We study phase-conjugate six-wave mixing spectroscopy based on electromagnetically-induced-transparency in a Doppler-broadened folded four-level system.It is found that the six-wave mixing spectrum can be either Doppler-free or very broad,depending on whether the interference between the polarisations of atoms with different velocities is constructive or destructive.To obtain the Doppler-free six-wave mixing spectrum in the folded four-level system,the conditions are more stringent in comparison with those in the cascade and N-type four-level systems.This polarisation interference can be controlled in the presence of a strong coupling field.
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.
