Disclosed is an add-drop multiplexer that receives an optical signal having a plurality of channels. The multiplexer spatially separates the channels, and a spatial light modulator within the multiplexer, which in some embodiments is a switched blazed grating, routes the channels along first or second paths according to whether the particular channels are to be sent along as a part of an output communication signal or “dropped” into a dropped-channel optical communications signal. The add-drop multiplexer is also operable to receive optical channels to be added to an optical signal and to use a spatial light modulator to add those optical signals to that optical signal.