飞秒脉冲技术的新进展

飞秒(fs)是这样一种概念,即1fs=10^-15s;光每秒行进3×10⁵km,它等于从地球到月球之间距离的四分之三。1fs内,光行进约300nm,小于人的头发丝直径的1%。迄今所获得的最短光脉冲为8fs,相当于脉冲的半宽度小于4个光学周期的持续时间。本文介绍fs脉冲技术的新进展、应用研究概况及其发展前景。     

最短的光脉冲

激光器出现五年之后,人们就获得了微微秒光脉冲,微汲秒光脉冲的出现,开拓了微微秒激光光谱学这一正在迅速发展中的领域.目前这项技术已成功地扩展至亚微微秒范围(10-13秒).这就为更精确地研究物理、化学、生物学中的快速过程提供了前所未有的工具. 据美国罗彻斯特大学的Mourou 和Sizer报道,现已产生了目前最短的激光脉冲,此光脉冲的持续时间不大于 70fs(7 ×10-13秒)[1].尽誉还没有准确测量这样短光脉冲脉宽的设备,但他们确信,实际上光脉冲宽度可能更短些. 贝尔实验室曾报道产生了脉宽为90fs的光脉冲,并经过压缩手段脉宽达到了50fs.Mourou…     

超短光脉冲宽度已被压缩到8fs

超短光脉冲技术除了在物质的超快过程的测量中有重要意义外,其本身的理论和技术也有重大科学和实际意义.自进入八十年代以来,超短光脉冲技术已从微微秒范畴深入到毫微微秒范畴.世界各国的科学家们为获得更短的光脉冲做了大量研究工作.1982年Shank等人[1]获得了30fs的光脉冲,1984年Fuji-moto等人[2]把光脉冲压缩到16fs.不久前有人报道已把光脉冲压缩到12fs.最近Bell实验室的Knox等人[3]又把光脉冲压缩到10fs以内,达到8fs(在半高全宽度内,光脉冲持续的时间不到四个光振动周期),向获得真正的1fs光脉冲的方向前进了一大步. Knox等人还是用光的…     

双色激光场中极端远紫外超连续谱相干辐射产生

通过脉宽为周期量级的超快强场激光脉冲与惰性气体原子的极端非线性相互作用,能够产生单个的阿秒量级的极端远紫外光脉冲。然而,现有的周期量级的激光脉冲的最短脉宽仍在两个光周期左右,尚不足以支持产生脉宽短于100as的单个极端远紫外脉冲。文章作者首次提出通过在一个周期量级(如6fs)激光脉冲上再叠加一个强度较弱但相对位相精确控制的二倍频激光,能够直接实现单个的65as光脉冲。如进一步经过位相补偿,将有望产生一个脉宽仅为23as的单脉冲,从而将阿秒光脉冲的时间宽度推进到短于一个原子时间单位。     

高功率钕玻璃啁啾脉冲放大系统光谱整形

增益窄化效应限制了啁啾脉冲放大系统输出的最短脉冲宽度,需要对注入放大系统的种子啁啾脉冲进行光谱整形,以保证放大后的啁啾脉冲光具有足够的带宽并满足特定的光谱分布.理论上假定放大后啁啾脉冲的光谱分布为高斯分布或超高斯分布,对"百焦耳级钕玻璃啁啾脉冲放大系统"进行逆运算.研究发现,所需种子啁啾光的能量、带宽以及压缩后脉冲的信噪比都与该分布密切相关.通过光谱整形,使放大后的啁啾脉冲光的光谱分布为二阶超高斯分布.能够获得小于400 fs高信噪比的超短脉冲输出,此时所需种子啁啾脉冲能量仅为33μJ,带宽14 nm,在装置前级注入能力的范围之内.为百焦耳装置光谱整形实验的开展提供了理论指导.     

用双色受激喇曼效应产生飞秒或更短光脉冲的一种新方法

介绍用双色受激喇曼效应产生飞秒或更短光脉冲的一种新方法。利用这种方案有可能产生比6fs 更短的 TW 级光脉冲。     

飞秒激光激发空气电离的阈值研究

报道在脉宽50fs—22ps，波长800nm脉冲激光作用下的空气电离阈值的研究结果.利用探测等离子体发光信号的方法，实验测量了激发空气电离所需的阈值激光强度.结果表明，当激光脉冲宽度从50fs增加到22ps时，阈值光强I_th从8.7×10¹⁴W/cm²下降到2.7×10¹³W/cm²；I_th经历了由迅速降低逐渐发展为缓慢降低的过程.在50fs—1p     

对撞脉冲锁模染料激光器产生75fs光脉冲

本文研究了对撞脉冲锁模染料激光器的运转特性.激光脉冲的宽度和波长随可饱和吸收介质的厚度和浓度变化;在最佳条件下获得的光脉冲宽度为75fs.     

