Analysis of High-Index Contrast Lithium Niobate Waveguides Fabricated by High Vacuum Proton Exchange

High-index contrast waveguides fabricated with precise control and reproducibility are of high interest for nonlinear and/or electro-optical highly efficient and compact devices for quantum and classical optical data processing. Here, we present a new process to fabricate planar and channel optical waveguides on lithium niobate substrates that we called high vacuum proton exchange (HiVacPE). The main purpose was to improve the reproducibility and the quality of the produced waveguides by limiting and controlling the water traces in the melt, which is used for the ionic exchange. Moreover, we discovered that, when the acidity of the bath is increased, depending on the substrate orientation (Z-cut or X-cut) the waveguides are completely different in terms of crystallographic properties, index profiles, and nonlinearity. The best-obtained channel waveguides exhibit a refractive index contrast as high as 0.04 without any degradation of the crystal nonlinearity and state-of-the-art propagation losses (0.16 dB/cm). We have also demonstrated that the HiVacPE process allows fabricating waveguides on Z-cut substrate with high-index contrast up to 0.11 without degrading the crystal nonlinearity but high strain induced propagation losses. On top of that, we proposed an original and very useful method of analyzing waveguides with complex index profiles. This method can be used for the analysis of any waveguides, whose core contains several layers.