Highly sensitive fiber Bragg grating refractive index sensors

Abstract

We combine fiber Bragg grating ͑FBG͒ technology with a wet chemical etch-erosion procedure and demonstrate two types of refractive index sensors using single-mode optical fibers. The first index sensor device is an etch-eroded single FBG with a radius of 3 m, which is used to measure the indices of four different liquids. The second index sensor device is an etch-eroded fiber Fabry-Pérot interferometer ͑FFPI͒ with a radius of ϳ1.5 m and is used to measure the refractive indices of isopropyl alcohol solutions of different concentrations. Due to its narrower resonance spectral feature, the FFPI sensor has a higher sensitivity than the FBG sensor and can detect an index variation of 1.4ϫ 10 −5 . Since we can measure the reflection signal, these two types of sensors can be fabricated at the end of a fiber and used as point sensors. Since the early 1990s, fiber Bragg grating ͑FBG͒ sensors have been intensively developed due to their many desirable advantages such as the small size, absolute measurement capability, immunity to electromagnetic interference, wavelength multiplexing, and distributed sensing possibilities. [1][2][3][4][5] Thus far, the FBG sensors' capability to measure physical quantities such as the temperature, strain, pressure, etc., has been studied extensively. [2][3][4][5][6][7][8] However, the use of FBG sensors for detection of environmental refractive index change has not been fully explored. Refractive index sensing is important for biological and chemical applications since a number of substances can be detected through measurements of the refractive index. [2][3][4][8][9][10][11][12][13] For normal FBGs, removal of the fiber cladding is required to increase the evanescent field interaction with the surrounding environment. This concept has been demonstrated using D-shaped fiber and sidepolished fiber. 9,11,12 In both cases, the strength and durability of the sensor were greatly reduced. Special fiber was also needed, which would raise the costs and limit the possible applications. Long-period fiber gratings have also been demonstrated to have high sensitivity to the refractive index of the ambient media, 2,3,13-15 however, their multiple resonance peaks and broad ͑typically tens of nanometers͒ transmission resonance features limit the measurement accuracy and their multiplexing capabilities. 9 In addition, the relatively long length of the grating limits their application as point sensor devices.In this letter, we first demonstrate a single etch-eroded FBG sensor using standard single-mode telecommunication fiber ͑Corning SMF-28͒. Fiber Fabry-Pérot interferometers ͑FFPIs͒ have also been widely used as sensors. 2,3,16,17 Compared to a single FBG, the FFPI sensors possess narrower resonance peaks and are more desirable for high accuracy wavelength measurement. 2,3,9,18 To that end we propose and demonstrate an etch-eroded FFPI sensor formed by two FBGs. The FFPI sensor is used to measure the refractive index of isopropyl alcohol ͑IPA͒ solutions of different concentrations, exhibiting the capabi...

Optofluidic refractometer using resonant optical tunneling effect

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Near-Field Nanofluid Concentration Measurement by Rayleigh Particle Scattering Bragg Grating Evanescent Wave

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