Imaging properties of photon sieve with a large aperture

We report the optimization design and experimental results for the imaging properties of a photon sieve, which is formed on a layer of metal film supported by a thin glass substrate. As an example, we considered a micro-optical element with parameters of diameter D=50 mm, 3,564,290 hole number, and 10 μm minimum micro-hole diameter, which was designed and fabricated by means of surface machining technique in the lab. To evaluate its imaging performance, both on-axis and off-axis imaging experiments were carried out using the element. Compared to a Fresnel zone plate lens with the same feature size, the photon sieve has super imaging performance. Some quantitative analyses and initial qualitative explanations were given for the imaging characteristics.     

Photon density of states in polarization-entangled laser operation

In the context of nonlinear interaction between a classical pump field and two polarization-entangled photon modes, we derive mathematical expressions involving the photon occupation number and photon density of states in polarization-entangled laser operation at low temperature.     

卫星激光通信跟瞄精度测试方法及其实验研究

卫星激光通信捕获、跟踪、瞄准(ATP)需要端机具备有微弧度(μrad)量级的高精度跟踪能力。如何检验通信端机的这一特性是一个很重要的问题。设计了一套端机跟瞄精度测试装置。装置模拟入射光方向的偏转,端机跟踪从偏转方向发出的回馈信标光。通过测量回馈光远场光斑的位置变化可以得到端机的跟瞄精度。测试实验表明,当入射光方向的变化小于4μrad,测试频率在0~250Hz时,测试系统的精度可达到0.2μrad。此项工作对卫星激光通信的光链路通信的研究以及对端机性能的整体评估具有重要意义。     

Joint influence of heterogeneous stochastic factors on bit-error rate of ground-to-ground free-space laser communication systems

Research methods for the formation of BER in the ground-to-ground Free-Space Laser Communication Systems (Free-Space Optics – FSO) with increased analytical effectiveness have been suggested. They correspond to physically rightful approximations with approbated quantity effectiveness. The methods have been used to study the influence of heterogeneous stochastic factors, such as: quantum noises, atmospheric transmittance fluctuations, mechanical vibrations of the transmitting laser antenna, atmospheric turbulence, on BER of FSO.

An algorithm for quantity estimation of the joint influence of all stochastic factors has been created. Numerical examples, illustrated by graphic material, have been shown.     

Impact of spatial modulation on the third harmonic wave propagation and far-field focal spot in frequency conversion system

Third harmonic generation (THG) of high power laser is discussed in KDP crystals utilized in frequency conversion systems of large laser facilities. The far-field focal spot of the third harmonic wave is presented based on numerical simulation of the nonlinear coupled-wave equations, in which the walk-off and paraxial diffraction are taken into account and the electric field of the fundamental wave (1053 nm, 1ω) pulse is phase and amplitude modulated in spatial domain. Impact of the modulation depth and frequency on the focal spot energy, the side lobes location and conversion efficiency are analyzed in detail. The results show that the side lobes location is equivalently determined by the modulation frequency of both phase modulation and amplitude modulation, while the energy-concentration is decreased mostly because of the 1ω modulation depth. Relatively, the phase modulation plays a more important role than the amplitude modulation in decreasing main lobe energy for different reasons. The phase modulation makes the energy flowing from the main lobe to side lobes, while amplitude modulation not only makes the energy flowing but also decreases tripling efficiency significantly.     

合肥光源单束团下束团长度和能散的测量

根据同步光与储存环中的束流信号具有相同的时间结构的原理,测量同步光脉冲的半高全宽值可以计算出束团的长度。根据合肥光源的特点和实际需要,选择快速光电接收器搭配高速高带宽示波器作为在线测量束团长度和纵向分布等的主要手段。对单束团模式下束团长度随流强和高频腔腔压的变化趋势进行了测量。测量结果表明:束团长度与腔压的0.3次方成反比,比理论值0.5小;而束团长度随流强的增长率为2.0 ps/mA。通过测量纵向量子寿命进行了能散随流强变化的间接测量,结果表明,束团的拉伸是能散变化和势阱效应共同作用的结果。     

Modal correction for fiber-coupling efficiency in free-space optical communication systems through atmospheric turbulence

High-speed free-space optical communication systems have recently used fiber-optical components. The coupling efficiency with which the received laser beam can be coupled into a single-mode fiber is noticeably limited by atmospheric turbulence due to the degradation of its spatial coherence. Fortunately, adaptive optics (AO) can alleviate this limitation by partially correcting the turbulence-distorted wavefront. The coupling efficiency improvement provided by Zernike modal AO correction is numerically evaluated. It is found that the first 3-20 corrected polynomials can considerably improve the fiber-coupling efficiency. The improvement brought by AO is compared with that brought by a coherent fiber array. Finally, a hybrid technique that integrates AO and a coherent fiber array is proposed. Results show that the hybrid technique outperforms each of the two above-mentioned techniques.     

