KR20010031525A - Apparatus and method for drawing waveguide fibers - Google Patents

Abstract

The melting furnace 12 has a muffle tube 22 coated with silicon carbide. The furnace is tubular. The furnace 36 is used to produce the fiber 36.

Description

Waveguide fiber drawing method and apparatus {APPARATUS AND METHOD FOR DRAWING WAVEGUIDE FIBERS}

In order to draw a high strength low loss fiber from a fiber base material or a blank with a high silica content, a relatively high heat source is needed. Two main heat sources used to draw such fibers are zirconia and graphite melting furnaces. In general, the fiber drawer is operated at a temperature above about 1900 ° C., typically about 2050 ° C.

Typically, a zirconia induction furnace includes a housing, the interior of which is provided with a tubular yttria-stabilized zirconia susceptor centrally surrounded by a cylindrical quartz beaker containing granular zirconia insulating material. The induction coil surrounding the insulating material provides an alternating electromagnetic field when activated. The electromagnetic field is coupled to the susceptor to raise the temperature of the susceptor to create a high temperature region. The distal end of the glass optical fiber base material is lowered into the high temperature region so that the distal end is melted, and fibers are drawn out from the distal end.

One disadvantage associated with zirconia induction furnaces is that extended use and thermomechanical stresses cause cracks in muffles and susceptors. This cracking causes the zirconia particles to diffuse from the inner surface of the furnace to the base material and / or fibers drawn from the base material, resulting in fragile fibers and many production losses.

Typically, the graphite induction furnace has a graphite muffle which is less susceptible to cracking, but the graphite muffle is oxidized at a high drawing temperature. In order to prevent the furnace muffle from oxidizing, it has been proposed that waveguide fiber drawing in the graphite induction furnace must be carried out in an inert protective atmosphere. According to the reaction scheme described below, oxidation occurs when gas from the ambient atmosphere reacts with the solid carbon muffle at high temperature.

C + O ₂ → CO ₂

C + CO ₂ → 2CO

Typical graphite temperatures for Scheme 1 for the graphite grade used in the drawing furnace are about 700 ° C. Scheme 2 is well above 900 ° C. Due to this reaction of the furnace muffle with oxygen and carbon oxides, the furnace muffle is lost at particularly high fiber drawing temperatures.

The graphite muffle material comprises graphite particles bonded to each other by a carbon binder matrix. It is believed that the binder material is more affected by oxidation than graphite particles. Thus, when the composite of the two materials is exposed to oxygen at temperatures above the onset of oxidation, the matrix binder material is preferentially oxidized. The graphite particles without binder to retain freely escape from the composite structure. Due to this mechanism, the graphite particles diffuse from the muffle wall into the fiber matrix and / or fibers during the drawing process.

Graphite particles coalescing into the fiber during the drawing process cause production loss due to the point bonding. Point defects appear when sharp attenuation increases in the signal transmitted through the fiber. The point defect production loss due to graphite particles from the draw losses can be about 5% or more, which is an unacceptably high loss. In addition, graphite particles adhering to the fiber during the drawing process also cause fiber breakage.

As mentioned above, it has been proposed that the oxidation of the graphite furnace muffle can be overcome by drawing in an inert protective gas atmosphere. The outer surface of the graphite muffle can be insulated by encapsulating the muffle inside the housing and flowing an inert gas between the housing and the outer wall of the muffle. However, it is difficult to remove all of the oxygen from the furnace muffle, and in particular, removing oxygen from the inner surface of the muffle exposed to oxygen in ambient air that may leak into the furnace during the mounting and removal of the waveguide fiber substrate. Very difficult. It is also believed that oxygen is present in the furnace because it is difficult to remove the oxidant from the furnace. For example, the upper portion of the muffle is subjected to oxidation by the oxygen-containing porous soot of the optical fiber blank remaining in the furnace muffle during the process of mounting the blank in the furnace. Oxygen present in the porous portion of the blank is believed to oxidize the muffle and produce graphite particles.

In view of such circumstances, it is desirable to provide a muffle with graphite fiber drawing, which greatly reduces the point defect loss of fibers without producing graphite particles.

The present invention relates to a waveguide fiber drawing method and apparatus therefor. In particular, the present invention relates to a melting furnace which greatly reduces the loss of point defects in the fibers generated during the drawing process.

1 is a schematic view showing an embodiment of the optical fiber drawing furnace of the present invention.

Accordingly, the present invention provides a heating apparatus for heating a glass waveguide fiber base material at a fiber drawing temperature, the apparatus including a tubular graphite muffle having an inner surface on which a high purity silicon carbide film is formed. Preferably, the coating includes a thickness of at least about 2 mils and less than about 900 impurities per billion.

In another aspect, the present invention provides a method for producing waveguide fibers in an drawing furnace comprising a tubular graphite muffle having an inner surface. The method includes providing a high purity silicon carbon film on the inner surface of the graphite muffle. The method further includes disposing a waveguide fiber base material inside the furnace muffle, heating the furnace to a fiber drawing temperature, and drawing fibers from the blank.

Many important advantages will be apparent from the description of the invention. An important advantage of the present invention is that it significantly reduces the point defects in waveguide fibers drawn in a furnace with graphite muffle. Other features and advantages of the present invention are described below. Both the foregoing general description and the following detailed description are exemplary and intended to explain the invention in detail as set forth in the claims. The various components in the accompanying drawings are not drawn to scale and are intentionally modified for ease of explanation.

Hereinafter, preferred embodiments of the present invention shown in the accompanying drawings will be described in detail.

