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For the surface plasmon resonance (SPR)-based optical fiber sensor with four-layer configuration (fiber core/metal/sensing layer/sample), the effects of different metals, different models (Lorentz-Drude model and Drude model) for the same metal, the thickness of metal layer, and the refractive index (RI) of sample on the transmitted spectra are investigated. The calculated results show that the real part of the refractive index of metal layer has the greatest effects on the sharp degree of the transmitted dip. Moreover, the dispersion function under four-layer configuration can be used to understand the relationship between the resonance wavelength and the thickness of metal layer.Recently, surface plasmon resonance (SPR)-based optical fiber sensors have attracted large attention for lots of advantages, such as real time and in situ monitoring. Gupta et al [1][2][3][4][5] have presented many theoretical studies on the SPR sensor in the wavelength interrogation mode and given the spectra of the transmitted power with wavelength. Balaa et al [6] have studied the SPR-based optical fiber sensor with four-layer mode, in which the biolayer is considered as a mixture of sample and analyte. In the theoretical calculations, Gupta et al [1][2][3][4][5] always adopted the Drude model for the refractive index (RI) of metal, but Rakic et al [7][8][9] presented the optical parameters of 11 metals (Ag, Au, Cu, Al, Be, Cr, Ni, Pd, Pt, Ti and W) in the Lorentz-Drude model, and showed that the Lorentz-Drude model was in good agreement with experiment data. In fact, the metal RI has great effects on the transmitted spectra and resonance wavelength of SPR-based optical fiber sensors. In addition, in order to explain the relationship between the resonance wavelength and sample RI, a simple resonance condition was used [1][2][3][4][5] . But this simple resonance condition cannot be used to explain the changes of the transmitted spectra with the thickness of metal layer and sensing layer. In this work, for SPR-based optical fiber sensors with four-layer configuration consisting of fiber core, metal, sensing layer and sample, the effects of Drude model and Lorentz-Drude model of metal on the transmitted spectra and the quantitative relationships between the resonance wavelength and the thickness of metal layer and sample RI based on the dispersion function of the surface plasmon wave (SPW) are investigated.For the SPR-based fiber optical sensor in the wavelength interrogation mode, the four-layer model is widely used to simulate the attenuated total reflection. The first layer is the fiber core, whose RI is denoted by n 1 ; the second layer is a metal layer with dielectric constant m and thickness d 2 ; the third layer a sensing layer with thickness d 3 , whose RI and dielectric constant are denoted by n sen and sen , respectively; the fourth layer is the sample, whose RI and dielectric constant are denoted by n sam and sam , respectively. As the light from a collimated source is launched into one end of the fiber at the axial point, th...

For the superstructure fiber grating (SFG) consisting of fiber Bragg grating (FBG) and long-period fiber grating (LPFG), there exist three kinds of coupling: the contra-directional coupling between forward propagating core mode and backward propagating core mode, the contra-directional coupling between core mode and cladding mode, and the co-directional coupling between core mode and cladding mode. By using the similarity transformation method for the coupled mode equations, the effects of three coupling coefficients κ co L, κ cl L, and κ S L on the transmission and reflection spectra of SFG have been investigated. The simulations show that the overall intensity of the transmission spectrum of SFG is decided by LPFG, that the main lobes of the transmission and reflectivity spectra of SFG are decided by coupling coefficients κ co L and κ S L, and that the side lobes of the transmission and reflectivity spectra are decided by coupling coefficients κ cl L and κ S L.