Blue five-level lasing-without-inversion system in rubidium

We demonstrate continuous coherent blue laser light production using a five-level lasing-without-inversion scheme in rubidium vapor. Two low-power lasers at 780 nm and 776 nm induce a strong atomic coherence in the 5S-5P-5D states. The atoms decay to the 6P excited state from which stimulated emission produces a coherent blue (420 nm) beam. We have coupled both ground-state hyperfine levels, effecting a coherent state between four levels. The coherent blue output is enhanced through several mechanisms, including stronger coupling to a larger fraction of the atomic population, operation at a detuning such that the vapor is nominally transparent to the 780 nm pump field, reduced losses due to optical pumping, and optimal phase-matching. We report experimental findings and compare with results from a semiclassical Maxwell-Bloch model. c © 2005 Optical Society of America