Average capacity optimization in free-space optical communication system over atmospheric turbulence channels with pointing errors

A model is proposed to study the average capacity optimization in free-space optical (FSO) channels, accounting for effects of atmospheric turbulence and pointing errors. For a given transmitter laser power, it is shown that both transmitter beam divergence angle and beam waist can be tuned to maximize the average capacity. Meanwhile, their optimum values strongly depend on the jitter and operation wavelength. These results can be helpful for designing FSO communication systems.     

Analysis of inter-satellite free-space optical link performance considering different system parameters

The welcome and adaptation of optical wireless technology by the modern era has brought forward the concept of an inter-satellite free-space optical communication system. In the present work, I study the combined effect of selection of different operating wavelengths and detector types along with the pointing errors at the transmitter and receiver side on the performance of an inter-satellite free-space optical link. The link performance has been optimized by measuring and analyzing the bit error rate and quality-factor of received signal under different scenarios. Performance of the inter-satellite link has also been investigated considering different modulation formats and data rates for LEO and MEO distances.     

Quantum-correlation-based free-space optical link with an active reflector

We present a quantum-correlation-based free-space optical(FSO) link over 250 m using an outdoor active reflector 125 m from the transceiver station. The performance of free-space optical communication can be significantly degraded by atmospheric turbulence effects, such as beam wander and signal fluctuations. We used a 660 nm tracking laser to reduce atmospheric effects, by analyzing the fast beam wander and slow temporal beam drift, using this information to correct the quantum channel alignment of the 810 nm signal photons. In this work, the active reflector consisted of a mirror, a 6-axis hexapod stage, and a long-range wireless bridge. The slow drift of the beam path due to outdoor temperature changes was steered and controlled using wireless optical feedback between the receiver units and the active reflector. Our work provides useful knowledge for improved control of beam paths in outdoor conditions, which can be developed to ensure high quality quantum information transfer in real-world scenarios, such as an unmanned FSO link for urban quantum communication or retro-reflective quantum communication links.     

Design and analysis of the optical transceiver for mobile atmospheric laser communication

The concept of mobile atmospheric laser communication (MALC) is introduced in this paper. Atmospheric attenuation, turbulence-induced scintillation and beam wander cause deep fades in the beam power and degrade the optical channel. The optical transceiver presented in this paper is designed for a MALC test system. Currently achievable hardware performance capabilities for the MALC terminals are used as input parameters to the analysis. A novel optical transceiver structure is designed. Link margin is analyzed using the MALC analysis software, our optical link analysis program. Data rate, bit-error rate, prime transmit     

空间激光通信中湍流信道光束传输仿真

对大气光信道特性的研究是空间激光通信研究的重要组成部分。首先以大气湍流理论为基础,阐述了激光在大气中传输时受到的大气湍流的影响和分析方法。接着重点对近年来应用日益广泛的空间激光通信的数值仿真方法进行了综述。不仅介绍了基本传输方程及其求解方法,还对采用数值方法模拟光波在随机介质中传播的若干关键技术进行了分析。最后结合现有的研究成果,说明了数值方法在空间激光通信中的应用。     

Experimental determination and comparison of rain attenuation in free space optic link operating at 532 nm and 655 nm wavelength

The demand for high data rate, security and reliable communication is driving the development of free space optic communication (FSO) technology. The atmospheric effects such as scintillation, absorption and scattering severely affect the availability and range of the FSO system. The atmospheric rain absorbs and scatters the laser beam energy resulting in attenuation of the propagating signal. Initial development of FSO technology primarily used wavelength from infrared spectrum. In the recent years, the interest in visible light carrier for FSO applications is consistently increasing. In this paper, the effect of rain over two optical wavelengths from the visible spectrum i.e. 532 nm and 655 nm has been experimentally evaluated and results for the specific rain attenuation at 532 nm and 655 nm wavelengths have been compared.     

