GB2041558A

GB2041558A - Light-guide terminations

Info

Publication number: GB2041558A
Application number: GB7940138A
Authority: GB
Original assignee: Raychem Corp
Current assignee: Raychem Corp
Priority date: 1978-11-29
Filing date: 1979-11-20
Publication date: 1980-09-10
Also published as: FR2443074A1; FR2443074B1; GB2041558B; DE2948157A1

Abstract

A polymer clad glass core wave guide 10 has a termination device at at least one end thereof to positively locate the waveguide core coaxially thereof and also to provide strain relief. Cladding 30 has a portion removed and replaced by polymine sleeve 40 and is thus centred by a second polymeric sleeve 54. <IMAGE>

Description

SPECIFICATION Improvements in or relating to optical waveguides The present invention relates to optical waveguides and more specifically to the type having a glass core and a polymeric cladding, such as PCS waveguides (polymer clad silica waveguides).

Optical waveguides of the type in question serve to transmit light at least in part by reflection at the interface between the polymeric cladding and the glass core and include both stepped index and graded index waveguides. Waveguides of this type offer considerable advantages over glass-clad glass core waveguides. For example, they are more resistant to ionizing radiation, they permit higher numerical apertures to be attained, they are more flexible and they are cheaper to manufacture. They do however suffer from the problem that they have hitherto proved extremely difficu It to terminate effectively.For example, it has proved difficult to optically finish the end surfae of the glass core, i.e. to provide the end surface of the core with an acceptably optically smooth surface, e.g. by controllabiy cleaving or more preferably polishing the core. In addition, in view of the lack of concentricity of the core within the cladding typical of this type of waveguide, optically acceptable low loss junctions via an optical connector, e.g. with another optical waveguide, with an optical emitter or with an optical receiver, have hitherto proved extremely difficult to achieve. The problems are particularly pronounced in the case of waveguides provided with a soft rubbery cladding as in the case of silicone elastomer claddings.

The present invention is directed to the problem of adequately terminating optical waveguides of the type in question.

Accordingly, the present invention provides an optical waveguide having a glass core and a polymeric cladding, at least one end portion of the glass core being free of cladding and sheathed with a termination device comprising a first sleeve, extending over the free end portion of the core and bonded to the glass surface thereof, having a refractive index lower than that of the glass surface and a second sleeve radially outwardly of and overlying the first sleeve and the end of the polymeric cladding whereby the end of the core free of cladding is positively located coaxially of the termination device and is strain relieved.

In order to ensure positive location and strain relief of the core, it is preferred that the materials of the first and second sleeves each have a greater 2% secant modulus at 230C than that of the polymeric cladding and more preferably have a 2% secant modulus at 23 C of at least 1000 Kg/cm2. The materials of the first and second sleeves are also preferably harder than that of the polymeric material as measured in terms of Shore hardness, e.g. the materials preferably have a significant hardness as measured on the Shore D scale.

The present invention also provides a method of terminating an optical waveguide having a glass core and a polymeric cladding, at least one end portion of the core being free of cladding, which comprises sheathing said end portion of the core free of cladding with a termination device as hereinbefore defined, either preformed or assembled in situ, whereby the free end of the core is positively located coaxially of the termination device and is strain relieved, and optically finishing the end surface of the core positively located within the termination device.

It is preferred that the termination device as hereinbefore defined is heat-recoverable and more preferably comprises a fusible polymeric first sleeve having a refractive index lower than that of the glass surface of the core to which it is to be bonded and a polymeric heat-recoverable second sleeve coaxially disposed about the first sleeve, the second sleeve being adapted to recover radially inwardly at a temperature at which the first sleeve is fusible to urge the fused first sleeve into contact with the glass core. Such a heat-recoverable termination device adapted for use with a waveguide of the type as hereinbefore defined also forms part of the present invention.

The properties and manufacture of heat-recoverable articles are known perse e.g. US Patents 2,027,962 and 3,086,242. Preferably the material of the first sleeve in the heat-recoverable termination, in fused form, is sufficiently low in viscosity as to wet the surface of the glass core to form a strong adhesive bond on contact, yet is sufficiently high in viscosity to remain uniformally disposed around the glass core.

In order to improve the strength of the adhesive bond between the first sleeve and the surface of the glass core of the waveguide, particularly in the case where the first sleeve is fusible and no other bonding agent is employed, it is preferred that a coupling agent particularly a silicone coupling agent be employed at the interface between the first sleeve and the glass core. Such a coupling agent may be coated onto the inner surface of the sleeve, and/or the outer surface of the glass core or, when the first sleeve is fusible, the coupling agent may be dispersed throughout the body of the sleeve.Examples of silane coupling agents are described in UK Patent No. 1,284,082 and in the trade bulletin "Silane Adhesion Promotor in mineral filled compositions", 1973 published by Union Carbide Corporation, Chemicals and Plastics, 270 Part Avenue, New York, USA, the disclosures of which are incorporated herein by reference. One preferred silane coupling agent is N-(2-amino-ethyl)-3 amino- propyl trimethoxysilane.

