JPH01275442A - Production of optical fiber preform - Google Patents

Abstract

PURPOSE:To obtain the title optical fiber preform free of moisture and capable of being doped with a large amt. of fluorine by sintering a porous preform consisting of fine quartz-based glass particles in an atmosphere contg. a reducing gas and gaseous fluoride. CONSTITUTION:The porous preform 1 is inserted into a furnace core tube 2 made of quartz, He and Cl2 are introduced 4 into the tube 2, and the furnace is held at 1,000 deg.C. Under such conditions, the soot 1 is pulled down at a specified velocity, and dehydrated. The tube 2 is then held at the maximum furnace temp. of 1,310 deg.C, He, Cl2, CO as the reducing gas, and SiF4 are passed through the furnace, and the soot 1 is pulled down to obtain a transparent member. When the refractive index difference of the preform thus obtained is examined, the F corresponding to 0.84% based on DELTA<-> can be doped. In addition, when the member is cutted into specified thickness and SiOH absorption is investigated by IR absorptiometric analysis, OH absorption is not detected. When sintering is carried out without CO, the F corresponding to 0.67% based on DELTA<-> is doped.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field] The present invention relates to a method for manufacturing an optical fiber preform.

[Prior art]

In recent years, optical fiber manufacturing technology has made remarkable progress, and it has become possible to obtain fibers with ultra-low losses that are close to the theoretical limit. In addition, recently, dispersion-shifted optical fibers with a dispersion value of zero in the 1.55 μ-band, where the lowest loss can be obtained among silica-based optical fibers, have been manufactured and are being put into practical use.

In this dispersion-shifted optical fiber, since the core is generally doped with germanium, the C9-scattering coefficient becomes thick, and the limit value for low loss is approximately 0.20 dB/+++. To obtain a dispersion-shifted optical fiber with even lower loss, for example, the germanium content in the core is reduced and the cladding is doped with a large amount of fluorine (Δ-0.7%).
). However, with conventional techniques, it has been difficult to dope a large amount of fluorine without contaminating the base material with moisture. Moreover, it is an object of the present invention to provide a method for manufacturing an optical fiber preform which can be doped with a large amount of fluorine.

[Structure of the invention]

In order to achieve the above object, the method for manufacturing an optical fiber preform of the present invention involves sintering a porous preform for an optical fiber made of silica-based glass particles in an atmosphere containing at least a reducing gas and a fluoride gas. It is characterized by this.

[Embodiments of the invention]

Embodiments of the present invention will be described in detail below with reference to the drawings.
The present inventor has arrived at the present invention as a result of various experiments. first,
A porous base material (hereinafter referred to as soot) of quartz-based glass particles was manufactured by a VAD method. This was obtained by introducing 5iC14 into a concentric quadruple tube burner (acid-hydrogen flame) and performing a flame hydrolysis reaction, and its diameter was 5011.
I111 length is 4501m1M, and its density is 0.
It was 2g/cm3. This soot 1 was introduced into a quartz furnace tube 2 as shown in FIG. Here, reference numeral 3 indicates an electric furnace, reference numeral 4 indicates a gas introduction pipe, and reference numeral 5 indicates an exhaust pipe.

First, He is 504! /sin, C1z to 1.51/
The maximum temperature in the furnace was maintained at 1000''C while flowing the soot 1 through the gas introduction pipe 4 into the quartz furnace tube 2, and in this state, the soot 1 was lowered at a speed of 350 mm+/h to perform dehydration. When the entire soot 1 passes through the highest temperature part of the furnace, it is pulled up, and then the maximum temperature in the furnace of the quartz furnace tube 2 is raised to 1310°C, and He @OA 7!/
sin, C1z 0.1 it/5in1SiFa 2j! The soot 1 was drawn down at a speed of 250 sn/h while flowing /win into the quartz furnace tube 2, and was allowed to pass through the highest temperature part of the furnace to achieve transparent vitrification while doping with fluorine. When the refractive index difference of the transparent vitrified base material obtained by this method was examined using a preform analyzer, it was found that Δ- was 0.67.
% of fluorine was doped. Furthermore, when this transparent base material was cut into a 10 cm thick piece and the absorption of 5iOll at 2.73 μm was examined by infrared absorption analysis, no OH absorption was observed. This transparent base material will be referred to as rod A (comparative example).

