Synthesis of Multilayer and Waveguide Filters for use in Optical Communication Systems

We report on a new synthesis method of reflection filters with multilayer and waveguide structures. The filter is characterized by bandwidth as small as ≤ 1 nm for effective use in wavelength-division-multiplexed optical communications. Both the transfer-matrix method and the Fourier transformation are employed to determine the filter reflection from a spatially varying refractive index. In contrary to Bragg reflectors that utilize periodic index variations, the present synthesis model employs aperiodic variation around an averaged index. The calculations show that the reflectivity spectrum for 25 pairs of periodic GaAs-AlGaAs quarter-wavelength Bragg stacks is a single band with broad plateau (≥ 100 nm) surrounded by high lobes (> 80%). We show that these side-lobes are suppressed to less than 40% when the same number of layers is used but with an aperiodic varying thickness. Moreover, by exceeding the filter length to 2 mm, we achieved reflectors with 99% reflectivity, bandwidth of ≤ 1 nm and low side-lobes (<-13dB). Similar characteristics are attained using a GaAs/AlGaAs waveguide with a corrugated structure on the upper interface of the core. The corrugated profile of the core-thickness is obtained for an asymmetric waveguide. We also demonstrate accomplishment of two-band reflectors for use as minus (transmission) filters.