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New sensor systems using surface plasmon resonance (SPR) have been developed for measuring the refractive index (RI) of an analyte. 1,2 The systems use an optical phenomenon in which incident light excites a charge-density wave at the interface between a metal and a dielectric. 1,2 An SPR sensor allows the realization of quick determination of the RI of a sample with high accuracy. However, the usual SPR sensor systems utilize bulk optical configurations that are limited by the use of a coupling prism. Furthermore, since the SPR system requires a spectrometer or a rotation stage, the resulting systems are relatively large and expensive, are inapplicable for remote sensing applications. 3 Optical fiber sensors based on the SPR phenomenon have been developed. These systems replace a coupling prism with an optical fiber, and the SPR phenomenon is applicable to analyses of liquid or gas samples. The systems have a sensing element of a deposited metal film on the core of an optical fiber and must scan the wavelength 3,4 or change the angle of incident light. [5][6][7][8][9][10][11][12][13][14][15] Another type of the sensor based on SPR in a thin deposited metal film on the core of an optical fiber implanted in a silica block and polished has been reported. [16][17][18][19][20][21][22][23] A gold island SPR sensor has also been investigated. [24][25][26][27][28][29] Most theoretical analyses for the optical fiber sensors using SPR are based on the traditional Kretschmann configuration and the response curves have been calculated. 3,6,12,14,[16][17][18][19][20][21][22][23][30][31][32][33][34] However, these analyses are made for the sensors in a restricted range of the refractivity (less than 1.45 RI units). Thus, a theoretical investigation of the response of the gold-deposited optical fiber sensor over a wider range of refractivity is necessary to understand the mechanism of the response.An optical fiber sensor system based on SPR without scanning the wavelength or changing the angle of incident light is desired for remote sensing, continuous analysis, and in situ monitoring using a small and inexpensive sensing element. We have already developed a small and simple gold-deposited optical fiber sensor system without scanning the wavelength or changing the angle of incident light. [35][36][37][38][39][40] The sensing element has a thin (30 -60 nm) gold film deposited all around the bare core of an optical fiber and a sample solution flows around it. When light with an intensity distribution is introduced into the optical fiber of the sensor with a focusing lens, the SPR occurs at the interface between the gold film and the sample solution. A part of the incident light disappears by the SPR and the intensity of the transmitted light varies depending on the refractivity of the sample solution. [35][36][37][38][39][40] We have performed the analyses of various alcohol solutions [35][36][37][38][39][40] and some esters in ethanol 36 and have demonstrated the usefulness of this sensor system. Theoretical analyses for the ...

Gold-deposited optical fiber sensors with film thicknesses from 30 to 60 nm were prepared, and the responses to a wide range of a refractivity (1.33 -1.54 refractive index (RI) units) were investigated both experimentally and theoretically. The response curve of the sensor has two minima in the refractivity range from 1.33 to 1.44 and at 1.462 RI units. The former minimum is due to surface plasmon resonance (SPR) in the thin gold film, and shifts to a lower refractivity as the film becomes thicker. The response curves of the sensors with film thicknesses of 45 and 60 nm agreed well with those calculated from SPR theoretical equations. Morphology observations of the surfaces of deposited gold films on glass by atomic force microscopy (AFM) and a variation in resistance of the films with various thicknesses show the structure of the gold films. We concluded that the thin deposited gold films have many defects, and that the core of the golddeposited optical fiber leaks light through the defects to the sample solution with the same refractivity (1.462 RI units) as that of the core.

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