GB2186992A - Preparing optical fibres for splicing - Google Patents

Abstract

A plastics coated optical fibre is prepared for splicing or termination by stripping the plastics from the fibre, optionally bonding hydroxyl groups to the fibre surface, and providing a coating on the surface whereby the strength of the fibre adjacent the fibre end is maintained or enhanced. Preferably the plastics coating is removed by plasma etching using O2 and/or CO2 as the reactant gas and optionally Cl2 or F2. H2 may be admitted to the plasma to render the fibre surface saturated with OH groups. The strengthening coating may be provided by TiCl4 vapour, silane and N2, or alloys. Alternatively, the plastics coating may be solvent stripped and the OH groups provided by an aqueous solution.

Description

SPECIFICATION Preparing optical fibres for splicing This invention relates to a method and an preparing optical fibre ends for splicing or termination ofthe fibres.

When coated optical fibres are to be spliced orterminated it is generally necessary to remove the coating from the fibre ends. This exposes the fibre to the risk of surface contamination. Further, it is known that the strength of an optical fibre splice is variable and, in general, lowerthan that of the fibre itself.

Under stress a spliced fibre fractures typically a few millimetres away from the natural joint. This fracture area remains solid throughout the splicing oper- ation. It is thought that the primary cause ofthis phenomenon is the change in the radial and axial stress pattern as a result of the different thermal and mechanical history of the splice in comparison with the fibre as originally drawn. During the fusion splicing process axial surface stress may be produced in the fibre near the joint which thus becomes very sensitive to surface contamination. Such contamination can act in two ways. Diffusion into the surface can increase the thermal coefficient of expansion thereby further increasing the tensile stress, and the presence of impurities can generate or increase Griffith cracking.

The object of the invention is to minimise orto overcome this disadvantage.

According to one aspect of the invention there is provided a method of preparing a coated optical fibre for splicing or termination, the method including removing the coating from the fibre end, bonding hydroxyl (OH) groups to the exposed surface, and coating the exposed fibre surface with an inorganic material whereby the strength ofthefibre adjacent the fibre end is maintained or enhanced.

According to another aspect of the invention there is provided an apparatus for stripping coating material from a coated optical fibre, the apparatus including means for exposing the coated fibre under reduced pressure to a high frequencyglowdis- charge plasma containing an etchant gas.

The technique minimises the risk of surface contamination by depositing athin protective surface coating on the fibre immediately after removal of the original coating. This coating remains on the fibre during and after subsequent fusion splicing. Preferably the coating is removed via a plasma etching technique.

Embodiments of the invention will now be described with reference to the accompanying drawings in which: Figure lisa plan view of an apparatus fortreatment of optical fibres; Figure2 is a cross-sectional view of the apparatus of Figure 1; Figures 3 and 4 show details of the fibre seals of the apparatus of Figure 1 and 2; and Figures 5to 8 illustrate a fibre treatment sequence intheformation of afusion splice.

Referring to Figure 1 and 2, the apparatus includes a two part reactor vessel 11 having an inlet port 12 and an outlet port 13 for gaseous reactants. The two parts 1 lea, lit of the reactor, when assembled, form a gas tight seal therebetween to allow reduced pressure conditions to be provided within the reactor.

Grooves formed in the mounting surfaces of the reactor parts provide for sealing engagement of plastics coated optical fibres 1 4whose ends are to be stripped. To facilitate maintainance of this seal the mating faces of the reactor parts 1 lea, 1 1t may be coated with a silicone grease. The reactdr is formed from an insulating material, e.g. PTFE,that has a low absorbance at microwave frequencies.

The reactor 11 is partly surrounded by a microwave cavity 15 which cavity may also be of two-part construction. Microwave power is supplied to the cavity from a generator (not shown). The arrangement is such that reactant gas inlet through the port 12 passes through the centre of the cavity 15 where dissociation takes place to form a plasma. The plasma extendsfromthecavitytocontactthefibre ends thereby reacting with the plastics coating so as to remove the coating. Typically we employ oxygen, carbon dioxide or mixtures thereof as the reactant gas. The products of the gas reaction are removed through the outlet port 13 via a vacuum pump (not shown).

The contour of the shoulderfound in the fibre coat- ing by the etching process is influenced by the shape of the inner reactor surface adjacent the seal. Thus, a surface protuberance 41 (Figure 3) into the reactor produces a substantiallyrectangularshoulder, whereas a recess 42 (Figure 4) results in a tapered shoulder. For many applications a tapered shoulder is preferred.

In a typical process, fibre ends are secured between the reactor parts and the reactor chamber is pumped down to a pressure of 10 mtorr or less.

Oxygen, carbon dioxide or a mixture thereof is then admitted at a rate sufficient to maintain a pressure of 0.1 to 2 torr. Typically the pumping speed is 60to 100 1/min. Microwave power is then applied to the cavity to generate a plasma thus effecting stripping ofthe plastics material from the fibre ends. Using a reactor power of 100 Wwe have optimised stripping rates in excess of 20 microns/minute.

In some applications a chlorine orfluorinecontaining gas may be added in a concentration of about 1 to 10% to remove impurities on or within the plastics coating. Typical additives are fluorine, chlorine, boron trichloride and sulphur hexafluoride.

