Study On Propagation Characteristics Of Coma Beams And Imaging Of Diffuse Reflection Light Of Extended Object In Oceanic Turbulence

With the progress and development of modern underwater laser technology,it is of great significance to study the transmission characteristics and imaging problems of laser in the ocean.Laser produces aberration in the process of generation and transmission.It will reduce the laser beam quality,such as spherical aberration,coma,astigmatism and so on.Compared with the research on the influence of spherical aberration and astigmatism on the transmission characteristics of beams,the influence of coma aberration on the transmission characteristics of beams is very little.Therefore,it is very important to study the beam with coma transmission characteristics in oceanic turbulence(such as scintillation index,beam width,energy Strehl ratio and?parameter etc.).On the other hand,the issue of diffuse reflection and back-light imaging by laser illumination target is one of the most concerned issues in image transmission.It is particularly important to study the influence of oceanic turbulence on the imaging of diffuse reflection.This thesis mainly uses the self-prepared numerical simulation program to carry out the research work in the following three aspects:1?The transmission characteristics of coma beam through oceanic turbulence are studied.The mean-squared beam width,the energy Strehl ratio and the?-parameter are studied as the research through the evaluation of oceanic turbulence coma beam transmission characteristics parameters.It shows that,oceanic turbulence can disrupt the comet-like distribution of the beam.And the position of the center of the beam does not coincide with the position of the maximum light intensity.The mean-squared beam width centered at the position of the center of the beam is smaller than the width at the maximum intensity position,while it is more affected by the oceanic turbulence.In addition,the larger coefficient of coma,the smaller influence of the energy concentration due to turbulence when the Strehl ratio of energy is used.Moreover,when the coefficient of coma is a specific value,the energy concentration of the beam with the?parameter as the evaluation parameter is most affected by turbulence,which should be avoided in practice.The physical meaning of the energy Strehl ratio is different from the?parameter.Namely the influence of turbulence on the energy concentration defined according to the radius of the given bucket contained within the energy(i.e.,energy Strehl ratio)and the certain power percentage(i.e.,?parameter)is different.It should be paid special attention in practical application.2?The influence of coma aberration on the aperture averaged scintillation through the oceanic turbulence was studied.It shows that,coma aberration can significantly suppress?_I~2(D)in weak oceanic turbulence,and?_I~2(D)decreases with the increase of coma aberration coefficient.However,in the moderate oceanic turbulence,the effect of coma on?_I~2(D)can be ignored.In particular,the saturation phenomenon of?_I~2(D)appears in strong oceanic turbulence.In addition,the study also found that in the oceanic turbulence,the aperture averaged scintillation?_I~2(D)dominated by salinity-induced turbulence is stronger than that dominated by temperature-induced turbulence.It is worth mentioned that:for the coma aberration Gaussian beam,the aperture averaged scintillation and the point scintillation are obviously different.In practice,the aperture averaged scintillation is more suitable for describing the intensity scintillation.3?The properties of diffuse reflectance imaging of extended objects in oceanic turbulence was studied.The spatial filtering method is used to satisfy the paraxial approximation wave propagation equation and the oceanic turbulence power spectrum proposed by Nikishov is adopted to generate the random phase screen,the computer code of propagation and imaging of diffuse reflection light of extended object in oceanic turbulence is designed.The dependence of the imaging quality on oceanic turbulence parameters and receiver size is studied in detail.It is shown that method proposed in this paper is feasible and correct,and is useful for studying imaging of diffuse reflection light of extended object in oceanic turbulence.As the ratio of temperature and salinity and the rate of dissipation of mean-squared temperature increase and the rate of dissipation of kinetic energy per unit fluid mass decreases,the see parameter in oceanic turbulence decreases and the imaging quality degrades.