Abstract
Long period gratings arc-induced in different types of fibre were annealed at 1000 degrees C for 24 h. During the annealing, the spectrum of the gratings written in Ge-doped fibres shifted towards longer wavelengths whereas an opposite shift was observed for gratings written in Ge-free fibres. A discussion on the mechanisms responsible for that behaviour is presented.

Abstract
An optical fibre sensor for determination of acetic acid is presented. The sensing probe is based on a fiber Bragg grating (FBG) Fabry-Perot cavity, coated with a thin film of sol-gel-PVP (polyVynil Pirrolidone) composite material. The polymeric thin film renders interferometric output sensitive to the presence of carboxylic acid species. Results show that the wavelength of the interferometric peaks change with acetic acid concentration, enabling its quantification. Coupling the fibre probe with a serrodyne modulated readout interferometer enables pseudo-heterodyne interrogation and the detection of acetic acid with a sensitivity of 1 deg/0.01% and a resolution of 0.2% v/v. The results demonstrate the potential of the proposed scheme to operate as a sensitive chemical sensor platform. © 2009 SPIE.

Abstract

Abstract
The von Mises-Fisher distribution is widely used for modeling directional data. In this article, we derive the discriminant rules based on this distribution to assign objects into pre-existing classes. We determine a distance between two von Mises-Fisher populations and we calculate estimates of the misclassification probabilities. We also analyze the behavior of the distance between two von Mises-Fisher populations and of the estimates of the misclassification probabilities when we modify the parameters of the populations or the samples size or the dimension of the sphere. Finally, we present an example with real spherical data available in the literature.

Abstract
Orthogonal Frequency Division Multiplexing (OFDM) systems guarantee transmissions free from intersymbol interference (ISI) and interchannel interference (ICI) over frequency selective channels, provided that the channel delay spread is less than the duration of the cyclic-prefix (or the zero-padding guard interval). However, it is realistic to consider practical implementation scenarios in which the channel delay spread exceeds this limit so that both ISI and ICI do arise. We show that in such cases, the objective of symbol synchronization should be to maximize the ratio of the total useful received power over all subcarriers to the total power of ISI and ICI for the given channel realization, and that this aim can be achieved through a proper choice of the symbol timing. We also present a practical low-complexity synchronization scheme for this case and show that its performance tops the results obtained by the best existing correlation-based timing estimators. Furthermore, we determine upper bounds on the achievable signal-to-interference ratio based on the ideal assumptions of either perfect channel state information or at least knowledge of the channel statistical description, and show that they are very closely approached by the proposed method. © 2009 IEEE.

Abstract
In this work, image segmentation is addressed as the starting point within a motion analysis methodology intended for biomechanics behavior characterization. First, we propose a general segmentation framework that uses Atanassov's intuitionistic fuzzy sets (A-IFSs) to determine the optimal image threshold value. Atanassov's intuitionistic fuzzy index values are used for representing the unknowledge/ignorance of an expert on determining whether a pixel belongs to the background or the object of the image. Then, we introduce an extension of this methodology that uses a heuristic based multi-threshold approach to determine the optimal threshold. Experimental results are presented.