基于X射线的新一代深空无线通信

在空间无线通信的研究应用方面,微波和激光通信技术已比较成熟,但是也存在传输距离、通信速率有限等瓶颈问题,严重制约未来的深空应用.X射线由于波长短、穿透能力强,当X射线光子能量大于10keV(λ<0.1nm)时,在太空中几乎是无衰减的传输.因此可望在较小的体积、重量、功耗下实现远距离太空传输.然而,在我国对于X射线通信的应用研究目前尚属空白,美国航空航天局也刚刚提出并将此技术称为"革命性的概念".因此,利用X射线作为信息载体的新概念空间通信方法的研究具有重要的科学意义及应用前景,X射线通信技术不仅将对已有的微波和激光通信技术起到很好的补充,而且还能在微波和激光受到屏蔽的通信场合发挥独特作用.开展基于X射线的深空无线通信技术研究,主要包括开展X射线空间通信的传输理论研究,开展大功率、宽频带(GHz)X射线脉冲调制发射技术的研究,开展快速、极微弱X射线探测技术的研究,开展X射线深空通信的捕获、跟踪与对准技术的研究,以及开展高效率纠错编码理论与技术的研究.     

空间X射线通信链路建模与功率分析

从空间X射线通信传输理论出发,首先对X射线通信的功率传输过程进行研究,以加性高斯白噪声信道模型与自由空间损耗为基础,分析了深空X射线通信的信道特征及功率传输过程、推导了功率传输方程,建立了通信链路模型;其次,分析了深空X射线通信系统的误差来源,建立了基于单光子探测器的噪声模型,并给出了主要噪声来源的数学表达式,从而确定了通信核心参数通信距离、通信速率、误码率与信号功率之间的关系;最后,对不同阳极高压与不同调制方式下探测器端信号光子数进行实验监测,对功率传输方程及误差模型进行验证.实验结果表明,所给出的功率传输方程与误差模型可以对空间X射线通信过程进行较好地解释.根据此模型,计算了在满足一定的误码率和传输距离要求时对X射线源出射功率的要求,为空间X射线通信研究建立了理论基础,为核心元器件的研究指明了下一步的方向,同时有望对空间X射线通信的工程化提供一定的参考.     

X射线通信系统的误码率分析

基于栅极控制脉冲发射X射线源与单光子探测技术的X射线通信语音方案已经在实验室实现, 为探索未来X射线深空通信应用打下了坚实的基础. 实验室X射线语音通信演示系统实现后, 迫切需要测试X射线通信系统的误码率性能. 在泊松噪声模型下对X射线通信演示系统的理论分析的基础上, 使用基于现场可编程门阵列的误码率测量方法对开关键控调制方式下X 射线通信误码率进行测量. 通过实验测量发现, 要实现语音通信, 系统误码率应该达到10^-4 量级; 分析、测量了现有系统在开关键控调制方式下不同速率对应的误码率, 论证了泊松噪声模型理论分析现有X射线通信系统的合理性; 分析提出了限制现有实验室条件下X 射线通信误码率性能的主要因素.     

一种X射线聚焦光学及其在X射线通信中的应用

基于栅控脉冲发射X射线源与单光子探测技术的X射线通信已经实现了实验室语音通信验证,并对通信系统的误码率性能进行了分析,为探索未来X射线深空应用打下了坚实的基础.针对目前X射线通信面临的信号发散角大、通信距离短、难以实现工程化应用的情况,迫切需要对X射线通信天线系统进行深入研究.为了提高信号增益、增大X射线通信的距离,提出了多层嵌套式X射线聚焦光学作为X射线通信的"收发天线",理论分析了X射线聚焦光学用于X射线通信"收发天线"的可行性,分析了X射线聚焦光学的理论基础与结构设计,对"发射天线"发散角、"接收天线"有效面积与焦斑尺寸、信号增益等性能做了探讨.结果表明:在信号发射端,"天线"的发散角为3 mrad左右,发射增益23 d B;在信号接收端,"接收天线"的有效面积5700 mm2@1.5 keV,焦斑直径为4.5 mm,接收增益为25 d B,通信系统总的增益可达48 d B.     

