Objective Fiber Bragg grating (FBG) is widely used in the structural health monitoring of military industry, water conservancy, aerospace and other fields. FBG needs to be fixed on the object to be measured in a certain way to form heat or strain transfer. Thus, temperature or strain could be measured. Polymer adhesive is widely used in traditional FBG sensor packaging. The drawback is that polymer is prone to creep and aging during long-term use. Therefore, the deformation transfer between FBG and the measured object is mismatched, which makes the sensor ineffective. To solve this problem, metal alloy is adopted as a stable packaging material for metalizing fiber Bragg grating. Because the existing metallization method has great influence on the transmission loss caused by FBG, ultrasonic welding is studied for metallization. The temperature and strain sensing performance of the metallized sensor is verified.
Methods The fiber grating was metallized by ultrasonic welding. The selected FBG is encapsulated on the beam of equal strength with tin alloy in the form of full package. The transmission and reflection spectra of FBG before and after package were observed. Temperature sensing experiments (−75-150 ℃) and strain sensing experiments (−560-560 με) were carried out to verify the temperature and strain sensing characteristics of the metallized fiber grating sensor.
Results and Discussions The shape of FBG reflection spectrum of full-wrap package is a little distorted when the package is just finished. But the degree of distortion does not affect the calculation of the central wavelength of reflection spectrum. The package transmission loss is approximately 0.04 dB(Fig.4). The full package fiber grating can realize the sensing of the entire experimental temperature experimental test range. Its temperature sensitivity is about 2.95 times that of unpackaged FBG (Fig.5). During the long, high temperature process of the temperature sensing experiment, stress release ameliorates the distortion of the reflection spectral shape of the FBG when the package is just completed (Fig.6). The shape of the reflection spectrum of the fully packaged fiber grating is affected by the strain load, but the degree of deformation does not affect the sensing effect (Fig.7). The strain sensing of the sensor has high measurement accuracy (Tab.2).
Conclusions The ultrasonic welding method was used to metalize FBG with full-wrap surface-mount type. The transmission loss of the metal-packaged FBG sensors were less than 0.04 dB. Compared with other metallization methods, the transmission loss is reduced by one order of magnitude. Low-loss package effect of FBG surface mount metallization package is achieved. Temperature sensing experiments and strain sensing experiments were carried out on the packaged FBG sensor. The experimental results showed that the temperature sensitivity coefficient and strain sensitivity coefficient of the full-wrap package FBG sensor are 26.44 pm/℃ and 1.18 pm/με, respectively. The correlation coefficient R-Squared of temperature and strain sensing are both above 0.999 6. The reliability of temperature sensing and strain sensing of the sensor is verified. It can be seen that the ultrasonic welding method could be effectively used to metallize FBG. It can not only play a role of protecting FBG and improving sensitivity, but also realize the production of FBG temperature and strain sensors in a simple, low loss and reliable way.
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