JPH0455984B2 - - Google Patents

Description

Claim 1: A step for axially introducing a waveguide preform into a furnace comprising a tubular susceptor, and for energizing an induction coil surrounding the susceptor to re-enter a portion of the preform into the preform. and withdrawing the fiber from the re-entering portion of the preform, the cylindrical susceptor is removed prior to introduction of the preform to prevent particles from migrating from the surface of the susceptor onto the waveguide fiber. producing a soot layer of waveguide preform material on at least a portion of the waveguide preform material;
A method for manufacturing waveguide fibers, characterized in that, before or simultaneously with said introduction, said soot is hardened into a thin coating.

2. A method according to claim 1, further comprising producing a soot layer of waveguide preform material between the susceptor and a cylinder placed between the susceptor and the induction coil;
A method characterized in that at least a portion of the soot is solidified into a thin coating.

3. A method according to claim 1, comprising vapor growing a soot-thin porous layer of waveguide preform material on the inner surface of the susceptor and on the outer surface of the susceptor or on the inner surface of the cylinder; and heating the susceptor to energize the induction coil surrounding the susceptor so that the soot layer solidifies to form a thin fused silica layer to prevent particles from migrating from the susceptor. A method characterized by:

4. An induction heating furnace for heating a waveguide preform to raise its temperature in order to pull out a fiber from the waveguide preform, comprising: a tubular susceptor surrounded by an insulating grain; an induction coil surrounding the susceptor; An induction heating furnace comprising: a thin coating of waveguide preform material on at least a portion of an inner surface of the susceptor.

5. An induction heating furnace according to claim 4, characterized in that a cylinder is placed between the insulating grain and the susceptor and spaced around the susceptor.

6. An induction heating furnace according to claim 4, characterized in that the soot layer of waveguide preform material is located between the cylinder and the susceptor.

7. An induction heating furnace according to claim 6, characterized in that a portion of the soot layer of the waveguide preform material hardens into a thin coating on at least a portion of the outer surface of the susceptor. .

8. An induction heating furnace according to claim 6, characterized in that a portion of the soot layer of waveguide preform material hardens into a thin coating on at least a portion of the inner surface of the cylinder.

9. An induction heating furnace according to any one of claims 4 to 8, characterized in that the preform material is fused silica and the thin coating material is silica.

10. An induction heating furnace according to claim 9, characterized in that the susceptor is zirconium dioxide, and a grain of insulating zirconium dioxide is placed between the coil and the susceptor.

TECHNICAL FIELD The present invention relates to a furnace for heating a waveguide preform to high temperatures in order to draw fibers from the waveguide preform.

BACKGROUND OF THE INVENTION Over the past several years, the development of low-loss, fused silica waveguide fibers has focused on the exploration of high temperature (e.g., approximately 2000° C.) heat sources to extract high strength fibers from waveguide preforms. Among the possible heat sources, oxyhydrogen torches, _CO2 lasers, and a few induction and resistance furnaces have been employed to draw high temperature silica fibers. Although the torch method is economical, it does not maintain a uniform diameter over the length of a long drawn fiber. CO ₂ lasers provide the cleanest extraction environment, but require special optical designs to radially distribute the extraction energy and are limited in power output. In the case of resistance furnaces, an inert protective environment is required to prevent oxidation of the heating elements.

An induction furnace basically consists of a hollow, centrally placed cylindrical susceptor surrounded by insulating material. The induction coil surrounds an insulating material to provide an alternating electromagnetic field when power is applied to the induction coil. The electromagnetic field couples to the susceptor and increases its temperature, creating a warm region within it. Therefore, glass waveguide fibers are introduced into the warm region so that the waveguide fibers are re-introduced into a portion of the warm region from where they are extracted.

High temperature induction furnaces provide high thermal inertia, stability, and radially symmetrical heating. However, most induction furnaces use graphite or refractory metal susceptors that require a protective environmental flow during operation to remove migrating contaminants from the interior surfaces of the susceptor. As a result, such furnaces have a finite susceptor life and the furnace contains some contaminants in the furnace atmosphere.

