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The origin of the nonlinear gain effect in 1.5 μm semiconductor active layers is investigated by using highly nondegenerate four‐wave mixing, where the pump‐probe detuning is extended up to 2 THz. From the signal intensity measured as a function of the detuning frequency we find that both the spectral hole burning and the dynamic carrier heating contribute to the four‐wave mixing. The dynamic carrier heating, however, creates the index grating rather than the gain grating, and hence, the spectral hole burning is the main origin of the nonlinear gain effect.