| 题名 | 部分相干光在星潜激光通信中的传输性质研究 |
| 作者 | 鲁伟 |
| 学位类别 | 博士 |
| 答辩日期 | 2007 |
| 授予单位 | 中国科学院上海光学精密机械研究所 |
| 导师 | 刘立人 |
| 关键词 | 部分相干光 光学湍流 星-潜激光通信 高斯－谢尔模光束 互谱密度 谱相干度 折射率空间功率谱 |
| 其他题名 | Theoretical studies of propagation of partially coherent beam in satellite-submarine laser communications |
| 中文摘要 | 星潜激光通信信道中的大气和海水湍流会对系统的工作性能产生严重影响。对于大气中的光通信，这种影响主要体现在激光光束的大气传输效应，包括大气对光束的吸收、散射和湍流效应，其中吸收和散射主要表现在传输途径上光波的能量损耗，大气湍流的影响则表现为接收平面上光强和相位起伏效应。对于能量损耗，可通过提高发射功率或采用高灵敏接收装置等技术进行补偿，对于如何克服湍流的影响，目前在已报道的方法中，自适应技术和多孔径技术分别存在波面探测以及电子系统复杂等困难，因此作为能有效克服大气湍流的部分相干光技术则逐渐成为人们的研究热点之一。 众所周知，完全相干的激光光束在自由空间中传输时，光束束宽和空间相干长度随传输距离不断增加，在远场仍维持其相干性；而当光束在大气湍流中传输时，湍流折射率起伏不仅使光束大大展宽，而且还导致波阵面畸变并进而导致空间相干性的下降，相干长度变短使得光束在远场内仅有很小的区域内仍维持相干性，其他区域则退化为非相干区域，严重削弱激光的光学质量。近来，已有研究显示，在湍流中传输的部分相干光比完全相干激光具有更强的抗湍流能力，部分相干光束在湍流中传输时也会产生明显的湍致展宽，但其相干性的退化却弱于完全相干光，即当传输至某特定通信距离时，发射源为部分相干光的光场内具有更大的相干区域，部分相干光的这一特性对于远距离激光通信以及激光雷达等技术来说是十分有利的。 部分相干光的谱密度、谱偏振度以及谱相干度在传输中会发生变化，围绕这些变化已经开展了诸多研究，其中强度、空间相干度和位相都呈轴对称高斯型分布且具有良好方向性的高斯－谢尔模（Gaussian Schell-model beam，GSM）光束倍受研究人员的青睐。Rochester大学的Wolf课题组已先后对GSM光束在大气湍流中传输时的偏振度演变、基于相关的相位空间因子、湍致光束展宽演变等光束特性展开了研究。与谱密度和谱偏振度理论研究的蓬勃发展形成鲜明对比的是，自从Wolf给出了在自由空间中传输的光束相干度的标量表达式以后，尚未见到对光波的谱相干度在大气湍流等随机介质中传输变化的报导。光波在湍流大气中的相干性退化是宏观物理特性改变的本质原因，因此需要对部分相干光在大气湍流中的相干度演化进行研究。此外，激光传输除了大气信道外还涉及了海水信道，激光在海水中传输时除了海水水体的强散射外，同样会受到海水湍流的影响，因此有必要对部分相干光束在海水湍流中的传输行为进行理论上的分析。针对上述背景，我们开展了如下几方面工作： 1. 在矢量理论框架下，对大气湍流中传输的GSM光束光波场矢量互谱密度随传输距离的演化进行了理论分析，结果表明：光束相干性演变的行为可看作是部分相干光源参数和湍流谱模型参数同时对光束产生影响的结果。同时，使用位置矢量表示的远场互谱密度可更为清晰地表征湍流中部分相干光场辐射能量分布的方向性。 2. 对部分相干的GSM光束的部分特性进行了理论分析，得到任意相干态的光源的相位－空间积因子，该参数在统计上同时反映了远场光束的角度发散情况和光源的部分相干特性；此外，分析了光源相干长度对在湍流大气中部分相干光束的自由空间角度扩展和湍流距离的影响，结果表明：当相干长度的取值分别取在临界值前后时，上述两个参数具有不同的变化趋势。 3. 对部分相干GSM光束在海水湍流信道中的传输进行了分析，得到光束相干度以及湍致光束展宽公式，结果表明：温度的耗散率、单位质量的湍流动能耗散率以及水体中温度和盐分贡献比等海洋参数是决定部分相干光在海水信道中的相干度退化、湍致展宽及湍流距离等特性的主要因素。 4. 在实验上验证了星潜激光通信中多通道二维图案传输理论模型的可行性，同时将这种多通道技术应用于基于BB84协议算法的量子密钥传输，可以实现自由空间中量子密钥的m×n通道传输，大大提高系统密钥传输效率。 |
| 英文摘要 | System performance of satellite-submarine laser communications is damaged significantly by atmosphere and seawater turbulence. As for optical communications in atmosphere, this degradation is mainly represented by effects on propagation of laser beam in atmosphere, which include effects of absorption, scattering and turbulence. Absorption and scattering lead to the power extinction of laser beam, which could be overcome by increasing transmitting power or using filter with high-sensitivity, while the atmospheric turbulence cause random fluctuations of intensity and phase at the receive terminal, for which several techniques have been reported to alleviate the effect of atmospheric turbulence. Due to its strong turbulence-resistance, partially coherent beam is being focused with its advantages over other methods such as adaptive optics and multi-aperture optics. As we all known, beam width and spatial coherent length of the completely coherent laser beam is increasing when it is propagating in free space and status of its coherence is keep going at far field. However, not only the width will be expanded greatly but also wave front is distorted leading to reduced spatial coherence by the random fluctuation of refractive index of turbulence. At far field, there is a very small area that keeps its coherence, which makes quality of laser beam degraded. Recently, studies showed that partially coherent beam has stronger turbulence-resistance than completely coherent laser beam. Though partially coherent beam has also a remarkable beam width expansion, reduction of its coherence is less than its coherent counterpart. That is to say, for a special link distance, partially coherent beam has a larger coherent area, which is very important for the applications of laser