Gain without inversion in the frequency up-conversion regime

We analyze the general conditions and limitations for the realization of frequency up-conversion via lasing without inversion (LWI). Based on this analysis we propose a class of schemes in which LWI can be achieved via optical- or microwave-frequency driving on a weakly allowed transition. A noninversion amplification process based on this system is studied with particular emphasis on propagation effects and Doppler broadening. Amplification of both monochromatic and bichromatic fields is considered and a possible implementation in realistic atoms and ions is discussed.