Improved detecting sensitivity of long period fiber gratings by polyelectrolyte multilayers: The effect of film structures

Novel polyelectrolyte multilayer (PEM) coated long period fiber gratings (LPFGs) are emerging as one of high-performance optical refractive index (RI) sensors. The influence of PEM film structures (i.e film thickness and chemical structures) on the transmission spectra and detecting sensitivity of LPFG sensors had been investigated theoretically and experimentally. The results demonstrated that the central wavelength of the attenuation bands of LPFGs will shift to the low value with increasing PEM film thickness. The optimized film thickness, where the highest variation of the resonant wavelength may take place, is strongly governed by the inner chemical structure of PEM film. For the dense and less-responsive film coated LPFG, its optimized film thickness will decrease when the RI of surrounding medium increases. This behavior is different with the responsive film coated one. In addition, the detecting sensitivity of PEM coated one is improved by two orders of magnitude as compared with the uncoated one. Therefore, these PEM coated LPFGs have great potentials in diverse fields of molecular diagnostic, environmental monitoring, food safety testing and homeland security.