Lasing without inversion based on magnetically assisted gain in coherently prepared cold atoms

We report on the observation of laser oscillation using the magnetically assisted gain in an ensemble of coherently prepared cold cesium atoms. This gain mechanism was recently demonstrated and is based on the modification of the Zeeman populations and coherences induced by a coupling and a probe beams by the presence of a transverse magnetic field. We have also investigated the enhancement of the gain due to the presence of a resonant ring cavity having different finesses. In the case of a low-finesse cavity operating below threshold, a 14-fold gain enhancement, in comparison with the single-pass gain, was measured and, finally, the system showed laser oscillation when a higher-finesse cavity was used. The present demonstration of laser oscillation constitutes an unconventional kind of lasing without inversion which is based on the manipulation of the dark state by an external magnetic field.