Abstract
It is described a large-core air-clad photonic crystal fiber-based sensing structure using the outer cladding as a light guide, which is highly sensitive to refractive index. The sensing head is based on multimodal interference, and relies on a single mode/large-core air-clad photonic crystal fiber/single mode fiber configuration. Using this configuration and controlling the light to travel in a segment of the outer cladding multimode fiber, it was possible to implement a sensing head and the results were obtained independently from variations of temperature, strain and refractive index. (c) 2012 Wiley Periodicals, Inc. Microwave Opt Technol Lett 54:10091011, 2012; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26726

Abstract

Abstract

Abstract
A Fabry-Perot (FP) cavity of simple design and based on a pure silica diaphragm-free hollow tube is proposed. Its operation is based on a first reflection of light at the end of the single-mode fiber that illuminates the silica rod and in a second reflection that takes place at the end of the rod. The FP cavity is characterized for high temperature, pressure and refractive index sensing, showing useful characteristics for the measurement of these three parameters. The diaphragm-free configuration simplifies the measurement of the refractive index of fluids. (C) 2011 Optical Society of America

Abstract
Palladium-based fiber-optic sensors have been one of the most promising configurations for hydrogen sensing. In the latest decade, fiber-optic sensors have indeed earned a strong interest owing to their ability to monitor molecular hydrogen at specific spatial points-either as a sensing tip device or in large areas via multiple sensing regions distributed along the optical fiber. This review focuses on the various types of optical fiber hydrogen sensors, containing specifically palladium as active element. Three distinct working principles are described, viz. interferometric-, intensity-, and fiber grating-based sensors; their characteristics and sensing performances are critically overviewed.

Abstract
In this Letter, a hybrid interferometer for simultaneous measurement of the partial pressures of O-2 and CO2 mixtures is reported. The sensing head consists in the combination of two interferometric structures, one a Fabry-Perrot cavity and the other a modal interferometer. The intrinsic Fabry-Perot was formed by splicing a single mode fiber with a graded index fiber length that was then subjected to chemical etching creating an air cavity. The second interferometer is based on a splice of a pure silica tube in series with the Fabry-Perot. It was observed for a particular gas that its refractive index changes with pressure variation in a specific way, a characteristic that permitted the simultaneous measurement of partial pressures of CO2 and O-2 with rms deviations of similar to +/- 48.7 kPa and similar to +/- 20.1 kPa, respectively. (C) 2012 Optical Society of America