CA2479863C - Modifying the coating on optical fibres - Google Patents

Abstract

This invention relates to the modifying or stripping of primary or secondary coatings on optical fibres by the application of heat such that the coating is entirely or partially removed from the surface over a given length of an optical fibre.

Description

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 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

[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.

[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 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 125-5 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 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 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 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.

Claims (11)

1 A method for removing at least part of a metal coating from an optical waveguide, said metal coating covering at least in part said optical waveguide, said method comprising:
- producing an electrical discharge substantially adjacent said metal coating;

and - heating said metal coating with said electrical discharge;
- wherein - heating said metal coating includes raising a temperature of said metal coating to a temperature high enough to volatilize, at least partially, said metal coating at a rate small enough to substantially prevent said optical waveguide from melting; and - producing said electrical discharge includes producing a pulsed electrical discharge, said pulsed electrical discharge including at least two pulses having each a predetermined duration, said at least two pulses being separated from each other by an inter-pulse interval.
- whereby at least partial removal of said metal coating from said optical waveguide is achieved.

2. A method as defined in claim 1, wherein producing said electrical discharge substantially adjacent said metal coating includes positioning said optical waveguide between two electrodes and applying a voltage between said two electrodes.

3. A method as defined in claim 2, wherein said electrodes are tungsten electrodes.

4. A method as defined in claim 2 or 3, wherein said optical waveguide defines a free end, said method including positioning said optical waveguides between said electrodes with said free end substantially adjacent said electrodes.

5. A method as defined in any one of claims 1 to 4, wherein heating said metal coating includes raising a temperature of said metal coating to a temperature high enough to remove substantially all of said metal coating along a portion of said optical waveguide.

6. A method as defined in claim 1, wherein said at least two pulses are square wave pulses.

7. A method as defined in claim 1 or 6, wherein said at least two pulses carry a current of from 1 mA to 500 mA.

8. A method as defined in any one of claims 1, 6 and 7, wherein said pulse duration is from 1 microsecond to 100 microseconds.

9. A method as defined in any one of claims 1 and 6 to 8, wherein said inter-pulse interval is between .1 second and one second.

10. A method as defined in claim 2, further comprising translating said optical waveguide and said two electrodes with respect to each other to heat different

11 portions of said metal coating along said optical waveguide.

11. A method as defined in any one of claims 1 to 10, wherein said optical waveguide is an optical fiber.