Preferably the waveguide cladding comprises a silicone elastomer or a fluoropolymer.

In general, the diameter of the glass core onto which the termination device is to be installed will be 1000 Fm or less, frequently 760,am or less, particularly in the range 5 to 6001lem, typical diameters being 5, 50,75, 125,200,250,400 and 600 Mm and accordingly this will govern the dimensions of the device employed. In relation to heat-recoverable termination devices, it follows that the internal diameter thereof after full recovery should, in general, be less than 1000calm.

Furthermore, in order to obtain satisfactory optical isolation of the core, the cladding and the first sleeve of the termination device should have a wall thickness of at least one, more preferably at least two, especially at least three, for example twelve, times the wavelength of the light to be transmitted through the waveguide.

Since most optical waveguides operate in a window (range of wavelengths of optimum transparency) of 600 to 1100 nm, this, inter alia, will determine the minimum thickness of the first sleeve of the termination device.

In general, it is preferred that the wall thickness of the first sleeve, after insulation, lies in the range 1 to 15 mm and more preferably in the range 1 to 5 mm.

Preferably the difference in refractive index between the glass core and the first sleeve of the termination device is at least 0.02. A preferred core material is silica glass having a refractive index of 1.46 and accordingly the refractive index of the first sleeve is preferably 1.44 or below, especially in the range 1.32 to 1.44.

A material having the preferred mechanical and optical properties for the first sleeve of the termination device is polyvinylidene fluoride, particularly in fusible, i.e. non-cross-linked, form.

A material having the preferred mechanical properties for the second sleeve of the termination device particularly when in heat-recoverable form is non-fusible, i.e. cross-linked, polyvinylidene fluoride.

As hereinbefore indicated, the first and second sleeves of the termination device may be assembled in situ on the waveguide core or may be preformed as a unit before application. When preformed, the first and second sleeves may be separately formed as discrete components and then assembled, e.g. by bonding together, or may be formed as an integral unit directly, e.g. by coextrusion.

It is preferred that the second sleeve of the termination device be greater in length than the first sleeve such that on installation, the first sleeve extends over the whole length of the end of the core free of cladding upto and into abutting relationship with the end of the polymeric cladding, the second sleeve extending over the junction so formed and overlying the polymeric cladding.

It is also preferred that the radially inner surface of the portion of second sleeve overlying the cladding is lined at least in part, with an adhesive, particularly a hot-melt adhesive, e.g. in the form of a band extending circumferentially around the cladding.

The optical waveguide according to the present invention may be provided with a jacket substantially over the length thereof to provide environmental protection, e.g. for mechanical protection purposes. In such a case, it is preferred that the waveguide be environmentally sealed within the jacket by the provision of a termination device as hereinbefore defined at each end thereof such that the second sleeve of each termination device also overlies and is sealingly bonded to an end of the jacket.

In bonding a termination device to an end of a waveguide free of cladding in accordance with the present invention, it has been found that the efficacy of the adhesive bond may be adversely effected by the presence of polymeric residues remaining after the removal of the polymeric cladding, particularly when the cladding is a silicone elastomer.

Whilst treatment of the stripped core end with solvents such as tetramethyl guanidine, aqueous hydrofluoric acid or a stripping solution available from Indust-R-Chem Laboratory, Richardson, Texas, USA under the tradename J-100, to remove to the extent possible the polymeric residue, and/or treatment of the core with an appropriate coupling agent, particularly a silane coupling agent, serve to enhance the adhesive strength of the bond between the termination device and the core, the surface of the core is nevertheless difficult to wet and accordingly optimum adhesive strength, i.e. to the degree exhibited by an analogous glass core which has never been in contact with the cladding, is seldom achieved.

The present invention accordingly further provides a method of preparing the surface of the glass core of a waveguide for termination as hereinbefore described which comprises stripping the cladding from at least one end of the core of the waveguide and pyrolysing the polymeric residues remaining on the surface of the stripped end of the core.

The pyrolysis of the polymeric residues preferably comprises a first heating step to embrittle the polymeric residues on the core facilitating their mechanical removal, e.g. by flaking off, and a second heating step at a higher temperature than the first step and which may be regarded as the pyrolysis proper, to render the core surface receptive to adhesive. Preferably, said first step involves heating the surface of the glass core to a temperature of at least 300 C, e.g. 31 5'C for a short period, e.g. of the order of 10 seconds. Preferably said second step involves heating the surface of the glass core to a temperature of at least 700 C, e.g. of the order of 760or, for a short period, e.g., of the order of 10 seconds.Although less preferred, said first heating step may be replaced with a solvent treatment step to at least soften and preferably substantially to dissolve the polymeric residues. The solvents hereinbefore described, particularly in the case of silicone elastomer clad quartz fibre, are appropriate.

