Air trench waveguide bend for high density optical integration Shoji Akiyama* a , Kazumi Wada a , Jurgen Michel a , Lionel C. Kimerling a , Milos A. Popovic b , Hermann A. Haus b a Massachusetts Institute of Technology, Department of Materials Science and Engineering, 77 Massachusetts Avenue, Cambridge, MA 02139; b Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 77 Massachusetts Avenue, Cambridge, MA 02139 ABSTRACT Air trench structure for reduced-size bends in low ( n=0.01-0.1) and medium ( n=0.1-0.3) index contrast waveguides is proposed. Local high index contrast at bends is achieved by introducing air trenches. An air trench bend consists of cladding tapers to avoid junction loss, providing adiabatic mode shaping between low and high index contrast regions. Drastic reduction in effective bend radius is achieved. We present FDTD simulations of bends in representative silica index contrasts, fabrication scheme and waveguide loss measurement results using Fabry-Perot loss measurement technique. We employed CMOS compatible processes to realize air trench bends and T-splitters to achieve low production cost and high yield. A simple, compact waveguide and T-splitter are fabricated and evaluated. The loss measurement results show that losses are consistent with theoretical simulations. By using air trench waveguides, other applications such as BioMEMS (e.g. Evanescent-field sensor) or EDWA can be realized. Keywords: air trench bend, cladding taper, silica waveguide, bend loss, low index contrast, enhanced lateral mode confinement 1. INTRODUCTION Low index contrast (LIC) silica bench technology which is often referred to as PLC (planar lightwave circuit) or SiOB (silica optical bench) has been widely used in practice in the fabrication of passive integrated optical components such as arrayed waveguides gratings (AWGs) by virtue of its use of well-tested CMOS processes and technology [1]. It has been reported that LIC platform provides low fiber-to-chip coupling and propagation losses due to its low index contrast between core and cladding [1]. But a major drawback of LIC platform is the relatively large footprint, where a critical factor is the minimum waveguide bend radius. This radius is as large as in the millimeter or centimeter order ( = 0.25-1.5%) [1] depending on its index contrast. Low density of integration keeps production cost high, and invites yield problems and also temperature instability for AWG application due to its large size. On the other hand, high index contrast such as Si/SiO 2 or SiN/SiO 2 material system where the index difference is as much as 2 or 0.5-0.7, respectively, while offering dense integration, poses challenges of fiber-to-chip insertion loss due to mode shape mismatch and misalignment between fiber and waveguide, scattering loss and vulnerability to other fabrication defects and tolerances, as well as fabrication processing challenges such as lithography capability. Air trench waveguides offer a drastic reduction in the bending radius, while offering simple fabrication, and the size problem of PLC platform will be overcome. Truly large-scale optical integration will be achieved on low, middle index contrast platform using our proposed structures. *akiyama@mit.edu ; phone 1(617)253-6907; fax 1(617)253-6782; http://photonics.mit.edu/ Integrated Optics: Devices, Materials, and Technologies VIII, edited by Yakov Sidorin, Ari Tervonen, Proceedings of SPIE Vol. 5355 (SPIE, Bellingham, WA, 2004) · 0277-786X/04/$15 · doi: 10.1117/12.529630 14