Optical and structural properties of annealed PE:LiNbO3 waveguides formed with pyrophosphoric and benzoic acids

The optical and structural properties of protonated lithium niobate waveguides have been investigated. The proton-exchange process was performed by immersing Z-cut substrates in molten pyrophosphoric and benzoic acids at 240 °C for 8 h. Surface refractive index changes of 0.148 (pyrophosphoric acid) and 0.124 (benzoic acid) were obtained at λ=632.8 nm. The exchanged samples were subjected to annealing for 1 h through each 20° starting from 200 °C. Reflection high-energy electron diffraction, optical measurements, and vibrational (IR and Raman) spectra were used at each stage of annealing in order to study the guiding characteristics. Immediately after the exchange process and at low-temperature annealings a two-step index profile was detected. The dependence of the profile parameters on the annealing temperature up to 360 °C was followed and correlated with the phase transformations in the LiNbO3-HNbO3 system. The hydrogen concentration in the waveguide layers was estimated by using optical measurements and Raman spectra.