GB1562112A

GB1562112A - Opticalwaveguide couplers

Info

Publication number: GB1562112A
Application number: GB3754476A
Authority: GB
Original assignee: Plessey Co Ltd
Current assignee: Plessey Co Ltd
Priority date: 1977-08-26
Filing date: 1977-08-26
Publication date: 1980-03-05

Description

(54) IMPROVEMENTS RELATING TO OPTICAL WAVEGUIDE COUPLERS (71) We, THE PLESSEY COMPANY LIMITED a British Company of 2/60 Vicarage Lane, Ilford, Essex do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which is to be performed, to be particularly described in and by the following statement: This invention relates to the coupling of two optical waveguides or two bundles or optical waveguides, and has for an object to provide improved coupling means of relatively simple construction which make it practicable for workers in the field to establish a coupling of high efficiency.

In theory a coupling of high efficiency between two optical waveguides may be simply obtained by directly butting the adjacent ends of the two waveguides together in close contact, but because of the small diameter of the fibres or bundles of fibres usually employed as waveguides, it is necessary in this case for the ends of the fibres to be aligned to within an accuracy of 5 to 10 microns. This presents considerable difficulty in the field, and as a further consequence of the very small diameter of the coupling surfaces even a small speck of dust between these surfaces is liable to cover a large proportion of the light-guiding region of the cross-section of the optical fibre. It must be appreciated that this region is frequently less than 150 microns in diameter and that, even if the two fibre ends to be coupled are secured in complementary connector members with a view to ensuring accurate fibre alignment, some amount of misalignment may still occur when the connector is subjected to mechanical stress or vibration.

Hereinafter, wherever the context permits, the term fibre is intended to cover both individual optical fibres or other optical waveguide elements and bundles of such waveguide elements or fibres.

According to the present invention there is provided a fibre optic connector including two tapered waveguides, the smaller diameter end of wach waveguide having substantially the same diameter as a fibre optic and being secured to the end thereof, each waveguide being rigidly and accurately located within a sleeve with its larger diameter end face substantially in alignment with the plane of the end of the sleeve, the two sleeves being adapted to abut with the larger diameter ends of the waveguides face to face and the axes of the removable collar adapted to surround the two sleeves and maintain them in abutting relationship.

If desired an air gap may be left between the two larger diameter ends of the tapered waveguides, and this air gap may be filled with a liquid having substantially the same index of refraction as the tapered waveguides sections. It will be readily appreciated that, owing to the larger diameter of the butted coupling surfaces of the two tapered sections compared to the diameter of the fibres, the detrimental effect of any transverse misalignment is very much less than in the case of a coupling in which the fibres themselves are butted in the coupling, and any speck of dust will, for the same reason, cover only a relatively small portion of the cross-sectional area of the butted ends of the tapered sections. The assembly of each fibre end with its associated tapered section will be carried out in a factory or laboratory, where absence of dust particles and high accuracy of alignment is readily achievable. Any angular mutual misalignment of the two tapered sections, which would be liable to impair the coupling effect, can be readily minimised by a suitable construction of the coupling device employed. The accompanying drawing is an axial section of a pair of fibre ends when coupled to each other in accordance with the present invention, together with one form of a suitable coupling device.

