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POFBG Sensor Applications
Published in Ricardo Oliveira, Lúcia Bilro, Rogério Nogueira, Polymer Optical Fiber Bragg Gratings, 2019
Ricardo Oliveira, Lúcia Bilro, Rogério Nogueira
Fiber Bragg gratings are inherently sensitive to temperature and mechanical deformation. In view of that, coating or packaging FBG in responsive materials can extend their range of sensing capabilities. Hydrogels are a class of hydrophilic polymers that can absorb water and swell or expel water and contract in the presence of an external stimulus. The volumetric change generates a mechanical response in the form of stress that can be readily assessed by combining the hydrogel with a stress-sensitive device [33]. FBGs are one of those devices that can easily measure the amount of strain generated by the polymer absorption or desorption upon an external stimulus. Regarding pH detection, protonation and deprotonation of acidic or basic pendant groups on the polymer material, cause a pH-dependent osmotic pressure difference, which leads to the swelling or shrinkage of the polymer relative to the external conditions [34]. Polymer optical fibers have been reported to have stress sensitivities 28 times higher than the ones reported for silica optical fibers [35]. Hence, the attractiveness of the combination of the swelling properties provided by some special hydrogels and the high-stress sensitivities achieved by POFBGs is highly interesting. This lead recently the Cheng et al. [36] and also Janting et al. [37] to report pH sensors based on the use of a POFBGs encapsulated in a hydrogel material. The configurations used in both works may be seen in Figure 5.7a and b, respectively.
Fibre Lasers
Published in Yu. N. Kulchin, Modern Optics and Photonics of Nano and Microsystems, 2018
The fibre Bragg grating is a segment of an optical fibre in the core of which, with the help of ultraviolet radiation, a periodic change in the refractive index with an amplitude of ∼ 10−5 - 10−3 with dashes oriented along the normal to the fibre axis and a period of the order of the wavelength of the propagating radiation. In its essence, FBG is a one‐dimensional photonic crystal (see Chapter 3), effectively reflecting radiation with a wavelength close to the Bragg wavelength of the crystal. The main characteristics of FBG are: the modulation period of the refractive index Λ, the amplitude of the induced change in the refractive index δn, the number of lattice dashes Np, and its length L. Two modes propagating in the optical fibre with propagation constants β1 and β2 effectively interact at the FBG if the phase matching condition is satisfied for them [13]
Optical Fibers and Accessories
Published in Daniel Malacara-Hernández, Brian J. Thompson, Advanced Optical Instruments and Techniques, 2017
In the phase-mask fabrication process, light from a UV source passes through a diffractive phase mask that is in contact with the fiber (Figure 18.30). The phase mask has the periodicity of the desired grating. Light diffracted in orders (+1, −1) of the mask interferes in the plane of the fiber, providing periodical modulation of the refractive index. The phase-mask technique allows fabrication of fiber gratings with variable spacing (chirped gratings). Fiber Bragg gratings can be routinely fabricated to operate over a wide range of wavelengths, extending from the ultraviolet to the infrared region.
Convergence deformation monitoring of a shield tunnel based on flexible long-gauge FBG sensors
Published in Mechanics of Advanced Materials and Structures, 2022
Tao Wang, Yongsheng Tang, Hao Yang, Xiangyang Xu, Wei Liu, Xizhi Li
The Fiber Bragg Grating (FBG) sensing technology has obvious advantages, such as high accuracy, multiplexing performance and long-term stability. Therefore, it is widely applied in structural monitoring [8, 9]. For example, a strain measurement system based on FBG was installed on Tsing Ma Bridge to monitor its structural performance under traffic loads [10], and the strains measured from FBG sensors have been used to obtain the strain mode shape to identify the damage of prestressed concrete structures [11]. FBG-based liquid-level sensors have also been used on a bridge to measure subgrade settlement at the time when the high-speed rail line was undercrossed by one tunnel [12]. In most cases of FBG application, the sensor has been used to obtain strains at a given point, which is similar the target of traditional strain gauges. However, these ‘point’ type sensors are difficult to accurately identify the local damage, such as concrete cracks, as the local damage often distributes randomly. A long-gauge FBG sensor is herein proposed to measure average strains of some across areas on a structure. This is essentially, measurement of macrostrain to improve damage identification [13]. With these long-gauge FBG sensors, it is possible to implement monitoring to specific segments of a structure in a distributed sensing mode.
Simultaneous measurement of refractive index and temperature based on a long period fiber grating inscribed in a photonic crystal fiber with an electric-arc discharge
Published in Instrumentation Science & Technology, 2019
Chao Du, Qi Wang, Sheng Hu, Yong Zhao
Since fiber grating sensors have been proposed, there have been many reports on cascading interferometer structures and fiber grating structures for multiple parameter measurements.[10–13] Fiber Bragg grating sensors have been widely used as temperature sensing devices for decades, but the well known temperature sensitivity of 10 pm/°C for bare fiber Bragg grating has limited the applications for high sensitivity measurement. In addition, fiber Bragg gratings are insensitive to external refractive index variations because of the insensitive property of the core mode. On the contrary, the long period fiber grating is sensitive to external temperature and refractive index variations. However, the limitation of cross-sensitivity may be caused if refractive index and temperature are measured at the same time. In 2015, Yin et al. proposed a liquid-filled photonic crystal fiber sensor to eliminate the cross-sensitivity of long period fiber grating inscribed on single mode fiber,[14] but the refractive index sensitivity was less than 50 nm/RIU (1.33–1.38), and the fabrication process of the proposed device is complicated.
Long-period fiber grating sensor induced by electric-arc discharge for dual-parameter measurement
Published in Instrumentation Science & Technology, 2018
However, the temperature sensitivity of fiber Bragg grating is only as low as 10 pm/°C because of the low thermal expansion coefficient of silica with temperature variation. In addition, there is only one resonance dip or resonance peak because of the coupling between forward-propagating core mode and back-propagating core mode for fiber Bragg grating, which will cause the limitation of cross-sensitivity for simultaneous measurement of temperature and strain. To eliminate the cross-sensitivity of temperature and strain in fiber Bragg grating measurement systems, various measurement schemes have been proposed, such as dual-wavelength fiber Bragg grating inscribed by femtosecond laser,[12] integration of Fabry–Perot fiber optic sensor and a fiber Bragg grating,[13,14] and a dual-fiber Bragg grating structure.[15,16] Although these methods diminish the cross-sensitivity, the cascaded sensing elements increase the complexity of the system.