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

Abstract

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 (≈10 7 ) 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.

Integrated active lithium niobate photonic devices

“…Very recently, electro-optically tunable dual-wavelength microlasers have also been demonstrated on a single weakly perturbated Er 3+ :TFLN microdisk. 36) The two laser modes nearly degenerated polygon dual modes with a small wavelength spacing in the same microdisk. A tunable lowphase-noise microwave signal is synthesized by beating the two lasers.…”

The formation of high-Q polygon modes in weakly perturbed microdisks on thin-film lithium niobate and its applications

“…Besides single-frequency microlasers, stable dual-wavelength microlasers have been also demonstrated from nearlydegenerate polygon modes [12]. The nearly-degenerate polygon modes were designed as octagon modes with high-order excited states in a 53 m radius erbium-doped LN microdisk.…”

“…The spectrum of single-frequency microlaser [11]. (m) The radio-frequency spectrum of the synthesized microwave signal [12].…”

“…To overcome these issues, mode trimming was performed to the generation of high-Q polygon modes and star modes by re-organizing the traditional WGMs with the assistance of weak perturbation introduced into the LN microdisk by a coupled tapered fiber [10][11][12][13][14]. The LN microdisks were fabricated with ultra-smooth surfaces by photo-lithography assisted chemo-mechanical etching (PLACE) technique [15], in erbium-doped thin-film LN or un-doped LN wafer.…”

“…And as the perturbation is weak and the polygon modes suffer from less scattering loss since their propagated paths are often far from the relatively rough periphery, the Q factors of the polygon modes reach up to 10 7 . Due to the suppression of densely distributed WGMs, the ultra-high Q factors, and the high spatial modal overlap, single-frequency microlasers and stable dual-wavelength microlasers have been demonstrated from polygon modes in erbium-doped LN microdisks under the optical pump of polygon modes [11,12]. Moreover, soliton Kerr comb generation has been observed in an un-doped LN microdisk displaying normal dispersion by mode trimming.…”