Electro-Optically Tunable Low Phase-Noise Microwave Synthesizer in an Active Lithium Niobate Microdisk

Photonic-based low-phase-noise microwave generation with real-time frequency tuning is crucial for a broad spectrum of subjects, including next-generation wireless communications, radar, metrology, and modern instrumentation. Here, for the first time to the best of the authors’ knowledge, narrow-bandwidth dual-wavelength microlasers are generated from nearly-degenerate polygon modes in a high-Q active lithium niobate microdisk. The record-high-Q (≈10⁷) nearly-degenerate polygon modes formation with independently controllable resonant wavelengths and free spectral ranges is enabled by the weak perturbation of the microdisks using a tapered fiber. Moreover, because a high spatial overlap factor between the pump and the dual-wavelength laser modes is achieved, the gain competition between the two lasing modes spatially separated with a π-phase difference is suppressed, leading to stable dual-wavelength laser generation with low threshold, and in turn, the low noise microwave source. The stable beating signal confirms the low phase-noise achieved in the tunable laser. Without the need of external phase stabilizers, the measured microwave signal shows a phase noise of −123 dBc Hz⁻¹ and an electro-optic tuning efficiency of −1.66 MHz V⁻¹. The linewidth of the microwave signal is measured as 6.87 kHz, which is more than three orders of magnitude narrower than current records based on integrated dual-lasers.