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 sideshifted 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 (<1% deviation about a mean), and that the effect can be explained by postulating the existence of precisely oriented, stress-induced gratinglike structures (with irregular periods in the 10–70-µm range) in the guides.
© 1986 Optical Society of America
M. De Micheli and Philip St. J. Russell, "Anomalous side-shifted multimode spectra in proton-exchanged LiNbO3 waveguides," Appl. Opt. 25, 3896-3898 (1986)