Operation of a new cleaved facet room‐temperature double heterostructure GaAs:GaAlAs laser utilizing leaky wave coupling through the substrate is reported. The coupling mechanism is analogous to that of prism coupling from a waveguide, but no external components are used. The laser produces a highly collimated output beam with approximately 2°×8° divergence. The peak pulsed beam power at 2 times threshold is 1.5 W from one end with approximately one‐third of this power in the collimated beam. The total external differential quantum efficiency is on the order of 35%. These desirable characteristics are obtained at the expense of threshold current increases of approximately 30%.