Optically stable, proton‐isolated, gain‐guided (AlGa)As double heterostructure injection lasers are described. A narrow stripe width (3.5 &mgr;m) and a very shallow implantation are employed (1.3 &mgr;m from the active layer); this results in a cosh−2‐like optical mode of ∼9.0‐&mgr;m full width at half‐maximum. The far‐field profiles possess strong sidelobes at high optical output power. The optical material loss coefficient and the antiguiding parameter are derived from the measured far‐field profiles, using the theory of gain guiding. Key parameters in the description of the optical mode distribution are calculated for several values of output power.