Measuring the phase of slow kerr-type nonlinearities: The role of phase modulation

We show theoretically and verify experimentally how the phase modulation of the laser pulse affects the phase of the various contributions to the nonlinear polarization in the optical phase conjugation geometry. This provides an explanation for the apparent sum rule violation observed by Jain and Lind in semiconductor-doped glasses. It also allows to define the conditions which must be fulfilled when one wants to measure the phase of a slow Kerr-type nonlinearity.