Abstract

The nonlinear amplitude and phase and distortions that arise from cascading of second-order nonlinear processes¹ (χ⁽²⁾-cascading) have received considerable attention mainly because the self induced effects can be significantly larger than those originated by third-order interactions. This phenomenon is investigated in various contexts including, e.g., solitons and optical switching. In particular, two techniques of passive mode-locking based on χ⁽²⁾-cascading have recently been demonstrated lor continuous-wave Nd lasers²⁻⁴. In the Cascaded Second order nonlinearity Modelocking (CSM) the spatial phase distortion causes power-dependent cavity mode changes which, in combination with an aperture, provide nonlinear resonator losses, like in the Kerr-lens modelocking. In the Nonlinear Mirror Mode-locking⁵ the nonlinear losses derive from the parametric interaction of the laser beam with its second harmonic in an intracavity crystal. Various problems remain open with these techniques, among them: resonator design, pulse duration limitation due to the group velocity mismatch in the second harmonic crystal, and application to different active materials.

© 1996 IEEE