Abstract
A fiber optic sensor for liquid refractive index measurement based on a Fabry-Perot interferometer is described. The interferometer is achieved between the reflection of a short fiber Bragg grating and the Fresnel reflection from the cleaved fiber end. This fiber end is then in contact with the liquid sample to provide refractive index measurements. The sensor is characterized by immersing the fiber tip in distilled water with different concentrations of ethylene glycol. A linear relation of the interferometer fringe visibility with refractive index variation is observed, and a resolution of similar to 10(-3) is obtained. It is also shown that the sensor operation is independent of temperature effects, other than the one related to temperature-induced change of the liquid refractive index. (c) 2008 Society of Photo-Optical Instrumentation Engineers.

Abstract
A long-period-grating-based fiber optic Michelson modal interferometer with coherence addressing and heterodyne interrogation is studied as a sensing structure for measuring environmental refractive index, temperature, and liquid level. The effects of several system parameters on the measurements are investigated. Experimental results show that the sensitivity to the external refractive index increases with the order of cladding mode and with a reduction of the fiber diameter. The decrease of the fiber diameter from 125 mu m down to 70 mu m enhances the sensitivity to the external index by a factor of 2.7. It is also shown that the use of a silica-core fiber increases the sensitivity to the external index by a factor of 1.4 and reduces the thermal sensitivity by a factor of 2.5 compared to a standard fiber. (C) 2008 Society of Photo-Optical Instrumentation Engineers.

Abstract
An optoelectronic system for detection and monitoring of methane has been developed and implemented. The signal processing technique used in the proposed system is based on Wavelength Modulation Spectroscopy (WMS). When associated with the revolutionary microstructured fibres, this scheme revealed an effective way to measure gas concentration. Aiming the optimization of the sensing head design, the methane diffusion time inside a hollow-core fibre was evaluated. An error of 2.8% between experimental and theoretical values was obtained, thus validating the adopted model. These results were very encouraging towards the implementation of a practical unit for remote gas monitoring applications. © 2008 IEEE.

Abstract
A fibre Bragg grating structure arranged in a twisted configuration is proposed for sensing applications. The characteristics of the sensing head for measuring temperature, longitudinal strain and transverse load are analysed. It is shown that this configuration is particularly applicable for transverse load measurement. In this case a resolution of 0.002 N mm(-1)Hz(-1/2) was achieved.

Abstract
A robust all-fibre interferometric interrogation technique based on the wavelength modulation of a pi-shifted fibre Bragg grating is proposed. The concept is demonstrated for strain measurement using a Bragg-grating-based Fabry-Perot interferometer. The strain-phase relationship is shown to be linear with a sensitivity slope of (2.19 +/- 0.02)degrees/mu epsilon. A strain resolution of approximate to 2.4 mu e is demonstrated.

Abstract
A scheme for the simultaneous determination of oxygen and temperature using quantum dots and a ruthenium complex is demonstrated. The luminescent complex [Ru(II)-tris(4,7-diphenyl-1,10-phenanthroline)](2+) is immobilized in a non-hydrolytic sol-gel matrix and used as the oxygen sensor. The temperature information is provided by the luminescent emission of core-shell CdSe-ZnS semiconductor nanocrystals immobilized in the same material. Measurements of oxygen and temperature could be performed with associated errors of +/- 2% of oxygen concentration and +/- 1 degrees C, respectively In addition, it is shown that while the dye luminescence intensity is quenched both by oxygen and temperature, the nanocrystals luminescent emission responds only to temperature. Results presented demonstrate that the combined luminescence response allows the simultaneous assessment of both parameters using a single optical fiber system. In particular, it was shown that a 10% error in the measured oxygen concentration, induced by a change in the sample temperature, could be compensated using the nanocrystals temperature information and a correction function.