Dynamics of a Q-Switched Laser near Threshold

The time evolution of the optical field produced by a $Q$-switched He:Ne laser has been investigated experimentally, in the neighborhood of the threshold of oscillation. The experiment was based on a measurement of the variation of the photoelectric counting probability with time, following the turn-on of the laser. $Q$ switching was achieved with the help of an external mirror to extinguish the laser, and a Pockels cell to switch out the mirror. The working point of the laser could be held anywhere in the threshold region, by use of a feedback arrangement which controlled the cavity length, and therefore the atomic gain. Results are presented showing the evolution of the mean light intensity, the variance of the light intensity, and the counting probability with time, for several different values of the laser pump parameter. The results are found to be in very good agreement with the calculations of Risken and Vollmer, based on a rotating-wave van der Pol oscillator model of the laser.