The invention relates to the field of optical parametric oscillators (OPO), especially to an essentially vertical monolithic system (S) for parametric conversion from a pump wave with a pump wavelength, said system comprising at least two resonant cavities (6, 7). Said cavities are coupled by at least one coupling mirror (3), at least one of the cavities comprising an active non-linear medium, and the at least one coupling mirror being arranged in such a way that the parametric frequencies associated with the pump wavelength are located in the stop line of the at least one mirror for an injection direction of the pump wave essentially according to the axis of the system.