Behaviour of asymmetric fused-tapered single-mode fibre couplers

Abstract

Fused-tapered single-mode fibre couplers are available which have a flattened wavelength response and which are therefore useful in applications requiring a near-constant power-splitting ratio over a range of wavelengths. They are made by inducing an asymmetry in the constituent fibres, for example by pre-tapering or pre-etching one or both fibres prior to coupler fabrication, or by using two dissimilar fibres. The maximum coupled power in these couplers has been shown to decrease with increasing asymmetry. Couplers made from dissimilar fibres could also be useful for integrating systems made from different fibres, and it may be necessary for such a coupler to exhibit complete power transfer despite the asymmetry in fibre type. Complete power transfer has only previously been shown to be possible in dissimilar fibre couplers by differentially etching the fibres used to make the coupler, in order to adjust the extent of cladding diameter asymmetry. It would be advantageous to be able to adjust the splitting ratio of these couplers without requiring close control over the cladding diameter asymmetry of the constituent fibres. We demonstrate that the maximum coupled power of an asymmetric coupler is strongly dependent on the degree-of-fusion in the coupler, and that it is possible to obtain near-total power transfer (or indeed any level of power transfer) for a given cladding diameter asymmetry by controlling the degree-of-fusion of the coupler. We therefore remove the requirement for a controllable differential etching process prior to coupler fabrication.

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