communication as well as laser radar. People has developed studies on change of spectral density, spectral degree of polarization and spectral degree on propagation, in which Gaussian Schell-model beam is much preferred due to its Gaussian-shaped intensity, spatial degree of coherence and phase as well as its good directionality. Research team directed by Emil Wolf in Rochester University had already studied change of spectral degree of polarization, phase-space factor based on correlation and turbulence-induced evolution of beam width of GSM beam. Compared with the flourish research of spectral density and spectral degree of polarization in turbulence, spectral degree of coherence in turbulence has not ever been reported since the scalar expression of degree of coherence of laser beam propagation in free space had been proposed by Wolf. Considering that reduction of coherence in turbulence is fundamental reason of change of macroscopic physic properties, it is necessary to study the evolution of degree of coherence of partially coherent beam in turbulence. In addition to the atmosphere channel, optical beam propagation often involves channel of sea water turbulence, which has been seldom researched. Thus, it is also necessary to study the behaviors of partially coherent optical beam propagating through sea water turbulence. Therefore, we developed theoretical and experimental studies as followings: 1. Based on vector theory, Evolution of vetorial cross spectral density of partially coherent electromagnetic optical beam in atmospheric turbulence with propagation distance is theoretically studied, and the expression of element of cross spectral density matrix has been derived. Expression of the spectral degree of coherence of optical beam has been given and its effective width has been defined. At the same time, we derive the far-field cross spectral density of partially coherent GSM beam by using position vector method and consider the far-field radiance intensity. 2. Some characteristic of partially coherent GSM beam has been theoretically studied. By using the principle of superposition of coherent mode in partially coherent source, we calculate the normalized widths of cross spectral density and angular correlation function of the radiant field. We also analyze the variation trends of the free space angular spreading and turbulence distance with different coherence lengths of the partially coherent source, both of which describe the behavior of partially coherent optical beam in turbulence. 3. Propagation of partially coherent GSM optical beam in sea water turbulence has been analyzed. According to the power spectrum of refractive index of sea water turbulence and cross spectral density of partially coherent optical beam, we derive the expressions of the degree of coherence and turbulence-induced beam spreading of the beam in sea water turbulence. The degradation of degree of coherence, turbulence-induced beam spreading and turbulence resistance are also being studied. 4. Feasibility of multi-channel 2-D pattern transfer model of satellite-submarine laser communications has been experimentaly examined. at the same time, the multi-channel scheme has been applied to the quantum cryptography key transfer based on BB84 protocal and further m×n channels transfer of quantum key in free space, which makes the effeciency of system improved greatly. |
| 语种 | 中文 |
| 内容类型 | 学位论文 |
| 源URL | [http://ir.siom.ac.cn/handle/181231/15446]  |
| 专题 | 上海光学精密机械研究所_学位论文 |
| 推荐引用方式 GB/T 7714 | 鲁伟. 部分相干光在星潜激光通信中的传输性质研究[D]. 中国科学院上海光学精密机械研究所. 2007. |
| 个性服务 |
| 查看访问统计 |
| 相关权益政策 |
| 暂无数据 |
| 收藏/分享 |
除非特别说明，本系统中所有内容都受版权保护，并保留所有权利。
修改评论