Single-etched fiber-chip coupler with a metal mirror on a 220-nm silicon-on-insulator platform for perfectly vertical coupling

Vertical couplers play a pivotal role as essential components supporting interconnections between fibers and photonic integrated circuits (PICs). In this study, we propose and demonstrate a high-performance perfectly vertical coupler based on a three-stage inverse design method, realized through a single full etching process on a 220-nm silicon-on-insulator (SOI) platform with a backside metal mirror. Under surface-normal fiber placement, experimental results indicate a remarkable 3-dB bandwidth of 99 nm with a peak coupling efficiency of -1.44 dB at the wavelength of 1549 nm. This achievement represents the best record to date, to the best of our knowledge, for a perfectly vertical coupler fabricated under similar process conditions.