Wide-Bandwidth and Low-Reflection High-Efficiency Grating Couplers

Grating couplers are attractive for use in photonic integrated circuits [1]. We previously proposed shift-pattern overlays [2], which enable high coupling efficiency of over 81% experimentally [3], but they had relatively poor optical bandwidth and large back-reflection. Grating couplers with 1 dB optical bandwidth of over 30 nm are required in many applications [4]. Wideband designs also relax the fabrication tolerances and improve the stability of coupling over a wide range of operational temperatures. Photonic integrated circuits with integrated lasers or optical amplifiers may have their laser linewidths degraded, or amplifier gain clamped because of back-reflections [5]. Back reflections also introduce undesired Fabry-Perot resonances [6].