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 (TM0) mode in the narrow input/output waveguide and the first higher order (TM1) 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 TE0 mode gets out from the through port while the launched TM0 mode is coupled completely to the cross port with the assistance of the first higher order (TM1) mode in the wide middle waveguide. The designed PBS has a compact footprint of 38 × 3.5 μm2. 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.
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