Sensitivity enhancement for a multimode fiber sensor with an axisymmetric metal grating layer

•A novel surface plasmon resonance (SPR) fiber sensor with an axisymmetric sub-wavelength metal grating layer was proposed.•The sensitivity S and quality factor Q of the SPR dip are investigated.•The proposed sensor has a sensitivity of 13,000nm/RIU (refractive index unit) at refractive index range from 1.3 to 1.4.