Abstract
A novel class of optical waveguides with a box-shaped cross section consisting of a low-index inner material
surrounded by a thin high-index coating layer is presented. This original multilayered structure widens the
traditional concept of index contrast for dielectric waveguides toward a more general concept of effective index
contrast, which can be artificially tailored over a continuous range by properly choosing the thickness of the outer
high-index layers. An electromagnetic analysis is reported, which shows that the transverse electric and transverse
magnetic modes are spatially confined in different regions of the cross section and exhibit an almost 90° rotational symmetry. Such unusual field distribution
is demonstrated to open the way to new intriguing properties with respect to conventional waveguides. Design
criteria are provided into details, which mainly focus on the polarization dependence of the waveguide on
geometrical parameters. The possibility of achieving single-mode waveguides with either zero or high birefringence
is discussed, and the bending capabilities are compared to conventional waveguides. The feasibility of the proposed
waveguide is demonstrated by the realization of prototypal samples that are fabricated by using the emerging
CMOS-compatible Si<sub>3</sub>N<sub>4</sub>–SiO<sub>2</sub> TriPleX technology. An exhaustiveexperimental
characterization is reported, which shows propagation loss as low as state-of-the-art low-index-contrast waveguides
(< 0.1 dB/cm) together with enhanced flexibility in the
optimization of polarization sensitivity and confirms the high potentialities of the proposed waveguides for
large-scale integrated optics.
© 2007 IEEE
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