  The effect of an ac modulation on a 2-level or 3-level system is studied
theoretically. The absorption spectrum is calculated by solving the optical
Bloch equations and is interpreted by reference to the Floquet quasienergy
spectrum. The dependence of the absorption sidebands on the intensity of the
coupling laser field in Rydberg systems submitted to a radio-frequency (rf)
field is analysed in detail. It is shown that for sufficiently strong coupling
fields additional sidebands appear in the probe absorption spectrum in ladder
3-level systems. These additional sidebands are induced by the coupling of the
intermediate state to the Floquet manifold spawned by the upper state under rf
modulation.
