Glass manufacturing – Processes of manufacturing fibers – filaments – or preforms – Process of manufacturing optical fibers – waveguides – or...
Reexamination Certificate
1999-05-06
2001-02-20
Hoffmann, John (Department: 1731)
Glass manufacturing
Processes of manufacturing fibers, filaments, or preforms
Process of manufacturing optical fibers, waveguides, or...
C065S430000
Reexamination Certificate
active
06189341
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to methods for drawing optical fiber from glass preforms and, in particular, to a method for drawing fiber with reduced particulate defects.
BACKGROUND OF THE INVENTION
The most widely used method for making glass optical fiber involves the use of refractory material induction furnaces to heat a glass preform to fiber drawing temperature. A typical induction furnace for drawing silica fibers is described in R. A. Runk, “A Zirconia Induction Furnace For Drawing Precision Silica Waveguides”,
Optical Fiber Transmission II Technical Digest
(TuB 5-1) (Feb. 22-24, 1977). The induction furnace uses a tubular susceptor of zirconia or other refractory material disposed within a radio frequency coil. After the susceptor is heated to a desired temperature of approximately 2100° C., a glass preform rod is introduced into the middle of the susceptor, known as the hot zone. The preform end is reflowed and a tip portion drops off, attached to a glass strand and permitting the drawing of fiber from the remaining reflowed end. The conventional zirconia draw furnace comprises an outer cylindrical beaker of fused quartz, an inner tube (susceptor) of zirconia and an insulating region therebetween filled with zirconia granules (grog).
Although this method has produced fibers for many years, there has been a persistent problem with particulate contamination of the fiber. Refractory particles can contaminate the fiber as it is drawn, weakening the fiber and causing breakage. The induction furnace, with its refractory susceptor and grog, is a significant source of refractory particles. Moreover facilities where such furnaces are used develop a small but nonetheless significant environmental concentration of such particles.
The standard approach to reducing particulate defects is to draw the fiber under cleanroom conditions. This approach, however, has been less than completely successful. One problem is that cleanroom conditions cannot be established in the critical drawing region where the molten glass is closely spaced with the particulate generating refractory material. Another is that even a minute concentration of refractory particles can present a contamination problem. Accordingly, there is a need for a new method of reducing particle-induced breakage in drawing optical fiber.
SUMMARY OF THE INVENTION
In accordance with the invention, particle-induced breakage in drawing optical fiber is reduced by coating the glass preform with a particle-digesting coating prior to introducing the preform into the induction furnace. At least the end of the preform should be coated and preferably all of the preform. When the preform passes through the hot zone, the coating melts and digests refractory particles. The coating material and digested particles dissolve into the silica glass at a low surface concentration that does not substantially change the performance of the drawn fiber. The preferred particle-digesting coating is lithium tetraborate.
REFERENCES:
patent: 4116653 (1978-09-01), Irven
patent: 4184860 (1980-01-01), Schneider et al.
patent: 4443239 (1984-04-01), Biswas
patent: 4589725 (1986-05-01), Dyott
patent: 4888101 (1989-12-01), Adams
patent: WO 92 21629 (1992-12-01), None
patent: WO 98 18733 (1998-05-01), None
English language Abstract of JP 63117922, May 21, 1998.
Hoffmann John
Lucent Technologies - Inc.
Mathews, Collins Shepherd & Gould P.A.
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