O-band-operated fiber-chip surface gratings for silicon nitride photonic integrated circuits

Advanced optical interconnects call upon innovative approaches to establish efficient photonic chip interfaces. Surface grating couplers are prime candidates for low-loss coupling between conventional optical fibers and photonic integrated circuits (PICs). Persistently, this remains a challenging task due to material, modal, and geometrical disparities. In this work, we report on recent advances in the design and fabrication of subwavelength grating (SWG) metamaterial-engineered silicon nitride (SiN) grating-coupled optical interfaces. The unique properties of SWG metamaterials offer an extra degree of freedom for controlling light in grating couplers, thus improving both diffraction and fiber-coupler field matching efficiencies. In turn, this enhances fiber-chip coupling performance, while maintaining cost-effective and foundry-compatible fabrication. Our SiN couplers, manufactured in open-access photonic prototyping facilities, have coupling losses as low as -3.1dB at the 1310nm wavelength. Moreover, hybrid SiN grating couplers with amorphous silicon (α-Si) overlays and SWG metamaterials achieve further reduced coupling losses of -2.4dB and -1.3dB for uniform and apodized layouts, respectively. These results underscore the potential of metamaterial-engineered couplers for low-loss fiber-chip interface integration within scalable SiN PICs for foundry-compatible optical and quantum systems.