Study Of Distributed Acoustic Sensing System And Its Experimental Research Based On Weak Fiber Bragg Grating

In the distributed vibration sensing system based on the weak fiber Bragg grating array,fiber serves as the transmission medium and the sensor element for optical signal.It can realize long distance signal detection with high sensitivity and is therefore widely used in such fields as border warning,earthquake prediction and others.In this paper,the system compared with the traditional vibration system based on Rayleigh scattering light,has effectively improved the signal to noise ratio(SNR),and it consists of 396 identical weak fiber Bragg gratings with spacing of 5m and the detection frequency can range from 5Hz to 25 kHz.It is successfully applied to the partial discharge signal detection of high voltage cable,and has obvious advantages of anti-electromagnetic interference and corrosion resistance compared with the traditional sensor and chemical sensor.The main content of this paper is as follows:(1)The optical path of the distributed vibration sensing system was designed.When building the optical system and the software and hardware environment,the design and optimization of the photoelectric detection and pulse modulation parameters were mainly carried out to increase the sensing distance,the upper limit of the dynamic range and the effective sampling rate of the system as much as possible.Then the influence of the crosstalk between the weak grating and the linewidth of light source on the signal-to-noise ratio of the system was analyzed and verified in the experiment.(2)In terms of signal algorithm processing,the advantages and disadvantages of the NPS(Naval Postgraduate School)algorithm and the arctangent algorithm in the 3×3 coupler phase demodulation method were compared.The results showed that arctangent algorithm was better than NPS algorithm in suppressing low frequency noise,but both of them produced serious harmonic distortion when the coupler was asymmetric.Therefore,this paper proposed a modified coefficient arctangent algorithm based on ellipse fitting method.The experimental results showed that the improved arctangent algorithm could effectively reduce the harmonic distortion ratio of demodulation signal.(3)The PZT(piezoelectric)experiment showed that the system could detect different frequency well and maintain the linear relationship between the applied strain and the demodulation phase signal.Finally,high-voltage cables and transformers were used to build partial discharge device,and optical fiber was wound around aluminum foil tape to enhance the sensitivity of detection.The results showed that when the voltage of 10 kV to 50 kV was applied to the high-voltage cable,the ultrasonic signal produced by partial discharge could be observed in the frequency range of 9kHz to 15 kHz.