Abstract This paper deals with the absorption spectra of a weak probe in a four-level Doppler-broadened system driven by three coherent fields. The main aim is to extend earlier studies of the spontaneous emission spectrum and to present a comprehensive survey of the spectral features of this system. In addition to a derivation of exact formulae for the spectra, we give an explanation with the help of an appropriate set of dressed atomic states. We also get a deeper insight into the physical origin of gain in view of the existence of a population inversion between the levels of the lasing transition. Finally, we explore the effect of Doppler broadening on the absorption profile of the weak probe.
Received: 27 July 2000
Accepted manuscript online:
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