Enhanced sucrose sensing sensitivity of long period fiber grating by self-assembled polyelectrolyte multilayers

The response laws of long period fiber grating (LPFG) coated with polyelectrolyte multilayers (PEMs) of poly(allylamine hydrochloride) (PAH)/poly(sodium-p-styrenesulfonate) (PSS) to a series of concentration of sucrose solutions were investigated. Experimental results showed that the central wavelengths of LPFG spectra move to short-wave as concentration of sucrose solution increase. Pure LPFGs show the optimal sensitivity to 55-60% of sucrose solutions. With increasing number of the layers, the most sensitive regions of LPFGs to sucrose solutions will shift from high concentration to low concentration by matching average refractive index of films and sucrose solutions with that of LPFG cladding. LPFGs assembled with 100 bilayers of PAH/PSS exhibited the optimal sensitivity to 20-25% of sucrose solutions. However, LPFGs coated with 115 bilayers were more sensitive to a lower concentration (<5%) of sucrose solution. Therefore, the sensitivity of LPFG to the small molecular weight analytes can be strongly improved by designing the refractive index and thickness of PEM overlay, as confirmed and interpreted by theoretical simulations.