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

Abstract

Thin-film lithium niobate (TFLN) based frequency doublers have widely been recognized as an essential component for both classical and quantum optical communications. Nonetheless, the efficiency (unit: %/W) of these devices is hindered by imperfections present in the quasi-phase matching (QPM) spectrum. In this report, we present a thorough experimental study of spectral imperfections in TFLN frequency doublers with varying lengths, ranging from 5 to 15 mm. A non-destructive diagnostic method based on scattered light imaging is proposed and employed to identify the waveguide sections and primary waveguide parameters contributing to the imperfections in the QPM spectrum. By applying this method, we obtain the evolution of the QPM spectrum along the waveguide’s length. Correlating this information with the measurements of the relevant geometric parameters along the waveguides suggests that the TFLN film thickness variation is the primary source for the measured spectral distortions. Furthermore, we numerically reproduce the QPM spectra with the mapped TFLN film thickness across the entire waveguiding regions. These findings align with and complement the simulation results from previous numerical studies, providing further evidence of the effectiveness of the developed diagnostic method. This comprehensive investigation offers valuable insights into the identification and mitigation of spectral imperfections in TFLN-based frequency doublers, paving the way for the realization of nonlinear optical devices with enhanced efficiency and improved spectral fidelity.