot also differ. Therefore, a multimode optical fiber preform having an ideal refractive index distribution of GI type can be stably obtained by simply adjusting the amount of the refractive index adjusting additive introduced into the burner along the α power distribution. It was difficult.

  An object of the present invention is to provide a method for producing a multimode optical fiber preform that can accurately form a refractive index distribution required for a GI type multimode optical fiber.

  In order to achieve the above object, a method for producing a glass base material according to the present invention generates silicon flame by introducing silicon tetrachloride, a refractive index adjusting additive and a combustion gas into a burner, and generates glass fine particles. A glass particulate deposition layer is formed by relatively moving the burner in the longitudinal direction of the starting material rotating around the axis, and a plurality of glass particulate deposition layers are formed outside the starting material by repeating the operation a plurality of times. A glass base material manufacturing method for manufacturing a glass base material for heating and sintering the glass microparticle stack after forming the glass microparticle stack so as to have a graded refractive index distribution Change the refractive index for each layer, set the generation and deposition conditions for the glass particles in each layer, leave a record, manufacture the glass base material, measure its refractive index distribution, and compare it with the target refractive index distribution. The actual crook If the refractive index distribution is different from the target refractive index distribution, the manufacturing conditions for the generation and deposition of the glass fine particles of the layers having different refractive index distributions are modified so as to approach the target refractive index distribution. It is characterized by manufacturing a glass base material.

  In the method for producing a glass base material according to the present invention, it is desirable that the difference between the target refractive index and the measured refractive index for each layer be 1% or less of the maximum refractive index difference in the graded refractive index distribution portion. .

  Further, the glass base material manufacturing method according to the present invention is new by interpolation approximation from the condition of the amount of each refractive index adjustment additive in the measured refractive index of a plurality of layers approximating the target refractive index for each layer. It is desirable to obtain the addition amount of the refractive index adjusting additive as a manufacturing condition.

  Moreover, the manufacturing method of the glass base material which concerns on this invention newly produces the average value of the addition amount of each refractive index adjustment additive in the measurement refractive index in which the difference with the target refractive index for every layer is less than 1%. It is desirable to set the addition amount of the refractive index adjusting additive as a condition.

  According to the method for manufacturing an optical fiber preform of the present invention, a new optical fiber preform is manufactured under manufacturing conditions in which the measured refractive index of each layer of the previously manufactured optical fiber preform is fed back. The refractive index can be accurately adjusted to the target refractive index. This makes it possible to manufacture a multimode optical fiber preform having a refractive index distribution that is very close to the target refractive index distribution by accurate feedback without being influenced by the air flow state in the reaction vessel, the manufacturing conditions, or the like.

Hereinafter, as an example of a glass base material manufacturing method according to the present invention, a multi-mode optical fiber base material manufacturing method will be used, and this embodiment will be described with reference to the drawings.
FIG. 1 shows a manufacturing apparatus capable of carrying out a method for manufacturing a multimode optical fiber preform according to the present invention.

A manufacturing apparatus 1 shown in FIG. 1 deposits glass fine particles on a mandrel 4 in a space inside a reaction vessel 2 by a so-called OVD method.
The reaction vessel 2 is formed using a material such as silicon dioxide, silicon carbide, nickel, or a nickel alloy that hardly corrode by chlorine gas or the like even under high temperature environmental conditions when generating and depositing glass particles. ing.

  A holding tool 3 that can be moved up and down in the vertical direction is housed in the reaction vessel 2. The gripping tool 3 grips the upper end of a long mandrel 4 as a starting material and supports the mandrel 4 in the vertical direction. Further, the gripping tool 3 can rotate the supported mandrel 4 around its axis. The mandrel 4 is made of any material among alumina, carbon, and silica. The mandrel 4 desirably has a cross-sectional outer shape close to a perfect circle, and the overall configuration may be a hollow cylindrical shape or a solid columnar shape.

