WDM transmission performance in optical networks can be monitored efficiently by first separating the electronic baseband channel mixtures that result after direct photodetection. We carry out this process through a blind signal separation (BSS) method based on higher-order statistics (HOS). Relative to a previously proposed method for WDM-channel extraction, the HOS-based procedure shows a reduced complexity with the number of channels, no stability problems, and an improved performance, as demonstrated by computer experiments.