Lasing on the resonance transition in sodium atoms under nonresonant optical excitation

Coherent emission on the 3 P –3 S resonance transition ( D line) of nonresonantly excited sodium atoms in a buffer gas atmosphere is studied experimentally and theoretically. Both forward and backward coherent emission on the D lines is observed relative to the propagation direction of a pump beam whose frequency is blue-shifted from resonance. The divergence of the emitted radiation does not exceed that of the pump beam. The emission is due to the population inversion created on the “operating” transition when the pump is far detuned from resonance and the buffer gas pressure is sufficiently high. It is found that both emission intensity and the detuning range where this phenomenon is observed increase with the buffer gas pressure.