Prism coupling was used to investigate the nonlinear properties of semiconductor-doped glass slab waveguides. Experimental data show a dramatic reduction in the coupling efficiency, accompanied by asymmetries and multiple all-optical switching. Up to five distinct peaks were observed within a single Gaussian envelope, confirming the validity of a travelling wave interaction model. The nonmonotonic behaviour of output versus incident pulse energies is interpreted in terms of interference between Kerr and absorptive nonlinearities. Finally, the presence of band-filling nonlinearities has been confirmed using a picosecond laser source.