Abstract
We report the development and characterizations of a fiber Bragg grating (FBG) sensor coated with different ultraviolet (UV) curable resins. The UV-curable resins were applied on the fiber after the FBG inscription and cured with an UV lamp. One set of samples used the NOA 68 resin and the other used NOA 88. The samples were characterized with respect to the temperature, moisture absorption and strain response. Furthermore, in order to understand the influence of the resin coating on the optical fiber’s mechanical properties, tensile tests were performed with the samples. Results show that all samples presented negligible sensitivity to moisture absorption in the 50-min long tests with the fibers immersed in a container filled with distillated water. Regarding the temperature responses, the coated FBGs presented higher sensitivity (13.84 pm/°C for NOA 88 and 12.76 pm/°C for NOA 68) than the uncoated FBGs due to the thermal expansion of the coatings. In the strain tests, all coated and uncoated samples presented similar sensitivities, but with a larger strain range applied for the coated samples (strains higher than 5500 µε) when compared with the uncoated samples (3500 µε). Moreover, the stress-strain curves of the coated samples indicated a Young’s modulus one order with magnitude lower than the one of the uncoated silica fiber, where the lowest Young’s modulus is 3.84 GPa and was obtained with the NOA 68 coating, which indicates the possibility of obtaining highly sensitive pressure and force sensors.
Highlights
Optical fiber sensors have been increasingly used in many applications due to advantages such as multiplexing capabilities, intrinsic safe operation, electromagnetic fields immunity, compactness and corrosion resistance [1]
This paper presented the development and characterization of fiber Bragg grating (FBG) coated with UV-curable resins
The FBGs were inscribed using the phase mask technique and the UV resins were applied in the uncoated region of the fiber, in which the FBG was inscribed: one sample with the NOA 68 and the other with NOA 88
Summary
Optical fiber sensors have been increasingly used in many applications due to advantages such as multiplexing capabilities, intrinsic safe operation, electromagnetic fields immunity, compactness and corrosion resistance [1]. Optical fiber sensors are able of measuring physical and chemical parameters using sensing approaches such as intensity variation [7], fiber Bragg gratings (FBGs) [8], interferometers [9] and nonlinear effects [10], among others. Among the many sensing approaches, FBGs are one of the most common optical fiber sensors due to its inherent high sensitivity and multiplexing capabilities, since it is possible to inscribe dozens of sensors in the same fiber [11]. FBGs use wavelength-encoded information, i.e., the sensing parameter is obtained from the wavelength shift in the reflected spectrum. This feature leads to a sensor that is insensitive to optical source power fluctuations [12]. The reflection mode operation of Sensors 2020, 20, 3026; doi:10.3390/s20113026 www.mdpi.com/journal/sensors
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