Light amplification without population inversion may be induced by optical coherence among the coupled states driven by laser fields or by the difference between the absorption and emission paths manifested by quantum interference. The optical coherence and quantum interference may lead to interesting statistical properties, such as narrower intrinsic linewidths and amplitude squeezing in inversionless lasers with coherently pumped atomic systems as active media. Here, we present an analysis of several three- and four-level coherently pumped atomic systems and show that steady-state lasing without inversion may be realized. In particular, lasing may be initialized from population inversion created by either incoherent or coherent pumping. As the laser intensity builds up, the excited-state population is depleted and the atomic coherence is enhanced. Eventually a transition from lasing with inversion to lasing without inversion occurs and the system evolves from an inversion laser into an inversionless laser. Such a transition is a general feature for a coherently pumped atomic system. We show that under suitable operating conditions, lasers without inversion may exhibit a nonlinear increase in the intensity and can be comparable in efficiency to conventional lasers with population inversion. Well above the threshold the inversionless laser may become amplitude squeezed and the maximum squeezing can be more than 50% below the shot-noise limit. There are two factors contributing to the noise reductions in the light output of these laser systems: first, the disappearance of the population inversion results in the depleted excited-state population, which reduces the spontaneous-emission noises; second, the fast coherent cycling of electrons driven by the coherent pump field and the lasing field leads to highly regulated absorption and emission processes.
