GB2210470A

GB2210470A - Inducing refractive index changes in localized regions of optical fibres

Info

Publication number: GB2210470A
Application number: GB8819466A
Authority: GB
Inventors: John Philip Dakin
Original assignee: Plessey Co Ltd
Current assignee: Plessey Co Ltd
Priority date: 1987-09-25
Filing date: 1988-08-16
Publication date: 1989-06-07
Also published as: GB8819466D0; GB8722615D0

Abstract

Localised changes in the refractive index of the core 2 of an optical fibre 1 are produced at a plurality of regularly spaced points along the fibre 1, by arranging for a plurality of minute localised regions of the core to be heated by a laser 4 and then allowed to cool in order to induce refractive index changes in the localised regions. <IMAGE>

Description

IMPROVEMENTS RELATING TO OPTICAL FIBRES This invention relates generally to methods of producing localised variations in the refractive index of an optical fibre at one or more points along the length of the fibre and relates more especially, but not exclusively, to such methods for producing in optical fibres refractive index gratings having predetermined longitudinal refractive index periodicities. Refractive index gratings having such longitudinal periodicities may be incorporated in interferometric sensors and also define variably reflective regions along their length which are strongly wavelength selective due to their different reflection coefficients. Such gratings may also have application in wavelength multiplexed sensor communication systems in view of their wavelength selectivity.

It is already known to produce refractive index gratings in optical fibres by photo-refractive processes using illumination of an optical fibre with low level light in such a way as to produce standing wave interference patterns in the optical fibre. After a long exposure, very small refractive index variations may be produced as a result of photo-refractive effects. The reasons for this are not at present well understood, but it is generally believed that electronic transitions occur in the glass, probably associated with non-bridging oxygen atoms. The major structure of the glass is not, however, affected by such processes which do not involve significant disturbance of the nuclear sites of the glass molecules.The standing waves may be produced from illumination of the side of the fibre by two parallel mutually-inclined beams of the same optical wavelength to set up a stationary interference pattern or they may be produced by arranging that the appropriate section of the optical fibre is illuminated by counter-propagating-beams guided by the optical fibre core. These known processes, however, are only capable of producing small variations in the refractive index.

The present invention comprises a method of producing localised changes in the refractive indexes of an optical fibre at points therealong in which one or more minute localised regions of the usual optical fibre core are arranged to be heated by laser means and then allowed to cool in order to induce refractive index changes in the localised regions.

In order to produce a predetermined decrease in the refractive index of the minute localised regions of the optical fibre the regions concerned may be melted by laser-absorption-induced fusion and then allowed to cool and soldify rapidly, the rate of cooling determining the level of decrease in the refractive index of the localised regions. The rapid cooling of the regions concerned and their relatively large changes in refractive index are possible due to the rapid heat loss from the heated regions to the surrounding bulk of the optical fibre core when the laser means is de-energised.

An increase in the refractive index of minute localised regions of the optical fibre, however, may be produced by heating predetermined minute regions of the optical fibre core close to the fusion point of the core material and then reducing the heat intensity slowly as by appropriate adjustment of the laser means to cause thermal annealing of the regions concerned. This slow cooling increases the refractive index of the minute localised regions above that of adjacent regions of the optical fibre core.

For the purpose of providing an optical fibre having longitudinal refractive index periodicity which may be utilised in optical interferometer sensors, the intensity of the light output from the laser means which may be arranged to focus the light output therefrom on to a minute region of an optical fibre core may be modulated whilst the optical fibre and laser means are moved relatively to one another. The optical fibre is preferably moved relative to the laser means and it may be moved longitudinally at constant velocity or it may be moved incrementally in constant steps.

As the laser means and optical fibre move relatively to one another the variations in intensity of the light output from the laser means caused successive minute localised regions along the optical fibre core to have their refractive indexes reduced or increased, as the case may be, in accordance with the instantaneous intensity of the laser means output and in dependence upon the particular method of varying the refractive index employed, that is to say whether the glass material at the localised regions of the fibre core is actually fused and then cooled rapidly or is simply heated near to the fusion point of the core material and then allowed to cool slowly to enable thermal annealing to take place. A combination of both index varying methods may be used to provide longitudinally spaced regions of lower and higher refractive indexes.

