The relative intensities of the Doppler-free signals in the transition from polarized and dispersive signals to absorptive signals of the Na D1 line are calculated theoretically from the approximation of a single cycle of opticalpumping in a four-level system. The transition from polarized signals to saturated signals is made by varying the angle of an analyzer. Since the signal in polarization spectroscopy is mixed with the signal in saturation spectroscopy during the transition, spurious signals due to saturation and polarization effects should be taken into account in the signal analysis of polarization and saturation spectroscopies, respectively. Both theories, of polarization spectroscopy and saturation spectroscopy are unified and applied to the Doppler-free signals of the Na D1 line. The results obtained from the four-level optical pumping theory which mixes polarization spectroscopy with saturation spectroscopy are in good agreement with experimental reports.
Doppler-free laser spectroscopy using polarization modulation has been carried out in Na D₁ and D₂ lines. The polarization of the pump beam is modulated between identically and orthogonally to a linear or circular polarization of the probe beam. The recorded signals of the hyperfine and crossover resonances are in agreement with a recent theoretical analysis of Nakayama which is based on velocity-selective optical pumping in a four-level system. Some exceptional discrepancy occuring in the D₂ is considered to be due to several relatively close lying resonances.