A compact and broadband asymmetrical polarization beam splitter on SiN-on-silicon platform

Silicon nitride (SiN) is an ideal material which is compatible with complementary metal-oxide-semiconductor (CMOS) technology. Its advantages include suitability for high power handing, a large spectral range and low thermo-optic coefficient, etc. A polarization beam splitter (PBS) is one of the significant polarization handling devices for integrated silicon photonic circuits. In this paper, a low insertion loss (IL), broadband and high extinction ratio (ER) PBS with a compact coupling length of 6 μm based on an asymmetrical directional coupler (ADC) was proposed. It consists of a fully-etched silicon (Si) strip waveguide and a silicon nitride (Si3N4) strip waveguide with a vertical gap of 100 nm. By carefully optimizing the parameters, the input transverse magnetic (TM) mode polarization from the Si waveguide will couple to the Si3N4 waveguide due to the phase matching, while the input transverse electric (TE) mode polarization will keep propagating in the silicon waveguide due to the large refractive index difference between the two waveguides. For the TE polarization mode, the simulation results show that the IL is less than 0.23 dB and the ER is higher than 56 dB in the wavelength range of 1300-1900 nm. For the TM polarization mode, the numerical results show that the IL is less than 1 dB and the ER is higher than 12 dB in the wavelength range of 1424-1712 nm. Meanwhile, our design also has high fabrication tolerance and is suitable for production on