Optical waveguides – With splice – Alignment of fiber ends prior to splicing
Patent
1990-10-24
1992-08-25
Jones, W. Gary
Optical waveguides
With splice
Alignment of fiber ends prior to splicing
65 421, 156158, 385 96, G02B 618
Patent
active
051426030
DESCRIPTION:
BRIEF SUMMARY
The invention relates to methods for connecting optical waveguides, such as optical fibres.
A conventional optical waveguide comprises a length of dielectric material having a cross-sectional refractive index profile which is peaked such that optical radiation travelling along the length of material is guided to remain within the material of the waveguide. The material within the peak region is generally known as the core, and the material outside the peak region is generally known as the cladding.
Conventionally, optical waveguides of this type are connected or spliced together by bringing adjacent ends of the waveguides together under force in the presence of an electrical arc which, after the ends have been brought together, is turned off. This causes the fibres to fuse together. In some cases, the fused joint is annealed to improve mechanical strength. It is important to take account of the major splice loss mechanisms which exist in order to minimise losses resulting from the fused connection. In general, these loss mechanisms can be divided into three:
Transverse core offsets--this is caused by the core being non-concentric with the waveguide so that even if the waveguides are accurately aligned, the cores are not.
Core deformation--this is caused by poor alignment of the waveguides before the splice and/or the fact that the adjacent ends of the waveguides are not aligned.
Mode field mismatch--the refractive index profile controls the cross-sectional width of an optical field propogating in the waveguide as measured between the 1/e intensity levels and is characterised by the "mode spot size".
In a typical monomode fibre having a core diameter of about 8 .mu.m, the mode spot size will be in the order of 9 .mu.m. If the mode spots have different shapes (or sizes) in the two waveguides then there will be significant losses at the joint.
If the two waveguides have significantly different mode spot sizes, then the mode mismatch will be the dominant loss mechanism. Various proposals have been made to deal with this. For example, EP-A-0076186 describes a method of heating the ends of the fibres to a temperature of 800.degree. C. to 1200.degree. C. for a period of 24 to 48 hours to cause migration of dopant in the core in to the cladding since this results in modification of the refractive index of the optical fibre and a corresponding change in the mode spot size. The main disadvantages of this process are the need for a lengthy pre-treatment (1 to 2 days) and the necessity to cleave the fibre in the correct place to match mode sizes.
The effect of fluorine migration from cladding to core on the refractive index profile has been described in "Theory of tapering single-mode optical fibre by controlled core diffusion" by C. P. Botham and "Tapers in single-mode optical fibre by controlled core diffusion" by J. S. Harper, C. P. Botham, and S. Hornung both in Electronics Letters Feb. 18, 1988, vol. 24 No. 4, pages 243-246. Both these papers are concerned with a single fibre and the effect on that fibre of a heating process and illustrate typical heating times in the order of 1 hour or more.
In accordance with the present invention, a method of connecting optical waveguides having different refractive index profiles and at least one of the waveguides having a high numerical aperture (NA) comprises fusing the ends of the waveguides together with their cores in alignment and subsequently heating the fused junction between the waveguides so as to cause dopant to migrate out of the core of the high-NA waveguide, the heating step being carried out for a time sufficient to achieve a predetermined level of matching between the mode spot sizes in the two waveguides.
We have found that in the proposals described above, all of which are concerned with low NA fibres, losses increase rapidly when the fibres are subjected to heat treatment. In contrast, we have found that a post fusion heat treatment is surprisingly effective when at least one of the waveguides has a high-NA. When splicing a high-NA fibre to a standard fibre
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Mathyseek, New Coupling arrangement Between Diode and Fiber, 10th European Cont. on Optical Communication, Sep. 1984, pp. 186-187.
Botham, Tapering Fibers by Controlled Core Diffusion, Electronic Letters, Feb. 18, 1988, vol. 24, No. 4, pp. 243-244.
Harper, Tapers in Fibre by Controlled Core Diffusion, Electronic Letters, Feb. 18, 1988, vol. 24, No. 4, pp. 245-266.
British Telecommunications public limited company
Bruckner John J.
Jones W. Gary
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