In WDM optical networks, signals have to be manipulated and monitored on a per wavelength basis. This requires that (de)multiplexing and wavelength processing functions are combined in the same optical component. The most efficient solution from a footprint size, cost and reliability point of view would be a combination of both functions in one photonic integrated circuit (PIC), fabricated by photolithography. Given that processing / detecting of individual wavelengths is an active function, in S, C and L communication bands naturally realized by InP and related semiconductor materials, InP-based PICs are widely agreed to be the superior choice.
This paper reports a practical design of such InP-based PICs. It is based on a building block approach, which allows a large variety of WDM optical components to be built from relatively few monolithically integrable elements, by using standard semiconductor fabrication technologies. These include: (i) (de)multiplexer based on a planar echelle diffractive grating with polarization compensation in the slab waveguide region; (ii) single-mode vertically integrated waveguide active devices with detecting, attenuating, amplifying and switching features, inserted in every (in)output waveguide channel of a planar (de)multiplexer, and (iii) passive waveguide circuitry required for coupling the light to and directing it through the InP-based photonic chip. Design and characterization examples of the building blocks and PICs for channel monitoring, variable attenuation / equalization and spatial switching are presented.