In this paper, we developed a four-layer theory that is capable of calculating LPFGs with real
metallic coatings using matrix method. The resonant spectrum characteristics of LPFGs
coated with various metallic films are investigated in detail. The resonant wavelength shifts of
LPFGs with the change of refractive index of surrounding media over different metallic
thicknesses are obtained. It is found theoretically that there exists an optimal range of metallic
thickness within which the maximal measurement sensitivity for the refractive index change
of surrounding media can be obtained.
A novel single-ended reflecting long period fiber grating (LPFG) sensor with thin metal film overlay and the sensing
system is described. An all-fiber reflection based surface plasma resonance (SPR) LPFG sensor with three-layer structure
(core, cladding and metal) is established experimentally and fabricated with a pulsed CO2 laser writing system and
vacuum evaporation coating system. Different nm-thick thin metal films are deposited on the reflected LPFG sensor for
the excitation of surface plasma waves (SPWs) and the characteristics of the reflection resonance spectra of the LPFG
sensor for measuring refractive index of fluids are studied. It is found that different thicknesses of different metal films
show different measuring sensitivity. Through the comparison of the resonance wavelength obtained in air, water,
alcohol, glycerin and their mixture solution, different sensitivities of the reflected SPR-LPFG sensor have been achieved.
The proposed SPR-LPFG sensing scheme offers an efficient platform for achieving high performance fiber sensors for
the measurement of ambient refractive index.
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