Probing time-resolved emission in laser-driven electron-multirescattering in a high-order harmonic generation

We present an ab initio study of quantitative extraction of the time–frequency spectra of multiple rescattering processes of the electrons driven by the mid-infrared laser field in a high-order harmonic generation (HHG). The HHG is calculated by solving the three-dimensional time-dependent Schrödinger equation by means of the time-dependent generalized pseudospectral method. We extend a synchrosqueezed transform (SST) technique to extract the individual contributions of multiple rescattering processes in HHG. Combining with an extended semiclassical analysis and the SST time–frequency spectrum, the role of quantum trajectories in multiple rescattering processes in HHG is clarified. We find that the SST allows us to distinguish the individual contribution of multiple rescattering processes in HHG and show the details of the spectral and temporal fine structures of the HHG, which provides an important tool for a deep understanding to the dynamics of multiple rescattering processes in HHG.     

脉冲调制下相噪测量的环路设计和定标方法研究

在窄脉冲调制下相位噪声测量的定标过程中,通过采用PRF滤波器提取主谱线技术,对主载频差拍频率的波形进行了还原,使定标过程较之传统方法简便快捷。在脉冲调制状态下的锁相环路设计中,采用PRF滤波器消除PRF谱线,避免锁相环路难以锁定或者偏锁,另外通过提高环路增益等方法解决由于窄脉冲调制造成的环路锁定能力降低。应用以上技术的脉冲调制波相位噪声现场测量装置分别以不同的环路带宽对不同占空比下的脉冲调制波源的相位噪声进行了测量,证明了上述方法的可行性。     

光电脉搏波信号处理中呼吸干扰的抑制

动态光谱的检测精度与光电脉搏波的幅值检测精度直接相关．为了获取较高的幅值检测精度,抑制光电脉搏波中的呼吸干扰,采用基于相干平均的脉搏波特征参数重构滤波和小波变换相结合的方法对光电脉搏波原始测量信号进行处理．首先用相干平均法获取脉搏波特征参数并重构光电脉搏波信号;然后对原始信号减去光电脉搏波后的剩余部分进行小波频率分析,并提取呼吸干扰,处理结果表明,本文的方法可以提取到高精度的光电脉搏波幅值信息,抑制大部分的呼吸干扰,从而改善动态光谱的检测精度．     

脉冲波声场的数值计算与测试研究

针对超声检测中影响缺陷定量准确性的声场问题进行了深入讨论,研究了脉冲波声场的计算方法,设计了相应的计算软件并绘制出实用换能器声轴线上的声压分布曲线,分析了不同材料和厚度的采样回波对频谱的影响,比较了连续波声场和脉冲波声场的分布特性及异同点。对脉冲波声场的测试结果表明,数值计算和实测结果有较好的一致性。本研究结果为提高超声检测中缺陷定量的准确性和科学性提供了依据。     

Stable supercontinuum pulse generated by modulation instability in a dispersion-managed fiber

We study on non-linear localized waves on continuous wave background in a dispersion and non-linearity management fibre. We find a stable supercontinuum pulse can be generated from a small modulation on continuous wave in a proper management way, for which the pulse spectrum width and its growth rate can be controlled well by the management parameters. Additionally, we demonstrate a Kuznetsov–Ma breather like non-linear localized wave can exist in a periodic dispersion management fibre, and its spectrum evolution is distinctive from the Kuznetsov–Ma breather’s.     

Phase-locked ultra-broadband supercontinuum generation in a mid-infrared polarization gating

We theoretically investigate the UV-assisted high-order harmonic generation (HHG) process in a mid-infrared polarization gating (PG) field. The ionization and recombination steps of the HHG process can be controlled simultaneously by the UV and PG pulses. The influences of the delay between the UV and PG pulses on the phase-locking degree and the intensity of the harmonics are presented quantitatively. The results show that the harmonics are phase-locked as well as the intensity is significantly enhanced by 3 or 4 orders at a proper delay. Then a phase-locked ultra-broadband supercontinuum with a bandwidth of 240?eV is produced efficiently, supporting the generation of a Fourier-transform-limited pulse duration below one atomic unit of time. In addition, efficient isolated 110 as pulses with tunable central wavelengths can be obtained directly by selecting a different range of the supercontinuum.     

Spectrum modification of XUV radiation in the presence of a delayed weak control field

We use a delayed weak laser beam to control the spectral features of extreme ultraviolet (XUV) pulses generated by a strong femtosecond laser beam through phase-matched high harmonic generation (HHG) in an atomic medium, e.g. krypton. The variation of the HHG spectrum reveals the influence of the free electrons on the propagation of the XUV field in the medium. In addition, a signature of the autoionization process is visible. Our findings provide a promising technique to study ultrafast dynamics of atomic and molecular gases.     

