Design Of Photonic Crystal Fiber For Orbital Angular Momentum Mode And Study On Its Magnetic Tunability

The orthogonality between the orbital angular momentum?OAM?modes and the theoretical infiniteness of its order can make it play an important role in future space division multiplexing?SDM?communication systems and greatly increase the capacity of the communication system.Although there is a special superposition relationship between the OAM mode and the intrinsic modes of the traditional fiber,there is a phase singularity and the center intensity of the OAM light field is zero.Therefore,the fiber supporting OAM mode needs to be specially designed.This paper focuses on the research on the design of photonic crystal fiber?PCF?supporting OAM mode,gives a PCF structure supporting 54 OAM modes,and analyzes its transmission characteristics in detail.Then,this PCF is combined with new material magnetic fluid?MF?and we analyze the magnetic tunability of PCF's transmission performance.The content and innovations of this article are as follows:1.Based on the photon's volatility,the ratio between the angular momentum and energy of the OAM beam and the mode characteristics of the OAM mode are described.The transmission characteristics of the OAM fiber are summarized,including effective refractive index,dispersion,confinement loss?CL?and nonlinear coefficient,etc.2.A new PCF structure supporting OAM mode transmission is proposed,in which the ring core is sandwiched between the inner cladding and the outer cladding.The inner cladding is a larger air hole,and the outer cladding is composed of periodically arranged air holes and basic material silica.The radii of the air holes of the outer cladding and the spacing between each out layer gradually increase in the radial direction.The advantage of this structure is that it can increase the refractive index difference between the transmission layer and the cladding,can effectively limit the light waves in the ring core,that is,reduce the confinement loss of the PCF,and can increase the effective refractive index difference between degenerate modes.The simulation using Comsol software shows that the PCF can stably support the transmission of 54 OAM modes in the wavelength range of 1200nm to 1850nm.Under linearity of the dispersion curves>0.986 and the error of<4.20 ps/?7?nm?km?8?,the PCF possesses a flat dispersion curve with a dispersion slope of less than 0.2p s/?7?nm²?km?8?.The confinement loss of the PCF is in the order of10^-11?25?10^-10dB/m,which is lower than the confinement loss of current PCF.At the wavelength of 1550nm,the nonlinear coefficient of the PCF is approximately in the range of 1.0?25?1.6W^-1/km,which is slightly more than that of the standard single mode fibre.3.The magnetic tunability of OAM mode was studied by filling magnetic fluid into the inner cladding of the PCF and adjusting the refractive index of the inner cladding by applying a magnetic field.The research results show that in the range of300Oe applied magnetic field strength,as the magnetic field strength increases,the effective refractive index difference between degenerate modes decreases,and the PCF's confinement loss becomes larger(about10^-5?25?10^-4dB/m).The range of the nonlinear coefficient is from 1.4W^-1/kmto 0.96W^-1/km.When the magnetic field strength is150Oe,at the wavelength of 1500 nm,the waveguide dispersion of the OAM mode is approximately in the range of 20ps/?7?nm?km?8?to 250 ps/?7?nm?km?8?.