空间激光通信最新进展与发展趋势

空间激光通信凭借其带宽优势,成为未来高速空间通信不可或缺的有效手段,是近年来国际上的研究热点。本文详细介绍了美国、欧洲和日本在空间激光通信技术领域的最新研究进展和未来发展规划,总结了国内外空间激光通信演示计划的主要参数指标。通过对空间激光通信最新研究计划的分析,归纳出空间激光通信高速化、深空化、集成化、网络化、一体化5个发展趋势,以及需要突破的高阶调制、高灵敏度探测、多制式兼容、"一对多"通信等关键技术。为我国激光通信设备及相关研究提供借鉴和参考。     

基于超导纳米线单光子探测器深空激光通信模型及误码率研究

高速深空通信是深空探测的关键技术之一,具备单光子灵敏度的激光通信系统将大大提高现有的深空通信速度.然而,单光子条件下的激光通信不仅需要考虑传输环境的影响,还需要考虑实际单光子探测器性能和光子数量子态的分布.本文在不考虑大气湍流影响的情况下,以光电探测模型为基础,引入超导纳米线单光子探测器(SNSPD)系统的探测效率和暗计数,建立了反应系统差错性能的数学模型,提出了系统误码率的计算公式.先对公式中的光强和激光脉冲重复频率对误码率的影响进行仿真,再通过实验结果验证仿真模型.结果表明,光强对误码率的影响最明显,随着光强从0.01光子/脉冲到1000光子/脉冲的增加,误码率从10~(-1)到10~(-7)量级明显下降;激光脉冲重复频率对误码率的影响受到不同光强的制约,但都随着脉冲重复频率的增加呈下降趋势.与此同时,当增加光强或者提高速度时,误码率高于仿真结果,约在10~(-4)量级,其原因可能是实际通信中调制光信号的消光比不足和光纤引入背景噪声提高了系统暗计数.以上模型和实验结果为进一步开展基于SNSPD的月球-地球、火星-地球等高速深空激光通信奠定了基础.     

太赫兹波通信技术研究进展

太赫兹波段位于红外和微波之间,在空间通信、近程战术通信等国防领域有巨大的应用前景。总结了太赫兹通信技术的特点、类别、方案及太赫兹波通信系统涉及的关键技术。介绍了近几年国内外太赫兹通信技术研究现状及取得的成果,在0.14 THz频段通信距离达到20 km,0.3 THz处通信传输速率高达105 Gb/s。分析了全电子学方法、光电子方法、量子级联激光通信等几种通信技术路线,并比较了各系统的优缺点。展望了太赫兹通信技术的发展趋势和应用前景。     

基于超快激光调制的纳秒脉冲X射线发射源

面向基础科学与空间应用研究领域对小型化超快脉冲X射线发射源的需求,设计并研制了基于激光调制光源与光电阴极X射线管的超快脉冲X射线发生器,解决了传统X射线调制发射装置重复频率低、时间稳定性差、脉冲特性差等应用难题.本文主要开展了脉冲X射线发生器的超快调制控制模块研究,并利用基于预调制的激光控制光源实现了高时间精度、高时间稳定度的超快时变光子信号以及纳秒脉冲X射线产生.理论方面,建立了脉冲X射线发生器时间响应模型,分析了出射脉冲X射线的时域时间特性.实验方面,搭建了基于超快闪烁体探测器的脉冲X射线时间特性实验测试系统,测试了激光控制光源及脉冲X射线发射源的时间特性参数.实验结果表明脉冲X射线发生器可同时实现高重频(12.5 MHz)、超快脉冲(4 ns)、高时间稳定度(400 ps)特性,且与所建立的理论模型高度符合.相比于传统X射线调制方案,脉冲时间参数指标得到了大幅提升、应用场景获得了极大拓展,本项研究有望为实现超高时间稳定性、超快脉冲X射线发射源提供新思路.     

