Anomalous side-shifted multimode spectra in proton-exchanged LiNbO_3 waveguides

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Abstract
We report the presence of a curious and highly reproducible effect in multimode lithium niobate waveguides fabricated by proton exchange (PE) in molten benzoic acid at temperatures ranging from 160°C to ~250°C. The spectral lines in the mode spectra of these guides (measured using a prism coupler) are anomalously side-shifted out of the expected geometrical plane. Transforming these measurements back into the plane of the waveguide, we find that the direction of scattering (relative to the crystal axis) is extremely precise (μm range) in the guides.