Research Of The Fiber Bragg Grating Geophone

Geophone is the special sensor that could convert signal of mechanical vibration into signal of electricity and could be used in seism prospecting or measure of project-field. It is also a special precision instrument. Its capability directly affects recording quality of seismic wave and explaining of seismic data. It is the most important part during collecting seismic wave data. The sensing probe of the fiber Bragg grating geophone is made up of fiber Bragg gating (FBG). The information which it collects is embodied in wavelength. The modulation-demodulation is accomplished by FBG geophone directly. Compared with traditional geophone, there are many advantages, such as higher resolution, wider bandwidth, higher accuracy and better immunity to electromagnetic interference, which meet the new needs in the seism probing field.The coupled mode equations are built by means of expanding perfect wave-guide modes, the relationships between the reflective spectrum characteristics of fiber Bragg gratings and their structural parameters(grating length, coupled coefficient)are obtained. The temperature, strain and pressure sensing mechanisms of fiber Bragg gratings are studied systematically. The emphasis was laid on the mechanism of using FBG sensors to measure vibrations.The demodulation technique of the FBG is currently a critical and difficult problem in nowadays FBG censoring technique fields. Although a large number of researchers in domestic research units have been doing a lot of research and investigations on the demodulation methods of FBG, most of the researches are still in the phase of lab experiments and have not yet met the requirements of the practical industrial mass productions. After describing various wavelength demodulation technologies, including filter method, interference method and tunable narrow-band light source method, This paper puts forward an original demodulation method of FBG sensors based on matching wave filter technology. Demodulation grating was controlled by PZT . By using DSP technology, the reflected wavelength-shift of sensor grating is monitored, and the sensor has the ability to automatically adjust the drive voltage of PZT to keep up with the dramatic change of the reflected FBG wavelength, an extraordinary wide demodulation range of frequency and a high sensitivity are achieved.The contributions of this dissertation are:(1) The principle of the FBG geophone is analyzed.(2) A comprehensive expatiation was given to expound the worldwide statusquo of the FBG transducers' demodulation technique. Some basic demodulation methods and theories were explained in detail, indicated their strong point, the limitation of each method was pointed out, thereunder we propose a novel demodulation method.(3)The operating principle and structure design of the sensing probe of the FBG geophone are explained in detail. We reckon the sensitivity and frequency characteristic of the sensing probe.(4) The system architecture of FBG geophone is described. Some component of the geophone is presented in details.(5) A set of measuring and processing system is made. It can test the sensitivity of the sensing probe of the FBG geophone. The outcome of vibration test is analyzed qualitatively.