Abstract
A long-period grating written in the SMF-28 fibre was heat treated at 1000 degrees C for 15 days. The spectrum of the grating shifted to longer wavelengths and the resonances depth decreased as a result of structural relaxation. The background loss increased considerably for times longer than 200 h, and this loss is caused by devitrification of the fibre.

Abstract
We present results on the characterization of the response of a dual resonance observed in the spectrum of a single long-period grating arc-induced 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. The behavior of the resonances is studied when the grating is subject to strain, bending, torsion, temperature, or external refractive-index changes. The strain, bending, and torsion sensitivities of the two resonances differ, whereas the temperature sensitivities are almost the same. The sensitivities to variation in external refractive index are the same for the two resonances when the long-period grating is straight and differ when the fiber with the grating is curved. (c) 2010 Optical Society of America

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.

Abstract
In this work an all-optical hot-wire flowmeter based on a silver coated fiber combining a long period grating and a fiber Bragg grating (FBG) structure is proposed. Light from a pump laser at 1480nm propagating down the fiber is coupled by the long period grating into the fiber cladding and is absorbed by the silver coating deposited on the fiber surface over the Bragg grating structure. This absorption acts like a hot wire raising the fiber temperature locally, which is effectively detected by the FBG resonance shift. The temperature increase depends on the flow speed of the surrounding air, which has the effect of cooling the fiber. It is demonstrated that the Bragg wavelength shift can be related to the flow speed. A flow speed resolution of 0.08m/s is achieved using this new configuration. (C) 2011 Optical Society of America

Abstract
A compact sensor based on step-changed are-induced long-period fiber gratings was implemented to discriminate between temperature and strain. The proposed sensor consists of a single long-period grating with two sections written consecutively in the SMF-28 fiber using the electric are discharge technique. The two sections have the same period but different fabrication parameters. The operation of the sensor relies on the existence of a difference between the values of temperature and strain sensitivity of two neighboring resonances observed in the spectrum of the step-changed grating. The temperature and strain resolutions obtained for the sensor are 0.2 degrees C and 35 mu epsilon, respectively. (c) 2007 Optical Society of America.

Abstract
In this work a modal interferometer based on arc-induced long-period gratings (LPGs) in a Mach-Zehnder configuration is evaluated as a sensing structure for environmental refractive index measurement. To interrogate this sensing device, coherence addressing and pseudo-heterodyne processing were used. The influence of geometric effects such as stretching, bending and twisting the interferometer on the sensitivity to refractive index changes was studied. It is shown that due to the antisymmetric nature of cladding modes in arc-induced LPGs, it is possible to tune the system sensitivity to external refractive index by simple mechanical action. The experimental results show that it is possible to tune the sensitivity to external refractive index by more than 50% by control of the curvature in the Mach-Zehnder interferometer.