This book explores the use of second order nonlinear optical processes for efficient passive mode-locking of laser. Second order processes help make the system fully solid state and compact. Two second order processes give the effect of third order, which is exploited here to realize self starting, stable and power-scalable mode-locking. The book also presents the realization of negative feedback required for the stability of mode-locking by the use of second order nonlinear process again. Starting from the basics of nonlinear optics and the theory of mode-locking, the experimental realization of mode-locking of lasers with cascaded quadratic nonlinearity is presented in a lucid way.