Research On OAM Mode Measurement Method Of Vortex Beam Space Communication

Since Allen discovered orbital angular momentum,scholars generally believe that using the vortex beam with orbital angular momentum as a new type of information carrier beam in optical communication can significantly improve the transmission rate and transmission capacity of the system,and is expected to break through the capacity limit of traditional communication technology.In practical application,OAM mode detection of vortex beam is the most basic technology.In this thesis,theoretical simulation and experimental research on OAM beam generation and mode detection are carried out:(1)Generation of vortex beam and its precise radius control.The corresponding diffraction phase diagram is loaded by the spatial light modulator,and the corresponding experimental links are designed and built to generate different modes of vortex beams.Through the use of a single lens,the radius of the generated vortex beam can be adjusted to avoid the problem of incomplete reception due to the large radius of the vortex beam.By comparing the simulation results with the experimental results,it is proved that the proposed method can be used to fully receive the OAM beam in the OAM detection system,and further simplify the experimental link of OAM mode detection.(2)The design of chessboard gradient grating(CGCG)and the study of OAM mode detection characteristics.In this thesis,a chessboard gradient amplitude grating is designed based on the gradient characteristics of chessboard grating and grating.Combining with the theory of grating diffraction and double beam interference,an OAM mode detection method is designed.Only by judging whether the distribution of central light intensity can detect OAM mode,avoids the problem of identifying OAM mode by identifying the fringes in diffraction chart.In this thesis,a chessboard gradient binary phase grating is designed by combining the characteristics of chessboard gradient amplitude grating and blazed grating.The simulation results show that the phase grating improves the diffraction efficiency compared with the amplitude grating,and transfers the light power to the effective information of OAM pattern in far-field diffraction.(3)The design of cubic phase grating(CPG)and its OAM mode detection characteristics are studied.In this thesis,a cubic phase grating is designed according to the phase characteristics of Airy beam and the structure function of the grating,and an OAM mode measurement method is designed according to(1).The simulation results show that the diffraction pattern of OAM beam diffracted by cubic phase grating contains one-to-one information corresponding to the OAM mode of incident beam.In this thesis,an experimental platform is built to verify the simulation results,which proves the effectiveness of CPG model for OAM pattern recognition.Based on the designed CPG model,this thesis proposes and proves the quantitative relationship between the waist,grating parameters and off-axis distance,and the adaptive adjustment of each parameter in OAM pattern recognition.In a word,aiming at the problem of OAM mode detection in free space optical communication,combined with grating diffraction method and double optical path interference method,a chessboard gradual change amplitude grating is proposed for OAM mode detection.The simulation results show that the grating has good effect in OAM mode detection.Combined with the characteristics of blazed grating,a chessboard gradual change binary phase grating is proposed.The simulation results show that the grating has high diffraction efficiency in OAM detection.By combining the phase characteristics of Airy beam and grating structure function,a cubic phase grating is proposed,and the corresponding experimental link is built.At the same time,the use of single lens not only makes the beam radius of OAM controllable,but also further simplifies the experimental link of OAM mode detection.The simulation and experimental results show that the cubic phase grating has a good effect in OAM mode detection.Finally,the quantitative relationship among grating parameters,off-axis distance and waist radius in OAM mode detection based on cubic phase grating is proved by simulation and experiment,which provides a new idea for adaptive adjustment of OAM mode detection experiment.