A monolithic transmitter photonic integrated circuit (TxPIC) chip and a monolithic receiver photonic integrated circuit (RxPIC) chip include a plurality of optical signal channels together with other active elements integrated on a semiconductor chip, which chips further include an optical combiner or decombiner that is a wavelength selective comprising a supergrating or an Echelle grating which provides for a more compact chip compared to an integrated on-chip arrayed waveguide grating functioning as a wavelength selective combiner or decombiner.