Glass manufacturing – Fiber making apparatus – With drawing means
Reexamination Certificate
1989-09-11
2001-10-02
Lovering, Richard D. (Department: 1712)
Glass manufacturing
Fiber making apparatus
With drawing means
C065S280000, C065S356000, C065S477000, C065S535000
Reexamination Certificate
active
06295844
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to glass fiber making and particularly to the type of fiber making in which a glass optical fiber is drawn from a preform.
Optical fibers are customarily drawn by feeding a glass preform longitudinally into a heating chamber, wherein one end of the preform is heated in a heating zone causing the preform to soften and begin to flow by gravity, thereby forming a fiber. The fiber is then drawn from the preform.
One of the most important considerations in the drawing of optical fibers is the total optical loss of the drawn fiber. In long distance communication systems, for example, a low loss characteristic (for example less than about 0.1 dB/km) is essential. Absorption and scattering losses, both intrinsic and extrinsic, contribute to the total optical loss in a glass fiber. In respect to intrinsic loss, intrinsic scattering loss results from density fluctuations in the glass, and intrinsic absorption loss arises from the glass multi-phonon structural vibration.
Fluoride glasses have been found theoretically to have an ultra low intrinsic optical loss, about 0.001 dB/km at a wavelength of 3.6 &mgr;m. Therefore, fluoride glass has much potential as a material for use in low loss optical fibers suitable for long range communication systems. However, fluoride glass is very unstable and is susceptible to moisture attack, thus forming OH and O groups upon heating the glass to the fiber drawing temperature. The hydroxyl groups are a source of unwanted extrinsic absorption loss at wavelengths of about 2.8 &mgr;m, whereas oxygen causes absorption of light at a wavelength of approximately 7.3 &mgr;m.
In addition to the loss due to contamination by moisture and oxygen, loss can also result when the glass is exposed to the drawing temperature for an extended period of time or to a non-uniform drawing temperature, conditions causing phase separation and microcrystallization. Such crystallization in the fluoride glass causes scattering loss.
Prior methods of making fluoride optical fibers have not been capable of producing fluoride glass fibers having low loss characteristics, primarily because of crystallization of the glass from extended heating, fluctuations in drawing temperature, and glass contamination by impurities such as moisture and oxygen.
Typically in prior techniques, the crucibles or preform from which the fiber is drawn are flushed with an inert gas such as argon, as shown in
FIG. 5
of “Fluorozirconate Glasses with Improved Viscosity Behavior for Fiber Drawing” by D. C. Tran, Ginther, and Sigel,
Materials Research Bulletin,
Vol. 17, No. 9 (1982), incorporated herein by reference. This inert gas serves only to purge the heating chamber of impurities such as moisture, oxygen, dust, transition metals, etc. Any contaminants which reach the surface of the glass remain in the glass, thereby causing undesirable optical loss in the finished fiber. In addition, prior drawing devices have heating zones, in which the preform is softened and drawn, which are too long, such that crystallization of the glass results. Finally, prior devices also do not provide adequate temperature stability for fluoride fiber drawing within the heating chamber, which also results in crystallization. As discussed, such crystallization in the finished fluoride fiber causes unwanted scattering loss.
SUMMARY OF THE INVENTION
Therefore, it is an object of this invention to provide an apparatus and method for drawing fluoride glass optical fibers wherein contamination of the drawn fiber is minimized such that a drawn fluoride fiber has a low optical loss.
It is another object of this invention to provide a fiber drawing apparatus having a heating chamber with a narrow heat zone and temperature stability so that crystallization of the drawn fiber may be minimized.
Various other objects and advantages will appear from the following description of one embodiment of the invention, and the novel features will be particularly pointed out hereinafter in connection with the appended claims.
The above and other objects are achieved by several features. First, the preform and fiber are flushed with a reactive gas which reacts with the fluoride glass to remove moisture from the preform surface and prevent O contamination. The gas also acts to purge the heating chamber of contaminants. To maintain temperature stability within the heating chamber wherein the fiber is drawn, an enclosure similar to a bell jar is provided for preventing convection currents which cause temperature fluctuations. Insulation around the heating chamber also contributes to temperature stability. In addition, a short heating zone of about 2 mm is provided by an induction furnace. Thus, the above features of the invention act to prevent contamination and crystallization of the glass fiber being drawn, such that a low loss fiber may be produced.
REFERENCES:
patent: 3076324 (1963-02-01), Morgan
patent: 3362803 (1968-01-01), Dannohl et al.
patent: 3890127 (1975-06-01), Siegmund
patent: 4030901 (1977-06-01), Kaiser
patent: 4202679 (1980-05-01), Cocito
patent: 4249925 (1981-02-01), Kawashima et al.
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patent: 4263030 (1981-04-01), Kobayashi et al.
patent: 0134721 (1979-10-01), None
Tran et al. “Fluoride Glass Preforms Prepared By A Rotational Casting Process”,Electronics Letters,Jul. 22, 1982, vol. 18, No. 15, pp. 657-658.*
Tran et al. “Fluorozirconate Glasses With Improved Viscosity Behavior For Fiber Drawing”,Mat. Res. Bull.,vol. 17, pp. 1177-1184, 1982.*
Cannon, “Optical Fibre Materials For Operating Wavelengths Longer Than 2 &mgr;m”,Journal of Non-Crystalline Solids,42 (1980) 239-246.*
Tran et al. “Rayleigh Scattering in ZrF4-Based Glasses”,Electron Lett.vol. 18, No. 24, pp. 1046-1048, 1982.*
Gbogi et al. “Surface and Bulk-OH Infrared Absorption in ZrF4-and HlF4-Based Glasses”,Communications of the American Ceramic Society,Mar. 1981, pp. C-51-C-53.*
Tran et al. “Fluoride Glass Preforms Prepared By A Rotational Casting Process” Electronics Letters, Jul. 22, 1982, vol. 18, No. 15, pp. 657-658.
Tran et al. “Fluorozircomate Glasses With Improved Viscosity Behavior For Fiber Drawing” Mat. Res. Bull., vol. 17, pp. 1177-1184, 1982.
Cannon “Optical Fibre Materials For Operating Wavelengths Longer Than 2 um” Journal of Non-Crystalline Solids 42 (1980) 239-246.
Tran et al. “Rayleigh Scattering In Zr F4-Based Glasses” Electron Lett. vol. 18, No. 24, pp. 1046-1048, 1982.
Gbogi et al. “Surface and Bulk-OH Infrared Absorption In ZrF4-and HfF4-Based Glasses” Communications of the American Ceramic Society, Mar. 1981, pp. C-51-C-53.
Pureza Pablo C.
Tran Danh C.
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