基于自适应光学补偿的自由空间光通信系统性能研究

自由空间光通信在未来全球通信网中具有潜在的应用价值,然而自由空间光通信的性能易受大气湍流影响,而自适应光学能够解决大气湍流问题.在gamma-gamma分布大气湍流中,用自适应光学技术对自由空间光通信系统进行补偿,进行通信系统误码率的分析,给出了上行链路和下行链路的模拟结果.结果表明:自适应光学误差补偿技术能够很好地提高自由空间光通信的性能,并且低阶自适应光学补偿就能达到很好的校正效果.     

Earth-to-deep-space optical communications system with adaptive tilt and scintillation correction by use of near-Earth relay mirrors

Performance of an Earth-to-deep-space optical telecommunications system is degraded by distortion of the beam as it propagates through the turbulent atmosphere. Conventional approaches to correcting distortions, based on natural or artificial guide stars, have practical difficulties or are not adequate for correction of distortions, which are important for Earth-to-deep-space optical links. A beam-relay approach that overcomes these difficulties is discussed. A downward-directed laser near an orbiting relay mirror provides a reference for atmospheric correction. Adaptive optics at the ground station compensate the uplink beam so that after it passes through the atmosphere uplink propagation effects are removed. The orbiting mirror then directs the corrected beam to the distant spacecraft.     

Investigation on the influence of intensity scintillation and beam wander in space optical uplink DWDM communication system

Abstract

Dense wavelength division multiplexing (DWDM) has been widely applied in ground optical communication. However, the technology of DWDM is still not mature enough in the space optical communication system. In order to further advance the use of DWDM into space optical communication, the probability density function (PDF) and the bit-error rate (BER) performance of DWDM is investigated in uplink communications under the influence of atmospheric turbulence, consisting of intensity scintillation and beam wander caused by atmospheric turbulence. Numerical results show that the atmospheric turbulence has a great impact on BER and PDF, and wavelength, divergence angel, and other relevant parameters should be carefully considered in this DWDM system. This work can be conducive for improving DWDM design of space optical uplink communication systems.     

DOE effect on BER performance in MSK space uplink chaotic optical communication

With the increasing demand for space optical communication security, space chaotic optical communication has attracted a great amount of attention. Compared with traditional space optical communication, a chaotic optical communication system has a higher bit error rate(BER) for its complex system design. In order to decrease the BER of space chaotic optical communication systems, we introduce two diffractive optical elements(DOEs)at a transmitting terminal(Tx). That is because the commonly used reflective optical antenna at Tx blocks the central part of the transmission beam, which leads to a great amount of power consumption. Introducing the DOEs into the optical subsystem at Tx can reshape the transmission beam from a Gaussian distribution to a hollow Gaussian distribution so that the block of the secondary mirror in the reflective optical antenna can be avoided. In terms of the DOE influence on communication quality, we give a BER model based on a minimumshift-key(MSK) space uplink chaotic optical communication system to describe the DOE function. Based on the model, we further investigate the effect of the DOEs through analyzing the BER relationship versus basic system parameters based on the BER model. Both different mismatch conditions of chaotic systems and different atmospheric turbulence conditions are considered. These results will be helpful for the scheme design of space uplink chaotic optical communication systems.     

Ultra-broadband indoor optical wireless communication system with multimode fiber

In this paper we experimentally demonstrate an ultra-broadband indoor full-duplex WDM optical wireless communication system with multimode fiber. The multimode fiber is used because it is employed in most of the already installed in-building fiber distribution networks. Simultaneous error-free (BER<10(-9)) transmission of 4×12.5 Gbps downlink and 800 Mbps uplink has been successfully demonstrated. The experimental results show that, although the use of multimode fiber will induce ~2.4 cm reduction in the maximum error-free beam footprint in the downlink, the bit rate of the uplink can be much higher compared to the system with single-mode fiber.     

Light Pipe Line Beam Shaper

A light pipe based laser beam shaper is proposed as a low loss approach to transfer a Gaussian laser beam into a long line beam with uniform distribution along the line direction for the applications of illumination or material processing. A telecentric relay optics has been used for solving the radiometric issues in the wide angle optics. In addition, the length of the light pipe is used as a parameter for manipulating irradiance distribution at the outlet to compensate for the non-uniformity resulting from the partial reflection at the optical interface in the relay optics, so as to achieve a highly uniform line beam at the target plane. The proposed scheme provides an economical and versatile solution for the wide angle beam shaping problem.     

Analysis of coherent gradient sensing (CGS) by fourier optics

A coherent laser beam reflected or transmitted from a deforming plate specimen acquires an optical path difference (phase change) which is related to the deformation and stress field. The optical method of Coherent Gradient Sensing (CGS) uses two parallel grating plates to displace (shear) and recombine the distorted light beam emerging from the specimen. Fringes are produced on the image plane by the interference of the shifted and unshifted beams. The fringe pattern is related to the spatial difference of the phase in the shearing direction. If the shearing distance is small enough, the spatial difference of the phase approximates to its gradient. Each fringe in the image represents a locus of equal gradient component of the phase in the shearing direction. The technique is interpreted by means of wave optics, using the Fraunhofer approximation and the paraxial theory of lenses. The assumptions made in earlier analyses have been removed here. A more precise analysis based on Fourier optics is presented. The simplicity of the optical setup and variable resolution of the technique have led to its frequent use in the area of solid mechanics, including fracture mechanics. Some examples are discussed in the second half of this paper.     

