Transmission Properties of the Femtosecond Pulse Through a Near-field Optical Fiber probe

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1 MHz repetition rate hollow fiber pulse compression to sub-100-fs duration at 100 W average power

We report on nonlinear pulse compression at very high average power. A high-power fiber chirped pulse amplification system based on a novel large pitch photonic crystal fiber delivers 700 fs pulses with 200 μJ pulse energy at a 1 MHz repetition rate, resulting in 200 W of average power. Subsequent spectral broadening in a xenon-filled hollow-core fiber and pulse compression with chirped mirrors is employed for pulse shortening and peak power enhancement. For the first time, to our knowledge, more than 100 W of average power are transmitted through a noble-gas-filled hollow fiber. After pulse compression of 81 fs, 93 μJ pulses are obtained at a 1 MHz repetition rate.     

大气导致的脉冲展宽对星载量子密钥分发的影响

分析比较了星地量子密钥分发系统中由于大气湍流和大气色散导致的脉冲展宽。分别推导了大气湍流和色散效应下脉冲展宽比。根据大气压强和温度与海拔间的实测数据,计算了到达脉宽与不同初始脉宽、工作波长、链路天顶角的关系。并根据到达脉宽计算了系统背景噪声量。数值模拟结果表明：当工作波长为1.06 μm,初始脉宽为15 fs时,到达脉宽有最小值。对于不同的链路天顶角,在无月晴朗的星空,探测器探测到的背景噪声约为1.9×10-10~2.7×10-10,而误码率约为0.5×10-5。     

激光在烟幕中传输的蒙特卡罗模拟

建立蒙特卡罗计算模型,模拟计算1.06μm脉冲激光在石墨烟幕中的传输.分析透过率与石墨粒子粒径、烟幕浓度、烟幕厚度的关系,用卷积方法计算一定宽度的脉冲激光穿过烟幕后的波形,数值仿真δ脉冲和矩形脉冲激光在各种石墨烟幕中的时间展宽特性.模拟结果表明:石墨粒子存在一最佳半径(0.16μm),该半径的石墨粒子烟幕不仅使激光能量衰减最强,而且产生的时间展宽也最显著;激光脉冲宽度越小,其波形变化和时间展宽越显著,微秒量级脉宽的激光脉冲穿过烟幕后波形变化和时间展宽效应不明显.     

Time-resolved measurements of high order harmonics confined by polarization gating

We investigate the temporal confinement of high order harmonic pulses generated by a femtosecond (fs) infrared (IR) pulse with a time varying polarization. We use a set of two birefringent quartz plates to modulate the IR polarization. It produces a short temporal gate of linear polarization where harmonics are efficiently generated during a small fraction of the IR pulse. By rotating one of the plates, the gate width can be continuously varied between 70 fs down to 7 fs. The XUV pulse duration is measured by cross-correlation with a probe IR pulse of 12 fs. When the gate width is decreased, a clear temporal confinement of the XUV emission is observed through the cross correlation signal. This experiment is the first direct experimental evidence in the temporal domain that the polarization gating technique can be used to significantly shorten the harmonic pulse duration. PACS 32.80.Wr; 42.65.Ky; 42.65.Re     

锥形镀膜光纤探针中飞秒激光脉冲的传输

采用三维时域有限差分法对飞秒激光脉冲照明下锥形镀膜光纤探针内的光场进行了计算,研究了飞秒脉冲在探针中传输时的时域、频谱和相位特性,并分析了不同锥角和不同长度的探针对这些特性的影响.研究结果发现飞秒脉冲在探针中传输时出现了脉冲展宽、幅值振荡以及频谱分裂等现象,并且利用频谱和相位的变化特性初步解释了脉冲的展宽和脉冲振幅随时间周期性变化等现象. 关键词： 时域有限差分法 飞秒脉冲 光纤探针     

OPA泵浦保偏光子晶体光纤产生超连续谱和非线性特性的研究

采用波长可调光参量放大器作为泵浦源,对保偏光子晶体光纤的超连续谱的产生和非线性特性进行了实验研究.将光参量放大器产生的中心波长为1.27 μm,脉宽约为250 fs,重复频率为250 kHz和单脉冲能量只有92 nJ的光脉冲耦合进0.2 m长的保偏光子晶体光纤,实验中观察到了光谱展宽和非线性效应,在1.3 μm 波长区域获得了谱宽为83 nm (1.2486 ～1.3318 μm)的超连续谱.     

