Abstract: The invention concerns an optical resonant frequency converter comprising two mirrors and a non-linear crystal in a ring resonator arrangement for converting laser radiation into frequency-doubled radiation. Because of the refractive effect of the prism-shaped, non-linear crystal two mirrors are sufficient to form a ring resonator with a total of three optical elements. A suitable choice of the orientation of the crystal axes in relation to the laser beam direction reduces scatter in the crystal. In one embodiment the entry surface of the crystal is at the Brewster angle while the exit surface is perpendicular to the beam and has an antireflection coating. In another embodiment the converted beam is coupled out through a polarisation beam splitter layer on one of the crystal surfaces. In a further embodiment the crystal surfaces are cylindrically curved. That produces an elliptical beam profile in the crystal, which reduces the walk-off effect.