无衍射光束的产生及其应用

近年来,随着激光技术的快速发展,相继产生了多种在远距离传输后中心光斑保持不变的无衍射光束,包括贝塞尔光束、高阶贝塞尔光束、马丢光束、高阶马丢光束、余弦光束、抛物线光束以及艾里光束.无衍射光束在激光打孔、激光精密准直、光学精密控制、光学微操控、光通信、等离子体导向、光子弹产生、光通信、自聚焦光束的合成以及非线性光学等领域中有着广泛的应用.本文介绍了各类无衍射光束的数学表达式、产生方法及对应的实验结果;就无衍射光束的特性和应用进行了归纳和讨论;并对其在未来的研究与应用前景中发挥的重要作用进行了简要总结与展望.     

高斯光束通过复杂光学系统的变换

使用矩阵光学技巧和方法详细研究了高斯光束通过复杂光学系统的传插问题。将结果推广于:(1)物像空间析射率不相等(n_1≠n_2);(2)光学系统矩阵元为复数或C元素为零的情况。就高斯限模光阑对光束传输的影响作了讨论。经典光学和高斯激光束光学(n_1≠n_2,n_1=n_2)的比较,清楚地说明了它们的区别和联系。     

Investigation of Near-Field Imaging Characteristics of Radial Polarization for Application to Optical Data Storage

Radially polarized incident light can generate a more confined longitudinal electric field on a focal plane in near-field (NF) optics than focusing circularly polarized light. Using this phenomenon, it is feasible to reduce beam spot size on storage media to increase the areal density of optical data storage. A radially polarized beam generates a beam spot which is 20% more confined on the 1st surface of medium than that of circularly polarized light. However, the peak intensity of total electric field sharply decreases and its transverse component is much more dominant inside the media stack. This confirms that radially polarized optics can be a candidate not for an NF recording system but for an NF read-only memory (ROM) system. Potentially, the results could be useful to understand the effect of radial and circular polarizations inside and outside medium for various applications of NF optics.     

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.     

Study on beam propagation through a double-adaptive-optics optical system in turbulent atmosphere

Beam propagation through a double-adaptive-optics optical system in turbulent atmosphere has been investigated. A typical model of optical systems with the double-adaptive-optics configuration is established, principles of the two adaptive optics and theory of beam propagation through the optical system in turbulent atmosphere are analyzed. Power efficiency and beam quality (BPF) of the received beam are introduced to evaluate performance of the optical system. Under the H-V 5/7 turbulent model, influences of the two adaptive optics installations are numerically calculated, respectively. Results show that the adaptive optics at the launcher can improve both power efficiency and beam quality of the received beam, the adaptive optics at the receiver can significantly improve beam quality of the received beam. Evolution of the system performance with the increase of adaptive optics correction order is numerically simulated, and results show that the double-adaptive-optics configuration has great advantages in improving beam quality of the received beam in optical systems with beam long-distance propagation through the turbulent atmosphere.     

Nonparaxial propagation of Hermite-Laguerre-Gaussian beams in uniaxial crystal orthogonal to the optical axis

Analytical expressions for the three components of the nonparaxial propagation of a Hermite-Laguerre-Gaussian （HLG） beam in uniaxial crystal orthogonal to the optical axis are derived. The intensity distribution of an HLG beam and its three components propagating in a uniaxial crystal orthogonal to the optical axis are demonstrated by numerical examples. Although the y and z components of an HLG beam in the incident plane are both equal to zero, they emerge upon propagation inside the uniaxial crystal. Moreover, the beam profile of the x component is relatively stable and the beam profiles of the y and z components have the same evolution law. If the ratio of the extraordinary refractive index to the ordinary refractive index is larger than unity, the beam profile of the HLG beam is elongated in the x direction and generally rotates clockwise. Otherwise, the beam profile of the HLG beam is elongated in the y direction and generally rotates anticlockwise. This research is beneficial to the optical trapping and nonlinear optics involved in the rotation of a beam profile.     