After the pyrolysis treatment, the core is preferably wiped with an appropriate reagent to remove any loose particles of oxidised polymer. Isopropyl alcohol is an appropriate reagent for such treatment.

To determine if adequate heat treatment has taken place, all that is necessary is to make a termination with a treated fibre core, and then pull on the termination with a device such as an Instron tester until failure of the bond or fibre core itself. The termination can be made with a termination as hereinbefore described. Criteria have been developed to determine if the resulting bond is sufficiently strong. One is whether or not the bond formed is substantially as strong as the bond that can be formed with a fibre core that has never been clad. If this is the result, the heat treatment is deemed successful. Another criterion is to determine if the bond that is formed is stronger than the waveguide itself. If this is the result, then the heat treatment is deemed successful.

For 200 micron diameter quartz fibres clad with polydimethyl siloxane, it has been found that pre-heating to 315 C (600 F) for ten seconds to aid in the removal of the silicone residue and subsequently heating the exposed fibre core at 7600C (1400 F) for ten seconds, allows a satisfactory bond to be formed between the quartz core and a termination device.

Heating has been successfully done either with a hydrogen gas flame or with a radiant oven.

When heating, it is desirable to protect the cladding that is not removed from the fibre core from damage.

This can be effected with an insulating shield, such as a shield made from asbestos or other insulating material, or by wrapping the cladding with an insulating material, such as asbestos tape. When a hydrogen flame is used, it is possible to localize the flame so that protection for the unremoved cladding is not necessary.

The effectiveness of the heat treatment may be the result of removing the polymeric residue, e.g. silicone elastomer. However, it is believed that the heat treatment is effective because the polymeric residue is oxidized, thereby increasing the polarity and cross-linking of the residue. This causes the residue to be easier to bond to and stronger. For example, in the case of silicone elastomer, partial or complete oxidation thereof to silicon dioxide may result, which would have the same high surface energy as pristine fused silica. Thus preferably the heat treatment occurs in the presence of oxygen.

Embodiments of the invention will now be described by way of example with specific reference to the accompanying drawings, wherein: Figure 1 is a partial side cross-sectional view of an optical waveguide according to the invention.

Figure 2 is an end view of the waveguide shown in Figure 1 as seen through the lines 2 - 2.

Figure 3 is a partial cross-sectional view of an end portion of a polymer clad glass core optical waveguide prior to termination.

Figure 4 is a partial cross-sectional view of the waveguide of Figure 3 with a portion of the polymer cladding removed.

Figure 5 is a partial cross-sectional view of the waveguide shown in Figure 4 with a first polymeric sleeve disposed around the core.

Figure 6 is a partial cross-sectional view of the assembly shown in Figure 5 around which a heat-shrinkable second polymeric sleeve has been disposed.

Figure 7 is a partial cross-sectional view of the bonded structure formed by heating the assembly shown in Figure 6.

Figure 8 is a partial cross-sectional view of the bonded structure shown in Figure 7 after the end of the glass core has been given an optically smooth finish.

Figure 9 illustrates a presently preferred intermediate structure wherein the first and second polymeric sleeves are pre-bonded together to form a sub-assembly.

Figure 10 through 13 inclusive illustrate a variation of the method of the invention which forms the inner polymeric sleeve directly on the glass core through a dip coating process.

Figure 14 is a partial cross-sectional view of an environmentally sealed polymer clad glass fibre waveguide in accordance with the invention, and, Figure 15 is a partial cross-sectional view of an optical waveguide connector with the waveguide of the invention self-centred and bonded within the connector's pre-aligned bore.

Referring now generally to the several figures, wherein the same elements are referred to by the same reference numerals, and specifically to Figure 1, there is shown a portion of a stepped index optical waveguide 10 which has been terminated in accordance with the invention. The waveguide 10 is formed with a glass core 20 possessing a flat, optically smooth end surface 26. The glass core 20 is typically pure, fused silica. As such, it is essentially homogeneous, the refractive index thereof being constant radially thereof. The core 20 is surrounded along most of its length by a polymeric cladding 30 which possesses a refractive index numerically lower than the refractive index of the fused silica core 20 (which is typically 1.46). A portion of the cladding 30 has been removed and replaced with a first polymeric sleeve 40 bonded to the exterior surface of the core 20.A second polymeric sleeve 50 is telescopically disposed radially outwardly of and bonded to the exterior surface of the first sleeve 40. The first and second sleeves 40 and 50 possess walls which are of substantially uniform thickness when viewed through any plane normal to their longitudinal centerlines, whereby the glass core 20 is substantially centered with respect to the exterior surface of the second sleeve 50 as is shown in Figure 2.

