GB1600010A

GB1600010A - Optical fibre couplings

Info

Publication number: GB1600010A
Application number: GB53436/77A
Authority: GB
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1977-12-22
Filing date: 1977-12-22
Publication date: 1981-10-14
Also published as: FR2412863A1; IT7830863A0; AU4253478A; FR2412863B1; IT1192600B; NL7811931A; DE2854496A1; JPS54109854A

Description

(54) OPTICAL FIBRE COUPLINGS (71) We, STANDARD TELEPHONES AND CABLES LIMrrED,a British Company of 190 Strand, London,W.C.2. England, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to optical fibre couplings, and in particular to the coupling of glass optical fibres that have no internal optical waveguiding structure within the glass but instead rely upon the use of lower refractive index plastics cladding to provide the requisite optical guidance.

In general it is found that such fibre is liable to lie appreciably off the axis of the cladding, and the cladding material tends to be mechanically unsatisfactory, and hence any method of coupling that proposes to align the fibres by bringing their claddings into alignment is liable to be unsatisfactory. It is therefore preferred to align the fibres themselves having first stripped the cladding to expose a short length of bare fibre. The disadvantage of this is that light is liable to leak away from the stripped fibre if it is allowed to come into contact with dielectric material of higher refractive index.

This problem may not be too severe if the fibre itself has a relatively high refractive index, however one of the materials otherwise found to be particularly suitable for glass fibre with no internal waveguiding structure is silica, and silica has a low refractive index compared with most other glasses and other rigid dielectric materials.

According to the present invention there is provided an optical fibre coupling between a pair of glass optical fibres that have no internal waveguiding structure within the glass, wherein the fibre ends are held in axial alignment by being fitted into opposite ends of a rigid dielectric sleeve wherein the bore of the sleeve is provided with a lining layer that acts as a waveguiding layer for the inserted fibre ends.

This lining layer may be a metallic layer that of itself provides a reflecting surface, or it may be a dielectric layer of lower refractive index than that of the fibres. In the latter instance the dielectric lining layer may be made of a nonrigid plastics material, sufficient rigidity of the composite structure being provided by the rigid sleeve. Alternatively the dielectric lining layer may be a glass layer. In this instance the presence of the lining layer lessens the amount of low index glass required. This is because the sleeve is required for its mechanical properties rather than its optical properties, and thus the lining layer used need only be thick enough to provide the requisite optical guidance, whereas much more of the low index glass would be required if it were to be made mechanically robust enough for the sleeve to be safely dispensed with.

Embodiments of the invention will now be described with reference to the drawings accompanying the Provisional Specification in which: Figure 1 depicts one form of optical fibre coupler Figure 2 depicts an alternative form of optical fibre coupler, and Figure 3 depicts an optical fibre coupling assembly incorporating the coupler of Figure 2.

A pair of plastics clad silica fibres 1 and 2 that have no internal waveguiding structure in the silica have the plastics cladding 3 stripped back to expose a short length of bare fibre at each end. These ends are fitted into opposite ends of a glass sleeve 4 whose refractive index is not less than that of the fibres 1 and 2. The tube ends are flared so as to be large enough to permit entry of the plastics cladding 3 into the end sections of the sleeve, while its central section is small enough to hold the bare fibres in axial alignment. The bore of the glass sleeve is provided with a lining layer 4a that acts as a waveguiding layer for the inserted fibre ends.

According to one embodiment this layer 4a is a metallic reflecting layer and according to another embodiment it is a dielectric layer of material lower refractive index than that of the silica of the fibres 1 and 2.

The reason for requiring this layer 4a is that, in its absence, light propagating in the fibres would leak directly into the main body of the sleeve 4 in the regions where the bare fibres are in contact with the sleeve, and also leak into it by means of frustrated total internal reflection in regions where the sleeve is extremely close to the fibres.

A convenient way of providing a metallic It ear 4a is bY the reduction of an unstable silver salt solution, such as silver nitrate in ammonium hydroxide, with for instance Rochelle Salt. The layer may be deposited upon the bore of the sleeve by forcing the mixture through the sleeve. If necessary the resulting deposited layer may then be polished by forcing though a fine suspension of a suitable polishing medium. An alternative way of providing the metallic layer is by evaporation of an aluminium coating from the surface of a fine wire passed through the sleeve.

A preferred way of providing a dielectric layer 4a is by deposition of a fluorine doped silica layer by a chemical vapour reaction. This deposition is performed on the bore of larger diameter tubing which which is then drawn down to the appropriate size for the sleeve.

The preferred deposition method is one in which silicon tetrafluoride, silicon tetrachloride, and oxygen are passed through the tube and caused to react in a hot zone created by localised heating of the exterior of the tube.

