Frequency up-conversion He-Ne laser without inversion

{A scheme for obtaining up-conversion lasing without inversion in the visible range is proposed using a Doppler-broadened He-Ne mixture as active medium. We consider a cascade scheme (2s(2) <––> 2p(2) <––> 1s(3)) with a frequency upconversion ratio of 1.91. The upper transition is inverted and driven by an external field and lasing without inversion occurs at 616.4 nm provided that the collisional decay rates of the 1s(3) and 1s(5) levels overcome a certain threshold.}