Air trenches for sharp silica waveguide bends

Air trench structures for reduced-size bends in low-index contrast waveguides are proposed. To minimize junction loss, the structures are designed to provide adiabatic mode shaping between low- and high-index contrast regions, which is achieved by the introduction of "cladding tapers." Drastic reduction in effective bend radius is predicted. We present two-dimensional (2-D) finite-difference time-domain/effective index method simulations of bends in representative silica index contrasts. We also argue that substrate loss, while present, can be controlled with such air trenches and reduced to arbitrarily low levels limited only by fabrication capabilities. The required trench depth, given an acceptable substrate loss, is calculated in three dimensions using an approximate equivalent current sheet method and also by a numerical solver for full-vector leaky modes. A simple, compact waveguide T-splitter using air trench bends is presented.

I. Introduction

Low-Index contrast silica bench technology—referred to as planar lightwave circuit (PLC) or silicon optical bench (SiOB)—has gained widespread use in practice in the fabrication of passive integrated optical components by virtue of its use of well-tested integrated-circuit industry manufacturing processes and technology [1]. Large silica waveguide cross sections offer low fiber-to-chip coupling and propagation losses. A major drawback of SiOB technology is the relatively large component size, where a critical factor is the minimum waveguide bend radius. This radius is large—normally in the millimeters—in the low-index contrasts − found in silica [1]. The low density of integration keeps production cost high and invites yield problems. On the other hand, high-index contrast, such as silicon-on-insulator (SOI)—while offering dense integration, poses challenges of fiber-to-chip insertion loss due to mode shape mismatch and misalignment, scattering loss, and sensitivity to other fabrication defects and tolerances, as well as fabrication processing challenges.