title: Germanium-on-silicon mid-infrared grating couplers with low-reflectivity inverse taper excitation.
abstract: A broad transparency range of its constituent materials and compatibility with standard fabrication processes make germanium-on-silicon (Ge-on-Si) an excellent platform for the realization of mid-infrared photonic circuits. However, the comparatively large Ge waveguide thickness and its moderate refractive index contrast with the Si substrate hinder the implementation of efficient fiber-chip grating couplers. We report for the first time, to the best of our knowledge, a single-etch Ge-on-Si grating coupler with an inversely tapered access stage, operating at a 3.8 μm wavelength. Optimized grating excitation yields a coupling efficiency of -11  dB (7.9%), the highest value reported for a mid-infrared Ge-on-Si grating coupler, with reflectivity below -15  dB (3.2%). The large periodicity of our higher-order grating design substantially relaxes the fabrication constraints. We also demonstrate that a focusing geometry allows a 10-fold reduction in inverse taper length, from 500 to 50 μm.
authors.forename: Carlos
authors.initials: C
authors.lastname: Alonso-Ramos
authors.forename: Milos
authors.initials: M
authors.lastname: Nedeljkovic
authors.forename: Daniel
authors.initials: D
authors.lastname: Benedikovic
authors.forename: Jordi Soler
authors.initials: JS
authors.lastname: Penadés
authors.forename: Callum G
authors.initials: CG
authors.lastname: Littlejohns
authors.forename: Ali Z
authors.initials: AZ
authors.lastname: Khokhar
authors.forename: Diego
authors.initials: D
authors.lastname: Pérez-Galacho
authors.forename: Laurent
authors.initials: L
authors.lastname: Vivien
authors.forename: Pavel
authors.initials: P
authors.lastname: Cheben
authors.forename: Goran Z
authors.initials: GZ
authors.lastname: Mashanovich