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题名:
自适应光学在自由空间光通信中的应用
作者: 杨昌旗
学位类别: 博士
答辩日期: 2007-06-03
授予单位: 中国科学院光电技术研究所
授予地点: 光电技术研究所
导师: 姜文汉
关键词: 自由空间光通信 ; 自适应光学 ; 相位起伏 ; 幅度起伏 ; 闪烁 ; 误码率
其他题名: Adaptive Optics in Free-space Optical Communication
学位专业: 光学工程
中文摘要: 由于有些场合无法架设光缆进行通信,特别是出于保密等等原因,自由空间光通信成为了下一代光通信系统的研究热点。自由空间光通信系统主要是以真空或者湍流大气为传输介质,本文专注于研究传输介质为湍流大气的情况。自由空间光通信的波长通常要远远小于大气折射率不均匀单元的尺度,这导致了自由空间光通信容易受到湍流大气的影响。光波在大气中传输时,相位起伏是如此之大,使得外差探测系统几乎不能够在大气传输的环境下正常的工作,以至于现有的光通信系统基本上都是基于直接检测的。大气对光波的影响有以下5 个方面:1,光束扩散;2,光束漂移;3,光束到达角的起伏;4,空间相干性的损失;5,闪烁。闪烁是这五个效应中对直接检测的光通信系统影响最大的。本文的研究内容紧紧围绕闪烁,通过在自由空间光通信系统中引入自适应光学相位校正来减小闪烁,进而达到减小自由空间光通信系统的误码率的目的,从而提高了自由空间光通信系统的性能。 在自由空间光通信和自适应光学中都必须要考虑湍流大气对光波的作用。光波在湍流大气中传输,Rytov 近似下的结果只能适用于弱起伏的条件。论文作者将闪烁调制理论应用到对相位起伏的研究中来,得到了上行、下行和水平三种链路的相位起伏的表达式。作者所得到的结果与Rytov 近似下的结果之间最大的不同之处就是考虑了闪烁的聚焦和饱和。 到目前为止,已经实现工程化的自适应光学都是直接校正相位的。对于在自由空间光通信中,自适应光学应该如何放置的问题,学术界存在不同的观点。主流的观点认为,可以将自适应光学放置在自由空间光通信系统的发射端或者接收端来减小闪烁,进而改善自由空间光通信系统的性能。本文作者对此持不同意见。论文通过研究几何光学近似下的相位起伏和幅度起伏,指出了光波在湍流大气中传输时幅度起伏产生的原因。作者认为,应该将自适应光学放置在自由空间光通信系统的发射端才能达到减小闪烁的目的。将自适应光学放置在自由空间光通信系统的接收端,只能校正相位起伏,而不能影响幅度起伏。 误码率是衡量自由空间光通信系统的性能的重要参量。大气闪烁其本质上存在如下特征:信号电平会出现正交叉和负交叉;每次正交叉和负交叉后,信号电平都会保持一段时间。大气闪烁的这些特征决定了它可以被用来研究误码率。作者基于闪烁统计学的三个基本参数,提出了一种计算误码率的新方法,并在其中考虑了自适应光学倾斜校正,得到了点接收器的结果。 在将闪烁调制理论应用到研究相位起伏的过程中,作者从能量守恒的角度,得到了Andrews 调制近似的显式表达式,明确指出了Andrews 调制近似在各种情况下的物理含义。通过把这些显示表达式做谱展开,并独立于Andrews 等人的工作,作者得到了大尺度和小尺度起伏的对数辐射方差和协方差。 点接收器的结果只能适用于某些特殊的通信链路,然而大多数情况下的自由空间光通信都必须考虑接收端孔径平均效应的影响。建立在点接收器的结果之上,同时将大尺度和小尺度对数辐射方差和协方差考虑进去,作者得到了考虑了孔径平均效应的误码率。 在研究自适应光学校正的自由空间光通信时,Tyson 的方法是间接测量误码率(也就是直接测量闪烁,再由闪烁和误码率的数学关系间接的得到误码率)。与之不同,本文作者研制了直接测量误码率的电路,为以后从事这个课题的研究人员打下了基础。
英文摘要: Because in some circumstances it is impossible to use optical fibre to establish normal communication, especially because of confidence, free-space opticalcommunication attracts more and more attention. Free-space optical communication uses vacuum or turbulent atmosphere as propagation medium. In this dissertation,the author focuses only on the case of turbulent atmosphere. Due to the fact that the optical wavelength is far more less than the scale of refractive index inhomogeneity, free-space optical communication is easily influenced by the turbulent atmosphere. When optical wave propagates through the turbulent atmosphere, the phase fluctuation is very serious, which makes the heterodyne system can hardly work in this situation. As a result, the nowadays used systems are mostly based on direct detection. The turbulent atmosphere influences the optical wave in such five aspects: 1) beam spreading; 2) beam wander; 3) fluctuation of angle of arrival; 4) loss of spatial coherence; 5) scintillation. Among these five factors, scintillation is the most serious one that affects the direct detection system. This dissertation mainly investigates the scintillation. By reducing the scintillation in free-space optical communication with adaptive optics correction, the bit-error rate for free-space optical communication can be reduced, and hence enhances the system performance. Both in free-space optical communication and adaptive optics, the impact of turbulent atmosphere on the optical wave should be considered. The result of Rytov Approximation is only appropriate in weak fluctuation. By using the theory of scintillation