Design and fabrication of low-loss broadband transmission filters for dense wavelength-division multiplexing networks

We present a method to design broadband optical transmission filters from complex spectrum profiles, for application in dense wavelength-division multiplexing (DWDM) systems. A discrete inverse scattering method, known as layer-peeling, is used to reconstruct a long-period fibre grating (LPG) structure from a specified realisable complex spectrum by a direct solution of the coupled-mode equations, while simultaneously determining the physical properties of the layered structure. The low algorithm complexity, and relative short computation time when compared to classical grating analysis techniques, are the main advantages of the layer-peeling method. Results obtained from the layer-peeling method, are optimised using the flip-flop refinement method to ease the fabrication process. We also present a method to manufacture symmtrically-written LPGs in optical fibre. The fabrication process is based on the point-by-point method using a continuous-wavelength (CW) carbon-dioxide (CO2) laser to induce the necessary index changes in the fibre core.