A wavelength tuneable external-cavity laser module comprises a gain medium in thermal contact with a thermally stabilized substrate; an end mirror, and a phase element for controlling the phase of the optical beam and being positioned within the external cavity between the gain medium and the end mirror, wherein said phase element comprises a material having a refractive index that varies in response to changes in temperature and has a transmissivity substantially independent of wavelength across said predetermined wavelength range. The thermally-controllable phase element is configured so as to induce a phase variation that compensates the drop in the output power due to ageing or to external temperature variation. A heating element is placed in thermal contact to the phase element. By thermally controlling an intra-cavity phase element it is possible to vary continuously the output power as a function of the injection current.