Fundamental Phase Noise in Thin Film Lithium Niobate Resonators

ABSTRACT

Fundamental phase noise in thin‐film lithium niobate (TFLN) photonic integrated circuits is governed by thermal‐charge‐carrier‐refractive (TCCR) dynamics arising from thermally driven carrier fluctuations. In contrast to the predominantly thermorefractive noise (TRN) in silicon photonic platforms, TCCR noise represents a distinct mechanism that becomes critical for applications requiring high frequency stability and phase coherence, including optomechanical sensing, low‐phase‐noise microwave synthesis, and on‐chip quantum squeezing. Here, we identify three dominant contributors to the TCCR noise in TFLN microresonators: material anisotropy, surface states, and temperature. Material anisotropy results in increased noise for extraordinarily polarized optical modes and leads to a geometry dependent phase noise. Surface‐state effects manifest as increased noise in higher‐order transverse modes as well as more than 120‐fold higher noise in suspended microresonators. Temperature‐dependent measurements reveal increasing frequency noise with temperature and distinct thermal scalings of the TCCR and TRN contributions. Additionally, we demonstrate that post‐fabrication annealing — widely used to reduce defect densities and recover crystal quality — suppresses frequency noise by a factor of 8.2 in cladded microresonators. Together, these results establish a practical pathway for noise engineering in TFLN integrated photonic devices and accelerate their deployment in next‐generation precision photonic systems. 2025 The Author(s)