On the other hand, the conditions up to the dehydration step were exactly the same, and only the conditions for the fluorine doping and transparent vitrification steps, the so-called sintering steps, were changed as follows. That is, the maximum temperature in the furnace of the quartz furnace tube 2 was maintained at 1310°C, and He was 0.117 uin and Ch was 0.1 in the furnace.
j! /+win, Co O, 1N/+in, SiF
.

While flowing 21/in, suit l is 25On+m/
A transparent base material was obtained by pulling down at a speed of h. Connect this to rod B (
When the refractive index difference of this rod B (Example) was examined using a preform analyzer, it was found that it was doped with fluorine corresponding to 0.84% in Δ-. Furthermore, this rod B was cut to a thickness of 10 cm, and 5i at 2°73 μm was determined by infrared absorption analysis in the same manner as in the case of drad A.
When OH absorption was examined, no 0■ absorption was observed.

As described above, rod A and rod B differ only in whether or not CO gas was included in the atmosphere during the fluorine doping and transparent vitrification steps, so-called sintering steps, and the other conditions are exactly the same.

As shown by the aforementioned rod B, CO is present in the atmosphere of the sintering process.
The reason why the amount of fluorine doped into the base material could be increased by mixing the gas is estimated as follows.

In other words, CO is a highly reducing gas.
The following reaction occurs with the soot made of SiO□.

C6+Mi5i-0-5i3--1-RaCO□13SiSi3・----1...-(1) At this time, if fluorine F is present, 5iFa-111---→ SiF', +F, -...
・−・−−−−−−(2)=Si s+= +h
~------ Su35i-F F-5i3 ---
--(3)'', and F can react more easily than in the absence of CO. In other words, due to the presence of CO, fluorine F
It is estimated that the amount of doping can be increased. still,
The above formula (2) is based on thermal decomposition.

In the above embodiment, CO gas is used to increase the amount of fluorine doped, but this is not limited to CO gas as long as it is a reducing gas, and other gases may also be used, such as NO.
1NO2, 5O1o, (deuterium gas), etc. can be used. Of course, H2 can also be used as it is a reducing gas, but using this gas leaves 011 groups in the base material, which is not very preferable. Using this point D2, O
There is no problem because the influence of the D group does not extend to the 1, 3 μm or 1.5 μm bands, which are normal communication wavelength bands.

Furthermore, in this example, the dehydration process is performed before the sintering process, but it goes without saying that this dehydration process can be omitted and dehydration, fluorine doping, and transparent vitrification can be performed all at once in the sintering process alone. stomach.

Furthermore, as the fluoride gas, 5
In addition to iFa, SF4, CCItFz, BF3, CF
, Sigma, etc. may be used alone or in combination.

According to the present invention as described above, it is possible to manufacture an optical fiber preform that can be doped with a large amount of fluorine and does not contain moisture.

〔Effect of the invention〕

As described above, according to the method for producing an optical fiber preform of the present invention, it is possible to obtain an optical fiber preform that contains very little water and is doped with a large amount of fluorine.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram showing an embodiment of an optical fiber preform sintering apparatus according to the present invention. 1 ~ Soot 2 ~ Quartz furnace core tube 3 ~ Electric furnace patent applicant Furukawa Electric Co., Ltd. Figure 1

Claims (1)

[Claims] A method for producing an optical fiber preform, comprising sintering a porous preform for an optical fiber made of silica-based glass fine particles in an atmosphere containing at least a reducing gas and a fluoride gas.