| With the rapid development of information technology,traditional communication systems that employ wavelength,polarization,phase and so on to modulate or multiplex are limited by bandwidth,which can not meet the demand for high-capacity and high-speed communication.Theoretically,using the orbital angular momentum(OAM)of the vortex beam to multiplex can effectively improve the communication capacity and the utilization rate of spectrum,realizing high-capacity,high-speed communication.OAM is a unique dimension of vortex beams.Different OAM modes are mutually orthogonal.Therefore,the multiplexed OAM modes can be separated well.In addition,the possible number of OAM modes are infinite,which can constitute an infinite Hilbert space and provide a high degree of freedom for information coding that could increase the data capacity of the communication system.Because of these excellent characteristics of the vortex beam,it has become a very competitive light source for FSOC.However,when the beam transmitted in free space,the OAM will be disturbed by atmospheric turbulence,which will cause crosstalk between channels and increase the bit error rate of the communication system.Those are detrimental to data transmission.In order to solve this problem and realize the high-capacity,high-speed communication,it is necessary to study the evolution characteristics of the OAM mode of vortex beams in turbulent atmosphere.Based on the Rytov theory and the generalized Huygens-Fresnel principle,we investigated the evolution characteristics of the OAM mode of twisted Gaussian-Shell model beams(TGSM)in the atmospheric turbulence using theoretical derivation and numerical calculation,and explored the potential application of the OAM spectrum of TGSM beam in Lidar detection.In addition,we proposed a method that can effectively reduce crosstalk between OAM modes caused by atmospheric turbulence.The main research projects and results include the following aspects: 1.In the typical weak level Kolmogorov atmospheric turbulence,the expression of the normalized energy weight of the OAM mode of the TGSM beam on the receiver is derived based on the Rytov theory.And we analyzed the effect of the beam parameters(wavelength,twist factor,coherence length,beam waist)and turbulence parameters(inner scale,the structure constant of the refractive index)on the distribution of OAM spectrum of TGSM beams.The results show that by increasing the beam waist size or decreasing the beam coherence length,the OAM spectrum of the TGSM beam will be broadened at the source plane,while its OAM mode is more robust when transmitted in atmospheric turbulence.In addition,the larger the value of the twist factor is,the better the TGSM beam can maintain the original OAM mode distribution when the beam transmitted in turbulence.2.On the basis of the first work,we explored the application of the OAM spectrum of the TGSM beam in Lidar detection.And the results of the first work are extended to weak and strong turbulent conditions using the tensor method.First,the model for detecting target objects using TGSM beam is built.Then,the influence of the parameters of target on the OAM mode distribution of TGSM beam is analyzed.We obtain the relationship between the size,roughness of the target and the parameters of the received TGSM beam,which has potential guidance value for Lidar detection.In addition,the variation of coherence length of TGSM beam with different wavelengths under different turbulence intensities are investigated.The results show that in weak turbulence conditions,the diffraction effect of the beam dominates when transmitting a short distance,which is manifested by the increase of beam coherence;as the transmission distance increases,the cumulative effect of turbulence increases,and the beam coherence begins to decrease.For strong turbulence conditions,turbulence effect on beams is always been dominant,and the degree of coherence gradually decreases with increasing transmission distance.3.Based on the vortex Gaussian beam,a method for reducing the cross-talk among different OAM modes caused by atmospheric turbulence is proposed.And we have quantitatively analyzed the improvement effect of this method with theoretical derivation and simulation.Let a light beam with vortex phase pass through a focusing mirror and then propagates in turbulent atmosphere.In this process,the interacted area between the beam and the turbulence decreases.Therefore,the disturbance on beam is decreased,and the cross-talk among OAM modes is reduced.Then,the numerical method is used to analyze the influence of beam parameters and mirror parameters on the OAM spectrum of the beams.The results show that by using a focusing mirror,the detection probability of the central OAM mode will be increased significantly,and the crosstalk between adjacent OAM modes will be significantly reduced.Besides,the larger the mirror size is,the more obvious the crosstalk between OAM modes will be reduced.The mirror has a better effect on the beams with a large number of OAM modes.Finally,using the random phase screens,atmospheric turbulence is simulated to further proving the validity of our method.The results will be useful in free-space optical communication. |
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