The invention describes an integrated-photonics arrangement, implementable in a multi-layer III-V semiconductor structure, which has a semiconductor substrate; an epitaxial semiconductor structure grown on this substrate in one growth step; a common waveguide; and a plurality of wavelength-designated waveguides; all the waveguides being formed in this epitaxial structure using conventional semiconductor processing techniques. Each waveguide being defined by the bandgap wavelength of its core region and all the waveguides being arranged vertically in order of ascending bandgap wavelength; with the common waveguide placed at the bottom of the structure and the wavelength-designated waveguide having the longest bandgap wavelength placed at the top of the structure. In use, the bandgap wavelength of the common waveguide being well below any operating wavelength, therefore providing conditions for low-loss propagation of each operating wavelength to its designated waveguide through the common waveguide. The invention discloses a method of wavelength demultiplexing (multiplexing) for optical signals in a plurality of wavelengths, which are co- or bi-directionally propagating within the integrated-photonics arrangement, by vertical splitting them from (combining them into) the common waveguide into (from) wavelength designated waveguides.