For the realisation of a distributed feedback (DFB) laser, the properties of a waveguide with a grating adjacent to the laser stripe are investigated. The results are applied to the metal-clad ridge-waveguide (MCRW) structure with DFB grating. It is found that, for small stripe widths, and if the optical field is mainly concentrated within the active region of the waveguide, enough optical feedback may be expected to realise laser operation with efficient longitudinal mode filtering.