Referring now to the drawing, 1 and 2 are end portions of two fibre cables of identical construction each consisting of a core 3 enclosed in cladding 4, the core and cladding terminating in a common, plane end surface 5 at right angles to the axis of the cable, and a protective sheath or cable casing 6, which terminates somewhat short of the end surface 5 In accordance with the present invention, each core 3 is extended by a tapered section 7 of light-guide material, preferably of the same material as the core 3, and preferably also provided with a cladding 8, each tapered section being connected at its small-diameter end 9 to the end surface 5 of the associated end of the cladded fibre 4 by a glued joint. The smaller-diameter end 9 of each tapered section can thus be secured, under laboratory conditions, in aligned butting engagement with the end surface 5 of the adjacent fibre core 3, and coupling in the field is effected by joining the largerdiameter ends 10 of the two tapered sections in aligned butting relation. In the illustrated embodiment a narrow air gap 11 is left between the adjacent ends of the two tapered sections. This air gap can be subsequently filled with a material, in liquid form, whose refractive index is substantially the same as that of the cores of the tapered sections 7; if desired a material may be used which subsequently solidifies. Each tapered section 7, 8 together with the adjacent end of the cable 1 or 2, is, in the factory or laboratory, secured in a metal sleeve 12. Each sleeve 12 has a stepped axial bore 13, 14 into the narrower portion 14 of which the end portion of the cable 1 or 2 is coaxially inserted while the associated tapered section 7 with its cladding 8 is inserted with a tight fit into the largerdiameter portion 4 of the stepped bore.

Some suitable glue is applied to one or both of the adjacent end surfaces 5 and 9, and when lateral alignment of the fibre end and tapered section has been established, which can be checked by measuring the light transmission achieved which will then be at a maximum, the relative position of the elements 3 and 7 is secured by an injection of epoxy resin to fill the cavity 15 left in the sleeve 12 between the end of the waveguide and the tapered section. Each of the metal sleeves 12 is provided at its outer side with a cylindrical surface 16 which is made coaxial with high accuracy in relation to the axis of the bore 13, 14 and thus to the common axis of the tapered section 7 and of the fibre end 3, and when the coupling connection is to be effected in the field, an alignment collar 17, in whose bore the cylindrical surfaces 16 of the two sleeves 12 employed are a tight sliding fit, is placed over the junction of the two metal sleeves 12, thereby ensuring both lateral and axial alignment of the axes of the two mutually facing large-diameter and surfaces 10. Conventional means of any suitable kind may be provided for retaining the two coupling members, respectively embodying the two metal sleeves 12 against axial movement away from each other, and means are also preferably provided permitting the injection of suitable material into the gap 11.

It will be readily appreciated that the mechanical construction of the connector may vary considerably from the specific example described with reference to the drawing, and that the form of the tapered section attached to the optical waveguide may also vary considerably; more particularly the tapered section need by no means be shaped as a true cone having a constant cone angle from one end to the other, provided that the diameter of the core of the tapered section at its thin end is equal to that of the core of the waveguide to be coupled.

Moreover, although the tapered sections in the illustrated embodiment are cladded, if desired an unclad dielectric body may be employed, provided that no light-absorbing medium or medium of higher refractive index is permitted to come into contact with it. The numerical aperture of the tapered sections should however be equal to or greater than the numerical aperture of the fibre cores to be coupled, in order to prevent loss of light. The term numerical aperture in this context is defined as the square root of n12 n22 wherein nl is the refractive index of the core and n2 is the refractive index of the cladding.

WHAT WE CLAIM IS: 1. A fibre optic connector comprising two tapered waveguides, the smaller diameter end of each waveguide have substantially the same diameter as a fibre optic and being secured to the end thereof, each waveguide being rigidly and accurately located within a sleeve with its larger diameter end face substantially in alignment with the plane of the end of the sleeve, the two sleeves being adapted to abut with the larger diameter ends of the waveguides face to face and the axes of the waveguides in substantial alignment and a removable collar adapted to surround the two sleeves and maintain them in abutting relationship.

2. A fibre optic connector as claimed in claim 1 in which each waveguide in frustoconical in shape.

3. A fibre optic connector as claimed in claim 2 in which each waveguide comprises an inner light transmissive core surrounded by a layer of cladding.

4. A fibre optic connector as claimed in any one of claims 1 to 3 in which any gap left between the larger diameter ends of the waveguides is filled with a liquid having substantially the same index of refraction as the light transmissive core of the waveguides.

