Modulation transfer processes in optical heterodyne saturation spectroscopy.

Transfer of modulation from a phase-modulated laser beam to an unmodulated, oppositely running beam occurs in a sufficiently nonlinear resonant gaseous medium. Two mechanisms account for this transfer: modulated hole burning and reflection from an induced population grating. Heterodyne detection of the transferred modulation reveals multiplet patterns that are distinct for the two mechanisms. In particular, a central dispersion feature in the dispersive phase pattern is a diagnostic indication of the reflection process.