Low-Loss Silica Edge Coupler Based on Cascaded Gratings With Multiple Duty Ratios

A silica edge coupler based on cascaded gratings with four different duty ratios is demonstrated. Finite-difference time-domain method is adopted in the coupler optimization. Ultraviolet photolithography, plasma etching and chemical vapor deposition have been adopted in the coupler preparation. When the refractive index difference between the silica core and cladding layers is 2%, the measured coupling loss is 0.52 dB/facet at the wavelength 1550 nm. The bandwidth with a penalty less than 1 dB/facet exceeds 318 nm (1300 nm - 1507 nm and 1509 nm - 1620 nm). Features of large alignment tolerance promise the edge coupler good potentials on silica platform.