A planar lightwave circuit including a slab waveguide, with a plurality of different diffraction filtering elements optically coupling a plurality of input and output ports, provides various optical functionalities including multiplexing, demultiplexing, diplexer and triplexer. In the basic configuration one or more output ports are optically coupled to an input port via grating filters etching in the cladding of the slab waveguide, and an additional port is optically coupled to the input port via a diffraction grating etched in an endwall of the slab waveguide. A triplexer platform can be provided by optically coupling photo-detectors to two output ports, which receive wavelength channels demultiplexed from an input signal by two cladding etched filters, and by optically coupling a laser to another input port, which launches an outgoing laser signal at the endwall etched grating filter for coupling into the same fiber that launched the original input signal.