Parasitic Mode Conversion in Z-Propagating Lithium-Niobate Waveguides

The propagation characteristics of near-Z-axis lithium-niobate waveguides are investigated, considering the finite crystallographic-alignment tolerances. We expand on the previous models that have considered only the departures from the Z-axis in the plane of the substrate ("yaw") to include the effects of "pitch" and "roll." We find that a combination of yaw and pitch couples the major electric-field components of the fundamental quasi-TE and quasi-TM modes through off-diagonal permittivity components. When these two rotations are both larger than ~0.1deg, the imaginary coupling coefficient associated with the overlap of the major field components becomes larger than the real coefficient, associated with the coupling of the longitudinal TM component to the transverse TE component, that results from yaw alone. We show numerically, as well as experimentally, that unintentional crystallographic-alignment errors can result in substantial mode conversion, affecting the extinction ratio and linearity of Z-propagating modulators. The results also indicate that waveguides with intentional yaw will have mode-conversion characteristics that are highly sensitive to any unintentional pitch resulting from the boule slicing/polishing process. We highlight the importance of crystallographic alignment as well as the role of modal birefringence in suppressing undesired mode conversion