A system includes: a first nonlinear crystal arranged to receive to a first laser beam having a first wavelength λ1 and operable to generate, by frequency doubling of the first laser beam, a second laser beam having a second wavelength λ2, in which the second beam propagates collinearly with the first beam; a second nonlinear crystal arranged to receive the first and second laser beams from the first crystal, in which the second crystal is operable to generate, by frequency mixing of the first and second laser beams, a third laser beam having a third wavelength λ3; and one or more lenses between the first and second crystals, in which the one or more lenses are operable to spatially separate the first and second beams. The first and second laser beams propagate at an offset and/or titled with respect to an optical axis in order to cause the spatial separation.