Molecular saturation and the criterion for passive Q switching

Rotational hole burning is known to be the main factor that determines saturation in molecular absorbers during a short pulse. Experimental results are presented that suggest that pulse transmission measurements, as opposed to CW transmission measurements, must be used in order to determine if a gas can be used as a saturable absorber in passive
Q
switching. Rate equations are developed for a five-state model to explain saturation in SF
6
. This model is found to predict CW and pulse transmission through SF
6
at various lines of the 10.4-μm band of the CO
2
laser. A simplified model is developed for pulse transmission through saturable gases and is solved analytically for the peak of the pulse. It is found that the absorption coefficient of the absorber at the peak of the pulse is given by a simple expression, which leads directly to a new criterion for passive
Q
switching. An experimental test of this criterion is carried out for several mixtures of SF
6
and He which act as saturable absorbers under varying conditions of irradiation. It is found that the transition from CW to pulsed operation is very sharp and occurs at a well-defined value of radiation intensity.