Ultrasmall Four-Channel Multiplexer Designed by Bayesian Direct Binary Search Method

We propose two kinds of mosaic-based four-channel multiplexers operating with 20-nm channel spacing in the C- and O-bands, respectively. Mosaic devices feature a complex refractive-index distribution including etched holes. A triangular lattice is introduced to maximize the hole density per area and facilitate fabrication. The hole configurations are optimized using the Bayesian direct binary search method. The multiplexers are the smallest among those validated through experimental demonstration with 20-nm channel spacing. The C-band multiplexer exhibits a low insertion loss of 3 dB and dimensions of 13.8 μm × 7.58 μm. Tolerance analyses indicate that spectral shifts caused by waveguide width deviations is 1/10 of that caused by hole diameter deviations. The O-band multiplexer, suitable for coarse wavelength division multiplexing systems, exhibits an insertion loss of 3.6 dB and dimensions of 12.7 μm × 7.9 μm. Mosaic devices are a powerful option as ultrasmall passive components for large-scale photonic integrated circuits.