We have demonstrated a sensitive new method of Doppler-free spectroscopy, monitoring the nonlinear interaction of two monochromatic laser beams in an absorbing gas via changes in light polarization. The signal-to-background ratio can greatly surpass that of saturated absorption. Polarization spectra of the hydrogen Balmer-$\beta$ line, recorded with a cw dye laser, reveal the Stark splitting of single fine-structure components in a Wood discharge.

• Received 25 March 1976

DOI:https://doi.org/10.1103/PhysRevLett.36.1170