DESHABILLAGE THERMIQUE DES FIBRES OPTIQUES

THERMALLY AIDED STRIPPING OF OPTICAL FIBER JACKETS

Abrégé anglais

THERMALLY AIDED STRIPPING OF OPTICAL FIBER JACKETS
Abstract of the Disclosure
A protective jacket around an optical fiber is rendered more
easily strippable by heating the fiber to a temperature causing
differential expansion of jacket and fiber and consequent breakage or
weakening of adhesive bonding between the fiber and its jacket.
- i -

Revendications

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A method for stripping a portion of a protective jacket
bonded to an optical fiber, the jacket material having a greater thermal
coefficient of expansion than the material of the fiber, the method
comprising heating the portion of the jacketed fiber, to promote
sufficient thermal mismatch at the interface of the fiber and the jacket
to cause breakage and weakening of adhesive bonds between the fiber and
the jacket along the length of said portion, the method further comprising
gripping said jacket portion and pulling it from the underlying fiber
portion.
2. A method as claimed in claim 1, specifically for
removing an end portion of the fiber jacket, in which, following heating
of the jacket end portion, said jacket end portion is pulled axially off
the end of the fiber.
3. A tool for stripping a part of a protective jacket
bonded to an optical fiber, the jacket material characterized by a higher
thermal coefficient of expansion than the material of the optical fiber,
the tool comprising a heat conducting body having a linear bore extending
thereinto, the bore having a diameter marginally greater than the diameter
of a jacketed fiber to be stripped, the block having heating means
associated therewith for heating the body to a temperature causing
differential expansion at the interface of the fiber and the jacket to

markedly reduce adhesive bonding therebetween, the tool having a thermally
insulating covering around the body permitting the tool to be manually
held.
4. A tool as claimed in claim 3, the body including a first
cylindrical block having a bore extending generally axially therethrough.
5. A tool as claimed in claim 3, in which an outer end of
the bore is of greater diameter than, and tapers towards, a remaining part
of the bore.
6. A tool as claimed in claim 4, including a second
cylindrical body concentric with said first cylindrical body, the first
and second cylindrical bodies having said heating means disposed
therebetween.
7. A tool as claimed in claim 6, in which the heating means
is a resistive heating element surrounding said first cylindrical body.
8. A tool as claimed in claim 3, in which said bore extends
through the block.
9. A tool as claimed in claim 3, in which the bore is
closed at one end.

Description

-` 1155642
This invention relates to a method and tool for stripping a
jacket from an optical fiber.
Optical fibers for use in telecommunications systems consist
usually of a central core of a relatively high refractive index glass, a
cladding around the core of a relatively low refractive index glass and a
protective jacket. Typically, the fiber jacket is a polymer such as a
fluoropolymer, polyurethane, a silicone or an acrylate which is applied to
the cladding surface usually either by extrusion or dipping. The coating
which may be anything from a few microns to few hundred microns is applied
immediately after the fiber is formed and protects the underlying glass
from abrasion, contamination and propagation of microcracks.
When terminating an optical fiber at an electro-optic device
such as a laser or photodetector, and when interconnecting or splicing
fiber ends together, it is necessary first to bare the fiber end by
removing an end piece of its jacket.
The polymer jacket bonds to the underlying glass and usually
cannot be easily removed by purely mechanical means without stressing the
fiber or leaving residual pieces of the jacket sticking to the fiber. The
conventional method of removing the jacket is by using a solvent, for
2~ example, tetramethylguanidine for room temperature vulcanizing (RTV)
silicone rubber or methyl chloride for ultraviolet curable urethane
acrylate. The solvents act to soften the sheath and render it more easily
strippable. Liquid chemicals for stripping fiber jackets are difficult to
handle and often are toxic. Moreover, the stripping process, if it
includes cleansing of the bared fiber ends is time consuming. A method of
stripping fibers which is rapid, easy and which renders a clean fiber end
is now proposed by the present invention for jacketed fiber in which the
~'

