Fundamental Phase Noise in Thin Film Lithium Niobate Resonators

TL;DR

The paper addresses fundamental phase noise in thin-film lithium niobate photonic circuits, identifying thermal-charge-carrier-refractive (TCCR) noise as a distinct mechanism from thermorefractive noise and showing its strong dependence on material anisotropy and surface states. A fluctuation–dissipation framework is used to model TRN, pyro-EO, and TCCR, with TCCR described by an effective RC-like relation $S_{\nu,\mathrm{TCCR}}(f,T)=\frac{n^4 r^2 \nu^2 k_B T}{4\pi^2 V_{eff}} \frac{\sigma(f,T)}{\epsilon_0^2 \epsilon_r^2 f^2}$, linking noise to anisotropy $r$, mode volume $V_{eff}$, and surface conductivity $\sigma(f,T)$. Experiments demonstrate that TE-polarized modes experience much stronger TCCR noise than TM modes due to $r_{TE}=(r_{13}+r_{33})/2$, and that larger mode volumes can mitigate noise albeit with geometry-dependent EO coupling; suspended (air-cladded) devices show dramatically higher noise due to surface effects, which can be mitigated by post-fabrication annealing that reduces noise by about 8.2×. The findings provide practical guidelines for engineering low-noise TFLN microresonators, including polarization and geometry optimization, surface-passivation strategies, and annealing procedures, enabling ultra-stable photonic systems for optomechanics, microwave synthesis, and squeezed-light generation.

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.

Figures (5)

• Figure 1: (a) Sources of the fundamental noise for TFLN microresonators. (b) Fundamental noise of microresonators on Si$_3$N$_4$ and TFLN platform, respectively. The free spectral range of the X-cut TFLN ring resonator and Si$_3$N$_4$ ring resonator are 140 GHz and 100 GHz, respectively. The input light is TE polarized. (c) Microresonators samples that are studied in this work.
• Figure 2: a) Simplified diagram for the noise measurement setup. b) Intensity and phase transmission diagram. The red arrow indicates the laser parking point where the frequency noise is measured. CW: continuous wave; PID: Proportional–Integral–Derivative control system. DUT: device under test. PBS: polarizing beam splitter.
• Figure 3: The nonlinear anisotropy of LN affects the fundamental noise of the microresonator, which is demonstrated by single-sided power spectral density (PSD) of frequency noise for: (a) an X-cut LN ring-based microresonator under the interrogation of the TE and TM light polarization and (b) an x-cut LN ring-shaped resonator (140-GHz FSR) and racetrack-shaped resonator (28-GHz FSR) under TE light polarization. Experimental results are plotted using solid curves while the simulation result of the TCCR and the summed TRN+pyroEO noise are shown in dashed and dashed dot curves, respectively. The noise in the TM-polarized case has comparable contributions from both the TRN+PyroEO noise and the TCCR noise at frequencies higher than 100 kHz, while the noise in the TE case is primarly dominant by the TCCR noise term. The EO coefficients used for the TE and TM mode in ring-shaped resonator are $r_{TE} = (r_{13}+r_{33})/2$ and $r_{TM} = r_{13}$, respectively, while the EO coefficient for the TE mode in racetrack-shaped resonator is $r_{33}$.
• Figure 4: The surface states of LN microresonators affect the fundamental noise, which is demonstrated by single-sided power spectral density (PSD) of frequency noise for: (a) an X-cut LN suspended ring-shaped resonator (air-cladded on both surfaces); The device is fabricated on a 300-nm-thick wafer with a 250-nm etch depth and suspended via a wet etch process. The TCCR model for the suspended device uses an effective conductivity of $\sigma = 4.32\times10^{-7},\mathrm{S/m}$ at a 1-kHz offset frequency, 120 times larger than the conductivity used in the original TCCR model. The peaks observed between 1 and 10 MHz correspond to mechanical modes supported by the suspended structure. (b) an X-cut LN cladded ring-shaped microresonators measured at TM00 and TM01 optical modes. The device is fabricated on a 600-nm-thick wafer with a 350-nm etch depth and cladded with PECVD silicon dioxide of 800 nm.
• Figure 5: Decreasing the fundamental noise of LN microresonators via annealing, demonstrated by single-sided power spectral density (PSD) of frequency noise for an X-cut LN ring-based microresonator measured with TE polarized mode before and after annealing (a) with a TE polarized optical mode; (b) with a TM polarized optical mode.