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Proceedings Paper

Graded-index coupler for microphotonic SOI waveguides
Author(s): Andre Delage; Siegfried Janz; Dan Xia Xu; Dan Dalacu; Boris Lamontagne; Alexei L. Bogdanov
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Paper Abstract

Coupling light into and out of small high index contrast waveguides is a fundamental challenge to implementing practical microphotonic waveguide and photonic crystal devices. Previous approaches include three-dimensional tapers, inverse taper waveguides, and grating based couplers. We propose and describe a much simpler coupler based on a short length of graded index (GRIN) material deposited on top of a silicon-on-insulator (SOI) microphotonic waveguide. The GRIN coupler has a refractive index that decreases from the index of silicon at the waveguide-coupler interface, to an optimized value at the coupler surface. Beam propagation method calculations are used to evaluate the coupling efficiency from a 4 μm thick coupler section to the fundamental mode of a 0.5 μm thick SOI waveguide. Coupling efficiencies are compared for couplers with smoothly varying quadratic index profiles and with one, two and three index steps. Coupling efficiencies of 75% (1.3 dB) or better are predicted using a three step GRIN structure with indices ranging from n=3.30 to 3.41 (Si). This index range is easily accessed using a-Si layers deposited by PECVD at varying deposition conditions, or by using composite digital alloys of high and low index films. With this method, microphotonic waveguide couplers can be designed and fabricated using only PECVD deposition and one patterning etch step with very modest tolerances. Efficiency increases to 87% (0.6 dB) when the index range of the 3-step coupler is extended to 3.0.

Paper Details

Date Published: 20 December 2004
PDF: 9 pages
Proc. SPIE 5577, Photonics North 2004: Optical Components and Devices, (20 December 2004); doi: 10.1117/12.567264
Show Author Affiliations
Andre Delage, Institute for Microstructural Sciences/NRC (Canada)
Siegfried Janz, Institute for Microstructural Sciences/NRC (Canada)
Dan Xia Xu, Institute for Microstructural Sciences/NRC (Canada)
Dan Dalacu, Institute for Microstructural Sciences/NRC (Canada)
Boris Lamontagne, Institute for Microstructural Sciences/NRC (Canada)
Alexei L. Bogdanov, Institute for Microstructural Sciences/NRC (Canada)

Published in SPIE Proceedings Vol. 5577:
Photonics North 2004: Optical Components and Devices
John C. Armitage; Simon Fafard; Roger A. Lessard; George A. Lampropoulos, Editor(s)

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