Method for manufacturing optical fiber preform

Abstract

Disclosed is a method for manufacturing an optical fiber preform that, specifically, is able to suppress outer diameter fluctuations in the preform in the area where the drawing of a preform ingot ends and to yield a preform of uniform diameter in the length direction. A preform manufacturing method, which consists of a step in which a porous glass preform (6) is manufactured by producing silica powder from a compound containing silicon, a step in which a preform ingot (10) is obtained by sintering and vitrifying the porous glass preform (6) at high temperatures, and a step in which the preform ingot (10) is drawn and the diameter thereof is reduced by exerting tension on a heated portion while sequentially moving a heating means in a relative manner along the length of the preform ingot (10), is characterized by leaving an unsintered portion (11), which has not been completely vitrified, at one end of the sintered preform ingot (10), and by beginning the drawing from the unsintered portion (11) side in the drawing and diameter-reducing step.

Description

technical field [0001] The present invention relates to a method for manufacturing a preform for an optical fiber, capable of suppressing the outer diameter of the preform especially at the end of drawing when heating and drawing a silicon ingot as a base material made of synthetic quartz to manufacture a preform for an optical fiber change. In addition, this application is related to the following Japanese application. With respect to the designated countries whose approved documents are incorporated by reference, the contents described in the following applications are incorporated by reference into this application as part of this application. [0002] Japanese Patent Application No. 2008-228909 was filed on September 5, 2008. Background technique [0003] For preforms for optical fibers, there are known vapor phase axial deposition methods and external vapor phase deposition methods, in which silicon compounds such as silicon tetrachloride are hydrolyzed in an oxygen-h...

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Problems solved by technology

[0009] In this case, there is a problem that in order to draw the base material silicon ingot with a large diameter and a variation in the outer diameter in the longitudinal direction, it is necessary to increase the size of the drawing furnace too much in accordance with the variation in the outer diameter. The airtight mechanism of the base material silicon ingot inserted into the furnace becomes complicated

The reason is that at the end of drawing, the thermal conductivity and specific heat are different between the vitrified part and the unsintered part, so even at the same temperature, the drawn part changes greatly

Moreover, if the conveying amount of the base material is too large, there will also be a problem that the base material breaks at the junction of vitrification.

[0016] As a method of preventing the upper part of the drawing end side from being drawn first, as described in Patent Document 2, a method of covering the upper part of the base material silicon ingot with a heat-resistant heat-insulating jig and performing drawing is described. However, in It takes man-hours to install jigs before drawing, and especially for base material silicon ingots with a diameter of φ150 mm or more, the effect of suppressing diameter fluctuations cannot be sufficiently obtained

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