In this paper, a detailed investigation on the modeling of long-period fiber grating (LPFG) sensors is discussed with the aim of providing a more realistic solution for their use in biosensing. Add-layer sensitivity, i.e., sensitivity of the sensor to an additional layer adhered onto the fiber surface, is quantified and a clear and complete analysis about the influence of the average thickness of the deposited biological sensing layers, as well as the change in refractive index of these layers, on the resonant wavelength of the cladding modes of an LPFG is provided. Add-layer sensitivity of LPFG sensors close to mode transition (MT) and also at turn-around point (TAP) are taken into account. Adsorbed layer thicknesses, as estimated from measured wavelength shifts of the LPFG, are found to have a good match with the values obtained through other measurement techniques.
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 (LP 0,12 ) 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.
We present our theoretical study on the design of LPFG sensor where its add-layer sensitivity is enhanced. addlayer sensitivity quantifies the sensitivity of the sensor to the changes taking place within few tens of nanometers around the receptor molecules. Two different methodologies: the use of dual overlay layer and tailoring of the intermodal separation between two cladding modes, have been used to enhance the add-layer sensitivity. Using coupled mode analysis we compute several examples to carry out a detailed comparative analysis between the results obtained, focusing on the cladding mode near mode transition.
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