Note: Descriptions are shown in the official language in which they were submitted.
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J. JJees/A. R. Gilbert/
~03~98t3 M. K. R. Vyas 17-4-4 Rev.
BACKGROUND OF THE INVENTION
The present inventions relates to a method of malcing a butt jolnt
between optical fibers and to the joint per se.
The need for forming butt Joints or splices for single optical fibers
in the field of fiber optics communications is well known. Reference is
made to U. S. Patents Nos. 3,768,146 and 3,810,802 for examples of slngle
fiber joints. The purpose of the present invention ls to provlde a ~oint whlch
produces very low light transmission losses and a method of making the ~oint.
SUMMA~Y OF THE INVENTION
According to the one aspect of the present invention, there is provided
an optical fiber jolnt including a pair of optical fibers whose ends are butted
together within the straight parallel-sided, uninterrupted bore of a metal sleeve.
The fibers have a sliding flt in the bore so that alignment of the butted ends is
provided by virtue of the fit. The fibers are secured withln the sleeve in theirbutting relationshlp.
A preferred way of making a suitable metal sleeve having a bore of the
required tolerance ls by casting, using a relatively low melting point alloy cast
around a wire held under tension. The optical coupling between the butted ends
may be enhanced by the use of a liquid, commonly referred to as an index match-
2~) ing liquid. In fact, the liquid does not have to have the same refractive index
as the fibers, since any refractive index greater than that of air will reduce, to
some extent, the reflection losses at the interfaces at the ends of the Eibers.
The fibers may be secured within the sleeve by any of a variety of methods lnclu-
ding by lightly crimping the sleeve in regions removed from the fiber ends, by
2-5 applying a fillet of adheslve around the points of emergence of the fibers rom
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M.K~R. Vyas l7-a-4 Rev.
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the sleeve, and by shrinking the ends of a length of heat
shrinkable plastic tubing extending around the sleeve and
the adjoining regions of the fibers. Preferably, the sleeve ~`
is made of an alloy that has a low enough melting point
for the joint to be oapable of being dismantled without
damage to the fibers by melting the sleeve. In-this case,
a further method of securing the fibers within the sleeve
is to fuse the ends of the sleeves onto the fibers. ~ `
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suitable alloy for typical glass fibers is Wood's metal, -
10 which melts in the`range 70-72 C. ~ -
BRIEF DESCRIPTION OF THE DR~WINGS `'
Fig. 1 is a sectional view of a mold used for maXing
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the sleeve used in the method of forming the joint accord- ;~
ing to the invention;
Fig. 2 is a perspective view of the components of
the joint before assembly; and ``~
Fig. 3 is a perspective view of a jig for holding `
the assembly in alignment for crimping.
DESCRIPTION OF THE PREFERRED EMBODIMEN~S .
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Referring to Fig. 1, there is illustrated a demount-
able mold for casting a metal joining or splicing sleeve,
approximately 10 mm long, 1.5 mm in external diameter with ~
a concentric cylindrical longitudinal uninterrupted bore -
80 ~m in diameter. The term "uninterrupted bore" means
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25 that the wall of the bore is continuous and is not inter- ~
tupted by a slot. The mold comprises a barrel portion 10,
a bottom plug 11, a top cap 12, and a wire 13. The~ -
wire may be made of stainless steel and the remaining ~;~
parts of polytetrafluoroethylene. The wire 13 is
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J. Lees/A. R. Gllbert/
31.~3~9~3~3 M. K. R. Vyas 1;'~4-4 Rev.
80 ~Im in diameter and extends through 80 ~Im axial holes in the plug and
cap. The plug 11 has a cylindrical splgot 14 which fits ln the bore of the
barrel 1û while the cap 12 has a llp 15 which fits over the top end of the
barrel. The bore diameter of the barrel is 1.55 mm. The wire is tensioned
with a weight 16 attached to lts bottom end, and the assembly immersed in
a beaker 17 of hot water 18, where lt hangs freely suspended on the wire.
The càp Is cllpped, or otherwise held, at a short distance above the barrel.
A hypodermlc syrlnye (not shown) containing pieces of Wood's metal is
immersed in the heated water. When the Wood's metal melts, it Is dispensed
from the syringe into the contalner formed by the barrel and bottom plug, where
it displaces the water. When the mold is full of molten metal 19, the cap is
released and pushed down over the top of the barrel, thereby holding the wire
on axis through the mold. At thls stage the mold, still suspended on its wire,
is removed from the hot water and allowed to cool. When the metal is cool,
the wire i9 cut on each side of the mold, the cap and plug are pulled off the
wire, and the cast sleeve 19 is pushed out of the barrel and the wire is pulled
out of the sleeve. 1~ the cutting of the wire does not result in a clean brea3c,but instead tends to produce a splaying of the end, it will be necessary to
remove this splaying from one end so that it does not deform the sleeve bore 19aas It passes through it on removal of the wire.
