Furnace for heating glass preform for optical fiber and method f

Details Furnace for heating glass preform for optical fiber and method f Furnace for heating glass preform for optical fiber and method f

1988-10-06

1990-11-13

Woodard, Joye L.

Glass manufacturing

Processes

With shaping of particulate material and subsequent fusing...

65 183, 65 301, 65157, 6537415, 65DIG16, C03B 37014

Patent

active

049699415

DESCRIPTION:

BRIEF SUMMARY
FIELD OF THE INVENTION

The present invention relates to a furnace for heating a glass preform for an optical fiber and a method for producing such glass preform. More particularly, it relates to a heating furnace and a method for heating a porous glass preform consisting of fine particles of quartz glass to add fluorine to the preform and vitrify it. The heating furnace of the present invention can prevent contamination of the glass preform with impurity elements and has good durability.


BACKGROUND ARTS

As one of general methods for mass producing a glass preform for use in the fabrication of an optical fiber, the VAD (Vapor Phase Axial Deposition) method is known. The VAD method comprises depositing fine particles of glass generated in oxyhydrogen flame on a rotating starting member such as a glass plate or rod to form a cylindrical porous preform (soot preform) and sintering said porous preform to obtain a transparent glass preform for use in the fabrication of an optical fiber.
In the VAD method, for sintering the porous preform to convert it into transparent glass, the preform should be heated in an atmosphere of an inert gas (e.g. helium and argon) to a temperature of 1,600.degree. C. or higher. As a heating furnace for sintering the preform, usually a heating furnace having a carbon heater is used. What should be taken care of when sintering the preform in such heating furnace is inclusion of transition metals such as copper or iron and water. When 1 (one) ppb or larger of the transition metal is included in the glass preform, transmission loss wavelength characteristics of the fabricated optical fiber is greatly deteriorated in an entire wavelength range. When 0.1 ppm or larger of water is included in the preform, the characteristics of the fabricated optical fiber is impaired in a longer wavelength range.
Therefore, the porous preform is usually dehydrated before or during vitrification. As a dehydration method, it is known to heat the porous preform at a high temperature in an atmosphere of an inert gas containing a chlorine-containing gas, a fluorine-containing gas, etc. When the fluorine-containing gas is used, not only the porous preform is dehydrated but also fluorine is added to the porous preform. When the fluorine is added to the porous preform, a refractive index profile which is essential to the optical fiber is advantageously adjusted. In this connection, Japanese Patent Publication No. 15682/1980 and Japanese Patent Kokai Publication No. 67533/1980 can be referred. These publications will be discussed below.
The treatment with the fluorine-containing gas is carried out in the heating furnace before or simultaneously with vitrification. To prevent wastage of the carbon heater due to moisture or oxygen which is generated during heating of the preform, a muffle tube is installed for separating the carbon heater and the sintering atmosphere. As the muffle tube, an alumina made one is conventionally used (cf. Japanese Patent Publication No. 40096/1982 and U.S. Pat. No. 4,338,111). However, when the alumina made muffle tube is used, alkali components contained in alumina float in the heating atmosphere at high temperature and adhere to a surface of the porous preform to form a cristobalite layer.
Then, a quartz made muffle tube has been practically used. In comparison with the alumina made muffle tube, the use of the quartz made muffle tube gives following advantages:
1. The quartz has better mechanical processing accuracy and therefore airtightness of the atmosphere is maintained so that the soot preform is effectively dehydrated.
2. The quartz made muffle tube contains few impurities such as iron and alkali and is much purer than the alumina made muffle tube.
3. The glass preform produced by means of the quartz made muffle tube does not suffer from surface devitrification caused by alkali.
4. The quartz made muffle tube hardly suffers from. thermal breakage (breakage due to thermal shock).
5. When the fluorine-containing gas is used, no contaminating gas such as AlF.sub.3 and the like i

REFERENCES:
patent: 4586943 (1986-05-01), Kyoto et al.

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