Abstract
The potential applications of luminescent semiconductor nanocrystals to optical oxygen sensing are explored. The suitability of quantum dots to provide a reference signal in luminescence-based chemical sensors is addressed. A CdSe-ZnS nanocrystal, with an emission peak at 520 nm, is used to provide a reference signal. Measurements of oxygen concentration, which are based on the dynamic quenching of the luminescence of a ruthenium complex, are performed. Both the dye and the nanocrystal are immobilized in a solgel matrix and are excited by a blue LED. Experimental results show that the ratio between the reference and the sensor signals is highly insensitive to fluctuations of the excitation optical power. The use of CdTe, near-infrared quantum dots with an emission wavelength of 680 run, in combination with a ruthenium complex to provide a new mechanism for oxygen sensing, is investigated. The possibility of creating oxygen sensitivity in different spectral regions is demonstrated. The results obtained clearly show that this technique can be applied to develop a wavelength division multiplexed system of oxygen sensors. (c) 2006 Optical Society of America.

Abstract
In this work we described an optical fibre sensing system for detecting low levels of methane. The properties of hollow-core photonic crystal fibres are explored to have a sensing head with favourable characteristics for gas sensing, particularly in what concerns intrinsic readout sensitivity and gas diffusion time in the sensing structure. The sensor interrogation was performed applying the Wavelength Modulation Spectroscopy technique, and a portable measurement unit was developed with performance suitable for remote detection of low levels of methane. This portable system has the capacity to simultaneously interrogate four remote photonic crystal fibre sensing heads. Copyright © 2009 J. P. Carvalho et al.

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]