The present invention includes an apparatus for heating waveguide fibers to a fiber drawing temperature. An exemplary embodiment of the invention is shown in FIG. 1 and indicated by reference numeral 10.

Referring to FIG. 1, the furnace 10 includes a cylindrical housing 12 having a side portion 14, an upper portion 16, and a lower portion 18. The upper part 16 has a central opening 22 perpendicular to the opening part 24 of the lower part 18. An insulating material 26 is disposed in the housing 12 in the axial direction, and may be formed of a plurality of segments. A tubular graphite muffle 28 is located centrally in the insulating material 26. To insulate the muffle from the bottom, the muffle 28 and the insulating material may be separated from the bottom 18 by a spacer ring 20 having a through hole 21 through which fibers pass through. The spacer ring 20 may be made of silica. In order to provide a heat source to the furnace 10, an induction coil 30 connected to a power source (not shown) surrounds the insulating material 26.

The water-cooled housing 12 may be made of stainless steel or equivalent. Preferably, the housing 12 extends axially by the entire length of the muffle 26 to completely enclose the muffle. An inert gas, such as argon, flows into the housing 12 to prevent oxidation of the outer surface of the muffle 26.

The waveguide fiber base material 32 (virtual wire) is axially inserted into the muffle 26 until the first end 34 is located in the ″ hot area ″ within the induction coil 30. After the high temperature region reaches a fiber drawing temperature of preferably 1900 ° C. or more, the optical fiber 36 is drawn from the end 34 of the base material 32. As an important feature of the present invention, the inner surface of the muffle 28 adjacent to the base material 32 has a high purity silicon carbide film to prevent deterioration of the graphite muffle. Since it is difficult to coat a long muffle of about 40 inches or more, the graphite muffle 28 preferably comprises at least two, more preferably three axial segments.

The thickness of the silicon carbide coating is preferably at least about 2 mils, up to about 100 mils. Thin films of about 2 mils or less do not adequately prevent contamination of graphite particles from fibers drawn from the furnace, and thick films of about 100 mils or more are subject to microcracks and thermal shock. In order to prevent peeling of the coating due to thermal expansion mismatch, the thermal expansion of the SiC coating must exactly match the carbon binder matrix material that binds the graphite particles of the muffle to each other.

Preferably, the silicon carbide film on the inner surface of the muffle is formed by chemical vapor deposition using a gas containing silicon. Such a coating may be produced by reacting a gas containing silicon such as silane with hydrogen to form SiC, wherein the silicon and carbon are present in a ratio of about 1: 1. The SiC is coated on the inner surface of the substrate heated to at least 1000 ℃. In order to prevent contamination of the fibers drawn in the furnace of the present invention, a high purity coating is preferred for the inner surface of the drawing muffle. The impurity in the silicon carbide coating is preferably about 900 or less per billion, more preferably about 200 or less per billion.

Another aspect of the invention relates to a method for producing waveguide fibers in an drawing furnace comprising a tubular graphite muffle having an inner surface. The method includes providing a high purity silicon carbide film on an inner surface of a graphite muffle, disposing a waveguide fiber base material inside the muffle, heating the furnace to a draw temperature and drawing fibers from the base material. do.

The furnace is preferably heated to a temperature of at least about 1900 ° C., more preferably at least about 2000 ° C., in order to allow the tip of the waveguide base material to soften so that fibers can be drawn from the tip. The high purity silicon carbide is preferably about 99.999% pure, more preferably about 900 or less impurities per billion. Since high impurity can lead to optical or mechanical defects in the fibers produced in the furnace, low impurity is an important feature of the present invention.

Waveguide fibers made using the furnaces and methods of the present invention exhibit very reduced point defect losses. The fibers drawn in a conventional graphite muffle drawer showed a production loss of about 5% from attenuation due to caking. Fibers made in the furnace of the present invention comprising a tubular graphite muffle having an inner surface coated with a silicon carbide layer about 5 to 8 microns thick exhibit a production loss of about 0.8% from attenuation due to caking.

It will be apparent to those skilled in the art that various changes and modifications can be made to the apparatus and method of the present invention without departing from the spirit and scope of the invention. Accordingly, the invention is intended to embrace all such alterations and modifications and equivalents thereof that fall within the scope of the appended claims.

Claims (11)

A heating furnace for heating a glass waveguide fiber base material at a fiber drawing temperature, the heating furnace comprising a tubular graphite muffle having an inner surface on which a high purity silicon carbide film is formed. The furnace of claim 1, wherein the muffle further comprises at least two tubular portions. 3. The furnace of claim 2 wherein the muffle comprises two tubular portions. The furnace of claim 1, wherein the coating has a thickness of at least about 2 mils. The furnace of claim 1, wherein the silicon carbide comprises up to about 900 impurities per billion. A method for producing waveguide fibers in a drawing furnace comprising a tubular graphite muffle having an inner surface, Providing a high purity silicon carbide film on an inner surface of the graphite muffle; Disposing a waveguide fiber matrix inside the muffle; Heating the furnace to a temperature sufficient to draw fibers from the substrate; And And drawing the fiber from the base material. 7. The method of claim 6, wherein the temperature of the furnace is at least about 1900 ° C. 7. The method of claim 6 wherein the temperature of the furnace is at least about 2000 ° C. The method of claim 6, wherein the silicon carbide comprises up to about 900 impurities per billion. 7. The method of claim 6, wherein the waveguide fibers drawn from the furnace have a point defect loss of about 4% or less. The method of claim 1, wherein the waveguide fibers drawn from the furnace have a point defect loss of about 1% or less.