Abstract
Palladium-based fiber-optic sensors have been one of the most promising configurations for hydrogen sensing. In the latest decade, fiber-optic sensors have indeed earned a strong interest owing to their ability to monitor molecular hydrogen at specific spatial points-either as a sensing tip device or in large areas via multiple sensing regions distributed along the optical fiber. This review focuses on the various types of optical fiber hydrogen sensors, containing specifically palladium as active element. Three distinct working principles are described, viz. interferometric-, intensity-, and fiber grating-based sensors; their characteristics and sensing performances are critically overviewed.

Abstract
In this Letter, a hybrid interferometer for simultaneous measurement of the partial pressures of O-2 and CO2 mixtures is reported. The sensing head consists in the combination of two interferometric structures, one a Fabry-Perrot cavity and the other a modal interferometer. The intrinsic Fabry-Perot was formed by splicing a single mode fiber with a graded index fiber length that was then subjected to chemical etching creating an air cavity. The second interferometer is based on a splice of a pure silica tube in series with the Fabry-Perot. It was observed for a particular gas that its refractive index changes with pressure variation in a specific way, a characteristic that permitted the simultaneous measurement of partial pressures of CO2 and O-2 with rms deviations of similar to +/- 48.7 kPa and similar to +/- 20.1 kPa, respectively. (C) 2012 Optical Society of America

Abstract
We propose and theoretically study a novel surface-plasmon-resonance sensor based on an H-shaped, elliptical-core optical fibre. The two grooves of the H-fibre are coated with a thin, uniform metal layer that in turn is covered with a high-index dielectric layer to allow broad spectral tunability. The sensor maintains linear polarization and facilitates effortless splicing. Electromagnetic mode analysis indicates a sensitivity of 1800 nm/RIU (refractive-index unit) for aqueous analytes.

Abstract
The study of the strain and temperature characteristics of a sensing head based on a four-hole suspended-core fibre in a Sagnac interferometric configuration is reported. It is shown that, for the case of using an uncoated suspended-core fibre, a relatively large strain sensitivity is obtained (similar or equal to 1.94 pm/mu epsilon), while the temperature sensitivity is small (similar or equal to 0.29 pm/degrees C), pointing to a temperature-independent strain sensor. When the fibre is coated, the strain sensitivity remains essentially the same, while the temperature sensitivity becomes much larger and with a value that changes with the localisation of the temperature variation range.

Abstract
This work presents a temperature independent strain/load sensor using a highly birefringent photonic crystal fibre loop mirror. The length of the sensing head is 38 centimetres and its corresponding wavelength spacing between two interferometer minima is 8 nm. The obtained strain and transverse load sensitivity were 1.21 pm/mu epsilon and 0.37 nm/Kg/mm, respectively, while is insensitive to temperature (0.3 pm/degrees C).

Abstract
In this work, a curvature sensor based on a highly birefringent (Hi-Bi) photonic crystal fibre loop mirror is presented. For this purpose, a novel Hi-Bi photonic crystal fibre was designed and fabricated. Half of the microstructured region of the photonic crystal fibre was composed by holes with large diameter, while the other half contained holes with small diameter. Due to this geometry, the fibre core was shifted from the centre and high birefringence appears in the optical fibre. The Hi-Bi photonic crystal fibre loop mirror was demonstrated as a curvature sensor. The curvature was applied for three different fibre directions for the range of 0.6 - 5 metres(-1). The configuration was also characterized for temperature and longitudinal strain, showing insensitivity for these two physical parameters.