Abstract
It is reported a LPG dynamic interrogation technique based on the modulation of fibre Bragg gratings located in the readout unit that permits to attenuate the effect of the 1/f noise of the electronics in the resolution of the LPG-based sensing head. The concept is tested to detect variations of the external refractive index and a resolution of 2.0x10 (4) NIR was achieved without system optimization. Additionally, the effect in the sensor resolution when introducing Erbium and Raman optical amplification is experimentally investigated.

Abstract
An electrical dynamic interrogation technique is reported for long-period grating sensors relying on the modulation of fibre Bragg gratings located in the readout unit that permits us to attenuate the effect of the 1/f noise of the photodetection, amplification and processing electronics on the sensing head resolution. The concept is tested to detect variations of curvature, and a resolution of 9.4 x 10(-3) m is achieved.

Abstract
It is reported a long-period grating (LPG) dynamic interrogation technique based on the modulation of fiber Bragg gratings located in the readout unit of the system. It permits to attenuate the effect of the 1/f noise of the electronics in the resolution of the LPG-based sensing head. The concept is tested to detect variations of the external refractive index and a resolution of 2.0 x 10(-4) NIR was achieved without system optimization. Additionally, the effect in the sensor resolution when introducing Erbium and Raman optical amplification is experimentally investigated.

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 hybrid interferometer for simultaneous measurement of the partial pressures of O-2 and CO2 mixtures is reported. The sensing head consist in two different interferometers based on a Fabry-Perot cavity and a modal interference configuration. The intrinsic FP cavity was created by splicing a single mode fiber (SMF28) with a graded index fiber section that was then subjected to chemical etching creating a cavity. The second interferometer is based on a splice of a pure silica tube in series with the Fabry-Perot. Due to the design, different sensitivities are achieved for the pressure inducing refractive index changes of each gas. The rms deviations were found to be +/- 0.079 kPa and +/- 0.029 kPa for CO2 and O-2 partial pressure measurements, respectively.

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]