采用多量子阱器件的无线光通信差拍接收方案研究

研究一种采用光折变多量子阱器件的无线光通信自差拍接收方案。与寻常相干接收方案相比,省去了中频跟踪电子系统;与量子阱器件零拍方案相比,采用中频窄带滤波器,提高了接收端机的输出信噪比。理论上研究了自差拍相干接收方案,从其物理机制出发,结合信噪比分析,探讨了影响相干接收方案的主要因素。建立了该方案的实验系统,并开展了有意义的实验研究。理论分析及实验结果表明,该方案对激光大气传输所引起的诸多不利影响皆有一定的抑制作用,因而有可能应用于卫星地面激光通信和地面无线光通信系统。     

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。     

孔径平均效应对采用相位补偿技术的空间相干光通信系统误码率的影响

基于大气湍流影响下的空间相干光通信系统模型和孔径平均效应的平面波模型,通过数值模拟研究了弱光强波动条件下孔径平均效应以及大气湍流内外尺度对相干光通信系统误码率和接收孔径直径最优值的影响。研究结果表明:孔径平均效应能够有效减小相干光通信系统的误码率,改善系统性能;原始信噪比越高,传输距离越短,波长越长,相位补偿模式的J值越大,接收孔径直径越接近最优值,孔径平均效应对误码率的改善效果越明显;孔径平均效应会影响接收孔径直径的最优值,相位补偿模式的J值越大,影响越明显;系统误码率和接收孔径直径最优值会随着大气湍流内尺度的增大而相应增大,随着大气湍流外尺度的减小而相应减小。研究结果将为空间相干光通信系统设计提供必要的理论依据。     

Photopolymer diffractive optical elements in electronic speckle pattern shearing interferometry

In this paper we present an electronic speckle pattern shearing interferometer using a photopolymer diffractive optical element in the form of a holographic grating, in combination with a ground glass to shear the images. The sheared images on the ground glass are further imaged onto a CCD camera. The distance between the grating and the ground glass can be used to control the shear and to vary the sensitivity of the system. The direction of sensitivity is easily controlled by rotation of the diffraction grating around its normal.Introducing photopolymer holographic gratings in ESPSI gives the advantage of using high aperture optical elements at relatively low cost. The fact that the diffractive optical element is a photopolymer layer on glass substrate with thickness of 2 mm makes for a compact optical system.The system was successfully used for detection of the resonant frequencies of a vibrating object.Most of the published work on vibration analysis is analytical. Very few experimental results are available in the literature. The well known laser Doppler vibrometers (LDV) and accelerometers used for modal analysis are pointwise measurement techniques, although multipoint LDV is available at significant cost.Electronic speckle pattern techniques suitable for experimental detection of the resonant frequencies of vibrating objects are very promising for vibration analysis because they are whole field and non-contact.A finite element model is developed for prediction of the vibration modes of the object under test. Detection of vibrational modes of aluminium diaphragm is demonstrated and compared with the theoretical model. The results obtained are very promising for future application of ESPSI systems with HOEs, for modal analysis. A significant advantage of shearography over electronic speckle pattern interferometry is that ESPSI is relatively insensitive to external disturbances. Another advantage of the proposed system is that it could be easily converted to a phase-shifting electronic speckle shearing interferometer.     

毫米级高分辨率的混沌激光分布式光纤测温技术

近年来,随着分布式光纤传感技术在各大基础设施健康监测领域的广泛应用,人们对能够实现毫米量级精准定位和监测技术的需求日益增长.本文提出了一种基于宽线宽混沌激光的高分辨率分布式光纤测温技术.实验通过改变光反馈混沌源的偏振匹配态和反馈强度等外部参数,产生了–3 dB线宽约为7.5 GHz的宽线宽混沌激光,并在300 m传感光纤实现了空间分辨率为7.05 mm的分布式温度测量.同时,为了抑制光源线宽增加造成的布里渊增益谱恶化,在泵浦路中引入了时间门控技术,其中经脉冲调制后的泵浦光峰值功率提高了约9.5 dB,同时脉冲调制使混沌互相关锁定于脉冲持续时间内,从而布里渊增益谱的信号背景噪声比由约2.28 dB提升为4.55 dB,最终实现了空间分辨率为3.12 mm的分布式温度测量.     

