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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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