Abstract: This invention relates to generally to semiconductor devices, for example lasers and more particularly to single frequency lasers and is directed at overcoming problems associated with the manufacture of these devices. In particular, a laser device is provided formed on a substrate having a plurality of layers (1,2,3,4,5), the laser device comprising at least one waveguide (for example a ridge) established by the selective removal of sections of at least one of the layers. The ridge (100;101) has at least one defect defining region (104), the at least one defect defining region of the ridge defining a defect in the ridge. The width of the ridge is greater in the at least one defect defining region of the ridge than in adjacent sections of the ridge.