Side-Polished Optical Fiber Grating-Based Refractive Index Sensors Utilizing the Pure Surface Plasmon Polariton

In this paper, we present the ambient refractive index (ARI) sensing characteristics of metal-coated side-polished optical fiber gratings based on the excitation of the pure surface plasmon polariton. The resonance wavelength shift as a function of the ARI and the grating lengths required for a fixed minimum transmittivity (30%) for the TM-like mode are obtained for different metal and residual cladding thicknesses. It is found that a long-period grating (LPG)-based sensor is about 5-20 times more sensitive to the change in the ARI and requires much shorter grating lengths for a given sensitivity than the one based on fiber Bragg grating (FBG). Further, unlike a FBG-based device, an LPG-based sensor is found to have maximum sensitivity at an optimum value of metal thickness, the reason for which is also explained. This paper should find application in the design of sensitive fiber-optic bio/chemical sensors.