modulation into the field of phase fluctuation, the author arrives at the expressions of phase fluctuation for three kinds of links. The difference between the results that obtained by the author and those under the Rytov approximation is that the former considers the scintillation convergence and saturation. For the moment adaptive optics in engineering is directly phase-correction. As to the problem that how to configure adaptive optics in free-space opticalcommunication, it still has different viewpoints. The dominant viewpoint thinks that “the addition of adaptive optics to the transmitter or the receiver can reduce the effects of scintillation and thereby improve system performance”. The author has different opinion. By investigating the phase and amplitude fluctuations in the geometrical region, the author points out the reason of amplitude fluctuation of wave propagation through the random medium. This dissertation deems that adaptive optics should be placed at the transmitter to reduce the effects of scintillation. Putting adaptive optics at the receiver of free-space optical communication system can only compensate phase fluctuation, and can not influence amplitude fluctuation. The bit-error rate is an important measure for the performance of free-space optical communication system. In essence the atmospheric scintillation has such characteristics that: the signal level has positive and negative crossings; whenever the level crossing is taken place, the signal level will hold on for a moment. These characteristics determine that it can be used to analyze the bit-error rate. Based on the three scintillation statistical parameters, the author proposed a kind of new method to analyze the bit-error rate, adaptive optics tip-tilt compensation is considered, and the result of point receiver is obtained. During the course of investigating the phase fluctuations with scintillation modulation approximation, the author arrives at the specific expressions for Andrews Modulation Approximation from the point of view of energy conservation, and points out clearly the definite physical meanings of Andrews Modulation Approximation under all kinds of situations. By expressing Andrews Modulation Approximation in its spectral representation, and independent of Andrews et al’s work, the author obtains the large-scale and small-scale log-irradiance variances and covariances. The result of point receiver is only appropriate in some special links, and in most of the situations, the effect of aperture averaging should be considered. Based on the result of point receiver and considering the large-scale and small-scale log-irradiance variances and covariances, the author obtains the bit-error rate with aperture averaging. Different to Tyson’s work, in which Tyson measures the bit-error rate indirectly,the author of this dissertation has developed a circuit to measure the bit-error rate directly, and with which the people taking over this research are well grounded.
语种: 中文
内容类型: 学位论文
URI标识: http://ir.ioe.ac.cn/handle/181551/216
Appears in Collections:光电技术研究所博硕士论文_学位论文

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Recommended Citation:
杨昌旗. 自适应光学在自由空间光通信中的应用[D]. 光电技术研究所. 中国科学院光电技术研究所. 2007.
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