| Free-Space Optical(FSO) communication has high communication capacity,strong anti-electromagnetic interference capability,and good confidentiality.In the FSO communication system,it is of great significance to improve the data transmission capacity,analyze and reduce the interference caused by atmospheric turbulence to information transmission,and optimize the structure of each part of the communication system.The research in the dissertation focuses on the application of Orbital Angular Momentum(OAM)beams in FSO communication systems.The main research contents are as follows:In the first part,the model of computational propagation of OAM beams through the Cassegrain antenna in the turbulent atmosphere is built.The analytic functions of the diffraction field of the Laguerre-Gaussian(LG)beam passing through the Cassegrain antenna,the average field distribution of the LG beam passing through the Cassegrain antenna and propagation in the turbulence atmosphere,and the OAM modes transmission probability spectrum are accurately derived.Using the Cassegrain antenna to transmit the LG beam can effectively reduce the crosstalk among different OAM modes after the beam passes through the atmospheric turbulence is proposed.The secondary mirror occlusion problem of the Cassegrain antenna is solved by utilizing the characteristic of the annular distribution of the OAM beam,and the transmission efficiency of the Cassegrain antenna is effectively improved.In the second part,the two-dimensional(2-D) and three-dimensional(3-D)ray tracing results of the parallel beam passing through the Cassegrain antenna are calculated using vector reflection theory.The propagation trajectory of the Bessel-Gaussian(BG)beam in free space after passing through the Cassegrain antenna and the crosstalk among different OAM modes caused by turbulence atmosphere is analyzed.The parameters of the optical system and the half-cone angles of the BG beam can control the transmission trajectory of the beam.After the BG beam passes through the collimating optical system,the beam occurs the focusing phenomenon.And when the BG beam passes through the focusing optical system,the focusing phenomenon and secondary focusing phenomenon will occur.The BG beam with focusing properties can effectively reduce the crosstalk among different OAM modes.In the third part,a method to optimize fibers'coupling efficiency in the turbulent atmosphere by using a focusing lens system is proposed.The coupling efficiency of the Gaussian vortex beam into the single-mode fiber and parabolic fiber in the turbulence atmosphere are investigated.The effects of the focusing lens system focal length on the fiber coupling efficiency are studied.It is an effective and straightforward method to improve the coupling efficiency under different turbulent atmospheric intensities or specific propagation distances by controlling the focal length of the lens.In the fourth part,two novel fiber structures are designed for FSO communication systems.An asymmetric large-mode field,bend-resistant photonic crystal fiber(PCF)with hybrid cladding,and new hollow-core photonic bandgap fiber(HC-PBGF)with two fiber background materials,which can support OAM modes are designed.Using the finite-element method,the bending loss and effective mode area(EMA)of the novel PCF fiber proposed in this dissertation are studied under different fiber structure parameters.The results show that the fundamental mode bending loss of the fiber is 0.0113 d B/m with the EMA larger than 1000?m2 when the bending radius is 10 cm,and the bending orientation angle isą100°.This fiber can be applied to high-power fiber lasers.In addition,a novel HC-PBGF is designed by adopting two kinds of fiber background materials and arranging annular air holes in the center of the fiber.The confinement loss and effective refractive index of the fiber are studied in the wavelength range of 1.3?m?2.0?m.The research results show that the HC-PBGF can support at least 48 OAM modes and the fiber confinement loss is less than 10-7 d B/m.This new fiber can applications to increasing communication capacity.The research in the dissertation builds a computational model for the OAM beam passing through the Cassegrain antenna and propagating in atmospheric turbulence.Using the field distribution characteristic of the OAM beam can solve the secondary mirror occlusion problem of the Cassegrain antenna and improve the transmission efficiency effectively.The transmission characteristics of the OAM beam propagation in atmospheric turbulence are researched,and the method to reduce the crosstalk among different OAM modes is proposed.A practical scheme to improve the fiber coupling efficiency is proposed.In addition,two new optical fibers are designed in the dissertation. |
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