We demonstrate multi/demultiplexers with a very flat passband and small excess loss using a multi-input arrayed waveguide grating combined with a cascaded Mach–Zehnder interferometer structure. We also analyze the impact of coupling between input waveguides just before the input slab on chromatic dispersion and investigate a waveguide structure for reducing the chromatic dispersion. We fabricated flat-passband multi/demultiplexers with 100-GHz channel spacing using 2.5%-Δ silica waveguides and experimentally investigated the dependence of their optical characteristics on design parameters to demonstrate the effectiveness of the proposed theoretical model. We obtained a very flat passband response having a 1-dB bandwidth of 0.645–0.658 nm and 20-dB bandwidth of 0.944–0.960 nm with a loss penalty due to the passband-flattening of 0.9–1.0 dB and chromatic dispersion of -22.1–17.5 ps/nm over 0.5 nm in the passband. The measured passband shape and chromatic dispersion generally agreed with the simulation results.
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