The waveguide termination shown in Figure 1 is formed by a series of steps which are illustrated sequentially in Figures 3 to 8. Figure 3 depicts an end portion of a polymer clad glass core optical waveguide prior to termination. The end surface 25 of the glass core 20 is shown in a rough, unpolished condition.

Figure 4 depicts the waveguide of Figure 3 with an end section of the polymer cladding 30 removed exposing an end portion of the glass core 20. In a typical situation where the polymer cladding 30 is a silicone rubber material, the end portion of the cladding can be easily stripped off by the use of finger nails alone. The length of cladding which is removed is approximately equal to the length of the sleeve 40 which can be of any convenient length but is preferably about 1 centimeter long. Where a silicone polymer cladding 30 is used there is frequently left behind a silicone residue on the exterior cylindrical surface 24. Such a residue renders the surface 24 difficult to bond to and limits the strength of any bond formed thereto.Although the invention will provide satisfactory results, even in the presence of such a residue, superior results will be obtained if the surface 24 is prepared, e.g. by carefully cleaning with a solvent such as tetramethyl guanidine to remove the silicone residue or preferably by pyrolysis treatment (see Examples below).

Figure 5 shows the first polymeric sleeve 40 disposed around the core 20, one end 48 thereof abutting end 36 of the cladding 30.

In selecting a material for the sleeve 40 the following criteria are employed. First, the sleeve must be fabricated from a material which possesses a refractive index numerically lower than the refractive index of the exterior surface 24 of the glass core 20. The refractive index of pure fused silica is 1.460, whereas, the commercially available polymer of lowest refractive index is believed to be perfluorinated ethylene propylene copolymer which possesses a refractive index of about 1.338. Second, the difference between the refractive index of the cladding and the refractive index of the core determines the numerical aperture of the waveguide. Third, when using a pure fused silica core 20 which possesses a refractive index of about 1.460, a measurable signal loss results when the material selected for the first sleeve 40 possesses a refractive index greater than 1.440.Test results indicate that the sleeve should be fabricated from a material which possesses a refractive index that is numerically at least 0.02 lower than the refractive index of the exterior surface 24 of the glass core 20 with which the first sleeve 40 is to be used. As a practical matter, this minimum differential in indices of refraction is not a critical consideration as other factors suggest the use of polymeric materials which provide even larger index of refraction differentials. Afourth consideration in selecting a polymeric material for the sleeve 40 is that it be capable of forming a bond with the exterior surface 24 of the glass core 20 preferably in the presence of some polymer cladding residue such as, for example, a silicone polymer residue.Fifth, although the material selected should soften and fuse upon heating so as to be able to wet the surface 24, it should remain sufficiently viscous and uniformally disposed around the fibre core 20. A material which is compatible with the foregoing considerations is substantially non-cross-linked polyvinylidene fluoride. Cross-linking of the polyvinylidene fluoride to any extent inhibits the polymer from flowing and wetting the surface of the waveguide. This polymer possesses a refractive index of 1.42 and is sold under the Trademark "KYNAR" by Pennsalt Chemical Corporation, Philadelphia, Pennsylvania USA. To enhance the ability of Kynar to bond to glass, a silane coupling agent can be applied to the glass prior to bonding. This silane functions by providing reactive sites on which the Kynar can bond.An example of such a coupling agent is N-(2-amino ethyl)-3 amino propyl-trimethoxysilane, (Z6020) supplied by Dow Corning Corporation, Midland, Michigan, USA. In a modification, instead of application of the coupling agent to the glass, the coupling agent may be dispersed throughout the body of the first sleeve.

Figure 6 shows the heat-shrinkable polymeric sleeve 50 located about sleeve 40. One end 55 of sleeve 50 is preferably aligned with the end 45 of the sleeve 40 and the unpolished end 25 on the glass core 20.

Alignment of these surfaces is not critical as they will all be brought into alignment with one another subsequently.

The assembly shown in Figure 5 is then heated with a hot-air gun, an infra-red heating device or the like, thereby causing the sleeve 40 to soften and fuse and the second sleeve 50 to shrink radially driving the softened sleeve 40 into uniform circumferential contact with the exterior surface 24 of the glass core 20. The softened end 48 of sleeve 40 is simultaneously urged into intimate contact with the end of the cladding 30 filling any interstices therebetween. Although the sleeve 40 is sufficiently fused to wet and bond to the surface 24 it remains viscous thereby uniformly distributed about the core 20 to facilitate the centering of the core 20 with respect to the exterior surface 54 of the sleeve 50. Figure 7 shows the bonded structure formed by heating the assembly shown in Figure 6.After cooling, the structure formed by the core 20, the sleeve 40 and the sleeve 50, is relatively stiff, i.e. stiffer than the cladded core in the remaining parts of the waveguide.

