Pulsations of the radiation emitted by multimode solid-state lasers

The kinetics of multimode free oscillation is analyzed on the basis of rate equations allowing for an inhomogeneous distribution of the population inversion. The model of two overlapping modes is considered. It is shown analytically and confirmed by computer calculations that transient pulsations (spikes) of the radiation emitted by such a system should be damped out and the steady-state emission should be stable. In the case of external modulation of the losses, undamped oscillations should appear only if the modulation coefficient is sufficiently large and the modulation frequency is close to the pulsation frequency. It is shown that undamped pulsations appear when the mode composition is altered by slow changes in the losses or stimulated emission cross sections. Such conditions are encountered in pulsed lasers when the hearing causes the mode frequencies and center of the gain profile to shift with time at rates which differ from one mode to another. Criteria for the appearance of damped or undamped pulsations are established. The theoretical results are supported by experimentally obtained slit scans of the free-oscillation emission of ruby at 100°K which exhibits undamped regular pulsations.