in this paper, we propose a microfiber-plane-grating composite optical waveguide (mpgcow), which is formed by immobilizing a tapered microfiber on the surface of a plane grating with one defect, for gas refractive index ( ri) sensing. its optical properties and gas ri sensing properties are investigated by the finite difference time domain method. results show that the mpgcow has a photonic stop band and is very sensitive to the ambient gas ri variation. the largest gas ri sensing sensitivity of 486.67 nm/riu and detection limit of 2 x 10(-6) are obtained by immersing the structure in the mixture gas of n-2 and he with various mixture ratios. (c) 2017 optical society of america