Ultrabroadband On-Chip Optical Circulator via Traveling-Wave-Driven Nonreciprocal Mode Conversion

Nonreciprocal optical devices such as isolators and circulators form the backbone of modern optical systems by enabling unidirectional light routing and signal isolation. The rapid development of on-chip photonic systems, driven by the cointegration of semiconductor lasers and optical components, demands fully monolithic nonreciprocal devices without reliance on magneto-optical materials. We propose and demonstrate a traveling-wave-driven optical circulator on the lithium niobate-on-insulator (LNOI) platform, leveraging electro-optically induced nonreciprocal TE0-TE1 mode conversion. The device architecture combines a traveling-wave electro-optic (E-O) nonreciprocal mode converter with two mode (de)multiplexers, while a square-wave radiofrequency (RF) signal is synchronized to introduce the direction-dependent phase modulation as desired. Experimentally, the fabricated multiport optical circulator with the 1.5 cm-long modulation region shows low excess losses of 0.5–2 dB as well as high extinction ratios exceeding 15 dB (counter-propagation) and 10 dB (copropagation) across the wavelength range of 1530–1605 nm, achieved with 21 dBm and 4.1 GHz RF driving signals. Note that this approach eliminates the need for magneto-optical materials and enables full circulation-path reconfigurability through controlling the injected RF signals. The present approach of merging an E-O nonreciprocal mode converter and mode (de)multiplexers opens a promising option for developing nonmagnetic nonreciprocity in photonic integrated circuits, with direct applicability to quantum photonics, optical communications, and programmable photonic networks.