In1−xGaxP1−zAszdouble heterojunctions (77 °K, yellow) are shown to exhibit either (or both) homogeneous or inhomogeneous line broadening when operated as lasers in an external grating cavity. Just above threshold, inhomogeneous line broadening is observed over much of the recombination spectrum. Well above threshold (large gain), homogeneous line broadening is observed when the grating is tuned onto the cavity modes in the region of line center.