Free-space laser communications with adaptive optics: Atmospheric compensation experiments

Refractive index inhomogeneities of the turbulent air cause wave-front distortions of optical waves propagating through the atmosphere, leading to such effects as beam spreading, beam wander, and intensity fluctuations (scintillations). These distortions are responsible for severe signal fading in free-space optical communications systems and therefore compromise link reliability. Wave-front distortions can be mitigated, in principle, with adaptive optics, i.e., real-time wave-front control, reducing the likeliness of signal fading. However, adaptive optics technology, currently primarily used in astronomical imaging, needs to be adapted to the requirements of free-space optical communication systems and their specific challenges.In this chapter we discuss a non-conventional adaptive optics approach that has certain advantages with respect to its incorporation into free-space optical communication terminals. The technique does not require wave-front measurements, which are difficult under the strong scintillation conditions typical for communication scenarios, but is based on the direct optimization of a performance quality metric, e.g., the communication signal strength, with a stochastic parallel gradient descent (SPGD) algorithm.We describe an experimental adaptive optics system that consists of a beam-steering and a higher-resolution wave-front correction unit with a 132-actuator MEMS piston-type deformable mirror controlled by a VLSI system implementing the SPGD algorithm. The system optimizes the optical signal that could be coupled into a single-mode fiber after propagating along a 2.3-km near-horizontal atmospheric path. We investigate characteristics of the performance metric under different atmospheric conditions and evaluate the effect of the adaptive system. Experiments performed under strong scintillation conditions with beam-steering only as well as with higher-resolution wave-front control demonstrate the mitigation of wave-front distortions and the reduction of signal fading.     

Atmospheric turbulence and numerical evaluation of bit error rate (BER) in free-space communication

Bit error rate (BER) is an important parameter in free-space optical (FSO) communication. This paper presents the numerical evaluation of BER by studying the propagation of an initial Gaussian laser beam through turbulent atmosphere. Beam scintillation, spreading and wandering were considered as the atmospheric turbulent effects on the laser beam degradation. Different turbulent conditions and laser beam characteristics were applied to the calculations. The results show that the BER can be influenced considerably by atmospheric turbulence. According to these results the laser beam size and wavelength have also significant effects in the BER values.     

基于混合光模式阵列的自由空间编码通信

光学涡旋的产生、传输与应用是当前光学领域的研究热点之一.光学涡旋具有轨道角动量,作为一种全新的自由度,丰富了目前光通信的方式.利用面向目标的共轭对称延拓傅里叶计算全息技术,基于空间光调制器,用单束激光直接产生混合光模式阵列进行编码通信.采用由单光涡和复合光涡构成的4种易于识别的模式组成2×2混合光模式阵列,进行灰度图像的编码传输.在接收端提取混合光模式阵列图的信息并进行解码,实现零误码的灰度图像再现.以传输一幅Lena图像为例,使用2×2混合光模式阵列进行编码通信,相对于传统单光涡编码通信,其信息容量可增加4倍.该方法光路简单易行,可扩展性强,进一步拓展使用4×4混合光模式阵列进行编码通信,信息容量提升16倍.提出的混合光模式阵列编码通信方法对于提高信息传输容量具有重要价值.     

可同时测量真温及光谱发射率的8波长高温计

基于提出的新的多光谱测温法的数据处理方法 ,研制了一种可以自动识别目标真温及光谱发射率的新型多波长高温计。其光路系统采用棱镜分光技术 ,克服了以往用光导纤维分光、干涉滤光片限定工作波长的缺点 ;数据采集系统采用 SA6 8D2 2模块 ,可以实现数据的无线传输。基于提出的新的发射率假设模型 ,使得此多波长高温计可用于大多数工程材料的目标真温及光谱发射率的测量     

Optimized multiemitter beams for free-space optical communications through turbulent atmosphere

