Differential intracavity phase spectroscopy and its application to a three-level system in samarium

A technique of differential intracavity phase spectroscopy is introduced. The measured beat frequency between the two counterpropagating fields in a ring laser directly yields the differential phase shift due to the presence of an atomic vapor. In an unstabilized laser cavity, phase differences near 10(-6) are measured. This technique is used to study the nonlinear dispersion associated with a three-level Lambda system in atomic samarium. Sub-Doppler resonances and population trapping are observed, both of which are attributed to the coherences between the two lower levels of the Lambda level structure. Solutions of the density matrix describing the system are also presented, and good qualitative agreement is found between the calculations and the measurements.