Such a deposition method is described in greater detail by for instance KAGE in the paper entitled 'Fluorine doped silica for optical waveguides' pages 59 to 61 of 'Second European Conference on Optical Fibre Communication', 27-30 September 1976, Paris.

In order to reduce unwanted reflections at the fibre ends these may be provided with reflection suppression interference coatings or moth's eye coatings as described in our Patent Specification No. 1 558 689. Alternatively reflection losses may be reduced by use of an index matching medium 5 in the form of a fluid that remains liquid or one that cures to form a cement. For this purpose the sleeve is filled with the medium 5, and the excess forced out by the insertion of the fibres 1 and 2.

The diameter of the bore of the sleeve at its central region is relatively critical, and should be a sliding fit around the fibres. If it is too small, the fibres are not able to enter far enough into the sleeve; whereas if it is too large, the fibre ends are not held positively in axial alignment. A convenient way of ensuring the correct size is to collapse the central region of the coated sleeve on to a short linke piece of silica fibre having the same diameter as the two fibres 1 and 2. The use of such a link piece is depicted at 6 in Figure 2. The collapse of the sleeve on to the link piece may be effected by heating the sleeve to soften it and allow the sleeve to collapse on to the link piece by the effects of surface tension. By using a localised heat source it is possible to arrange for the collapse to start near the mid-point of the link piece and then to progress in a controlled out toward its ends. In this way the bore immediately beyond the ends of the link piece can be matched very closely with the diameter of the fibres 1 and 2. Preferably any residual spaces between the ends of fibres 1 and 2 and the ends of the link piece are filled with an index match- ing medium 7 in the form of a fluid that remains liquid or one that cures to form a cement. The disadvantage of this collapse of the sleeve on to a link piece is that the lining is provided prior to collapse and hence has to be able to withstand the temperatures encountered in the collapse process. Moreover it is necessary to ensure that the collapse is effected without serious distortion of the link piece.

In certain constructions of glass optical fibre with no internal waveguiding structure within the glass the surrounding cladding of lower refractive index plastics material is relatively thin, and is itself surrounded by a secondary plastics coating which provides mechanical protection. A typical fibre of this construction has a 150 pm diameter core, the external diameter of the lower refractive index cladding layer is 250 pm while that of the secondary plastics coating layer is 1.0 mm. The core may be made of silica, the cladding of a silicone resin, and the secondary plastics coating of polypropylene. In making a coupling between a pair of secondary coated fibres it is convenient to secure the fibres together by their secondary coatings. To this end a sleeve of the type previously described with reference to Figure 1 or 2 may be located in the bore of a larger tubular member whose ends are secured around the ends of the secondary coatings of the two fibres. Such an arrangement is depicted in Figure 3.

Referring to Figure 3, the principal components of a coupling assembly are the sleeve 4, complete with its coating 4a, and optionally with a link piece 6, two pierced watch jewels 13 and a tubular member which in this instance is made in three portions, a central portion 14 and two end portions 15 and 16.

The central portion 14 of the tubular member has a bore in which the sleeve 4 is a sliding fit. The two ends of the central portion 14 are counterbored to receive the watch jewels 13 which are push-fitted into the ends with their oil cups facing outwardly. The jewel aperatures are chosen to be smaller than the entry apertures of the sleeve so that there is an unobstructed passage from each jewel into each sleeve aperture. Secured to the central portion are the two end portions 15 and 16 of the tubular member. These end portions have bores that are a sliding fit around the secondary plastics coatings 8 of fibres 1 and 2 complete with their lower refractive index plastics cladding layers 3.

The ends of the fibres 1 and 2 are exposed by stripping back the secondary plastics coatings 8 and the claddings 3. With coatings of polypropylene and claddings of silicone resin this may be achieved mechanically using a suitably adjusted wire stripping tool. Instead of mechanically stripping the silicone resin, it may alternatively be removed by dissolving it in sulphuric acid. The coatings and claddings are stripped back for a distance just sufficient to allow the bared ends to be inserted fully into the ends of the sleeve 4. The two ends of the sleeve 4 are filled with the index matching cement or liquid 7 and then the fibre ends are inserted. This insertion is relatively straight forward since the fibres are brought into approximately correct alignment by the engagement of their secondary coatings within the bores of the end portions 15 and 16 of the tubular member. This approximate alignment is either sufficient to cause the fibre ends to pass straight through the apertures in the jewels, or it is at least sufficient for the ends to land on the curved surfaces of the oil cups. In the latter instance the polish in the cups is sufficient for them to act as guides guiding the fibre ends through the jewel apertures and into the ends of the sleeve 4. Then, when the fibres are fully inserted, the ends of the tubular member are secured to their secondary plastics coatings for instance by means of a crimp as depicted at 1 5a or by means of a fillet 1 6a of cement.

