Study On Micro-vibration Measurement System Based On Laser Heterodyne Interferometry

MEMS devices realize relative functions from mobile structures.The ongoing development of MEMS devices is that the frequency is become higer.The performance of the devices is mainly determined by the structure and manufacturing process.For a better understanding of wave motion in complex micro-and nanostructures,direct information of the vibration field is needed.Not only amplitude and phase information at the basic resonant frequency needs to be measured,but also vibration information at resonant frequencies of other modes.According to the measurement demands,the vibration measurement instrument should have large frequency range,high sensitivity and large amplitude range.Laser heterodyne interferometers are widely used in microvibration measurement because of its high sensitivity and high resistivity to environment change.However,the existing instruments have some limitations,which cannot meet the measurement requirements of various devices,such as large measurement bandwidth,high measurement sensitivity,and large amplitude measurement range and so on.In view of this problem,the thesis focuses on the construction,improvement,performance testing and applications of a laser heterodyne interferometer system.The specific contents can be summarized as follows:1.A scanning probe laser heterodyne interferometer system was constructed including the optical system and detection system based on the basic principles of heterodyne interferometry.The optical system is the vibration signal modulation measurement and mixing part,and the detection system is used for signal demodulation.The detectable frequency range of the interferometer system is only limited by the bandwidth of the detector and demodulation circuit electronics.The control and data processing software were programmed for the system,and the data acquisition and realtime processing are realized.2.The performance and parameters of the system were tested experimentally.Firstly,the calculation of measurement bandwidth and equivalent noise bandwidth is given according to the transfer function of the detection system.The frequency depent efficiencies of the circuit system were calibrated and tested.The noise of the system was analyzed and the method to improve the SNR of the system was put forward accordingly.The measurement accuracy and frequency range of the system were verified by experiments.The measurement sensitivity of the system is tested.3.A method was proposed to expand the amplitude measurement range of the system.And it was based on the extension of the basic principle of heterodyne interferometry.Bessel functions of the first kind would be introduced and the amplitude information could be obtained by solving the simultaneous equations.The method was validated experimentally.4.Different modes of piezoelectric micromechanical ultrasonic transducer were tested by the system.And this is the first vibration measuring instrument that can meet these two measuring ranges in the existing reports.The device was analyzed according to the measured results.Firstly,the propagation mode of sound wave in the device was analyzed theoretically,and the device was simulated using FEM software according to the theory.By comparing the measurement results of the device with the simulation results,the quality factor Q of the device under test was analyzed and its dispersion curve was obtained,which provides the performance of the device making its analysis possible.