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。     

Airborne laser radar for mapping two-dimensional contours of aerosol concentration

An improved version of a compact dye laser radar system with real-time data analysis capability has been developed for aircraft installation. The system is designed for use in mapping two-dimensional contours of aerosol backscattering coefficients both above and below the aircraft. The laser radar is part of a larger system which has been developed to characterize the optical properties of aerosols as well as their spatial and temporal variability.     

Probability of fade for airborne laser communication system over exponentiated Weibull distribution under aperture averaging

In this paper, the probability of fade is performed for the airborne laser communication systems considering both the atmospheric and aero-optic effects. The atmospheric fluctuation is characterized by the exponentiated Weibull (EW) fading channel. The novel analytical expression is derived for probability of fade according to Meijer’s G function. The probability of fade is demonstrated to the simulation data with the Gamma–Gamma and log-normal distributions. The probability of fade is obtained for different flight altitudes and propagation distances. The probability of fade is analyzed in the airborne laser communication systems under the effect of aperture averaging in weak-to-strong turbulence regime. Results suggest that the proposed EW model is valid in airborne laser communication with high altitude, especially in the lower values of the irradiance under atmospheric turbulence and aero-optics effect. The fading of outage performance can be effectively mitigated by aperture averaging. Furthermore, this work is helpful for the compensate technique of system performance on airborne optical communication system.     

Terahertz imaging through self-mixing in a quantum cascade laser

We demonstrate terahertz (THz) frequency imaging using a single quantum cascade laser (QCL) device for both generation and sensing of THz radiation. Detection is achieved by utilizing the effect of self-mixing in the THz QCL, and, specifically, by monitoring perturbations to the voltage across the QCL, induced by light reflected from an external object back into the laser cavity. Self-mixing imaging offers high sensitivity, a potentially fast response, and a simple, compact optical design, and we show that it can be used to obtain high-resolution reflection images of exemplar structures.     

An improved method of detecting and processing for the beacon in satellite-to-ground laser communication links

In satellite-to-ground laser communication links, the motion and vibration of the satellite are important factors that affect the real-time requirements of pointing, acquiring, and tracking. In order to acquire again the beacon quickly and accurately after losing it for a short time, we propose an improved method of detecting and processing for the beacon. In contrast to the conventional design of processing systems for the beacon, the advantage of the system we designed is provided by the spatial environment, where the optical communication terminal is located, and the motion trace of the beacon can be predicted effectively. We perform numerical simulations and simulation experiments, with a sampling frequency of the CCD in a system of 60 Hz and frequency and amplitude of the simulation vibration signal of 1 Hz and 0.1 mrad, respectively. We assume the motion of the satellite to be of translation type. The prediction precision is not more than 6 pixels of the CCD after the algorithm converges and the experimental results correspond fairly to the numerical simulation.     

基于FPGA的激光光束自对准电气自动化系统设计

激光光束技术运用的越来越广泛,包括光通信技术,激光测距技术,激光瞄准技术等,其中光通信技术范围最广;在光通信过程中,需要面对光束发射端与接收端中心对齐的问题,增加了操作的难度;针对光通信激光光束对准难的特点,研究并设计了一套基于FPGA的激光光束对准系统,利用在FPGA芯片上设计软硬件速度快、稳定可靠、研发周期短等特点;整个系统以FPGA芯片为核心,辅以操作电路、自适应算法;实验表明, 该系统较好地符合了光束对准的要求。     

