In this work, sensitivity to strain and temperature of a sensor relying on modal interferometry in hollow-core photonic crystal fibers is studied. The sensing structure is simply a piece of hollow-core fiber connected in both ends to standard single mode fiber. An interference pattern that is associated to the interference of light that propagates in the hollow core fundamental mode with light that propagates in other modes is observed. The phase of this interference pattern changes with the measurand interaction, which is the basis for considering this structure for sensing. The phase recovery is performed using a white light interferometric technique. Resolutions of +/- 1.4 microepsilon and +/- 0.2 degrees C were achieved for strain and temperature, respectively. It was also found that the fiber structure is not sensitive to curvature.

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A fiber-optic Fabry-Pérot sensing structure based on the utilization of a suspended-core fiber is presented. The interferometric structure is formed when a small length of the suspendedcore fiber is spliced to the end of a standard single-mode fiber. The interfering waves are generated by the refractive-index mismatches between the two fibers in the splice region and at the end of the suspended-core fiber. Thermal and strain responses of two different sensing heads associated with suspended-core fibers with three and four holes are characterized.Index Terms-Fabry-Pérot (FP) interferometer, microstructured fiber, optical fiber sensor. I. INTRODUCTIONO PTICAL fiber Fabry-Pérot (FP) interferometric sensors have been demonstrated in the past for many sensing applications. These sensors can be classified as extrinsic or intrinsic depending on their geometry. For extrinsic sensors, an air gap between two cleaved end faces is used. On the other hand, when dealing with intrinsic sensors, the mirrors are fabricated inside the fiber. Kersey et al. [1] published in 1983 a fiber FP formed by the two extremes of the fiber. In 1988, Lee and Taylor [2] proposed a fiber FP where the FP reflectors were formed on the single-mode fiber by a fusion splicing technique. Farahi et al.[3] demonstrated a multiplexing network using this type of Manuscript

We report on the fabrication of a high pressure extrinsic Fabry–Perot interferometric (EFPI) fiber optic sensor for downhole applications by using a mechanical transducer. The mechanical transducer has been used for increasing the pressure sensitivity and the possibility of installation of the sensor downhole. The pressure–temperature cross-sensitivity (PTCS) problem has been solved by replacing the reflecting fiber with a metal microwire in the EFPI sensor. In this way the PTCS coefficient of the sensor was decreased from 47.25 psi °C−1 to 7 psi °C−1. By using a new EFPI design, a temperature sensor was fabricated. Further improvement in the pressure and temperature sensor has been done by developing fabrication technique and signal processing.