Abstract: A high-efficiency non-collinearly phase matched parametric oscillator is provided, wherein a laser pumps a nonlinear optical material with a plural number of flat reflection surfaces that zigzag at least one parametrically generated off-axis radiation about the pump laser beam axis via multiple reflections from the surfaces. The off-axis zigzag oscillation of the radiation establishes parametric oscillation and improves energy coupling among mixing waves in a monolithic nonlinear optical material. Preferably the pump laser has a transverse beam size covering the area of the zigzagging parametrically generated radiation. To further enhance the performance of the off-axis zigzag parametric oscillator, the other parametrically generated radiation can be seeded by an external laser source or resonated in a cavity. The present invention also includes a double-side pumped off-axis zigzag parametric oscillator installed inside a standing-wave pump-laser cavity.