Study On Rytov Variance Of Optical Wave And Characteristic Parameters Of Optical Wave And Adaptive Optical Imaging System In Oceanic Turbulence

Since recently the interest in underwater communications,imaging,sensing and lidar appeared,it is important to study the light propagation and optical imaging in oceanic turbulence.It has been verified experimentally the significant effect of oceanic turbulence on image-quality degradation.The strehl ratio,the Greenwood time constant and the isoplanatic angle are the characteristic parameters of adaptive optical imaging systems,but these parameters in oceanic turbulence haven't been studied.The spatial coherence length and the Rytov variance are the important parameters in theoretical studies of optical wave propagation in turbulence media,but the two parameters based on the new spatial power spectrum model of oceanic turbulence?i.e.,Elamassie's power spectrum model?haven't been involved.In addition,the influence of double-pass oceanic turbulence on performance indicators of laser radar systems should be considered.However,until now,only single-pass propagation of optical waves in oceanic turbulence was studied.In this thesis,the above problems are studied,the main research results are summarized as follows:1.The characteristic parameters of the adaptive optical imaging system in oceanic turbulence are studied.The approximate analytical expression of the strehl ratio for the short-exposure imaging is derived.It is illustrated that this approximate expression is accurate enough except the near field when D_G/r₀=1?where r₀ is the seeing parameter in oceanic turbulence,D_G is the pupil diameter of the optical system?.Furthermore,the analytical expressions of the isoplanatic angle and the Greenwood time constant in oceanic turbulence are also obtained.It is shown that the values of the three characteristic parameters decrease when salinity-induced optical turbulence dominates gradually.The values of the three characteristic parameters also decrease when the rate of dissipation of kinetic energy per unit mass of seawater decreases or the rate of dissipation of mean-squared temperature increases.2.The characteristic parameters of optical wave and short-term beam spreading in oceanic turbulence are studied.Based on the Elamassie's power spectrum model of oceanic turbulence,the analytical formula of the wave structure function,the spatial coherence length of optical wave and the Fried parameter in oceanic turbulence are derived,and the correctness of each of these formulae is verified.It is shown numerically that the results obtained by using the Elamassie's power spectrum model are quite different from those obtained by using the Nikishov's power spectrum model.If the Nikishov's power spectrum model is adopted,the strength of turbulence is underestimated when oceanic turbulence is dominated by the temperature fluctuations,while the strength of turbulence is overestimated when oceanic turbulence is dominated by the salinity fluctuations.If the Elamassie's power spectrum model is adopted,it is shown that the Kolmogorov five-thirds power law of the wave structure function is also valid for oceanic turbulence in the inertial range,and 2.1 times the spatial coherence length of optical wave is the Fried parameter,which is in agreement with that in atmospheric turbulence.Furthermore,based on the Elamassie's power spectrum model,the semi-analytical formula of the short-term beam spreading of Gaussian beams is derived in this paper,and its correctness is also verified.It is shown that the difference in short-term beam spreading is very large,whether the stable stratification of seawater is assumed or not.3.Rytov variance of spherical wave and performance indicators of laser radar systems in oceanic turbulence are studied.The analytical expression of the spherical wave Rytov variance ?₀² in oceanic turbulence is derived by using the Elamassie's power spectrum model?EPSM?,and its correction is confirmed.The performance indicators of underwater laser radar systems,including the double-pass scintillation index ?_I²,the aperture averaging factor A,the mean signal-to-noise ratio ?SNR? and the mean bit-error-rate ?BER? of monostatic system?MS?and bistatic system?BS?in oceanic turbulence are investigated.On the other hand,the difference of?₀²,?_I² and ?SNR? between EPSM and Nikishov's power spectrum model?NPSM?is also studied.It is found that,in weak and moderate turbulence,?_I² is underestimated by NPSM when oceanic turbulence is dominated by the temperature fluctuations,while?_I² is overestimated by NPSM when oceanic turbulence is dominated by the salinity fluctuations.However,it is inverse situation in strong turbulence.Furthermore,the behavior of ?SNR? between EPSM and NPSM is different from that of?_I².The relative difference of?_I² and ?SNR? between EPSM and NPSM is larger for MSthan that for BS.The relative difference of?_I² and ?SNR? between EPSM and NPSM is larger in weak and moderate turbulence than that in strong turbulence.