Effects of Cr⁴⁺ ions on forming Ince–Gaussian modes in passively Q-switched microchip solid-state lasers

The effects of Cr⁴⁺ ions on forming Ince–Gaussian (IG) laser modes in a tilted pumped Cr⁴⁺:YAG passively Q-switched (PQS) Nd:YAG microchip lasers has been studied experimentally and theoretically. The formation of Cr⁴⁺-ion domains in Cr⁴⁺:YAG crystal has been proposed based on the spatial distribution of Cr⁴⁺ ions in Cr⁴⁺:YAG crystal. The spatial modulation effect of Cr⁴⁺-ion domains on selecting a transverse mode has been investigated based on their periodic distribution and nonlinear absorption of the Cr⁴⁺ saturable absorber. The formation of IG modes in the Nd:YAG/Cr⁴⁺:YAG PQS microchip laser under tilted pumping has been demonstrated theoretically with the spatial modulation of the Cr⁴⁺-ion domains in Cr⁴⁺:YAG crystal and the inversion population distribution, by taking account of the thermal lens effect. The IG modes selected theoretically in the Nd:YAG/Cr⁴⁺:YAG PQS microchip laser are in good agreement with experimentally obtained IG modes. This work provides a theoretical model of the selection of IG modes with Cr⁴⁺-ion domains in a tilted beam pumped Nd:YAG/Cr⁴⁺:YAG PQS microchip laser to produce efficient, stable and controllable IG laser modes. (letter)