Temperature Self-compensated Fiber Bragg Grating Strain Sensors

According to sensing characteristics of fiber grating, fiber Bragg grating is sensitive to the temperature and strain, and its center wavelength will shift with the environmental temperature and strain change. When fiber Bragg grating is applied to strain measurement, the fiber Bragg grating is difficult to distinguish the wavelength shift caused by change in temperature and strain respectively, resulting in temperature strain cross-sensitive issue. Therefore, in strain measurement, it needs temperature compensation for fiber Bragg grating, and then eliminates effect of temperature change on the Bragg wavelength shift.The fiber Bragg grating strain sensor which used bi-metallic tube structure on the fiber Bragg grating package is designed, and the two ends of fiber Bragg grating connecting the fiber are pasted between two inner tubes with a hole, then two inner tubes with hole and screw are fixed on the two ends of outer tube. The inner tube with hole and screw uses a larger thermal expansion coefficient material, and outer tube with a smaller thermal expansion coefficient material, and the outer tube is thin, the inner tubes are thick. When the temperature rises, the outer tube will along the axial outward direction, and the free end of inner tube with hole and screw will expand along the axial inward direction. Using thermal expansion coefficient difference between inner and outer tube offsets the deformation due to thermal expansion and optical thermal effect of fiber Bragg grating, in order to eliminate the influence of temperature change on the fiber Bragg grating peak wavelength, and achieves Bragg wavelength only affected by strain. The main completed work as follows:1. According to bi-pipe structure, the model of fiber Bragg grating strain sensor is established.2. Materials of fiber Bragg grating strain sensor are selected, and with material selection parameters of the strain sensor are calculated theoretically. When the outer tube material is chosen as steel, outside diameter takes 10 mm, inside diameter takes 8 mm, length takes 100 mm; the inner tube with a screw material is chosen as copper, outside diameter takes 8 mm, inside diameter takes 3 mm, length takes 20 mm; outside diameter of non-screw inner tube takes 5 mm, inside diameter takes 3 mm, length takes 10 mm, achieving the temperature self-compensation effect, at this time the theory temperature sensitivity is 0 pm/℃, strain sensitivity is 1.215 pm/με.3. After various devices are assembled, through the heating water method temperature sensitivity of packaged fiber Bragg grating strain sensor is verified, and temperature sensitivity of experiment is 2.64 pm/℃. The center wavelength of Bragg grating used in this study is 1550 nm, and its theory temperature sensitivity is 10.8 pm/℃, the temperature sensitivity of this experiment is only the bare grating 1/5.4. Strain experiment is executed using weights:calibration method, and the experimental data indicates, the experiment strain sensitivity of the fiber Bragg grating strain sensor as follows:load 1.392 pm/Kg, unload 1.394 pm/Kg; nonlinearity error: load 2.14%, unload 3%; repeatability error:load 3%, unload 3%.