Variable threshold Q-switching of high gain lasers

Abstract

We report a technique of Q-switching a high-gain, flash-pumped laser which is capable of extracting, within a single pumping flash, an arbitrary number of output pulses with individually predetermined energies and times of occurrence. Conventional Q-switched lasers which vary the reasonator from a high loss to a fixed low loss state cannot achieve this because the energy of a Q-switched laser pulse is a function of both the population inversion at the onset of Q-switching and the threshold inversion established by the switch. Since the former varies during the pumping flash and the latter is usually fixed by mirror transmissions, etc., it is not possible to independently predetermine the energy and time of occurrence of an output pulse. This can be accomplished, however, with an acousto-optic beam deflector placed in a resonator whose optical arrangement is such that the undeflected portion of the laser beam goes into output and the deflected portion goes into the feedback loop. Because the deflection efficiency of an acousto-optic device is proportional to the acoustic power therein, the round trip, feedback of the resonator (and correspondingly the threshold population inversion) can be electronically controlled by varying the rf power applied to the deflector. This in turn allows the extraction of a pulse of any desired energy (subject to a maximum, of course) at any arbitrary time by applying a suitable rf pulse to the A-0 deflector. Furthermore, because the low Q-state of the laser involves almost 100% round-trip loss and no power to the deflector, high standoff capability and low rf power requirements are realized.