Effect of Doppler broadening on the transparency and dispersion of the generalized double dark resonance

Spectroscopy of the Doppler broadened Rb atomic vapour coherently driven by two coupling fields is presented. Double dark resonances have been observed in different geometries of the coupling fields. A simple four-level model explains the basic counter-intuitive features of experimental spectra. The eigenvalues of the atom-field system distinguishing between the various geometries clarify the physics.