Chip-fiber grating couplers have been widely used in siliconon-insulator platforms. In silicon nitride waveguides, the lower index contrast yields gratings with a weaker radiation strength and poor directionality, thereby limiting the coupling efficiency. Here we propose and design an apodized self-imaging grating coupler in silicon nitride that images an in-plane waveguide input to an output optical fiber placed at a specific distance above the chip. Both amplitude and phase apodizations are employed to engineer the transfer function of the self-imaging grating to produce an image field matching the fiber mode profile. Two-step etch staircase grating teeth are used to achieve directionality as high as 93%. Full three-dimensional finite-difference time-domain simulations show coupling from a 40 mu m x 40 mu m grating to an SMF-28 single mode fiber with a record calculated efficiency of 86% (0.66 dB loss) and a 3 dB bandwidth of 40 nm near the 1550 nm wavelength. (C) 2016 Optical Society of America
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