X射线二极管时间特性研究

研究X射线二极管(XRD)时间特性.XRD 是构成软X射线能谱仪的主要部件，它用于激光等离子体发射软X射线谱测量.实验利用激光聚变研究中心的200TW激光器(激光能量～6J，脉冲宽度～30fs)打金箔靶产生的X射线发射谱,用滤片(Al)-XRD探测系统测量，探测信号由高频电缆(SUJ-50-10)传输和宽带示波器(TDS694C和TDS6604B)记录.实验数据进行了线性拟合和比对分析. 关键词： X射线二极管 时间特性 软X射线能谱仪 数据处理     

超稳光生微波源研究进展

随着科技的进步以及精密测量应用技术的不断提高,超稳微波源的稳定度和噪声水平等技术要求不断提高,应用范围愈加广泛,包括高性能频标研究、网络雷达研制、深空导航系统等方面.基于超稳激光和飞秒光梳的超稳光生微波源是目前频率稳定度最高的微波频率源,相对频率稳定度可达10~(16)@1 s量级.该装置也是未来频率标准(光频标)推广应用的基础,无论是时间的产生还是绝大多数的精密测量,都需要将光频标的输出激光变换为超稳的基带频率信号后才能够实现.本文介绍了超稳光生微波源技术的发展、现状和应用需求.以国家授时中心研制的国内首套超稳微波频率源技术为主线,介绍了超稳光生微波源的原理和结构以及各组成部分的技术发展情况:超稳激光方面,着重介绍超稳光学腔研究和研制的进展以及Pound-Drever-Hall锁频技术、剩余幅度调制等噪声抑制技术;飞秒光梳方面,着重介绍目前最常用的掺铒光纤光梳系统的激光锁模、频率控制等技术发展;低噪声光电探测方面,着重介绍宽带光电探测噪声抑制技术和激光幅度噪声引起微波相位噪声的抑制技术.最后对光生超稳微波技术进行了总结和展望.     

Guided-Wave Two-Dimensional Acousto-Optic Scanner Using Proton-Exchanged Lithium Niobate Waveguide

Two-dimensional scanning of a 0.6328 mum guided-light beam has been realized using noncolinear acousto-optic (AO) coplanar Bragg diffraction together with colinear AO guided-mode to substrate radiation-mode conversion in a Z-cut Xpropagation LiNbO3 proton-exchanged (PE) waveguide. The two surface acoustic (SAW) waves utilized are at the center frequencies of 500 and 200 MHz, propagating in the Y and X axes, respectively. Two-dimensional scanning of approximately 720 resolvable light beam spots, namely, 18 40 (horizontal vertical) scanning, has been demonstrated using a light beam of 1.0 mm aperture. The total number of resolvable beam spots can be greatly increased from 720 by simply utilizing SAW transducers of larger bandwidth and a light beam of greater aperture. It should also be possible to significantly increase the diffraction efficiency from 3 % by optimizing the parameters of the PE waveguide and the SAWs.     

Effect ofγ-radiation on the spectral luminescence characteristics of impure cadmium tungstate crystals

Results of a study of the effect of γ-radiation on the spectral luminescence properties of cadmium tungstate crystals doped with silver, bismuth, and molybdenum cations are presented. Spectral characteristics of the nondnnnoped crystals are briefly described. Absorption and photo and X-ray luminescence spectra of the crystals taken before and after exposure to γ-radiation (5.5·10⁴ Gy) are compared. It is found that the spectral characteristics of the crystals doped with silver, bismuth, and molybdenum cations do not change markedly after the exposure. The relation between the type of impurity-induced defects, individual characteristics of the impurity cations, and the character of the effect of γ-radiation on the spectral luminescence properties of impure crystals is analyzed (preliminarily). Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 1, pp. 55–60, January–February, 1997.     

Femtosecond Transform-Limited Pulse Generation by Compensating for the Linear Chirp of SPM Spectra in Dispersion Shifted Fibers

A simple technique of pulse compression, based on the linear chirp compensation of self-phase modulation (SPM) spectra in dispersion shifted fibers, is demonstrated. The optimization procedure is carried out, for a short span of a single-mode fiber, using a parabolic law, which describes the behavior of the squared output pulse width versus the pump peak power in the case of Gaussian pulses. The experimental results give a minimum pulse duration of 233 fs, which is in good agreement with the model. Shorter and coherent pulses, down to 90 fs, have been obtained by inserting an interference filter at the optical output.     

