Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to a method and apparatus for use
ln coating an elongate filament and particularly to such a method and
apparatus for coating an optical waveguide drawn directly from
preceding waveguide manufacturing equipment.
Dielectric optical waveguides should have sufficient
strength and integrity that they are not damaged by operations
occurring subsequent to fiber drawing such as fiber take-up onto
reels, fiber characterization, fiber cabling and cable installation.
The waveguide integrity can be markedly reduced if the various
handling processes introduce surface defects into the fiber.
Plastic coatings can provide good protection against
surface defects resulting from abrasion and other mechanical stresses.
Such coatings can also reduce microbending loss and cross talk
between fibers. The coating should be sufficiently thick and
resilient to protect the underlying fiber in spite of any bending of
the fiber. The coating should also be mechanically strippable,
non-hydroscopic, concentric with respect to the fiber and uniform
thickness. Suitable materials for coating optical fibers are
silicone, epoxy-acrylates, tetrafluorethylene, ethylene-vinyl-acetate
copolymer, perfluorinated ethylenepropylene and perfluoro-vinyl-
methyl ether. A variety of coating methods may be used for this
purpose a common techinque being shown in U.S. patent 3,980,390
(Yamamoto et al) in which filament immediately after it has been
drawn is passed into a reservoir of the suitable coating material and
out of the base of the reservoir through a nipple or coating die.
In order to obtain higher fiber drawing speeds, tapered
dies have been proposed, the tapered dies tending to give a more
concentric coating than the simple apertured reservoir known
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previously. Tapered bnre arran~Jements are described for example in
U.S. Defensive publlcation number T963002 (Albarlno et al).
Even with these tapered dies there is a problem of
turbulence resulting when a fiber passes rapidly through a chamber in
which viscous coating fluid is substantially static. In the contact
zone some of the fluid must be accelerated to the fiber speed
relative to the rest of the fluid in the chamber. If the turbulence
persists to the die exit aperture, it can result both in a variation
in coating thickness and in the introduction of air bubbles into the
coating so weakening the jacketed fiber.
One attempt to solve the problem of turbulence is
described in U.S. patent 4,294,190 (Ohls). As described in this
patent specification a die and reservoir are configured such that the
fiber passes through a tapered die full of coating fluid. Additional
fluid is supplied from a reservoir not into the top of the die where
the fiber enters the fluid, but some way down the tapered die through
a series of radial ports extending through the die and providing
fluid communication between the reservoir and the tapered bore. The
provision of individual fixed aperture ports does not permit easy
adjustment of the equipment to compensate both for changing coating
fluid viscosity or change in the speed at which fiber is drawn
through the die. Consequently, if the coating fluid viscosity
increases or the fiber draw speed is increased, the fiber may be
imperfectly coated. In addition, eccentricity of the fiber within
the coating can result from using several discrete inlet ports
especially if they are not exactly radially symmetric.
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An alternative arrangement ls now proposed whlch
overcomes the problems of fiber coating eccentricity and turbulence
discussed previously. In addition, the arrangement can be easily
adjusted to take account of changes both in fiber drawing speed and
coating fluid viscosity.
According to one aspect of the invention, there is
provided a fiber coating mechanism comprising means defining a
passage, the passage having an upper vertical zone of substantially
uniform cross-sectional area communicating at a lower end thereof
with a downwardly tapering lower zone, the lower zone substantially
centro-symmetric about a vertical axis, the lower zone at an upper
end thereof greater in cross-sectional area than the upper zone and
at a lower end thereof less in cross-sectional area than the upper
zone, the lower zone having an annular fluid inlet zone at an outer
extremity thereof at said upper end and an exit aperture at the lower
end thereof, the exit aperture vertically aligned with a central axis
of the upper zone, means for feeding fiber vertically downward
through the passage, and means for feeding fluid into the passage
- through the inlet aperture to fill the lower zone and at least
partially to fill the upper zone.
The passage can be defined by a regulating valve and a
cup, the upper zone defined by a bore through a stem of the valve the
lower zone defined by a lower part of the cup and the annular
aperture defined by an inner wall of the cup and an outer surface of
the valve stem.
The cup can have a nipple in a base thereof, the nipple
having a tapered coating chamber therein. The cup can have a tapered
fluid storage chamber above the annular aperture.
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The valve stem can be vertlcally reclprocal whereby the
size of the annular gap can be adjusted, The valve stem can screw
engage a bore through a lid for the cup.