应用在缓冲包装设计中的冲击谱

根据包装件的冲击力学模型,推导出来正弦波脉冲、矩形波脉冲、后峰矩齿波脉冲等冲击波形的冲击响应,并证明了在缓冲包装设计中应用的缓冲材料最大变形冲击谱和内装物最大加速度冲击谱的一致性,绘制了三种冲击作用的冲击谱。     

Radiative model of post-breakdown laser-induced plasma expanding into ambient gas

The dynamics of the radiative plasma expansion into an ambient gas is considered. The model describes the evolution of the plasma emission spectrum and the dynamics of the resulting shock wave. The time frame for the applicability of the model is in the tens of nanoseconds after the laser pulse is terminated, until a few microseconds later when the plasma ceases to emit. It is assumed that local thermodynamic equilibrium is established and that the plume expands with spherical symmetry. The model outputs are spatial and temporal distributions of atoms, ions, and electron number densities, evolution of atom and ion line profiles, and the shock wave. The model should be applicable to spectroscopic analysis of the initial plasma state and plasma dynamics.     

Electron and photon beams interacting with plasma

A comparison is made between the interaction of electron bunches and intense laser pulses with plasma. The laser pulse is modelled with photon kinetic theory, i.e. a representation of the electromagnetic field in terms of classical quasi-particles with space and wave number coordinates, which enables a direct comparison with the phase space evolution of the electron bunch. Analytical results are presented of the plasma waves excited by a propagating electron bunch or laser pulse, the motion of electrons or photons in these plasma waves and collective effects, which result from the self-consistent coupling of the particle and plasma wave dynamics.     

Spatial chirp in the focusing of few-optical-cycle pulses by a mirror

We calculate the electric field of few-optical-cycle pulses at the focus of a perfectly conducting mirror by adding coherently the Airy diffraction patterns for each pulse frequency. We will show that the pulse suffers temporal spreading generated by a change in the spectrum of the pulse as a function of position in the focal plane, which introduces spatial chirp to the pulse. A double pulse appears near to the diffraction minima of the carrier frequency due to the variations in the spectra.     

Field ionization effects on ion acceleration in laser-irradiated clusters

Field ionization is included in our two-dimensional particle-in-cell simulations of femtosecond laser pulse interactions with submicron water clusters. Though laser field does not penetrate into the interior of submicron clusters, field ionization is shown to be efficient there due to the electric fields induced by quasineutrality violation due to expelled electrons by laser wave together with hot electron bunches which cross the cluster in the next half-period of the laser. Thus, oxygen atoms are ionized up to Z≥6 in the whole submicron cluster by 40 fs laser pulses of amplitude a₀≥0.5. The impact of oxygen ion charge state on the spectrum of accelerated protons is shown and comparison of the spectrum with simulations with fixed ion charge states is presented.     

频域提取法提高动态阻抗谱测量精度

血液成分的无创检测对于糖尿病等疾病的诊断和治疗具有重要意义．在血液成分无创检测领域,阻抗谱法血液成分无创检测技术具有重要应用价值．但血液成分复杂,并且各成分浓度变化对血液的阻抗特性影响较小．为了实现基于生物电阻抗的血液成分分析必须提高阻抗测量精度．以动态脉搏波阻抗谱原理为依据,并结合过采样技术,研究了动态阻抗谱的频域提取方法．用FFT频域提取基波分量代替幅值,克服了时域峰峰值提取的困难,同时采用过采样技术,可以实现过采样频率每提高一倍,信噪比提高3dB．实验结果表明,这两种方法结合可以显著提高动态阻抗谱测量精度．     

光孤子通信系统的色散控制研究

光纤色散特性是支撑光纤孤子通信的决定性因素。比较了目前的几种色散补偿方案后,着重对相位共轭控制的色散控制方式进行了分析研究,该方式利用光纤非线性四波混频产生与输入光脉冲信号相位共轭的输出光脉冲信号,光脉冲的频谱以泵浦光频为中心进行反转,构建成相位共轭器,实现了对放大自发辐射(ASE)噪声、孤子相互作用与色散波等的控制,达到提高系统传输速率、距离和通信容量的目的。     

Analytical Study of Moving Excitations for One-dimensional Homogeneous Lattice with Quartic Anharmonicity

