Knowledge Bank

Passive Q-switching has resulted from the introduction of various saturable absorbers into the cavity of a carbon dioxide laser. The use of $SF61$, $BCl32$, heated $CO23$, vinyl $chloride4$, and other $materials4,6$ has been reported. We explain this behavior using the following models: (1) The role of the absorber is explained by a kinetic four-level $model6$ used previously to explain saturation effects. (2) The differential equations presented by Wagner and $Lengyel7$ have been expanded to include the effect of continued inversion pumping and are used to describe the processes in the laser. By coupling these two models, a system of differential equations is obtained whose solution gives the time dependent behavior of the laser. The equations are solved numerically using reasonable parameters $\mathrm{<mrow\; data-mjx-texclass="ORD">6,8</mrow>}$ and good agreement is found with experimental measurements. The two main conclusions drawn are: (1) A narrow hole at the laser frequency is burned into the spectrum of the absorber and corresponds to the depletion of those rotational levels capable of absorbing the laser radiation. (2) The relaxation of this hole and the properties of the passively Q-switched laser are best explained by rotational relaxation rates rather than vibrational ones.