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We describe an optical pulse mixing experiment with a two‐segment GaAs/AlGaAs single quantum well, graded‐index separate confinement heterostructure laser to determine the effective lifetime of photogenerated carriers in a short reverse‐biased segment with subpicosecond resolution. This lifetime is of importance if such a segment is used as a fast photodetector or as a saturable absorber in a monolithic mode‐locked laser structure. We found that the lifetime depends not only on the applied bias but also on the excitation pulse energy. Lifetimes shorter than 5 ps have been observed. The strong dependence on excitation pulse energy is attributed to screening effects of the escaped carriers. A simple model based on an exact solution of the one‐dimensional Schrödinger equation for a particle in a quantum well in an electric field together with an electrical equivalent circuit verifies this assumption. © 1994 American Institute of Physics.