Abstract: A method of stabilising a laser in order to maintain the laser in SLM operation is disclosed. The resonator cavity of the laser is modulated by a modulation signal applied to a piezo-electric transducer 10 coupled to the output coupler 2 of the laser. The modulation signal is also multiplied by a signal indicative of the energy of the laser radiation within the resonator cavity. The product of these two signals is then low-pass filtered to provide an error signal which is additionally applied to the piezo-electric transducer 10 so as to further vary the optical length of the resonator cavity.