Doppler-free spectroscopy in driven three-level systems
  We demonstrate two techniques for studying the features of three-level systems driven by two lasers (called control and probe), when the transitions are Doppler broadened as in room-temperature vapor. For $\Lambda$-type systems, the probe laser is split to produce a counter-propagating pump beam that saturates the transition for the zero-velocity atoms. Probe transmission then shows Doppler-free peaks, which can even have sub-natural linewidth. For V-type systems, the transmission of the control beam is detected as the probe laser is scanned. The signal shows Doppler-free peaks when the probe laser is resonant with transitions for the zero-velocity group. Both techniques greatly simplify the study of three-level systems since theoretical predictions can be directly compared without complications from Doppler broadening and the presence of multiple hyperfine levels in the spectrum. 
Umakant D. Rapol and Vasant Natarajan