Modification of proton exchanged LiNbO3layers for guiding modes with ordinary polarization

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Abstract
To complete preliminary data recently published, a detailed description is given for the fabrication procedure of guiding layers with ordinary polarization on proton exchanged LiNbO3, waveguides. The process of “reverse exchange” of a substratepreviously exchanged in benzoic acid is carried out in a melt very rich in lithium. Theinfluence of several parameters on the process is studied. Melts with different concentrations of LiNO3 produce the same ordinary index profiles: they are parabolic-like for X- and Y-cut substrates and gaussian-like for Z-cut substrates. On the otherhand, 10% lithium benzoate melts produce sharper, steplike profiles on Y-cut substratesand gaussian like on Z-cut. On increasing the treatment temperature from 300°C to 375°C, the process speeds up and ordinary profiles appear smoothed. By choosing appropriated treatment parameters, losses for ordinary modes haw been found to be below 0.7dB/cm. The occurrence of buried extraordinary modes induced by the reverse exchange treatment has been confirmed here. Also, protons introduced during normal exchange in benzoic acid partially go out of the guide during reverse exchange, as monitored by the OH− infrared absorption band.
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