Optical fiber manufacture

Details Optical fiber manufacture Optical fiber manufacture

1999-04-08

2001-04-24

Silverman, Stanley S. (Department: 1731)

Glass manufacturing

Processes of manufacturing fibers, filaments, or preforms

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

C065S385000, C065S384000, C065S399000, C065S413000, C065S416000, C065S417000, C065S419000, C065S420000

Reexamination Certificate

active

06220060

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to optical fiber manufacture and more specifically to improved preform fabrication techniques.
BACKGROUND OF THE INVENTION
The manufacture of optical fiber typically uses one of two fundamental approaches. Both use rotating lathes, and accumulate pure glass material on a rotating preform by chemical vapor deposition or a modification thereof. The earliest technique deposited material on the outside of a rotating preform, and the preform usually started as a hollow tube with a slowly increasing diameter as the vapor deposited glass material accumulated on the outside of the solid tube. A significant advance in this technology occurred with the introduction of the so-called Modified Chemical Vapor Deposition (MCVD) process of MacChesney et al. in which the glass forming precursers are introduced into a rotating hollow tube and the glass material is deposited on the inside wall of the hollow tube. In this way exceptionally pure material can be produced at the critical core region. It also allows better control over the reaction environment.
The MCVD process has evolved to a highly sophisticated manufacturing technique and is widely used in commercial practice today. However, problems in the tube collapse phase of the MCVD process persist. One of those is the spontaneous formation of a bubble in the core of the preform near the end of the collapse cycle. The bubble essentially destroys the preform. From 5-10% of preforms typically suffer this fate.
The dopant used to increase the core index in most commercial MCVD preforms is germanium in the form of GeO
2
. Studies of the bubble formation phenomenon have established that the source of the bubbles is GeO vapor from the GeO
2
in the doped core. GeO vapor is emitted during the entire collapse cycle but as collapse proceeds the internal tube pressure due to accumulated GeO vapor increases, sometimes to the point where it exceeds the surface tension of the softened silica tube. When that occurs the tube wall distorts in a bubble, and there is no satisfactory way of reversing the distortion. Such tubes are scrapped, resulting in a significant reduction in process yield and substantially increased cost. Collapse techniques which avoid bubble formation would represent an important advance in the commercial practice of MCVD.
PRIOR ART
Many techniques for controlling the thermodynamics of the collapse process have been proposed in the prior art. For example, U.S. Pat. No. 5,868,815, issued Feb. 9, 1999, describes a preform collapse process that employs a gas jet of air impinging on the hot zone, where the torch softens the glass, to control ovality as the preform collapses. U.S. Pat. No. 5,160,520, issued Nov. 3, 1992, describes using jets of air in the hot zone to control the temperature profile in the hot zone. In each case the gas jets contact the preform where the glass has been softened and is susceptible to deformation.
SUMMARY OF THE INVENTION
We have developed a technique for eliminating bubble formation due to excessive GeO vapor pressure in the MCVD preform tube during the final stages of collapse. Cooling means are provided just ahead of the torch used for collapse. Resulting cooling of the preform tube increases the rate of transport of the GeO vapor thereby eliminating the source of the bubble.


REFERENCES:
patent: 5868815 (1999-02-01), DiGiovanni

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