Double-frequency doppler-free spectroscopy of $^{87}$Rb atoms

We investigate Doppler-free absorption lines induced by counter-propagating polychromatic optical fields in the D1 line of 87Rb atoms. Cases of orthogonal and parallel polarizations are considered. The resonance associated with ${{F}_{e}} = 1$ demonstrates unusual behavior with two-photon detuning: the amplitude grows while the width becomes smaller. The crossover resonance involving transitions to both sublevels ${{F}_{e}} = 1,2$ of the excited state is more pronounced at parallel polarizations than at orthogonal and has smaller width compared to the lines corresponding to atomic transitions. A qualitative explanation of experimentally observed features is given.