a method is provided for controlling the output frequency of a laser having a lasant gain medium which is characterized by at least two gain curve distributions over a range of lasant frequencies and having a laser cavity which is characterized by at least one resonant frequency within one gain curve distribution . the method comprises the steps of : locating in the laser cavity a first lyot filter which has a transmission band which favors the frequency range of one gain curve distribution in preference to the frequency range of the other gain curves ; and locating in the cavity a second lyot filter which has a transmission band which favors said one resonant frequency within the frequency range of the one gain curve , whereby an essentially single frequency output is obtained .