The vortex beam has a spiral phase structure and carries orbital angular momentum(OAM)with orthogonal characteristics,and the OAM mode number can take any value,so vortex beam mode multiplexing can increase the capacity of free-space optical communication.However,the light intensity and phase of the vortex beam will be distorted by the atmospheric turbulence in the actual communication link,which in turn generates mode crosstalk and reduces communication quality.This distorted vortex beam can be solved fundamentally by using wavefront correction techniques to solve the beam wavefront distortion problem.The principle of wavefront correction technique is to correct the beam by generating a phase conjugate to the distorted phase.It is important to improve the quality of optical communication systems.In this paper,a stochastic parallel gradient descent algorithm(SPGD)and a genetic algorithm(GA)are used to correct OAM beams with different distortion levels in a wavefront-free correction system to achieve high quality light intensity distribution and low crosstalk patterns.The main research of this paper is divided into the following areas.Firstly,the basic principles of wavefront correction for vortex beam aberrations are outlined.This paper introduces the composition and working principle of the OAM beam wavefront correction system,and then describes the important part of the correction system,which includes the basic characteristics of the OAM beam,the method of wavefront aberration generation,and two classical wavefront correction algorithms.In addition,the effect of different combinations of operators on the quality of wavefront correction by genetic algorithm is investigated.The above section provides the necessary theoretical basis for the subsequent study of OAM beam wavefront correction.Secondly,wavefront correction techniques for high-quality light intensity distribution are investigated.The degree of wavefront distortion affects the quality of optical intensity distribution of OAM beams,and the optical intensity correlation coefficient can be used as a representation of the optical intensity distribution relationship between distorted and ideal beams.Therefore,in this paper,the optical intensity correction coefficient is selected as the evaluation function for the wavefront correction of high-quality optical intensity distribution.and the SPGD algorithm and GA algorithm are used to compensation the single-mode and multi-mode OAM beams with different aberration intensities.The results show that under weak distortion(-6rad-6rad),this method can increase the intensity correlation coefficient of a topologically charge l =1 single-mode beam from 0.73 to 0.94,and the intensity correlation coefficient of a topologically charge l =1,-2 multimode OAM beam from 0.66 Raised to 0.96.Under strong distortion(-15rad-15rad),the intensity correlation coefficient of single-mode vortex beam of topological charge l =3 can be increased from 0.35 to 0.78,and the intensity correlation coefficient of three-mode vortex beam of topological charge l =1,3,5 can be increased from 0.27 to 0.69.Third,the wavefront correction technology of low crosstalk mode is studied.Wavefront distortion can easily cause crosstalk in the mode of the OAM beam,which will inevitably increase the difficulty of OAM mode demultiplexing,and the correction algorithm that uses the light intensity correlation coefficient as the evaluation function to reduce the mode crosstalk is not obvious.This paper directly selects the spiral spectrum distribution as the evaluation function of the low crosstalk mode wavefront correction,and also uses the SPGD algorithm and GA algorithm to correct the OAM beam under different distortion intensities.The results show that under weak aberrations(-6rad-6rad),this method can increase the mode purity of a single-mode OAM beam with topological charge l=3 from 0.56 to 0.98.The relative power of the topological charge l =1 in the beam of the multimode beam with topological charge l =1,3,5 can be increased from 0.24 to 0.30,the relative power of l =3 can be increased from 0.14 to 0.32,and the relative power of l =5 increased from 0.13 to 0.32.Under strong distortion(-15rad-15rad),the mode purity of single-mode OAM with topological charge l =1 can rise from 0.48 to 0.89,The relative power of topological charge l =-2 in the multimode OAM beam with topological charge l =1,-2 can be increased from 0.11 to 0.42,and the relative power of topological charge l =1 can be increased from 0.32 to 0.44.The above results show that the OAM wavefront correction for low-crosstalk modes can be better achieved by the method adopted in this paper. |