Fundamental Phase Noise in Thin Film Lithium Niobate Resonators

TL;DR

This paper investigates the fundamental phase noise mechanisms in thin-film lithium niobate resonators, identifying key contributors like material anisotropy and surface states, and demonstrates noise reduction via annealing.

Contribution

It provides a detailed analysis of TCCR noise sources in TFLN resonators and offers practical methods for noise suppression to enhance photonic system performance.

Findings

Material anisotropy increases phase noise for certain polarizations.

Surface states cause higher noise in higher-order modes.

Annealing reduces frequency noise by a factor of 8.2.

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 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. A quantitative understanding of the deterministic parameters that control TCCR noise is therefore essential for engineering the next generation of low-noise TFLN photonic systems. Here, we identify two dominant contributors to the TCCR noise in TFLN microresonators: material anisotropy and surface states. 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. Finally, 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.