High-efficiency broadband grating out-coupler for thick silicon waveguides with a tuned SiGe layer

We design a novel grating out-coupler while maintaining a Gaussian-shaped far-field output. The approach is based on the patterning of a grating in a SiGe layer on the top surface of a 3-μm-thick Si waveguide. The grating aims at coupling the guided signal into a beam vertically out from the surface of the chip, i.e., out-coupling. The use of a high refractive index alloy, i.e., silicon-germanium, allows a tunability of the effective index of the guided mode and to modify the field distribution enabling a higher efficiency of the grating. It yields a theoretical efficiency of 75% at the central wavelength of 1583 nm. The overall length of this grating coupler is 500 μm, and the output light is detected at 1 μm above the grating. This type of long grating coupler is intended to be eventually used as sensor in LiDAR (Light detection and ranging) applications.