A theoretical model describing noise transfer characteristics of all-optical wavelength converters based on cross-gain modulation in semiconductor optical amplifiers (SOA) is given in this paper. The analytical results show that the noise transfer is related with the input pump light power. the input probe light power, and the internal loss of the SOA. After wavelength conversion, the noise at the on-level of the input signal can be effectively suppressed while that at the off-level is enhanced. the overall effect is that the signal quality is improved. The theoretical model yields a good agreement with the previous experimental results. (C) 2002 Elsevier Science Ltd. All rights reserved.