| In recent years,with the rapid development of various information technologies,optical communication networks are facing problems such as spectrum utilization and system capacity approaching the Shannon limit.As a new type of multiplexing technology,orbital angular momentum beam has attracted the attention of many researchers.Optical fiber supporting OAM mode can view these modes as independent channels and transmit information in one fiber in parallel,which greatly improves the transmission capacity of optical fiber communication.At the same time,due to the mutually orthogonal characteristics between the OAM modes,the mutual interference between the modes is effectively avoided in the transmission process,so the optical fiber has an efficient information transmission quality.However,due to the special optical characteristics,the OAM mode is converted over a distance in a normal fiber and eventually coupled to the LP mode.Therefore,the fiber supporting OAM mode transmission needs to be specially designed.The effective refractive index difference between the vector modes of the fiber needs to be larger than 10-4 to ensure the stable transmission of the OAM mode.On the other hand,optical communication parameters such as dispersion and confinement loss are also need to be considered.Therefore,in view of the problems faced by current OAM fiber communication,this paper designs several different types of OAM micro-structured fibers.By optimizing the specific parameters of the fiber structure,the proposed fiber can support stable transmission of multiple OAM modes,and each mode has lower dispersion and confinement loss.The main research are as follows:1.A high-index ring-core microstructured fiber which supports 22 OAM mode transmissions is designed.The effective refractive index difference between each vector mode is larger than 10-4,which satisfies the transmission condition of the OAM mode.In the wavelength range of 1500-1600 nm,by optimizing the physical parameters of the air holes inside the cladding,the modal dispersion of the supported OAM mode can be controlled within the range of 0-50 ps.nm-1.km-1,and the dispersion variation of each mode is lower than12.8ps.nm-1.km-1.In addition,the mode confinement loss of each supported OAM mode is less than 1.3549×10-9 dB/m at 1550 nm.By adjusting the parameters of the fiber cladding air hole and the core air hole,the influence of the change of the fiber structure parameters on the transmission characteristics of the OAM mode is further analyzed in detail.The proposed fiber has the advantages of low flat dispersion,low confinement loss,etc.,and has potential application value in the field of OAM communication.2.A pure silica microstructure fiber,which can support 30 orbital angular momentum?OAM?modes transmission and possess relatively flat and low dispersion is proposed.The OAM modes can be well-separated due to the large effective refractive index difference(above10-4)between the eigenmodes.The only material of the designed fiber is silica.The dispersion of each OAM mode is controlled in the range of 50-100 ps·nm-1·km-1 and the total dispersion variation is below 10 ps·nm-1·km-11 from 1500 nm to 1600 nm.Moreover,the confinement loss of each OAM mode is below 8.17×10-1010 dB/m at 1550 nm,and the nonlinear coefficients are less than 0.71W-1/km for all modes at 1550 nm.With all these good features,this proposed optical fiber is promising to be applied in fiber-based OAM communication systems. |
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