A frequency converter for converting a single mode input beam, which is emitted from a laser light source at a fundamental frequency, to an output beam at a converted frequency, comprising: a plurality of spaced optical components (22, 24, 26, 28, 31) defining an external resonant cavity and configured to shape the input beam with at least one beam waist in the cavity, the shaped beam having a section selected from the group consisting of at least one divergent section, collimated section and a combination of these, and a non-linear crystal located within the cavity, characterized in that the single mode input beam is emitted at a power of the single mode input beam varying over a range higher than 12W, preferably between 12W and a kW-level, the frequency converter operates over said range of input beam powers and further comprising, the non-linear crystal located within the cavity in: the divergent beam section of the shaped beam with a Rayleigh range smaller than a round trip length of the cavity, the crystal being located so that a center of the crystal is spaced from the beam waist along a beam path, or the collimated beam section with a Rayleigh range greater than the round trip length; and the frequency converter further characterized in that the non-linear crystal is displaceable within the resonator along the divergent section among a plurality of locations, which correspond to respective input powers of the range, such that at each location the resonator is impedance matched and the nonlinear crystal generates the second, third, fourth and/or higher order harmonic at a maximum conversion efficiency. a plurality of spaced optical components (22, 24, 26, 28, 31) defining an external resonant cavity and configured to shape the input beam with at least one beam waist in the cavity, the shaped beam having a section selected from the group consisting of at least one divergent section, collimated section and a combination of these, and a non-linear crystal located within the cavity, characterized in that the single mode input beam is emitted at a power of the single mode input beam varying over a range higher than 12W, preferably between 12W and a kW-level, the frequency converter operates over said range of input beam powers and further comprising, the non-linear crystal located within the cavity in: the divergent beam section of the shaped beam with a Rayleigh range smaller than a round trip length of the cavity, the crystal being located so that a center of the crystal is spaced from the beam waist along a beam path, or the collimated beam section with a Rayleigh range greater than the round trip length; the divergent beam section of the shaped beam with a Rayleigh range smaller than a round trip length of the cavity, the crystal being located so that a center of the crystal is spaced from the beam waist along a beam path, or the collimated beam section with a Rayleigh range greater than the round trip length; and the frequency converter further characterized in that the non-linear crystal is displaceable within the resonator along the divergent section among a plurality of locations, which correspond to respective input powers of the range, such that at each location the resonator is impedance matched and the nonlinear crystal generates the second, third, fourth and/or higher order harmonic at a maximum conversion efficiency.