Attosecond temporal gating with elliptically polarized light

Temporal gating allows high accuracy time-resolved measurements of a broad range of ultrafast processes. By manipulating the interaction between an atom and an intense laser field, we extend gating into the nonlinear medium in which attosecond optical and electron pulses are generated. Our gate is an amplitude gate induced by ellipticity of the fundamental pulse. The gate modulates the spectrum of the high harmonic emission and we use the measured modulation to characterize the sub-laser-cycle dynamics of the recollision electron wave packet.     

Generation of double pulses in-line by using reflective Dammann gratings

Doubled femtosecond laser pulses in-line are needed in the collinear pump-probe technique, collinear second harmonic generation frequency-resolved optical gating (SHG FROG) and the spectral phase interferometry for direct electric-field reconstruction (SPIDER), etc. Normally, it is generated by using a Michelson's structure. In this paper, we proposed a novel structure with two-layered reflective Dammann gratings and the reflective mirrors to generate doubled femtosecond laser pulses in line without transmission optical elements. Angular dispersion and spectral spatial walk-off are both compensated. In addition, this structure can also compress the positive chirped pulse, which cannot be realized with a Michelson's structure. By adopting triangular grating and blazed gratings, the efficiency of the system would in principle be increased as the Michelson's scheme. Experiments demonstrated that this method should be an alternative approach for generation of the double compressed pulses of femtosecond laser for practical applications.     

Pulse-amplitude-equalized output from a rational harmonic mode-locked fiber laser

We report, for the first time to our knowledge, the demonstration of amplitude equalization of high-repetition-rate pulses generated from a rational harmonic mode-locked Er-doped fiber laser. The output pulses are injected into another fiber laser with a nonlinear optical loop mirror. This scheme provides pulse-amplitude equalization up to the ninth rational harmonic mode-locked pulse train.     

Generation of 88 as Isolated Attosecond Pulses with Double Optical Gating

Isolated attosecond pulses with a duration of 88 as are generated in the spectral range of 29-72 eV using double optical gating technique.The gate width is set to be shorter than half the optical cycle to avoid carrier envelop phase stabilization of the 4.2 fs driving laser pulses centered at 800 nm.The attosecond pulse duration is measured with the technique of frequency resolved optical gating for complete reconstruction of attosecond bursts.     

Generation of shaped ultraviolet pulses at the third harmonic of titanium-sapphire femtosecond laser radiation

We experimentally demonstrate a method to generate shaped femtosecond laser pulses in the ultraviolet at a central wavelength of 267 nm, the third harmonic of conventional titanium-sapphire femtosecond laser systems. Employing a 128-pixel liquid-crystal spatial light modulator, we impose variable spectral phase modulations upon the near-infrared laser pulses. By this, complex laser pulses can be shaped whose overall spectrum is still conserved. Our experiments show that it is possible to easily transfer these pulses into the ultraviolet at 267 nm via sum-frequency mixing in nonlinear crystals and to predictably generate multistructured ultraviolet femtosecond laser pulses. We analyze the temporal and spectral composition of these pulses after frequency conversion into the ultraviolet using difference-frequency cross-correlation and XFROG (cross-correlation frequency-resolved optical gating) techniques with an unmodulated fundamental laser pulse. The method can be employed to facilitate adaptive quantum control experiments in the ultraviolet wavelength regime, where the major absorption bands of many organic molecular systems are located. PACS 42.65.Re; 42.72.Bj; 32.80.Qk     

飞秒激光脉冲的谐波频率分辨光学开关法测量研究

建立了一台谐波频率分辨光学开关法(FROG)飞秒脉冲测量装置，利用该装置进行了掺钛蓝宝石飞秒激光脉冲的测量研究.在二次谐波自相关测得的时域和频域信号基础上，结合对信号光强度分布的计算机迭代处理，得到了有关飞秒激光电场、光谱及其相位的信息，所得脉宽与干涉测量的结果基本一致. 关键词： 频率分辨光学开关法(FROG) 迭代计算 飞秒激光 自相关     

