1. A method of designing a mode-selective add-drop (MSADU) unit having an optical ADU waveguide for coupling signals from an input optical waveguide to a multi-mode waveguide, the optical ADU waveguide having a region for coupling said input optical waveguide, said method comprising: specifying a gap value d κ between said optical ADU waveguide and said multi-mode waveguide, a wavelength value λ l and, an allowable optical ADU waveguide width ω a x ; calculating one or more transfer matrices and propagation constants for modes in a coupling regions and quasi phase matching region between said optical ADU waveguide and said multi-mode waveguide; forming a representation of an output width of said ADU waveguide as a function of said calculated one or more transfer matrices and propagation constants for modes in coupling regions and quasi phase matching regions; minimizing a value of said representation such that a difference between a desired modal power distribution and an actual modal power distribution is minimized; calculating an optimum ADU waveguide width based on said minimized value; and, optimizing a profile of said coupling region of said ADU waveguide to preserve continuity in the width and at least a first derivative of said profile to minimize power loss, wherein a program using a processor unit executes one or more of said calculating one or more transfer matrices and propagation constants, forming, minimizing, calculating ADU waveguide width, and optimizing.