Abstract
A 253km ultralong remote displacement sensor system based on a fiber loop mirror interrogated by a commercial optical time-domain reflectometer is proposed and experimentally demonstrated. The use of a fiber loop mirror increases the signal-to-noise ratio, allowing the system to interrogate sensors placed 253km away from the monitoring system without using any optical amplification. The displacement sensor was based on a long period grating spliced inside of the loop mirror, which modifies the mirror reflectivity accordingly to the applied displacement. (C) 2011 Optical Society of America

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
An electrical dynamic interrogation technique previously reported by the authors for long-period grating sensors is now progressed by relying its operation exclusively on the modulation of a DFB Laser. The analysis of the detected first and second harmonic generated by the electrical modulation of the DFB Laser allows generating an optical signal proportional to the LPG spectral shift and resilient to optical power fluctuations along the system. This concept permits attenuating the effect of the 1/f noise of the photodetection, amplification and processing electronics on the sensing head resolution. This technique is employed in a multiplexing sensing scheme that measures refractive index variations.

Abstract
In this work, we demonstrate the possibility of fabricating short-length long-period gratings and rocking filters in highly birefringent Photonic Crystal Fiber using a CO2 laser. In our experiments both kinds of gratings were made in the same Boron doped highly birefringent PCF using similar exposure parameters. We also present the sensing capabilities of both fabricated gratings to temperature, strain and hydrostatic pressure by interrogation of the wavelength shifts at different resonances. Crown Copyright

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
A sensing head based on a hollow-core photonic crystal fiber for in-reflection measurement of micro-displacements is presented. The sensing structure takes advantage of the multimodal behavior of a short segment of hollow-core photonic crystal fiber in-reflection, being spliced to a single mode fiber at its other end. A modal interferometer is obtained when the sensing head is close to a mirror, through which displacement is measured.