Subpicosecond UV pulse generation for a multiterawatt KrF laser

A 180-fs UV pulse has been generated based on a hybrid synchronously pumped mode-locked dye laser for a multiterawatt KrF laser system. The pulse width was measured by the single shot autocorrelation technique with the three-photon fluorescence of the XeF C-A transition. The pulse width broadening due to dispersive media was investigated. The results show that the observed pulse width broadening from 210 fs to 390 fs through the entire system is explained mostly by the linear dispersion of the optical elements for near-transform-limited input pulses.     

The Pulse Displacement Technique — a single particle counter with a size range larger than 1000: 1

This paper describes the pulse displacement technique(PDT) to simultaneously measure particle size and velocity in applications characterized by a large size range and high particle concentration. PDT is based on the detection of scattered refraction and reflection pulses which sweep past a detector at different times as a particle traverses a narrow probe volume. Basic analytical relationships are presented which allow the calculation of the spatial and temporal widths and separations of the reflection and refraction pulse as a function of particle diameter and velocity. Two implementations of PDT are discussed using one or two receivers with two laser sheets having the same or different wavelengths. This paper also discusses several methods to measure particle velocity with PDT, discusses the limitations associated with signal broadening in practical systems, and briefly presents experimental results which show that the temporal separation between the refraction and reflection and reflection pulse maxima is independent of collecting lens f-number from f/3 to f/20 for particle sizes ranging from 250 to 2000 μm.     

Generation of single intense short optical pulses by ultrafast molecular phase modulation

Pulses with durations below 4 fs have been generated using the method of ultrafast molecular phase modulation. A laser pulse shorter than the molecular vibrational or rotational period obtains spectral broadening during propagation along a hollow waveguide filled with previously impulsively excited Raman active gases. The induced time dependent phase, frequency, and frequency chirp are controllable by changing the delay between excitation and probe pulse within the molecular vibrational period.     

Experimental evidence of 25-fs laser pulse distortion in singlet beam expanders

We report the measurement of spatiotemporal distortions of an ultrashort pulse in singlet beam expanders. With a simple second-order autocorrelator the temporal broadening of the pulse from 23 to 40 fs, due to propagation time difference (PTD), is determined. The delay due to PTD between different parts of the beam is also measured. This effect was theoretically studied for the first time by Bor [J. Mod. Opt. 35, 1907 (1988)]. These experimental results are in good agreement with the calculations of a dedicated three-dimensional ray-tracing program developed to simulate the spatial and temporal transformation of femtosecond pulses in optical systems.     

Generation of subpicosecond electrical pulses by optical rectification

We describe the generation of subpicosecond electrical pulses by optical rectification of ultrashort optical pulses. The electrical pulses are generated by the second-order nonlinear response of a LiTaO(3) crystal bonded to a coplanar transmission line. A bipolar temporal waveform with a width of 875 fs was measured after a propagation distance of 175mum . This pulse width was limited by the response time of the photoconductive sampler. We observed both broadening and amplitude reduction in the temporal waveform owing to propagation.     

Rabi Oscillations and Coherence Dynamics in Terahertz Streaking-Assisted Photoelectron Spectrum

We present an approach,a Terahertz streaking-assisted photoelectron spectrum(THz SAPS),to achieve direct observations of ultrafast coherence dynamics with timescales beyond the pulse duration.Using a 24 fs probe pulse,the THz SAPS enables us to well visualize Rabi oscillations of 11.76 fs and quantum beats of 2.62 fs between the 5S_1/2 and 5P_3/2 in rubidium atoms.The numerical results show that the THz SAPS can simultaneously achieve high resolution in both frequency and time domains without the limitation of Heisenberg uncertainty of the probe pulse.The long probe pulse promises sufficiently high frequency resolution in photoelectron spectroscopy allowing to observe Autler-Townes splittings,whereas the streaking THz field enhances temporal resolution for not only Rabi oscillations but also quantum beats between the ground and excited states.The THz SAPS demonstrates a potential applicability for observation and manipulation of ultrafast coherence processes in frequency and time domains.     

基于交叉偏振波产生的脉冲净化技术研究与应用

本文利用交叉偏振波产生技术(XPW)对800 nm波段钛宝石飞秒激光器输出的激光脉冲进行时域净化, 提高脉冲时域对比度, 并测量验证了10¹¹对比度的脉冲, 达到测量仪器的动态范围极限, 比初始脉冲时域对比度有三个量级的提高, XPW的效率为22%. 同时发现净化后脉冲光谱宽度也得到一定展宽, 进一步利用啁啾镜对和补偿片对净化后的脉冲进行色散补偿, 得到25 fs脉宽的脉冲. 利用该净化后的激光脉冲作为种子注入已有的太瓦级钛宝石啁啾脉冲放大系统中, 在输出脉冲能量250 mJ, 宽度50 fs, 对应峰值功率5 TW的情况下, 在主脉冲前100 ps以外的范围内测量验证了10¹¹的脉冲对比度.     

