We compare state-of-the-art sub-wavelength grating couplers (SWGCs) based on their coupling efficiencies, 1-dB bandwidths and back reflections. Three types of SWGC, the uniform SWGC, the apodized SWGC, and the broadband SWGC, which use one-dimensional sub-wavelength gratings, are compared for the silicon-on-insulator platform. The SWGCs are designed for the fundamental TE mode, operating at 1550nm.