Ocean Turbulence Effect Of The Orbital Angular Momentum Mode Of Vortex Beams

With the rapid increases in the requirements of application of optical information transmission and detecting technologies in turbulent environments,scholars have further deepened and improved researches on the use of special laser beam structures to resist turbulence.The most commonly encountered turbulent media in optical information transmission and detecting technologies are the turbulent ocean,the turbulent organism,and the turbulent atmosphere.However,the laser beam is affected by medium absorption and turbulent scattering when it is propagating in these three types of turbulent media.Therefore,it is very necessary to study the influence of turbulent media on beam transmission.In this dissertation,the turbulent ocean is selected as the main channel media to study their effects on the propagation of the partially coherently modified Bessel-correlated vortex beam.At the same time,the research results are generalized to the turbulent biological tissue.Firstly,in the channel of the paraxial and anisotropic weak ocean turbulence,according to the statistical law of ocean turbulence,a model was established about the probability density of orbital angular momentum(OAM)modes carried by the partially coherently modified Bessel-correlated vortex beam.Then,based on the statistical laws of turbulence in biological tissues,we extended the theory of orbital angular momentum(OAM)propagation carried by the partially coherently modified Bessel-correlated vortex beam to turbulent biological tissues,a model for the signal-to-noise ratio(SNR)and noise equivalent intensity of the partially coherently modified Bessel-correlated vortex beam was established.Based on the above models,the effects of ocean turbulence factors,such as the ratio of temperature and salinity,anisotropy factor and receiving aperture,and partial consistency on the signal transmission quality in the ocean under ocean turbulent environment were studied.The effects of factors such as temperature fluctuation intensity,cutoff correlation length,external scale of tissue index inhomogeneity,and fractal dimension of particle size distribution on biological tissue turbulence are analyzed.And the following main conclusions are drawn:(a)We can choose to carry information transmission modes with lower OAM quantum numbers to deliver information more effectively.(b)Larger turbulence internal scale,smaller temperature fluctuation intensity,and stronger anisotropy are important factors for improving the transmission quality of vortex beams carrying orbital angular momentum in oceanic turbulence.(c)There is an optimizing receiving aperture in the specific interval of the ratio of temperature and salinity,which can reduce the effect of the ratio of temperature and salinity on the normalized energy weights and signal reception of the OAM mode.(d)Ocean turbulence with higher anisotropy has less influence on OAM signals than turbulence with lower anisotropy,so the anisotropic ocean turbulence is more suitable for signal transmission.(e)We can use OAM multiplexed free-space link to reduce mode interference and increase the information capacity of optical communication systems,the method is to select the OAM mode and the non-diffraction signal with large differences in quantum numberbetween modes to transmit information.(f)In the case of other parameters have been determined,there is a value of transmission distance that makes the transmission quality to be worst during the light beams propagation of in biological tissue turbulence.Analogously,the values of the fractal dimension of particle size distribution and the value of temperature fluctuation intensity are all so.Therefore,these values should be avoided as much as possible during the study of biological tissue detection and diagnosis and treatment.Values can be obtained at parameter values with higher ratio of signal-to-noise,which can effectively avoid more trouble in the research process.Our theoretical experimental results can provide important theoretical basis for high-capacity underwater wireless communication and design or research on medical devices.