One induction furnace designed to eliminate the above problems is the Optical Fiber Transmission Technical Digest (Optical Fiber Transmission Technical Digest).
Fiber Transmission technical digest)
(Tu B5-1), February 1977 issue 22-24, R. “A・” by B Rank (RBRunk)
“A Zirconia Induction Furnace for Drawing Precision Silica Wave Guys”
Furnace For Drawing Precision Silica Wave
The furnace is
It uses a cylindrical susceptor made of zirconium dioxide stabilized with yttrium oxide. The susceptor has a long life expectancy and contains minimal contaminants in the furnace atmosphere in an oxygen partial pressure atmosphere. Such furnaces have been shown to be most effective for drawing waveguide fibers from preforms. However, after prolonged use, microscopic particles of zirconium dioxide have been found to migrate from the susceptor to the preform and/or to the fibers being drawn from the waveguide preform. In effect, the zirconium dioxide particles weaken the drawn fiber, resulting in an unacceptable product.

Therefore, a long life, high temperature induction furnace is required that is virtually free of contaminants during the drawing process to produce high strength fibers.

SUMMARY OF THE INVENTION The present invention comprises an induction furnace having a centrally located cylindrical susceptor surrounded by an insulating material and an induction coil and having a thin coating of waveguide preform material on at least a portion of its inner surface. This solves the above problems. Also, the cylinder can optionally be placed between the insulating material and the susceptor, at a distance from the susceptor. Furthermore, the waveguide preform material can optionally be spaced between the susceptor and the tube.

[Brief explanation of the drawing]

FIG. 1 is a sectional view of an induction heating furnace according to the prior art. FIG. 2 is a cross-sectional view of an embodiment of an induction heating furnace according to the present invention. FIG. 3 is a sectional view of another embodiment of the present induction heating furnace. Figures 4 and 5
6 and 6 are cross-sectional views of various other embodiments of the present induction heating furnace.

DETAILED DESCRIPTION FIG. 1 shows a high temperature induction furnace according to the prior art, referenced by the number 10. Furnace 10 includes a cylindrical vessel 12 having sidewalls 14 and a top 16.
and a bottom portion 18. The top 16 has a central aperture 22 that is vertically aligned with an aperture 24 in the bottom 18 . The silica beaker 26 is open at the top 28 and has openings 22, 2 at its bottom.
4 and a circular aperture 3 axially aligned with
It has 2. A cylindrical susceptor 34 is centered within the beaker 26. The annular volume between the inner surface of beaker 26 and the outer surface of cylindrical susceptor 34 is filled with an insulating grain 36 of zirconium. Induction coil 3 with circular or square cross section
8 is connected to a power source (not shown) and mounted around the beaker 26. The container 12 is water cooled and made of copper or similar material to act as a shield to reduce stray radio frequency electromagnetic fields. Susceptor 34 is made of zirconium stabilized with 8% by weight yttrium oxide, namely Zircoa, Solon, Ohio.
Zirconium with composition 1372 from Company).

Zirconium dioxide susceptor 34 (at room temperature >
The low temperature resistivity of ¹⁰⁴ ohm-cm) is too high for direct coupling to the alternating electromagnetic field of the powered RF coil 38. For this reason, the zirconium dioxide susceptor 34 is preheated at room temperature by coupling an electromagnetic field to an axially inserted carbon rod (not shown). At temperatures above 1000°C, the susceptor 34 begins to couple to the electromagnetic field, and up to approximately 1400°C the carbon rod can survive without thermal shock to the susceptor.

