CMOS-Compatible Silicon Etched U-Grooves With Groove-First Fabrication for Nanophotonic Applications

We propose a method for process integration to passively edge-couple the optical fibers with on-chip integrated waveguides. By fabricating U-grooves before the waveguide formation, the groove-first process provides a universal platform for hybrid, large-scale fiber-to-waveguide interconnection. This integrated method simplifies the groove formation and avoids unwanted waveguide damage during the conventional groove formation. Besides, the groove-first process can be utilized with the pre-defined layers for cost reduction. The demonstrated groove-first fabrication helps to improve the coupling stability and suppress the power variation. Time-dependent measurements exhibit > 3.5 dB improvement in the power stability. We also show that this groove structure can provide stable coupling with a high-power input up to 300 mW. In addition, a naturally process-induced inverse-taper with sub-micrometer resolution is formed under conventional UV contact lithography, which potentially provides efficient conversion between the fiber-to-waveguide mode. The groove-first method builds a universal, simple, and flexible integration process for silicon photonics.