The sleeve r with its 80 um~bore is designed for joining plastic coated
glàss optical fihers, which when stripped of their plastlc coatlng, have a dia-
meter of 72-75 llm. The diameter of the coated fibers is typically about 1 mm,
A length of the plastic coating is strlpped from the ends of the two such flbers21 and 22 to expose regions 23 and 24 of bare fiber, as seen in Fig. 2. The
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J, ~ ees~h, R. Gllbe t/
M. K. R. Vyas 17-4-4 Rev.
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manner of stripping wlll depend upon the particular plastic material used
to coat the fibers. With polypropylene coated fibers, it is preferred to melt
the coating off rather than to dissolve it off. .A hot wire, soldering iron, or
preierably a hot air gun may be used fDr this purpose.
The two bared regions 23 and 24 of silica fiber are prepared for buttin~
together by providing them with flat end faces 25 and 26 by any suitable tech-
nique. One way is to score lightly, the bared fiber surface and then subject
the flber to tensile stress until it breal;s. Another way involves placing the
bared fiber on a sharp edge, such as that of a razor blade, applying the spark
of a tesla coil to the fiber at the sharp edge, and then breaklng the fiber at r,,
this point by bending it or applying tension. These flat end faces 25 and 26
are prepared at positions having in each case a length of bare fiber 23 and 24
that i~s just longer than half the length of the case Wood's metal sleeve 19 so
that when the fibe~s are inserted into the sleeve bore 19a, their ends will butttogether leavillg only a short distance of bare fiber at either end. First, how-ever, a length of heat shrinkable plastic tubing 27 i9 slipped over the end of
one of the fibers. Then the two bared fibers are inserted by hand into the
sleeve, until their ends bu'-t together somewhere near the midpoint. It is pre-
ferred to insert a drop of liquid into the sleeve before the fibers to provide aglass-liquid interface instead of a ~lass-air one at the fiber ends in order to -
reduce reflection losses. A liquid that has a lo~,v volatility, and toxicity, and
that will not attack the plastic coatings, is desired for this purpose. One
example of a suitable liquid is a silicone oil having a viscosity of about l0û
centipoise. With care, the instant of the two fiber ends butting together can
be felt. The assembly ls then placed between the Jaws of a two part ~ig 30
J, Lees/A, R. Gilbert/
M. K. R. Vyas 17-4-4 Rev.
1C~3~
depicted in Fig. 3. Thls jlg has a groovèd center section 31 for gripping
the central portion of the sleeve 19, and grooved end sections 32 for
gripping the plastlc coating of the fibers. The two parts of the jlg are
clamped together to hold the assembly in position while the coupling
eficiency of the joint is tested by observing how much of the light launched
into one of the fibers emerges from the far end of the other~ If this is satis-
factory, the fibers are then secured within the sieeve. One method of
securing the fibers is to crimp the sleeve onto the bare fibers. Care has to
be taken with such crimping in order not to damage the fibsrs. A suitable
tool for crimping is a pair of parallel ~aw pllers havlng a ~aw width of aboul:
1 mm, and having the travel limited by a suitable spacer of 1.3 mm placec~
between the ~aw~ to act as a stop,
~s an alternative to crimping, the ends of the sleeve may be heated
with a hot air gun so that they melt and collapse onto the fibers. Yet another
lS possible method of securing the fibers is to apply fillets of a quick setting
cement such as cyanoacrylate adhesive around the bare fibers where they
emerge from the ends of the sleeve
Finally the length 27 of heat shrinkable plastic tubing, which was
slipped over one of the fibers before their insertion into the sleeve, ls slipped
back down the fiber and positioned over the sleeve with its two ends extending
a short distance over the plastic coatings of the two Eibers. A hot air gun ls
then used to collapse the tubing. If the temperature required to shrink the
tubing is very close to the melting point of the underlying metal sleeve, the
heating 1s confined to shrinking only the end ~ortlons to secure it to the plastic
coatlng on each fiber.
J. Lees/A. R. Gilbert/
M, K . R . Vya s 17 -4 -4 Rev .
~9~
The dirnensions of the sleeve 19 and optical fibers disclosed herein
are given by way of example only, and not by limitation. Further, flbers
other than plastic coated glass flbers could be used in the ~olnt of the
present invention.
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