Note: Descriptions are shown in the official language in which they were submitted.
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TITLE OF THE INVENTION
Modifying the coating on optical fibres.
FIELD OF THE INVENTION
[001] This invention relates to the modifying or stripping of specialist
primary or
secondary coatings on optical fibres such that the coating is removed from the
surface over a region of the optical fibre without substantially affecting the
properties of the optical fibre.
BACKGROUND OF THE INVENTION
[002] Glass based optical fibres are generally coated with a polymer layer to
protect the surface of glass, which would otherwise deteriorate over a period
of
time. This deterioration process is primarily induced by the action of water
vapour, chemicals or mechanical damage from contact with other surfaces.
Normally for optical communications the protective coating is an acrylate
polymer or soft silicone, depending on the type of cable that the fibre is
ultimately housed in. For other applications such as fibre pigtails which need
to
remain flexible, the primary coating is tightly sheathed in a secondary
polymer
jacket which protects the primary coating from mechanical damage and adds
strength to the lead. For optical fibre jumper cables, the secondary coated
fibre
may be surrounded by Kevlar fibres and cabled in a plastic tube to provide a
rugged structure.
[003] Optical fibres can also be coated with a thin, hard, hermetic coating of
carbon to allow the fibre to be used in environmentally harsh conditions such
as
at elevated temperatures and/or in corrosive surroundings. Recently, polyimide
has featured as a specialist coating. This material has excellent mechanical
and chemical resistance properties, and has been used widely in industry as a
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masking material or for providing electrical insulation. Coating optical
fibres, for
example, allows them to be used in sensing applications. These coating may
also reduce the diffusion into the glass of gases such as hydrogen that affect
performance of the fibre. These speciality coated fibres make a more rugged
fibre structure and are therefore attractive for a number of applications in
devices that are used in difficult environments.
[004] It is necessary to remove any such coatings prior to splicing two fibres
together, as the polymer may contaminate the fibre end and block the coupling
of light from one optical fibre to the other. Generally, the coatings are not
exactly concentric with respect to the fibre core, and therefore cannot be
used
for alignment between two fibre ends. Polymer coating on optical fibres can be
removed by mechanical stripping with a wire stripper. This process removes
the secondary and primary coating together, leaving the glass fibre bare for
cleaving and splicing. Cleanliness and mechanical integrity of the optical
fibre
are of prime importance when preparing them for splicing. Additionally, any
serious degradation of the mechanical or optical properties of the optical
fibre
may compromise performance of the splice over the long term. Mechanical
stripping is difficult for stripping the coating of metal, carbon or polyimide
from a
coated optical fibre.
[005] Another method of stripping-off most coatings the optical fibre is by
immersion of the coated fibre into a bath of hot sulphuric acid. This is a
very
successful technique but is not generally preferred as it poses severe hazard
for the operator in the field. A safer method is needed and this is the
subject of
our current invention.
SUMMARY OF THE INVENTION
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[006] The present invention provides a novel method for the removal of most
primary coatings from the surface of an optical fibre. This is accomplished by
applying localized heating to the tip of the fibre or any other region. This
may
be applied, for example, by a series of weak or continuous electrical
discharges
or, alternatively, by pulses of light from a tightly focussed laser beam. Such
modification can be carried out in a controlled manner so as to allow precise
removal of just the coating, without substantially affecting the properties of
the
optical fibre. This method has been demonstrated to not only remove standard
polymer based primary coating, but also metal and polyimide coatings.
[007] The object of the invention may be achieved by applying a controlled
electrical discharge or laser light to a local region of the fibre. In a
preferred
embodiment of the invention, the discharge or laser light treatment is applied
digitally, in short pulses or continuously so that the coating bears the brunt
of
the heating affect, rather than the underlying optical fibre. The heat
supplied to
the fibre is only sufficient to remove the coating without melting the fibre.