Using laser beams with less than perfect spatial coherence is an effective way of reducing scintillations in free-space optical communication links. We report a proof-of-principle experiment that quantifies this concept for a particular type of a partially coherent beam. In our scaled model of a free-space optical communication link, the beam is composed of several partially overlapping fundamental Gaussian beams that are mutually incoherent. The turbulent atmosphere is simulated by a random phase screen imprinted with Kolmogorov turbulence. Our experiments show that for both weak-to-intermediate and strong turbulence an optimum separation between the constituent beams exists such that the scintillation index of the optical signal at the detector is minimized. At the minimum, the scintillation reduction factor compared with the case of a single Gaussian beam is substantial, and it is found to grow with the number of constituent beams. For weak-to-intermediate turbulence, our experimental results are in reasonable agreement with calculations based on the Rytov approximation.     

星地激光链路中光束发散角与跟瞄误差的最佳比值

 实际星地光通信系统的发射光束为高斯型的情况下，跟瞄误差和大气闪烁是星地激光链路中的主要信道噪声源。在结合两者对系统性能影响的前提下，对星地激光通信链路进行了理论分析和模型建立。基于该模型对星地激光链路中光束发散角与跟瞄误差之比的优化选择问题进行了研究。结果表明：对于上行链路，在不同误码率需求下都存在一个光束发散角与跟瞄误差的最佳比值，使得上行链路余量最大；对于下行链路，在不同误码率要求下均存在一个光束发散角与跟瞄误差的最佳比值，使得下行发射光功率的需求最小。利用最优化方法和最小二乘法拟合，得到星地激光链路中光束发散角与跟瞄误差的最佳比值和系统误码率的经验公式。     

双向大气信道激光传输的信道互易性研究

在大气信道激光传输中,大气湍流对系统性能会产生较大影响,主要体现为降低传输速率和增加误码率。在具有信道互易性的双向激光传输链路中,两终端光斑信号强度的变化相关,可以在终端提取信道状态信息,以对信道影响进行补偿,从而提高传输速率。本文首先在弱湍流条件下,根据Rytov近似理论推导了平面波双向传输链路接收到的光斑信号的相关系数与传输路径的关系,并给出解析式。结果表明,两终端接收的光斑信号的光通量具有相关性,且相关系数与传输路径有关。进一步搭建了双向收发共轴激光传输系统,并进行了外场试验,试验结果不仅验证了双向大气信道激光传输链路具有互易性,且两接收端光斑信号光强的实时变化趋势相同。本文结论对实现大气信道高速率、低误码率激光传输具有重要意义。     

Reliability analysis of an auto-tracked FSO link under adverse weather condition

In this paper, the effects of auto-tracking subsystems on the Bit-Error-Rate (BER) and reliability of free-space optical (FSO) communication links are investigated. Considering environmental influences on the laser beam intensity distribution in receiver, we study the propagation properties of Gaussian laser beams through a complete optical path. Then it is tried to derive an analytical formula for intensity distribution along the complete optical path as well as on the receiver's detector plane based on the Collins integral. Based upon the intensity formula, we calculate the values of signal to noise ratio (SNR), BER, and power while considering the effects of atmospheric losses due to absorption, scattering and turbulence on them. Using mentioned values, the role of auto-tracking subsystems on link reliability improvement in adverse weather condition is described. The related results are illustrated by graphs obtained by calculations and simulations.     

Capacity for non-Kolmogorov turbulent optical links with beam wander and pointing errors

The performance of optical wireless communication links depends strongly on the atmospheric conditions and the parameters of the link such as the propagation distance, the operation wavelength, jitter variance, attenuation coefficient and effective beam spot radius at the receiver. The analytical expression for the evaluation of the average capacity of optical wireless communication systems is derived, using the gamma-gamma distribution in the non-Kolmogorov atmosphere turbulence. The impact of atmospheric attenuation, beam wander and pointing errors on the average of the optical wireless communication link is investigated. It is shown that the capacity has a fluctuation curve, when power law α increases, and the power law α of minimum point in fluctuation curve is bigger as the non-Kolmogorov turbulence strength is stronger.     

