High performance on-chip polarization beam splitter at visible wavelengths based on a silicon nitride small-sized...

Due to sensitive scaling of the wavelength and the visible-light absorption properties with the device dimension, traditional passive silicon photonic devices with asymmetric waveguide structures cannot achieve polarization control at the visible wavelengths. In this work, a simple and small polarization beam splitter (PBS) for a broad visible-light band, using a tailored silicon nitride (Si₃N₄) ridge waveguide, is presented, which is based on the distinct optical distribution of two fundamental orthogonal polarized modes in the ridge waveguide. The bending loss for different bending radii and the optical coupling properties of the fundamental modes for different Si₃N₄ ridge waveguide configurations are analyzed. A PBS composed of a bending ridge waveguide structure and a triple-waveguide directional coupler was fabricated on the Si₃N₄ thin film. The TM excitation of the device based on a bending ridge waveguide structure shows a polarization extinction ratio (PER) of ≥ 20 dB with 33 nm bandwidth (624-657 nm) and insertion loss (IL) ≤ 1 dB at the through port. The TE excitation of the device, based on a triple-waveguide directional coupler with coupling efficiency distinction between the TE₀ and TM₀ modes, shows a PER of ≥ 18 dB with 50 nm bandwidth (580-630 nm) and insertion loss (IL) ≤ 1 dB at the cross port. The on-chip Si₃N₄ PBS device is found to possess the highest known PER at a visible broadband range and small (43 µm) footprint. It should be useful for novel photonic circuit designs and further exploration of Si₃N₄ PBSs.