Optical strength of $YAG$ and $LiNbO_3$ crystals irradiated with giant pulses from a $YAG:Er^{3+}$ laser $(\lambda=2,94\mu m)$

Measurements were made of the bulk damage threshold $I_d$ of yttrium aluminum garnet $(YAG)$ crystals, either free of deliberately introduced impurities or activated with $Er^{3+}$ ions in different concentrations (up to 80 wt.procent), and also of $LiNbO_3$ crystals. This threshold was determined using giant pulses from a $YAG:Er^{3+}$ laser emitting at $(\lambda=2,94\mu m)$; the energy per pulse was 70 mJ and the pulse duration was 120 nsec. In the case of $YAG$ crystals it was found that $I_d$ varied considerably from sample to sample in the range $I_d=(2,5-18)\cdot10^{10}$ W/cm${}^2$, which was not associated with the presence of $Er^{3+}$ but was clearly due to the presence of some accidental impurities. A considerable increase in $I_d$ was observed for $YAG:Er^{3+}$ samples characterized by low damage thresholds when these samples were annealed in air. In the case of $LiNbO_3$, a typical value of the damage threshold was $I_d\approx10^{11}$ W/cm${}^2$. The results obtained indicated a sufficiently high optical strength of $YAG:Er^{3+}$ and $LiNbO_3$ crystals, used respectively as the active and modulating elements in $YAG:Er^{3+}$ lasers.