Doppler-Free Laser Polarization Spectroscopy

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.