超紧凑型飞秒电子衍射仪的设计

由于空间电荷效应的限制,产生百飞秒的极短电子脉冲是超快电子衍射技术的一大难点.同时,电子的穿透深度随着电子能量的增加而增加,而电子的散射几率却具有相反的规律.因而,除了时间分辨的提升,还需要可宽范围调节的电子能量以优化不同厚度样品对其的需求.基于此,提出并设计了一种新型超紧凑电子枪,结合均匀场阴极和可移动阳极的配置,可在10-125 kV加速电压范围内实现100 fs量级时间分辨率.通过优化设计高压电极轮廓,使得其轴上和整个阴极面的场增强因子在不同阴阳极间距下均小于约4%,从而保证了不同加速电压下最大轴上场强均可达10 MV/m量级,有效地抑制了电子脉冲的展宽效应;进一步将阳极小孔设计成可放置致密电镜载网的阶梯孔,一方面可将载网支撑的样品紧贴小孔后方放置,最大程度上缩短了电子从阴极到样品的时间弥散,同时也可以有效地减弱阳极孔对电子束的散焦效应,提升电子束的横向聚焦性能.     

Oscillation of the refractive index at the focal region of a femtosecond laser pulse inside a glass

The temporal evolution of refractive-index change produced by a tightly focused femtosecond (fs) laser pulse inside a soda-lime glass plate was investigated by use of a transient lens method with subpicosecond time resolution. An oscillating behavior of the light intensity in the central region of the probe beam was observed 0-1500 ps after irradiation of the plate. The oscillation was interpreted in terms of a rapid temperature increase and the ensuing propagation of the pressure wave. This study is to our knowledge the first real-time observation of refractive-index change inside a glass induced by a fs laser pulse.     

Spatiotemporal stability of a femtosecond hard-x-ray undulator source studied by control of coherent optical phonons

We report on the temporal and spatial stability of the first tunable femtosecond undulator hard-x-ray source for ultrafast diffraction and absorption experiments. The 2.5-1 Angstrom output radiation is driven by an initial 50 fs laser pulse employing the laser-electron slicing technique. By using x-ray diffraction to probe laser-induced coherent optical phonons in bulk bismuth, we estimate an x-ray pulse duration of 140+/-30 fs FWHM with timing drifts below 30 fs rms measured over 5 days. Optical control of coherent lattice motion is demonstrated.     

Optimal pulse compression in long hollow fibers

The spectral broadening performance of 1 m and 3 m long hollow fibers are compared. The 3 m capillary clearly outperforms the 1 m one in terms of both transmission and achievable spectral broadening. Starting from 1.1 mJ 71 fs pulses at 780 nm, a spectral broadening ratio of 26 was achieved using a single 3 m long argon-filled hollow fiber. After compression the measured pulse duration was 4.5 fs corresponding to a compression ratio of 16 at an energy of 0.42 mJ. Both the pulse duration and the pulse energy were limited by the applied chirped mirrors.     

采用光谱展宽技术的钛宝石飞秒激光放大器

报道了飞秒激光啁啾脉冲放大器中的激光脉冲光谱展宽技术，实验中使用了几种光谱展宽元件，结果表明使用激光光谱展宽元件能有效地将飞秒激光脉冲 光谱展宽，抑制了飞秒激光放大器中常见的激光脉冲光谱增益窄化现象，从飞秒放大系统的振荡级输出28fs种子脉冲，放大后得到了重复频率为10Hz、峰值功率为1．6TW、脉冲宽度37．5fs的超短强飞秒激光脉冲 。     

Temporal broadening and scintillations of ultrashort optical pulses

The purpose of this paper is to study the temporal broadening and temporal scintillations of ultrashort optical pulses. We present the general formulation of the temporal broadening and scintillation index of ultrashort (femtosecond) space-time Gaussian pulses propagating through weak optical turbulence and derive analytic approximations for the near- and far-field zones. We then apply the results to cross-link, uplink and downlink communication channels in a laser satellite communication system. We show that both the temporal broadening and scintillations depend mainly on the strength of optical turbulence and the outer scale and increase with these parameters. We also show that 10-30 fs pulses broaden significantly more and have greater scintillations than wide pulses. Furthermore, the horizontal and vertical/slant path temporal broadening are identical and the temporal scintillation indices exhibit similar behaviour.