Damage threshold studies of glass laser materials

In Section I, results of studies of Q-switched laser damage to transparent uncoated dielectric surfaces are presented. It is concluded that surface scratches and digs on some conventionally polished glasses are not of prime importance in damage. Rather, contaminants left by the polishing process determine the damage threshold. Removal of these contaminants can be effected by treatment in hot nitric acid or hot water, raising the threshold to as high as 500 J/sq cm (30 ns). The effectiveness of the treatment is dependent on polishing history. An analysis of pulse cutoff by the damage site is also presented. It is concluded that the shape of the cutoff is not related to the damage mechanism, but rather to the manner in which the plasma expands across the laser beam. In Section II, results of experiments on the feasibility of a high energy density Nd:glass laser are reported. Damage-free operation at more than 4.4 GW/sq cm (70 J/sq cm in a 15 ns pulse) was achieved. Restraints on higher irradiance operation and possible improvements are discussed.