The longitudinal spatial hole burning (LSHB) in gain-clamped semiconductor optical amplifiers (GCSOA's) is investigated by means of a numerical model, which is based on position-dependent rate equations for the carrier density and the propagation equations for the optical power, The simulation results show that the carrier densities are nonuniformly distributed within the active layer of GCSOA's, The nonuniformity can be large, especially for high currents and optical signal powers near the saturation. It is found that the LSHB induces a gain nonlinearity, which causes interchannel cross talk when GCSOA's are used in wavelength division multiplexing (WDM) applications. In order to reduce this gain nonlinearity, two methods are analyzed: the use of low resistivity devices and the use of unbalanced Bragg mirror reflectivities.