Current sensors using highly-birefringent bow-tie fibres

Abstract

Faraday-effect optical-fibre current sensors have a number of well-known advantages for use in high-voltage transmission systems and other hostile environments. However, these applications have been restricted by the presence of linear birefringence in the fibre which adversely interacts with the Faraday polarisation rotation produced by magnetic fields.

Linear birefringence results from (a) intrinsic effects within the fibre (ellipticity, inbuilt thermal-stress) and (b) packaging and coiling which introduces transverse pressure and bends. The first of these can be eliminated by spinning a moderately-birefringent conventional fibre during the draw to produce a fibre with negligible intrinsic linear birefringence. However, the fibre remains just as sensitive to externally-induced birefringence caused by coiling. A complete solution to this problem is to design fibres with a large circular birefringence which overwhelms the perturbations caused by linear birefringence and thus permits the full Faraday rotation to be observed. Helical fibres have this property, but are large in diameter which restricts their application to coils of about 15cm radius. Moreover, care is required in launching and splicing.

We report here an alternative approach which appears particularly suited to applications requiring multi-turn, small-diameter coils. A highly-birefringent Bow-Tie fibre is rapidly spun during the draw to produce a fibre which exhibits a large elliptical birefringence. Unlike the unspun Bow-Tie fibre, the fibre now exhibits a magnetic field sensitivity. In addition, just as in a normal birefringent-fibre, the fibre is resistant to polarisation mode-coupling effects and therefore the current sensitivity is largely unaffected by packaging.

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