Abstract
A review of refractive index measurement based on different types of optical fiber sensor configurations and techniques is presented. It addresses the main developments in the area, with particular focus on results obtained at INESC Porto, Portugal. The optical fiber sensing structures studied include those based on Bragg and long period gratings, on micro-interferometers, on plasmonic effects in fibers and on multimode interference in a large spectrum of standard and microstructured optical fibers.

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 refractometric sensor based on a phase-shifted long-period fiber grating written by electric-arc discharges is presented. Transmission and reflective configurations for refractive index measurements are studied. It is observed that the reflective topology permits better performance compared with the transmission one, which is the approach normally utilized in the context of long-period fiber sensing. The resolution achieved in the measurement of refractive index enables the application of this sensing head structure in demanding situations, such as the measurement of the level of salinity of water. (c) 2006 Optical Society of America.

Abstract
In this work we describe the characterization of high Q optical microresonators using an all fiber based system. Silica microspheres fabricated on a fiber tip by electric arc discharge are characterized using a simple interrogation system based on an adiabatic fiber taper coupler and on the collection of scattered radiation by a multimode fiber.