Frequency-resolved optical gating of femtosecond pulses in the extreme ultraviolet

Femtosecond extreme ultraviolet (XUV) pulses were fully characterized for the first time by using a newly developed cross-correlation frequency-resolved optical gating (FROG) technique in the XUV region. This method utilizes laser-assisted two-photon ionization as a nonlinear optical process. Near-infrared pulses characterized by FROG were used as a reference. The amplitude and phase of XUV pulses with a pulse duration of 10 fs were found to be in good agreement with a model analysis, taking into account phase modulation by ionization, self-phase modulation, and the atomic dipole phase.     

振幅和相位涨落对锁模脉冲列特性的影响

 用傅里叶变换的方法分析了锁模激光器中模式间的相位和振幅涨落对锁模脉冲列特性的影响。将锁模激光器中各模式的相位和振幅涨落作傅里叶展开，再通过增益谱抽样的傅里叶变化得到其时域行为。分析结果表明振幅系统涨落及随机涨落均不引起脉宽的明显变化，只引起脉冲间背景的加大，但相位涨落随机会使脉冲列强度产生波动。所用物理图像简明，结果对获得稳定的锁模脉冲列的实验研究有参考意义。     

Coherent measurement of short laser pulses based on spectral interferometry resolved in time

We propose a new method for measuring the phase and the amplitude of a short laser pulse that is based on shearing interferometry in the spectral domain combined with time gating. The method has several features in common with spectral interferometry for direct electric-field reconstruction, in particular, fast and direct reconstruction of the phase. Accurate measurement of the phase added to an 80-fs pulse by a block of F4 glass demonstrates the technique.     

Estimation of ultrasound attenuation and dispersion using short time Fourier transform

Determination of the acoustic attenuation and dispersion has important applications in ultrasound tissue characterization and non-destructive material testing. Current signal processing methods Fourier transform of ultrasound signals to get the spectra of amplitude and phase to estimate respectively the attenuation and dispersion of a given medium. These methods are frequency domain method and obsessed with ambiguity issue in the phase unwrapping calculation. Conventional ultrasound velocity measuring method detects the time of arrival of a pulse (or echo) signal, which is a time domain method to compute group velocity (not phase velocity). This paper presents a novel approach based on the short time Fourier transform (STFT)--a time-frequency analysis, to estimate the ultrasonic dispersion and attenuation. Only the amplitude information of the pulse-signal spectra is used. Based on the time-frequency presentation, the attenuation coefficient of the signal is obtained by computing the amplitude decay of pulse spectrum in time domain, while phase velocities are obtained based on the "time-of-flight" (TOF) of the mono frequency component of the pulse signals. As a result, we eliminate the ambiguity issue in phase angle calculation. Furthermore, the proposed method makes the phase velocity pedagogically intuitive for novice users. The paper presents experiments to evaluate demonstrate the performance of the proposed method.     

Amplitude and Phase Retrieval of Ultrashort Optical Pulse Using Optical Spectrogram Scope

We demonstrate the retrieval of the amplitude and phase of an ultrashort optical pulse from an optical spectrogram obtained by optical spectrogram scope (OSS). OSS is an interferometric method of obtaining a single-shot optical spectrogram without nonlinear optical effects. Since the optical spectrogram obtained by OSS retains information on the amplitude and phase of a signal pulse, we can retrieve this information without an iterative inversion algorithm from the spectrogram. In our demonstration, we retrieved the quadratic variation of a spectral phase of a linear chirped ultrashort laser pulse from a mode-locked Ti: sapphire laser.     

