Optical couplers play an integral role in coupling of the guided mode from an optical fiber into a Photonic Integrated Circuit (PIC). Apart from the edge couplers, grating couplers are the most extensively used optical I/O's. Grating couplers have limited application owing to their polarization selectivity and a small coupling 3-dB bandwidth of approx. 70 nm. By engineering the average effective index of the grating coupler using sub-wavelength gratings, we increased the coupling bandwidth while introducing polarization insensitivity. With the a uniform sub-wavelength grating coupler designed in this thesis, we attained 3-dB bandwidth of 105nm and 121 nm for the TE and TM polarizations, respectively. The proposed device attains a simulated coupling efficiency of -4.88dB for both TE and TM polarization at 1550 nm, resulting in zero polarization dependent loss. To the best of our knowledge, the proposed SWGC offers the best reported performance in bandwidth and polarization sensitivity.