Very-high-resolution spectroscopy carried out using a ring laser

A method is suggested for the determination of the frequency separation between two fine-structure components of lines and of their relative intensities. Use is made of the dependence of the width of the single-wave emission region on the frequency separation between the line components and on the ratio of their intensities. The resolution of the method is less than the homogeneous line width and the precision is several percent of the measured quantity.