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In this paper, we present a numerical investigation of the phase shift experienced by the detected envelope of a sinusoidally modulated optical carrier propagating in a nonlinear semiconductor optical amplifier. The model we employed accounts for high-order coherent population oscillations. As such, it allows for large gain levels and arbitrary modulation indexes of the input field. We highlight the role of the fields that are generated by high-order nonlinearities and compare the results with those of more common small-signal nonlinear models. We find that conventional models suffice only for low powers, small modulation indexes, and moderate amplifier gain levels.