Separation of coherent and incoherent nonlinearities in a heterodyne pump-probe experiment

We demonstrate that the transient coherent nonlinearity (coherent artifact) affecting the pump-probe response of semiconductor optical amplifiers an b e experimentally separated from the incoherent transient. The technique is based on measuring the mirror component of the coherent artifact which is a background-free four wave mixing signal at a different frequency with resp ect to the transmitted probe in a heterodyne detection scheme. Measurements on amplifiers of different length reveal strong deviations from the common ly expected symmetric shape of the coherent artifact in case of long wavegu ides. (C) 2000 Optical Society of America.