Fiber Bragg grating has embraced the area of fiber optics since the early days of its discovery, and most fiber optic sensor systems today make use of fiber Bragg grating technology. Researchers have gained enormous attention in the field of fiber Bragg grating (FBG)-based sensing due to its inherent advantages, such as small size, fast response, distributed sensing, and immunity to the electromagnetic field. Fiber Bragg grating technology is popularly used in measurements of various physical parameters, such as pressure, temperature, and strain for civil engineering, industrial engineering, military, maritime, and aerospace applications. Nowadays, strong emphasis is given to structure health monitoring of various engineering and civil structures, which can be easily achieved with FBG-based sensors. Depending on the type of grating, FBG can be uniform, long, chirped, tilted or phase shifted having periodic perturbation of refractive index inside core of the optical fiber. Basic fundamentals of FBG and recent progress of fiber Bragg grating-based sensors used in various applications for temperature, pressure, liquid level, strain, and refractive index sensing have been reviewed. A major problem of temperature cross sensitivity that occurs in FBG-based sensing requires temperature compensation technique that has also been discussed in this paper.

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FBG based refractive index sensor are widely used in sensing applications such as structural health monitoring, oil exploration, refractive index measurement of chemical solutions, etc., owing to bene ts of FBG such as small size, real time monitoring, high resolution, resistant to high temperature and various chemicals, etc. In this paper, simple and high sensitive method employing a double D-shaped FBG (DDSF) for refractive index (RI) measurements of glucose solution is proposed and investigated. The operating principle is based on the exploitation of the evanescent wave interaction of the propagating light with the surrounding external environment, resulting in shift of Bragg wavelength. Wavelength shift is measured, when the refractive index of external medium changes from 1.33 to 1.43 approximately for glucose solution with varying concentrations, exhibiting a good linear relationship between them. By changing the depth of the D-shaped groove, the sensor's sensitivity can optimize. A high sensitivity of 47.37 nm/RIU is achieved, which indicates that proposed sensor has provided high sensitivity and linearity.

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