  Further, a burner 5 for generating glass fine particles is provided in the reaction vessel 2. The burner 5 has a plurality of ports through which gas is blown out, and the combustion gas and the glass raw material gas are blown out from the ports, respectively, and the glass raw material is hydrolyzed in an oxyhydrogen flame generated by the combustion of the combustion gas, so that the glass It produces fine particles. Further, the burner 5 is disposed laterally toward the mandrel 4 so that the generated glass fine particles are deposited on the mandrel 4.

The combustion gas contains hydrogen (H ₂ ) and oxygen (O ₂ ). The glass raw material gas contains silicon tetrachloride (SiCl ₄ ), and appropriately contains an additive for adjusting the refractive index. As the additive for adjusting the refractive index, an additive for increasing the refractive index of silica glass is used. For example, germanium, phosphorus, boron, or the like is used. In this embodiment, germanium is used as an additive for adjusting the refractive index. When this is introduced into the burner 5, germanium tetrachloride (GeCl ₄ ) gas is introduced. When silicon tetrachloride and germanium tetrachloride are used as the glass source gas, glass fine particles mainly composed of silica (SiO ₂ ) and germanium dioxide (GeO ₂ ) are generated in the flame of the burner 5.

  Further, the reaction vessel 2 has an exhaust port 7 at a position facing the burner 5, and the internal exhaust gas containing excess glass fine particles not deposited on the mandrel 4 is sent out from the exhaust port 7 through the exhaust pipe 8. It is.

The gas pipe 14 for introducing various gases into the burner 5 includes a flow rate regulator (MFC) 16 that regulates the flow rate of the gas introduced into the burner 5, and a control computer 13 is connected to the flow rate regulator 16. ing.
A setting device 15 is connected to the control computer 13, and the control computer 13 outputs a control signal to the flow rate regulator 16 based on the set value set by the setting device 15, and the burner 5. Adjust the flow rate of various gases to be introduced.

  When a multimode optical fiber preform is manufactured by the manufacturing apparatus 1 having the above configuration, first, the mandrel 4 suspended in the reaction vessel 2 by the gripping tool 3 is rotated around its axis. Then, silicon tetrachloride, germanium tetrachloride, hydrogen, and oxygen are introduced into the burner 5 to generate an oxyhydrogen flame toward the rotating mandrel 4. In the oxyhydrogen flame, glass fine particles containing germanium dioxide are generated by a hydrolysis reaction. Further, the mandrel 4 is moved back and forth in the longitudinal direction while rotating around the axis. In this embodiment, the burner 5 is fixed and the mandrel 4 is moved. Conversely, the burner 5 is moved in the longitudinal direction of the mandrel 4 to move the burner 5 and the mandrel 4 relative to each other. You may let them.

  In this way, glass fine particles are generated from the burner 5 and reciprocated in the longitudinal direction while rotating the mandrel 4 so that the generated glass fine particles are deposited in a layered manner around the mandrel 4 to deposit glass fine particles. The body 6 is formed. Thereafter, the mandrel 4 is removed, sintered and made transparent, and, for example, the inner surface is subjected to vapor phase etching with fluorine gas, and further solidified to obtain a multimode optical fiber preform.

  Here, in the manufacturing apparatus 1, when the glass particles are deposited on the mandrel 4, the control computer 13 controls the flow rate regulator 16 based on the set value from the setter 15, whereby the burner 5. The amount of the refractive index adjusting additive to be supplied to is adjusted along the α power distribution for each layer, and the content of the refractive index adjusting additive is gradually reduced for each layer as the deposit is formed. Accordingly, a glass having a low refractive index is formed by containing a large amount of refractive index adjusting additive and depositing high refractive index glass particles at a location close to the mandrel 4 and reducing a content of the refractive index adjusting additive at a location far away. Fine particles are deposited. As a result, a multi-mode optical fiber preform having a refractive index distribution that gradually decreases toward the outside in the radial direction, that is, a graded refractive index distribution, is manufactured.

  However, as the glass fine particles are deposited and the diameter of the glass fine particle deposit 6 is gradually increased, the production conditions such as the air flow state and temperature in the reaction vessel 2 in which the glass fine particles are deposited change every moment. In addition, due to deterioration of the equipment that implements the OVD method, the manufacturing conditions for each lot also differ. Therefore, by simply adjusting the amount of the refractive index adjusting additive introduced into the burner 5 along the α power distribution, a multimode optical fiber preform having an ideal refractive index distribution of GI type can be stabilized. Difficult to get.

