Abstract

Lasing without inversion (LWI) has attracted attention in recent years since it opens new perspectives for laser physics via the effects of atomic coherence and interference. We will describe an experimental demonstration of a cw laser oscillator without population inversion. It is based on the scheme and is closest to the concept proposed by Imamoglu, Field and Harris. The active medium is a sodium atomic beam and the transitions are within the sodium D₁ line. By using a weak probe laser, we first demonstrate complete transparency and then inversionless gain. Next, a laser cavity is installed and aligned. With the probe blocked, we find that the laser starts spontaneously from vacuum fluctuations. This is the first demonstration in which the lasing medium was an atomic beam. It is an approach which, in addition to elucidating the physics, lays a foundation for extensions into the ultraviolet. Calculations using realistic atomic structure were critical to the choice of experimental approach. Observations agree with full density- matrix calcualtions and clearly show there was no population inversion.

© 1996 Optical Society of America