| With the development of science and society,people put forward higher requirements for data transmission capacity.In order to meet the demand of increasing data transmission capacity,various multiplexing technologies have been widely used in optical fiber communication systems.Among them,orbital angular momentum?OAM?mode multiplexing is a new way of space division multiplexing?SDM?technology.Different OAM modes have orthogonality and infinite topological charges in theory,so they can improve the transmission capacity of optical fiber communication system greatly.In order to realize the long-distance transmission of OAM modes,high-performance vortex fiber amplifiers play a very important role in the optical fiber communication system.The paper studies the performance of vortex fiber amplifier as for the aspects of increasing the number of OAM modes that transmitting stably,reducing the differential mode gain?DMG?and the noise figure?NF?,and ensuring the quality of OAM modes in long-distance transmission.The main contents are as follows:?1?The OAM mode theory and the theoretical model of erbium-doped fiber amplifier?EDFA?are introduced.The calculation formula of the noise figure of OAM-EDFA is derived.?2?A ring-core fiber that erbium ions are doped partially is designed,the gain of OAM modes,the differential mode gain,and the noise figure as a function of fiber length,pump power,input power of signal,erbium-doped distribution and concentration is studied combined with the rate equation.The results show that the optical fiber can support 22 OAM modes transmit stably,and the effective index difference of different OAM modes is greater than 10-4 at C-band?1530?1565 nm?.By optimizing the above parameters,the performance of the fiber amplifier is improved,that the gain of all OAM modes is greater than 20.39 d B,the differential mode gain is less than 0.03 d B and the noise figure is less than 3.8 d B in the whole C-band.?3?The influence of the bend of the fiber on the number of OAM modes,and the relationship between the loss coefficient of OAM modes and the bending radius or the signal wavelength is studied based on the OAM fiber designed above.Combined with the rate equation,the influence of the fiber length,the bending radius and the signal wavelength on the gain characteristics of the amplifier is studied when the ring-core fiber is bending.The results show that only 10 OAM modes?|l|=1,2,3?are supported for stable transmission when the bending radius of the fiber is 5 mm.The gain of all OAM modes of l?3 is greater than 23.4 d B,the differential mode gain is less than0.2 d B,and the noise figure is less than 5.6 d B at C band when the fiber length is 8.5 m and the bending radius is 5 mm.The vortex fiber amplifier proposed has lower differential mode gain and noise figure compared with the research results reported so far.In this paper,the gain characteristics of the vortex fiber in bending state are studied,which has a certain reference value for the practical application of the vortex fiber. |
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