An optical amplifier arrangement comprises a splitter for providing an input WDM optical signal on at least first and second output paths. An optical amplifier is provided in at least one of the paths, and a signal routing arrangement is provided for routing individual channels or groups of channels of the WDM signal within the at least one of the paths. A switch selects the signal from one of the at least first and second output paths. In this arrangement, there are two paths between the input and output. Whilst one path is being used, the components in the other path can be upgraded. For example, a switching arrangement may be provided in the other path, and an amplifier may also be upgraded, without disrupting service in the path in use. This enables an amplifying node within an optical communications system to be incrementally upgraded.