[008] In an alternative embodiment, the quality of the stripping may be
monitored on a video camera for precise removal of difficult coatings,
providing
visual inspection during the removal of the coating as well feedback to the
discharge to control the rate of stripping.<
BRIEF DESCRIPTION OF THE DRAWINGS
[009] Fig. 1 is a schematic representation of a cleaved optical fibre with a
specialist primary coating such as polyimide.
[0010] Fig. 2 is a schematic representation of a typical arrangement used for
stripping of coatings on the optical fibre using an electrical discharge by
the
method of this invention.
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[0011] Fig. 3 is a schematic representation of a typical arrangement used for
removal of coatings on the optical fibre using a focussed laser beam by the
method of this invention.
[0012] Fig. 4 is a schematic representation of the tip of the optical fibre,
indicating for this embodiment, the area in which the coating removal occurs.
[0013] Fig. 5 is a photographic representation after the application of 2
discharge pulses by the method of this invention at the end of a fibre.
[0014] Fig. 6 is a photographic representation of the region of optical fibre
in
which the local removal of the coating takes place in the middle of a fibre.
[0015] Fig. 7 is a photographic representation of the fibre after an extended
region of the coating has been removed.
[0016] Fig. 8 is a schematic representation of the device that transports the
optical fibre through the region of the heat zone synchronously with the
application of the electrical discharge.
DETAILED DESCRIPTION
[0017] Figs. 1 shows the cleaved end (1) of an optical fibre (2) with a
coating
(5).
[0018] Fig. 2 is a schematic representation of one embodiment of the
arrangement used to realize the removal of the coating (5), of this invention.
In
the prior art, electric-sparks have been used to remove debris loosely
deposited on ends of optical fibres prior to fusion splicing of optical fibres
by
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melting the two ends. These sparks are intended only to "kick" off any dirt
the
end. An optical fibre (2) may have a core (4) and may have a cleaved end (1).
The core (4) could for example have a diameter of 1 to 100 microns or greater,
while the uncoated fibre could have an overall diameter on the order of 125-
500
5 microns. The cladding could be a single layer, or could be fabricated with
two
or more layers and both the core and the cladding could have refractive
indices
which are graded in the radial direction. The optical fibre cladding (3) may
be
encapsulated in a protective glass or polymer or other coating as shown in
Figure 2, and it may be metallized for soldering or other purposes. The fibre
end (1) by which the fibre is terminated could be a cleaved end or a fibre
lens
fabricated by polishing, etching, drawing, or any other known method, and it
could be wedge-shaped or of any other shape suited to the application for
which it is intended.
[0019] In the embodiment of the invention of Fig. 2, an electrical discharge
is
established between two electrodes positioned near the tip of the fibre (1).
The
electrodes (6a and 6b) may be of tungsten, graphite or any other suitable
material capable of sustaining a repeated electrical discharge. Representative
dimensions are shown in Fig. 2, but these could be adjusted by a person
skilled
in the art, combined with selection of the electrical parameters of the
process,
as required to provide the required degree of processing. The electrical
pulses
causing the electrical discharge between the electrodes (6a and 6b) may be of
any suitable intensity and duration, with the geometry selected, for giving a
stepwise removal of the coating on the fibre and without melting the fibre.
For
example, pulses could be in the form of a square wave or any other shape
having typically amplitude between one and 500 milliamperes and duration on
the order of 1 to 100 microseconds or even continuous. Time between pulses
is typically on the order of one tenth of a second but may be less or several
seconds or longer, and this time may be controlled either automatically or by
manually triggering the treatment pulses. Different types of materials used to
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make the optical fibre may require either shorter or longer duration
discharges
as well as greater or smaller discharge currents. It will be evident to a
person
skilled in the art that the precise geometrical and electrical parameters
necessary to achieve the desired result will depend on humidity, atmospheric
pressure, type of fibre end, fibre size, fibre type, ambient temperature and
many other parameters. Any combination of suitable geometric and electrical
parameters that achieves the objects of this invention falls within its scope.
