Abstract
This work presents two distinct configurations based on phase-wavelength conversion using a ring fibre laser with two different long period gratings interferometer topologies. The sensors are interrogated by analysing the wavelength change of the emission laser, which is directly dependent on the interferometer phase change. The first configuration integrates a Mach-Zehnder interferometer, which is based on a pair of long period gratings and is used as sensing head for bending radius and longitudinal strain measurement. The second configuration, comprehends a Michelson interferometer, which is based on a single LPG and a fibre end mirror and is used as a liquid level sensor or as an optical refractometer.

Abstract
A high birefringent D-type fibre loop mirror used as refractometer is reported. A chemical etching was applied to the D-type fibre and presented high sensitivity for the surrounding medium refractive index. The refractometer was tested with different refractive indices in liquids and characterized in temperature, showing a linear and independent response to both parameters. With these results the refractometer can perform simultaneous measurement of refractive index and temperature.

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.