Sorting of bending magnets for the SSRF booster

The Shanghai Synchrotron Radiation Facility(SSRF)booster ring,a full energy injector for the storage ring,is deigned to accelerate the electron beam energy from 150MeV to 3.5GeV that demands high extraction efficiency at the extraction energy with low beam loss rate when electrons are ramping.Closed orbit distortion(COD)caused by bending magnet field uniformity errors which affects the machine performance harmfully could be effectively reduced by bending magnet location sorting.Considering the affections of random errors in measurement,both ideal sorting and realistic sorting are studied based on measured bending magnet field uniformity errors and one reasonable combination of bending magnets which can reduce the horizontal COD by a factor of 5is given as the final installation sequence of the booster bending magnets in this paper.     

Measurement of nonlinear coefficient of optical fiber based on small chirped soliton transmission

We measure the waveform and phase curves of short optical pulses before and after transmission over different lengths of fibers by use of the pulse analyzer with the frequency-resolved optical gating (FROG),and numerically simulate pulse evolution under the experimental conditions.The nonlinear coefficient of the fiber is given by comparing the experimental results with the numerical ones.Difference between the experiment and numerical simulation is analyzed.     

Resonance scattering of canonical elastic shells in absorbing fluid medium

Resonance scattering of elastic spherical shell and cylindrical shell while the surrounding fluid medium has absorption is studied. The normal mode solution derived using exact elastic theory and the separation of variables is still applicable. However, the scattering form function has to be modified for the absorbing medium, otherwise the unreasonable result would be obtained. The backscattering form function in the absorbing medium is redefined, and the form function of elastic spherical and cylindrical shell with vacuum or solid matter filled is calculated in various absorption conditions. The results show that the absorption of surrounding fluid leads to notable attenuation of the coincidence resonances in the mid-frequency, but it has a little influence on the low-frequency resonance scattering induced by the filler inside the shell.     

Single-beam self-referenced phase-sensitive surface plasmon resonance sensor with high detection resolution

A versatile and low-cost single-beam self-referenced phase-sensitive surface plasmon resonance(SPR)sensing system with ultra-high resolution performance is presented.The system exhibits a root-mean-square phase fluctuation of ±0.0028.over a period of 45 min.i.e.a resolution of±5.2×10-9 refractive index units.The enhanced performance has been achieved through the incorporation of three design elements:a true single-beam configuration enabling complete self-referencing so that only the phase change associated with SPR gets detected,a differential measurement scheme to eliminate spurious signals not related to the sensor response,and the elimination of retardation drifts by incorporating temperature stabilization in the liquid crystal phase modulato .Our design should bring the detection sensitivity of non-labeling SPR biosensing closer to that achievable by conventional fluorescence-based techniques.     

Influences of SiO2 protective layers and annealing on the laser-induced damage threshold of Ta2O5 films

Ta2O5 films are prepared on BK7 substrates with conventional electron beam evaporation deposition.The effects of SiO2 protective layers and annealing on the laser-induced damage threshold (LIDT) of the films are investigated.The results show that SiO2 protective layers exert little influence on the electric field intensity(EFI)distribution,microstructure and microdefect density but increase the absorption slightly.Annealing iS effective on decreasing the microdefect density and the absorption of the films.Both SiO2 protective layers and annealing are beneficial to the damage resistance of the films and the latter is more effective to improve the LIDT.Moreover,the maximal LIDT of Ta2O5 films is achieved by the combination of SiO2 protective layers and annealing.     

A stereo matching algorithm using multi-peak candidate matches and geometric constraints

Gray cross correlation matching technique is adopted to extract candidate matches with gray cross correla- tion coefficients less than some certain range of maximal correlation coefficient called multi-peak candidate matches. Multi-peak candidates are extracted corresponding to three closest feature points at first. The corresponding multi-peak candidate matches are used to construct the model polygon. Correspondence is determined based on the local geometric relations between the three feature points and the multi-peak candidates. The disparity test and the global consistency checkout are applied to eliminate the remaining ambiguous matches that are not removed by the local geometric relational test. Experimental results show that the proposed algorithm is feasible and accurate.