Study On Propagation Properties Of Autofocusing Airy-function-related Beams In Free Space

Free-space optical communication is an important part of space-air-ground integrated information transmission,and it is expected to solve the problem of “ the last kilometer ” of optical fiber,so it also plays a great role in promoting the information industrialization.In addition,as an auxiliary communication under special conditions,it can provide highly secure communication links for the military because of its convenient installation.Vortex beams carry orbital angular momentum,which has the potential to greatly increase the channel capacity.Vortex beams,generally obtained by means of laser phase modulation,have certain spatial distribution.So they are more vulnerable to the influence of inhomogeneous media such as atmospheric turbulence,leading to the optical topological charge random fluctuations at the receiving end of communication system and the information transmission distortions.Therefore,the selection of background light field of vortices is extremely important.Boasting the advantages of no-diffraction,self-healing and abruptly auto-focusing before the target,the autofocusing Airy beam can be used as the background light field to load the optical vortices to reduce the distortions of turbulence to the optical vortices propagations.Starting from the propagation dynamics properties of autofocusing Airy vortex beams,this dissertation utilizes optical field control technology to regulate and control the phase,amplitude,polarization state and other parameters of the autofocusing Airy vortex beams,so as to reduce the crosstalk and vortex splitting of the orbital angular momentum modes in the free space transmission and optimize the communication system performances.Main contents and conclusions of this dissertation are as follows:1.The propagation dynamics of optical vortices embedded in autofocusing Airy beams are studied.Based on the angular spectrum theory and the beam propagation method,the influences of waveform parameters and topological charge of autofocusing Airy vortex beams on the evolution of light intensity distribution and self-focusing ability of the circular Airy vortex beam with the vortex center located on and outside the transmission axis are numerically analyzed.Analysis indicates that with a given focal length,the smaller the main ring radius of autofocusing Airy beam is,the stronger the optical field focusing ability is;When the vortex center is located on the propagation axis,the larger the topological charge is,the greater the optical field energy losses during propagation.When the vortex center deviates from the propagation axis,the light field rotates during the propagation and the focus position changes,and the Gouy phase shift theory is needed to confirm the position of the focal plane.2.The propagation properties of autofocusing Airy vortex beam with controllable intensity gradient in free space are studied.By modulating the intensity gradient,the self-healing effect of autofocusing Airy vortex beams can be further stimulated,and then under the combined effects of self-healing and self-focusing,the energy of the optical field flows inward radially.Therefore,the intensity gradient of background optical field can be introduced as a new degree of freedom to modulate the propagation properties of autofocusing Airy vortex beam in free space.For this to be effective,the right intensity gradients need to be selected for different degrees of turbulence,thus reducing the vortex splitting effectively.3.The propagation properties of autofocusing Airy beam carrying power-exponent-phase vortex in free space are studied.Based on the Rytov theory,the probability densities of orbital angular momentum modes of the autofocusing Airy beam carrying powerexponent-phase vortex after passing through the weak anisotropic non-Kolmogorov turbulent atmosphere are theoretically formulated and numerical investigated.Analysis shows that a relatively small beam waist and longer wavelength are good for separating the detection regions between signal orbital angular momentum mode and crosstalk orbital angular momentum modes;The autofocusing Airy beam carrying powerexponent-phase vortex with a given topological charge is,in fact,the result of the weighted superposition of multiple propagating traditional orbital angular momentum modes,and the weight coefficient of each traditional orbital angular momentum mode is determined by power order.Therefore,the power order can be introduced as an extra steering parameter to modulate the probability density distributions of orbital angular momentum modes.These results provide guidelines for the design of an optimal detector,which has potential application in optical vortex communication systems.4.The propagation properties of a circular Airy vortex beam array through atmospheric turbulence are studied.By analogy with the differential geometry thought,the autofocusing Airy vortex beam can be regarded as a circular phased-locked coherent Airy vortex beam array with sufficient beamlets.Therefore,through adjusting the topological charge values and locations of these vortices embedded in the array elements,not only the cross-section power distribution of the resultant light beam can be tailored without energy loss,but also the local focusing improvement is achieved to balance the beam wander and beam spreading effects.These results provide guidelines for the design of an optimal signal transmitter,which has potential application in optical vortex communication systems.5.The propagation properties of a vectorial autofocusing Airy vortex beam in free space are studied.By using high order Poincare spheres,the wavefront structure and crosssectional polarization state of autofocusing Airy vortex beam are redesigned,and the influences of polarization parameters on beam focusing ability are investigated.By comparing the vortex splitting ratio of scalar autofocusing Airy vortex beam with that of vector autofocusing Airy vortex beam under the same turbulence condition,the results indicate that the vector autofocusing Airy vortex beam has a smaller vortex splitting ratio under strong turbulent atmosphere.Therefore,as an information carrier,the vectorial autofocusing Airy vortex beams are more reliable when they are transmitted in free space.