A Three-Dimensional Silicon Nitride Polarizing Beam Splitter

A 3-D polarizing beam splitter based on a silicon nitride (Si₃N₄) vertical directional coupler is experimentally demonstrated. A new planarization technique by incorporating conventional chemical-mechanical lapping with a dry-etching process is developed, in order to obtain a flat film surface for the second Si₃N₄ core deposition after the first-layer waveguide is formed. Both the Si₃N₄ layer thicknesses are 200 nm. As there is a material refractive index mismatch between the vertically separated waveguides, the polarization splitter can be realized with a bottom waveguide width of 1.55 μm and a top core width of 1.35 μm. The transverse electric (TE) polarized light can be transmitted completely to the cross-layer output-port, whereas the transverse magnetic (TM) polarized wave outputs mostly from the port at the input layer. A high-extinction ratio and a wide operation bandwidth can be achieved. An extinction ratio of 26 dB for the cross-layer output-port at 1550-nm wavelength and that of 16 dB for the input-layer output-port are obtained. There is an excess coupling loss of for the TE light, but a 1-dB loss for the TM wave.