Measurements of the variation of differential gain, refractive index, and linewidth enhancement factor with carrier density in InGaAs‐GaAs strained‐layer quantum well lasers are presented for the first time. These results verify predictions of improvement over unstrained bulk or quantum well lasers, but only at certain carrier densities. Differential gain (dg/dN) is found to vary from 7.0×10−16to 2.5×10−16cm2over the range of carrier densities studied, while the carrier dependence of the real part of the refractive index (dn/dN) ranges from a peak of −2.8×10−20down to −7.0×10−21cm3. From these measurements the resulting linewidth enhancement factor (&agr;) is found to vary from 5 to a minimum of 1.7. This information is critical to successfully exploiting the potential advantages of strained‐layer lasers for such devices as high‐frequency or narrow linewidth lasers.