Optical Field Scintillation And Speckle Inhibition Of Laser Active Imaging Analysis

It is known as laser active imaging that targets illuminated by laser and imagedthrough atmospheric turbulence. Compared to ordinary sources of light, laser has faroperation range, small divergence and high directionality, because of its coherence.And also due to this property, the uniformity of illumination filed, experiencingatmospheric disturbance and rough surface modulation, both of which lead totemporal and spatial intensity fluctuation, is serious undermined, finally, influence theimaging system property and reducing system resolution. Therefore, in order toimproving the imaging system performance, this dissertation focuses on improvingthe uniformity of target filed and inhibiting the scintillation and speckle phenomenonbased on the in-depth analysis of statistics properties of target field. The mainly workstudied in the thesis is arranged as follows:(1) The study on target field of laser active imaging is resolved into two processes,based on Born and Rytov perturbation approximation: first, analysis the turbulenceeffect of illumination field after propagation through atmosphere, including the meanirradiance, coherence radius, beam wander and scintillation index; second, study thestatistic properties of return speckle field, reflected by rough surface in free spacepropagation, deriving the influence of roughness on field uniformity, and establishingthe relationship between speckle contrast and surface RMS height and correlationlength.(2) Considering the nature of intensity fluctuation during the imaging process, this thesis adopts multi-beam emitting and polarization diversity of illumination field toweaken the coherence, based on the statistical property of illumination field andspeckle field. According to this principle, the multi-beam emitting model, in whichboth energy of each emitting beam and the polarization state overlapped illuminationfield are adjustable, is founded, meanwhile the inhibition effects of this model, provedfeasible, on turbulent scintillation and speckle phenomenon is discussed in terms ofprobability distribution density of the illumination field. The optimal inhibition effect,the scintillation index and speckle contrast of N beams emitting are N and√Ntimessmaller than single beam, respectively, can be achieved by multi-beam emitting withequal energy of each one, and the influence of polarization diversity on irradiancefluctuation is also discussed.(3) Under theoretical analysis of illumination field, the simulation of field propagationis conducted based on split-step method, whose sampling interval is adjustable, andFFT phase screen, which is determined by turbulence properties, in Kolmogorovspectrum atmosphere; and the return speckle field, reflected by different roughnesssurface, is also simulated based on surface random height function. The comparison,between the simulation results of single beam illumination and those of multi-beamillumination, demonstrates that the multi-beam emitting does have restraining effectson scintillation and speckle, and that the optimal inhibition effect as expected bytheoretical analysis is obtained.(4) According to the multi-beam emitting model, the experimental system isestablished, and the atmospheric turbulence simulation system with static phase plateis also established in lab. With these two system, some experiments are carried out, asfollows: first, the scintillation and speckle phenomenon with linearly polarized singlebeam illumination; second, the scintillation and speckle phenomenon with linearlypolarized multi-beam illumination; third, the scintillation and speckle phenomenonwith partially polarized multi-beam illumination. All the experimental results, agreedwith theoretical expectations and simulation results, demonstrate that the multi-beamemitting with equal energy of each one produces the optimal restraining effects on scintillation and speckle phenomenon, and increasing the polarization diversity, usingmulti-beam, of overlapped field in target field can further reduces the speckle contrastof return field, compared with linearly polarized multi-beam illumination, but uselessfor turbulence scintillation.