Practical Sagnac interferometer based fibre optic current sensor

A theoretical and experimental investigation of a practical new Sagnac interferometric optical fibre current sensor using a 3×3 fibre coupler is presented. A simple analytical expression for the output response of the sensor has been derived in the presence of bend-induced linear birefringence and twist-induced circular birefringence in the fibre sensing coil. This represents the first analytic formulation for this type of current sensor. The effects of the bend-induced linear birefringence and twist-induced circular birefringence on the phase bias and scale factor have been analysed in detail. Numerical simulation was also carried out to study the effect of various disturbances and imperfections such as intrinsic linear birefringence and localised birefringence in the sensing fibre. It is shown that our proposed structure minimises the effects of variations, as might be caused by temperature changes, for example, in the bend-induced linear birefringence and twist-induced circular birefringence, on the sensor stability. Preliminary experimental results show good temperature stability without the use of any passive or active compensation arrangement