三类构型激光脉冲压缩器光栅拼接误差容限比较

建立了包含拼接光栅的三类典型构型(双程Z型、X型及菱型)激光脉冲压缩器的理论模型. 利用光线追迹法计算压缩器引入的相位变化,结合傅里叶变换的方法仿真分析并比较了各压缩器相应的光栅拼接误差容限. 以远场焦斑峰值能量下降到理想值的0.9倍为限确定了三类压缩器光栅拼接误差容限,明确了不同构型激光脉冲压缩器对拼接误差的敏感程度. 这对脉冲压缩器的选型和设计具有指导意义. 关键词： 光栅拼接 脉冲压缩 误差容限 光线追迹     

双轴压缩下颗粒物质剪切带的形成与发展

本文采用离散元方法,研究了双轴压缩的颗粒体系在刚性边界约束下,局部剪切带的形成和发展过程,注重分析了细观的体积分数、配位数、颗粒旋转角度等参数以及力链结构形态的演变.并从颗粒体系jamming 相图中J点附近的边壁压强和配位数随体积分数的标度规律出发,分析了剪切带内外的体积分数和配位数的变化.结果表明:剪切带形成于颗粒体系的塑性变形开始阶段,此时体系发生剪胀,颗粒体积分数减小,颗粒体系抵抗旋转的能力降低,开始出现细小剪切带,随着轴向应变的继续,细小剪切带发生连接,最终导致贯穿性优势剪切带形成 关键词： 颗粒物质 力链 双轴压缩 剪切带     

Compensation of dispersion-induced power fading for highly linear radio-over-fiber link using carrier phase-shifted double sideband modulation

A carrier phase-shifted (CPS) double sideband (DSB) modulation technique in radio-over-fiber (RoF) system is proposed and experimentally demonstrated. By tuning the biases in a single-drive dual parallel Mach-Zehnder modulator (SD-DPMZM), the optical carrier in the DSB spectrum acquires additional phase shift. The transmittance response of a dispersive RoF link is thus being controlled and shifted in the frequency domain. Experiments successfully turned the maximum transmission frequency to 10?GHz and 15?GHz for both 25 and 39?km fiber links. This is also a highly linear scheme, of which a spurious-free dynamic range (SFDR) of 111.3?dB·Hz^2/3 is experimentally obtained.     

Reconstruction for limited-projection fluorescence molecular tomography based on projected restarted conjugate gradient normal residual

Limited-projection fluorescence molecular tomography (FMT) can greatly reduce the acquisition time, which is suitable for resolving fast biology processes in vivo but suffers from severe ill-posedness because of the reconstruction using only limited projections. To overcome the severe ill-posedness, we report a reconstruction method based on the projected restarted conjugate gradient normal residual. The reconstruction results of two phantom experiments demonstrate that the proposed method is feasible for limited-projection FMT.     

Tunable chirped microwave photonic filter employing a dispersive Mach-Zehnder structure

A Mach-Zehnder structure with modulation in one arm and dispersive time delay in the other is proposed to implement highly flexible single-bandpass chirped microwave photonic filters based on broadband optical sources. Both the amplitude response and the time delay slope can be fully reconfigured via control of the optical spectra and dispersion. The passband can also be widely tuned without changing the shape. A chirped filter with a bandwidth of ～4?GHz, a delay slope of ～-0.6?ns/GHz, and a tunability up to 40?GHz is demonstrated experimentally.     

Tunable 360° photonic radio frequency phase shifter based on optical quadrature double-sideband modulation and differential detection

We propose a novel structure of a photonic RF phase shifter based on the vector-sum principle. The optical signal with quadrature double-sideband modulation passes through a dual-output Mach-Zehnder interferometer (MZI), and the two outputs are differentially detected. Two phase-quadrature RF terms are generated, and their amplitudes can be controlled in a triangularly complementary way by changing the phase of the MZI. A full tuning range of 0°-360° at 14?GHz is demonstrated experimentally accompanied by nearly constant RF amplitude. The validity of using our scheme in all-optical RF phase modulation is also verified.     

Novel fabrication of net-like and flake-like Fe doped TiO2 thin films

New morphologies of net-like and flake-like TiO₂ thin films with different concentrations of Fe dopant were successfully fabricated by micro-arc oxidation (MAO) process of Ti plates and a subsequent chemical treatment of the as-prepared MAO-TiO₂ thin films. It was found that Fe ions can be easily introduced into the MAO-TiO₂ samples with the increase concentration of K₄(FeCN)₆·3H₂O precursor, and the amount of Fe determined the morphologies of TiO₂ thin films after chemical treatment; net-like morphology was observed with low Fe dopant, while it transformed to a flake-like one when Fe exceeds 1.7 at.%. UV-vis spectroscopy test showed that the absorption edge of the Fe ions doped TiO₂ thin films with new morphologies has an obvious red shift.     

The role of surface terminations on the band structure and optical properties of silicon nanonets

The ab initio calculations are carried out to investigate the effect of hydrogen, oxygen and nitrogen terminations on the properties of the band edge and the values of the band-gap, as well as the oscillator strength of the silicon nanonets (SiNNs). The oxygen functional groups are found to effectively preserve the direct band-gap nature of the SiNNs, and even change the luminescence properties of the silicon nanowires (SiNWs) to the direct band-gap transition. The appreciable oscillator strength of the first direct transition is obtained for the oxygen terminated nanostructure. The study on the electronic states indicates that the variation of the band edge caused by the surface terminations is attributed to the change of the state compositions. These surface modifications are thought to be useful for silicon band-gap engineering in the area of optoelectronics.     

Molecule oxygen-driven shaping of gold islands under thermal annealing

Fabrication of the gold micro/nano-structure in a controlled manner has recently attracted considerable interest for its potential applications. With the help of the AFM and XRD measurements, our research on the annealing of the evaporated thin gold films under different thermal environments reveals that O₂ molecule, of which the influence has been generally ignored in the prevenient literatures, can play a very important role in the formation of gold island film. It is experimentally found that the molecule oxygen-driven migration of gold atoms can only occur at a high enough temperature. The time-dependent morphological development of the thin gold film annealing in O₂ is also observed, which is determined by the reduction of the whole interface energy. The morphological features of the Au islands, such as the flat top surfaces and the steep edges, point to the essential role of the interaction between oxygen and the specifically oriented gold surface during the structural evolution.     

微通道局部加热气泡动力学实验研究

实验研究了在水平放置的低高宽比聚二甲基硅氧烷(polydimethylsiloxane,PDMS)微槽道(300μm×60μm)内的局部位置给予恒定热流密度条件下气泡的核化沸腾、生长和运动情况,实验中的工质采用FC-72,并用真空泵抽至室温下的饱和气压。研究发现微通道内起始沸腾需要比常规槽道更大的壁面过热度,液体流量和加热速率对气泡的生长和运动有很大的影响。