Coherent Doppler Lidar Based On Mode-locked Laser

Coherent Doppler LIDAR (laser and infrared radar) has been proved aneffective tool for measuring the velocity of wind fields and moving targets by meansof remote sensing. Single longitudinal mode (SLM) laser is generally used in thecoherent Doppler LIDAR system as an emitter, and mode-locked laser, with thecharacteristics of wideband spectrum, narrow pulse width and high peak power, is apotential source for coherent detection. In this dissertation, a coherent DopplerLIDAR system based on the mode-locked laser is studied theoretically andexperimentally.According to the heterodyne detection theory of SLM laser, a theoretical modelof coherent beat frequency between two mode-locked lasers is discussed, in whichthe longitudinal mode number of the local oscillator is m+1, the longitudinal modespacing is ω, and the frequency shift of the signal laser is Δω. The derivation resultindicates that mode-locked laser is able to realize coherent frequency mixing whenthe phase difference is constant, and the difference frequency signals can beobtained after low-pass filtering or FFT spectral analysis. Accordingly, thenumerical simulation method has been employed, the waveforms of signal laserschanging with different frequency shifts have been analyzed, and the conditions torealize coherent detection have been discussed as well.The experimental studies on the mode-locked laser have been conducted for thepotential of being a laser source of coherent Doppler LIDAR. Mode-locked laseroutputs have been obtained by applying different mode-locked devices. For theactively mode-locked laser as well as the actively mode-locked Q-switched laser,steady outputs with the characteristics of hundreds of picoseconds duration,100%modulation depth, and over95%mode-locked probability have been obtained. Thecomparison between the actively mode-locked laser and the actively mode-lockedQ-switched laser indicates that the actively mode-locked pulse train has a longduration and the pulse peak power is low, while the actively mode-lockedQ-switched pulse train has a high peak power and a short duration. Mode-lockedlasers with different output characteristics provide different emitters for thesubsequent coherent velocity measurement experiments.The coherent beat frequency experiment has been conducted by using anacousto-optic frequency shifter (AOFS) simulating Doppler shift in the heterodynedetection. In the30~80MHz frequency shift range, the beat frequency waveformsas well as the results after signal processing are consistent with the theoreticalanalysis, and the relative errors of the results measured are less than0.5%, and the beat frequency experiments have been conducted by using Q-switched SLM lasersof10nanoseconds pulse width and16nanoseconds pulse width respectively, and therelative errors obtained are3.7%and1.6%accordingly when the frequency shiftvalue of the signal laser is set at150MHz. The results indicate that the envelopenumbers of beat frequency waveforms are limited due to the narrow pulse width ofthe Q-switched SLM laser, and the relative errors are large. For the mode-lockedlaser, with a wide envelope, the measurement accuracy is high even when measuringsmall frequency shift, which means the mode-locked laser is more dominate inmeasuring low velocity moving targets. In addition, the mode-locked laser has highpeak power and narrow pulse width. The coherent beat frequency experiment basedon fiber coupling has been conducted. The FFT spectrum analysis results of beatfrequency waveforms obtained are consistent with the set values as well astheoretical analysis.A laser Doppler velocimetry has been designed and set up. In the system, ahigh-speed rotating disk is used to simulate the moving target, and the Doppler shiftvalue has been obtained after beat frequency and filtering. The radial velocity erroris less than0.4m/s through calculation. By means of changing the initialtransmittance of the Q-switch crystal, the effects of different envelope widths of theprobe lasers on measuring results have been discussed. The echo signal of the probelaser with a wider envelope is weaker, but the measuring result is more stable; theecho signal of probe laser with a narrower envelope is stronger, and the relativeerror of single measurement is probably bigger, which can be reduced by averagingthe values of repeated measurements. A superheterodyne method is used by adding afrequency shift to the local oscillator, since moving directions of the target areunable to be distinguished according to the beat frequency waveforms. Experimentalresults indicate that the direction and velocity information are available. Thecoherent beat frequency experiment indicates that the frequency shift value isconsistent with the actual value no matter the disk rotates clockwise orcounterclockwise. The average relative errors are less than2%and1%respectively.The gained line-of-sight velocity curve based on the frequencydifference fits well with actual velocity curve.