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InGaAsSb/AlGaAsSb quantum well (QW) diode laser structures with either 1% or 1.5% compressively strained QWs were grown on GaSb substrates by molecular beam epitaxy. Wide-stripe lasers fabricated from structures of both types have room-temperature operating wavelengths near 2.3 microns. The room-temperature threshold current density of 1-mm-long uncoated devices with 1.5% strained QWs was lower than threshold current density of the 1.0% strained QW devices by nearly a factor of two (120 A/cm2 versus 230 A/cm2 ). Experiment shows that the reduction in threshold current density with increasing QW strain is related to the increase in differential gain and decrease in transparency current density. Optical gain calculations prove that improvement of the QW hole confinement reduces the threshold carrier concentration in laser structures with heavily strained low arsenic content quantum wells.