飞秒脉冲振幅和相位的无干涉条纹重构法测量

对作者所提出的无干涉条纹直接电场重构测量飞秒脉冲的振幅和相位的新方法作出进一步理论分析,并通过实验测量说明该方法的优越性.该方法克服了传统的SPIDER方法的弊病,能得到一组无干涉条纹的图像,排除传统方法必须使用傅里叶变换滤波消除干涉条纹而引进的系统误差,使得该方法能够采用较简便设备且能较准确测量飞秒脉冲强度轮廓和相位.最后给出同一条件下新方法和传统SPIDER方法分别重构的脉冲强度自相关曲线与实验测量结果的比较,以说明新方法的有效性和优越性. 关键词： 光谱相位相干直接电场重构法 飞秒脉冲测量 超快信息光学     

啁啾光脉冲的振幅调制和相位扰动对压缩光脉冲的影响

 通过采用分步傅里叶变换法求解非线性薛定谔方程,模拟了啁啾光脉冲有振幅调制和相位扰动下的自相位调制（SPM）对压缩光脉冲对比度和预脉冲宽度的影响。结果表明：啁啾光脉冲的振幅调制深度和相位扰动深度越大,则压缩后的光脉冲对比度越小;啁啾光脉冲的振幅调制周期和相位扰动周期越小,则压缩光脉冲的预脉冲宽度越大。     

飞秒脉冲振幅和相位的无干涉条纹重构法测量

对作者所提出的无干涉条纹直接电场重构测量飞秒脉冲的振幅和相位的新方法作出进一步理论分析，并通过实验测量说明该方法的优越性.该方法克服了传统的SPIDER方法的弊病，能得到一组无干涉条纹的图像，排除传统方法必须使用傅里叶变换滤波消除干涉条纹而引进的系统误差，使得该方法能够采用较简便设备且能较准确测量飞秒脉冲强度轮廓和相位.最后给出同一条件下新方法和传统SPIDER方法分别重构的脉冲强度自相关曲线与实验测量结果的比较，以说明新方法的有效性和优越性.     

Time-resolved phase spectroscopy from femtosecond free-induction decay

We report a novel two-channel experimental scheme to record femtosecond amplitude and phase dynamics for dephasing studies. The measurement of free-induction decay in one channel and pulse autocorrelation for the central wavelength of the exciting pulses at the other channel as the reference allows the recording of time-resolved amplitude and phase dynamics of the free-induction decay throughout the dephasing process. The technique is applied to the study of coherent dynamics of the photocarriers in semiconductor GaAs. The observed phase dynamics is well simulated with optical Bloch vector components analysis for the homogeneously broadened two-level model and the experimental and theoretical study is extended for the three-level system. The two-pulse-induced phase spectroscopy should be generally applicable to different kinds of materials by recording the population difference for the two-level system with pulse-induced fluorescence.     

行波电光相位调制器输出响应的定量分析

从标量波动方程出发,采用傅里叶变换的方法,导出了行波电光相位调制器的时间域输出响应的计算公式。以高斯脉冲调制信号为例,定量计算并讨论了速度不匹配和微波衰减对输出相位调制信号的脉冲形状,峰值,半值宽的影响,所得到的结果,对行波电光调制器的实验研究有很好的参考价值,所给出的公式和方法适用于行波电光调制器的时间域定量分析。     

光温双控太赫兹波调制特性研究

基于多频带金属开口谐振环结构,利用GaAs材料的光敏特性和VO₂薄膜的热致相变特性,设计了一种既能实现光控又能实现温控的太赫兹(Terahertz, THz)波调制器,研究了光强和薄膜温度对 THz波调制特性的影响。结果表明,随着光强的增加,谐振频率均出现蓝移且谐振强度减小,当光强达到0.2 μJ·cm-2时,第二个谐振点(0.52 THz)蓝移了0.14 THz,透射幅度增加达50%;随着VO₂温度增加至相变温度以上,THz波透射幅度急剧减小,在0.63 THz处透射幅度减小达45.5%;当光强和温度同时控制时,随着光强和温度的增加,谐振点频率蓝移且谐振点处的THz波透射幅度增加,但在温度超过相变温度后,则温度控制起主导作用。设计的THz波调制器能通过光控和温控实现对THz波的明显调制效果,可为实现多功能的THz波功能器件的设计及应用提供参考。     

Dispersion-Managed Vector Solitons in Optical Fibers

The dynamics of optical solitons propagating through birefringent optical fibers in a strong-dispersion managed system with damping and amplification is studied. A multiple-scale perturbation expansion method is used to analyze the vector nonlinear Schro¨dinger's equation. The pulse is decomposed into a rapidly varying phase and a slowly varying amplitude. The fast phase is calculated exactly and the coupled nonlocal evolution equation of the amplitude is derived.     

