Temperature And Strain Sensing Properties Of Fiber Optic Sensor Packaged By Laser Welding

Fiber Bragg grating(FBG)has the advantages of small size,light weight and the set of information transmission and sensing in one,becoming the preferred carrier of intelligent information transmission and sensing structure.FBG become one of the components in the intelligent metal structure,widely used in various engineering fields.However,the fiber itself is so fragile with the slender quartz fiber which has poor bending strength,especially after writing to the grating,so it is necessary to protect it before embedded in the metal matrix.In this paper,FBG was metal coated and the associated sensing properties were tested.The temperauure and strain properities of FBG after welding encapsulation are systematically studied.The main contents are as follows:1)In order to protect the FBG,metallization with a metal coating using an electroless-electroplating method are conducted.Results show that Ni and Cu coated FBGs by chemicalplating are smooth and continuous,and the coating is uniform and compact,with no obvious cracking and peeling.Surface of Ni-Ni coated and Ni-Zn coated FBG are smooth and bright,without obviously particles and cracks.It is suggested that the FBG can be well protected by this metallization method.2)Temperature and strain properities of metal coated FBG were studied.Results show that wavelength shifts of Ni-Ni and Ni-Zn coated FBGs under the thermal loads show a linear trend,with the temperature sensitivity of the 20.23 pm /? and 34.36 pm /?,respectively.The temperature sensitivities of Ni-Ni and Ni-Zn coated FBGs can be enhanced about 2.04 times and 3.55 times than the bare FBG.The wavelength shifts of Ni-Ni and Ni-Zn coated FBGs under the applied load shows linear trend as well,with the stain sensitivities of 4.98 pm/g and 5.12 pm/g,respectively.The response tendency of the Zn coated FBG to a transient temperature change is similar to that of bare FBG,which could be attributed to the small thermal capacity and thickness of the metal coating.The central wavelength stability of the Zn coated FBG is lower than that of the bare FBG due to the high temperature sensitivity,with the deviations of Zinc coated FBGs and bare FBG are 0.013 nm and 0.008 nm,respectively.3)A stress calculation model based on optical grating sensing was built and the intrinsic stress evolution generated during the electroless Cu plating and electroless Ni plating processes are monitored in real time.The experimental results show that the center wavelength of the Cu coated FBG show blue shift during electroless plating processes,which serves as compressive stress on the FBG.The intrinsic stress produced during the three chemical plating experiments are 17.80 Mpa,23.65 Mpa and28.04 Mpa,respectively.During chemical plating Ni processes centre wavelength show red shift and the accumulated intrinsic stress serves as tensile stress on the FBG.The intrinsic stress produced by the electroless Ni plating is 12.92 MPa.The stress sensitivity of acquired from the monitoring FBG is 4.10 pm/MPa,with a measurement precision of 0.24 Mpa.4)FBG was nickel coated and then packaged on surface of a Ag-Cu alloy substrate by laser welding and the temperauure and strain properities of FBG after welding encapsulation were systematically studied.Results show that the Ni coated FBG can be well bonded on the Ag-Cu alloy substrate and the weld bead show uniform and compact.Temperature sensitivity of the packaged FBG is 16.94 pm /?,which is about 1.76 times as that of the bare FBG.The wavelength shifts of the packaged FBG and strain show linear trends,with the strain sensitivity of 0.197 pm /g.The center wavelength stability of the packaged FBG after the laser welding processes is studied.Results show that the center wavelength stability of package FBG shows slightly lower than that of the bare FBG at room temperature.In the recording period of 9 hours: for the zinc coated FBG and bare FBG,the deviations are0.091 nm and 0.006 nm,respectively.The reason for this phenomenon may be that Residual stress may be exist in the pavkaged FBG when the Ag-Cu alloy deformed during the welding process the FBG which affects the center wavelength stability of FBG.In addition,the higher the temperature sensitivity also affects the center wavelength stability of the entire welding package FBG sensor.