Abstract

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
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.

Abstract
We report on the use of a frequency-reflectometry domain technique for referencing optical intensity sensors located between two fibre Bragg gratings. The experiment combines the concept of frequency modulated continuous wave with the spectrally selective mirror proper-ties of fibre Bragg gratings to interrogate with referencing properties intensity based sensors. Multiplexing two of these sensors using this technique in a parallel topology system was also experimentally demonstrated. Due to the use of different fibre lengths (delay lines), signals from the fibre Bragg gratings located at different positions in an array are separated in the frequency-domain.

Abstract
A curvature sensor based on a highly birefringent (Hi-Bi) photonic crystal fiber inserted into a Sagnac interferometer is demonstrated. For this purpose, a novel Hi-Bi photonic crystal fiber was designed and fabricated. Half of the microstructured region of the photonic crystal fiber was composed by large diameter holes, while the other half contained small diameter holes. Because of this geometry, the fiber core was shifted from the center and high birefringence appears in the optical fiber. Curvature was applied for three different fiber directions for a range of 0.6-5 m(-1). Temperature and longitudinal strain was also characterized for constant curvature. The configuration showed insensitivity to these two physical parameters. (C) 2008 Optical Society of America.

Abstract
A Sagnac interferometer with a section of a polarization maintaining side-hole fiber for multiparameter measurement is proposed. The sensor was experimentally demonstrated to be sensitive to torsion, temperature, and longitudinal strain, simultaneously The birefringence in the investigated side-hole fiber is induced simultaneously by the elliptical shape of a germanium-doped core and by field overlap with the air holes surrounding the core. The latter effect is purely geometrical and causes high chromatic dispersion of the group birefringence in the long wavelength range, which results in a different period of spectral interference fringes. A different wavelength response is obtained for each interference fringe peak when the fiber is subjected to torsion, temperature, or longitudinal strain. A matrix equation for simultaneous measurement of the three parameters-torsion, temperature, and longitudinal strain-is also proposed. (C) 2008 Optical Society of America