Circulator-less millimeter-wave transceiver front end in thin-film lithium niobate

For decades, electro-optic modulation of an optical carrier with microwave signals has been proposed as a fully-photonic alternative to electronic receiver front ends. However, the typically low microwave-to-optical conversion efficiencies have led to high noise figures, limiting practical use of this technique. Here, we implement a doubly-resonant electro-optic modulator in a thin-film lithium niobate integrated circuit that converts 200 MHz-wide signals from 10-100 GHz into the optical domain. High-Q racetrack resonators provide sufficient enhancement to theoretically achieve on-chip receiver noise figures below 5 dB without preamplification. Due to phase matching conditions, the optical whispering-gallery modes undergo modulation only by co-propagating millimeter-waves (mmWaves) traveling in the reception direction, allowing counter-propagating transmitter waves to pass undetected. This unidirectional behavior enables a compact photonic transceiver front end when paired with a power amplifier and antenna, eliminating bulky circulators and low-noise amplifiers. Though narrowband, this front end could support stable mmWave links for time transfer between remote optical clocks, paving the way for integrated optical clocks with built-in photonic-enabled wireless synchronization.