By using the technique of noncollinear nearly-degenerate four-wave mixing, ambipolar diffusion coefficients and carrier lifetimes were directly determined for a compressively-strained InGaAsP multiple quantum well semiconductor optical amplifier operating at 1.3 &mgr;m. A diffusion coefficient of8.0 cm2/sand a carrier lifetime of 1.33 ns, were obtained at the amplifier current density of 1.88kA/cm2.The current-density dependent measurements show that the diffusion coefficient will drop with increasing amplifier pumping current, which is consistent with the prediction of the conventional semiconductor theory. ©1997 American Institute of Physics.