Study On Temperature Independent Pressure Sensor Based On Tapered Fiber Bragg Grating

A fiber Bragg grating(FBG) is a periodic perturbation of the refractive index along the fiber length which is formed by exposure of the core to an intense optical interference pattern. Optical sensors using FBG have many advantages over conventional electronic sensors for sensing applications,such as small size,light weight,immunity to electromagnetic interference(EMI),resistance to chemical corrosion,high sensitivity,large bandwidth,etc. However,the problem of temperature-strain cross-sensitivity restricts the application of FBG in sensing.In this paper,a temperature independent pressure sensor is proposed,which is based on tapered FBG.For tapered FBG,the cladding of grating section is tapered,and then there'll be a strain gradient along the fiber when it's stretched,which makes the bandwidth broadened. The dimension of bandwidth is only dependent on subjected to applied tension but not temperature.Thus the problem of cross-sensitivity can be solved by detecting the bandwidth.FBG with different reflectivity and length were fabricated by different excimer laser,and were etched by HF solution.The etching rate of HF solution with different concentration was observed,and the FBG were tapered by 40%HF solution.Tapered FBG with different taper were fabricated,and were researched in order to get the ideal value of taper.The characteristics of bandwidth and reflected power of tapered FBG with different reflectivity and length were researched both theoretically and experimentally,and it is proved that for tapered FBG fabricated of high-reflectivity FBG,the reflected power increases linearly with tension and is independent on temperature.Thus the applied tension can be obtained by power detecting,which further simplify the complexity of demodulation of system.The high-reflectivity FBG based tapered FBG was applied onto a kind of metal thin-wall strain cylinder to fabricate a temperature independent pressure sensor.The sensor was tested under different pressure and temperature.Experimental results indicate that the strain sensitivity of the high-reflectivity tapered FBG is 48.67μW/N,and the pressure sensor provides a measurement range of 0～40MPa with a pressure sensitivity of 0.056μw/MPa.The sensor introduced in this paper is simple to fabricate and operate,and is promising in further application.