InAsSb/InAsSbP double heterojunction lasers have been grown by liquid phase epitaxy in which free carrier absorption loss was investigated and minimized by the introduction of two undoped quaternary layers on either side of the active region. The diode lasers operate readily in pulsed mode at elevated temperatures and emit near 3.45 mu m with a threshold current density as low as 118 A/cm(2) at 85 K. Compared to the conventional 3-layer DH laser, reducing the optical loss increases the maximum lasing temperature by 95 K to similar to 210 K in the optimized 5-layer structure.