On the Transverse Mode in an Optically Pumped Far-Infrared NH3 Laser

It is found that the resonance cavity-lengths for transverse modes observed at 81.5 and 263.4 µm of an optically pumped far-infrared NH3 laser agree well with existing theory. It is observed that the number of modes in the 81.5-µm line decreases from five to one with the increasing NH3 pressure. This effect may be attributed to the pressure dependence of the high gain region in the far-infrared cavity. The large gain coefficient of this laser is discussed. The observed strong correlation between the 81.5- and the 263.4-µm lines is reasonably explained by a simple model based on the cascade transition, implying that in this experiment the collision-induced transition is masked by the cascade transition.