On determining the spot size for laser fluence measurements

Energy fluence, defined as pulse energy over irradiated area, is a key parameter of pulsed laser processing. Nevertheless, most of the authors using this term routinely do not realize the problems related to the accurate measurement of the spot size. In the present paper we are aiming to approach this problem by ablating crystalline Si wafers with pulses of a commercial KrF excimer laser (λ = 248 nm, τ = 15 ns) both in vacuum and at ambient atmosphere. For any pulse energy, the size of the ablated area monotonously increases with increasing number of pulses. The difference in the ablated area could be as high as a factor of three when 2000 consecutive pulses impinge on the surface. The existence and extent of the gradual lowering of multi-pulse ablation threshold queries the applicability of routinely used procedure of dividing the pulse energy with the size of the ablated area exposed into either carbon-paper or a piece of Si with one or a few pulses when determining the fluence. A more quantitative way is proposed allowing comparison of results originating from different laboratories.     

光谱扫描滤波法提升超强飞秒激光相干信噪比分析

光谱调制所形成的预脉冲和后沿脉冲的时频谱图将等间距、 平行地分布于啁啾主脉冲时频谱图的两侧, 基于光谱调制啁啾光脉冲的这种时频谱特点, 采用扫描滤波方式来提升其对比度. 数值模拟分析了扫描滤波过程中时间抖动、 扫描率差异和通带宽度等因素对扫描滤波效果的影响, 结论表明, 在扫描轨迹与信号光频率相同的情况下, 控制通带宽度, 相干信噪比提升率接近0.41F²倍, 而啁啾信号光以超过90%的透光率通过光谱扫描滤波器. 关键词： 超快激光技术 相干信噪比 光谱扫描滤波 短时傅里叶变换法     

湍流对地-空光通信系统性能的影响及自适应光学补偿

空间通信在地 空通信中具有广阔的应用前景。大气湍流引起的相位噪声会严重地影响系统的性能,而自适应光学补偿可使问题得到很好的解决。分析了湍流引起的相位噪声问题,并以Hufnagel湍流模型为例,借助结构函数的概念对这一噪声进行了计算。论证了自适应光学补偿方案。     

表面损伤衍射双向反射分布函数模型建立及分析

建立了包括划痕和坑点在内的表面损伤的衍射双向反射分布函数(BRDF)模型,并分析了模型在各领域中的应用.通过使用非傍轴标量衍射理论,提出了采用相干窗口函数滤波的方法得到非相干光条件下的表面损伤衍射BRDF模型,得到了表面划痕和坑点的散射特性.该方法在表面损伤检测、表面损伤杂光分析以及图像渲染技术等领域都有重要的应用价值...     

Investigation of the properties of a sharp-focusing schlieren system by means of Fourier analysis

The sharp-focusing schlieren system is analysed by means of a Fourier approach. The signal received in the imaging plane is governed by the phase distribution in a particular object plane normal to the optical axis, while the contributions from other planes in the object that are out-of-focus can be considered as noise. A numerical simulation performed for a specific test object shows that the best spatial resolution is obtained with a system of five channels and that further increase of the number of channels does not improve the signal quality.     

A simple technique for quickly measuring the spot size of Gaussian laser beams

A simple and accurate technique for measuring the spot size of Gaussian laser beams by scanning the beam with an opaque ribbon is proposed. The usefulness of this technique is verified experimentally.     

