Subpicosecond heterodyne four wave mixing experiments on InGaAsP semiconductor laser amplifiers

We give a detailed description of a recently proposed femtosecond heterodyne four-wave mixing experiment to investigate ultrafast coherent dynamics in waveguides and discuss similarities to femtosecond heterodyne pump-probe experiments. We demonstrate and discuss the potential and the limitations of the new technique on the basis of first experiments. Examples of measurements on an InGaAsP semiconductor amplifier are presented, that include transient four-wave mixing between pulses of the same and of orthogonal polarisations. Implications of our four-wave mixing results on the ultrafast dynamics in semiconductor amplifiers are discussed.