Patent
1987-09-29
1990-02-13
Lee, John D.
350 9621, G02B 626
Patent
active
049001140
DESCRIPTION:
BRIEF SUMMARY
This invention relates to optical fibre joints.
It is important in optical fibre communication systems to be able to provide joints with very low losses between optical fibres. At present, standard single mode fibre is used both for transmission purposes and for connection to devices such as lasers, receivers and integrated optic components. Low loss joints between fibres of the same type are readily achievable, with fusion splicing, for example. By using this common fibre, low loss joints between the devices and the transmission fibre can be obtained.
Unfortunately, however, the standard system fibre is not usually the optimum fibre for connecting to these devices as a large proportion of optical power is lost due to difficulties in coupling the optical mode of the device to the fundamental mode of the standard fibre. An improved coupling can be achieved by carefully selecting the refractive index profile of the fibre to suit the particular device which is being coupled to. Although this special fibre allows low loss coupling of the modes this power advantage is lost due to the higher joint losses when this fibre is spliced to a standard transmission fibre.
To obtain a low loss joint between two optical fibres the mode fields of each fibre must match at the splice junction. In practice this means that the fibres must have identical refractive index profiles with no relative tilt or offset between them. Losses due to tilt and offset when jointing fibres of the same type can be reduced to small values by using high quality splicing techniques. Such a technique which reduces jointing losses between fibres of the same kind is described, for example, in published European Patent Application No. 82401668.7 (Thomson CSF). The technique described there consists, essentially, in tapering the two fibre ends and jointing the reduced diameter ends of the fibres. The reduction in core diameter at the joint face commensurate with the reduction in overall fibre diameter is said to reduce the sensitivity of the joint to angular and lateral misalignments. However even with such high quality splicing techniques, losses due to dissimilar refractive index profiles are still present. Such losses will occur when a special fibre of the type referred to above is spliced with the standard transmission fibre.
An object of the present invention is to provide a jointing technique in which fibres of different propagation properties can be joined with a low loss joint.
According to one aspect of the present invention there is provided an optical fibre joint between two generally co-axial fibres having different refractive index profiles in which the adjacent end portions of the fibres are tapered to such an extent that at least a proportion of the optical energy which is normally guided in the fibre core is guided by the cladding of the fibres, at least in the waist of the tapered region. The guidance by the cladding results in a greater proportion of optical power being coupled into the fundamental mode of the receptor fibre.
The expression "is guided by the cladding" should be interpreted on the basis that, as will be readily understood by the person skilled in the art of optical fibres, in an unmodified single mode fibre the energy is guided by the core, and the core also carries the major part of the transmitted optical energy, with however, a small fraction being carried by the cladding in the vicinity of the core. However, in this case the cladding has no guiding function. As the energy progresses into the tapered region, the relative proportions of the optical energy carried by the core and cladding progressively change as the core region gets smaller, with the result that an increasing amount of energy propagates through the cladding. Below a certain core size the field has expanded sufficiently for the cladding to assume itself a guiding function.
The extent to which the fibres are tapered depends upon the difference of the field profiles of the two fibres and the joint loss which is acceptable. With increasing taper of the f
REFERENCES:
patent: 3579316 (1971-05-01), Dyott et al.
patent: 4201447 (1980-05-01), Thompson et al.
patent: 4252403 (1981-02-01), Salisbury
patent: 4798436 (1989-01-01), Mortimore
Electronics Letters, "Low-Loss Joints Between Dissimilar Fibres by Tapering Fusion Splices", D. B. Mortimore et al., vol. 22, No. 6, Mar. 13, 1986, pp. 318-319.
Electronics Letters, "Tapered-Beam Expander for Single-Mode Optical-Fiber Gap Devices", K. P. Jedrzeuewski et al., vol. 22, No. 2, Jan. 16, 1986, pp. 105-106.
Electronics Letters, vol. 22, No. 6, Mar. 13, 1986 (Hitchin Herts, GB), D. B. Mortimore et al., "Low Loss Joints Between Dissimilar Fibres by Tapering Fusion Splices", p. 318.
Electronics Letters, vol. 22, No. 2, Jan. 16, 1986 (Hitchi Herts, GB), K. P. Jedrzejewski et al., "Tapered-Beam Expander for Single-Mode Optical-Fibre Gap Devices", pp. 105, 106.
Applied Optics, vol. 21, No. 7, Apr. 1, 1982, (New York, U.S.A.), Yasuyuki Kato et al., "Arc-Fusion Splicing of Single-Mode Fibers.1: Optimun Splice Conditions", pp. 1332-1336.
Mortimore David B.
Payne David B.
British Telecommunications, plc.
Heartney Phan T.
Lee John D.
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