A dual-loop Sagnac interferometer

A new type of all-fibre Sagnac interferometer composed of two loops is proposed and analysed in detail. It can be used with a very long transmission line while maintaining excellent performance characteristics due to the automatic compensation of any birefringence effects in the trunk fibre. Preliminary experiments at 1310 nm wavelength with a 70 km long trunk fibre demonstrated an interference visibility as high as 98%, indicating that this scheme has promising potential applications.