Several Practical Research On Fiber Seismometer Signal Demodulation System

Fiber seismometer is a new seismic measure method with high sensitivity, strong,anti-interference ability and etc. which is widely used in earthquake precursor monitoring.Fiber seismometer is based on the phase interferometer, which changes the acceleration,velocity,displacement and other physical quantities into the interferometer phase variation to measure the actual physical value more accurate. Although, the fiber sensing technology has been developed for many years, there are several problems still in the fiber sensing measure system, such as static working point and bandwidth. These problems are the limitation of the fiber seismometer all-area application and industrial mass production. In this paper,we analyze the question of the fiber seismometer in practical application from the principle and the production process and design the modeling of several parameters. The main content is as follows:Firstly, we analyze the optical fiber interferometer phase demodulation algorithm at present and make the comparison of advantages and disadvantages exist in these algorithms.We design the model to figure out the right static working point in phase generate carrier(PGC) demodulation method. The light intensity disturbance (LID), the modulation depth and the parameters of the modulation waves are discussed in this paper. The PGC working parameters are optimized based on the above analysis, which include the right modulation depth and the LID effect. We also express a PGC demodulation method combined with the 3×3 coupler to suppress the LID and the drift of the static working point.Secondly, the phase demodulation hardware which includes the analog amplifier with light-voltage circuit and the digital sample circuit is designed. The circuit noise and the work bandwidth are researched. We also discuss the noise factors in analog circuit and the high-speed digital circuit signal lines laying claim. The phase demodulation algorithm software, which contains the PC, DSP and FPGA program, is coded on the hardware above. It realizes the function of network data reception and data display in real-time. The UI is friendly and easy to start.Thirdly, we manufacture the integration prototype by the phase demodulation system.The light source and the demodulation circuit are packaged into the prototype to increase the practical ability. The GPS timing function, power supply hot backup function and temperature acquisition compensation process and all designed at the same time.At last,the fiber seismograph data acquisition and recording experiments are based on the system above. The phase resolution and the phase noise are analyzed, after that, the system is improved and be manufactured several times to finish the outer field experiment.Several earthquake signals are sampled during the experiment. Some experimental data are compared with the seismometer in use, the SNR of the fiber seismometer are more higher 6-10dB to the traditional seismometer.According to the record, the phase resolution of the phase demodulation is 4×10-5 rad/?Hz with the 10kHz work bandwidth. In experiment of seismic exploration, we can get the similar waveform with the seismometer in use.