Energy-level difference and laser absolute frequency measurement by polarization beats spectroscopy with phase-conjugation geometry

We have studied the polarization beats spectroscopy with phase-conjugation geometry in a cascade three-level system. It was found that the accuracy fo r the energy-level difference measurement is determined by the homogeneous linewidths of the optical transitions. Under the condition for the occurren ce of beating, the frequency difference of the beat between two independent light sources could exceed the laser linewidth. The measurement accuracy f or laser absolute frequency could reach the same order of magnitude as the laser linewidth.