Abstract
A high birefringent D-type fiber loop mirror used as refractometer is reported. The D-type fiber section was etched by hydrofluoric acid (HF) being sensitive to the surrounding-medium refractive index allowing the measurement of different refractive indices in liquids. The refractometer was also tested in temperature, and simultaneous measurement of refractive index and temperature was demonstrated. © 2008 IEEE.

Abstract
The application of hollow-core photonic crystal fibers for gas sensing is reviewed and discussed. Some problems with splicing to standard fibers and gas diffusion into the core are addressed. Current work at INESC Porto in the development of sensing heads and interrogation units is also reported.

Abstract
We present, for the first time to our knowledge, results on the characterization of response of a dual resonance observed in the spectrum of a single long-period grating arcinduced in a B/Ge co-doped fiber to different physical parameters. The dual resonance is formed by two overlapping resonances corresponding to coupling of the core mode to symmetric and antisymmetric cladding modes. Therefore, these resonances may behave differently when strain, bending, torsion, or temperature is applied to the grating. We show that the bending, strain, and torsion sensitivities of the two resonances are very different, while the temperature sensitivities are almost the same. © 2008 IEEE.

Abstract
A review of optical fiber sensing demonstrations based on photonic crystal fibers is presented. The text is organized in five main sections: the first three deal with sensing approaches relying on fiber Bragg gratings, long-period gratings and interferometric Structures; the fourth one reports applications or these fibers for gas and liquid sensing; finally, the last section focuses oil the exploitation of nonlinear effects in photonic crystal fibers for sensing. A brief review about splicing with photonic crystal fibers is also included.

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.