This paper describes a theory of amplified spontaneous emission in index-guided multimodal waveguide structures. It is based on a pertubation approach taking into account the correlated excitation of the different guided modes of the unperturbed waveguide. The coupling between these modes due to the propagation through a perturbed waveguide is taken into account by deducing evolution equations of the cross coorelations between the modal amplitudes. Eventually, a beam propagation method program, including the spontaneous emission model, has been used for a comparison of the noise characteristics of a single-mode SOA with a 1-by-1 multimode interference semiconductor optical amplifier. Experiments have been performed comfirming the obtained simulation results.