Q Switching of Lasers by Means of a Passive Shutter

Abstract

As stated in the Introduction, the Q switching of lasers can be realized by means of a passive shutter. The operating principle of this type of shutter entails the following. A passive element, whose transmissivity at the emission wavelength depends on the power of the light flux incident upon it, is placed in the laser cavity which is formed by external mirrors and is only partially filled with the active medium. The insertion of this element into the cavity increases the initial losses and, hence, the pumping threshold level. As a result, the population inversion at the initial instant of emission will be considerably higher than in the absence of the passive element. The onset of emission inside the cavity (prior to the instant of “bleaching” of the passive element this is the so-called free-oscillation regime) causes a rapid increase in the light flux density through the element, so that when this density attains a certain threshold level the transmissivity of the element drops to an insignificant value, i.e., “bleaching” of the passive element takes place. Inasmuch as the oscillation buildup time is normally small (relative to the pumping time, spontaneous decay time of the upper level, etc.) and only a minute portion of the population inversion is spent in bleaching of the passive element, the entire remaining store of particles after bleaching undergoes de-excitation in the form of a powerful short light pulse, i.e., a giant pulse.