Advances in Lithium Niobate Photonics Unlock 120-Fold Improvement in Frequency Stability

Phase noise, a fundamental limitation in precision photonic devices, currently restricts the performance of integrated optical systems, and researchers are actively seeking ways to minimise its impact. Ran Yin, Yue Yu, and Chunho Lee, alongside colleagues at Ian Christen and Zaijun Chen’s institutions, now demonstrate a detailed understanding of the origins of this noise in thin-film lithium niobate resonators. Their work reveals that thermal-charge-carrier-refractive dynamics, driven by fluctuations in charge carriers, govern phase noise in these devices, differing significantly from the noise mechanisms in silicon photonics. By identifying material anisotropy and surface states as key contributors, the team not only explains the behaviour of noise in these systems, but also shows that a simple post-fabrication annealing process dramatically reduces frequency noise, paving the way for improved performance in applications like optomechanical sensing and microwave synthesis.

Lithium Niobate Microresonators, Noise Sources, and Limits

Research details investigations into noise sources and performance limitations in lithium niobate (LiNbO3) integrated photonic circuits, focusing on microresonators. LiNbO3 is a strong candidate for integrated photonics due to its excellent electro-optic properties, enabling functionalities such as frequency combs and quantum information processing, but realizing its full potential requires overcoming noise limitations. Key noise sources include thermorefractive noise, charge noise stemming from material defects, two-level system losses from surface and bulk defects, and fundamental thermal noise, which sets a lower bound on achievable noise levels. Reducing noise is crucial for building ultrastable lasers, generating high-quality frequency combs for applications like optical clocks and spectroscopy, and enabling quantum technologies. Current research focuses on improving material quality to reduce defects, optimizing device design to minimize noise impact, and fully understanding the origins of charge noise and two-level systems within LiNbO3, ultimately paving the way for more stable and efficient photonic devices.

Thermal-Charge Carrier Noise in Lithium Niobate Microresonators

Scientists have identified thermal-charge-carrier-refractive (TCCR) dynamics as the primary mechanism governing fundamental phase noise in thin-film lithium niobate photonic integrated circuits, distinguishing it from silicon photonics. A systematic investigation of TCCR noise was conducted by experimentally characterizing microresonators with varied geometries and optical polarizations, providing critical insights into noise control. Researchers used a balanced homodyne detection system and a laser locking system to measure and transduce frequency noise into optical phase noise. The team discovered that optical modes polarized along the crystal’s polar axis exhibit higher TCCR noise due to the stronger electro-optic coefficient, and that device geometry influences noise intensity.

Analysis revealed that surface-defect density significantly impacts charge noise, with suspended microresonators exhibiting much higher noise than those clad with silicon dioxide. A fluctuation, dissipation model was adopted to simulate resonance-frequency noise, and post-fabrication thermal annealing suppressed frequency noise by a factor of 8.2 in cladded microresonators, establishing a practical pathway for noise engineering.

TFLN Phase Noise Originates From Thermal Effects

Detailed investigations into noise sources within thin-film lithium niobate (TFLN) photonic integrated circuits have identified thermal-charge-carrier-refractive (TCCR) dynamics as a key factor governing fundamental phase noise. This work distinguishes TFLN from silicon photonics, where thermorefractive noise typically dominates, and establishes TCCR noise as a critical consideration for applications demanding high frequency stability and phase coherence. Experiments revealed that material anisotropy and surface states are primary contributors to TCCR noise. Measurements confirm that extraordinarily polarized optical modes exhibit increased noise due to material anisotropy, and surface-state effects manifest as elevated noise in higher-order transverse modes, with suspended microresonators displaying significantly higher noise compared to cladded counterparts.

Post-fabrication annealing, a standard process for improving crystal quality, suppresses frequency noise by a factor of 8.2 in cladded microresonators, delivering a practical pathway for noise mitigation. Data shows that TCCR noise in TFLN resonators exceeds thermorefractive noise in silicon nitride resonators at lower frequencies, and exhibits a unique frequency scaling. These results establish a foundation for noise engineering in TFLN integrated photonic devices, accelerating their deployment in next-generation precision photonic systems.

Charge Carrier Noise in Lithium Niobate Photonics

This research provides a comprehensive understanding of phase and frequency instability in thin-film lithium niobate photonic integrated circuits, identifying thermal-charge-carrier-refractive dynamics as the governing mechanism behind fundamental noise. Unlike silicon photonics, where thermorefractive noise dominates, these circuits exhibit noise strongly influenced by charge fluctuations, a critical factor for applications demanding high stability and coherence. Investigations reveal that material anisotropy and surface states significantly contribute to this noise, with anisotropy increasing noise for specific light polarizations and surface states exacerbating noise in higher-order modes and suspended structures. The team demonstrated a practical method for reducing this noise through post-fabrication annealing, achieving an 8.2-fold reduction in frequency noise for cladded microresonators, and providing a pathway for engineering noise characteristics in these devices. The analytical framework developed is broadly applicable to other lithium niobate configurations, allowing for optimization of device geometry and circuit layout to improve performance, and offers critical insights for designing optimized microresonators for applications requiring ultrahigh frequency stability.