An optical cavity is combined with non-linear optical crystals and a laser source to generate light radiation at a frequency about 1.5 times the frequency of the initial laser source while having a wavelength 2/3 that of the initial laser source. The optical cavity comprises mirrors of relatively high reflectivity for optical radiation at a frequency of F/2, superior transmission for radiation at a frequency of 3/2*F to extract the final radiation and mirrors with relatively immediate or high reflectivity, or generally high transmission for radiation at a frequency F. The effective optical length of the cavity is precisely tunable by operating at least one of the mirrors that form the cavity. The optical cavity houses at least two non-linear optical crystals: at least one of which satisfies the phase-matching--quasi phase-matching--condition for non-linear conversion F 2*F/2, and at least one of which satisfies the phase-matching--or quasi phase-matching--condition for non-linear conversion F+F/23/2*F The length of the optical cavity, and its resonance modes, are actively stabilized such that the only process of frequency division is that by a factor of around 2.