GB2180232A

GB2180232A - Optical fibre

Info

Publication number: GB2180232A
Application number: GB08522720A
Authority: GB
Inventors: John Irven
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1985-09-13
Filing date: 1985-09-13
Publication date: 1987-03-25
Also published as: GB2180232B; DE3630223A1; GB8522720D0; AU6211386A; AU585060B2

Abstract

A method of manufacturing a rod preform from which polarisation maintaining optical fibre can be drawn, which method enables the achievement of stress anisotropy without geometrical asymmetry as in a conventional method. Following the formation of two orthogonal stripes of a stress cladding material in a substrate tube, a differential deposition process is employed first to fill the gaps between the stripes with inner cladding layer material and then to cover the filled gaps and exposed stripes with a uniform layer of inner cladding layer material. Core layers are subsequently deposited. During collapse of the substrate tube to a preform the asymmetric stress cladding causes stress anisotropy but without distorting the core layers as in the conventional method.

Description

SPECIFICATION Optical fibre This invention relates to optical fibre and in particular but not exclusively to methods of manufacturing polarisation maintaining optical fibre or rod preforms from which such fibre may be drawn.

According to one aspect of the present invention there is provided a method of manufacturing a rod preform including the steps offorming one or more strips of a first material internally of a substrate tube, and then performing a differential deposition process within the substrate tube with a second material whereby the second material presents a uniform geometry for subsequent deposition in the substrate tube.

According to another aspect ofthe present invention there is provided a method of manufacturing polarisation maintaining optical fibre, including the steps of depositing a stress cladding layer internally of a substrate tube, etching said layerwherebyto form two orthogonal stripes of stress cladding material separated by troughs, performing a differential deposition process whereby to fill the troughs with a further cladding material and to coverthefilled troughs and stress cladding material stripes with the further cladding material such that the further cladding material presents a uniform geometry, depositing core layer material on the further cladding material, collapsing thesubstratetube into a rod preform and drawing the rod preform into optical fibre.

According to a further aspect of the present invention there is provided a method of manufacturing polarisation maintaining optical fibre, including the steps of selectivelydepositing of selectively depositing two orthogonal stripes of stress cladding material internally of a substrate tube, performing a differential deposition process whereby to fill gaps and stress cladding material stripes with the further cladding material such that the further cladding material presents a uniform geometry, depositing core layer material on the further cladding material, collapsingthesubstratetube into a rod preform and drawing the rod preform into optical fibre.

Aconventional method of fabrication of polarisation maintaining optical fibre comprises the steps of depositing an outer cladding layer, at approximately 1 740 C, with a refractive index similar to siiica on the innerwall of a silica substratetube.A "stress" cladding layer of silica containing a high concentration of B203,Fand possibly GeO2 is then deposited on the outer cladding layer, this also being carried out at approximately 1 740"C. The stress cladding layer normally has a depressed refractive index relative to silica and a large expansion coefficient.Two orthogonal stripes (troughs) are then etched in the stress cladding layer, by SF6 at approximately 1400"C. This leaves two orthogonal stripes of the stress cladding layer on the outer cladding on the inner wall of the su bstrate tu be. A matched index inner cladding layer is then deposited to extend over the stripes of stress cladding layer and into the troughs. This layer is of a uniform thickness and thus "depressions" still persist in the vicinity of the troughs even after its deposition. A core layer of SiO2/GeO2 is subsequently deposited and the tube is then collapsed to form a rod preform which can be drawn into fibre in a conventional manner. During collapse the asymetric stress cladding layer transforms into two "lobes" which surround the core, each lobe being substantially in the shape of a "butterfly" or "bow-tie".The asymmetric stress induced in the core by these lobes causes stress birefringence and polarisation maintaining properties within the fibre.

This conventional method relies on the asymmetric stress cladding layer to generate anisotropic properties, however it can also lead to asymmetric core geometry during collapse due to the troughs still being present as depressions after deposition of the inner cladding layer.

The present invention thus proposes to fill the troughs prior to deposition ofthe inner cladding layer proper. Thus asymmetric layer geometry is restored before the core layer is deposited.

This may be achieved in that following etching of the "high " stress cladding layer to produce the troughs a differential deposition of "low" stress cladding material (inner cladding material) is performed. Thusthetroughs arefilled with inner cladding material and the stress cladding layer stripes and filled troughs are covered with an inner cladding layerstripes and filled troughs are covered with an innercladding layersoastorestoreuniform geometry. After collapse the asymmetric stress cladding layer still causes stress anisotropy, as with the conventionally manufactured fibre, but this is achieved without distorting the core.The resulting control of polarisation and other properties of the fibre is thus improved. Stress anisotropy in optical fibre preforms is thus produced whilst maintaining near symmetric deposit geometry, instead of the geometrical asymmetry of the conventional method.