中红外偏振态门驱动产生高效的极宽超连续谱

研究了在中红外偏振态门驱动下高效产生极宽超连续谱的过程.利用波长为2000 nm的偏振态门激光场可以将电子的回复控制在半个光周期内,并且将谐波谱的截止区拓展到270 eV.在偏振态门中加入0.5 fs的紫外控制脉冲,可以对谐波的电离过程进行控制,把谐波的强度提高了4—5个数量级,并得到了谱宽为240 eV的极宽超连续谱.该极宽超连续谱在傅里叶变换极限下支持脉宽仅为16 as的单个脉冲的产生.通过过滤超连续谱中不同次数的谐波,可以得到一系列波长可调的110 as的高强度单脉冲. 关键词： 高次谐波 阿秒 偏振态门 超连续谱     

飞秒激光脉冲的频率分辨偏振光学开关法测量研究

建立了一台频率分辨偏振光学开关（PG FROG）法飞秒脉冲测量装置，利用该装置对“极光Ⅱ号”飞秒激光放大系统进行了测量.在利用偏振光学开关法测得的时域和频域信号基础上，结合对信号光强度分布的计算机迭代处理，得到了有关飞秒激光电场、光谱及其相位的信息；并且对系统工作在不同状态时的激光脉冲进行了测量和比较分析，给出了有关该系统较详细的电场、光谱、相位以及啁啾状况.结果显示，当系统工作在零啁啾附近时，该系统输出的激光脉冲的电场、光谱和相位分布较规则，相位起伏较小；当系统偏离零啁啾状态时，虽然电场和光谱变化不很明显，但相位分布变化剧烈. 关键词： 频率分辨偏振光学开关(PG FROG)法 飞秒激光 自相关     

利用三色组合脉冲激光获得孤立阿秒脉冲发射

孤立阿秒脉冲因可以跟踪和控制原子及分子内电子的运动过程而备受关注.本文从理论上开展了氦原子在3束飞秒脉冲激光组合场辐照下产生的高次谐波和阿秒脉冲辐射的研究.组合激光场由16 fs/1600 nm,15 fs/1100 nm和5.3 fs/800 nm的钛宝石脉冲构成.与前两束脉冲合成的双色场产生谐波谱相比,附加钛宝石脉冲的三色场产生的高次谐波发射谱呈现出高转换效率及宽带超连续特性,超连续谱范围覆盖从230—690次谐波,傅里叶变换后实现了128 as高强度孤立短脉冲的产生.该结果归因于合成的三色场呈现出高功率及少周期的中红外飞秒脉冲激光特性,可以有效控制原子电离以及复合发生在中红外飞秒脉冲的一个有效光学周期内.     

Blind frequency-resolved optical-gating pulse characterization for quantitative differential multiphoton microscopy

We use a unique multifocal multiphoton microscope to directly characterize the pulse in the focal plane of a high-NA objective using second-harmonic generation frequency-resolved optical gating (FROG). Because of the nature of the optical setup, femtosecond laser pulses of orthogonal polarization states are generated in the focal plane, each acquiring a different spectral dispersion. By applying an additional constraint on the phase extraction algorithm, we simultaneously extract both the gate and probe pulses from a single spectrogram with a FROG error of 0.016.     

Phase matching and focussing effects in noncollinear sum frequency mixing in the near VUV region

We demonstrate that focussing effects are extremely important in determining the bandwidth in sum frequency mixing of ultrashort pulses in the near VUV region. This effect is demonstrated with noncollinear sum frequency mixing performed between the fundamental and the third harmonic subpicosecond pulses of a Ti:sapphire laser. The spectrum of the generated fourth harmonic radiation is significantly broader (by 33%) than the theoretical spectrum obtained if focussing is not taken into account. We have developed a method of calculating the output bandwidth for sum frequency mixing of broadband spectral envelopes whose bandwidths correspond to ultrashort optical pulses, including focussing. The calculated and the measured spectra show excellent agreement.     

亚10飞秒脉冲的诊断与压缩

采用二次谐波-频谱分辨光闸法对克尔透镜锁模钛宝石激光器输出的飞秒脉冲的振幅和相位 进行诊断，测得其脉冲包络是双曲正割型的，而相位则近似为时间的三次幂形式. 通过对脉 冲频域相位的拟合，诊断出二阶群速度色散是造成脉冲啁啾、导致脉冲展宽的主要原因. 利 用棱镜对补偿二阶群速度色散，进一步将17 fs啁啾脉冲压缩至半高全宽为8.5 fs的近似转 换极限脉冲. 关键词： 飞秒脉冲 二次谐波-频谱分辨光闸法 群速度色散     

用SHG-FROG方法测量超短光脉冲的振幅和相位

介绍了二次谐波振荡频率分辨光学快门(SHGFROG)测量超短脉冲的实验系统和二维相位恢复算法,这种方法可以实现对超短脉冲振幅和相位的完全测量.利用矩阵方法数值模拟了几种常见超短脉冲的SHGFROG迹线,并用主元素广义投影算法(PCGPA)从这些无噪音的SHGFROG迹线中恢复了脉冲的振幅和相位,误差接近收敛的标准.研究结果表明,这种测量方法结构简单,算法可靠,是测量超短脉冲的理想方法之一.     

