Abstract: The apparatus extracts from an optical transmission path bidirectionally-propagating groups of wavelength-division multiplexed optical signals, arranges the groups to propagate unidirectionally via an output port, receives a corresponding unidirectional group via an input port, sorts the corresponding unidirectional group into two groups corresponding to those extracted from the transmission path, and returns the two corresponding groups of wavelength-division multiplexed signals to the transmission path so that each group propagates in its original direction. The apparatus comprises first and second bidirectional ports for coupling in series between first and second sections of the transmission path, and a series of optical devices each comprising first, second, third and fourth ports and an optical bandpass filter or other wavelength-selective element. Each of the filters selects a different wavelength.