Design Of Fiber For Orbital Angular Momentum And Its Brillouin Scattering Characteristics

With the vigorous development of the communication industry,the demand for channel capacity in communication system is increasing.Multiplexing and photonic crystal fiber（PCF）have been widely used to improve communication capacity.Because orbital angular momentum（OAM）modes have infinite topological charge in theory and different OAM modes are orthogonal to each other,the utilization of frequency spectrum and the capacity of communication system are improved greatly by multiplexing orbital angular momentum at the same bandwidth.In order to meet the application requirements of long-distance and large-capacity optical communication,it is necessary to conduct a comprehensive study on the synchronous transmission and amplification of various orbital angular momentum modes.Therefore,a systematic study is carried out around orbital angular momentum in this paper,the main content is as follows:（1）A novel structure of photonic crystal fiber for supporting 84 orbital angular momentum modes is designed.The dual-guided PCF is achieved through the nested design of circular air holes and rectangular air holes.The properties of the proposed PCF are systematically analyzed through the finite element method.The results show that the proposed PCF has great transmission characteristics and the influence of bending on its performance is little,the transmission characteristics are relatively stable.（2）Combining orbital angular momentum with nonlinear optical effect,the Brillouin gain spectrum,Brillouin gain coefficient and signal gain coefficient suitable for the amplification of orbital angular momentum mode are derived.It is shown that the topological charges carried by the orbital angular momentum mode have a certain influence on the Brillouin gain spectrum for the stimulated Brillouin scattering effect.（3）A photonic crystal fiber amplifier for OAM modes based on stimulated Brillouin amplification is designed and optimized in this paper.The transmission properties of the amplifier are systematically analyzed by using the finite element method in the C-band.The results show that this photonic crystal fiber amplifier can support the transmission and amplification of 66 orbital angular momentum modes,and all values of the purity of the orbital angular momentum modes supported by this amplifier are higher than 99.4%.By systematically analyzing the Brillouin gain spectra of orbital angular momentum modes with different topological charges,it is found that they all have high Brillouin gain coefficients(>7×10^-9 m/W),thus higher signal gain can be obtained.