Research On Orbital Angular Momentum Detection Algorithm And Communication Application Of Multiplexing Vortex Beams

The development of information and communication technology is considered to be the first major scientific and technological revolution since the 21st century.With the laying of submarine optical fiber,the launch of communication satellites,and the popularity of the Internet,by the beginning of this century,almost the whole world is wrapped and connected by the invisible network called‘Internet’,and our life has changed dramatically.With the progress of science and technology,the5th-Generation Mobile Communication Technology（5G）,Big Data,and Internet of Vehicles have become the most popular new terms.These new technologies put forward further requirements for the communication capacity of the communication system.Vortex beams are special beams with spiral wavefronts carrying orbital angular momentum（OAM）.Because OAM beam has infinite mode states in theory and different eigenstates are orthogonal to each other,it could be used as a new dimension to further improve the capacity of communication systems.In order to apply vortex beam in the communication system,it is inevitable to determine the mode of vortex beam.This thesis aims to measure the modes of vortex beams.By designing a special phase plate,which is based on pure phase modulation technology and could be uploaded onto the core experimental device spatial light modulator,the mode detection of large-capacity multiplexed vortex beams is realized.Furthermore,the problems of the measurement of vortex beams under limited aperture size and the measurement of fractional-order vortex beams are also solved.The main research contents and achievements are as follows:Two algorithms are proposed to measure high-capacity multiplexing vortex beams,which solve the problems in the current OAM mode measurement scheme including the narrow OAM mode measurement range,the destruction of the original signal and the ineffectiveness of measurement of multiplexed vortex beams,and realize the instantaneous mode detection of wide-range multi-mode vortex beams.In the first scheme,a three-dimensional multifocal vortex measurement array generated under tight focusing condition is used to realize the high-capacity OAM mode measurement of vortex beams.The vortex measurement array contains 4 focal planes and 64 vortex focal spots,which can be used to measure the vortex beams with topological charges（TCs）from-32 to 32 at the same time.In addition,by using the linear polarization response characteristics of the spatial light modulator,a tight focusing three-dimensional multifocal polarization vortex measurement array generated by employing two spatial light modulators is proposed to measure the x-polarization and y-polarization vortex beams with TCs from-16 to 16.The second scheme generates a two-dimensional 64-focus vortex array based on a dense phase stitching algorithm.The dense phase stitching algorithm can integrate 4 phase masks that can generate four 16-focus arrays respectively and form a stitched phase mask that can generate a 64-focus vortex array.The stitched phase mask can also be used to measure the single or multiplexed vortex beams with TCs from-32 to 32.In addition,the dense phase stitching algorithm can measure vortex beams without mode crosstalk,which is an ideal scheme to increase the measurement range of vortex beams.An OAM-based optical communication scheme suitable for multi-ring OAM fibers is proposed,which solves the problem that the vortex radius does not match the refractive index distribution of the fiber core when the vortex beam is coupled into the fiber and realizes the integrated modulation of the spatial position,mode,and radius of the vortex beam at the transmitter.In order to efficiently couple the vortex beams into the multi-ring OAM fiber,a scheme that can generate perfect vortex arrays is introduced.The position,mode,and radius of each perfect vortex in this array can be flexibly controlled to match various multi-ring OAM fibers.At the receiving end,a phase plate is designed to measure and demodulate the multiplexed perfect vortex beams.In addition,an efficient OAM coding scheme based on multiplexing vortices is proposed to increase the data capacity from log₂（L）bits to L bits as there are L available OAM states in the fiber.A scheme of measuring vortex mode based on synthetic aperture receivers is proposed,which solves the problem that it is difficult to measure the mode of divergent vortex beam transmitted through long-distance free space,and realizes the demultiplex of vortex beams under limited aperture size.For the vortex beam propagating in free space,its radius will increase with the increase of propagation distance,which may make it difficult to effectively receive and detect the mode of vortex beam at the receiving end.Here,inspired by the concept of synthetic aperture radar,we propose a scheme of using synthetic aperture receivers that consist of two partial aperture receivers with different sizes to measure vortex beams.By using synthetic aperture receivers,which could evenly distribute the crosstalk on the adjacent modes caused by incomplete receiving aperture in several modes,so as to reduce the system crosstalk,the problem of measuring the mode of vortex beam propagating over a long distance in free space OAM-based communication can be effectively solved.An algorithm is proposed to calculate the TC of fractional vortex by using the intensities of the multiplexed integer vortex beams,which solves the problems of complex device and low accuracy of measurement results in the traditional scheme of measuring fractional vortex,and realizes the accurate measurement of TC of the single-mode fractional vortex and TCs of the multiplexed fractional vortices with integer mode interval.Fractional vortex can be equivalent to multiplexed integer vortex beams.By measuring the energy of each vortex beam in the multiplexed integer vortex beams,the average OAM of fractional vortex can be calculated.Based on this principle,a scheme using a two-dimensional integer vortex array to measure the mode of fractional vortex is proposed.In addition,considering that the average OAM of fractional vortex is not equal to its TC,a correction function is proposed.The scheme uses the energies of multiple integer vortex modes to accurately measure the TC of a single-mode fractional vortex.Thereafter,an optimized fractional vortex evaluation scheme is established,and the accurate calculation of fractional vortex mode is realized by using the energies of two integer vortex beams.Based on the optimized fractional vortex evaluation scheme,the TCs of multiplexed fractional vortices with integer mode interval are further detected.