Modal Characteristics Of OAM In A Modified Ring Fiber

Orbital Angular Momentum(OAM) based mode division multiplexing is an effective approach to increase the transmission capacity of optical fibers. Therefore, OAM multiplexing scheme is presented applied to optical communications. Theoretically, due to OAM has an infinite number of different quantum states orthogonal basis. It can provide a novel multiplexing mechanism. At the same time, it can greatly enhance the capacity of optical communication after a high-dimensional coding. In addition, OAM has been widely used in other areas. Research in the field of optical communications will promote the development of other aspects of OAM.Currently, there are many problems stable transmission OAM mode in the fiber. In this paper, we study in detail two of them. First, how to stabilize the OAM transmission in the optical fiber. Second, how to effectively motivate OAM mode in the optical fiber. For the first question, we proposed a modified ring fiber by introducing a high refractive index layer. Then with the effect of mode reorganization, modal index differences between the orbital angular momentum(OAM) modes are enlarged, thereby lifting the degeneracy among OAM modes. Parameters of the modified ring fiber are designed properly by numerical simulations and modal index difference of 10-3 are achieved, which helps effectively minimizing the coupling between OAM modes and thus improving the transmission property of ring fiber. For the second question, we presented an effective method to generate VBs by using a helical fiber Bragg grating(H-FBG). We derived the H-FBG coupled mode equation, and then reveal the excitation mechanism by using H-FBG to realize OAM mode. Through the preliminary numerical simulation, the results show that the structure can excite high purity OAM mode.