1. A mode-selective add/drop unit (MSADU) for a mode division de/multiplexing (MDM) device comprising: an optical ADU waveguide adapted for coupling to an input optical waveguide, said optical ADU waveguide having: at least one region providing optical signal coupling between the ADU waveguide and a multi-mode waveguide; and, one or more phase matching regions for controlling a phase difference between an electromagnetic wave (EMW) carried in the ADU waveguide and the multi-mode waveguide.