We report an investigation of the dependence of the temperature and strain coefficients on the grating type for fiber Bragg gratings that are UV inscribed in B/Ge-codoped fiber with and without hydrogenation. The results reveal that all types of grating exhibit similar strain sensitivities but markedly different temperature sensitivities, greater for gratings inscribed in hydrogen-free rather than hydrogenated fiber and substantially less in type IA gratings than all others. The sensitivity characteristics of these gratings have been used to implement a new type of dual-grating sensor for simultaneous measurement of temperature and strain that has properties superior to those of previously reported structures.

The fabrication and characterization of long-period gratings (LPGs) in fiber tapers is presented alongside supporting theory. The devices possess a high sensitivity to the index of aqueous solutions due to an observed spectral bifurcation effect, yielding a limiting index resolution of ±8.5 × 10 −5 for solutions with an index in the range 1.330-1.335. Index Terms-Long-period fiber gratings, optical fiber devices, optical fiber tapers, refractive-index measurement, temperature measurement. I. INTRODUCTION L ONG-PERIOD gratings (LPGs) and both adiabatic and nonadiabatic tapers in fibers are all devices that couple light from the fiber core into cladding modes. An LPG is an axially periodic refractive-index variation inscribed in the core of a photosensitive single-mode optical fiber by ultraviolet (UV) radiation or other means, which couples light from the core to cladding modes at discrete wavelengths [1], [2]. In a fiber taper, it is the change of the waveguide radius that enables coupling between the modes [3], producing applications in chemical/biological and strain sensing [4], [5]. The study of LPGs has yielded many potential uses in the area of telecommunications and numerous sensing applications. This is because the mean wavelengths of the attenuation bands are susceptible to strain, temperature, curvature, and the refractive index of the surrounding medium [1], [2], [6], [7]. In particular, there has been strong interest in using LPGs as chemical sensors [6], [7], for example, in the detection of organic aromatic compounds in paraffin [7]. Recently, there has been some interest in combining both types of fiber sensors for chemical sensing [8], [9], leading to high-resolution index measurements. We report for the first time the new spectral behavior of LPGs recorded in fiber tapers and propose a suitable mechanism responsible for this behavior. These tapered-fiber LPG devices

Abstract-A femtosecond laser has been used to asymmetrically modify the cladding of fiber containing long-period gratings. Following modification, devices in single-mode fiber are shown to be capable of sensing the magnitude and direction of bending in one plane by producing blue and red wavelength shifts depending upon the orientation of the bend. The resulting curvature sensitivities were −1.62 and +3.82 nm · m. Devices have also been produced using an elliptical core fiber to study the effects of the cladding modification on the two polarization eigenstates. A cladding modification applied on the fast axis of the fiber is shown to affect the light in the fast axis much more significantly than the light in the orthogonal state; this behavior may ultimately lead to a sensor capable of detecting the direction of bending in two dimensions for applications in shape sensing.Index Terms-Curvature measurement, long-period fiber gratings, polarization dependence.

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