用于宽带啁啾脉冲光参量放大系统精确同步的新方法

基于非共线光参量放大,以波长800 nm飞秒激光的倍频光为抽运光,以激光二极管640 nm连续波激光为信号光,产生了与800 nm飞秒激光精确同步的无直流本底的1064 nm脉冲光。实验结果显示该1064 nm的光脉冲在脉冲能量和空间光束质量上均可作为1064 nm波长脉冲放大器的种子光。仅通过一级简单紧凑的非共线参量放大就可实现光周期量级的光参量啁啾脉冲放大系统的抽运激光和种子光的精确同步。     

Development of a laser-driven proton accelerator for cancer therapy

Recent advances in laser technology have made proton (light ion) acceleration possible using laser-induced plasmas. In this work, we report our work for the last few years on the investigation of a new proton therapy system for radiation oncology, which employs laser-accelerated protons. If successfully developed, the new system will be compact, cost-effective, and capable of delivering energy-and intensity-modulated proton therapy (EIMPT). We have focused our research on three major aspects: (1) target design for laser-proton acceleration, (2) system design for particle/energy selection and beam collimation, and (3) dosimetric studies on the use of laser-accelerated protons for cancer therapy. We have performed particle-in-cell (PIC) simulations to investigate optimal target configurations for proton/ion acceleration. We also performed Monte Carlo simulations to study the beam characteristics and the feasibility of using such beams for cancer treatment. Since laser-accelerated protons have broad energy and angular distributions, which are not suitable for radiotherapy applications directly, we have designed a compact particle selection and beam collimating system for EIMPT beam delivery. We also proposed a new gantry design to make the whole system compact to retrofit existing linac vaults. We have compared Monte Carlo calculated dose distributions using X-ray IMRT and laser-proton EIMPT. Our results show that EIMPT using laser protons produces superior target coverage and much reduced critical structure dose and integral dose compared to X-ray IMRT.     

主三镜一体化离轴三反光学天线设计

为提高空间激光通信离轴三反光学天线的装调效率,需要对反射镜总的装调自由度进行优化设计。基于共轴三反无焦系统的设计原理,提出一种主三镜一体化的设计方法,并推导了光学系统结构参数之间的关系,利用Zemax光学软件设计了一种结构紧凑、主三镜一体化的离轴三反光学天线。设计结果表明:全视场光学像质优于衍射极限,主镜和三镜空间位置相接近,可共母板加工和面形共基准检测,为主三镜的一体化加工提供了方法。光学天线的装调自由度减少了6个,降低了装调难度,提高了装调效率。     

基于拉盖尔-高斯光束的单光子捕获概率研究

单光子源通常采用基于高斯光束的高度衰减激光脉冲,假设激光束具有初始高斯时域脉冲波形和TEM01模拉盖尔-高斯空域分布.基于折射率起伏的Rytov近似和修正von Karman谱模型,研究了大气湍流对星地量子通信单光子捕获概率的影响;建立了上行信道和下行信道的单光子捕获概率理论模型;针对低轨卫星-地面站间激光链路,对单光子捕获概率进行了分析.结果表明:上行信道的单光子捕获概率强烈依赖于地面折射率结构常数C2n(0),且随着C2n(0)的增加而减小;然而,下行信道的单光子捕获概率并不依赖于C2n(0),即大气湍流对其没有影响.     

星地激光通信中接收光信号的仿真方法

星地激光通信中, 由于大气湍流等影响, 接收光信号将出现包络起伏现象, 严重时将引起接收系统突发误码。为研究其中突发误码的发生规律及其对通信有效性的影响, 对接收光信号包络起伏的仿真方法进行了研究。针对常规的仿真方法在计算量、资源消耗、仿真时间长度等方面存在的诸多弱点, 提出了一种使用Gauss随机过程的幂级数拟合接收光信号幅度的仿真方法, 该方法利用接收光信号均值、方差、偏态系数和峰度系数计算各阶系数, 最高取到3次方项, 并使用FFT/IFFT实现时间功率谱特征拟合。仿真结果证明了该仿真方法的合理性。     

