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Waveguide grating couplers based on complementary metal-oxide-semiconductor (CMOS) poly-silicon gate layers are designed and fabricated. Sharing the same etching profile as that of the CMOS poly-silicon gate layer, the fabrication of the grating couplers is fully embedded in the CMOS process without adding any additional masks or process steps. Peak coupling efficiency of 40% and 3 dB bandwidth of ~60 nm, as well as a low reflection, are experimentally achieved. The coupling efficiency can be further improved to ~70% by using silicon-on-insulator wafers with optimal parameters.