Encoding/Decoding Of Composite Vortex Beams With Spaced Orbital Angular Momentum

An key foundation for optical communication is the encoding of digital signals using the Orbital Angular Momentum(OAM)dimension of vortex beam.This encoding technique can increase the capacity of the channel,the security of communication systems,and the efficiency of encoding digital signals.One of the research areas for OAM encoded communication technology is how vortex beam can efficiently reliably and increase the data transmission rate.In this research,intensity encoding/decoding and grating diffraction properties of composite vortex beam are investigated.The following is the precise substance of the work:(1)A composite vortex beam encoding method with an OAM interval is studied.The Laguerre-Gaussian(LG)vortex beams with fixed OAM modes and radial modes were co-axially superimposed with eight OAM modes(l=±3,±5,±7,±9)and four LG vortex beams with radial modes(p=0,1,2,3).32 distinct types of composite vortex beams’ intensity profiles were generated.It is found that the intensity distribution of the composite vortex beam presents bright multi-ring spots,and with the increase of the OAM mode and radial mode of the superimposed vortex beam,the radius and number of each ring of the intensity map also increase gradually.(2)The encoding and feasibility verification methods of composite vortex beam are presented.A 5-bit binary sequence(00000～11111)was used to encode 32 sets of composite vortex beam intensity distributions,in order to simulate the influence of atmospheric turbulence during transmission,the encoded composite vortex beam intensity distributions were superimposed with Gaussian white noise with mean 0 and variance 0.1 to propagate 600 times in free space.The receiver can determine if the image transmitted by the sender is effectively detected based on the correlation degree of the intensity map of the two composite vortex beams sent and received,and sent and received,and the correlation coefficient is utilized as the judgment foundation.The results show that the minimum probability of successful detection at the receiver is 96.3%,which verproves the feasibility of the proposed encode.(3)The OAM modes and radial modes were detected and decoded based on the far-field diffraction spot of the gradually-changing-period grating.According to the mapping relationship proposed in this paper,32 sets of composite vortex beams were converted into 32 sets of single LG vortex beams,which were then irradiated on the gradually-changing-period gratings along the x and y axes in turn.The OAM mode and radial mode values of incident LG vortex beams can be detected and identified by the+1 or-1 level node-line number,direction and 0 level ring number of the far-field diffraction pattern of the periodic gradient grating,and the corresponding complex vortex beams can be deduced,and then decoded into the initial propagation information sequence.This work provides a theoretical basis for the application of vortex beam OAM modes and radial modes in the field of information encoding/decoding.Among them,the combination of the gradually-changing-period grating and the single/composite vortex beam mapping relationship provides an effective method to detect the OAM modes and radial modes of the composite vortex beam.