The cup preferably has an inner wall which is steep at
the top of the cup and becomes shallower towards the annular
aperture. The nipple preferably has a conical inner surface. The
fluid coatiny mechanism can further include means for supplying fluid
to the storage chamber to a level whereby to set the pressure of
fluid at the annular aperture. The nipple can be screw threaded into
the base of the cup.
According to another aspect of the invention there is
provided a method of coating a fiber comprising feeding the fiber
vertically downward through a passage filled with coating fluid, the
passage having an upper zone within which air bubbles and turbulence
are inevitably introduced into the fluid the upper zone being of
sufficient length and cross-sectional area to permit the introduced
bubbles to float upwardly therethrough, the passage further having a
downwardly tapering lower zone, the method further comprising
directing fluid uniformly radially inwardly and downwardly into the
lower zone to suppress the introduced turbulence at an exit aperture
at a lower end of the passage.
An embodiment of the invention will now be described by
way of example with reference to the accompanying drawings in which:-
Figure 1 is a sectional view through a coating mechanismaccording to the invention; and
Figure 2 is a sectional view to a larger scale of a part
of the Figure 1 mechanism.
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Referring ln detail to Figure 1, there is shown d
coating mechanism having a cup lO, a nipple 12 and a valve 14. A
fiber 16 is drawn vertically downwards through a bore 18 in the valve
14 and out through an aperture 20 at the lower tip of the nipple.
Within the nipple 12 the fiber 16 is coated with a protective fluid
22. The cup 10 has an inner surface 24 which is steep at the top end
of the cup but which becomes shallower towards an aperture 26 within
the base of the cup. Essentially the inner surface of the cup is
paraboloid. The nipple 12 which is of stainless steel composition,
screw engages within a recess 32 at the base of the cup 10. On a lip
28 of the cup is mounted a stem support 30 through which extends an
internally threaded bore. The valve stem is externally threaded and
engages within the bore.
As shown to a larger scale in Figure 2, the nipple 12
has a conical inner surface 34 and the valve stem has a convex lower
surface 36. An annular gap or restriction 38 is defined by an
angular projection at the junction of the cup 10 and the nipple 12
which opposes the outer convex surface 36 of the valve 14.
As shown in Figure 2, viscous coating fluid 22 stored
within the cup 10 is forced by gravity through the annular gap 38
into a conical chamber 40 defined by the nipple. The level of fluid
within the cup is sufficiently high that the fluid extends somewhat
up the bore in the valve stem. The shape of the valve 14, the cup 10
and nipple 12 at the gap 38 determine that the fluid 22 is directed
radially symmetrically into the chamber 40.
As illustrated in Figure 2 turbulence and air bubbles
are introduced into the fluid 22 by the passage of fi'ber 16 into and
through it. By ensuring a sufficient fluid height and a relatively
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wide bore 18 within the valve stem 14, air bubbles entralned by the
fiber are permitted to float freely up the bore to the surface of the
coating fluid within the stem. However, with such a wide bore 18,
turbulence tends to be transmitted down through the passage. The
persistence of turbulence is discouraged by the flow of fluid through
the annular gap 38. As shown in Figure 2 the relatively large
turbulent eddies emitted from the bottom of the valve stem 14 are
gradually suppressed within the nipple tapered chamber 40.
Consequently immediately above the exit aperture 20 of the nipple,
substantially lamellar flow is restored. Thus as the coating fluid
exits the die in contact with the fiber 16, it is devoid both of
turbulent eddies and air bubbles and thus uniformly coats the fiber.
The pressure and viscosity of fluid 22 within the cup 10
and the width of the annular gap 38 determine the rate at which
coating fluid enters the conical chamber 40 within the nipple 12.
That rate is set equal to the rate at which fluid exits the nipple 12
with the chamber 40 filled. If the rate at which fiber is being
pulled through the coating mechanism is raised and consequently a
greater flow of coating fluid into the coating chamber 40 is desired
then the annular gap is increased by rotating the valve 14 to lift it
through the stem support 30 and increase the size of the annular gap.
If the viscosity of the coating fluid changes then compensation can
be made either by increasing the height of fluid 22 within the
storage reservoir or by changing the size of the annular gap 38, or
both. These two operating parameters can be tuned for the desired
fluid dynamic activity within the coating chamber 40.
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After coatlng with flu~d, tne fluid coating is cured for
example by heat in the case of a silicone or by ultra-violet light if
using acrylates or other ultra-violet curing material.
Although the invention has been described in terms of
applying a plastic coating to glass clad optical fiber to protect the
fiber, the coating method described can also be used in other
circumstances where it is desired to apply a uniform coating to a
solid fiber, for example, for applying lower refractive index plastic
cladding directly to a fused silica core.
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