Wavelength dependence of coherently induced transparency in a Doppler-broadened cascade medium

The effect of changing the coupling laser transition, and hence the wavelength, on the coherently induced transparency seen by a probe beam in a Doppler-broadened cascade electromagnetically induced transparency (EIT) configuration is studied experimentally and theoretically. The transparency of the vapor is shown to be an effect not just of EIT, but of the varying Autler-Townes splitting for each velocity group within the vapor. Consequently, the best overall transparency for Rabi splittings less than the Doppler width is not at the position of matched coupling and probe wavelengths, but for coupling wavelengths less than that of the probe.