Research On Signal Processing Technology Of Laser Coherent Vibration Measurement

The laser coherent detection technology has been widely applied in the detection of weak vibration signals because of its advantages of high accuracy,non-contact and anti-interference,as well as better sensitivities and signal-to-noise ratio than the direct detection since it first came out in 1970.It mainly includes surface roughness measurement,MEMS detection,camouflage target recognition and laser speech measurement.The signal processing technology of laser coherent detection has also undergone a transformation from a traditional analog demodulation mode to a digital demodulation mode.As a result of a backward state in terms of the detection range and detection accuracy,the promotion space of signal processing technology is relatively large.Therefore,researching on the signal processing technology of laser coherent vibration measurement is of great significance.In order to realize the high-precision and real-time measurement of microvibration signals by the laser coherent detection system,this paper,on the basis of fully investigating the relevant research,has carried out a deep research on the signal processing technology of laser coherent vibration measurement by combining theoretical analyses,numerical simulations and experimental researches.When the target is a single-frequency vibration target,the laser coherent detection system based on analog demodulation achieves a detection range of 35 meters and a minimum detectable displacement of 100 nm.The laser coherent detection system based on digital demodulation achieves a detection accuracy of 8 nm at a detection range of 100 meters and realizes the real-time output of complex speech vibration signals.The main research contents of this paper are as follows:(1)This paper introduces the basic theories of laser coherent detection,PGC demodulation technology and laser linewidth.The advantages and disadvantages of two PGC demodulation algorithms including differential cross multiplication and arctangent in ideal and noisy environment are compared and analyzed.In this paper,a new method of calculating the photocurrent power spectrum of the coherent detection by using mathematical expectations is proposed.The influence of linewidth on the photocurrent power spectrum is discussed by numerical simulations.The results show that the greater the laser linewidth in noise environment is,the more difficult the IF signal detection is.The results of theoretical analysis lay a foundation for the followup experimental research.(2)The influence of an acoustooptic modulator and the target surface roughness on the displacement resolution of coherent detection system is analyzed by calculating the intensity of interference field and modeling rough surfaces.The phase error of the echo light is caused by the diffraction light order crosstalk in an acousto-optic modulator and the phase error varies periodically with the phase delay between the signal light and the local oscillator light.The theoretical calculation shows that the existence of phase error will reduce the minimum detectable displacement of coherent detection system.The numerical simulation of photocurrent on rough surface shows that the bigger the correlation length of rough surface is,the bigger the IF current is and the bigger the mean square root of the height difference of rough surface is,the smaller the IF current is.(3)According to theories of signal and noise,the change of phase noise is a Wiener process.The probability distribution density functions of phase noise of Gauss and Lorentz spectral lines are obtained by theoretical deductions.On this basis,the minimum detectable displacement model and the noise-equivalent mean square displacement model of coherent detection system are established.It is pointed out that the laser wavelength,detection distance and laser linewidth are main factors determining the minimum detectable displacement,and the laser wavelength and signal-to-noise ratio are main factors determining the noise-equivalent mean square displacement.(4)Based on the signal processing technology of analog demodulation,an analog quadrature demodulation circuit is designed.According to the requirement of system parameters,the minimum order of Butterworth analog filter is determined to be 7 by theoretical calculations.The isolation of transmission lines is simulated and analyzed under the ADS2013 environment,and the minimum level of PCB is determined to be 4 layers.(5)The AD sampling process of digital demodulation system and the principle of orthogonal mixing low-pass filter are briefly introduced.A prototype design model of fast FPGA based on Simulink is established by the ISE 14.7 and Simulink.The digital demodulation system is simulated and analyzed.The verilog hardware language is automatically generated by System Generator and on-line programming of signal processing flow of digital demodulation system is completed.An arctangent demodulation algorithm based on phase-locked loop is proposed.The cumulative errors and high-order harmonic components of demodulation output caused by the frequency jitter of the acoustooptic modulator are eliminated by the frequency locking of intermediate frequency carrier signal.On this basis,the digital demodulation circuit of laser coherent detection system is designed.(6)Two laser coherent detection systems based on the analog demodulation and the digital demodulation are built,and coherent vibration measurement experiments of single frequency vibration targets and complex speech vibration targets are carried out respectively.The experimental results show that both the analog demodulation system and the digital demodulation system can detect the micro-vibration signal,but the digital demodulation system has more advantages in the detection distance and detection accuracy,and in real time.The exploration of the previous analog demodulation technology lays a foundation for the smooth development of the fulldigital demodulation scheme.