Inversionless gain enhancing due to Doppler broadening in a closed lambda-type system

An analysis is made of the effect of Doppler broadening on gainwithout inversion (GWI) from different aspects in a closedlambda-type three-level system with an incoherent pump. It is shownthat, regardless of the driving field being on resonance or not, forthe counter- or co-propagating of the probe and driving fields (PDF),GWI does not monotonically decrease or increase with increasingDoppler width. Except for the case of counter-propagating PDF withoff-resonance driving field, at a suitable Doppler width one canobtain a gain maximum value much larger than that without Dopplerbroadening; especially in the situation of the resonant drivingfield, the co-propagating geometry leads to a larger GWI. Inaddition, for the counter-propagating geometry, when Doppler width islarger enough, GWI oscillation occurs, and the oscillation amplitudeand region increase with increasing Doppler width. These conclusionsare very different from that obtained in previous investigation(Lukin {it et al}, {it Laser Phys.} {bf 6} 436 (1996)).