Understanding Intrinsic Loss in Thin-Film Lithium Niobate Ring Resonators via Adiabatic Coupling

TL;DR

This paper investigates the statistical distribution of intrinsic loss rates in monolithic thin-film lithium niobate ring resonators, revealing a baseline loss and discrete loss events through extensive resonance characterization.

Contribution

It demonstrates that intrinsic loss in TFLN ring resonators follows a statistical distribution and introduces an adiabatic coupling method for high-quality spectral measurements.

Findings

Intrinsic loss rates follow a distribution with a baseline and tail.

Most probable intrinsic loss rate is 2 pi x 10.4 MHz.

Operation is in a low-loss regime comparable to silicon nitride platforms.

Abstract

Thin-film lithium niobate (TFLN) has emerged as a versatile integrated photonics platform, combining strong electro-optic and nonlinear effects. Among TFLN devices, ring resonators play a central role in filtering, modulation, and nonlinear optical processes. However, intrinsic loss, which ultimately limits ring performance, is most often summarized by single-valued metrics, and its statistical variability across resonances has received limited attention. Here, we show that intrinsic loss rates in monolithic TFLN ring resonators follow a statistical distribution, comprising a baseline loss and a tail arising from discrete loss events. This behavior is revealed by characterizing 2233 resonances, using an adiabatic waveguide-ring coupling architecture that selectively excites the fundamental mode and yields clean spectra in the ultra-high-Qi regime. We find the most probable intrinsic loss rate ki = 2 pi x 10.4 MHz, indicating operation in a low-loss regime comparable to state-of-the-art thick silicon nitride platforms.