Study Of Laser Interferometric Fluid Velocity Measurement Technology Based On Acousto-Optic Modulation

Laser measurement has always been an indispensable method in the field of test and measurement,the traditional flow rate measurement method has been unable to meet the needs of modern industrial production,the development status and trend of laser velocity measurement technology at home and abroad were analyzed,precision measurement requirements for flow rate in industrial production have been met,a laser interferometric fluid velocimetry technique based on acousto-optic modulation has been proposed in this paper.First of all,based on the scattering model of particles in fluids and the mathematical method of laser interference,mathematical models of laser interferometry systems based on acousto-optic modulation have been developed.Measurement models were based on the scattering of particles in water,using Doppler's principle,the frequency difference in the measurement was calculated;by mixing diodes in photodetectors,the mathematical model was derived by heterodyne detection principle,the conversion from light to electricity was completed;acousto-optic modulator was used to modulate the frequency of reference light,an optical heterodyne measurement path with a stable frequency difference of 140.3MHz was obtained,the anti-jamming ability of the system was guaranteed.The measurement optical path was improved by using the double probe structure,the signal receiving strength was improved,and the stability of the system was enhanced.Secondly,the key components of the system have been selected and the anti-interference design has been completed.A semiconductor laser was used as a light source and a constant-current driving circuit was designed for this purpose,Bragg type acousto-optic devices were selected to modulate the signals,and photodetectors satisfying the spectral sensitivity,frequency response characteristics and quantum efficiency of the signals were selected;the electromagnetic environment anti-interference(EMC)technology was applied to the electronic circuit of signal processing to block the transmission path of interference,suppress the noise of interference source,and strengthen the anti-interference ability of the circuit.Then,the output signal of photodiode mixing was collected and processed.The signal to noise ratio improvement analysis was carried out on the collected weak signals to effectively extract the signals submerged by noise by reducing the system bandwidth,based on the principle of superheterodyne measurement,the modulated high-frequency signal was moved to the intermediate frequency band and then demodulated,the harmonic response and noise can be effectively eliminated after the signal was converted twice;according to the characteristics of time-frequency conversion of Doppler frequency shift signals,an improved spectrum refinement algorithm for Fourier transform was proposed,after complex modulation,filtering and resampling,FFT calculation of the signal combined with windowing and ratio correction was performed,spectral leakage and interspectral interference were avoided to achieve high resolution velocity measurement.Finally,the measurement experiment and error analysis control have been completed.An acousto-optic measurement experimental device was built to measure the relative light intensity,driving frequency and power of Bragg diffractive light.The central frequency of the acousto-optic modulated device was determined to be 140.3MHz,the driving current was80 m A,and the diffraction efficiency could reach 70%;the performance of the system was tested.Compared with the standard velocity and the zero difference method,the system error is less than 1%,which can achieve high-precision measurement;the factors that may lead to errors in the experiment were analyzed and improved.The standard uncertainty of the synthetic system was 0.73%,which met the measurement requirements.