A group of waveguide grating couplers is disposed on a semiconductor substrate. The grating couplers are all within a spot illuminated on the substrate by a light from an optical fiber. The light propagating in the fiber is wavelength division multiplexed (WDM) and consists of several channels. Within the group of grating couplers, at least one grating coupler is designed to be tuned to each of the channels. The group of grating couplers demultiplexes the channels propagating in the fiber. A group of waveguide grating couplers can also be used to multiplex several channels of light into an optical fiber. Single mode and multimode fiber can be used to carry the multiplexed channels of light in an optical multiplexing and demultiplexing system.