On the use of wavelength and time diversity in optical wireless communication systems over gamma–gamma turbulence channels

Optical wireless communication or free space optical systems have gained significant research and commercial attention in recent years due to their cost-effective and license-free high bandwidth access characteristics. However, by using the atmosphere as transmission media, the performance of such a system depends on the atmospheric conditions that exist between transmitter and receiver. Indeed, for an outdoor optical channel link, the existence of atmospheric turbulence may significantly degrade the performance of the associated communication system over distances longer than 1 or even 0.5 km. In order to anticipate this, particular attention has been given to diversity methods. In this work, we consider the use of wavelength and time diversity in wireless optical communication systems that operate under weak to strong atmospheric turbulence conditions modeled by the gamma–gamma distribution, and we derive closed form mathematical expressions for estimating the system's achievable outage probability and average bit error rate. Finally, numerical results referred to common practical cases are also obtained in order to show that wavelength and time diversity schemes enhances considerably these systems’ availability and performance.     

曝光时间对上行传输光束强度起伏的影响

上行传输光束的闪烁起伏影响星地激光链路的稳定性,而利用时间平滑效应,增加光电探测器曝光时间是克服大气闪烁的有效途径之一。考虑曝光时间的平滑效应,分别导出了小天顶角和大天顶角下的时间平滑因子表达式,在此基础上,分析了接收终端离地高度和天顶角对时间平滑效应的影响。研究结果表明,随着曝光时间的增大,上行传输光束的闪烁强度显著降低;随着接收终端离地高度和天顶角的增大,大气闪烁时间平滑作用降低。     

Optimization of free space optics parameters: An optimum solution for bad weather conditions

Free Space Optics Systems (FSO) is one of the most effective solutions, especially for atmospheric turbulence due to the weather and environment structure. Free space optics system suffers from various limitations. A well-known disadvantage of FSO is its sensitivity on local weather conditions-primarily to haze and rain, resulting in substantial loss of optical signal power over the communication path. The main objective of this article is to evaluate the quality of data transmission using Wavelength Division Multiplexing (WDM) with highlighting several factors that will affect the quality of data transmission. The results of these analyses are to develop a system of quality-free space optics for a high data rate transmission. From the result analysis, FSO wavelength with 1550 nm produces less effect in atmospheric attenuation. Short link range between the transmitter and receiver can optimize the FSO system transmission parameters or components. Based on the analysis, it is recommended to develop an FSO system of 2.5 Gbps with 1550 nm wavelength and link range up to 150 km at the clear weather condition of bit-error-rate (BER) 10⁻⁹.     

湍流大气传输高斯谢尔光束的到达角起伏

研究了在弱大气湍流起伏环境下以窄带宽高斯谢尔光束为激光光源的大气通信问题,分析了大气湍流强度和光源空间相干度对通信光束到达角起伏的影响.采用窄带宽光场的交叉谱密度函数代替光场互相干函数的近似方法和采用包含大气湍流内外尺度的简化折射率谱密度函数,得出了湍流大气中传输高斯谢尔光束的波结构函数(WSF) 和到达角起伏方差解析近似关系.分析表明,光源的空间相干度和传输光束的湍流扩展是影响高斯谢尔光束的相位起伏结构函数和传输光束到达角起伏的重要因素.     

多模光纤产生用于大气光通信的部分相干光源

 克服大气湍流对近地大气光通信质量影响,可归结为在接收端的照明均匀化,而采用部分相干光照明则是提高照明均匀性的一种有效途径。提出利用多模光纤的模间色散产生一个具有部分相干光特性的合成光源的方法,分析了该光源的产生原理,表明光束发散角由光纤芯径和发射透镜的焦距决定,由模间色散产生的光程差越大,该合成光源的空间相干性越差。仿真结果表明形成1 mrad远场发散角的合成光源是可行的,因而适于在空间光通信中应用。