A method of practicing the invention includes at least the following steps of generating pump laser radiation having a spectrum containing a plurality of equidistant spectral lines; passing the pump laser radiation through a non-linear medium of a singly resonant, single-frequency optical parametric oscillator, wherein the pump laser radiation is continuously a wave or is pulsed, wherein the pulse duration in the latter case is longer than the time the optical parametric oscillation requires to reach its steady state; and coupling out the non-resonant idler or signal laser radiation from the optical parametric oscillator as usable frequency converted laser radiation. Further provided is a laser device used to carry out the method of the invention.