An apparatus for generating electromagnetic radiation includes a pump laser so adapted that in operation of the apparatus it generates electromagnetic continuous-wave pump radiation; an optical parametric oscillator which is arranged in the beam path of the pump radiation and has a non-linear optical crystal, and is so adapted that in operation of the apparatus it generates signal and idler radiation from the pump radiation, and a non-linear optical device having a non-linear optical crystal, being arranged at least in a beam path of the signal radiation or idler radiation, and being so adapted that in operation of the apparatus it generates from the signal or idler radiation electromagnetic radiation at a frequency greater than a frequency of the signal or idler radiation. The non-linear optical crystal being heated in a furnace so that the crystal has a temperature gradient in the beam direction of the signal or idler radiation.