Design and lasing characteristics of λ=1.3 μm GaInAsP twin-stripe (TS) metal-clad ridge-waveguide lasers, with a varying degree of index guiding between the channels are reported. The TS devices operate stably in the TE polarised mode, even if under single-stripe injection TM polarisation is observed. Effective longitudinal mode selectivity has been obtained for the TS lasers with an effective index step in the range of 1×10−2to 5×10−3between the channels. This mode selectivity can be explained by assuming coupling of lateral modes of the twin waveguide structure via the common gain-medium.