CN1276772A

CN1276772A - Apparatus and method for drawing waveguide fibres

Info

Publication number: CN1276772A
Application number: CN98810448A
Authority: CN
Inventors: S·C·鲍尔; J·M·巴纳德; J·A·斯萘匹
Original assignee: Corning Inc
Current assignee: Corning Inc
Priority date: 1997-10-31
Filing date: 1998-10-15
Publication date: 2000-12-13
Also published as: WO1999023040A1; JP2001521871A; KR20010031525A; EP1030824A1; AU1189699A; EP1030824A4; BR9813139A; CA2306269A1; TW407217B; AU734347B2

Abstract

A furnace (12) has a muffle tube (22) that is coated with silicon carbide. It is tubular. They make a fiber (36) with it.

Description

Apparatus and method for drawing waveguide fiber

Field of the invention

The present invention relates to a method and apparatus for drawing waveguide fiber. More particularly, the present invention relates to a furnace that significantly reduces the loss of point defects generated within the fiber during drawing.

Background of the invention

Drawing high strength, low loss fibers from a high silica content fiber preform or blank requires a relatively high heat source. The two primary sources of heat used to draw such fibers are zirconia and graphite furnaces. The furnace for drawing the fiber is typically operated at temperatures above about 1900 deg.C, and somewhat up to about 2050 deg.C.

Zirconia induction furnaces typically comprise a housing in the central portion of which is mounted a tubular yttria-stabilized zirconia inductor surrounded by a cylindrical quartz beaker containing granular zirconia insulation. The induction coil surrounds the insulating material and generates an alternating electromagnetic field when energized. The electromagnetic field couples with the inductor, and the temperature of the inductor is increased to form a heating zone. One end of the optical glass fiber preform is dropped to a heating belt to melt the end, and a fiber is drawn from the melted end.

One disadvantage of zirconia induction furnaces is that: long term use and thermo-mechanical stresses can cause cracks in muffle and inductor. The cracks can cause the zirconia particles to migrate from the inner surfaces of the furnace to the preform and/or fibers drawn from the preform, creating very weak fibers and undesirable loss of the finished product.

The graphite induction furnace generally has a graphite muffle furnace which is not easy to generate cracks, but the graphite furnace has a defect that oxidation occurs at the high temperature of drawing. It has been suggested that: the drawing of the waveguide fiber in a graphite furnace must be carried out under an inert protective atmosphere to prevent oxidation in the muffle furnace. Oxidation occurs when the ambient gas reacts with the solid carbon in the muffle furnace at high temperature according to the following reaction:

(1)

(2)

the graphite used in the drawing furnace generally has a starting temperature of about 700 ℃ for reaction (1). Above 900 ℃, reaction (2) becomes evident. These reactions of the muffle with oxygen and carbon dioxide consume the muffle, especially at the high temperatures at which the fiber is drawn.

Graphite muffle materials are composites of graphite particles bound together by a carbon binder matrix. It is believed that: the binder material is more easily oxidized than the graphite particles. Thus, when the composite of these two materials is exposed to oxygen at a temperature above the oxidation onset temperature, the binder matrix material preferentially oxidizes. The graphite particles are free to move away from the composite structure without a binder holding them in place. It is believed that: this mechanism causes graphite particles to migrate from the muffle wall to the fiber preform and/or the fiber being drawn.

Graphite particles that enter the fiber being drawn can result in unacceptable product loss due to point defects. Point defects appear as a significant attenuation of the fiber transmission signal. The losses due to point defects caused by the graphite particles of the drawing furnace can be higher than about 5%, which is an unacceptably high loss. Graphite particles adhering to the fibers during drawing also tend to break the fibers.