The differential deposition of the inner cladding layer may be achieved in the conventional glass working lathe employed for manufacturing rod preforms by not rotating the substrate tube and so only heating it in the vicinity of the troughs whilst passing suitable reactant gases therethrough, thus only in the troughs is the cladding material first deposited, and subsequently rotating the substrate tube to provide the inner cladding layer proper. Stress anisotropy without geometric asymmetry is thus achieved, the effects ofanisotropicstress and geometry being separated by the method ofthe present invention.

Whereas troughs are described above as produced by etching a continuous deposited "High" stress cladding layer, their equivalents may alternatively be formed "automatically" as a result of selectively depositing stripes of "high" stress cladding material, inasimilarmannertothatdescribedforfillingthe troughs by "stripes" deposition of the cladding material into the troughs. Thus, in the glass working lathe the substrate tube is not rotated and is only heated where the stripes of "high" stress cladding material are to be formed, whilst passing suitable reactantgasestherethrough.Thegaps between the stripes, which gaps are equivalentto the etched troughs, are subsequently filled as part ofthe differential deposition princess.

Claims

1. A method of manufacturing a rod preform including the steps offorming one or more stripes of a first material internally of a substrate tube, and then performing a differential deposition process within the substrate tube with a second material whereby the second material presents a uniform geometry for subsequent deposition in the substrate tube.

2. A method of manufacturing a rod preform as claimed in claim 1 wherein the first material is a stress cladding material which is deposited as a continuous layerwithin the substrate tube and wherein two troughs are etched in the deposited stress cladding material layer whereby to form two orthogonal stripes of stress cladding material, which rod preform is for use in the manufacture of polarisation maintaining optical fibre.

3. A method of manufacturing a rod preform as claimed in claim 2, wherein the differential deposition process serves first to fill the troughs with the second material and then to coverthe filled troughs and the stripes.

4. A method of manufacturing a rod preform as claimed in claim 1, wherein the first material is a stress cladding material, wherein two orthogonal stripes of said stress cladding material are formed internally of the tube by selective deposition, which stripes are separated by gaps, and which rod preform is for use in the manufacture of polarisation maintaining optical fibre.

5. A method of manufacturing a rod preform as claimed in claim 4, wherein the differential deposition process serves first to fill the gaps with the second material and then to cover the filled gaps and the stripes.

6. A method as claimed in any one ofthe preceding claims including the step of depositing an outer cladding layer in the substrate tube priorto said first material stripe formation and wherein the subsequent deposition is that of a core layer.

7. A method as claimed in claim 6 as appendant to claim 2 or claim 4 including the step of collapsing the substrate tube, following said subsequent deposition, into a preform, the stress anisotropy required for polarisation maintenance being achieved without geometrical asymmetry of the core.

8. A method as claimed in claim 2 or claim 4 wherein the substrate tube is of silica, wherein the stress cladding material is of silica containing a high concentration of B203,F and possibly GeO2 and wherein the core layer is of SiO2/GeO2.

9. A method of manufacturing polarisation maintaining optical fibre, including the steps of depositing a stress cladding layer internally of a substrate tube, etching said layerwherebyto form two orthogonal stripes of stress cladding material separated by troughs, performing a differential deposition process whereby to fill the troughs with a further cladding material and to coverthe filled troughs and stress cladding material stripes with the further cladding material such that the further cladding material presents a uniform geometry, depositing core layer material on the further cladding material, collapsing the substrate tube into a rod preform and drawing the rod preform into optical fibre.

10. A method of manufacturing polarisation maintaining optical fibre, including the steps of selectively depositing two orthogonal stripes of stress cladding material internally of a substrate tube, performing a differential deposition process whereby to fill gaps and stress cladding material stripes with the further cladding material such that the further cladding material presents a uniform geometry, depositing core layer material on the further cladding material, collapsing the substrate tube into a rod preform and drawing the rod preform into optical fibre.

11. A rod preform manufactured by a method as claimed in any one of claims 1 to 8.

12. A polarisation maintaining optical fibre drawn from a rod preform manufactured by a method as claimed in any one of claims 1 to 8 or 11.

13. A polarisation maintaining optical fibre manufactured by a method as claimed in claim 9 or claim 10.

14. A method of manufacturing a polarisation maintaining optical fibre from a rod preform manufactured as claimed in claim 1 and substantially as herein described.