If the coated fibres are secured in the tubular member by cementing them it may conveniently be made entirely of any suitable hard plastics material with good machining properties such as acrylic resin. If the coated fibres are to be secured by crimping them at least the end portions of the tubular structure will be made of a suitable metal such as steel.

The coupling assembly of Figure 3 shows the situation in which the cladding 3 protrudes far enough beyond the stripped back portions of the secondary coatings 8 for the cladding 3 to pass through the jewel apertures and enter the apertures of the sleeve. In an alternative arrangement (not shown) the cladding 3 is stripped back further because the jewel apertures are too small to admit the cladding.

This allows the use of a sleeve with a smaller flare to its bore. The disadvantage of this is that the bare fibre comes very close to, or touches, the material of the jewel. Normally the jewel will have a higher refractive index than the bare fibre and hence some light will be coupled out of the fibre at this point. However the amount of light loss will normally be small because of the smallness of the axial length of the aperture. In cases where the adhesion between the coating and the fibre is relatively small the jewel itself may be used to push back instead of strip the last few millimetres of cladding. This pushing back of the cladding produces a build up of material in the oil cup which firstly helps to seal off the ends of the sleeve and secondly may help to centralise the bare fibre within the jewel aperture and reduce the risk of it coming to rest in contact with the aperture wall.

It should be noted that although reflection losses would be reduced by using an index matching medium 5,7 that has an index of refraction only marginally larger than that of the fibres 1 and 2, such a medium would not be suitable because light would tend to be coupled into the medium from the fibres. For this reason the refractive index of the medium must be at least marginally less than that of the fibres. Preferably it is less by an amount that provides the region with a numerical aperture as large as the effective numerical aperture of the fibres being coupled. In calculating the effective numerical aperture it will normally be necessary to take account of the fact that a fibre with a cladding that is more lossy than its core will more heavily attenuate the higher order modes than the lower ones. One preferred material for the index matching medium is the silicone resin used for the fibre cladding.

This is applied in a mixed but uncured state, and is allowed to cure completely once the fibres have been inserted fully into the sleeve.

At the end faces of the fibres reflection losses are relatively well suppressed because the refractive index difference is small while at the exposed curved surfaces of the fibre in contact with the medium it will be large enough to guide all the modes propagating in the fibre.

WHAT WE CLAIM IS: 1. An optical fibre coupling between a pair of glass optical fibres that have no internal waveguiding structure within the glass, wherein the fibre ends are held in axial alignment by being fitted into opposite ends of a rigid dielectric sleeve, and wherein the bore of the sleeve is provided with a lining layer that acts as a waveguiding layer for the inserted fibre ends.

2. A coupling as claimed in claim 1 wherein the optical fibres are made of silica.

3. A coupling as claimed in claim 1 or 2 wherein the lining layer is a metallic reflecting layer.

4. A coupling as claimed in claim 3 wherein the lining layer is a silver layer.

5. A coupling as claimed in claim 4 wherein the silver layer is deposited by reduction of a silver salt.

6. A coupling as claimed in claim 3 wherein the lining layer is an aluminium layer.

7. A coupling as claimed in claim 6 wherein the aluminium layer is deposited by evaporation.

8. A coupling as claimed in claim 1 or 2 wherein the lining layer is a dielectric layer of material having a lower refractive index than that of the pair of fibres.

