The so-called single-longitudinal mode of operation of a tunable laser beam source requires active control of the band-pass filters mounted in the laser resonator. As a rule, this entails one, or a combination of several filter elements, as for example birefringent filters, as well as etalons. In either case, the band-pass filter needs to be so adjusted as to locate precisely one of the feasible longitudinal modes of the laser at the exact maximum transmission of the band-pass filter. The invention consists in regulating such filter setting by way of a program-controlled digital signal processor DSP in such a way that the DSP modulates the filter element with an adjustable frequency, and such regulation exploits the phase of the laser intensity fluctuations induced thereby to regulate the optimal position of the band-pass filter.