High Sensitive Refractometric Sensor Using Symmetric Cladding Modes of an FBG Operating at Mode Transition

We report a high sensitive refractive index (RI) sensor using fiber Bragg grating (FBG) where azimuthally symmetric resonant cladding modes were used for RI sensing. We enhanced the sensitivity by tailoring the effective index of a cladding mode to an optimum value so that the mode has maximum sensitivity around a specific surrounding RI. We tailored the effective index of a clad mode by controlled deposition of a thin polymer layer having RI higher than the cladding. Experiments show that the sensitivity of cladding mode (LP0,12) of a standard FBG with overlay coating to the surrounding medium having RI around 1.333 is ~ 2600 pm/RIU. The proposed sensing mechanism will be useful for biological sensing application.