Noise properties of nonlinear semiconductor optical amplifiers.

We demonstrate experimentally and describe analytically two important noise contributions in saturated semiconductor optical amplifiers. The first is spontaneous emission power enhancement that is due to the increased and spatially dependent inversion factor. The second is caused by a nonlinear interaction between the saturating signal and the amplifier noise that, in the optical domain, causes a distinct modification of the noise spectrum over a narrow bandwidth. This modification manifests itself as a noise power decrease in the detected electronic domain.