Abstract

We consider the behaviour of a gain switched Cr³⁺: LiSAF pumped by a flash with a pulse duration so that the laser output is made up of one spike only. We demonstrate that a mechanism of intensity-dependent losses can be created without relying on any bulky component inside the laser. This mechanism is based on the combination of a hard aperture and a nonlinear time-dependent lensing effect originating from the amplifying medium. The latter is caused by the direct coupling of the average lattice strains to the excited ions. Indeed, Cr³⁺ ions slightly change their coupling to the crystal field with changes in energy state and produce a variation in the refractive index which is found proportional to the inverted population [1]. Since saturation due to the laser beam induces a radial variation of the population inversion throughout its cross section, it results a lensing effect in the amplifying medium which is time-dependent during the transient behaviour of the laser. It results that the geometrical characteristics of the laser beam also change and consequently the loss level becomes a function of time when a limiting aperture is present in the laser [2]. The overall results is a loss avalanche as in the case of a saturable absorber. In fact, it is observed that the behaviour of the laser is a sort of a self-Q-switching. Figure 1 and 2 shows the characteristics of the output pulse with (solid line) and without (dotted line) the nonlinear lensing effect. One can conclude that more the aperture is closed more the laser is well Q-switched. The results of our modeling are in agreement with experimental observations.

© 2001 EPS