5. A fibre optic connector substantiallE as described with reference to the accom

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. end portions of two fibre cables of identical construction each consisting of a core 3 enclosed in cladding 4, the core and cladding terminating in a common, plane end surface 5 at right angles to the axis of the cable, and a protective sheath or cable casing 6, which terminates somewhat short of the end surface 5 In accordance with the present invention, each core 3 is extended by a tapered section 7 of light-guide material, preferably of the same material as the core 3, and preferably also provided with a cladding 8, each tapered section being connected at its small-diameter end 9 to the end surface 5 of the associated end of the cladded fibre 4 by a glued joint. The smaller-diameter end 9 of each tapered section can thus be secured, under laboratory conditions, in aligned butting engagement with the end surface 5 of the adjacent fibre core 3, and coupling in the field is effected by joining the largerdiameter ends 10 of the two tapered sections in aligned butting relation. In the illustrated embodiment a narrow air gap 11 is left between the adjacent ends of the two tapered sections. This air gap can be subsequently filled with a material, in liquid form, whose refractive index is substantially the same as that of the cores of the tapered sections 7; if desired a material may be used which subsequently solidifies. Each tapered section 7, 8 together with the adjacent end of the cable 1 or 2, is, in the factory or laboratory, secured in a metal sleeve 12. Each sleeve 12 has a stepped axial bore 13, 14 into the narrower portion 14 of which the end portion of the cable 1 or 2 is coaxially inserted while the associated tapered section 7 with its cladding 8 is inserted with a tight fit into the largerdiameter portion 4 of the stepped bore. Some suitable glue is applied to one or both of the adjacent end surfaces 5 and 9, and when lateral alignment of the fibre end and tapered section has been established, which can be checked by measuring the light transmission achieved which will then be at a maximum, the relative position of the elements 3 and 7 is secured by an injection of epoxy resin to fill the cavity 15 left in the sleeve 12 between the end of the waveguide and the tapered section. Each of the metal sleeves 12 is provided at its outer side with a cylindrical surface 16 which is made coaxial with high accuracy in relation to the axis of the bore 13, 14 and thus to the common axis of the tapered section 7 and of the fibre end 3, and when the coupling connection is to be effected in the field, an alignment collar 17, in whose bore the cylindrical surfaces 16 of the two sleeves 12 employed are a tight sliding fit, is placed over the junction of the two metal sleeves 12, thereby ensuring both lateral and axial alignment of the axes of the two mutually facing large-diameter and surfaces 10. Conventional means of any suitable kind may be provided for retaining the two coupling members, respectively embodying the two metal sleeves 12 against axial movement away from each other, and means are also preferably provided permitting the injection of suitable material into the gap 11. It will be readily appreciated that the mechanical construction of the connector may vary considerably from the specific example described with reference to the drawing, and that the form of the tapered section attached to the optical waveguide may also vary considerably; more particularly the tapered section need by no means be shaped as a true cone having a constant cone angle from one end to the other, provided that the diameter of the core of the tapered section at its thin end is equal to that of the core of the waveguide to be coupled. Moreover, although the tapered sections in the illustrated embodiment are cladded, if desired an unclad dielectric body may be employed, provided that no light-absorbing medium or medium of higher refractive index is permitted to come into contact with it. The numerical aperture of the tapered sections should however be equal to or greater than the numerical aperture of the fibre cores to be coupled, in order to prevent loss of light. The term numerical aperture in this context is defined as the square root of n12 n22 wherein nl is the refractive index of the core and n2 is the refractive index of the cladding. WHAT WE CLAIM IS:

1. A fibre optic connector comprising two tapered waveguides, the smaller diameter end of each waveguide have substantially the same diameter as a fibre optic and being secured to the end thereof, each waveguide being rigidly and accurately located within a sleeve with its larger diameter end face substantially in alignment with the plane of the end of the sleeve, the two sleeves being adapted to abut with the larger diameter ends of the waveguides face to face and the axes of the waveguides in substantial alignment and a removable collar adapted to surround the two sleeves and maintain them in abutting relationship.

panying drawings.