Wavelength dependence of the ordinary and extraordinary index change in LiNbO3 proton-exchanged waveguides

.The importance of LiNbO3 proton exchanged (PE) waveguides in integrated optics is well consolidated. In such waveguides, only extraordinary modes are supported, since the ordinary refractive index is decreased by proton exchange. However, the knowledge of the ordinary refractive index profile is of great importance, being related to the crystallographic phase of the exchanged layer. This work provides a wavelength dispersion curve for both ordinary and extraordinary index changes of PE-LiNbO3 waveguides. The dispersion curve of the ordinary index change was obtained by means of a non-conventional method, developed in our laboratory, based on radiation modes, that of the extraordinary index change by standard m-lines spectroscopy. A multimode PE waveguide was designed and fabricated specifically to obtain a very accurate determination of the index changes up to the 1.55mum spectral range. The waveguide was realized on a Z-cut LiNbO3 substrate, by a 26h proton-exchange at 247degreesC in benzoic acid Muted with 1% lithium benzoate, resulting in a depth of 2.75mum. The measurement of both index changes was performed at wavelengths ranging from the visible to the near infrared and the corresponding Sellmeier curves were obtained. It is worth noting that the two index changes exhibit a significantly different behaviour.