Evaluation of low index contrast in lithium niobate waveguides at telecom wavelengths

For a wide range of applications in modern photonics, the low index contrast lithium niobate waveguides still offer the most established solutions to integrate a number of functions on a single optical chip. These waveguides are mostly single-mode at telecom wavelengths and knowing their index contrast (IC) in this spectral range is not straightforward. Unfortunately, the single mode behavior of the propagation at telecom wavelength makes impossible to use the M-lines technique which allows retrieving the IC only if the waveguide is multi-mode. Here, we propose and test a new hybrid (experimental and numerical) method to determine the IC at 1310 nm and 1550 nm for typical low-IC lithium niobate waveguides by fitting the experimental in-depth optical mode profiles with the Finite Element Method simulated ones. Then, by using different versions of Sellmeier equation, we reconstruct the dependence of IC on the wavelength in VIS and near-infrared (NIR) range. This hybrid method can be of use in the analysis of any surface graded-index waveguides fabricated in low-IC range.