Method and apparatus for fiber cooling

A method and apparatus for cooling a glass optical fiber drawn from a glass preform, prior to applying a protective coating to the fiber, wherein the fiber is transported through a cooling zone containing coolant consisting of a solid or liquid dispersion of a condensible gas, the gas being a chemical element or compound having a vapor pressure of at least 1 atmosphere at 25.degree. C. such that rapid coolant vaporization at ambient temperatures is insured, are provided.

BACKGROUND OF THE INVENTION 
The present invention relates to a method and apparatus for cooling hot 
glass fibers or filaments as they are drawn from molten glass in a 
furnace. 
In processes such as the manufacture of glass optical waveguides, glass 
fibers are drawn at very high speeds from a molten glass preform 
positioned in a drawing furnace. Very rapid cooling of the glass fiber is 
required during such manufacture in order to reduce the temperature of the 
glass to a level at which the fiber may be protectively coated. The speed 
of cooling can in fact become a critical rate-limiting step for optical 
fiber production, since the technology of rapidly coating optical fibers, 
as well as the technology of rapid fiber drawing, is well developed. 
A number of different methods for cooling optical fibers at rapid rates 
have been proposed. U.S. Pat. Nos. 4,437,870 and 4,514,205 utilize cold 
flowing gases to cool the fibers, while U.S. Pat. No. 4,664,689 utilizes a 
combination of radiative cooling and cryogenic gas cooling to further 
increase the cooling rate. 
In addition to radiative and gas cooling of optical fibers, it has also 
been proposed to utilize conductive cooling via the direct application of 
cooling liquids to the surfaces of the fibers. U.S. Pat. No. 4,583,485 
employs a preapplication of liquid coating components to achieve 
quench-cooling of the fiber surfaces, while U.S. Pat. No. 5,043,001 uses 
water or a vaporizable organic liquid for fiber cooling. The apparatus 
used in the latter patent includes a provision for removing the coolant 
liquid from the surfaces of the fiber prior to coating. 
The use of liquid-phase quenching media to cool optical fiber is attractive 
because of the relatively high heat extraction rates available through the 
vaporization of cooling liquids. However, the disadvantages of such 
cooling include a substantial risk, particularly at higher fiber surface 
temperatures, that surface contact with substantial masses of liquid 
applied at temperatures much lower than fiber surface temperatures could 
introduce thermal stresses or cause breakage of the optical fiber. 
Moreover, as noted in U.S. Pat. No. 5,043,001, for most cooling liquids it 
is important that complete liquid removal from the surfaces of the cooled 
fiber be achieved prior to the time fiber coating material is applied to 
the fiber. The temperature range through which optical fiber must be 
cooled in order to prepare it for coating is a range extending from about 
1500.degree. C. or above down to approximately 50.degree. C.. At 
temperatures near the upper extreme of this range, radiative cooling is 
efficient and rapid cooling of the fiber can readily be achieved. 
In the mid-range of temperatures, e.g., from about 1000.degree. to about 
500.degree. C., radiative cooling becomes less efficient. Therefore, in 
this range, convective or conductive cooling of the fiber by gases of high 
thermal conductivity, typically supplied at cryogenic temperatures, is 
preferred to achieve high cooling efficiencies. 
An important shortcoming of present cooling procedures, however, is that of 
achieving rapid cooling through the lowest temperature regime, i.e., 
through the temperature range of about 500.degree.-50.degree. C. In this 
range, neither forced gas convection cooling nor radiative cooling 
provides rapid enough energy transfer to achieve an efficient reduction of 
fiber temperatures. This is due to the fact that the differences in 
temperature between the glass fiber and the cooling medium are relatively 
small, such that heat transfer rates between the fiber and the cooling 
medium are low. 
For these reasons it is evident that major improvements in cooling 
efficiency over the lower ranges of fiber surface temperature will be 
required if further large increases in fiber drawing rates are to be 
achieved. 
It is therefore a principal object of the present invention to provide 
improved fiber cooling methods, and apparatus for practicing those 
methods, which offer accelerated fiber cooling particularly at the lower 
extremes of the conventional fiber cooling range. 
Other objects and advantages of the invention will become apparent from the 
following description. 
SUMMARY OF THE INVENTION 
The present invention achieves a substantial acceleration of fiber cooling 
at low to moderate fiber temperatures through the use of a condensible gas 
cooling medium which is used as a fiber coolant in its condensed or 
liquified form. The condensed or liquid phase material, which is supplied 
directly to the surface of the fiber or closely adjacent thereto, is 
sufficiently vaporizable at temperatures in the region of the fiber 
surface that it is rapidly volatilized even at surface temperatures in the 
lower end of the fiber cooling range. 