  For this reason, in the manufacturing method of the multimode optical fiber preform of this embodiment, the optical fiber preform is manufactured as follows.

(1) First, an optical fiber preform having a predetermined profile A is manufactured in advance under manufacturing conditions in which the amount of the refractive index adjusting additive is changed for each layer.

(2) As shown in FIG. 2, the refractive index of the obtained optical fiber preform of profile A is divided for each formation layer at the time of manufacture and measured.
Note that the refractive index of an optical fiber obtained by drawing an optical fiber preform may be measured.

(3) Next, a manufacturing condition in which the measured refractive index of each layer is matched with the target refractive index of each layer of the target profile which is the refractive index distribution of the α power distribution of the newly manufactured multimode optical fiber preform is determined. .
For example, the manufacturing conditions 1 to 9 in the measured refractive index for each layer of the profile A are set to the manufacturing conditions 1, 1, 2, 2, 3, 4, 5, 7, and 9 in accordance with the newly manufactured target profile. .

(4) Thereafter, an optical fiber preform is manufactured according to the above-described procedure under the determined manufacturing conditions.
As a result, the obtained optical fiber preform has a refractive index distribution very close to the target profile.

As described above, according to the method of manufacturing the multimode optical fiber preform, a new optical fiber preform is manufactured under the manufacturing conditions in which the measured refractive index of each layer of the optical fiber preform manufactured previously is fed back. The refractive index of each layer can be accurately adjusted to the target refractive index. As a result, a multimode optical fiber preform having a refractive index distribution that is very close to the target refractive index distribution can be manufactured by accurate feedback without being influenced by the air flow state in the reaction vessel 2 or manufacturing conditions. .
In addition, the refractive index is changed in units of layers so as to obtain a graded refractive index distribution, the conditions for generating and depositing glass fine particles in each layer are set, the record is left, a glass base material is manufactured, and the refractive index distribution is determined. Measure and compare with the target refractive index distribution. If the actual refractive index distribution is different from the target refractive index distribution, target the manufacturing conditions for the generation and deposition of glass particles in the layers with different refractive index distributions. Therefore, a glass base material having a refractive index distribution very close to the target refractive index distribution can be manufactured.

  In particular, the difference between the target refractive index of each layer and the measured refractive index is set to 1% or less of the maximum refractive index difference of the graded refractive index distribution portion, so that a multi-layer that approximates the target refractive index distribution with higher accuracy can be obtained. A mode optical fiber preform can be obtained.

It should be noted that the addition amount of the refractive index adjustment additive that becomes a new production condition by interpolation approximation from the condition of the respective refractive index adjustment additive addition amount in the measurement refractive index of the plurality of layers approximating the target refractive index for each layer. You can ask for it.
Further, the average value of the addition amounts of the refractive index adjustment additives at the measured refractive index within 1% of the target refractive index for each layer is used as the addition amount of the refractive index adjustment additive as a new production condition. You can also.

Note that the measurement data used as the new manufacturing conditions is not limited to the optical fiber preform manufactured immediately before, but may be a plurality of optical fiber preforms manufactured previously.
Further, the above embodiment is not limited to the one having the same profile as that of the optical fiber preform manufactured before, but can be applied to the case of manufacturing optical fiber preforms having various profiles.

Adjust the amount of refractive index adjustment additive consisting of germanium tetrachloride for each layer, and manufacture a reference optical fiber preform with the refractive index distribution that gradually decreases in the radial direction as the target refractive index distribution. did.
Next, the refractive index of the manufactured reference optical fiber preform is measured, the measurement data is divided for each layer, and the manufacturing conditions are determined using the measured refractive index of each layer based on the target profile. A new optical fiber preform was manufactured while adjusting the addition amount of the refractive index adjusting additive made of germanium tetrachloride under the manufacturing conditions for each layer.
The refractive index distribution of the new optical fiber preform thus obtained was measured, and the difference from the target refractive index at each radial position was calculated and evaluated.

