Scalable compact on-chip multimode circulator with low loss and broad bandwidth.

High-performance multimode optical devices are critical for photonic integrated circuits (PICs) and mode-division multiplexing (MDM), but conventional fiber-based multimode circulators (MMC) lack compactness and most on-chip circulators only support single-mode operation, limiting on-chip multimode transmission. To address this, we propose and demonstrate a simplified, N-mode-scalable on-chip MMC. Incorporating two symmetric N-junctions and ⌊N/2⌋ tapered π-phase shifters, it supports off-chip multi-port and on-chip multi-mode circulation. The fabricated silicon waveguide 3-mode prototype with chip size ~4.5 × 223.5 μm2 shows insertion loss <1.9 dB at wavelength 1550 nm (<3.8 dB over 1500-1620 nm), and crosstalk <-12.1 dB. It provides a novel solution for on-chip MDM directional transmission and isolation, promoting the advancement of multi-mode PICs.