The heat of vaporization needed to vaporize the condensed phase is 
relatively high, thus insuring an extremely high heat transfer rate from 
the fiber to the vaporizing coolant even at the aforementioned low fiber 
surface temperatures. Further, since complete vaporization of the 
condensible gas coolant from the fiber surfaces occurs even at low fiber 
temperatures, there is no problem of residual fluid on the fiber surface 
to interfere with the subsequent coating process. 
In a first aspect, then, the invention comprises a method for cooling a 
glass fiber, such as an optical waveguide fiber being drawn from a glass 
preform, which comprises the step of transporting the fiber through a 
cooling zone containing a coolant in the form of a condensed gas. 
Typically, the coolant is a liquified or solidified condensed gas, the gas 
having a chemical composition such that it rapidly vaporizes and remains 
in the gas phase without significant condensation at normal room 
temperatures (e.g., temperatures of 25.degree. C. and above). 
Suitable gases may thus be characterized as consisting essentially of one 
or a mixture of chemical elements or compounds having a vapor pressure of 
at least one atmosphere at 25.degree. C. In the preferred process, the 
condensed gas will be introduced into the cooling zone as a liquified gas 
in finely divided or sprayed form, although introduction in the form of 
one or more streams of liquified gas would effect the required rapid 
cooling. 
The invention further comprises apparatus for the cooling of glass optical 
fibers which is particularly useful for cooling fibers in the low 
temperature regime, but which is well suited for fiber cooling at higher 
temperatures as well. Included in the apparatus is a cooler which 
effectively utilizes a condensed gas coolant to achieve rapid continuous 
cooling of glass fiber drawn therethrough. 
The central element of the apparatus is an elongated walled enclosure which 
defines and surrounds a fiber cooling zone wherein a glass fiber 
traversing the zone may be exposed to the condensed coolant. Inlet and 
outlet openings disposed at opposite ends of the elongated enclosure 
provide means for drawing fiber continuously through the enclosure. 
Positioned exteriorly of and encircling at least a portion of the walled 
enclosure are refrigeration or other cooling or condensing means such as a 
reservoir for a cryogenic liquid. The condensing means are spaced away 
from the outer wall of the walled enclosure such that an annular space is 
formed therebetween. This annular space, typically enclosed at each end, 
provides a condensate reservoir or chamber wherein condensation and/or 
redistribution of condensed coolant prior to its transport into the fiber 
cooling zone can be achieved. 
Communicating with the annular condensate reservoir is a fiber coolant 
inlet, this inlet providing the means for introducing a condensible or 
condensed fiber coolant into the reservoir for condensation and/or 
transport into the fiber cooling zone. Once introduced into the condensate 
reservoir, the condensed fiber coolant is directed into the fiber cooling 
zone through multiple fluid inlet perforations in the enclosure wall 
between the reservoir and the cooling zone. These inlet perforations thus 
provide multiple paths for condensate transport into the fiber cooling 
zone. 
In the preferred embodiment, the perforations are provided in the form of a 
plurality of horizontal slots communicating with the fiber cooling zone 
which offer a cross flow entry path for coolants fed from the condensate 
reservoir and into the cooling zone. This cross-flow coolant pattern 
results in minimal fiber disturbance and a cooling rate which is not 
highly sensitive to the lateral position of the fiber in the fiber cooling 
zone. 
If desired, the perforated inner wall surrounding the cooling zone may 
further comprise a surface finish or coating enhancing radiation 
absorption, for effecting more efficient radiative cooling of the fiber. 
This is particularly useful in cases where it is desired to adapt the 
apparatus for use in the higher temperature fiber cooling regime. 
As suggested above, then, the apparatus of the invention offers multiple 
operating modes for fiber cooling depending upon the temperature of the 
optical fiber to be cooled therein. Where the fiber is at a high 
temperature, the apparatus may provide pure radiative cooling or radiative 
cooling plus conductive cooling by means of a conventional heat transfer 
gas introduced through the slots. 
At moderate fiber temperatures, where radiative cooling is slow, heat 
conduction or convective cooling with a refrigerated heat transfer fluid, 
e.g., argon gas supplied at cryogenic temperatures, may be provided. 
Finally, at temperatures in the lower portion of the fiber cooling range, 
liquid phase cooling with a condensible gas in accordance with the method 
of the invention may be utilized to greatly speed up the fiber cooling 
rate toward a suitable temperature for subsequent coating processes. 
