Double heterostructures consisting of InAs0.91Sb0.09active layers with Al0.5Ga0.5Sb cladding layers were grown by molecular beam epitaxy on GaSb substrates. Optically pumped laser emission at 3.9 &mgr;m was observed from 80 to 135 K with an exponentially dependent threshold withT0=17  K. At 80 K the threshold corresponds to an effective current of 4 kA/cm2. This value represents a significant reduction when compared with previous results. For a 1‐&mgr;m‐thick active layer the full angular width of the far field at the half intensity point normal to the junction is 40° and is in reasonable agreement with the width calculated from the guide and cladding refractive indices.