Influence of the quadratic nonlinearity on the necessary conditions for spontaneous mode locking in an YAG:Nd3+ laser

The influence of the quadratic polarization of a nonlinear medium on the necessary conditions for selflocking of longitudinal and transverse modes in an active medium is studied experimentally and theoretically. It is shown that doubling of the fundamental radiation and simultaneous generation of the difference frequency between one of the second harmonics and one of the fundamental frequencies induces a cascade cubic polarization. This polarization is comparable with the cubic polarization due to the interaction of modes in an active medium of an YAG:Nd³⁺ laser under the same conditions. The modelocking range is governed by the relative positions of the active medium and nonlinear crystal, and can both be increased or completely compensated.