Observation of highly nondegenerate four-wave mixing in 1.5 μm traveling-wave semiconductor optical amplifiers and estimation of nonlinear gain coefficient

Nondegenerate four-wave mixing is measured in the 1.5 μm traveling-wave semiconductor optical amplifier medium as a function of the pump-probe detuning frequency ranging from a few GHz to 400 GHz. It is found that two different sources are responsible for the four-wave mixing: the carrier density modulation and the nonlinear gain effect. The latter is clearly observed when the detuning frequency increases above 100 GHz. The nonlinear gain coefficient ε, which induces a gain grating through the pump-probe beating, is estimated to be 1.75×10^-23 m³. The change in the real refractive index associated with the nonlinear gain effect, which generates an index grating, is negligibly small. The relaxation time involved in the nonlinear gain effect is found to be less than 0.3 ps. These results support the role of the spectral hole burning rather than the carrier heating in the nonlinear gain effect