Abstract
A theoretical and experimental characterisation of a self-referenced fibre optic intensity sensor based on a multiple beam Sagnac configuration is conducted. Via the definition of the measurement parameter (R parameter) sensor linearity and sensitivity are analysed. Theoretical and experimental results are compared, being considered the problem of sensor design and optimisation.

Abstract
In this work a wavelength multiplexing concept was demonstrated for frequency based self-referenced fibre optic intensity sensors relying on the utilisation of Bragg gratings and WDM couplers. The experimental results obtained showed a fairly good agreement with those predicted from the theory. It turned out that the system had negligible crosstalk between the two sensors. The resolution obtained for the sensors was found to be ˜0.05 dBvHz. It should be emphasised, that the sensing concept described in this work is particularly favourable in terms of the minimisation of system noise. This happens because what is monitored is the amplitude of two sinewaves, i.e., the detection bandwidth can be made as narrow as practically feasible, with the consequent decrease of the system noise level. The power budget of the sensing network can be improved if shorter lengths of delay fibre are used, with the penalty, however, of working with higher frequencies. On the other hand, if the reflectivity of the FBGs is optimised, the power received by the detectors will increase correspondingly. Finally, a proper choice of the coupling coefficient of the couplers in the reflective ladder topology will have a strong impact on the optical power levels reaching the detector unit. © 2002 IEEE.

Abstract
A new wavelength multiplexing configuration for self-referenced fiber optic intensity sensors using fiber Bragg gratings and wavelength division multiplexing (WDM) couplers is investigated. First, the network multiplexing concept is characterized, and then the self-referenced intensity sensor is presented, which is the basis of each individual sensor in the network. The implemented experimental setup of the multiplexing network is described, and results are presented considering the crosstalk, resolution, and power budget of the sensing multiplexing network. The characteristics and features of the configuration proposed are addressed. (C) 2004 Society of Photo-Optical Instrumentation Engineers.

Abstract

Abstract
A general overview of the R&D activity in fiber optic sensing developed over the last fifteen years in Portugal is given. Different topics are addressed, including interferometric, intensity and Bragg grating based fiber optic sensors, signal processing and multiplexing techniques, optical current sensors, together with some references to field trials and applications. Possible guidelines for present and future national R&D activity on this subject are outlined.

Abstract
An evaluation of two optical fiber sensing heads for measuring transversal load is presented. Their construction introduces power loss without the need of a pair of deformer plates and is based on crossing two fibers in braid-type (braid twisted), and of an optical fiber stretched out which is wrapped up by another one in spiral form (spiral twisted), respectively. For a higher sensitivity, the braid twisted configuration is preferable. Nevertheless, in both sensing heads, the sensitivity can be enhanced by decreasing the enrollment period and/or by choosing the appropriate encapsulation of the sensing head. Relative to resolution and hysteresis, the spiral twisted sensing head presented the best result. However, further work on the plasticity of the coatings is needed to decrease hysteresis. These configurations can be embedded in composite materials and used in many applications. (c) 2005 American Institute of Physics.