The combined effect of spatial compounding and nonlinear filtering on the speckle reduction in ultrasound images

Recently, a spatial compounding ultrasound imaging method was presented that utilizes a conventional 64-element phased array transducer with two unfocused pistons, each placed at one of the sides of the phased array transducer. This method is augmented here by inclusion of nonlinear filtering of the compounded images. The combined effects of the specific spatial compounding and nonlinear filtering on speckle reduction in the generated ultrasound images are studied and evaluated in two stages: First, the image quality is studied when nonlinear filtering is used as part of the spatial compounding. The study is performed by simulations using the Field II program, by processing several B-mode images of a kidney. The second stage compares the results obtained by the simulations to those obtained by in vitro laboratory experiments. Five different compounding strategies and two nonlinear filters, Gaussian and anisotropic diffusion, are investigated and evaluated in terms of image quality parameters-contrast and signal-to-noise ratio. It is shown that the combination of "averaging+nonlinear Gaussian filtering" produces the greatest improvement of image quality. When compared to a conventional phased array imaging system, the spatial compounding method that includes the conventional 64-element phased array transducer with two unfocused pistons, and employs the "averaging+nonlinear Gaussian filtering" strategy, obtains improvement in SNR that has reached 334%. Thus, though this method necessitates a somewhat wider probe, it produces significantly improved images.     

Computer generated optical holographic fringe patterns for the analysis of vibrating structures

Optical holographic interference fringe patterns on rods and bars vibrating in ultrasonic resonance (standing waves) are predicted by computer simulations. The simulations are based on a combination of finite element calculations with calculations based on the theory of holographic interference fringe generation. The hypothetical fringe patterns are then compared to holographic fringe images obtained by the experiment (time average holograms). The holographic arrangement and the mechanical part of the ultrasonic system (transducer, horn and specimen) were fixed on a commercial rigid optical table supported on air bags to isolate it from ground vibrations. Thus it is shown that the scope of the study can be reached with a minimum of experimental effort, i.e., with basic laser optical equipment. It is shown that provided the system operates in the desired mode, good agreement between the theoretical and the real image is given. Finally examples of the possible influence of defects or of parasitic modes of vibration on the appearance of the fringe patterns are given.     

Ultrafast three-dimensional submicrometer-resolution readout of coherent optical-phonon oscillations with shaped unamplified laser pulses at 20 MHz

An ultrafast three-dimensional readout of coherent optical-phonon oscillations from a diamond film is demonstrated using temporally and spectrally shaped ultrashort laser pulses, delivered by a compact, oscillator-only laser system. This system integrates a long-cavity ytterbium-fiber-laser-pumped 30?fs Cr:forsterite oscillator with a photonic-crystal-fiber soliton frequency shifter and a periodically poled lithium niobate spectrum compressor, providing coherent Raman excitation and time-delayed interrogation of optical phonons in diamond at a 20?MHz repetition rate with a submicrometer spatial resolution.     

灰霾背景下基于最优链路的低轨道量子卫星星间切换策略

为了应对灰霾对星地量子通信信道所带来的突发性干扰,根据灰霾天气下量子信号在自由空间中的衰减指数,本文提出了一种基于信号功率衰减最低的最优链路量子卫星切换策略。当目前的星地链路参数满足切换条件时,地面用户通过对不同信道中量子信号功率衰减系数的比对,测试到最小衰减卫星链路,基于对光子态Bell基的测量,完成卫星间的量子纠缠交换,建立新的纠缠信道,保证通信在切换过程中的连续性。仿真结果表明,当系统呼损率为5%,能见度为1km,地面用户数分别为50、100、300,轨道高度为300km和1400km时,量子卫星的切换成功率分别为95%、93.6%、91.8%和92%、90%、87%。由此可见,本策略能够在保证通信可靠性的前提下,实现星地间量子信道的平稳切换,提高量子卫星通信系统在灰霾背景下链路的有效性。