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

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