The Research On The Generation And Measurement Of Vortex Beams With Orbital Angular Momentum In Free Space Optical Communication

With the development of optical communication,the basic dimensions of light waves such as time,wavelength,amplitude,phase,polarization and other resources have gradually reached the limit.In order to break through the bottleneck of optical communication technology,and achieve high-capacity and high-rate stable transmission of optical communication,a new optical communication technology has been proposed to use the vortex beams carrying orbital angular momentum(OAM)as an information carrier.With this new dimension,the communication capacity of the system can be effectively increased,thereby breaking the capacity limit of the prior technology.In recent years,not only in the field of optical communication,but also the research and application of vortex beams have attracted widespread attention in the fields of optical manipulation,biomedicine and quantum information technology and so on.As we know,the generation of vertex beams is the precondition for its practical application,and the OAM mode measurement is the key for the multimode multiplexing and demultiplexing technology in optical communication.However,there are some problems at present,such as the equipment technology and the phase mask design method,that make the high-order vortex beams difficult to generate and accurately detect.Based on the above problems,an experimental scheme of stably generating high-order vortex beams can be designed by using phase diffraction equipment in this thesis.On this basis,in order to further solve the OAM mode measurement problem of the vortex beams,we design two experimental schemes to measure the OAM mode.One of them is to use superimposed interference of the conjugate vortex beams for measuring the OAM mode of the vortex beams.Although this scheme can measure the OAM mode of the vortex beam,it is difficult to obtain the sign of its topology charge.The other is an experimental scheme for measuring the OAM mode by grating diffraction,which can capture the OAM mode of the vortex beams more accurately than the interference measurement scheme.We carry out relevant theoretical and experimental researches on the generation and the mode measurement of vortex beams.The main work is as follows:(1)The study on the generation method of vortex beams.By analyzing the structure function of vortex beams,the structure characteristics can be studied by simulating the field distribution.A phase mask with two-dimensional modulation characteristics can be developed based on the phase distribution of vertex beams.The Fresnel diffraction can be used to simulate the field reconstruction experiment,in which the Gaussian beam can be modulated by the phase mask.In this way,the experiment of generating vortex beams can be designed.By using an optical diffraction equipment to load the phase mask,the light field distribution of the incident Gaussian beam can be controlled;combining with the fast response speed and wide spectral range of spatial light modulator,the vortex beams can be efficiently and flexibly generated.The simulation and experimental results show that the proposed scheme has the superior performance of stably generating high-order vortex beams,and the OAM mode value of the vortex beams obtained in the experiment can be up to 100.(2)The scheme for measuring OAM mode by the interference between conjugate vortex beams.By analyzing the light field function of the vortex beams and combining the principle of double light interference,the light intensity distribution of the superposed interference of the double vortex beams is deduced theoretically.In the case of mode conjugate,the intensity of vortex beams after interference can have a periodic distribution characteristic.According to that,the scheme of using conjugate vortex beams superposition interference for measuring the OAM mode can be designed.Through numerical simulation,it is found that the distribution of the spot after the conjugate vortex beam superimposed interference and its topological charge carry a certain regularity,and the OAM mode of vortex beams can be detected by analyzing the numerical relationship between the number of spots and the topological charge of the vortex beams.In order to further verify the feasibility of the scheme,we build an experimental platform base on the designed experimental scheme,and finally through CCD capture the spot distribution in the interference patterns.The experimental and simulation results are compared and analyzed to verify that the interference between conjugate vortex beams can achieve the OAM mode measurement.(3)The grating design and its diffraction measurement scheme.In order to accurately capture the OAM mode of the vortex beams,a new annual grating will be developed to measure the high-order topological charge in this thesis.By using far-field diffraction,the diffraction phenomenon can be achieved through the designed grating.In the way,the scheme of measuring the OAM mode can be designed.Numerical and experimental results show that a far-field diffraction pattern is formed by collimating the vortex beams onto the new annual grating.And it also shows that the variation of the patterns accurately corresponds to the OAM mode of vortex beams.As a result,the number of the topological charge measured in the experiment can be up to ±30.The grating design and its diffraction measurement scheme not only solve the shortage of the scheme for measuring OAM mode by the interference,but also provide a new approach to measurement applications of vortex beams.