We analyze both experimentally and theoretically the phase shift of a fixed
coupling laser field in a Lambda -type system induced by a tunable probe l
aser field. The measurements are performed with a heterodyne interferometer
on a beam of cesium atoms in the D-2 line. This system exhibits electromag
netically induced transparency with a rapidly varying refractive index. The
parametric dispersion of the coupling field over a range of six decades of
coupling laser intensities is investigated and found to be in accord with
the predictions from the semiclassical model.