Abstract
We report measurements and theoretical calculations which describe a new and powerful method of measuring the time-dependent relative phase and thereby obtaining the instantaneous frequency of an ultrashort frequency-swept optical pulse. 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 simple picture which describes this technique is the following. 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 ω0. 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 Φ (t) between the propagating pulse and the oscillating polarization. From knowledge of Φ (t), the instantaneous frequency is obtained.
© 1985 Optical Society of America
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