2 ?Fiber Bragg Grating StructuresFiber Bragg gratings

2.?Fiber Bragg Grating StructuresFiber Bragg gratings this research are simple, versatile, small and intrinsic sensing elements that can be written in optical fibers and, consequently, have all the advantages normally attributed to fiber sensors. In addition to the fact that typically the measurand information is encoded in the resonant wavelength of the structure, which is an absolute parameter, these devices are inherently self-referenced. Moreover, there are several intrinsic advantages associated with FBG technology such as operation in reflection, narrowband spectral response and their compatibility with standard telecom technology, and the intrinsic multiplexing capability, particularly important in the context of remote, distributed and multi-parameter sensing [8].
Typically, FBG-based refractometers rely on the evanescent field of the core mode under fiber etching conditions, which enables interaction Inhibitors,Modulators,Libraries with the surrounding medium.The first demonstration of an FBG as a refractometer was done in 1997 by Asseh et al. [3], and it was based on the application of chemical etching to the fiber region where the grating was located. After etching the fiber had a diameter of ~11 ��m, thus ~1 ��m of cladding still remained. The sensor was tested in different solutions of sucrose, inferring a variation of refractive index between 1.333�C1.345. The estimated Inhibitors,Modulators,Libraries sensitivity was 1 nm/RIU (RIU��Refractive Index Unit) and the measured resolution was 5 �� 10?4 RIU. In Portugal the first approach to this type of sensor was presented by Pereira et al. [9].
The sensing head contains two in-line FBGs, with one of them sensitive only to temperature, while the other is also sensitive to salinity through the associated changes in the refractive index of the water. The refractive index sensitive grating was etched and presented a highly reduced diameter (~8 ��m), allowing great interaction Inhibitors,Modulators,Libraries of the evanescent field with the surrounding liquid; the second grating was used for temperature measurements. The sensor was subjected to different concentrations of salt which implies a variation of refractive index in the range 1.330�C1.341. The sensor showed sensitivity to refractive index of roughly 727 nm/RIU and a resolution of 10?4 RIU. Frazao et al. [10] demonstrated another scheme using a single FBG written in side-hole fiber for simultaneous measurement of refractive index and temperature.
Etching the fiber cladding at the position of the Bragg grating enabled the evanescent field of the guided mode to interact with the immediate surrounding environment of the fiber. When the fiber grating was immersed in a liquid sample, Inhibitors,Modulators,Libraries this resulted in a wavelength response of the Bragg grating that was Carfilzomib affected by the refractive index of the solution to be measured. In this scheme, the sensing head was based on a FBG written on selleck catalog an H-shaped fiber.

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