發(fā)布時間：2018-05-03 09:03

  本文選題：自由空間光通信 + 大氣湍流　； 參考：《中南民族大學》2015年碩士論文

【摘要】：近年來,軌道角動量已成為自由空間光通信(free-space-optical-communication,FSO)領域內的研究熱點,并取得了較快進展。利用OAM光束進行信息交換、復用,并以此為基礎,與其他傳統復用技術相結合,可大幅度提高光通信數據傳輸容量和光譜效率。然而OAM光束在大氣空間傳播時,由于湍流的作用,會引起一系列湍流效應,如誤碼率增加、光強波動以及FSO系統信道容量降低。因為任意拓撲荷軌道角動量的光束都可以通過具有螺旋相位波前的拉蓋爾—高斯光束(LG)線性組合表示,因此對此類光束的研究可以歸為對LG光束的研究。本論文重點研究LG光束在大氣的傳播過程中特性變化情況,分析有無湍流的性能差異。首先,基于傅里葉光學理論推導出渦旋光束的菲涅爾衍射積分的角譜形式;然后采用分諧波的方法改進Monte-Carlo大氣相位屏;據此利用MATLAB工具對LG在湍流大氣中的傳播進行建模分析,獲得其傳播特性。通過數值分析討論光束光強和相位分布與湍流、OAM值、距離之間的關系,利用桶中功率(Power In Bucket,簡稱PIB)評估LG光束與高斯光束、點光源三者的抗湍流能力,研究結果表明,LG光束在大氣湍流中的傳播特性與無湍流條件相比差異較大:在湍流中傳播時,當增加LG光束的傳播距離,會降低光束的聚焦能力,光束有明顯擴散;光束本身特有環(huán)狀光強分布和螺旋相位分布受湍流作用而發(fā)生嚴重畸變,畸變的程度與OAM值的大小、湍流強度、傳播距離均緊密相關。最后,通過對比PIB曲線,還發(fā)現攜帶不同OAM值的LG光束的抗湍流能力具有明顯的差異。本文利用傅里葉光學,結合“光束分步傳播法”和相位屏法,設計了一套現實可行的MATLAB理論分析與數值仿真方法,并推導出具體角譜形式的光束衍射模型及其對應的抽樣限制條件,從而獲取最精確的仿真結果;明確了LG光束與湍流的關系。這些方法和結論對較全面地了解OAM光束大氣通信特性具有重要意義,可為OAM光通信系統仿真及其應用領域的深入研究提供思路和理論參考。
[Abstract]:In recent years orbital angular momentum has become a research hotspot in free-space-optical-communication FSOs and has made rapid progress. The data transmission capacity and spectral efficiency of optical communication can be greatly improved by using OAM beams for information exchange and multiplexing and combining with other traditional multiplexing techniques. However, when the OAM beam propagates in atmospheric space, it will cause a series of turbulent effects due to turbulence, such as the increase of bit error rate, the fluctuation of light intensity and the decrease of channel capacity in FSO system. Since beams with arbitrary topological charged orbital angular momentum can be expressed by a linear combination of Laguerre- beams with helical phase wavefront, the study of such beams can be classified as the study of LG beams. In this paper, the characteristics of LG beams propagating in the atmosphere are studied, and the performance difference of turbulence is analyzed. Firstly, based on Fourier optics theory, the angular spectrum of Fresnel diffraction integral of vortex beam is derived, then the phase screen of Monte-Carlo atmosphere is improved by subharmonic method. Based on this, LG propagation in turbulent atmosphere is modeled and analyzed by using MATLAB tool. To obtain its propagation characteristics. The relationship between the intensity and phase distribution of the beam and the value and distance of the turbulence is discussed. The anti-turbulence ability of LG beam, Gao Si beam and point light source is evaluated by using the power in bucket Bucket. The results show that the propagation characteristics of LG beams in atmospheric turbulence are different from those without turbulence. When the propagation distance of LG beams in turbulence is increased, the focusing ability of LG beams will be reduced and the beam diffusion will be obvious. The intensity distribution and helical phase distribution of the beam itself are distorted by turbulence, and the degree of distortion is closely related to the magnitude of OAM value, turbulence intensity and propagation distance. Finally, by comparing the PIB curves, it is found that the anti-turbulence ability of LG beams with different OAM values is obviously different. In this paper, a set of practical MATLAB theory analysis and numerical simulation method is designed by using Fourier optics, combining with "beam step propagation method" and "phase screen method". The beam diffraction model in the form of angular spectrum and its corresponding sampling limiting conditions are derived to obtain the most accurate simulation results, and the relationship between LG beam and turbulence is clarified. These methods and conclusions are of great significance in understanding the atmospheric communication characteristics of OAM beams, and can provide a theoretical reference for the further study of the simulation of OAM optical communication systems and their applications.
【學位授予單位】：中南民族大學
【學位級別】：碩士
【學位授予年份】：2015
【分類號】：TN929.1
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本文編號：1837877