Abstract
In this work, sensitivity to strain and temperature of a sensor relying on modal interferometry in hollow-core photonic crystal fibers is studied. The sensing structure is simply a piece of hollow-core fiber connected in both ends to standard single mode fiber. An interference pattern that is associated to the interference of light that propagates in the hollow core fundamental mode with light that propagates in other modes is observed. The phase of this interference pattern changes with the measurand interaction, which is the basis for considering this structure for sensing. The phase recovery is performed using a white light interferometric technique. Resolutions of +/- 1.4 mu epsilon and +/- 0.2 degrees C were achieved for strain and temperature, respectively. It was also found that the fiber structure is not sensitive to curvature. (C) 2009 Optical Society of America

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 theoretical and experimental study of a new fiber loop mirror based on a "figure-of-eight" configuration, is reported. For the theoretical model, the Jones matrix analysis is analyzed. The configuration is tested as an interrogation system where the spectral response arises from the combination of the reference signal modulated by the sensor signal. The configuration is characterized in mechanical strain and presents a phase sensitivity of 8.2 mrad/mu epsilon.

Abstract
In this work, a novel high birefringent (HiBi) fiber loop mirror sensor based on a "figure-of-eight" constructed with a 3x3 fiber coupler, is presented. The "figure-of-eight" is formed by two fiber loop mirrors (FLM's) made by four of the six fiber arms of the 3x3 fiber coupler. The other two remaining fiber ports of the 3x3 coupler are used as input and output fibers of the compound sensor. The sensing head is located in the one of the FLM and it is formed by a spliced section of HiBi elliptical core fiber. The spectral response of this "figure-of-eight" configuration presents two interference optical signals that can be easily tuned by a polarization controller that is located in the other FLM, and which is made only of standard singlemode fiber from two arms of the 3x3 coupler. The sensor head was optically characterized both in temperature and strain, showing wavelength dependence sensitivities of -0.23 nm/degrees C and - 2.6 pm/mu epsilon, for temperature and strain, respectively. It is noticed that these sensitivities are practically the same for the two interference signals. Future work will explore the possibility to use the singlemode FLM to interrogate the sensor head made by HiBi fiber section, and providing elimination of phase fluctuations that can occur, increasing its potential for remote sensing applications.

Abstract
Fiber optic modal interferometry has been around as a sensing concept since the outcome of fiber optic sensing. Initially supported by the utilization of standard Hi-Bi fibres associated to polarimetric modal interference, later this sensing approach evolved to modal interference based on spatial modes propagating in the core, on spatial modes propagating in the core and in the cladding with coupling performed by fibre devices such as long period gratings and tapers, and more recently on several types of modes propagating in photonic crystal fibers. This paper will address fiber optic sensing based on modal interferometry, and configurations of different type researched in last years will be presented and their performance compared.

Abstract
A multimode interferometer based-fiber optic sensor with a silica tube section aimed to measure refractive index (RI) variations of surrounding liquids is presented. The sensing head is a silica tube section fusion spliced to single mode fibers operating in transmission. In the splice regions tapers were made to allow the light to be guided in the silica tube while the core is formed by air. This configuration permits measurements of refractive index variations with sensitivities of 101.1, 106.29, and 107.97 nm/RIU considering resonances with different wavelengths. The same resonances were tested with temperature variations with sensitivities achieved of 7.8, 8.7, and 9.3 pm/ degrees C, respectively. The spectral variation associated with one degree temperature change corresponds to a refractive index change of similar to 8 x 10(-5), proving the low temperature dependence compared with sensitivity to RI variations. (C) 2011 Society of Photo-Optical Instrumentation Engineers (SPIE). [DOI: 10.1117/1.3646393]