By this method, the temperature of the furnace 10 can be brought up to operating temperature in about 60 minutes. During operation, the temperature of the zirconium susceptor 34 is monitored and controlled to within ±2° C. of the desired set point with an infrared pyrometer (not shown). The commonly used fiber drawing temperature is 1900°C, depending on the preform size and fiber drawing speed.
It is between 2300℃. At these temperatures, typically 7 kilowatts of power is required to maintain steady state operation. Frequencies on the order of approximately 4 MHz are required for effective operation at these temperatures. For grains 36, such as large particles and coarse grains, no RF electromagnetic field is coupled. Thus, grain 36 acts as an insulator to maintain elevated temperatures within susceptor 34 during operation. Glen 36 is an electrically fused zirconium dioxide manufactured in Niagara Falls, New York.
Tee. A. M Ceramics (TAM)
Zirconium dioxide [2.38-1.19 mm (8-14 mesh)] manufactured by Ceramics).

Once the temperature inside the susceptor 34 reaches the desired level (e.g. 2000°C), the solid, virtually cylindrical silica waveguide preform 4
4 (imagined and drawn) is its first end 46
is inserted axially inside the RF coil 38 until it is located in a "temperature zone" located in the center of the susceptor. At the end of the preform from the point of withdrawal of the waveguide fiber 52, the increased temperature heats the preform 44 as it reenters the small volume 48.

Although such techniques are most effective, it has been found that after some time, very small particles of zirconium dioxide are deposited on preform 44 and/or fiber 52.
Such particles are then drawn into the waveguide fiber 52 and contaminate the waveguide fiber 52. Such contamination results in a weaker fiber, effectively reducing fiber production yield. It is widely believed that small zirconium dioxide particles are generated from microcracks that form in susceptor 34 under elevated temperatures.

The present invention solves the above problems by depositing a thin layer of soot of the same material (i.e., SiO ₂ ) as the thin layer of soot of the solid state preform 44 on the inner surface of the susceptor 34 before assembling the furnace 10. It is something to do. Next, as shown in FIG. 2, a susceptor 34 is placed inside the furnace 10 to recycle a portion of the preform 44 after fixing the previously deposited soot into a fixed layer 54 (FIG. 2). Coil 38 is energized to create a warm region in the center of the interior of the susceptor having an elevated temperature of approximately 2000° C. for entry. The re-entering portion 48 of the preform 44 then withdraws the fibers 52 as shown in FIG. The drawn fiber 52 has been found to be substantially free of contamination and, as a result, is substantially stronger and results in a greater production yield than fiber drawn in the furnace 10 with an uncoated susceptor 34. I'm there.

Conversely, the susceptor 34 with soot thereon can be placed in a separate furnace and heated to a temperature of approximately 1700° C. to fix, or sinter, the soot into the fixed soot layer 54. next,
Susceptor 34 can be removed and inserted into induction furnace 10. Although the mechanism is not fully understood, a portion of the fixed soot layer 54 diffuses into microcracks within the susceptor 34.
(1) Form a seal. (2) Believed to act as an adhesive. Additionally, a very thin layer of fixed soot 54 remains on the surface of susceptor 34, making it virtually impossible to remove particles from the susceptor surface and undesirably migrate into preform 44. Preferably, fixed soot 54 that can be deposited from the surface of susceptor 34
does not affect the drawn fiber 52 as long as the preform 44 and the fixed soot are the same material.

The concept of depositing silica soot ( _SiO2 ) on the inner surface of the susceptor 34 is not readily considered by those skilled in the art, as it would appear that a eutectic of the material would form. Such eutectics have an undesirable effect on the coupling efficiency of the electromagnetic field to the susceptor 34, resulting in low and/or uncontrollable temperatures in the temperature range. Surprisingly, the deposition and fixation of silica soot into the fixed soot layer 54 on the surface of the susceptor 34 has a detrimental effect on the operation of the furnace 10.

Soot was deposited on the inner surface of the susceptor 34 by rotating the susceptor on the lathe while pointing the soot deposition torch in the lathe. Once the desired thickness (i.e., 1-2 mm) was deposited, the soot deposition torch was closed and the coated susceptor 34 was placed in the furnace as described above.

