Abstract
First implementations of subwavelength gratings (SWGs) in silicon-on-insulator (SOI) waveguides are discussed and demonstrated by experiment and simulations. The subwavelength effect is exploited for making antireflective and highly reflective waveguide facets as well as efficient fiber-chip coupling structures. We demonstrate experimentally that by etching triangular SWGs into SOI waveguide facets, the facet power reflectivity can be reduced from 31% to <2.5%. Similar structures using square gratings can also be used to achieve high facet reflectivity. Finite difference time-domain simulations show that >94% facet reflectivity can be achieved with square SWGs for 5 μm thick SOI waveguides. Finally, SWG fiber-chip couplers for SOI photonic wire waveguides are introduced, including design, simulation, and first experimental results.
Highlights
Subwavelength gratings (SWGs) have been known and used for many years [1], most commonly as an alternative to antireflective (AR) coatings on bulk optical surfaces
For a SOI waveguide thickness of ∼1 μm or less, these radiative losses are prohibitive for practical devices, as we have found with three-dimensional finite difference time-domain (FDTD) simulations
With rigorous coupled wave analysis (RCWA) we find that the grating reflectivity for plane waves drops from >99.9% to 81% when the angle of incidence is increased from 0◦ to 10◦
Summary
Subwavelength gratings (SWGs) have been known and used for many years [1], most commonly as an alternative to antireflective (AR) coatings on bulk optical surfaces. Since the SWG effect allows one to engineer artificial materials with intermediate effective indices by lithographic patterning, it has Advances in Optical Technologies the potential to circumvent this limitation We demonstrate this on two specific examples, namely, the control of the Fresnel reflectivity of the waveguide facets and the fiber-tochip coupling, both relying on the SWG effect. We have recently proposed the use of the SWG effect as a general tool for waveguide mode modifications, including light coupling between an optical fiber and high index contrast waveguides of submicrometer dimensions [19] and modification of facet reflectivity [20]. All SWG patterns discussed here, both for facet reflectivity modification and for fiber-to-chip coupling enhancement, can be fabricated by standard lithography and vertical etching processes We have carried out experiments on SOI waveguides and compared the experimental results with reflectivity calculations using effective medium theory and finite difference time-domain (FDTD) simulations
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