9. A coupling as claimed in claim 8 wherein the lining layer is a glass layer deposited by a chemical vapour reaction.

10. A coupling as claimed in claim 9 wherein the lining layer is made of a fluorine doped silica.

11. A coupling as claimed in any preceding

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. may be achieved mechanically using a suitably adjusted wire stripping tool. Instead of mechanically stripping the silicone resin, it may alternatively be removed by dissolving it in sulphuric acid. The coatings and claddings are stripped back for a distance just sufficient to allow the bared ends to be inserted fully into the ends of the sleeve 4. The two ends of the sleeve 4 are filled with the index matching cement or liquid 7 and then the fibre ends are inserted. This insertion is relatively straight forward since the fibres are brought into approximately correct alignment by the engagement of their secondary coatings within the bores of the end portions 15 and 16 of the tubular member. This approximate alignment is either sufficient to cause the fibre ends to pass straight through the apertures in the jewels, or it is at least sufficient for the ends to land on the curved surfaces of the oil cups. In the latter instance the polish in the cups is sufficient for them to act as guides guiding the fibre ends through the jewel apertures and into the ends of the sleeve 4. Then, when the fibres are fully inserted, the ends of the tubular member are secured to their secondary plastics coatings for instance by means of a crimp as depicted at 1 5a or by means of a fillet 1 6a of cement. If the coated fibres are secured in the tubular member by cementing them it may conveniently be made entirely of any suitable hard plastics material with good machining properties such as acrylic resin. If the coated fibres are to be secured by crimping them at least the end portions of the tubular structure will be made of a suitable metal such as steel. The coupling assembly of Figure 3 shows the situation in which the cladding 3 protrudes far enough beyond the stripped back portions of the secondary coatings 8 for the cladding 3 to pass through the jewel apertures and enter the apertures of the sleeve. In an alternative arrangement (not shown) the cladding 3 is stripped back further because the jewel apertures are too small to admit the cladding. This allows the use of a sleeve with a smaller flare to its bore. The disadvantage of this is that the bare fibre comes very close to, or touches, the material of the jewel. Normally the jewel will have a higher refractive index than the bare fibre and hence some light will be coupled out of the fibre at this point. However the amount of light loss will normally be small because of the smallness of the axial length of the aperture. In cases where the adhesion between the coating and the fibre is relatively small the jewel itself may be used to push back instead of strip the last few millimetres of cladding. This pushing back of the cladding produces a build up of material in the oil cup which firstly helps to seal off the ends of the sleeve and secondly may help to centralise the bare fibre within the jewel aperture and reduce the risk of it coming to rest in contact with the aperture wall. It should be noted that although reflection losses would be reduced by using an index matching medium 5,7 that has an index of refraction only marginally larger than that of the fibres 1 and 2, such a medium would not be suitable because light would tend to be coupled into the medium from the fibres. For this reason the refractive index of the medium must be at least marginally less than that of the fibres. Preferably it is less by an amount that provides the region with a numerical aperture as large as the effective numerical aperture of the fibres being coupled. In calculating the effective numerical aperture it will normally be necessary to take account of the fact that a fibre with a cladding that is more lossy than its core will more heavily attenuate the higher order modes than the lower ones. One preferred material for the index matching medium is the silicone resin used for the fibre cladding. This is applied in a mixed but uncured state, and is allowed to cure completely once the fibres have been inserted fully into the sleeve. At the end faces of the fibres reflection losses are relatively well suppressed because the refractive index difference is small while at the exposed curved surfaces of the fibre in contact with the medium it will be large enough to guide all the modes propagating in the fibre. WHAT WE CLAIM IS:

1. An optical fibre coupling between a pair of glass optical fibres that have no internal waveguiding structure within the glass, wherein the fibre ends are held in axial alignment by being fitted into opposite ends of a rigid dielectric sleeve, and wherein the bore of the sleeve is provided with a lining layer that acts as a waveguiding layer for the inserted fibre ends.

4. A coupling as claimed in claim 3 wherein the lining layer is a silver layer.

11. A coupling as claimed in any preceding

claim wherein the ends of the fibres are provided with reflections suppression interference coatings.

12. A coupling as claimed in any claim of claims 1 to 10 wherein the ends of the fibres are provided with reflection suppression moth's eye coatings.

13. A coupling as claimed in any claim of claims 1 to 10 wherein the ends of the fibres are immersed in an index matching liquid or cement.

14. A coupling as claimed in claim 13 wherein the fibres are coated with a layer of material that is the same as said cement.

15. A coupling as claimed in any preceding claim wherein the sleeve is flared at both ends to facilitate insertion of the fibre ends.

16. A coupling as claimed in claim 15 wherein the bore of the sleeve has a middle portion within which the fibres are a sliding fit.

17. A coupling as claimed in claim 15 wherein the middle portion of the sleeve bore is obstructed by a link piece of optical fibre of the same diameter as that of said pair of fibres, on to which link piece the sleeve has been collapsed.

18. A coupling as claimed in any preceding claim wherein the fibres are provided with plastics coatings and wherein the sleeve is retained within the central region of the bore of a tubular member into the ends of which bore are received the ends of the plastics coatings.

19. A coupling as claimed in claim 18 wherein the ends of the plastics coatings are secured within the ends of the tubular member.

20. A coupling as claimed in claim 19 wherein the ends of the plastics coatings are secured within the ends of the tubular member by crimping.

21. A coupling as claimed in claim 19 wherein the ends of the plastics coatings are secured within the ends of the tubular member with cement.

22. A coupling as claimed in claim 18, 19, 20 or 21 wherein the sleeve is secured between two pierced watch jewels within the bore of the tubular member wherein the watch jewels are provided with oil retaining recesses and are arranged such that these recesses are outwardly facing.

23. A coupling as claimed in claim 22 wherein the fibres are provided with primary and secondary plastics coatings and wherein the primary coating is a sliding fit through the jewel apertures.

24. A coupling as claimed in claim 22 wherein the fibres are a sliding fit through the jewel apertures.

25. An optional fibre coupling substantially as hereinbefore described with reference to Figures 1, 2, or 3 of the drawing accompanying the Provisional Specification.