As described above, there is proposed: the oxidation problem of graphite muffle furnaces can be solved by drawing in an inert protective atmosphere. The outer surface of the graphite muffle is insulated by placing the muffle in a jacket and passing an inert gas between the jacket and the outer wall of the muffle. However, it is difficult to remove all oxygen from the muffle furnace. In particular, the muffle interior surfaces are exposed to oxygen in the ambient air, which enters the furnace during assembly and disassembly of the waveguide fiber preform. In addition, it is believed that oxygen is present in the furnace because of the difficulty in removing the oxidant from the furnace. For example, the upper portion of the muffle furnace is oxidized by the oxygen-containing porous soot portion of the optical fiber preform residing in the muffle furnace during charging of the preform into the furnace, and it is believed that oxygen present in the porous portion of the preform oxidizes the furnace to produce graphite particles.

In view of the above, there is a need for a fiber drawing graphite muffle furnace that does not produce graphite particles to significantly reduce point defect loss within the fiber.

Summary of the invention

Thus, in general, the present invention provides an apparatus for heating a waveguide glass fiber preform to a temperature sufficient to draw the fiber, the apparatus comprising a generally tubular graphite muffle, the inner surface of which is coated with high purity silicon carbide. The coating thickness is preferably at least about 2 mils and contains less than about 900 parts per billion impurities.

In another aspect, the present invention provides a method of making a waveguide fiber in a draw furnace comprising a generally tubular graphite muffle having an inner surface. The method includes the step of coating high purity silicon carbide on the interior surface of a graphite muffle furnace. The method further includes placing the waveguide fiber preform in a muffle furnace, heating the furnace to a temperature sufficient to draw a fiber from the preform, and drawing the fiber from the blank.

Several important advantages can be recognized from the above summary. A major advantage of the present invention is the significant reduction in point defect loss in waveguide fibers drawn in the furnace of a graphite muffle furnace. Additional features and advantages of the invention will be set forth in the description which follows. It should be understood that: both the foregoing description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. The parts of the drawings are not to scale and are sometimes intentionally distorted to better illustrate the invention.

Brief description of the drawings

FIG. 1 is a schematic diagram illustrating an exemplary manner of an optical fiber draw furnace of the present invention.

Detailed Description

Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings.

The invention includes an apparatus for heating a waveguide fiber to a temperature sufficient to draw the fiber. An exemplary embodiment of the present invention is shown in fig. 1 and is designated generally by the reference numeral 10.

As described herein and shown in FIG. 1, the furnace 10 includes a generally cylindrical outer jacket 12 having a side wall 14, a top 16 and a bottom 18. In the center of the top portion 16 there is an opening 22 which corresponds vertically to an opening 24 in the bottom portion 18. The insulation material 26, which may be formed of multiple segments, is disposed axially within the outer jacket 12. A graphite muffle 28, generally tubular in shape, is centrally disposed within the insulation 26. The muffle 28 and the insulating material may be separated from the bottom 18 by a gasket 20, the gasket 20 having a hole 21 through which the fiber is drawn, the gasket also separating the muffle from the bottom. The gasket 20 may be made of silicon dioxide. An induction coil 30, which is connected to a power source (not shown), surrounds the insulation material 26 and provides a heating source for the furnace 10.

The water-cooled jacket 12 may be made of stainless steel or the like. The housing 12 preferably axially completely surrounds the entire length of the muffle 26. An inert gas, such as argon, is flowed into the jacket 12 to prevent oxidation of the outer surface of the muffle 26.

Waveguide fiber preform 32 (shown in phantom) is axially placed into muffle 26 until first end 34 is positioned in a "heating zone" within induction coil 30. After the "heating tape" reaches a temperature sufficient to draw the fiber from the preform, preferably above 1900 ℃, the optical fiber 36 is drawn from the end region 34 of the preform 32. In an important aspect of the present invention, the inner surface of muffle 28 adjacent to preform 32 has a coating of high purity silicon carbide thereon to prevent degradation of the graphite muffle. The graphite muffle 28 preferably includes at least two, and more preferably three axial segments because coating segments of the muffle longer than about 40 inches is difficult.