The versatility of the apparatus above described is such that several of 
the units can be used cooperatively as the components of a multi-unit 
cooling system in a tandem cooling arrangement. Each cooler will have the 
same basic configuration, but each unit in the series will operate in a 
different cooling mode in order to optimize the fiber cooling rate for the 
particular temperature of the fiber passing therethrough.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring more particularly to the FIG. 1 of the drawing, which is a 
schematic illustration (not in true proportion or to scale) of a preferred 
design for a cooling apparatus 8 provided in accordance with the 
invention, a glass optical fiber 10 is shown moving downwardly through an 
elongated enclosure 20 formed by a substantially cylindrical perforated 
metal tube 22. The apparatus will typically be 2 to 3 times longer than 
shown in the drawing, but with the omitted central portion (as shown by 
the break lines) having a structure the same as that of the structure 
immediately adjacent the break. 
Tube 22, which may be formed of stainless steel or the like, incorporates a 
multiplicity of perforations in the tube sidewall, as represented by 
selected perforations marked 24 in the drawing. All perforations such 
those marked 24 are preferably provided in the form of slots transverse to 
the central axis of the tube, as shown. These slots are cut through the 
tube sidewall and provide the means for introducing a fiber coolant into 
enclosure 20 for fiber cooling. Representative paths for fiber coolant 
ingress are indicated by arrows 26; other paths would be associated with 
each of the other slots such as slots 24 shown in the drawing. 
Positioned externally of tube 22 in the apparatus is an annular condensate 
chamber or reservoir 30. That reservoir provides a space for uniformly 
distributing a selected condensed or condensible fiber coolant, introduced 
for example via coolant bottom inlet port 32. The coolant thus introduced 
is distributed about the exterior of tube 22 such that even feeding 
thereof through slots 24 into enclosure 20 can be achieved. Chamber 30 
also acts as a condensation zone, if needed, wherein the fluid selected to 
serve as the fiber coolant may be condensed if not already condensed as it 
is uniformly distributed prior to introduction into enclosure 20. 
Condensate reservoir 30 is bounded by a cooling or refrigeration system, 
such as an annular cryogenic liquid reservoir 40 adapted to contain a 
cryogenic cooling liquid between inner and outer walls 42 and 44, 
respectively. When so filled, this reservoir constitutes an efficient 
means for cooling chamber 30 and fiber coolant contained therein. 
A schematic cross-sectional view of the apparatus 8 taken along line 2--2 
of FIG. 1 is provided as FIG. 2 of the drawing. 
If containing a cryogenic cooling liquid, the walls of reservoir 40 may be 
constructed, for example, of stainless steel. Referring again to FIG. 1, 
the cryogenic cooling liquid may be introduced into bottom port 45 at the 
bottom of the fluid reservoir and exit therefrom through top port 46. 
The cryogenic cooling liquid provided in reservoir 40 will be one having a 
temperature sufficiently low to cool condensate reservoir 30 to a level 
sufficient to maintain coolant present in reservoir 30 in a condensed 
state, or to achieve condensation of at least some fraction of the 
selected fiber coolant introduced into condensation chamber 30 if such is 
introduced therein as a non-condensed gas. 
During operation of the cooler, condensate formed or maintained by 
refrigeration of the fiber coolant will take the form of a particulate 
dispersion of a liquid or solid chemical element or compound. This 
dispersion is schematically illustrated by particles 34 in FIG. 1. The 
dispersion will be sufficiently fine that the condensate can easily be 
transported into cooling tube 22 by uncondensed vapors or other carrier 
gases. The temperature required for adequate condensation will of course 
depend on the fiber coolant, and is readily determined by routine 
experiment. 
Condensed fiber coolant transported into fiber cooling tube 22 via slots 24 
will be substantially re-vaporized in the fiber cooling zone. The 
re-vaporized fiber coolant can thereafter be allowed to exit the cooling 
tube, for example at the top or fiber inlet end thereof. Alternatively, 
vapor collection means may be disposed at an outlet from the apparatus, 
positioned for example near the fiber inlet end thereof or at top port 
32a, so that the condensible fiber coolant may be collected for 
recondensation and reuse. 
While as previously noted, any refrigerant or refrigeration apparatus 
having the capability of cooling chamber 20 to a temperature sufficiently 
low to achieve condensation of that coolant can be used in the invention, 
the preferred refrigerants are liquified gases or gas mixtures such as 
air. The particularly preferred refrigerant for chamber 20 is liquid 
nitrogen. 
The condensible coolant used for fiber cooling in the method of the 
invention will be a chemical element or compound which vaporizes rapidly 
at ambient temperatures (i.e., at 25.degree. C.). More preferably, the 
coolant will have a vapor pressure in excess of 1 atmosphere (i.e., will 
boil) at ambient temperatures and above. An example of a particularly 
preferred fiber coolant is argon, which can be condensed to a liquid at 
liquid nitrogen temperatures. 
