Ultra-compact silicon nitride grating coupler for microscopy systems

Grating couplers have been widely used for coupling light between photonic chips and optical fibers. For various quantum-optics and bio-optics experiments, on the other hand, there is a need to achieve good light coupling between photonic chips and microscopy systems. Here, we propose an ultra-compact silicon nitride (SiN) grating coupler optimized for coupling light from a waveguide to a microscopy system. The grating coupler is about 4 by 2 μm2 in size and a 116 nm 1 dB bandwidth can be achieved theoretically. An optimized fabrication process was developed to realize suspended SiN waveguides integrated with these couplers on top of a highly reflective bottom mirror. Experimental results show that up to 53% (2.76 dB loss) of the power of the TE mode can be coupled from a suspended SiN waveguide to a microscopy system with a numerical aperture (NA) 0.65. Simulations show this efficiency can increase up to 75% (1.25 dB loss) for NA = 0.95.