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24 February 2006 Low loss optical interconnects to silicon waveguides
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Waveguides fabricated in high-index-contrast material systems offer very strong light confinement compared to that achieved in low-index-contrast material systems. A core layer of silicon (refractive index n~3.5) surrounded by silica cladding (n~1.5) on a silicon-on-insulator (SOI) substrate is an example of a high-index-contrast material system. This enables miniaturization of functional optical components and enhances dense integration of devices on waveguide chips. Some physical effects, such as, Raman and Stimulated Brillouin Scattering (SBS) are much stronger in silicon than in glass. In view of the above two reasons, it is possible to use short (a few centimeter long) silicon waveguides to amplify light or modify its wavelength, instead of using kilometers of glass optical fibers. A large mismatch between the common optical fiber dimensions and that of the high-index-contrast waveguides makes it difficult to couple light in and out of the chip. A number of techniques have been utilized for this purpose, including prism couplers, grating couplers, tapered fibers and micro-lens mode transformers [ 1, 2]. A better option to effectively couple light in this situation is by incorporating a waveguide section that is tapered vertically, as well as laterally between the fiber and the waveguide. This tapered section acts as a classic adiabatic modal transformer [ 3, 4, 5, 6] that transforms the input fundamental mode shape to that of the waveguide mode. In this paper, coupling losses between optical fibers and rib-loaded SOI waveguides with lateral only (1-D) and combination of lateral and vertical (2-D) tapers are presented. The waveguide fabrication process down to 0.75 μm size with the tapers is discussed and the measured coupling losses are compared to predictions. Measured coupling loss values for waveguides with 2-D tapers (~1.8 dB) show a significant improvement over those for waveguides with 1-D tapers (~4 dB) or no tapers (~8 dB), and are in excellent agreement with predictions. A qualitative analysis of the Free Carrier Absorption (FCA) phenomenon in narrow silicon waveguides that suppresses the Raman amplification and SBS is also shown.
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Ariela Donval, Ram Oron, Moshe Oron, A. N. M. Masu Choudhury, Tom R. Stanczyk, and Dean Richardson "Low loss optical interconnects to silicon waveguides", Proc. SPIE 6123, Integrated Optics: Devices, Materials, and Technologies X, 61231E (24 February 2006);


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