  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.
