High-sensitive Distributed Transverse Load Sensor Based On Brillouin Dynamic Gratings

In recent years, mechanical quantity sensors have played an important role in industrial production and civil engineering fields, and they attract considerable interests in the research field. During the usage period, the huge buildings undergo the axial stress and transverse load combined impact and the local stress will lead to potential risks. Developing an efficient, stable and reliable optical fiber mechanical quantity sensing technology has a practical research value and economic value. It has been proved that the Brillouin dynamic grating(BDGs) can be used to measure the birefringence of the polarization-maintaining fiber, which has a high sensitivity to the external physical changes exhibiting advantages in distributed, high-sensitivity and high-spatial-resolution sensing.In this thesis, we propose and experimentally demonstrated a high-sensitivity distributed transverse load sensor based on BDGs. The sensing principle is to measure the transverse-load induced birefringence through exciting and probing a BDG in an elliptical-core polarization-maintaining fiber(ECORE-PMF). A distributed measurement of transverse load is experimentally demonstrated with a 20-cm spatial resolution, and this sensing technique features a high-sensitivity to transverse load with a measurement accuracy of 1.3x10-3 N/mm improving almost three orders of magnitude compared with former technique.First, the birefringence properties of the temperature and stain for ECORE have been discussed showing the temperature-insensitive property of the ECORE fiber. The results show the linear relationship between the birefringence and temperature, and strain with a fitting temperature and strain coefficients of the birefringence frequency shift(Bire FS) of =-6.53 MHz/oC and =-0.0606 MHz/με, respectively. The experiment results indicate that, different with the stress-induced PMF, such as PANDA and BOWTIE fibers, this kind of shape-induced PMF has a lower sensitivity to the temperature and strain changes, with a negative relationship between the Bire FS and strain.Second, using the home-made transverse load loading platform, the transverse load sensing experiment has been conducted. The fiber coating effects on both fiber intrinsic birefringence and transverse load sensitivity has been discussed. Fiber coating is made up with polymer materials, such as acrylate, which acts as a kind of cushioning layer, and it will reduce the sensitivity of fiber to transverse load and extend the load transfer time to the fiber.A 10.3m-long ECORE fiber was used as the sensing fiber with a 20-cm coating-stripped segment under test. The changes in sensitivity as a function of the orientation of the applied load has been obtained, and the distributed transverse load sensing with different loads and different load directions based on BDG has been characterized. The Bire FS difference responses to transverse load of the sensing fiber in both the birefringence-increasing direction and birefringence-decreasing direction have been obtained with a measurement accuracy of 1.3x10-3 N/mm improving almost three orders of magnitude compared with former technique.