Efforts are made to correlate the hydrogen presence in proton-exchanged waveguides with their modal properties and vibrational spectra. The exchange process was performed by immersion Z-cut substrates in molten pyrophosphorlc acid at 240 °C for 8 h, thus realizing technological conditions similar to those for waveguides made previously by us from benzoic acid. The number of modes obtained in such type of waveguides was high enought (10-20) to ensure precise profile reconstructions by using the IWKB method. A surface refractive index change of 0.148 was obtained at y=632.8 nm. The exchanged samples were subjected to subsequent annealings for 1 h through 20° starting from 200 °C. Reflection High Energy Diffraction (RHEED), optical measurements and vibrational (IR and Raman) spectra were used at each stage of annealing in order to follow the guide characteristics. Seme correlations between optical profiles and vibrational spectra have been found which allow to make conclusions about the substitution degree and the structure of exchanged layers.
New results on " as-grown" and protonated lithium tantalate crystals are given. Special attention is paid to crystals having different stoichiometry. The proton exchange in X- and Z-cut substrates was processed in pure benzoic acid for different time intervals. Optical waveguide measurements and infrared absorption studies have been performed to study the incorporation of protons. Definite correlation between the lithium concentration of the original crystals and the refractive index change in the exchanged layers has been observed.
Virgin and protOn-exchanged LiNbO3 planar waveguldes are investigated by different spectroscopic methods: ir absorption, Raman scattering and luminescence. Absorption spectra in the region of OH-vibrations are used to study the relationship between the refractive index profile and hydrogen content in the exchanged waveguides. An attempt to support the model suggested on the base of ir data is made by waveguide Raman techniques. In some pure and proton-exchanged samples strong luminescence bands in the region 600-650 nm are observed which make difficult the detection of the Raman signals. The appearance of the luminescence features could be connected with the presence of free lithium metal in the crystal lattice. The increased amount of defects in the wavegulde layers (Initially intrinsic Impurities and extrinsic ones produced by the proton-exchanged process) causes disorder effects in the crystal lattice. The disordered state is eflected in the Raman spectrum of the Internal vibrations of LiNbO3 which can be observed In the spectral region with frequency less than 900 cm1.The observed spectrum consists of narrow "crystalline" lines superimposed on a background resembling to phonon density of states, The crystalline component is induced probably by the penetration of guided light in the crystal substrate while the broad band background is characteristic for the "quasi-lattice" of the wavegulde layer.