Instead of moving the optical fibre relative to the laser means to provide the longitudinal refractive index periodicity in the optical fibre core, the laser means may be of relatively high power and arranged to illuminate the length of the optical fibre over which the periodicity is to be induced with a spatially varying light intensity pattern. This pattern may be produced along the core of the fibre by means of a mask through which light from the laser means passes before being focussed on to the core by optical lens means. The mask may comprise a ruled diffraction grating, slotted opaque plate or transparent plate having opaque strips thereon.

The periodicity may alternatively be produced by interference between two or more planar light beams at different angles. In the latter case the beams would be sufficiently intense to cause fusion or a glassy-viscous transformation of the minute localised core regions, which on subsequent cooling undergo relatively permanent changes in refractive index which are greater than those achievable by causing minor electronic transitions to occur as has heretofore been the practice.

In order to focus the output energy of the laser means on to the minute localised regions of the optical fibre core so as to cause preferential absorption in that region, it is advantageous to use a laser having a wavelength selected to result in relatively high absorption in the core region, which may, for example comprise germanium or phosphorus - doped silica, but relatively low absorption thereby giving rise to relatively low losses, in the usual core cladding (eg. silica) or any polymer sheathing material with which the optical fibre may be coated for mechanical strength and protection.

By way of example the present invention will now be described with reference to Figures 1, 2(a) and 2(b) and 3 of the diagrammatic accompanying drawings.

Referring to Figure 1 of the drawings, this shows a greatly enlarged cross-sectional view of an optical fibre 1, having a core 2, which may be composed of germanium or phosphorus doped silica, for example, and a cladding 3 which may comprise silica.

According to the invention, a laser 4 (eg. semiconductor laser) may be electrically energised from an electrical signal source 5 to provide a light beam output 6 which may be focused by means of a convex lens 7 on to a minute localised region 8 of the optical fibre core 2. The heat intensity of the laser output may melt the core material in the region 8 at temperature 15000C to 2000 C, the level of energy absorption by the core material by the selection of the laser wavelength being high relative to the energy absorption of the cladding material so that the energy beam suffers relatively low losses in passing through the cladding 3 thereby selectively concentrating the heat energy into the core region 8. After the core material of the region 8 has been melted, the laser is de-energised so that the region 8 will cool rapidly by absorption of the heat by the surrounding bulk of core material . This fusion, followed by rapid cooling of the region 8, causes the refractive index of the region to be significantly decreased.

Alternatively, the level of heating of the region 8 may be arranged to be kept just below the fusion point (i.e. transformation point) and the region may be cooled gradually, as by progressively reducing the heat intensity of the laser 4. In this way the material of the region 8 will be thermally annealed, whereby the refractive index of the region will be increased relative to the surrounding core material.

In order to produce periodic variations in the refractive index of the optical fibre core 2 along the length of the fibre 1, the latter may be moved relative to the laser 4 in the direction A indicated.

This movement of the optical fibre may be continuous, or it may be incremental with the fibre being moved successively by equal steps.

In this way minute localised regions spaced along the optical fibre core may be provided with varying refractive indexes simply by varying the intensity of the laser output fag pulse output) since the refractive index of the localised regions depends both upon the intensity of heating and the rate of cooling of the region after heating, the index being decreased or increased according to whether the core is actually melted and rapidly cooled or thermally annealed by slow cooling.

Referring now to Figures 2(a) and 2(b) of the drawings, in this arrangement planar light beams 10 and 11 derived from suitably energised two lasers 12 and 13 are directed on to an optical fibre core 15 of an optical fibre 16 having cladding 17 so that the planes of the beams are orthogonal relative to one another in the radial direction of the optical fibre 16 (see Figure 2(b)).

