A systematic study has been made of the effect of the middle active region width in double heterojunction Pb1−xSnxTe/PbTe laser diodes on the lasing threshold. A fourfold reduction in pulsed threshold over that of comparable single heterojunction lasers is demonstrated in lasers having 4‐&mgr;m‐wide middle regions. A comparison is also made between lasers fabricated by liquid phase epitaxy using undoped melts and substrates, and using thallium‐dopedp‐type PbTe layers; the latter is shown to yield thresholds as much as two times lower.