Temporal confinement of the harmonic emission through polarization gating

We show experimentally that a simple optical setup is able to reduce the duration of the harmonic emission generated by long (many optical cycles) laser pulses. By controlling the time-dependent ellipticity during the laser pulse, one can confine the harmonic emission to a narrow temporal window where the polarization is quasi-linear. The results obtained and their comparison with a simple model show a shortening of a factor 2 for harmonics located in the plateau region. Using shorter laser pulses of 15-20 fs duration should make it possible to isolate a single attosecond pulse.Received: 25 November 2002, Published online: 8 July 2003PACS: 42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation - 32.80.Wr Other multiphoton processes     

利用调制的偏振态门控制阿秒脉冲的产生

提出一种利用多光周期驱动脉冲获得极紫外宽带超连续谱的新方法.利用波长为1600nm的基频场组成的偏振态门增强高次谐波产生效率对椭扁率的依赖,并叠加上波长为800nm的倍频场来调制电离比率.计算结果表明,采用脉冲宽度为6个光周期(32.4fs)的基频脉冲就能够将高次谐波辐射限制在0.5个光周期内,获得了带宽为280eV的超连续谱,这个谱宽支持傅里叶极限为10as的单个脉冲输出.直接选取超连续谱上的一段,可以获得100as的脉冲输出.此外,在调制的偏振态门中可以使用相对延迟较小的两束基频光组成偏振态门,提高了 关键词： 阿秒脉冲 超连续谱 偏振态门     

Diode pumped Ho:YAG and Ho:LuAG lasers, Q-switching and?second harmonic generation

Direct diode pumped Ho:YAG generated laser pulses at 2.12???m with an optical to optical slope efficiency of 0.24. Ho:YAG and Ho:LuAG laser rods were evaluated with both wide and narrow bandwidth pump diodes. The laser wavelength varies with the level of pumping and optical design. This variation was found to be predictable. Second harmonic at 1.06???m was produced in a 6.0?mm long BBO crystal.     

Measurement of the intensity-dependent atomic dipole phase of a high harmonic by frequency-resolved optical gating

The temporal profile and phase of the fifth harmonic of a Ti:sapphire laser were fully characterized by two-photon ionization frequency-resolved optical gating technique for the first time. The fifth harmonic was found to have negative chirp and the pulse compression was demonstrated. The negative chirp is well explained by using a zero-range potential model. This technique is scalable to extreme ultraviolet (XUV) and soft x-ray regions by using currently available light sources, making it possible to measure the pulse duration and phase of vacuum ultraviolet, XUV, and soft x-ray pulses.     

Electric-field reconstruction of femtosecond laser pulses from interferometric autocorrelation using an evolutionary algorithm

We present an evolutionary algorithm for reconstructing a femtosecond laser pulse from its interferometric autocorrelation trace and laser spectrum. The algorithm is optimized for the intensity and phase characterization of several-cycle optical pulses. We tested this algorithm with numerically-generated femtosecond pulses and then applied it to experimental data. In the experiment, a negatively chirped 31-fs pulse and a sub-10-fs pulse containing high-order phase distortion were characterized. Frequency-resolved optical gating measurements, performed for comparison, confirm the reliability of our technique.     

储存环自由电子激光相干谐波实验的同步系统

 为了获得短波长自由电子激光(FEL),可以使储存环中的相对论性电子与外加强激光脉冲在光学速调管中充分耦合,从而产生高次谐波的相干辐射。为使耦合充分,必须使电子束团与激光脉冲在时间和空间上完全同步。其中空间上的同步可通过调节外激光的光路来实现,给出了实现时间上同步的一种方案。     

Effects of Pulse Durations on High-order Harmonic Generation

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