Sub-decibel silicon grating couplers based on L-shaped waveguides and engineered subwavelength metamaterials

The availability of low-loss optical interfaces to couple light between standard optical fibers and high-index-contrast silicon waveguides is essential for the development of chip-integrated nanophotonics. Input and output couplers based on diffraction gratings are attractive coupling solutions. Advanced grating coupler designs, with Bragg or metal mirror underneath, lowand high-index overlays, and multi-level or multi-layer layouts, have proven less useful due to customized or complex fabrication, however. In this work, we propose a rather simpler in design of efficient off-chip fiber couplers that provide a simulated efficiency up to 95% (−0.25 dB) at a wavelength of 1.55 μm. These grating couplers are formed with an L-shaped waveguide profile and synthesized subwavelength grating metamaterials. This concept jointly provides sufficient degrees of freedom to simultaneously control the grating directionality and out-radiated field profile of the grating mode. The proposed chip-to-fiber couplers promote robust sub-decibel coupling of light, yet contain device dimensions (> 120 nm) compatible with standard lithographic technologies presently available in silicon nanophotonic foundries. Fabrication imperfections are also investigated. Dimensional offsets of ± 15 nm in shallow-etch depth and ± 10 nm in linewidth’s and mask misalignments are tolerated for a 1-dB loss penalty. The proposed concept is meant to be universal, which is an essential prerequisite for developing reliable and low-cost optical couplers. We foresee that the work on L-shaped grating couplers with sub-decibel coupling efficiencies could also be a valuable direction for silicon chip interfacing in integrated nanophotonics. © 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

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osed grating co podized region th grating (SW hes are filled b rength can be optimization ca nd NP u are nu = NP a + NP u ), r Λ a,i and Λ u and i is the inte ng directional cal fiber and th ep-etch (l d ) and strength is alte propagation (h ating mode an ode field diam of 1.55 µm. Sy o called sub-w n a scale near ragg resonance non-resonant m tion (along the s is determined core (Si) and su ls based on F Expansion (F-15 µm >> h w s (2-D). Yet r culations [52,5 sional (3-D)

Optimizat
L-shaped grat efficiency to D FDTD calc of 98% and a 2(c) and 2(d) nd the grating he grating cou fixed to 120 nm ed according t g coupler with he grating diff ex of the Bloch ng. The local radiated grating of L-shaped g onform amplitu control over th directionality ide range of la s. Moreover, a owered index er region. On t ting coupler i increases, i.e hich in turn, in ngth along the 3 Fig. 4(b), a -chip coupling his is also asso an ultra-wide more, the increa up to −0.25 dB ) We also e fabrication im wavelength fo analysis is pe minimum feat considered. T dimensions. T encompass ch teeth (l n ), in w between the f offsets of ± 2 coupler design order to perfo of fabrication where h e,0 ; l d,0 As can be variations in e level variation and blue spec that errors in couplers are m trenches and misalignment  Fig. 7, the prop that to variatio yield a coupli operation at a f ± 15 nm yiel to in-plane per eth, in widths Fig. 7 dB and it is accompanied by noticeable spectral shift. For a 1-dB coupling penalty, dimensional offsets of ± 10 nm are tolerated, as shown in the inset of Fig. 7(b).

Conclusions
We proposed an efficient chip-to-fiber grating couplers to enable a low-loss interconnectivity between integrated SOI nanophotonic circuits and standard single-mode optical fibers. The grating couplers were formed with L-shaped waveguide profile and synthesized SWG metamaterials. This device arrangement is favorable for providing enough degrees of freedom to alter the grating directionality and radiated field profile, with overall fiber-chip coupling efficiency approaching 95% (−0.25 dB) at a wavelength of 1.55 µm. Moreover, apodized Lshaped grating couplers were designed for robust sub-decibel coupling and device layouts compatible with lithographic technologies for mass-scale production (> 120 nm). Tolerance analysis suggested that dimensional offsets up to ± 15 nm can be tolerated, with a 1-dB loss penalty. Overall, our work holds promises to further the development of robust, reliable, and low-cost off-chip fiber couplers within available silicon-foundry-compatible processing nodes. This result may provide a crucial edge in building future optical interfaces in largevolume chip-integrated nanophotonics.

Funding
The European Union's Horizon 2020 research and innovation program (ERC POPSTARgrating agreement N o 647342), partially founded by Agence Nationale de la Recherche (ANR) MIRSPEC, Nano 2020 under Important Projects of Common European Interest (IPCEI).