1 155642
jacket material has a greater thermal coefficient of expansion than the
material of the fiber. The method comprises heatins a jacketed fiber
portion to a temperature at which the consequent thermal mismatch caused
by a differential expansion at the interface of the fiber and the jacket
promotes breakage and weakening of adhesive bonds between the fiber and
the jacket, and the method further comprising gripping the jacket portion
and pulling it from said fiber portion.
Usually the fiber portion heated to be bared will be a fiber
end portion and after heat treatment, the loosened jacket portion can be
merely slid off the end of the fiber. However if an intermediate part of
a fiber should need to be bared, then following heat treatment the jacket
portion can be ruptured to detach it from the underlying portion of the
fiber. According to another aspect of the invention a tool for stripping
a part of d protective jacket banding to a portion of an optical fiber
comprises a heat conducting body having a bore extending therein, the bore
having a diameter marginally greater than the diameter of a jacketed fiber
to be stripped, the block having heating means associated therewith for
heating the body to a temperature causing differential expansion at the
interface of the fiber and the jacket to reduce adhesive bonding
therebetween, the tool having a thermally insulating covering around the
body permitting the tool to be manually held or clamped in place.
The body can comprise a pair of concentric cylindrical
copper members having a resistive heating element located between them.
- The bore can have a belled orifice to aid insertion of a fiber into the
bore. The thermally insulating covering is preferably polybenzimidazole
(PBI).

- ~ \
l 15564~
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 perspective view with part cut away of aheating tool according to the invention; and
Figure 2 shows operation of removing a heated optical fiber
jacket portion.
Referring in detail to Figure 1, the tool 10 comprises a
central copper cylinder 11 having a 450 micron diameter bore extending
axially therethrough. The block has a belled orifice 12 for aiding
insertion of 300 micron outer diameter jacketed fiber. The cylinder 11
has a resistive heating element 16 wrapped around it. Concentric with the
inner cylinder is an outer copper cylinder 18 and a thermally insulating
covering 20 of po1ybenzimidazole (PBI).
In operation a jacketed fiber end portion is inserted into
the bore and is maintained there while current is supplied to heat the
element. For a RTV silicone jacket, 90 ~m in thickness, surrounding a 125
micron outer diameter fiber, a temperature of 350C for a period of 10
seconds has been found sufficient to cause sufficient differential
expansion of the jacket 24 around the fiber 26 that adhesive bonds between
the jacket and the fiber are weakened or broken sufficiently for the
heated jacket portion subsequently to be easily mechanically stripped from
the underlying fiber 26. The silicone has a coefficient of thermal
expansion of approximately 300 x 10 6/oC whereas that of glass is
approximately 0.6 x 10 6/oC. Urethane acrylate which is also a
common jacket material has a coefficient expansion of
200 x 10 6/oC which is somewhat less than that of silicone but is
still high enough that there will be thermal mismatch between the glass

1 155642
and the urethane acrylate which can weaken or break the adhesive bonds
between jacket and fiber. It will be appreciated that there is
differential expansion both axially and radially. The actual temperature
to which the portion of the optical fiber jacket is raised is not critical
but it should be sufficiently high to maximize differential expansion yet
not high enough that, for example it becomes brittle since this would
lessen the chance of it being pulled cleanly from the fiber as a single
jacket section. Additionally, at high temperatures the jacket may ignite
or bake onto the fiber thus increasing the difficulty of removing it. The
property of the jacket compound to inhibit microcrack propagation within
the fiber is destroyed and at such high temperatures there is an increased
tendency for such crack propagation.
As shown in Figure 2 the heated jacket portion 22 is readily
stripped after extracting the fiber from the tool of Figure 1 simply by
gripping the jacketed fiber in one hand and the jacket end portion in the
other and pulling the two apart. The exposed bare fiber surface 28 is
found to be quite clean and free of contamination. To ensure scrupulous
cleanliness, precleaned grips (not shown) may be used.
.