A broadband and CMOS-compatible polarization beam splitter and rotator (PSR) built on the silicon nitride-on-silicon multilayer platform is presented. The PSR is realized by cascading a polarization beam splitter and a polarization rotator, which are both subtly constructed with an asymmetrical directional coupler waveguide structure. The advantage of this device is that the function of PSR can be directly realized in the SiN layer, providing a promising solution to the polarization diversity schemes in SiN photonic circuits. The chip is expected to have high power handling capability as the light is input from the SiN waveguide. The use of silicon dioxide as the upper cladding of the device ensures its compatibility with the metal back-end-of-line process. By optimizing the structure parameters, a polarization conversion loss lower than 1 dB and cross talk larger than 27.6 dB can be obtained for TM-TE mode conversion over a wavelength range of 1450 to 1600 nm. For TE mode, the insertion loss is lower than 0.26 dB and cross talk is larger than 25.3 dB over the same wavelength range. The proposed device has good potential in diversifying the functionalities of the multilayer photonic chip with high integration density.

We present a wavelength multiplexing and polarization multiplexing device assisted by subwavelength grating (SWG), implemented on a 300-nm-tall silicon nitride (SiN) platform. The multiplexing device consists of three SiN waveguides- WG1, WG2 and WG3. The bridging waveguide WG2 between WG1 and output waveguide WG3 is designed into SWG structure, which executes the mode coupling thus multiplexing functions according to the principle of phase matching. The results are as follows: for wavelength multiplexing, the insertion loss (IL) is 0.54 dB and 0.58 dB at the center wavelength of O-band and C-band, which is 1310 nm and 1550 nm, respectively. The crosstalk (CT) is -12.5 dB and -23 dB at these two wavelength. The 1-dB bandwidth is larger than 170 nm for both bands. For polarization multiplexing, the IL is 0.44 dB and 0.64 dB at the wavelength of 1310 nm and 1550 nm, with the crosstalk lower than -16.2 dB and -22.8 dB. The 1-dB bandwidth is larger than 180 nm for the two bands.

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 commerical foundry.