Study On Demodulation Method Of Intensive Weak Grating Array Based On OFDR

With the advantages of high sensitivity,fast measuring speed,large information capacity and strong adaptability,fiber optic sensor has become an investigation hotpot in sensing field.In all fiber optic sensing technology,fiber grating sensing technology is a typical representative,which has been widely used in many ways.The most prominent advantage of fiber grating sensors is the ability to form distributed sensing systems to achieve multi-parameter distributed measurements.The important performance indexes of the distributed fiber grating sensing system are gratings' multiplexing number and spatial resolution.In this paper,the weak grating array is used as the optical fiber sensing,and the reflectivity of each grating is about 0.1%,which greatly improves the multiplexing number.Meanwhile the optical frequency domain reflection(OFDR)technique is applied to the demodulation to achieve a distributed sensing system with large capacity and high spatial resolution.The main research achievements are as follows:(1)The basic working principle of OFDR is expounded by theoretical model.The theoretical model of phase noise of OFDR is established,and the influences on the spatial resolution and test distance of sensor system are explained theoretically.The main factors limiting the spatial resolution and test distance of the system are analyzed.(2)The demodulation algorithm of OFDR system is simulated.The basic working principle of OFDR in strictly linear sweep of light source is introduced by simulation.The influence of the nonlinear effect of the light source on the system is analyzed and the method to compensate the nonlinear effect is summarized.A windowed FFT algorithm based on cubic spline interpolation is used to compensate the nonlinear effect of the light source,whose feasibility is then verified by simulation.The influence of crosstalk signal in distributed sensing system is analyzed and the method to eliminate it is given.(3)The experiment of intensive weak grating sensing system based on the OFDR is conducted.An OFDR system with spatial resolution of 1mm is implemented by using the windowed FFT algorithm based on cubic spline interpolation to compensate the nonlinear effect.The experiments were carried out on 110 identical weak grating arrays with the interval of 10 cm,which proved the validity of adding the delay fiber to eliminate the crosstalk signal and the correctness of the algorithm to demodulate the central wavelength.The temperature experiment shows that the gratings' center wavelength in the fiber link is independent and the temperature sensitivity is 11.0pm/?.The experiment to measure the cantilever beam strain distribution displays that the fiber grating sensing system based on OFDR with high spatial resolution can monitor the health condition of the beam better.