After stripping of the plastics coating is completed, hydrogen is admitted to the plasma in place of the oxidising gas. This produces a hydrophilicsurface which is substantially chemically saturated with hydroxyl (OH) groups. The plasma is switched off and the chamber is evacuated. The activated fibre surface is next provided with a protective inorganic coating. Typically titanium tetrachloride (TiCI4) vapour is admitted to the evacuated chamber. This material reacts with the surface hydroxyl groups to form -OTiCin groups attached to the surface. Subsequent evacuation of the chamber and backfilling with air replaces the chlorine atoms in these surface groups with oxygen.Auger examination has confirmed thatthistechnique produces titanium con centration of approximately one third of a monolayer corresponding to the estimated density of surface hydroxyl groups.

We have found that such a surface treatmentstabi- lises and passivates silica or quartz surfaces against impurity attack. For example, metallic aluminium films do not attack surfaces thus treated even at temperatures approaching the melting point ofalumi- nium.

Whilsttitanium tetrachloride has beenfound particularly effective for this purpose, other volatile metal halides and/or alkoxides may be employed.

In an alternative embodiment the fibre ends may be solvent stripped. The based fibre ends are then exposed to an aqueous solution whereby hydroxyl (OH) groups are bonded to the silica surface. The treated fibre ends may then be provided with a protective coating as previously described.

Afurthertechnique provides the fibre ends with a protective non-metallic coating. In this process the fibre ends are plasma stripped with an oxidising gas.

After removal ofthe organic coating silane is added to the oxidising gas. The oxidising gas is next replaced with nitrogen. Finally the gas is turned off and the chamber is backfilled with nitrogen. The effect of this sequence is to provide the fibre ends with a vary thin SiO2 interface layer and a Si3N4 surface layer.

Both layers are typically less than 1 nm thick.

Aftertreatment,the based fibre ends may befusion spliced by any ofthe standard techniques that are well known in the art. In this respect the reactor may be used as a separate piece of equipment or it mayform part of fibre splicing apparatus.

In an alternative embodiment the protective coating may comprise an oxynitride, e.g. silicon oxynitride. Such a coating is applied bythefollowing technique.

After stripping of the plastics coating, silane is added to the oxidising plasma. The oxidising gas is then switched off and replaced with nitrogen. The silane supply is next switched offand the microwave power is disabled. Finally the chamber is back-filled with nitrogen.

In afurther embodimentthe stripped fibre ends are coated with a material having a very lowcoeffic- ient ofthermal expansion. Typical of such materials are the alloys Al203.TiO2 with a coefficient close to thatofsilica, and 63HfO2.37TiO2with a coefficient close to zero. Starting compounds may be chosen from aluminium alkyls,titanium tetrachloride and hafnium butoxide. Coatings having a low thermal expansion coefficient partthefibre surface into a state of compression and thus enhance the fibre strength.

After su rface treatment of the fibre ends, they may befusionsplicedtogetherusing anyoftheknown techniques. In view of the relativelyshorttem- perature cycle during the splicing operation, only a thin coating is required to provide a chemically and physically inertsurface. This coating is advan tageouslybetween 1 and 50nm inthickness.Asthe protective coating is deposited on all exposed parts ofthefibre end, it may in some applications be advantageous to leave the fibre end after coating to provide an uncoated end surface to minimise reflectance losses.

Atypical fusion splicing sequence is shown in Figures 5 to 8 of the accompanying drawings. Afibre 51 (Figure 5) coated with a plastics material 52 is treated to strip the plastics material and expose the fibre end. thin protective coating 53 (Figure 6) is then deposited on the based fibre as previously described. Preferably the fibre end is cleaved to provide an uncoated end surface 54 (Figure 7). Finally the prepa redfibreendisfusionsplicedtoasimilarfibre81 (Figure 8), the splice being protected by 9 quantity of plastics material 55.

Claims (15)

1. A method of preparing a coated optical fibre for splicing or termination, the method including removing the coating from the fibre end, bonding hydroxyl (OH) groups to the exposed surface, and coating the exposed fibre surface with an inorganic materialwherebythestrength ofthefibreadjacent the fibre end is maintained or enhanced.

2. A method as claimed in claim 1 ,wherein the coating comprises an interface layer of silica and a surface layer of silicon nitride.

3. A method as claimed in claim 2, wherein said layers are each less than lnmthick.

4. A method as claimed in claim 1, wherein the coating comprises a titanium oxide, or a mixed oxide oftitanium and aluminium oroftitanium and hafnium.

5. A method as claimed in claim 1 or 4, wherein the coating is from 1 to 50 nm in thickness.

6. A method as claimed in claim 1,4 or 5, wherein the plastics coating is removed from the fibre by plasma etching.

7. A method as claimed in claim 6, wherein the plasma incorporates oxygen or an oxygen containing gas.

8. A method as claimed in claim 7, wherein the plasma incorporates from 1 to 10 volume % of a gas containing fluorine or chlorine.

9. A method of preparing an optical fibre end substantially as described herein with reference to and as shown in the accompanying drawings.

10. A method as claimed in any one of claims 1 to 9, and which includes the step of fusion splicing the fibreto asimilarfibre.

11. An optical fibre prepared or spliced by a method as claimed in any preceding claim.

12. An apparatus for stripping coating material from a coated optical fibre, the apparatus including means for exposing the coated fibre under reduced pressure to a high frequency glow discharge plasma containing an etchantgas.

13. An apparatus as claimed in claim 12, and comprising a two part housing between the two parts of which the fibre is mounted.

14. An apparatus as claimed in claim 13, wherein the cavity is provided with a recess in registerwith the fibre wherebythe plastics coating of the fibre is provided with a tapered profile.

15. An apparatus for stripping a coated optical fibre,which apparatus is substantially as described herein with reference to and as shown in Figures 1 to 40ftheaccompanying drawings.