神龙二号多脉冲电子束束参数的时间分辨测量技术

神龙二号是一台三脉冲强流脉冲电子束直线感应加速器,就其多脉冲的电子束束参数的测量而言,基本要求是单个脉冲可分辨,进一步的要求是脉冲内时间可分辨。基于光学渡越辐射原理及瞬态发射度测量系统原理,发展了一种束斑与发散角可以分开测量的光学布局结构,结合多台高速分幅相机,成功研制了一套完整的多脉冲电子束束参数的测量系统,其特点是灵活的组合测量方式,全面满足了神龙二号复杂艰难的调试及参数测量工作要求。测量系统最高时间分辨测量能力达到约2 ns的水平,单个脉冲可以获得至少8个时间分辨的束参数测量结果。     

Application of the holographic carrier fringe and FFT technique for deformation measurement

This paper describes a holographic system based on the use of fibre optics and automatic spatial carrier fringe pattern analysis. Carrier fringes are generated by simply translating the object beam between two exposures. Single-mode optical fibres are used to transfer both the object and reference beams. The fast Fourier transform method is used to process the interferograms: it extracts phase from fringe patterns resulting from the interference of tilted wavefronts. The method is illustrated by measuring the deformation of an arbitrarily clamped, uniformly loaded circular plate. The results are given for the perspective plot of the out-of-plane deformation field, the maps of wrapped and unwrapped phase, and a contour map of the unwrapped phase.     

Telescope illumination and beam measurements for submillimeter astronomy

Submillimeter receivers which operate in the highest frequency atmospheric windows must still be mounted on large infrared and optical telescopes. Finding the optimum submillimeter wave illumination for these large telescopes is simplified by their effectively perfect optical surfaces and long focal lengths, but is complicated by telescope optics which have been optimized for non-tapered beams. In order to determine the effects of changing illumination edge taper, we calculated idealized aperture and beam efficiencies for telescopes which are used for submillimeter astronomical observations. The optimum illumination edge taper for the large infrared telescopes is near 10 dB, somewhat less than for classical radio telescopes, and is not very critical as long as it is between 10 and 13 dB.We also present an accurate method for measuring the sizes of, and deviations from, Gaussian beam pattern distributions by using astronomical sources which have sizes comparable with the beam.     

喇曼感生克尔效应光谱的琼斯矩阵分析

介绍喇曼感生克尔效应光谱(RIKES)的琼斯(Jones)矩阵分析方法.探测光束的传输强度不仅由所经过的每一个光学器件的琼斯矩阵所决定,而且还受到强的泵波在非线性介质样品中感生依赖于强度的二向色性和双折射(克尔效应)对琼斯矩阵的影响.同时计及样品和光学器件由强泵波作用下感生应力和其他外部产生的线双折射对喇曼感生克尔效应光谱观察的不利影响,导出测量系统的功率传输函数的完整表达式和喇曼感生克尔效应光谱的实现观察条件,最后简述以甲笨(C_7H_8)液体为试样的喇曼感生克尔效应光谱实验结果分析.     

Systematical study of the trapping forces of optical tweezers formed by different types of optical ring beams

The technique of optical tweezers has been improved a lot since its invention, which extends the application fields of optical tweezers. Besides the conventionally used Gaussian beams, different types of ring beams have also been used to form optical tweezers for different purposes. The two typical kinds of ring beams used in optical tweezers are the hollow Gaussian beam and Laguerre--Gaussian (LG) beam. Both theoretical computation and experiments have shown that the axial trapping force is improved for the ring beams compared with the Gaussian beam, and hence the trapping stability is improved, although the transverse trapping forces of ring beams are smaller than that of Gaussian beam. However, no systematic study on the trapping forces of ring beam has ever been discussed. In this article, we will investigate the axial and transverse trapping forces of different types of ring beams with different parameters systematically, by numerical computation in which the ray optics model is adopted. The spherical aberration caused by the refractive index mismatch between oil and water is also considered in the article. The trapping forces for different objectives that obey the sine condition and tangent condition are also compared with each other. The result of systematical calculation will be useful for the applications of optical tweezers formed by different types of ring beams.     

机载激光通信链路中光场二阶特性的分析计算

为了评估大气湍流对机载激光通信系统的影响,理论分析和数值计算了机载激光通信链路中传输光场的二阶特性,包括接收光斑尺寸、光斑偏移量、光束相干半径、光束到达角起伏。计算结果表明:大气湍流对空间激光通信系统的影响主要在低空对流层,并随着机载平台高度的增加而降低。由于受到低空大折射率结构常数的影响,上行链路的接收光斑尺寸、光斑漂移量比下行链路大。下行链路光束到达角抖动比上行链路大,其值为若干rad。提出的模型及计算结果,可以为机载激光通信系统设计及性能评估提供参考。     

轴对称静电场的束流光学模拟与设计

为了对高压倍加器和静电加速器等低能加速器中一些具有特殊结构的轴对称静电元件进行模拟和设计,采用传输矩阵法编写了直流束的束流光学计算程序,并利用该程序对高频离子源预聚焦系统和移动式加速器中子源的束流光学系统进行了模拟和设计。程序将整个轴对称静电场区域看作厚透镜,并均分成若干个小区间,先利用其他电磁场软件计算区域内的轴上电势分布,然后根据该电势分布计算每个小区间内的束流传输情况得到束流的包络曲线。该程序可以用于计算非线性效应可忽略的复杂轴对称静电场中强流和弱流束的传输,且所需计算时间很短。