Abstract
A fiber-optic sensor for torsion measurement, based on a two-LP-mode operation in ultra high birefringent photonic crystal fiber (PCF) is described. The structure of the photonic crystal fiber presents two large asymmetric holes adjacent to the core fiber. When linearly polarized light is injected in x or in y directions, respectively, two different interferometers can be obtained. In one of these cases, as torsion is applied to the ultra Hi-Bi PCF a beat between the two interferometers is formed due to the simultaneous excitation of the two polarization states. The detection technique to read the torsion sensor is based on the analysis of the Fast Fourier Transform (FFT), which is an alternative and simple solution. The sensor exhibited reduced sensitivity to temperature and also to strain. © 2009 SPIE.

Abstract
In this work, the authors present an alternative Sagnac interferometer based on a suspended twin-core fibre. Using the suspended twin-core fibre, the pattern fringe is due to the different optical paths of the light in the two cores. In this case, the value of the refractive index difference between the two cores is 10(-3), which indicates an advantage of this approach, namely the possibility to use a small length of the suspended twin-core fibre. The sensing configuration was characterized for strain, temperature, curvature and torsion, respectively.

Abstract
In this work, we propose to take advantage of properties of a Bragg fibre for optical sensing. The Bragg fibre exhibits three concentric high refractive index layers embedded in pure silica and surrounding a 35 mu m diameter core. A short (0.3 m long) piece of Bragg fibre slightly multimode, is used to elaborate an intermodal interferometer, the spectral response of which exhibits a fringe pattern that depends on the operating wavelength, which can therefore be used as a sensor. The two modes considered were found to be the fundamental LP(01) and the high-order mode LP(02). The sensor has been characterized in strain and temperature and presents a sensitivity of - 1.09 pm/mu epsilon and 14.1 pm/degrees C respectively. The sensor demonstrated insensitivity to curvature thanks to well known Bragg fibre properties.

Abstract
In this work, we demonstrate the possibility of fabricating short LPGs and rocking filters in highly birefringent Photonic Crystal Fiber using CO(2) laser. In our experiments both kinds of gratings were made in the same Boron doped highly birefringent PCF using similar exposure parameters. We also present the sensing capabilities of both fabricated gratings to temperature, strain and hydrostatic pressure by interrogation of the wavelength shifts at the different resonances.

Abstract
Birefringence monitoring of a Hi-Bi fibre under chemical etching is conducted. The Hi-Bi fibre (a bow-tie type) was inserted in a fibre loop mirror and the optical spectral response was measured while chemical etching of the Hi-Bi fibre was taking place. The birefringence of the bow-tie fibre has decreased due to the etching and the wavelength spacing of the fringe minima of the fibre loop mirror has increased.

Abstract
We report on the analysis and experimental validation of the strain sensitivity dependences of a fiber Bragg grating written in standard optical fiber when combined with fused tapers. By controlling the difference between the cross sections of the fused taper and the Bragg grating, the strain sensitivity of the Bragg wavelength can be changed by acting on the gauge length. The strain sensing characteristics of an interferometric structure formed by fabricating a fused taper in the middle of a fiber Bragg grating are also reported. (C) 2007 Optical Society of America.