In this case the two beams 10 and 11 from the lasers 12 and 13 will interfere with one another in order to cause a spatially varying pattern of heat intensity at longitudinally spaced regions 18, 19, 20 and 21, for example, of the optical fibre core. When the lasers are switched off after heating the core regions above the fusion temperature of the core material, the rapid cooling of the regions will produce varying refractive indexes at the spaced regions 18 to 21.

Referring to Figure 3 of the drawings this shows an alternative arrangement for producing a spatially varying light intensity pattern along the core 22 of a fibre 23. A semiconductor laser 24 is driven from an electrical signal source 25. The light output from the laser 24 falls on a mask 26 which may comprise metal strips 27 applied to a glass plate 28. The light output from the mask 26 is then focussed by a convex lens 29 on to the core 22 so as to heat localised regions along the core and thereby produce changes in the refractive index of these regions as cooling takes place in the manner previously described.

The method of the invention as described enables larger index variations in refractive index to be achieved as compared with those achievable using electrical transitions, as previously described, and, moreover, the refractive index variations achieved are of a more permanent nature.

Claims

1. A method of producing localised changes in the refractive index of the core of an optical fibre at a plurality of regularly spaced points along the fibre, in which a plurality of minute localised regions of the optical fibre core are arranged to be heated by laser means and then allowed to cool in order to induce refractive index changes in the localised region.

2. A method as claimed in claim 1, in which the minute localised regions of the optical fibre core are melted as a result of laserabsorption induced heating and then allowed to cool and solidify rapidly in order to produce a decrease in the refractive index of the localised region.

3. A method as claimed in claim 17 in which the minute localised regions of the optical fibre core are heated by the laser means to a + tnneranlre close to the melting point of the core 'material and the the laser intensity at the point(s) of heating is reduced slowly to cause slow cooling of the heated regions resulting in thermal annealing of the regions concerned so that the refractive index of the minute localised regions are increased above that of adjacent regions of the optical fibre core.

4. A metnoa as claimed in claim 1, in which the localised regions of the optical fibre are in heated sequentially.

5. A method as claimed in claim 4, in which the refractive index of a multiplicity of points along the core of the optical fibre is changed to provide the core with longitudinal refractive index periodicity by focussing the light output from the laser means on to a minute region of the optical fibre core and by modulating the laser light output whilst the optical fibre and laser means are moved relatively to one another, whereby minute localised regions along the optical fibre core have their respective refractive indexes changed.

6. A method as claimed in claim 5, in which the optical fibre is moved at constant velocity relatively to the laser means.

7. A method as claimed in claim 1, in which the refractive index of a multiplicity of points along the core of the optical fibre is changed to provide the core with longitudinal refractive index periodicity by arranging that light from the laser means illuminates the length of the optical fibre core over which refractive index changes are to be made with a spatially - varying light intensity pattern having a series of relatively high intensity zones causing localised relatively intense heating of the core regions.

8. A method as claimed in claim 7, in which the spatially - varying light intensity pattern is produced along the core of the fibre by means of a mask through which light from the laser means passes before being focussed on to the core by optical lens means.

9. A method as claimed in claim 7, in which the laser means provides two planar beams of light at different angles in order to illuminate a length of optical fibre core over which changes in refractive index are to be made and which interfere with one another effectively to provide a spatially - varying light intensity pattern which in turn produces changes in the refractive index at points along the optical fibre core by melting the core material at said points and then cooling the core material rapidly to provide a decrease in the refractive index of the core.

10. A method as claimed in claim 7, in which the laser means provides two planar beams of light at different angles in order to illuminate a length of optical fibre core over which changes in refractive index are to be made and which interfere with one another effectively to provide a spatially - varying light intensity pattern which in term produces changes in the refractive index at points along the optical fibre core by heating the core at said points to near melting point and then allowing the core material to cool slowly in order to provide an increase in the refractive index of the core at said points.

11. A method as claimed in claim 1, in which the light output from the laser means is selected to have a wavelength which exhibits relatively low absorption in the usual cladding material surrounding the fibre core but which has relatively high absorption in the core material itself.

12. A method of producing localised changes in the refractive index of the core of an optical fibre at points therealong substantially as hereinbefore described with reference to the accompanying drawings.