Study On The Technology Of Laser Doppler Reference Demodulation

To detect mineral resources and other information underground, many detection methods have been researched and developed, in which the method of seismic prospecting is one of the most effective methods. It has been applied widespreadly and the technology about it has developed rapidly. However, on one hand seismic detector is important tool of seismic prospecting. Traditional seismic detectors have not satisfied the requirement of seismic prospecting developing increasingly due to low measurement accuracy, low response and sensitive to electromagnetic interference. On the other hand, because of high accuracy, wide band, quick dynamic response, good linearity, large measurement range and untouched measurement, laser Doppler measurement develops rapidly. In this paper, based on a new method of seismic prospecting, that is remoter measurement scheme to seismic wave, laser Doppler measurement of vibration is researched, and laser Doppler reference optic structure of demodulation system is put forward to measure the vibration caused by seismic waves, and the principle experiment is carried out. The following achievements are completed: 1. Theory of the measurement of vibration using Laser Doppler effect are researched and reference Laser Doppler optical system are designed. Especially, acousto-optic modulator is used in this system to implement laser heterodyne, which can improve the signals' SNR and anti-jamming ability of the optical system. 2. According to theory of laser Doppler measurement, optical measurement system is optimized. Introducing Trent reversed reflection structure to improve measurement accuracy of optical system and the ability of anti-traversing and anti-beating. 3. PLL (phase locked loop) circuit is researched to detect and demodulate weak Doppler signals with monolithic PLL circuit NE568A. NI's DAQ card is used to acquire data. The adjustment of the whole system, acquisition of data and analysis of experiment data are completed. In the end, the software labview is applied to get amplitude and frequency parameters of vibration.