Influence Of Oceanic Turbulence On Optical Imaging And Spatial Coherence Of Beams

Until now,lots of work about atmospheric turbulence effects on optical imaging has been carried out,but the influence of oceanic turbulence on imaging are seldom explored.Unlike that in the atmosphere,the fluctuation of refractive index in oceanic turbulence are both induced by fluctuation of the temperature and salinity,so the spectra of the refractive index fluctuation has‘double bumped'profile.Recent research has showed that,oceanic turbulence is a key factor for the limitation of optical imaging in natural underwater circumstances.So,researches about oceanic turbulence effects on optical imaging and propagation properties of laser beams are of great significance for underwater applications,such as optical communication,imaging,ladar,and so on.Oceanic turbulence effects on long-exposure and short-exposure imaging and coherence of laser beams are explored in this thesis,and the main works are listed as follow:1.Based on the power spectrum of oceanic turbulence,the modulation transfer functions for long-exposure and short-exposure imaging through oceanic turbulence are studied in detail,the seeing parameter in oceanic turbulence are derived and the model of underwater imaging put forward by Weilin Hou are improved.It shows that,the seeing parameter is 2.1 times of the spatial coherence width in oceanic turbulence,and the relationship between them is just like that in atmospheric turbulence.In addition,from the model of underwater imaging shown in this paper,which considers both temperature and salinity fluctuations in oceanic turbulence,it can be found that,compared with the dominating salinity-induced turbulence,the range of angular spatial frequency where oceanic turbulence plays the major role rather than particle scattering is higher for the dominating temperature-induced turbulence.2.The differences between modulation transfer functions for long-exposure imaging in oceanic turbulence and in the atmosphere are compared.And,the validity of modulation transfer functions SEMTF for short-exposure imaging is discussed.Moreover,the effects of oceanic turbulence parameters on resolution are studied in detail.It is shown that the quality of imaging is better in the dominating temperature-induced turbulence than that in the dominating salinity-induced turbulence,i.e.,the image contains higher-frequency components of the original image.Even though the Fried's theory of short-exposure imaging for SEMTF has defects in high-frequency regions,it will show better correctness with increasing the ratio of lens diameter to the seeing parameter,and increasing the path length.In addition,compared with in weak or strong turbulence,the improvement of resolution by shorten the exposure time is more obvious in medium oceanic turbulence.3.Based on the extended Huygens-Fresnel principle,the analytical expression for the coherence degree of partially coherent annular beams propagating through oceanic turbulence is derived.The changes of the coherence degree and the width of the coherence degree of partially coherent annular beams versus the propagation distance,the oceanic turbulence parameters and the beam parameters are investigated in detail.It is shown that the oscillatory behavior of the coherence degree may appear when partially coherent annular beams propagating in weak oceanic turbulence,but the coherence degree takes a Gaussian-like profile as the strength of oceanic turbulence increases.On the other hand,under weak oceanic turbulence condition,the width of the coherence degree reaches a maximum at a certain propagation distance,which depends on the obscure ratio and the correlation parameter of annular beams.However,under strong oceanic turbulence condition,the coherence degree is nearly independent of the obscure ratio and the correlation parameter of annular beams.