Coherence phenomena in phase-modulation laser spectroscopy

The technique of phase-modulation laser spectroscopy is used to detect a coherent four-wave interaction in ${\mathrm{I}}_2$ vapor. A phase-modulated probe beam induces in the sample an oscillating population which in turn modulates a counterpropagating pump beam that reveals a characteristic two-resonance spectrum. The observations fully confirm an earlier density-matrix calculation.