Where less rapid cooling of the fiber is required other condensible 
materials which require less extensive cooling to condense to liquid or 
solid phases in the condensation zone may be used. Examples of alternative 
cooling materials include carbon dioxide or even halogenated hydrocarbons 
having high vapor pressure at ambient temperatures. 
The fiber coolant selected can be introduced into chamber 30 as a pure gas 
stream or as a mixture of a condensible gas with another gas which may 
serve as a carrier. It is generally desirable that, when delivered through 
slots 24 and into enclosure 20,the fiber cooling medium consist of a 
mixture of a gas and a particulate liquid or solid wherein the 
particulates are reasonable uniformly distributed. 
The invention may be further understood by reference to the following 
detailed hypothetical Example illustrating the use of the apparatus shown 
in the drawing for the rapid cooling of an optical fiber. 
EXAMPLE 
Apparatus having a configuration substantially as shown in the drawing is 
first provided. In that apparatus, the fiber cooling tube 24, composed of 
stainless steel, has a length of about 24 inches and includes slots in 
opposing positions across the axis of the tube as shown in the drawing. 
The slots employed have a slot width of about 0.010 inches (0.25 mm). This 
slot positioning generates cross-flow circulation patterns for the coolant 
entering the tube, thus minimizing the disturbance of fiber being drawn 
through the tube. The staggering of the slots as shown provides greater 
fiber coverage. 
A supply of liquid nitrogen is connected to fluid inlet port 45 at the 
bottom of reservoir 40 so that the reservoir may be continuously filled 
and replenished with liquid nitrogen. When liquid nitrogen is present in 
reservoir 40 the temperature of the inner reservoir wall 42 approaches 
77.degree. K. 
After the reservoir has filled with liquid nitrogen, a glass fiber is 
dropped through tube 22 and drawing of the fiber from a preform located in 
a drawing furnace positioned above the cooling apparatus is commenced. 
Once the draw rate of the fiber has been stabilized, a flow of condensible 
fiber coolant consisting of 50% helium and 50% of argon by volume into top 
inlet port 32 is commenced at a flow rate of about 50 slpm. (For this 
mixture of condensible argon in a helium carrier, the argon content must 
be at least about 29% by volume in order to achieve condensation of the 
argon at liquid nitrogen temperatures). 
As the fiber coolant enters condensation chamber 30, condensation of the 
coolant commences to provide a suspension of fine droplets of coolant in 
the coolant stream. The stream with condensed coolant then enters 
enclosure 20 through sidewall slots 24 and is directed against the fiber 
being drawn through the enclosure at multiple points along its length. 
Although the flow rate of fiber coolant into port 32 may be relatively 
high, condensation of the coolant in chamber 30 reduces the volume of the 
vapor stream. This reduction, together with the configuration of coolant 
delivery slots 24 in tube 22, are such that the velocity of the gas 
streams being directed into enclosure 20 remain somewhat low, as well as 
relatively constant over a wide entrance angle into the enclosure. Thus, 
again, gas jets which would disturb the fiber traversing the tube are 
avoided. 
Fiber cooling by the entering gas streams carrying coolant condensate is 
expected to be very high even in the low temperature fiber cooling regime, 
due to the very high heat capacity of the mixture. At a fiber draw rate of 
20 m/sec, calculations indicate a reduction in fiber temperature from 
about 500.degree. C. to at least about 50.degree. C. close to ambient 
should be achieved in a cooling enclosure having a length of not more than 
about 1 meter. 
A further specific advantage of this cooling procedure is that 
post-treatment of the cooled fiber prior to the application of a 
protective coating is not required. Hence, vaporization of residual 
coolant from the fiber surface upon exiting the cooling enclosure is 
extremely rapid at normal ambient temperatures, so that no processing to 
remove residual coolant is needed. 
Of course, the foregoing example is merely illustrative of methods and 
apparatus which may be employed in the practice of the invention within 
the scope and spirit of the appended claims. Thus, for example, when using 
the apparatus of the invention at substantially higher fiber temperatures 
where radiative cooling is efficient, less cooling of the fiber coolant is 
needed since the delta T driving force cooling the fiber is large. And, at 
intermediate temperatures in the 500.degree.-1000.degree. C. range, any 
radiation-absorbing coating on the interior wall of the cooling chamber 
will continue to assist in fiber cooling, so that a vapor stream provided 
at cryogenic temperatures but not necessarily including condensed coolant 
can be efficient. 
Finally, in the low temperature regime, extending from about 500.degree. C. 
to about 50.degree. C., the nature of the surface on the perforated tube 
defining the cooling zone is relatively unimportant since conduction 
constitutes the predominant cooling mechanism and the presence in the 
cooling zone of liquid or solid condensate to increase the heat capacity 
of the cooling fluid becomes the most important factor governing efficient 
fiber cooling.