We report a high power mid-infrared interband cascade laser operating at temperatures up to 170 K. The threshold current densities of this laser are considerably lower than the previously reported values in cascade lasers. The structure was grown by molecular beam epitaxy on a GaSb substrate and comprises 23 periods of active regions separated by digitally graded multilayer injection regions. A peak optical output power of∼0.5 W/facetand a slope of 211 mW/A per facet, corresponding to a differential external quantum efficiency of 131&percent;, are observed at 80 K and at a wavelength of∼3.9 &mgr;m.©1997 American Institute of Physics.