Sincesomephysicalpr0perties0fs0lidsmaymainlyorpartlydependoncrystalanharmonicity,s0thatmuchattenti0nhasbeenpaidtothenonlinearexcitations(intrinsiclocalizedvibrationalmodes0rphon0nlocalizedstates)inducedbytheanharm0nic-ityinpurecrystallattice.Theseexcitationsmayhavesizableamplitudesandlocalizedononlyafewlatticesiteswitheither0dd[1J0re.en[2]typeformsinthehighlydiscreteandsufficientlystronganharmonicitycaselbutgenerallyinthel0ngcharacteristicwave-lengthenvelopeform[3~6].Theimpurityexcitati0nsonly…     

Fraunhofer-type absorption lines in double-pulse laser-induced plasma

We studied the confocal double-pulse laser-induced plasma in the very beginning of its life. It was found that the second laser pulse fired 0.7 to 5 μs after the first pulse produces plasma which, during the first 0 to 20 ns, resembles solar configuration. There is a very hot and compact plasma core that radiates a broad continuum spectrum and a much larger and cooler outer shell. The light from the hot core passes through the cold outer shell and is partly absorbed by atoms and ions that are in ground (or close to ground) states. This produces absorption lines that are similar to Fraunhofer lines observed in the sun spectrum. The possibility to use these absorption lines for new direct and calibration free laser-induced breakdown spectroscopy analytical applications, both in laboratory and industrial conditions, is proved.     

不同应力波穿过多条非线性变形节理时的透射特性

基于节理非线性位移不连续模型,分析不同弹性纵波正向入射多条节理时的透射规律。利用半解析半数值解法得到透射初至波质点振动速度一维数值差分解,进而研究不同类型、不同振幅的入射纵波穿过多条节理时,初至波质点速度幅值、能量、频谱及时间延迟的变化特性。研究表明,透射能力由大到小的顺序为：矩形波、半正弦波、三角形波。速度透射系数、能量透射系数、透射波中高谐波频域幅值均随入射脉冲幅值增大而增大;延迟时间随入射波振幅的增大而减小,且三角波入射时的时间延迟大于半正弦波。随着节理条数的增加,速度和能量透射系数逐渐降低,且下降速度逐渐减慢;高谐波频域幅值先增大后下降,且下降速度逐渐减慢;时间延迟增大的速度逐渐加快。     

脉搏波的频域特征提取与自动识别技术

脉搏波的频谱蕴含丰富的病理信息,但其复杂的频谱计算和分类是临床应用的瓶颈之一．本文引入模式识别技术,建立了心血管疾病的自动识别专家系统,为脉搏波频谱分析在临床中的应用开辟了新的研究思路．首先采用小波变换在多分辨率层次上提取脉搏波的频域特征,不仅获得了各个频带的谱能分量,而且得到了频谱分布参数小波熵;然后采用贝叶斯判别分析法建立自动识别模型,对频域特征进行分类．临床采集了30例冠心病人和30例正常人的脉搏波信号,对识别模型进行了训练,最后对模型进行了交互验证．结果表明,该识别模型对冠心病人的识别准确率为83．3％,对正常人的识别准确率为70．0％．该方法具有较好的识别效果,为脉搏波自动识别技术的发展提供了借鉴．     

强载波干扰条件下的罗兰-C相位编码识别研究

针对强载波干扰能够淹没罗兰-C信号,甚至严重影响罗兰-C接收机的信号处理的情况,分析了罗兰-C载波信号初相位的apFFT识别原理,对强载波干扰条件下利用全相位谱分析(apFFT)的罗兰-C信号相位编码软件识别方法进行了研究.基于apFFT谱分析方法用MATLAB对罗兰-C接收信号在强载波干扰等情况下的初相位识别进行了仿真.结果表明,apFFT谱分析方法既能识别出罗兰-C载波信号的初相位,又能分析出干扰频率及其强度,尤其信干比(SIR)在-40dB情况下仍可准确地识别出相位编码.     

理论研究垂直腔半导体光放大器脉冲放大特性

从速率方程和薄膜光学理论出发,对垂直腔半导体光放大器(Vertical Cavity Semiconductor Optical Amplifiers,简称VCSOAs)在脉冲工作情况下的动态增益进行了数值模拟.在计算中考虑了载流子和光强沿光轴方向的不均匀性,以及腔内介质折射率的不连续性对光波传输的影响.详细分析了反射模式下VCSOA在脉冲通过时载流子密度和瞬时增益的变化、输出光脉冲的畸变以及抽运光功率、分布布拉格反射镜(Distributed Bragger Reflector,简称DBR)周期数、输入脉冲能量以及脉冲宽度等参量对脉冲放大中的能量增益的影响.结果表明,能量增益随抽运光功率增大而增加;在峰值功率一定时输入脉冲宽度的增加将减小能量增益;输入脉冲能量的增加也会引起能量增益的下降,而适当减少顶部DBR的周期数可以改善这种脉冲放大的能量增益饱和特性.