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 Λ 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 Λ 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.