How well centered the core 20 is with respect to the exterior surface 54 of the second sleeve 50 is a function of the initial uniformity of wall thickness possessed by the termination sleeves and the care used in heating them. For example, better centering is achieved when heat is employed uniformally around the sleeves.

Similarly, better core centering is obtained if the fibre is held vertically during the heat to shrink and bond procedure. It is to be understood that the method of the invention will provide an easy to handle low light loss waveguide termination entirely satisfactory for many applications even if the sleeve walls are not uniform and the heat to shrink and bond procedure is conducted carelessly. However, where sleeves with uniform wall thicknesses are used and care in heating is employed, applicants have been able to repetitively terminate waveguides with 200 micron diameter cores into a bonded structure approximately 1000 microns in diameter such that the core is centered within 25 microns. This centering capability allows waveguides terminated in accordance with the invention to be disposed and bonded into pre-aligned bores of inexpensive optical waveguide connectors.In many fibre optic transmission system, the signal losses associated with connections so made are fully acceptable. It is to be further understood that the exterior surface 54 on the bonded structure shown in Figure 7 need not be a right cylindrical surface. For example, to achieve better core centering within a pre-aligned bore of a waveguide connector, a portion of the surface 54 proximate the unpolished end 25 can be slightly tapered (cone shaped). When so configured, the terminated waveguide end can be pressed/wedged into a pre-aligned connector bore with less alignment-dependance on a close tolerance relationship between the inside diameter of the connector bore and the outside diameter of the terminated waveguide end. Such a tapered termination can be formed, for example, by the use of a first sleeve 40 which possesses a uniform wall at the end proximate the unremoved cladding and tapers into a thinner uniform wall at the end proximate the core end 25.

Figure 8 shows the bonded structure shown in Figure 7 after the end of the glass core 20 has been given an optically smooth finish which is shown as polished end surface 25. During the polishing operation the ends 45 and 55 of the sleeves shown in Figure 7 have been made substantially flush with the polished end surface 26 of the case and are shown as flush end surfaces 46 and 56 respectively. The flush end surfaces 26,46 and 56 preferably all lie in a plane which is substantially perpendicular to the longitudinal axis of the core 20. As mentioned above, there are a variety of ways ofopticallyfinishing the end of the core 20 such as, for example, cleaving, polishing with a series of increasingly fine abrasives, cutting, fire polishing, etching, coating and the like.Because the termination of the invention holds the terminal portion of the core 20 rigidly within a pair of concentric sleeves, the core cannot bend back and forth while a series of increasingly fine abrasives are used to produce the polished end surface 26. Therefore, there is little tendency for the core edge to chip and break and little tendency for the surface 26 to be slightly convex.

Figure 9 illustrates a preferred termination device before installation wherein the polymeric termination sleeves are pre-bonded together to form a sub-assembly 100. The use of such a sub-assembly eliminates the difficulty of maintaining the desired longitudinal relationship between the two sleeves and the fibre core during the heat to shrink and bond procedure. The stripped fibre to be terminated can be positioned vertically and the sub-assembly 100 slipped over the fibre core as shown. This ensures that no appreciable gap will be left between the surface 36 on the unremoved portion of the cladding 30 and the annular end surface 48 on the sleeve 40. Preferably, the end surface 48 is slightly tapered and dimensioned with respect to the unremoved portion of the cladding 30 as shown.Such a configuration facilitates the insertion of the stripped core 20 into the central bore of the first sleeve 40 and allows the annular end surface 48 to function as a longitudinal locating surface which contacts the unremoved portion of the cladding 30.

