Compact single air interface bends in PFCB polymer waveguides

Compact waveguide bends and splitters are important components to enable dense integration of many functions on a single photonic chip. A common approach is to use a waveguide material system in which a large refractive index contrast between the core and clad materials is available. This permits a small bend radius to be used while still achieving high optical efficiency for the bend. However, such material systems generally have higher propagation loss than is possible with low refractive index contrast material systems such as silica. In this presentation we examine an approach to make the bend size essentially independent of the core/clad refractive index contrast using total internal reflection from a planar interface. We show through both 2D and 3D finite difference time domain (FDTD) simulation that very high bend efficiencies are possible when the correct bend design principles are adhered to. We illustrate this in practice with single air interface bends (SAIBs) in a PFCB material system with approximately 1% refractive index contrast. We experimentally demonstrate 45 degree bends with 0.3 dB loss per bend, and discuss the effects of fabrication issues such as misalignment, etch undercut, and etch roughness.