Picosecond pulses are observed to exhibit large frequency shifts (∼50 GHz) and spectral distortion on propagation through semiconductor laser optical amplifiers. The spectral changes are shown to result from self‐phase modulation occurring due to the carrier‐induced index changes that invariably accompany gain saturation. An analytic model for pulse amplification is presented by including the self‐phase modulation effects. Its predictions are in good agreement with the experimental results.