GB2214650A

GB2214650A - Forming a grating on an optical fibre by heat shrinking

Info

Publication number: GB2214650A
Application number: GB8800960A
Authority: GB
Inventors: Peter George Hale
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1988-01-16
Filing date: 1988-01-16
Publication date: 1989-09-06
Anticipated expiration: 2008-01-16
Also published as: GB2214650B; GB8800960D0

Abstract

An optical grating is formed on the surface of an optical fibre by winding on it a monofilament of a highly axially-oriented polymer. This is done either throughout the whole of the fibre length, or at the locations at which gratings are needed. Then the monofilament and fibre are heated at the point(s) at which grating(s) are needed. This makes the polymer de-orient, which in turn causes shrinkage which is maintained on cooling. The shrinkage causes undulations on the fibre, thus producing a grating, which can be part of a non-intrusive tap. <IMAGE>

Description

FORMATION OF A GRATING ON AN OPTICAL FIBRE This invention relates to forming optical gratings on optical fibres. Such gratings can be used to provide non-intrusive taps on the fibre.

The principle used to provide such taps on an optical fibre is to bend the fibre to force light from the fibre core into the cladding from which it may be extracted. In practice a single bend is not used as it would be too severe for a silica based fibre to survive. Hence a series of more mild undulations is used. This involves pressing a grating against the fibre and using a prism placed against the fibre to collect the radiating light. Reciprocity dictates that light may be injected into the fibre core by such a tap.

An object of the invention is to provide a grating on the outer surface of the fibre, thus producing the undulations required to allow light to leave the core or to be injected thereinto.

According to the invention there is provided a method of forming an optical grating on an optical fibre, which includes winding a filament of a heat shrinkable material around the fibre, and heating the filament and the fibre in the region at which the grating is to be formed, which heating causes shrinkage of the filament, which shrinkage is maintained on cooling and causes radial pressure on the fibre to produce periodic deformation of the fibre, thus producing the grating.

An embodiment of the invention will now be described with reference to the highly schematic accompanying drawings, which are not to scale.

The principle of the method defined above is to wind highly axially-oriented polymer monofilament of an appropriate diameter and elastic modulus around a primary coated fibre, under a low tension, see Fig. 1.

For an optical fibre whose outside diameter is 125t plus a protective coating to 250t diameter, a suitable diameter for the monofilament is 3OO. Note that these are metric microns. Suitable polymers for use in the present method are, for example, oriented polyamides.

The winding of the monofilament around the fibre can be done continuously along a fibre length prior to cabling, or at a chosen tapping point after exposing the fibre. The primary coating preferably has sufficient compliance to avoid unwanted periodic loads being applied to the fibre at this stage, as this may cause unwanted radiative loss. The pitch of the winding is preferably equal to, or approximately equal to, the diameter of the monofilament, as this simplifies the winding operation. Suitable materials for the primary coating, which overlays the immediate cladding of the fibre, include u.v. radiation cured acrylates.

When a tap is required, the fibre and helically wound monofilament are exposed and in the locality of the tapping point and heated to a temperature which causes the oriented polymer to de-orient. With the materials referred to above, a suitable temperature is

The previously aligned polymer chains then assume a random orientation, witn tne cnain ends closer together. This causes axial shrinkage, which is maintained on cooling. This axial shrinkage causes radial pressure to be applied to the fibre, and since the monofilament is helically wound, causes helical periodic pressure to be applied to the fibre. This in turn causes helical periodic distortion of the optical fibre, as can be seen from Fig. 2. Using appropriate materials and dimensions, as indicated above, this distortion will cause controlled radiative loss. Hence light is led out of the fibre via the cladding and the primary coating. Such light can be channelled away, e.g. by a prism with one face adjacent the fibre, to a photo-detector. By replacing the photo-detector by an optical source such as a laser, light may be injected into the fibre.

Thus the method described above leads to the production of a grating on the surface of the optical fibre, which can be used to provide a non-intrusive tap for that fibre.

Claims

CLAIMS:

1. A method of forming an optical grating on an optical fibre, which includes winding a filament of a heat shrinkable material around the fibre, and heating the filament and the fibre in the region at which the grating is to be formed, which heating causes shrinkage of the filament, which shrinkage is maintained on cooling and causes radial pressure on the fibre to produce periodic deformation of the fibre, thus producing the grating.

2. A method of forming an optical grating on an optical fibre, which includes winding a highly axially oriented polymer monofilament around the optical fibre, and heating the monofilament in the region at which the grating is to be formed, which heating causes the previously aligned polymer to take up random orientations with concomitant shrinkage, which shrinkage causes radial pressure on the fibre which pressure produces periodic deformation of the fibre, thus producing the grating.

3. A method as claimed in claim 2, and in which the monofilament is wound on the fibre throughout the whole length of the fibre.

4. A method as claimed in claim 2, and in which the monofilament is wound on the fibre only at the points along the length thereof at which gratings are to be formed.

5. A method as claimed in claim 1, 2, 3 or 4, in which the fibre with its outside coating has an outside diameter of 25(41, in which the monofilament has an outside diameter of 30at, in which the monofilament is of oriented polyamide, and in which the heating is to a temperature in the range of 120 - 15OOC.

6. A method as claimed in claim 5, in which the pitch of the winding is of the same order as the thickness of the monofilament.

7. A method of forming an optical grating on an optical fibre, substantially as described with reference to the accompanying drawings.

8. An optical grating made by the method of any one of the preceding claims.

9. A non-intrusive tap on optical fibre which includes an optical grating on the fibre, as claimed in claim 8, and means to feed light into, and/or extract light from, the optical fibre.