Plasma-based accelerator diagnostics based upon longitudinalinterferometry with ultrashort optical pulses

All-optical ultrafast time-resolved plasma diagnostics of plasma-based accelerators (PBA's) are described, with emphasis on the laser wakefield accelerator (LWFA). Specifically, the diagnostic techniques involve replacing the trailing particle bunch in the LWFA with a trailing photon bunch: a weak ultrashort laser pulse. Since this photon pulse is derived directly from the intense pump pulse, practical difficulties such as synchronization and dephasing are eliminated. The interaction of the photon bunch with the plasma wake is essentially a simple time-domain shift in optical phase, which can produce both “DC” phase shifts and frequency blue/red-shifting of the probe pulse spectrum. These phase/frequency shifts are recorded in frequency domain interferograms, which are formally equivalent to time-domain holograms. Experimental results of longitudinal plasma density profiling are presented in which plasma density oscillations (Langmuir waves) in the wake of an intense (I_peak~3×10 ¹⁷ W/cm²) laser pulse (~100 fs) were measured with ultrafast time resolution. Phase shifts consistent with large amplitude (~80%) density oscillations at the electron plasma frequency were observed in a fully tunnel-ionized He plasma, corresponding to longitudinal electric fields of ~10 GV/m. Strong radial ponderomotive forces enhance the density oscillations. Finally, proposed single-shot schemes for simultaneous transverse and longitudinal profiling are discussed     

Compact direct space-to-time pulse shaping with a phase-only spatial light modulator

A very compact and innovative pulse shaper is proposed and demonstrated. The standard architecture for pulse shaping that is composed of diffraction gratings associated with an amplitude-phase spatial light modulator (SLM) is replaced by a single phase-only SLM. It acts as a pulse stretcher and as an amplitude and phase modulator at the same time. Preliminary experiments demonstrate the accurate control of amplitude and phase of shaped pulses.     

光孤子对的振幅比与相位差对传输的影响

利用分步傅里叶变换法分别求解含三阶色散效应和不考虑三阶色散情况下的光孤子非线性薛定谔（NLS）方程,通过数值求解发现三阶色散效应会使孤子对脉冲发生单边振荡,并在振荡侧逐级产生次脉冲。讨论孤子对的两束孤子脉冲之间的振幅比与相位差对传输的影响,发现在不考虑三阶色散的情况下,振幅比与相位差均对孤子对的传输有显著影响,在考虑三阶色散效应时,只有相位差对孤子对的传输产生影响,并可以导致脉冲能量转移。     

Solving nonlinear Schrödinger equation based on discrete split-step multi-wavelet method

In this paper, the discrete split-step multi-wavelet method (DSSMWM) is used to solve nonlinear Schrödinger equation. When the relative amplitude error is below10⁻³magnitude, relative error of amplitude evolution, relative error of pulse broadening ratio, relative phase error, and computing time is respectively achieved. Because multi-wavelet is extraordinary effective for data compression, it only needs to deal with very little data. It can be seen that although the relative amplitude error, relative error of amplitude evolution, relative error of pulse broadening rate and relative phase error changes little, but the computing time are greatly reduced.     

Spatially resolved amplitude and phase characterization of femtosecond optical pulses

Ultrabroadband pulses exhibit a frequency-dependent mode size owing to the wavelength dependence of free-space diffraction. Additionally, rather complex lateral dependence of the temporal pulse shape has been reported for Kerr-lens mode-locked lasers and broadband amplifier chains and in frequency-domain pulse shapers, for example. We demonstrate an ultrashort-pulse characterization technique that reveals lateral pulse-shape variations by spatially resolved amplitude and phase measurements by use of spectral phase interferometry for direct electric-field reconstruction (SPIDER). Unlike with autocorrelation techniques, with SPIDER we can obtain spatially resolved pulse characterization even after the nonlinear process. Thus, with this method the spectral phase of the pulse can be resolved very rapidly along one lateral beam axis in a single measurement.     

