Unveiling the origins of quasi-phase matching spectral imperfections in thin-film lithium niobate frequency doublers

TL;DR

This paper investigates the spectral imperfections in thin-film lithium niobate frequency doublers, identifying their causes through non-destructive imaging and numerical modeling, to improve device efficiency and spectral quality.

Contribution

It introduces a non-destructive diagnostic method and demonstrates the significant impact of film thickness variations on spectral imperfections in TFLN frequency doublers.

Findings

Spectral imperfections are linked to film thickness variations.

Waveguide sections contributing to imperfections are identified.

Numerical models reproduce the observed spectral features.

Abstract

Thin-film lithium niobate (TFLN) based frequency doublers have been widely recognized as essential components for both classical and quantum optical communications. Nonetheless, the efficiency of these devices is hindered by imperfections present in the quasi-phase matching (QPM) spectrum. In this study, we present a thorough analysis of the spectral imperfections in TFLN frequency doublers with varying lengths, ranging from 5 mm to 15 mm. Employing a non-destructive diagnostic method based on scattered light imaging, we identify the sources and waveguide sections that contribute to the imperfections in the QPM spectrum. Furthermore, by mapping the TFLN film thickness across the entire waveguiding regions, we successfully reproduce the QPM spectra numerically, thus confirming the prominent influence of film thickness variations on the observed spectral imperfections. This comprehensive investigation provides valuable insights into the identification and mitigation of spectral imperfections in TFLN-based frequency doublers, paving the way toward the realization of nonlinear optical devices with enhanced efficiency and improved spectral fidelity.