Change in the shape of a weak, ultrashort light pulse passing through a medium with a population inversion and a slow phase relaxation

Variations in the shape of a weak, ultrashort light pulse during coherent interaction with an amplifying medium have been studied experimentally. The model amplifying medium was a ruby crystal. The output pulse from a ruby laser with a self-locking mode was applied to the entrance of the amplifying element with a single-pass gain of about 500. Shape changes were detected even when the effect of the pulses on the level populations was negligible. It it suggested that the shape changes were the result of an emission caused by a macroscopic polarization in the amplifying medium. Densitometer traces of the pulses before and after amplification in the ruby crystal at 80 K are provided.