The present invention relates to the control or stabilisation of the lasing wavelength of a source of laser radiation (8), comprising a semiconductor material laser (2), a substrate (4) to which the laser (2) is mounted, a resonant optical cavity (15) within the semiconductor material (16,18,20), the cavity (15) having an active medium (16) for generating laser radiation (8) and one or more gaps (28) in the semiconductor material (16,18,20) within the cavity (15), wherein the substrate (4) is deformable by the application of a mechanical stress (14) to vary the size of said gap(s) (28) in order to change the optical size of said gap(s) and hence to vary the wavelength of laser radiation (8) generated by the semiconductor laser (2).