Diffusion characteristics and waveguiding properties of protonexchanged and annealed LiNbO3 channel waveguidesThe structural properties of annealed proton-exchanged waveguides in Z-cut LiTaO, have been investigated. Observations made in a polarizing microscope together with infrared-absorption measurements indicate the formation of monoclinic L4,-,,H,TaO, after proton exchange in pure pyrophosphoric acid. This monoclinic phase disappears after annealing at 300 "C. High-resolution x-ray diffraction shows that the c lattice parameter is 0.57% larger in the exchanged layer than in the bulk. During annealing the c parameter relaxes and in addition there appears a second diffraction peak in the x-ray rocking curves. This peak is attributed to a-phase Li,, -X,H,TaO,. A model for the increase of the refractive index is suggested. It shows that the main reason for the difference in index increase between proton-exchanged waveguides in LiTa03 and LiNbO, is the stronger spontaneous polarization of LiNb03. The model predicts an increase of the refractive index of LiTaO, after annealing which is experimentally observed. A short-term postannealing index decrease is suggested to be related to a ferroelectric domain reversal. 0 199.5 American Institute of Physics.
Experimental results on proton-exchanged waveguides in lithium tantalate are reported. Proton-exchanged z-cut samples are characterized optically and using secondary-ion-mass spectroscopy (SIMS). The SIMS profiles are used to obtain the temperature-dependent diffusion coefficient. The increase of the extraordinary index is obtained using the prism-coupling technique and the decrease of the ordinary index is deduced from the spectral modulation of the transmission at normal incidence. A series of proton-exchanged samples are annealed and the postannealing changes are followed over time. It is found that for certain parameters the refractive index decreases rapidly the first days after the annealing. There is also a smaller long-term change. Further, on repeated heat treatment this aging process will recommence. A second series of samples are fabricated by proton exchange in sulphate-salt melts. It is found that above a certain proton concentration in the melt, the refractive index decreases when the proton concentration is increased. This has the impact that certain samples with a monotonously decreasing proton concentration will have their maximum refractive index below the surface. This increases the difficulty in accurately reconstructing the index profile from measured effective indices.
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