Second harmonic generation in periodically poled lithium niobate waveguides with stitching errors

Depending on the chosen fabrication process, nonlinear waveguides realized in periodically oriented material such as periodically poled lithium niobate (PPLN) can present different fabrication errors. In this paper, we present a detailed numerical study of the impact on the nonlinear performances of one or several stitching errors occurring during the realization of the periodic domains by e-beam writing. This study shows that contrary to what was expected, a single finite stitching error does not simply decrease the nonlinear efficiency but splits the second harmonic (SH) signal into a double peak spectrum, where the position of the peaks and their width at half-maximum depend not only on the poling period, the total length of the grating, and the waveguide parameters, but also on the amplitude and the position of the defect. The numerical results are confirmed by second harmonic generation (SHG) experiments performed in PPLN waveguides obtained by Soft Proton Exchange and where the nonlinear grating was composed of one to four e-beam-written 1.5-mm-long PPLN sections between which important stitching errors can occur. In this case the spectrum can be quite complicated. It is worth noting that similar errors (domains merging or missing domains) can occur also in the e-field poling process but they are randomly distributed along the propagation path. Therefore, this study is a first attempt to take them into account to explain experimental results and indicate the steps that have to be taken to improve the quality of the components. (C) 2018 Optical Society of America