The invention relates to the design and processing of a semiconductor optical device. The device is formed of at least four layers of III-V compounds in which one consists of the penternary AlGalnAsSb material. The structure is wet etched in order to form optical ridge waveguides. One such device has incorporated two waveguides which are connected through a new junction design which can be made by wet etching. In one design the junction and waveguides consists of wet etched AlO .90GaO .10AsSb cladding around a core of AlO .28GaO .72AsSb in which an active layer composed of AlO.22InO.22GaO.55AsSb/InO.29GaO.71AsSb quantum wells is embedded. The resulting device is a erdge junction laser which has single mode emission and emits a narrow line width. We made and tested a device in the 2.34 müm to 2.375 müm wavelength area and found it to have an emission line width of around 0, 5 nm.