The optical fiber preform was molded according to the following procedure.
(1) While maintaining the surface temperature of the deposit within ± 7 ° C., the glass particulates are deposited on a carbon mandrel to form a glass particulate deposit.
(2) Pull out the mandrel from the glass particulate deposit and sinter and clarify in a helium (He) atmosphere to form a pipe-like glass sintered body.
(3) Vapor phase etching is performed on the inner surface of the glass sintered body with fluorine gas and solidified by a collapse method.

The results are shown in FIGS.
FIG. 3 is a graph showing the refractive index distribution of the reference optical fiber preform, and FIG. 4 is a graph showing the refractive index distribution of the new optical fiber preform manufactured under the determined manufacturing conditions.
As can be seen from FIG. 3, the reference optical fiber preform is introduced when, for example, the manufacturing conditions such as the air flow state and temperature in the reaction vessel in which the glass particles are deposited, the deterioration of each equipment, the sintering / transparency, etc. The refractive index distribution obtained with respect to the target refractive index distribution is greatly deviated particularly on the inner and outer peripheral sides due to the influence of gas phase etching with helium or fluorine gas, and the difference from the ideal refractive index is ± 0.045%.

  On the other hand, in the new optical fiber preform manufactured with reference to the refractive index distribution of the reference optical fiber preform, as shown in FIG. Almost identical, the difference from the ideal refractive index was reduced to ± 0.010%.

It is a schematic block diagram which shows the manufacturing apparatus which can implement the manufacturing method of the multimode optical fiber preform which concerns on embodiment of this invention. It is a schematic diagram explaining the manufacturing method of the multimode optical fiber preform of this embodiment. It is a graph which shows the refractive index distribution of a reference | standard optical fiber preform | base_material. It is a graph which shows the refractive index distribution of a novel optical fiber preform.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Manufacturing apparatus 2 Reaction container 3 Holding tool 4 Mandrel (starting material)
5 Burner 6 Glass particulate deposit 7 Exhaust port 8 Exhaust pipe 13 Control computer 14 Gas pipe 15 Setter 16 Flow controller

Claims (4)

  Glass is produced by introducing silicon tetrachloride, a refractive index adjusting additive and a combustion gas into the burner to generate a flame, generating glass fine particles, and relatively moving the burner in the longitudinal direction of the starting material rotating around the axis. A fine particle deposition layer is formed, and the operation is repeated a plurality of times to form a plurality of glass fine particle deposition layers outside the starting material to form a glass fine particle deposit, and then the glass fine particle deposit is heated. A glass base material manufacturing method for manufacturing a sintered glass base material, in which the refractive index is changed in units of layers so as to obtain a graded refractive index distribution, and the conditions for generating and depositing glass fine particles in each layer are set. , Leaving the record, producing a glass base material, measuring its refractive index distribution, comparing with the target refractive index distribution, if the actual refractive index distribution is different from the target refractive index distribution, Refractive index distribution Process for producing a glass preform, characterized in that to correct the production conditions at the time of the glass particles produced deposition of the different layers in the manufacturing conditions so as to approach the refractive index distribution and the target, to produce a glass preform. It is a manufacturing method of the glass base material of Claim 1,
A method for producing a glass base material, characterized in that a difference between a target refractive index and a measured refractive index for each layer is 1% or less of a maximum refractive index difference in a graded refractive index distribution portion. It is a manufacturing method of the glass base material according to claim 1 or 2,
The amount of addition of the refractive index adjustment additive, which becomes a new manufacturing condition by interpolation approximation, from the condition of the addition amount of each refractive index adjustment additive in the measured refractive index of multiple layers approximating the target refractive index for each layer A method for producing a glass base material characterized in that it is obtained. It is a manufacturing method of the glass base material according to claim 1 or 2,
The average value of the addition amounts of the respective refractive index adjustment additives at the measured refractive index within a difference of 1% with respect to the target refractive index for each layer is used as the addition amount of the refractive index adjustment additive as a new production condition. A method for producing a glass base material.

Images