Focused-ion-beam fabricated vertical fiber couplers on silicon-on-insulator waveguides

Focused-ion-beam technology is a versatile micro- and nanostructuring tool, in which material is sputtered locally by a finely focused ion beam. But for photonic applications this process has the inherent inconvenience of generating optical losses due to ion induced damage. Process optimization is needed to reduce these losses. In this letter the authors present the focused-ion-beam fabrication of shallow gratings in silicon-on-insulator waveguides for the coupling of light to a vertically positioned fiber. Using Al₂O₃ as mask and I₂ as selective etchant this resulted in a 24% fiber-to-chip coupling efficiency, which is comparable to similar devices fabricated with conventional etching techniques.