SELF-INDUCED HETERODYNE MEASUREMENT OF THE PHASE OF AN ULTRASHORT OPTICAL PULSE.

Summary form only given. A method of measuring the time-dependent relative phase and thereby obtaining the instantaneous frequency of an ultrashort frequency-swept optical pulse is described. For this method the frequency-swept pulse is passed through a resonant vapor (ensemble of two-level systems), and the consequent time-dependent intensity modulation of the output pulse from the cell is measured by a cross-correlation technique. A description of this technique is presented. When the frequency sweeps through the resonance of the two-level systems, they are excited in the impulse limit and thereafter oscillate with the resonance frequency omega //0 . The resulting macroscopic polarization radiates an electric field at the resonant frequency which interferes with the propagating pulse and thereby causes a strong oscillation in the intensity of the transmitted pulse. This self-induced heterodyne signal provides the measure of the time-dependent relative phase PHI (t) between the propagating pulse and the oscillating polarization. From knowledge of PHI (t), the instantaneous frequency is obtained.