A semiconductor optical device comprises an active waveguide having a tapered portion, and a passive waveguide extending beyond the end of the active waveguide and optically coupled to the tapered portion of the active waveguide. The passive waveguide beyond the end of the active waveguide supports an optical mode of larger size than the optical mode supported by the active waveguide. The tapered portion of the active waveguide is truncated and the separation between the active waveguide and the passive waveguide is greater than in previously known devices in order to minimize or at least reduce the truncation loss at the truncation.