皮肤组织容积脉搏波400~1000 nm光谱特性仿真研究

根据皮肤组织解剖结构特性建立了六层层状模型,并给出了皮肤组织各层的特性参数;考虑了氧合血红蛋白和还原血红蛋白的吸收特性,依据皮肤组织各层的水、血、脂肪、血氧饱和度含量以及血管大小给出了皮肤组织各层的光谱吸收系数;对不同波长散射系数做了适当简化,给出了皮肤组织各层的光谱散射系数。利用蒙特卡罗方法仿真血管组织在收缩与舒张两种状态下, 400~1 000 nm波长光在皮肤组织多层模型中的传输过程,并通过统计大量光子的分布特性,获得了皮肤组织光谱反射系数,并利用模拟所得的两种状态下的反射系数计算得到了光谱容积脉搏波幅度。仿真结果表明,当入射光强一定时,绿光的容积脉搏波幅度优于红光和蓝光。通过计算不同波长光沿皮肤组织深度方向光能流率衰减为1/e时对应的皮肤组织深度,获得了皮肤组织光谱穿透深度。结果显示,血管舒张状态下蓝光和绿光的穿透深度较小,蓝光大部分只能达到表皮层,绿光能到达微循环层,红光可直达真皮层。考虑到光在皮肤组织中传播包含了一个从收缩到舒张的动态过程,基于此,根据穿透深度定义了脉搏波信号产生深度,利用血管舒张与收缩两种不同状态下的穿透深度计算得到了光谱产生深度。结果表明,不同波长光产生深度大于其穿透深度,蓝光产生深度较浅,且其受到的血液吸收调制较小,因而其获得的脉搏信号易受噪声干扰;红光的容积脉搏波产生深度较大,但是相比于绿光其受血液吸收调制较小,且绿光产生深度足够达到真皮血管层,因而红光容积脉搏波的幅度小于绿光。上述仿真结果明确了皮肤组织部分光谱特性,为皮肤组织多光谱容积脉搏波的精确获取及其他相关研究提供了一定的理论基础。     

Characterization of a broadband pulse for phase controlled multiphoton microscopy by single beam SPIDER

We present what we believe to be a new version of spectral phase interferometry for direct electric field reconstruction (SPIDER) using only a single-phase and polarization controlled laser beam. Two narrow pulses and one broadband pulse are selected out of an ultrafast laser pulse by a polarization and phase control technique to generate second harmonic generation (SHG) signals, which are equivalent to a spectral shear interferogram in the conventional SPIDER method. The spectral phase of the broadband laser pulse is extracted analytically with double quadrature spectral interferometry (DQSI). An arbitrary spectral phase can be retrieved with great precision and compensated in situ at the sample position of a microscope. This new method requires no separate reference beam and is suitable for nonlinear optical microscopy with a phase controlled laser pulse.     

Extracting the phase distribution of the electron wave packet ionized by an elliptically polarized laser pulse

We use an interferometic scheme to extract the phase distribution of the electron wave packet from above-threshold ionization in elliptically polarized laser fields. In this scheme, an electron wave packet released from a circularly polarized laser pulse acts as a reference wave and interferes with the electron wave packet ionized by a time-delayed counter-rotating elliptically polarized laser field. The generated vortex-shaped interference pattern in the photoelectron momentum distribution enables us to extract the phase distribution of the electron wave packet in the elliptically polarized laser pulse with high precision. By artificially screening the ionic potential at different ranges when solving the time-dependent Schördinger equation, we find that the angle-dependent phase distribution of the electron wave packet in the elliptically polarized laser field shows an obvious angular shift as compared to the strong-field approximation, whose value is the same as the attoclock shift. We also show that the amplitude of the angle-dependent phase distribution is sensitive to the ellipticity of the laser pulse, providing an alternative way to precisely calibrate the laser ellipticity in the attoclock measurement.