The thickness of the silicon carbide coating is preferably at least about 2 mils and less than about 100 mils. A coating thickness of less than about 2 mils is insufficient to prevent graphite particles from contaminating the fiber drawn from the furnace; coating thicknesses greater than about 100 mils tend to produce microcracks and thermal shock. The coefficient of thermal expansion of the SiC coating must be closely matched to that of the carbon binder matrix material that holds the graphite particles of the muffle together to prevent delamination of the coating due to thermal expansion mismatch.

The silicon carbide coating on the interior surface of the muffle is preferably formed by a chemical vapor deposition process using a silicon-containing gas. Such coatings may be formed by reacting a silicon-containing gas, such as silane, with hydrogen to form SiC, where the ratio of silicon to carbon is about 1: 1. SiC is coated on the inner surface of the substrate that has been heated to above 1000 ℃. The high purity coating is preferably located on the inner surface of the muffle to prevent contamination of the fiber drawn in the furnace of the present invention. The content of impurities in the silicon carbide coating is preferably less than about 900 parts per billion, more preferably less than about 200 parts per billion.

Another aspect of the invention is a method of making a waveguide fiber in a drawing furnace that includes a graphite, generally tubular muffle having an inner surface. The method includes the steps of providing a coating of high purity silicon carbide on the interior surface of a graphite muffle furnace, placing a waveguide fiber preform in the muffle furnace, heating the furnace to a temperature sufficient to draw a fiber from the preform, and drawing the fiber from the preform.

The furnace is preferably heated to a temperature of at least about 1900 deg.C, more preferably at least about 2000 deg.C, to soften the tip of the waveguide preform and draw the fiber from the end. High purity silicon carbide preferably has a purity of about 99.999%, and more preferably contains less than about 900 parts per billion impurities. A low impurity content is an important aspect of the present invention, since a higher impurity content can cause optical or mechanical defects in the fibers produced in the furnace.

Waveguide fibers made using the furnace and method of the present invention exhibit significantly reduced point defect loss. Fibers drawn in a conventional graphite muffle furnace exhibit about 5% loss of the article due to point defects. The fibers produced in the oven of the present invention exhibited about 0.8% of product loss due to point defects. The furnace comprises a generally tubular graphite muffle having an inner surface coated with silicon carbide and a coating of about 5-8 microns thick.

It will be apparent to those skilled in the art that modifications and variations can be made in the method and apparatus of the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the scope of the invention include modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.

Claims

1. A furnace for heating a glass waveguide fiber preform to a temperature sufficient to draw the fiber, comprising a generally tubular graphite muffle including an inner surface coated with a high purity silicon carbide coating on the inner surface of the muffle.

2. The furnace of claim 1, wherein the muffle further comprises at least two generally tubular portions.

3. A furnace as claimed in claim 2 wherein the muffle comprises three generally tubular sections.

4. The furnace of claim 1 wherein said coating has a thickness of at least about 2 mils.

5. A furnace as set forth in claim 1 wherein said silicon carbide containsless than about 900 parts per billion impurities.

6. A method of making a waveguide fiber in a drawing furnace, the furnace comprising a generally tubular graphite muffle having an inner surface, the method comprising the steps of:

coating a high-purity silicon carbide coating on the inner surface of the graphite muffle furnace;

placing a waveguide fiber preform in a muffle furnace;

heating the furnace to a temperature sufficient to draw a fiber from the preform; and

the fibers are drawn from the preform.

7. The method of claim 6 wherein the furnace temperature is at least about 1900 ℃.

8. The method of claim 6, wherein the furnace temperature is at least about 2000 ℃.

9. The method of claim 6, wherein the silicon carbide contains less than about 900 parts per billion impurities.

10. The method of claim 6, wherein said waveguide fiber drawn from the furnace has a point defect loss of less than about 4%.

11. The method of claim 1, wherein said waveguide fiber drawn from the furnace has a point defect loss of less than about 1%.