Abstract
A Mach-Zehnder sensor based on a large knot fiber resonator with a diameter of a few millimeters is designed using a single long taper. The long taper of some centimeters is fabricated with a CO2 laser technique. In air, light cannot couple between adjacent sections in the knot, and no signal is observed. However, in liquid, light is less confined and there is coupling between adjacent sections of the knot, resulting in a phase difference and consequent interference. The Mach-Zehnder is formed by the two contact points in the knot. The refractive index sensing of liquid compounds is achieved by monitoring the wavelength shift of the spectra. A sensitivity of 642 +/- 29 nm/refractive index unit (RIU) is achieved for refractive index sensing in the range of 1.3735-1.428 with a resolution of 0.009 RIU. For temperature sensing, a sensitivity of -42 +/- 9 pm/degrees C is observed. A low influence of temperature in the refractive index change is observed: 6.5 x 10(-5) RIU/degrees C.

Abstract
This work demonstrates the viability of using a cavity ring-down (CRD) technique for remote sensing. A conventional CRD configuration is used where an optical circulator is added inside the fiber loop to couple 19 km of optical fiber with a gold mirror at its end with the purpose of remote sensing. As a proof-of-concept, an intensity sensor based on an eight-figure configuration is used at the end of the 19 km of fiber for displacement sensing. (C) 2016 Wiley Periodicals, Inc.

Abstract
A curvature sensor based on a Fabry-Perot interferometer is proposed. A capillary tube of silica is fusion spliced between two single mode fibers, producing a Fabry-Perot cavity. The light propagates in air, when passing through the capillary tube. Two different cavities are subjected to curvature and temperature. The cavity with shorter length shows insensitivity to both measurands. The larger cavity shows two operating regions for curvature measurement, where a linear response is shown, with a maximum sensitivity of 18.77pm/m(-1) for the high curvature radius range. When subjected to temperature, the sensing head produces a similar response for different curvature radius, with a sensitivity of 0.87pm/degrees C.

Abstract
Temperature-independent strain and angle measurements are achieved resorting to a taper fabricated on a Bragg fiber using a CO2 laser. The characteristic bimodal interference of an untapered Bragg fiber is rendered multimode after taper fabrication and the resulting transmission spectra are analyzed as a function of strain, applied angle, and temperature variations. The intrinsic strain sensitivity exhibited by the Bragg fiber is increased 15 fold after tapering and reaches 22.68 pm/mu epsilon. The angle and temperature measurements are also performed with maximum sensitivities of 185.10 pm/deg and -12.20 pm/K, respectively. The difference in wavelength shift promoted by variations in strain, angle, and temperature for the two fringes studied is examined. Strain and angle sensing with little temperature sensitivity is achieved, presenting a response of 2.87 pm/mu epsilon and -57.31 pm/deg, respectively, for strain values up to 400 mu epsilon and angles up to 10 degrees. Simultaneous angle and strain measurements are demonstrated.

Abstract
A curvature sensor based on an Fabry-Perot (FP) interferometer was proposed. A capillary silica tube was fusion spliced between two single mode fibers, producing an FP cavity. Two FP sensors with different cavity lengths were developed and subjected to curvature and temperature. The FP sensor with longer cavity showed three distinct operating regions for the curvature measurement. Namely, a linear response was shown for an intermediate curvature radius range, presenting a maximum sensitivity of 68.52 pm/m(-1). When subjected to temperature, the sensing head produced a similar response for different curvature radii, with a sensitivity varying from 0.84 pm/degrees C to 0.89 pm/degrees C, which resulted in a small cross-sensitivity to temperature when the FP sensor was subjected to curvature. The FP cavity with shorter length presented low sensitivity to curvature.

Abstract
An optical fiber interferometer taper fabricated with a CO2 laser is proposed for strain and curvature-independent temperature measurement. Variations in temperature produce changes in the conditions of the interference between light traveling along the core and cladding and a linear behavior is verified for the relation between the wavelength of the resonant loss peak and temperature, yielding a sensitivity of 110 pm/degrees C for a range between 25 and 510 degrees C. Both the applied strain and curvature only promote significant changes in the transmitted power, leaving the wavelength of the resonant loss peak approximately constant and rendering this optical sensing device a good strain and curvature-independent temperature sensor. (c) 2016 Wiley Periodicals, Inc.