Efficient TE-Polarized Mode-Order Converter Based on High-Index-Contrast Polygonal Slot in a Silicon-on-Insulator Waveguide

We proposed a new type of transverse electric (TE) polarized mode-order converter based on a deeply-etched polygonal slot on a silicon-on-insulator waveguide. Along the transverse direction of the waveguide, two irregular boundary surfaces of the slot can introduce high-contrast index modulation on guided modes, leading to multimode interference in the slot. Therefore, when the slot is optimized, we can achieve efficient mode conversions based on the multimode interference. As examples, mode converters from the fundamental TE mode (TE0) to the first-order TE mode (TE1) and to the second-order TE mode (TE2) have both been demonstrated with a short device length (<;24.0 μm), a high mode conversion efficiency (>97.6%), and a low modal crosstalk (<; -20.0 dB) over a broad wavelength range from 1500 to 1600 nm (~100 nm). In addition, based on different polygonal slots, other types of mode conversions such as from TE1 to TE3 and from TE2 to TE1 have be realized. Fabrication tolerance of the proposed structure is analyzed. Owing to the high efficiency and compact size, the proposed structure could be applied to on-chip mode division multiplexing systems for high-density integration.