Heterostructure injection lasers in which the GaAs active region is the center layer of a five‐layer dielectric slab waveguide have been investigated. The GaAs active layer is bounded on each side by an AlxGa1−xAs layer to confine the carriers. The two outside layers which are AlyGa1−yAs(y>x) confine the optical field. Lasers with this structure have been fabricated and room‐temperature threshold current densitiesJth300as low as 650 A/cm2have been obtained for 1‐mm cavity lengths. Differential quantum efficiencies &eegr;Dfor these separate optical and carrier confinement heterostructure (SCH) lasers were higher than generally encountered for double‐heterostructure (DH) lasers with values as high as 65% for[inverted lazy s]300−&mgr;−longcavities. The external quantum efficiency of several typical units was determined as a function of input current, and for one representative unit a maximum value of 39% was obtained at about four timesJth300. Emission in the fundamental TE mode was obtained for symmetrical structures with optical waveguide thicknesses in excess of 1 &mgr; at currents many timesJth300. The angle of beam divergence perpendicular to the junction plane was found to depend on the thickness of the optical waveguidewwith half‐power point values varying from 51° to 33° forwbetween 0.8 and 1.56 &mgr;. Calculation of the optical intensity distributions for the symmetrical SCH structure demonstrated the influence of the AlAs mole fraction and layer thickness on confinement of the optical intensity and suggest that it should be possible to achieve still lower threholds.