Robust single-etch surface grating couplers for silicon nitride waveguide platform

Silicon nitride (SiN) has emerged as an important waveguide platform to implement integrated photonic circuits for a wide range of applications, including telecommunications, nonlinear optics, and quantum information. The SiN platform is compatible with a CMOS fabrication and provides attractive properties such as low propagation loss and increased tolerance to fabrication errors. However, a comparatively low index contrast is a challenge for coupling light off-chip using surface grating couplers. Compared to silicon waveguides, reduced grating scattering strength limits attainable coupling efficiency since the size of the radiating beam is significantly larger compared to near-Gaussian optical mode of standard SMF-28 optical fibers. In this work, we present, both theoretically and experimentally, a set of robust uniform and apodized grating couplers implemented in 400 nm SiN platform. Grating couplers operate with TE polarization at telecom C-band, with measured coupling losses between -4 dB to -3 dB near 1550 nm wavelength. Prospectively, our designs can be further optimized by using sub-wavelength grating metamaterial engineering and self-focusing topology, with simulated fiber-chip coupling loss as low as -1.6 dB. Our results pave the way towards development of highly efficient and robust off-chip coupling interfaces in SiN platform.