高功率激光装置光束准直系统新型远场监测技术

利用高功率激光装置空间滤波器小孔成像和取样光栅的衍射,设计出一套新型光路远场监测方案,并且在实验平台上进行了实验验证.实验结果表明：相对传统的远场监测方法,该远场监测系统通过侧面离轴光栅取样灵活利用空间,其调整平均误差为空间滤波器小孔直径0.9%,能够满足准直系统远场调整精度(<小孔直径5%)的要求. 关键词： 激光技术 光束准直 光栅 远场     

大气环境下激光光斑瞄准偏差的补偿方法研究

针对大气环境下激光光斑瞄准偏差的问题,设计了激光光轴偏差测试系统,讨论了大气环境中由于诸多因素的影响而导致激光光斑几何中心与目标中心发生偏移而不能对准的现象。提出了数字图像处理技术与重心法相结合的一套新的偏差补偿算法。充分利用激光光斑全场数据,从理论和实验上对大气环境下的激光光斑瞄准偏差进行了分析和研究。外场实验结果表明,该方法可以对瞄准偏差进行有效补偿,具有较高的工程应用价值。     

对称分布的相位型互补衍射屏的夫琅禾费衍射

为了验证巴比涅原理在相位型互补衍射屏的应用,建立了相位型互补衍射屏的数学模型,由理论分析得出了二者复振幅透过率的表达式,与用傅里叶光学方法得出其在频谱面上光强分布的表达式相同.由计算机模拟得到抛物柱面相位型互补衍射屏的夫琅禾费衍射光强的分布规律,与理论分析一致,得出分布规律为偶函数的相位型互补衍射屏的衍射符合巴比涅原理.     

频域近红外光谱法研究生物组织内部光子传输特性

基于光子在生物组织中的辐射传输理论以及Feng模型,应用频域近红外光谱法研究生物组织中异质体位置的变化,对出射光的光强和相位变化的影响及规律。设计了一仿真实验,用牛奶代替强散射性质的生物组织,并在牛奶中放置一个高度可控的具有一定吸收系数和散射系数的小球。移动小球在牛奶中的高度,检测出射光的交流幅度AC、直流光强DC和相位延迟Phase的值,绘制小球位于不同深度时AC,DC和Phase的曲线,并探讨其变化规律。结果表明,随着小球在牛奶中深度的变化,光强AC,DC和相位Phase呈现一定的相关性;随着光源和检测器之间距离的增加,检测到的光强和相位曲线的波谷点均向右偏移;当小球偏离光源和检测器越远,对检测到的光强和相位的影响越小。验证了光子在生物组织中的传输规律,为用频域近红外光谱法进行组织光学参数的检测及组织中异质体位置的定位奠定了基础。     

A two-dimensional diffraction pattern sampling system for determining the optimum parameters of a matched spatial filter

The synthesis of a matched spatial filter based on holographic methods requires that the spatial frequency band in which the holographic fringe pattern with the best modulation is formed be tuned to the appropriate weighting at the Fourier transform plane of the spatial frequencies of the input object to be studied. For this purpose both the spectral intensity distribution of an input object at the Fourier transform plane and the intensity distribution of a reference beam at that plane must be accurately known beforehand. A two-dimensional diffraction pattern sampling system has been constructed which enables the Fourier spectral intensity distribution of the input object and the reference beam intensity distribution to be faithfully recorded. A preliminary experimental study shows the usefulness of this system.     

Photon statistical properties of the cavity field in the two-atom Jaynes-Cummings model

The model that two two-level atoms interact with a singel-mode cavity is studied. The exact solution of the time evolution operator for the two-atom Jaynes-Cummings model is presented by the bare-states approach. Furthermore, we investigate the dynamical     

激光光斑形状和尺寸对扫描显示中散斑对比度的影响

计算了圆形、矩形激光光斑对于激光扫描显示系统中散斑对比度的影响.对于圆形光斑,当光斑尺寸大于散射表面相关长度时,得到的散斑对比度随着光斑尺寸变小而下降;当光斑尺寸接近表面相关长度大小时,由于光斑中包含的散射颗粒变少,得到的是非高斯散斑,散斑的对比度反而会变大;随着激光斑进一步变小,由于镜面反射效果散斑的对比度会很快下降.对圆形光斑部分结果给出了实验验证.为了保证光斑中有足够的散射颗粒,只在一个方向压窄光斑,分别计算了平行于扫描方向和垂直于扫描方向压窄光斑得到的散斑对比度.平行于扫描方向压窄光斑,散斑的时间相关性会下降,平均效果变明显,散斑的对比度变小;垂直于扫描方向压窄,散斑对比度变化不大.