Crosstalk And Detection Of Orbital Angular Momentum Of Vortex Beams In Complex Channels

Free space optical communication is an important means of high-speed information transmission in space information network.Compared with traditional communication technology,it has more security and anti-interference capability of information transmission.At present,the technology has the problems of low frequency utilization and insufficient communication channel capacity.In theory,the above problems can be solved by using the vortex beam with orbital angular momentum(OAM)as the information carrier of optical communication.However,in real optical communication applications,the vortex beam will be interfered by atmospheric turbulence,rainfall and other environment in the space when it transmits in the complex channel,which will lead to the crosstalk of the orbital angular momentum of the beam,reduce the information transmission efficiency,and increase the communication error rate.Therefore,it is of great theoretical significance and application value to study and detect the orbital angular momentum crosstalk of vortex beams in complex channels.In this paper,the classical Laguerre Gauss(LG)vortex beam and focused LG vortex beam are used as models,and the theoretical analysis and numerical simulation are carried out.Theoretically,the spiral spectrum expression of vortex beam is derived,and the influence of beam parameters and turbulence related parameters on the beam orbit angular momentum crosstalk is analyzed;according to Fraunhofer diffraction theory,the diffraction intensity distribution of LG beam after phase distortion passing through the elliptical aperture is derived.In the aspect of simulation,atmospheric turbulence and turbulence raindrop mixed phase screens are constructed to model the beam propagation in the channel and analyze the characteristics of the beam propagation in the complex channel.Finally,the simulation results are compared with the theoretical results,and the method of detecting the orbital angular momentum information carried by the vortex beam is studied.The main work of this paper is as follows:(1)Based on the spiral spectrum analysis theory,using anisotropic non-Kolmogorov turbulence power spectrum,the spiral spectrum of focused and unfocused(collimated)Laguerre Gaussian vortex beams in anisotropic atmospheric turbulence is derived.The influence of different beam parameters(wavelength,orbital angular momentum number,waist radius),turbulence parameters(turbulence intensity,anisotropy coefficient,turbulence inner scale,turbulence outer scale)and other parameters(propagation distance,propagation height)on the received power of the beam are analyzed.The results show that different parameters have different effects on the received power.Compared with collimated and focused LG beams,it is found that focused LG beams have higher receiving power and stronger anti turbulence ability in turbulence.(2)Based on the fast Fourier transform power spectrum inversion method,the mixed phase screen of atmospheric turbulence and turbulence raindrop is constructed,and the beam transmission model is established by using the multi-layer phase screen method.Firstly,the intensity phase distributions of collimated and focused LG beams in atmospheric turbulence are simulated and compared;secondly,the intensity phase distributions of LG beams in turbulent raindrop mixing are simulated;finally,the orbital angular momentum crosstalk characteristics of LG beams under different parameters are simulated,and the simulation results are in good agreement with the theoretical analysis.The results show that the propagation quality of the focused beam in turbulence is better than that of the collimated beam,and the propagation quality of the beam is worse in the case of rainfall.(3)Based on the diffraction principle,the elliptical diaphragm is used to detect the orbital angular momentum of LG beams after phase perturbation.Simultaneous interpreting of the intensity of the diffraction of LG beams passing through the aperture under different turbulence intensities and different transmission distances is made.The results show that: in vacuum or weak turbulence conditions,the orbital angular momentum of LG beam can be determined according to the characteristics of diffraction intensity map,but with the increase of propagation distance and turbulence intensity,the beam distortion is serious,which makes it impossible to detect the orbital angular momentum information of the beam.