An external differential quantum efficiency exceeding 200&percent; has been observed from 4 &mgr;m InAs/InGaSb/AlSb interband cascade lasers under 1 &mgr;s pulses and 0.1&percent; duty cycle at 80 K. By increasing the pulse lengths and the repetition rates, average powers>16mW have been measured with 5 &mgr;s pulses at 10&percent; duty cycle, the internal quantum efficiency and the internal loss are determined to be 220&percent; and 14cm−1,respectively. ©1998 American Institute of Physics.