Integrated electro-optic circulator on thin-film lithium niobate for bidirectional optical fibre transmission

Optical circulators are a key component for optical systems, routing light between ports to manage signal flow and protect components from back-reflection. As integrated photonics advances towards chip-scale systems, incorporating key optical building blocks on chip is increasingly important. Optical circulators have been slow to follow this process. Although conventional circulators break Lorentz reciprocity using magneto-optic materials, they are impractical to integrate into devices. Here we realize a broadband electro-optic travelling-wave optical circulator based on thin-film lithium niobate that achieves non-reciprocity without magnetic materials, resonant structures or wavelength conversion. Our architecture uses cascaded travelling-wave Mach–Zehnder modulators driven by electrical sinusoidal signals, achieving up to 37 dB peak isolation, spanning the full O- and C+L optical telecom bands (14 THz bandwidth). The device shows excess losses of 0.1 dB (port 1 to 2) and 4.9 dB (port 2 to 3), with 0.84 dB of passive losses. To demonstrate practicality, we deploy this circulator in same-wavelength, bidirectional coherent transmission, achieving 400G/polarization with industry-compliant hardware, digital signal processing and forward-error-correction codes. These results establish a scalable route to broadband on-chip circulation, with relevance to industry-standard optical fibre transmission systems and position electro-optic non-reciprocity as a manufacturable, foundry-compatible building block for the next generation of integrated optical transceivers. A thin-film lithium niobate integrated circulator with peak isolation up to 37 dB and a broad transmission bandwidth supports bidirectional transmission through single-mode fibre at 800G standard throughput.