High extinction ratio, low loss and broadband on-chip TE-pass polarizer for optical gyroscope

Optical gyroscope is an important high-precision inertial measurement instrument, which plays significant role in defense, geological exploration and navigation. For high accuracy optical gyroscope system, a high-performance polarizer is strongly needed to filter only one polarization from Amplified Spontaneous Emission (ASE) source. Here, the plasmon-assisted structure is introduced by setting a thin metal layer above the silicon nitride (Si₃N₄) waveguide. The copper (Cu) is chosen as the metal layer material as it is compatible with the complementary metal-oxide-semiconductor (CMOS) fabrication process. The surface plasmons effect can simply be excited by TM polarization due to the electric field of TM polarization perpendicular to the metal layer, so that it can bring extra propagation loss. The TE polarization, on the contrary, fail to excite the surface plasmons effect as the electric field is parallel to the metal layer. To reduce the reflection caused by mode mismatch, the distance between metal layer and waveguide is increased by inserting a SiO₂ layer. The chemical mechanical planarization (CMP) process allows precise thickness control of the SiO₂ layer. The spiral waveguide structure is utilized to fully suppress TM polarization while the TE polarization can be well confined in broadened Si₃N₄ core with negligible propagation loss. The numerical results show that the working wavelength range is as large as 60 nm from 820 nm to 880 nm with the polarization extinction ratio > 30 dB and the insert loss < 0.5 dB. As far as we known, this is the first time to achieve ultra-high extinction ratio, ultra-low insertion loss, ultra-low reflection at the same time, and also achieve a working wavelength range larger than 60 nm at the center of 850 nm. Moreover, the proposed structure doesn't require high alignment accuracy and is compatible with silicon-on-insulator fabrication technology.