Figures 10 to 13 illustrate a variation on the method of the invention which forms the first polymeric sleeve directly on the glass core through a dip coating process. A portion of the cladding 30 is again removed from the core 20 and preferably an effort is made to remove most of the cladding residue from the exterior surface of the core 20. The exposed portion of the core is then dipped into a solution of, for example, polyvinylidene fluoride in DIMETHYL FORMAMIDE or a solution of polyvinylidene fluoride tetrafluoroethylene copolymer in acetone, and slowly withdrawn. Evaporation of the solvent carrier from the polymer can be facilitated through the use of, for example, a hot air gun. The polymer layer left behind forms a thin sleeve 60. Typically, the interior surface of the sleeve 60 is incompletely bonded to the surface of the core 20 thereby forming pockets or voids 12.Figure 11 shows the waveguide of Figure 10 around which the heat-shrinkable polymeric sleeve 50 is again disposed. In a fashion analogous to the procedure described in connection with Figure 6 above, the assembly shown in Figure 11 is heated with a hot air gun, an infra-red heating device or the like thereby causing the sleeve 60 to soften and fuse and the sleeve 50 to recover and shrink radially thereby compressing and driving the softened sleeve 60 into uniform circumferential contact with the exterior surface 24 of the glass core 20. This radial compressive force tends to squeeze out the pockets or voids 12 and enhance the ability of the fused first sleeve 60 to more thoroughly wet the exterior surface 24 of the core 20. Figure 12 shows the bonded structure formed by heating the assembly shown in Figure 11 and allowing that structure to cool.Again, the portion of the sleeve 50 overlying and heat-recovered around the remaining portion of the cladding 30 functions to strain relieve the junction between the cladding and the first sleeve 60 thereby protecting the fragile core 20 from breakage in that vicinity. Figure 13 shows the bonded structure of Figure 12 after the end of the glass core 20 has been given an optically smooth surface, shown again as polished end surface 26. Again, during the polishing operation the ends 66 and 56 of the sleeves become substantially flush with the polished surface 26. Although a dip coating process can be used in the practice of the present invention, its use is not preferred as it is more time consuming, craft sensitive and does not function as reliably as a slip-on type sleeve to center the fibre with respect to the exterior surface 54 of a bonded waveguide termination.Figure 14 shows an environmentally sealed polymer clad glass fibre waveguide 80. Silicone based polymers are frequently used as a waveguide cladding material to form waveguides with large numerical apertures and low inherent attenuations for light with wavelengths around 820 nanometers. However, it is also well known that small glass fibres are particularly sensitive to moisture enhanced stress cracking when bent through small radii. Unfortunately, silicone based polymers are not particularly effective as barriers against moisture nor are they particularly abrasion resistant. In order to circumvent these shortcomings, silicone clad waveguides are frequently enclosed within an outer protective jacket 70 as illustrated in Figure 14.Such a jacket provides abrasion protection for the underlying cladding 30 but is not sealed at the ends therefore can allow moisture to migrate through the cladding and reach the exterior surface of the glass core 20. Optical waveguides used in aircraft applications are particularly susceptible to this moisture problem because of the pervasive condensation which results from altitude cycling. The termination of the invention provides a simple and effective approach to environmentally seal the open ends of the protective jacket 70. The material used to form the protective jacket 70 is preferably a polymer which will act as a good moisture barrier such as, for example, polyethylene, polypropylene, polyvinylidene fluoride, polyethylene tetrafluoroethylene copolymer or polyvinylidene chloride. The protective jacket 70 possesses an exterior surface 74 which partially underlies and is bonded to a portion of the interior surface 52 on the sleeve 50 as shown. The materials used to fabricate the sleeve 50 and the protective jacket 70 can be selected such that they will bond with each other when the end of the waveguide is heated during the heat to shrink and bond procedure described above. For example, the second sleeve 50 can be fabricated from cross-linked Kynar and the protective jacket 74 can be fabricated from non-cross-linked Kynar. Alternatively, an adhesive 73 can be interposed between the overlapping portions of the exterior surface 74 and the interior surface 52 as shown. Such an adhesive 73 can be predisposed as a ring on a selected portion of the interior surface 52.It isto be understood that such a ring of adhesive can be predisposed on the sub-assembly 100 of first and second sleeves shown in Figure 9.

Preferably the adhesive 73 is a hot-melt adhesive such as, for example, the adhesive described in US Patent Application Serial No. 882,391,filed 1 sot March 1978 and entitled "Hot-Melt Adhesives" the disclosure.of which is incorporated herein by reference.

Figure 15 shows a portion of an optical waveguide connector 90 with the terminated waveguide of the invention self-centered and bonded therein. The waveguide connector 90 shown in Figure 15 is illustrative of connectors that achieve axial alignment between a pair of waveguides to be connected through the use of pre-aligned concentric surfaces. In particular, the connector 90 employs a ferrule member 91 possessing an interior bore 92 which is centered with respect to a concentric exterior surface 93. A pair of ferrules 91 (only one of which is shown) are adapted to be received into opposite ends of a bushing 94. The bushing 94 possesses an interior reference surface 95 concentrically disposed about the sleeves longitudinal axis.The exterior surface 93 on each ferrule 91 is adapted to engage a portion of the reference surface 95 thereby aligning the bore 92 with the longitudinal axis of the bushing 94. A cap 96 can be used to hold the ferrule 91 and the bushing 94 together. Such a connector 90 is more fully described in US Patent No. 3,999,837 issued on 28th December 1976 to T.P. Bowen et al, the disclosure of which is incorporated herein by reference. The sleeve 50 is dimensioned to fit snugly within the terminal portion of the pre-aligned bore 92 and can be secured in place by an adhesive 97 such as a cyano-acrylate or an epoxy resin. As described above, the longitudinal axis of the glass core 20 is substantially centered with respect to the exterior surface 54 on the sleeve 50.Therefore, because the sleeve 50 fits snugly within the terminal portion of the pre-aligned bore 92, the longitudinal axis of the core 20 is substantially aligned with the longitudinal axis of the bushing 94 thereby ensuring substantial alignment between juxtaposed waveguides.

Examples illustrating the effect of surface treatment of the waveguide core before termination will now be described.

