Research On Remote Fiber Laser Doppler Micro-Vibration Detection

Since the emergence of laser technology in the 1960 s,the measurement technology based on laser has been developed rapidly.Laser Doppler measurement is a precision measurement technique based on the principle of laser Doppler effect and optical interference mixing.It has become the research hotspot because of its long-distance,high precision,non-contact,and high spatial resolution.With the wide application of optical fiber laser in coherent Doppler wind measurement,fiber laser Doppler vibrometer measurement technology has also developed rapidly.Remote vibration measurement not only requires the high performance of the system but also need to consider the performance degrade due to speckle and turbulence.Therefore,designing a high-performance laser Doppler vibrometer and clarifying the influence of speckle and turbulence on long-range vibration measurements have become an urgent problem to be solved.In this paper,with the theme of remote laser Doppler vibration measurement,the research on the design of high-performance vibrometer,carrier-to-noise ratio improvement technology,signal fading influencing factors and suppression,and long-range voice interception is carried out by theory,simulation,and experiment.The main results obtained are as follows:1.This paper introduces the theoretical basis of laser Doppler micro-vibration detection and analyzes the carrier-to-noise ratio,heterodyne efficiency,and fiber coupling efficiency of the detection system.On this basis,a set of all-fiber LDV system is designed and built with the consideration of device selection and main parameters.The influence of some devices on the performance of the detection system is discussed.The theoretical derivation,simulation analysis and experimental verification of the effect of interference competition caused by the device isolation defect in the fiber LDV system show that multi-beam interference competition will bring the demodulation signal with ripple or spike modulation which is affected by the modulation depth,the amplitude ratio of competing signals,and their relative phases.In the case of weak echoes,the residual carrier frequency caused by parasitic reflections even leads to failure of intermediate frequency demodulation.2.This paper discusses the method of improving the carrier-to-noise ratio of the optical fiber LDV system from two aspects of the optical transceiver system and signal processing.In terms of the structure of the optical transceiver system,one is the bistatic architecture which uses dual telescopes to achieve complete isolation between transmitter and receiver,the other is the monostatic architecture which replaces the fiber circulator with the angled TR switch composed by a polarizing beam splitter and a quarter-wave plate.The experimental results show that both transceiver schemes can eliminate the residual carrier frequency caused by parasitic reflection,greatly improving the system carrier-to-noise ratio and realize the intermediate frequency demodulation of weak echo signals.In terms of signal processing,this paper introduced an all-fiber pulsed LDV system,where the separation between parasitic reflection and vibration signal is achieved by the all-digital range-gated signal processing method.The thesis details the prototype of the pulsed LDV system,establishes the mathematical model of the echo signal,designs the pulse drive signal,introduces the processing flow of the intermediate frequency signal,discusses the system carrier-to-noise ratio and the demodulated signal-to-noise ratio and finally conduces experimental test on single-frequency narrowband signal and wideband voice signal,etc.The results show that the allfiber pulse LDV system can achieve the shot noise limited performance by adopting a fully digital range gating method,significantly improving the carrier-to-noise ratio of the system.The remote target vibration is detected by demodulating the weak echo which is backscattered from the hard target's surface originated from the ultra-low energy pulse.3.The thesis analyzes the mechanism of the effect of speckle,turbulence,and their coupling on signal fading.According to the diffraction theory,a free space speckle transmission model is established.The static speckle intensity and phase distribution under different focusing conditions are simulated.The behavior difference between speckle translation and speckle boiling is given by discussing the speckle cross-correlation function.In order to consider the influence of turbulence,the free space speckle transmission model is modified into a double-pass laser transmission model.The turbulence effect of phase fluctuation,beam wandering,and beam spread are discussed by putting the simulated turbulence phasescreen with different scales into the double-pass laser transmission model.Then,a turbulence-driven-speckle evolution is observed on the receiving plane.The statistical characteristics of the speckle field on the detector plane formed by the secondary interference of the local oscillator light and speckle light are declared that the intensity obeys a Rician distribution while the phase obeys a Gaussian distribution centered on the local oscillator phase under strong local oscillator condition.The IF signal encountered an amplitude decay when there are more than one speckles within the aperture.This decoherence effect in the case of multiple speckles in the receiving aperture is analyzed,as well as the effect of signal fading on intermediate frequency signals.Finally,polarization diversity reception and spatial diversity reception to suppress carrier fading are discussed.4.The research of long-range voice interception using the LDV system is carried out.Since the vibration characteristics of the target are very important in long-range voice interception,the forced vibration model of the plates is introduced,and the simulation and experiment of different materials such as paper,aluminum alloy,cloth,and PVC are carried out.The results show that paper and PVC materials have better vibration response and are more suitable as listening targets.The thesis conducts a comparative experiment on the listening effect of the self-developed pulse LDV at a distance of 23 m and 120 m.It is found that the echo energy is another key factor affecting long-distance voice interception.The secondary auxiliary focusing method can effectively improve the echo energy,which greatly improves the listening effect.The experiments show that shot noise and speckle noise are generally present in listening speech.In order to improve the quality of intercepted speech,a time-frequency mask noise reduction method is proposed to effectively suppress the shot and speckle noise.