A STUDY OF OPTICAL GAIN DUE TO COHERENCE EFFECTS IN POTASSIUM

Experimental results on gain due to coherence effects in a potassium-helium mixture are discussed. A strong pump laser drives the potassium D-2 transition, and gain is observed for a weak probe laser tuned to the D-1 transition. Population is supplied to the probed level via collisional transfer between the 4P(J) levels. The data agree in many details with a theoretical four-level Raman-driven lasing-without-inversion (LWI) model. In model calculations, we have accounted for dephasing collisions (line broadening) and transit relaxation. We report preliminary experimental results for crossed linear polarization between the pump and probe beams, probing on the violet 4S(1/2)-5P(3/2) transition, and probing out of the upper 4P(J) levels to a higher-lying 6S(1/2) level. We discuss the problem of measuring upper level populations while the driving field is present.