[0020] Fig. 3 is a schematic representation of a second embodiment of the
arrangement used to realize the coating modification of this invention. A
laser
beam (7) is focussed by a lens or system of lenses (8) such that the focussed
beam (9) is incident on the fibre that is to be stripped. As for the
embodiment
of Fig. 2, the laser light may be pulsed with pulses of any suitable intensity
and
suitable duration or continuous, with the geometry selected, for giving a
stepwise or continuous removal of the coating on the fibre. Pulses could have
duration on the order of 1 to 100 microseconds or more, and time between
pulses may be on the order of one tenth of a second or longer and may be
controlled either automatically or by manually triggering the treatment
pulses.
Different types of materials used to make the optical fibre may require either
shorter or longer duration pulses as well as greater or smaller intensity of
the
treatment light. A carbon dioxide laser is well suited to this application. It
will
be evident to a person skilled in the art that the precise geometrical and
laser
parameters necessary to achieve the desired result will depend on humidity,
atmospheric pressure, type of fibre-end, fibre size, fibre type, ambient
temperature and many other parameters. Any combination of suitable
geometric and laser parameters that achieves the objects of this invention
falls
within its scope.
[0021] Fig. 4 shows schematically the region (10) of a fibre at which
stripping is
to be carried out by the method of this invention. The fibre may have a
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metalization coating or some other coating such as carbon or polyimide coating
(5). This metalization may for example be an electrolytically-deposited
coating
of a few microns of nickel and a thin flash of gold (less than 1 micron).
Alternatively, it may be a vacuum deposited coating such as, for example, 50
nm of titanium, 100 nm of platinum and 200 nm of gold. All such metallization
coatings can be removed precisely and locally with application of a single or
a
few electrical discharges or light pulses or by continuous exposure to
electrical
discharge or laser light, by the method of this invention. The power level is
such that a first single, several discharges, light pulses or continuous
exposure
to electrical discharges or light, do not measurably affect the glass of the
fibre,
but volatilize the thin metal/polyimide or other coating on the surface of the
fibre. Continuing application of discharge or light pulses results in
progressive
removal of the coating, for example in the region (11).
[0022] Fig. 5 shows the end of a fibre that has been stripped of its coating.
In
this case a polyimide coated fibre having a coating of a few microns thick was
used. Successive discharges were applied until the best conditions were found
to allow the coating to be stripped successfully.
[0023] During modification of fibre coating by the method of this invention,
it is
sometimes useful to monitor the surface visually as shown in Fig. 5, as
certain
coatings may be difficult to remove and for which a video camera may be used,
a technique which also falls within the scope of this invention.
[0024] Fig. 6 shows a photograph of a fibre that has been stripped of its
polyimide coating in the middle of a coated region using the technique
descried
in this invention.
[0025] By translating the optical fibre relative to the electrical-discharge
at the
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electrodes (6a, 6b) such that the coated section of the fibre enters or leaves
the
discharge area, subsequent sections of the optical fibre may be stripped
synchronously, thereby extending the region of the stripped fibre to an
arbitrary
length. Fig. 7 shows an extended stripped region using the technique of
translating the fibre. It is clear to a person skilled in the art that the
fibre needs
to move relative to the discharge or light, so that the fibre could for
example be
stationary and the electrodes are moved relative to the fibre.
[0026] Fig. 8 shows the schematic of the system used to modify extended
regions of the coating. The fibre (2) is held in a carriage formed by two
optical
fibre chucks (12) mounted on translation stages below (12), separated by a
distance (11) and linked with a rigid adjustable connector (14). The glide
rail
(13) allows the stages to move in a given direction perpendicular to the
direction of the discharge, so that the fibre remains in the discharge region
as
shown by the direction arrow (15). It should be understood that this invention
is
not limited to the specific embodiments described above but that various
modifications obvious to those skilled in the art, including the use of the
method
with optical fibres fabricated from polymer or from different glass
compositions,
may be made therein without departing from the scope of the following claims.
