The plasma electrons in pulsed far ir lasers constitute a temporary divergent refractive element. This lens element can be strong enough to prohibit stable laser modes and serves to explain the delay commonly reported between current and laser pulses. With electron recombination the cavity rapidly becomes stable, Q switching the laser pulse. The lens effect and Q-switching rate are calculated in terms of tube dimensions, laser frequency, electron density, and radial electron density distribution. Higher laser frequencies appear earlier when this effect is operative. A simple criterion is given to distinguish this delay effect from other sources of laser pulse delay. Experimental measurements are presented of cavity stability as a function of mirror curvature and electron density; wall reflections are shown to be important. The radial electron density distribution is compared with plasma theory results. Lens effects associated with the laser gain are shown to be negligible.
B. W. McCaul, "Plasma Self-Q Switching in Far Infrared Lasers," Appl. Opt. 9, 653-663 (1970)