Just as light have different polarization when propagating in free space, there are similar properties in integrated optical waveguides, we call it 'TE' polarization and 'TM' polarization. The electric field direction of TE polarization in silicon photonics chip is perpendicular to the substrate, while that of TM polarization is parallel to the substrate.
The orthogonality of the electric field implies that the two modes have no crosstalk while propagating in a waveguide simultaneously, thus realizing the advanced functions such as polarization division multiplexing. The slot waveguides, which owns the special structure compared to the convention strip waveguides, are irreplaceable in many applications. However, the approach to reduce the optical insertion loss or the reflection at the transition area between the slot waveguide and strip waveguide becomes a great issue.
Recently, researchers led by Prof. Xingjun Wang at Peking University proposed a novel structure to solve the above problem. A novel structure as sinusoidal-profile with a compact size (1.1 μm × 1.2 μm) is proposed for the transition area to excite the multimode interference (MMI) effect, which not only decreases the optical mode field mismatching but also has an ultra-compact device size.
The TE polarization and TM polarization are supported simultaneously. The insertion losses for TE0 and TM0 polarizations are measured as low as 0.40 and 0.64 dB, respectively. The polarization-dependent loss is 0.24 dB, indicating that the converter can handle a variety of applications where both TE0 and TM0 polarizations are needed. This study entitled "An ultra-compact polarization-insensitive slot-strip mode converter" was published in Frontiers of Optoelectronics on Apr. 2, 2022.
Reference: Tao, Z., Wang, B., Bai, B., Chen, R., Shu, H., Zhang, X., Wang, X.. An ultra-compact polarization-insensitive slot-strip mode converter. Front. Optoelectron. 15, 5 (2022). https://doi.org/10.1007/s12200-022-00008-5