Laser action below threshold inversion due to coherent population trapping

The possibility of 'lasing without inversion' on the 1079.8 nm line of a HeNe laser is investigated. It is shown that a modified double-Lambda scheme can be realized by use of the 877.9 nm line for introducing Zeeman coherence. It is shown experimentally that Zeeman coherence can enforce laser action, even if the inversion is below threshold. A different choice of the polarization of the driving beam can result in suppression of laser action above threshold. The underlying mechanisms are examined. On the 611.8 nm line, laser action below threshold inversion is obtained with the driving beam tuned to 824.9 nm ('up-conversion').