Study On Modulation And Detection Of Optical Communication Using Spatial Singular Beam

Nowadays with the popularity of the Internet,the application of big data,Internet of things and other technologies,the communication capacity requirement of the whole society was increasing fast.Existing modulation technologies are mainly based on linear momentum,but with consume of spectrum resources,the improvement of communication capacity are becoming more and more difficult.So find new dimensions to develop new modulating technologies has become a urgent problem for researchers in communication field.The spatial singular beam refers to a structured beam with dark hollow distribution,just like the orbital angular momentum(OAM)beam and the cylindrical vector beam(CVB).As phase or polarization is uncertain in the singular beam light field center,we called them the phase singularities or polarization singularities,phase or polarization direction around the singular point has topological structure and continue changed.We use topological charge to represent the change degree,and different topological charge corresponding to different beam modes.Moreover,the topological charge can take infinite values,and the singular beam with different topological charges is orthogonal to each other.Therefore make the topological charge of singular beam as a new degree of freedom to loading information can greatly improve the communication capacity of existing optical communication system.The modulation and demodulation of information are the foundation of data communication,the information modulation can be realized through high speed key control of the spatial singular beam mode,and we can realize demodulation of received information by detecting and decoding the mode at the receiving end.Therefore,how to control and detect the mode of singular beam mode is the key problem that we must to solve.At present,the detection of OAM beam is usually achieved by converting the OAM beam into ordinary beam,but the demodulation part of the detection technology is complex and inefficient.And the method of generating CVB is mainly using liquid crystal to transform the beam with uniform spatial distribution of polarization state to uneven spatial distribution,this method also has the shortcomings of complex system structure,low stability and high cost.Because of the complexity and low efficiency of the singular beam modulation and detection,using singular beam in the optical communication is seriously restricted.In this paper,for the difficulty of singular beam using information modulation/demodulation,we studied the mode control and detect technologies of OAM and CVB start with the effective generation of singular beam and studies the feasibility of its application in modulation/demodulation of communication.The main contents of this paper including:(1)A CVB polarization order control method based on two dimensional dielectric metasurface was proposed and realized.Using metasurface to generate CVB is efficient,but it can only generate single polarization order CVB because of it's fixed structure.In this paper,we made CVB polarization order's control by cascading metasurface and half wave-plate(HWP)to realize CVB polarization order's addition and subtraction operation.(2)An OAM mode recognition scheme based on coherent detection was proposed and applied to OAM beam modulation/demodulation communication.The most important technology of applying OAM beam to communication is the modulation and demodulation technology of OAM's mode.In this paper,an OAM detection method based on coherent detection was proposed,we realized OAM's detection by detecting the energy of the mixed beam after the signal beam coherent with the reference beam.So we can realize high speed detection of the OAM mode.