This patent document provides optical processing and switching of optical channels based on mode-division multiplexing (MDM) and wavelength division multiplexing (WDM). In one implementation, a method is provided for processing different optical signal channels to include receiving different input optical signal channels in different optical waveguide modes and in different wavelengths; converting input optical signal channels in multimodes into single-mode optical signal channels, respectively; subsequent to the conversion, processing single-mode optical signal channels obtained from the different input optical signal channels to re-group single-mode optical signal channels into different groups of processed single-mode optical signal channels; and converting different groups of the processed single-mode optical signal channels into different groups of output optical signal channels containing one or more optical signal channels in multimodes multimode signals to direct the groups as different optical outputs.