Summary form only given. We report on experimental and theoretical results concerning resonator design for cascaded second-order nonlinearity mode-locking (CSM) Nd:YAG lasers and pulse-duration optimization. The CSM critically depends on the nonlinear resonator losses. Since the nonlinear phase distortion arising from /spl chi//sup (2)/:/spl chi//sup (2)/ cascading is quite different from that resulting from Kerr effect, an original model, based on a perturbation theory, has been developed to analyze and design the resonator. The results have been tested experimentally.