Examples la, lb, 2a and 2b Examples la, 1 b, 2a and 2b show the limitations of the bond obtained when the core of silicone clad waveguide is bonded to a termination device using conventional surface preparation techniques.

The waveguide tested comprised a fused silica (quartz) fibre core having an outer diameter of about 200 microns. Around the fibre was an inner optical cladding 38 microns thick, and an outer protective cladding of 49 microns thick. Both claddings were made of RTV polydimethyl siloxane. Both cladding layers were removed from a portion of the fibre core. The exposed fibre core surface portions for Examples 1 a and 1 b were cleaned with tetramethyl guanidine and for Examples 2a and 2b with J-100 stripping solution. Each fibre was then bonded to a termination device and then installed in a connector, Model No. 530954-5, available from AMP Incorporation of Harrisburg, Pennsylvania (hereinafter referred to as an AMP connector) using an epoxy adhesive.For examples la and 2a, the inner sleeve of each termination device which was bonded to the exposed surface portion of each fibre core was substantially non-cross-linked polyvinylidene fluoride. For controls 1 b and 2b, the same material was used, in combination with N-(2-amino-ethyl)-3aminopropyl trimethoxysilane which functions as a coupling agent at 0.3% by weight. In all examples, the outer sleeve comprises substantially cross-linked polyvinylidene fluoride in heat-recoverable form. After heating each termination sleeve during installation, the resulting assemblies were allowed to cool before being installed in AMP connectors. The strength of bonds between termination devices and fibre cores were tested in an Instron tester at a rate of pull of five millimeters per minute. The resulting bond strengths are presented in Table 1.In all four examples, the bond failed before the optical fibre broke.

Examples 3a and 3b This test was conducted to show that the problem of inadequate bond strength is more of a problem with silicone cladding then with other types of cladding. In this test, the fibre core was a quartz fibre having a diameter of about 200 microns. The cladding was about 18 microns thick and made of Viton polymer from Dupont. The cladding was stripped from the fibre. No solvent and no heat were used to prepare the fibre for termination. Termination devices as in Example 1 were emloyed having substantially non-cross-linked polyvinylidene fluoride inner sleeves with and without a coupling agent, (Examples 3b and 3a respectively).

Each terminated waveguide was bonded into an AMP connector. The bond strengths were tested on an Instron tester using the same method used for Examples la, 1 b, 2a and 2b and the results set out in Table 1.

As reported in Table 1, each bond in Examples 3a and 3b did not fail, but rather failure was due to breakage of the waveguide fibre.

Examples 4a, 4b 5a and 5b The surface treatment method of the present invention was used for these Examples, using lengths of the same waveguide used for Examples 1 a, 1 b 2a and 2b. After the cladding was removed from an end portion of each fibre, the exposed surface portions of fibre cores 4a and 4b were treated with tetramethyl guanidine and the exposed surface portions of fibre cores 5a and 5b were subjected to pre-cleaning by exposure to a temperature in a radiant oven of about 315 C for ten seconds. After fibres 5a and 5b were allowed to cool to room temperature, portions of somewhat embrittled silicone residue were stripped from the fibre cores with fingernails.Exposed surface portions of fibre cores 4a, 4b, 5a and 5b were then subjected to a hydrogen flame generated with a Henes water welder gas generator. The temperature of treatment was about 8000C and the time of treatment was about 8 seconds. After the fibre cores had been allowed to cool to room temperature the exposed surface portions were wiped with isopropyl alcohol to remove any loose particle of oxidized silicone polymer.These four treated test fibres were then bonded into termination devices, reported in Table 1 and then terminated waveguides so produced assembled into an AMP TABLE 1 Cladding Surface Inner Sleeve Bond Strength Failure Test Material Preparation Material (kg) Mode Example 1A Silicone Tetramethyl Kynar .58 Pull out guanidine 460(') Example 1B Silicone Tetramethyl Kynar 460 .76 Pull out guanidine and coupling agent Example2A Silicone J-100 Kynar460 .56 Pull out Example2B Silicone J-100 Kynar 460 .79 Pullout and coupling agent Example3A Viton None Kynar 460 1.16 waveguide broke Example3B Viton None Kynar460 1.16 Waveguide broke Example 4A Silicone Tetramethyl Kynar 460 .71 Waveguide guanidine broke and high heat Example 4B Silicone Tetramethyl Kynar 460 1.6 Waveguide guanidine and coupling broke and high heat agent Example 5A Silicone Low heat Kynar 460 .75 Waveguide pre-clean and high heat Example 5B Silicone Low heat Kynar 460 1.4 Waveguide pre-clean and coupling broke and high heat agent (1) Substantially uncrosslinked polyvinylidene Fluoride from Pennwalt (2) All termination sleeves tested possessed an outer heat-shrinkable sleeve made of crosslinked polyvinylidene fluoride (Kynar 460)

Claims

1. An optical waveguide having a glass core and a polymeric cladding, at least one end portion of the glass core being free of cladding and sheathed with a termination device comprising a first sleeve extending over the free end portion of the core and bonded to the glass surface thereof having a refractive index lower than that of the glass surface and a second sleeve radially outwardly of and overlying the first sleeve and the end of the polymeric cladding whereby the end of the core free of cladding is positively located coaxially of the termination device and is strain relieved.

