Study On Photoacoustic Doppler Measurement Of Ultrashort Pulse Intensity Modulation

Pulsed photoacoustic Doppler measurement technology uses pulsed lasers to directly excite,including methods based on time-domain cross-correlation,bandwidth broadening,and photoacoustic correlation spectroscopy.They have higher requirements for particle concentration and linear motion of particles.Velocity measurements are greatly affected.This paper presents a new pulsed photoacoustic Doppler current measurement technique that combines ultra-continuous laser and electro-optic amplitude modulation with ultrashort pulse intensity modulation of a lock-in amplifier to study these photoacoustic Doppler measurements,which can alleviate these limitations,Can measure flow velocity and depth information while measuring the absorption spectrum of the sample.In order to prove the feasibility of this method,the paper analyzes the theoretical and experimental results of pulse-amplitude-modulated photoacoustic Doppler measurement.In this kind of pulsed photoacoustic Doppler flow velocity measurement,an electro-optic modulator is used to amplitude-modulate an ultra-short pulsed laser.Filters can be used to select different wavelengths and bandwidths,and an electric lift is used to control the one-dimensional motion of the sample.Using a photodetector to receive the the amplitude-modulated pulsed laser,use a digital oscilloscope to observe amplitude-modulated pulsed laser and the demodulated forehead signal,and use a planar ultrasonic transducer to receive it.The photoacoustic signal is amplified by a preamplifier,The photoacoustic signal and the reference signal are input to a lock-in amplifier for demodulation and Fourier transform processing to obtain a demodulated time domain graph and a spectrogram.The data of the lock-in amplifier and digital oscilloscope are stored in a computer for digital signal processing.In the experiment,the frequency shift value can be read directly in the lock-in amplifier,where the magnitude of the value indicates the magnitude of the frequency shift and the sign indicates the direction of the flow velocity.In the ideal case,there should be only one peak inthe demodulated spectrum,but in the actual measurement,the some peak appears and it shows an odd multiple relationship,which is reasonably explained in this paper,and reasonable methods are used to eliminate these effects.When using a phase-locked amplifier demodulation process,different 3d B cut-off bandwidths have different effects on the rate measurement.A reasonable choice of 3d B cut-off bandwidths for different rates can improve the measurement accuracy.In this experiment,six different samples were measured,and the laser with different wavelengths and the corresponding photoacoustic signal were used to quantitatively calculate the absorption spectrum of the actual sample.This method hopes to be used to study the blood of the microcirculation system,and to obtain the metabolic level of the microcirculation system by measuring the flow velocity of different blood vessels and the concentration of different biological tissues and substances,and it will become a new and effective biomedical research method.