Spectral line broadening in semiconductor lasers is studied experimentally through the gain saturation characteristics of 1.5 &mgr;m InGaAsP traveling‐wave laser amplifiers. Wide signal gain spectrum under the injection of an intense saturating signal is measured using a weak probe signal. The saturated signal gain spectrum is found to coincide exactly with the unsaturated spectrum under a less biased condition, thus verifying that the semiconductor laser gain saturateshomogeneouslyover the entire gain spectrum. Cross‐saturation characteristics between the two signal channels having identical input powers are also investigated and found to be in good agreement with theoretical calculations based on the homogeneous gain model. The degree of gain saturation is confirmed to be uniquely determined by the total output power from both channels.