Compact Polarization Beam Splitter Based on a Three-Waveguide Asymmetric Coupler With a 340-nm-Thick Silicon Core Layer

A compact polarization beam splitter (PBS) is presented for the case with a 340-nm-thick silicon core layer by using an asymmetric evanescent coupling system. It consists of a narrow input waveguide, a narrow output waveguide, and a wide middle waveguide between them. In this design, the widths of these three waveguides are chosen optimally, so that the phase matching condition is satisfied for the fundamental (TM
₀
) mode in the narrow input/output waveguide and the first higher order (TM
₁
) mode in the wide middle waveguide. Meanwhile, there is moderate phase mismatch for TE polarization. Then, the length of the coupling region is also optimized, so that the launched TE
₀
mode gets out from the through port while the launched TM
₀
mode is coupled completely to the cross port with the assistance of the first higher order (TM
₁
) mode in the wide middle waveguide. The designed PBS has a compact footprint of 38 × 3.5 μm
²
. In theory, the designed PBS works very well with a low excess loss (<;0.5 dB) and a high extinction ratio (>15 dB) in a broad band of 1510-1570 nm. For the fabricated PBS, the extinction ratios are >10 dB and >20 dB in the wavelength range of 1535-1610 nm, for TM and TE polarizations, respectively.