Propagation Characteristics Of High-order Bessel-gaussian Beams In Atmospheric Turbulence

The Bessel–Gaussian beam(BGB)has non-diffraction and self-healing properties comparing with other beams.BGBs have better anti-turbulence ability than laser beams and Gaussian beams,and are one of the best representative beams in the transmission theory.It has important application in wireless optical communication.BGB can be used as an "optical wrench",and it also have widely applications in cell manipulation,image processing,and information confidentiality.The thesis mainly analyzes the transmission characteristics of a BGB in atmospheric turbulence in detail.The specific content is as follows:The classic optical transmission theories,Generalized Huygens-Fresnel principle,and the multi-phase screen transmission theory based on the power spectrum inversion are introduced to calculate the average intensity of BGB field.In Chapter 3,according to the basic integral formula,in vacuum,the analytic expression of the vector BGB propagating in the far-field is derived based on the plane wave vector angular spectrum expansion.In the same way,by adding the focus factor,the analytical expression of the vector focused BGB is derived.Based on the plane wave angle spectrum expansion theory and the turbulence statistics obtained under the Rytov method,the propagation characteristics of high-order BGBs in atmospheric turbulence are simulated.Combining the turbulence statistics obtained in the Rytov method with the expression of the BGB field,the average intensity,broadening and drift of the beam in the atmospheric turbulence are analyzed.We find that beams with larger orbital angular momentum are more likely to produce broadening drift,and the amount of drift of BGBs caused by strong turbulence will change in magnitude compared to weak turbulence.The drift of Gaussian beam is less than the BGB.We also simulate the light intensity distribution of the multiplexed beam,and obtain the light field structure of the multiplexed beam under different topological charges.The light field distribution of the multiplexed beam also is obtained based on the turbulence statistics under different turbulence intensities.It can be seen that in strong turbulence,the beam is severely distorted,and the attenuation is very serious.We not only make a study of the line-of-sight,but also derive the helical spectrum characteristics of the vector focused and unfocused BGBs in the slant paths based on the Rytov phase structure and the ITU-R atmospheric structure constant model.Results show that the upward spiral spectrum is always lower than the downward spiral spectrum under the same conditions,and the variation of the slope spiral spectrum characteristics of the beam with the propagation distances under different initial parameters is analyzed.We find that in different turbulence intensities,the magnitude of the spiral spectrum propagating along an uplink path and a downlink path will cross,mainly because the spiral spectrum of uplink is smaller than the downlink and the turbulence intensity has the greatest influence on the vector BGB.The spiral spectrum of the slant transmission is larger than the line-of-sight transmission.At the same time,the helical spectrum characteristics of the vector focused BGBs are consistent with the vector non-focused BGBs in different parameters.However,the focused beam has a stronger anti-turbulence effect than the unfocused beam after a longer distance,and the focused beam has a larger spiral spectrum.The polarization characteristics of the radially polarized BGB are studied based on the diffraction integral formula.We find that the polarization of the beam is along the radial direction in the source plane,which is in good agreement with the theory,while as the propagation distance increases,we see that its polarization direction has changed and is no longer along the radial direction.Meanwhile,we find that the beam has non-diffraction characteristics within a certain propagation distance.Similarly,we also analyze the polarization characteristics of the multiplexed BGB,and the result is similar to a single beam,but the polarization structure of the multiplexed beam is closely related to the combination of the topological charges.We also study the discretized scintillation index,the pointing deviation,and the spiral spectrum of the BGB in atmospheric turbulence based on the multi-phase screen method.We get that the stronger the turbulence,the higher the scintillation index,and the scintillation point is minimum near the maximum light intensity.And the larger the turbulence scale,the greater the scintillation.At the same time,we also compare the pointing deviation of the scalar and vector BGBs,and obtain that the pointing deviation of the vector beam is smaller than the scalar beam,so the vector beam is more steady.Using the phase screen theory,we study the crosstalk characteristics of the multiplexed BGBs.Results show that the orbital angular momentum which carries the information can not be distinguished in strong turbulence.At this time,the information is destroyed and the beam is seriously distorted.This article provides a theoretical support for the transmission of BGBs in atmospheric turbulence,and also lays a foundation for the application of BGBs in the optical communications.