Research And Design Of The Fiber For Generating Multiple Orbital Angular Momentum Modes

Orbital angular momentum(OAM)multiplexing technology provides a new multiplexing form for free-space optical communication,which can be used to improve the channel capacity of communication system.Studying the generation and transmission of vortex beams carrying OAM is a key issue in OAM multiplexed communication system.In many generation methods,the vortex beam generated by the fiber has high phase purity.It is particulary important to design fiber that can generate multiple OAM modes and support transmission.In this paper,by optimizing the circular photonic crystal fiber(PCF),a fiber structure that can generate and support multiple OAM modes is obtained,and the performance of the fiber is studied and analyzed.The specific work is as follows:1.The theoretical formula of mode in the fiber is derived.Firstly,the vector mode solution and the scalar mode solution in the fiber are solved.Secondly,the light intensity and polarization distribution of vector mode and scalar mode are numerically simulated.Finally,the relationship between vector mode and OAM mode in the fiber is studied,which provides a theoretical basis for the optimization of fiber structure.2.The fiber structure is designed.The structure of PCF with a circular air hole cladding is optimized and improved.A rectangular air hole is introduced into the fiber cladding.By analyzing the effects of the fiber parameters on the number of OAM modes and the effective refractive index difference of the modes,a PCF that can generate and support multiple OAM modes is obtained.3.The fiber characteristics are analyzed.Through the simulation,the effective refractive index difference of the vector mode is obtained.The normalized electric field intensity distribution of the mode and the intensity distribution in the z direction are studied.It is proved that PCF obtained in this paper can generate and support 50 OAM modes at the wavelength range of 1.15?2.0 ?m.Finally,the optical characteristics such as dispersion,confinement loss and nonlinear coefficient are analyzed,and compared with the corresponding optical characteristics of PCF in other literatures.The results show that:The fiber structure can generate and support 50 OAM modes transmission at the wavelength range of 1.15?2.0?m.And the fiber has good performance,such as the effective refractive index difference between all vector modes are greater than 1×10-4,and the mode dispersion is flatter in the 850nm wavelength range.Also,the confinement loss is smaller,the larger mode field area and the smaller the nonlinear coefficient.These characteristics can prove that the fiber has better performance,which is conducive to the stable transmission of OAM mode in the fiber.