2. A waveguide according to Claim 1 wherein the difference in refractive index between the glass surface and the first sleeve is at least 0.02.

3. A waveguide according to either of the preceding claims wherein the refractive index of the glass surface is 1.46.

4. A waveguide according to any one of the preceding claims wherein the refractive index of the radially inner sleeve is from 1.32 to 1.44.

5. A waveguide according to any one of the preceding claims wherein the first sleeve is fusible.

6. A waveguide according to Claim 5 wherein the first sleeve comprises non-crosslinked polyvinylidene fluoride.

7. A waveguide according to any one of the preceding claims wherein the second sleeve, before installation of the termination device, is heat-recoverable radially inwardly thereof.

8. A waveguide according to any one of the preceding claims wherein the second sleeve comprises substantially cross-linked polyvinylidene fluoride.

9. A waveguide according to any one of the preceding claims wherein the first sleeve extends over the length of that portion of the core free of cladding upto and into abutting relationship with the end of the polymeric cladding.

10. A waveguide according to any one of the preceding claims wherein the radially inner surface of that portion of the second sleeve overlying the polymeric cladding is lined, at least in part, with an adhesive.

11. A waveguide according to claim 10 wherein the adhesive is a hot melt adhesive.

12. A waveguide according to any one of the preceding claims wherein a coupling agent is present at the bond interface between the first sleeve and the glass core.

13. A waveguide according to Claim 1, substantially as described herein with specific reference to the accompanying drawings.

14. An environmentally sealed optical waveguide which comprises an optical waveguide as defined in any one of the preceding claims provided with a jacket substantially over the length thereof and having a termination device as defined at each end thereof such that the second sleeve of each termination device overlies and is sealingly bonded to an end of the jacket.

15. An environmentally sealed waveguide as claimed in Claim 14, substantially as described herein with specific reference to Figure 14 of the drawings.

16. A heat recoverable termination device adapted for use in an optical waveguide as defined in any one of the preceding claims which comprises a fusible polymeric first sleeve which has a refractive index lower than that of the glass surface of the core to which it is to be bonded and a polymeric heat recoverable second sleeve coaxially disposed about the first sleeve, the second sleeve being adapted to recover radially inwardly at a temperature at which the first sleeve is fusible to urge the fused first sleeve into contact with the glass core.

17. A termination device according to Claim 16, wherein the inside diameter of the device after full recovery is 1000tom or less.

18. A termination device according to Claim 17, wherein the inside diameter of the device after full recovery lies in the range 5 to 300vim.

19. A termination device according to either of Claims 17 or 18, wherein the second sleeve is longer than the first sleeve and extends past the first sleeve at one end of the device.

20. A termination device according to any one of Claims 17 to 19 wherein, the first sleeve comprises non-crosslinked polyvinylidene fluoride.

21. A termination device according to any one of Claims 17 to 20 wherein the second sleeve comprises substantially crosslinked polyvinylidene fluoride.

22. A termination device according to Claim 17, substantially as described herein with specific reference to Figure 9 of the drawings.

23. A method of terminating an optical waveguide having a glass core and a polymeric cladding, at least one end portion of the core being free of cladding, which comprises sheathing said end portion of the core free of cladding with a termination device as defined in any one of the preceding claims whereby the free end of the core is positively located coaxially of the termination device and is strain relieved, and optically finishing the end surface of the core positionally located within the termination device.

24. A method according to Claim 23, wherein the end surface of the core is optically finished by polishing.

25. A method according to Claim 24, substantially as described herein with reference to the accompanying drawings.

26. A method of preparing the surface of the glass core of an optical waveguide for termination as defined in any one of the preceding claims which comprises stripping the cladding from at least one end of the core of the waveguide and pyrolysing the polymeric residue remaining on the surface of the stripped end of the core.

27. A method according to Claim 26 which comprises a first heating step to a temperature of at least 3000C and a second heating step to a temperature of at least 700 C.

28. A method according to Claim 26, substantially as described herein with specific reference to the Examples.

29. An optical waveguide system comprising an optical waveguide termination at at least one end thereof as defined in any one of Claims 1 to 15 and an optical connector having a bore in which a terminated end of the waveguide is located and whereby the core of the waveguide is positively located coaxially of the bore.

30. An optical waveguide system according to Claim 29, substantially as described herein with specific reference to Figure 15 of the drawings.