Compact high extinction ratio high-order mode pass filter based on inverse-designed ultra-compact unidirectional mode...

Mode division multiplexing (MDM) technology provides a pathway to enhance channel capacity beyond wavelength division multiplexing, positioning it as a pivotal advancement for next generation optical communications. Mode filters are essential for the low-loss transmission of specific modes and the reduction of modal crosstalk, thereby enhancing the feasibility of MDM systems. Although suppressing high-order mode is relatively straightforward, effectively blocking low-order modes poses a more intricate challenge. In this paper, we introduce a high-order mode pass strategy, effectively blocking low-order modes using the unidirectional mode converters. Specifically, a TE₁ high-order mode pass filter (HOMPF) is demonstrated on a silicon-on-insulator platform, utilizing a unique inverse-designed ultra-compact unidirectional TE₀-TE₁ mode converter. Experimental results show the TE₁-TE₁ insertion loss of the HOMPF of below 1.0 dB and an average TE₀-TE₀ extinction ratio of 36.8 dB (42.1 dB for 2-cascaded HOMPF) within the C-band range of 1525-1565 nm. Additionally, the scalability of the HOMPF structure is explored, with simulations demonstrating a TE₂ HOMPF. The proposed HOMPFs feature simplicity, compactness, low loss, and high extinction ratio, making them promising components for mode manipulation in MDM systems.