In another embodiment, as shown in FIG. 3, the susceptor 34 is comprised of a plurality of stacked susceptor tubes 62. Only the central susceptor tube 62 has a soot coating applied to the inner surface of the susceptor, which is then fixed into the fixed soot layer 54 set above. Central susceptor tube 6
The center position of 2 corresponds in fact to the "warm region",
In the "warm region", a portion of preform 44 is reflowed into fiber 52 drawn from the preform as shown in FIG. Again, such a coating has been found to substantially prevent zirconium dioxide particles from migrating into the preform 44.

As noted above, while the above techniques work satisfactorily when the cracks are relatively small (eg, 0.038 cm wide [0.015 cm wide]), large cracks cannot be contained using such techniques. Microscopic particles of zirconium dioxide from grain 36 are drawn through a large crack by the gas flow caused by the chimney effect inside susceptor 34. Therefore, according to the invention, very small particles of zirconium dioxide
Yet another modification (see FIG. 4) has been implemented to virtually eliminate the build-up of fibers 44 and/or fibers 52 through such large cracks. FIG. 4 shows a furnace 10 with a susceptor 34 consisting of zirconium dioxide tubes 53 stacked in three layers, with silica soot deposited on the inner surface of only the middle tubes. has been done. Further, the zirconium dioxide cylinder 62 is arranged with respect to the susceptor tube 53 with a space 64 maintained between the cylinder and the susceptor tube 53. cylinder 62
has a sufficiently lower density than the high-density susceptor tube 53 so as not to inductively couple to the electromagnetic field of the energized coil 38 (for example, 20
~30% porous). Cylinder 62 is comprised of coarse grains of calcium stabilized zirconium dioxide, which are manufactured in Solon, Ohio.
Zirconia Company of Solon
It is of composition #1247 manufactured by

As the furnace 10 heats up, the silica soot is deposited in a layer 54 on the inside of the central susceptor tube 53 to prevent particulate zirconium dioxide from migrating onto the waveguide preform 44.
It becomes fixed inside. Furthermore, the susceptor pipe 53
The cylinder 62 acts as a barrier to microscopic particulation from the grain 36 which is drawn through the large, unconfined crack in the grain 36.

In yet another embodiment shown in FIG. 5, the central susceptor tube 53 is coated with porous silica soot 68 on the outer surface in addition to the soot on the inner surface of the susceptor. Soot 68 is also deposited on the outer surfaces of only the top and bottom susceptor tubes 53.
(The distance between the outer surface of susceptor tube 53 and the inner surface of cylinder 62 has not been measured, but is exaggerated for clarity.) Coil 38 is energized to raise the temperature. , and both sides of the central susceptor tube 53 are fixed to the layers 54, 54'. As is clear from FIG. 3, the glass soot 68 on the outer surface of the susceptor tube 53 at the top and bottom remains unfixed and
Concentric alignment is maintained between the cylinder 62 and the cylinder 62.

In yet another embodiment of the invention, shown in FIG. 6, silica soot 68 is deposited on the inside surface of cylinder 62 and slipped neatly around susceptor tube 53. The central susceptor tube 53 was coated with silica soot on its inside surface. When energizing the coil 38, soot or central susceptor tube 53
The inside of the susceptor tube 53 and the cylinder are fixed in the layer 54, and a portion of the silica soot fixed on the central inner surface of the cylinder 62 is fixed in the fixed soot layer 54'. 62 and in concentric alignment, the soot residue is kept porous, ie, unfixed.

It is to be understood that the embodiments described herein are merely illustrative of the principles of the invention. Various modifications to the embodiments described above may embody the principles of the invention and are within the spirit and scope of the invention.
This can be done by a person skilled in the art.
For example, although described with respect to waveguides, the inventive concepts can be advantageously used in other furnaces (i.e., resistance furnaces) with centrally located tubular heating elements (e.g., Matsufuru furnaces).