  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 our knowledge, narrow-bandwidth
dual-wavelength microlasers are generated from nearly degenerate polygon modes
in a high-Q active lithium niobate microdisk. The high-Q polygon modes
formation with independently controllable resonant wavelengths and free
spectral ranges is enabled by the weak perturbation of the whispering gallery
microdisk resonators using a tapered fiber. The stable beating signal confirms
the low phase-noise achieved in the tunable laser. Owing to the high spatial
overlap factors between the two nearly degenerate lasing modes as well as that
between the two lasing modes and the pump mode, gain competition between the
two modes is suppressed, leading to stable dual-wavelength laser generation and
in turn the low noise microwave source. The measured microwave signal shows a
linewidth of ~6.87 kHz, a phase noise of ~-123 dBc/Hz, and an electro-optic
tuning efficiency of -1.66 MHz/V.
