High efficiency diffractive grating structures to interface a single mode optical fiber and a nanophotonic integrated circuit fabricated on silicon-on-insulator are presented. The diffractive grating structures are designed to be inherently very directional by adding a silicon overlay before grating definition. 55% coupling efficiency at a wavelength of 1.53 m is experimentally demonstrated on devices fabricated using standard complementary metal-oxide semiconductor technology. By optimizing the grating parameters, we theoretically show that 80% grating coupling efficiency can be obtained for a uniform grating structure.High refractive index contrast optical waveguide structures hold promise for large scale integration of optical functions on a single substrate. This is due to the fact that the high refractive index contrast allows realizing wavelengthscale optical components ͑e.g., photonic crystal cavities, 1 ring resonators, 2 modulators, 3 etc.͒ which can be interconnected by nanophotonic integrated waveguides. Silicon-oninsulator ͑SOI͒ is emerging as the dominant platform for this integration because the refractive index contrast between the silicon waveguide layer ͑n Si = 3.45 at a wavelength of 1.55 m͒ and the underlying buried oxide layer ͑n SiO 2 = 1.45͒ is very high. Moreover, these nanophotonic structures can be defined using state-of-the-art complementary metaloxide semiconductor ͑CMOS͒ technology. 4 While the high omnidirectional refractive index contrast allows realizing wavelength-scale optical functions, the interfacing between a nanophotonic waveguide and a standard single mode fiber is far from trivial due to the large mismatch in dimensions between the 9 m diameter core of a single mode fiber and the cross section of an integrated high index contrast waveguide, which is typically 0.1 m 2 for a single mode waveguide at telecommunication wavelengths. In this paper, we present the use of a diffractive grating structure defined in the waveguide layer to efficiently interface with a single mode optical fiber. The operation principle of the device is based on the Bragg diffraction from the grating. The optical fiber is slightly tilted off vertically in order to avoid second order Bragg reflection into the waveguide. 4 While the optical coupling properties of one-dimensional grating structures are very polarization dependent, it was shown that a twodimensional grating coupling approach allows tackling the issue of the polarization dependent loss of high index contrast photonic integrated circuits by applying a polarization diversity configuration, 5 without the need of integrating a polarization splitter and rotator on the photonic integrated circuit. 6The fiber-to-waveguide coupling efficiency is determined by the directionality of the grating, being the ratio of the power that is diffracted upward ͑P up ͒ to the total diffracted power ͑P up + P do ͒, as shown in Fig.
We report ultra-low propagation losses in silicon submicrometric waveguides on a 200 mm CMOS compatible photonics platform. We show losses in C-band (O-band) as low as 0.1 dB/cm and 0.7dB/cm (0.14dB/cm and 1.1dB/cm) in monomode rib and strip waveguide geometries, respectively, thanks to a H2 smoothing annealing. In addition to optical losses down to unprecedented levels in silicon waveguides, we show that the performance characteristics of the main passive and active building blocks of the photonics platform are preserved or even improved by the smoothing process.
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