Plastic and nonmetallic article shaping or treating: processes – Optical article shaping or treating – Optical fiber – waveguide – or preform
Reexamination Certificate
2000-12-15
2003-05-20
Vargot, Mathieu D. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Optical article shaping or treating
Optical fiber, waveguide, or preform
C065S435000, C264S002600, C264S002700
Reexamination Certificate
active
06565775
ABSTRACT:
The present invention relates to a method of cooling an optical fiber during drawing through contact with at least one cooling fluid in at least one cooling area.
BACKGROUND OF THE INVENTION
There are various categories of optical fiber: optical fibers based on oxide glass, optical fibers based on fluoride glass, and plastics material optical fibers based on polymer materials. Optical fiber based on oxide glass, generally silica glass, is manufactured by drawing a heated preform, which is a large cylinder of silica glass, optionally at least partly doped, whose diameter generally lies in the range 20 mm to 200 mm and whose length generally lies in the range 300 mm to 2000 mm.
FIG. 1
is a diagrammatic view of a drawing tower
1
. A preform
2
is melted in a drawing furnace
3
which heats the preform to a temperature of approximately 2000° C. A fiber
7
obtained in this way is cooled initially by the surrounding air, then in at least one cooling device
4
, and finally by the surrounding air again, before it is fed into a coating device
5
. The position of the cooling device
4
in the drawing tower
1
is generally optimized to obtain the correct fiber temperature for resin coating. The coating device
5
forms the coating of the fiber
7
from at least one coating resin which is usually cured by ultraviolet light. The device
5
generally includes at least one injection device (
5
a,
5
c
) followed by at least one curing device (
5
b,
5
d
). In the situation shown in
FIG. 1
, the device
5
includes a primary resin injection device
5
a
followed by a device
5
b
for curing said resin by ultraviolet light, and then a secondary resin injection device
5
c
followed by a device
5
d
or curing said resin by ultraviolet light. Finally, a coated optical fiber
8
is pulled by a capstan
6
and then wound onto a take-up spool
9
.
The devices under the drawing furnace
3
, which are on a common downward vertical axis Z, are generally identified by their position relative to the bottom of the drawing furnace
3
, as indicated by the dimension z. All the components of the device shown in
FIG. 1
are well-known to the person skilled in the art. Others, which are not shown, are also well-known to the person skilled in the art. Thus, for example, means for measuring the diameter of the bare and/or coated fiber, means for measuring the eccentricity of the fiber within its primary and/or secondary coating, and means for measuring the temperature of the fiber at a given distance along the axis are part of the prior art.
Cooling must reduce the temperature of the fiber leaving the drawing furnace to a temperature compatible with application of the coating resin, i.e. a temperature of the order of 50° C. The temperature of the fiber leaving the drawing furnace is high, generally of the order of 1000° C. to 2000° C. for a silica-based fiber, depending on the drawing furnace and the drawing speed used. Cooling the fiber between leaving the drawing furnace and entering the coating device is one of the major problems to be solved in drawing fibers, especially if it is required to increase the drawing speed. It is known that the attenuation of the fiber depends on the cooling conditions, and moreover, if the temperature of the fiber on entering the coating device is too high, this can lead to problems both with the eccentricity of the fiber in its coating and with the quality of said coating. The speed at which silica-based fibers are drawn industrially, which was 300 meters per minute (m/min) a few years ago, has increased more and more, and is now of the order of 1500 m/min or more. This tendency is still apparent, associated with increasing productivity, which is one of the major objectives of the optical fiber industry.
The principle of the process for fabricating optical fibers based on fluoride glass is the same, but the preform is generally smaller, generally having a diameter of 15 mm to 20 mm and a maximum length of a few centimeters to a few tens of centimeters, for example 10 cm, and the temperature on leaving the drawing furnace generally lies in the range 300° C. to 450° C. The same technical problem can arise in this case. Similarly, the same technical problem can arise in the fabrication of optical fibers based on polymer materials, in which the preform is generally smaller, generally having a diameter of a few tens of millimeters, for example 80 mm, and a maximum length of a few tens of centimeters, for example 50 cm, and the temperature on leaving the drawing furnace generally lies in the range 200° C. to 250° C. The remainder of the description refers to optical fibers based on silica, but identical reasoning applies to other types of optical fiber, including optical fibers based on glasses other than silica.
Various devices have been used to cool silica-based fiber. One solution would be to increase the area of heat exchange between the fiber to be cooled and the surrounding air, in particular by increasing the distance between the drawing furnace and the coating device. However, this would entail increasing the height of the drawing towers currently used, which would be much too costly, especially in terms of the investment required.
Another solution is to improve the efficiency of cooling over the existing distance between the drawing furnace and the coating device. In addition to simple cooling by the surrounding air, which proves to be highly inadequate for the drawing towers currently used, the common principle of various devices used in the industry (as illustrated by European Patent Application EP-A1-0 079 186, for example) consists in injecting a gas radially towards the surface of the fiber at a given distance from the outlet of the drawing furnace and causing said gas to flow upwards or downwards over a particular length of the fiber, inside a heat exchange tube. As is well-known to the person skilled in the art, heat is transferred because of the thermal conductivity of said gas, which gas is generally air, carbon dioxide, nitrogen, argon, or helium, and is preferably helium possibly mixed with nitrogen. The periphery of the tube is preferably cooled by a cooling fluid, which is generally water. By way of example, U.S. Pat. No. 4,761,168 describes an improvement to such systems in which the gas is caused to flow along the fiber in a heat exchange tube of particular shape, which ensures regular renewal of the boundary layer of gas flowing along the fiber. The improvement is aimed at improving the efficiency of heat exchange.
One of the main problems encountered in subsequent use of optical fiber cooled in the above way is that the cooling imposed on the fiber during its fabrication, on leaving the drawing furnace and before passing through the coating device, significantly increases the level of Rayleigh back scattering associated with the fiber and therefore increases the major part of the attenuation of the optical fiber ready for use. It is known in the art that the attenuation of optical fiber at the wavelengths used, whether close to 1310 nm or to 1550 nm, must be as low as possible for optimum transmission of optical signals in said fiber.
That is why several solutions have been proposed to the problem of defining cooling profiles which are obtained by particular methods and/or devices and which minimize Rayleigh back scattering in the fiber. At least partial use of slow cooling profiles is generally proposed, meaning profiles that are slower than those obtained for cooling by the surrounding air. Patent Application DE-A1-3 713 029, for example, teaches slow cooling on leaving the drawing furnace.
Such methods are not satisfactory, however, in that they do not achieve sufficient reduction of the attenuation compared to the theoretical minimum attenuation, without degrading the mechanical strength of the fiber.
OBJECTS AND SUMMARY OF THE INVENTION
An object of the present invention is to alleviate the above drawbacks of prior art cooling systems by improving the cooling of an optical fiber during drawing. One particular object of the invention is to c
Bourhis Jean-François
Dubois Sophie
Orcel Gerard
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