Process of producing preforms for silica glass optical waveguide

Details Process of producing preforms for silica glass optical waveguide Process of producing preforms for silica glass optical waveguide

1995-02-16

1997-03-11

Hoffmann, John

Glass manufacturing

Processes of manufacturing fibers, filaments, or preforms

Process of manufacturing optical fibers, waveguides, or...

65422, 65426, 65424, 6537413, C03B 37018

Patent

active

056096666

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

The invention pertains to a process for the production of preforms for quartz glass optical waveguides, which is applicable to the production of optical transmission devices for communications technology. More particularly, the invention relates a method for purging a porous quartz glass tube formed in a hydrolysis process.
In optical communications technology, widespread use is made of optical waveguides based on quartz glass. These waveguides are drawn from quartz glass rods with a length of 1 m to 2 m and with a diameter between 2 cm and 10 cm, which are referred to as "preforms". One of the essential requirements of the preform material is that it must be of extremely high purity, because a concentration of impurities even in the ppb range (10.sup.-9) can cause severe absorption losses in the waveguide.
Several processes are known for the production of waveguides. These are described, for example, in the book "Optische Telekommunikationssysteme". The four most important processes in order of their economic importance are:
OVD (outside vapor deposition),
MCVD (modified chemical vapor deposition),
VAD (vapor axial deposition), and
PCVD (plasma-activated chemical vapor deposition).
In the MCVD and PCVD processes, a substrate tube of quartz glass is required, in which the waveguide material is then deposited. Although the same purity requirements are not imposed on the material of the substrate tube, it is nevertheless necessary to ensure very low OH concentrations, which means that the process used to produce the substrate tubes is correspondingly expensive. The same is true for the so-called "casing" tubes.
Because most of the light-related performance is concentrated in the core of the waveguide and in a thin, adjacent jacket zone, for economic reasons it is becoming increasingly common to produce only this interior region of highly pure waveguide material and to fuse a thick-walled tube (casing tube) over it, which consists of quartz glass with a somewhat higher concentration of impurities. Nevertheless, the OH content of a casing tube of this type must still be very low.
The OVD process (Corning/USA) is the most economically important process for the production of preforms and is used to produce most preform material. The process is characterized by the simplicity of the production process and by the possibility of producing very large preforms.
The process steps for the production of preforms by the OVD process are shown schematically in FIGS. 1 and 2. As FIG. 1 shows, first a large quartz glass body 1 of fine-pored material is built up in stages. For this purpose, a burner 2, which is operated with a mixture of methane (CH.sub.4) and oxygen to which silicon tetrachloride (SiCl.sub.4) is also added, is moved back forth along a rotating ceramic rod 3 (Al.sub.2 O.sub.3).
A chemical reaction in the flame, which is referred to as hydrolysis, leads to the formation of quartz glass. This glass is deposited in the form of layers of a white, porous material. This material, also called "glass soot" has under the control of the hydrolysis reaction, a density ranging from 10% to as much as 25% of solid quartz glass and consists of tiny particles of small diameter, which are fused to each other at individual points.
To produce doped quartz glass, germanium tetrachloride (GeCl.sub.4), for example, is added to the burner gases. As a result, a GeO.sub.2 -doped material with an increased index of refraction is produced.
During hydrolysis, both quartz glass and a large amount of water vapor are produced. High concentrations (about 1,000 ppm) of this water vapor are incorporated in the form of OH ions into the porous quartz glass.
After the desired amount of quartz glass has been deposited, the burner is turned off. Because of its higher coefficient of expansion, the substrate rod contracts during cooling to a greater extent than the quartz glass body, with the result that the rod can be pulled out from the quartz. Because of the high OH content of the material produced in this way is, it

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