Integrated optical anisotropic waveguides: cutoff conditions and anisotropy-based devices

Dielectric planar waveguides are essential components in integrated optical devices, circuits, and systems. These waveguides are typically fabricated in anisotropic materials such as lithium niobate. Furthermore, the anisotropic nature of the waveguides is usually ignored due to the complexity of the analysis required to include these effects. In the present work, the allowed hybrid guided modes in anisotropic planar waveguides are calculated electromagnetically rigorously (without approximations). Three types of mode are identified: (1) homogeneous pure guided modes; (2) inhomogeneous pure guided modes; and (3) leaky guided modes. Furthermore, within these three types, there can be unconditionally stable and critically stable modes. Cutoff can occur starting from either of these latter two types through a transition to either a leaky guided mode or a leaky unguided wave. The cutoff conditions for these transitions are quantified. Active and passive cutoff anisotropy-based devices can be constructed simply by changing the direction of propagation on the anisotropic substrate. Polarizers, filters, and temperature sensors (performing an absolute rather than a differential measurement) are described that can be constructed in very simple configurations using the anisotropic propagation characteristics.