飞秒光纤激光器

我们利用同步泵浦光纤喇曼环形激光器在1.396μm处得到195fs 的光脉冲,并利用时间-色散调谐技术在1.396μm附近实现了47nm 的波长调谐.我们采用1.313μm的 Nd:YLF 锁模脉冲泵浦,其喇曼光处于单模光纤的负色散区,利用孤子效应得到195fs 的光脉冲.实验中我们采用一台 Nd:YLF(Quantronix Model 4216d)激光器作泵浦源,输出波长为1.313μm,主动锁模时输出脉宽约50ps.泵浦光经一枚双色镜反射后耦合进光纤.测试中使用的单模光纤是普通通讯用单模石英光纤.其零色散波长在1.305μm,芯径约为9.5μm.由于喇曼效应的作用,在光纤中产生1.395μm左右的喇曼光.喇曼光从光纤出来后经透镜准直、延迟后穿过双色镜反馈回光纤,其中一部分被反射作为输出.用 PIN 来监测泵浦脉冲与喇曼光脉冲的同步情况,并调节延迟线达到同步泵浦的要求.     

掺Er3+飞秒光纤放大器的特性研究

依据二能级速率方程和光传输方程,理论计算了Er3+光纤放大器中Er3+光纤的最佳长度Lc. 以全光纤飞秒激光器输出的孤子脉冲作为种子源,以理论值Lc作为实际采用的光纤长度,实验测量了Er3+光纤放大器在同向泵浦和反向泵浦下信号光的输出功率,得到同向泵浦的增益为20.0dB, 反向泵浦的增益为20.2 dB.同时测量了脉冲宽度随泵浦功率的变化,实验结果表明,随着泵浦功率的增大,输出光脉冲的宽度的压缩效应经历逐渐增强后逐渐减弱的过程.在同向泵浦方式下,可以得到更短的输出脉宽,其最短脉冲宽度为260 fs.     

周期量级激光脉冲作用下原子电离不对称性研究

应用不对称性参量对周期量级强激光脉冲下原子电离分布的反演不对称性进行了定量分析.采用非微扰的散射理论解析方法和三个激光模式模拟超短脉冲,研究不对称性参量随激光强度、包络位相和脉冲宽度的变化.计算表明,这种不对称性是随着波包的绝对位相以正弦形式变化而变化,其最大不对称程度依赖于脉冲强度和脉冲宽度.随着激光强度的提高,不对称性参量是先降低到最小值然后增加.对脉冲宽度相对长、有几个周期量级的高强度激光,其不对称性具有显著的特点.因此,提高脉冲强度有助于对包络绝对位相变化的观察.     

Intensity-dependent asymmetric photoionization in few-cycle laser pulses

The asymmetric photoionization of atoms irradiated by intense, few-cycle laser pulses is studied numerically. The results show that the pulse intensity affects the asymmetric photoionization in three aspects. First, at higher intensities, the asymmetry becomes distinctive for few-cycle pulses of longer durations. Second, as the laser intensity increases, the maximal asymmetry first decreases then increases after it has reached a minimal value. Last, the value of the carrier-envelope phase corresponding to the maximal asymmetry varies with the pulse intensity. This study reveals that the increasing of pulse intensity is helpful for observing the asymmetric photoionization.     

Photoionization of H atoms in few-cycle laser pulses

Photoionization of hydrogen atoms in few-cycle laser pulses is studied numerically. The total ionization probability, the instantaneous ionization probability, and the partial ionization probabilities in a pair of opposite directions are obtained. The partial ionization probabilities are not always equal to each other which is termed as inversion asymmetry. The variation of asymmetry degree with the CE phase, the pulse duration and the pulse intensity is studied. It is found that the pulse intensity affects the asymmetry degree in many aspects. Firstly, the asymmetry is more distinct at higher intensities than that at lower intensities when the pulse duration exceeds 4 cycles; secondly, the maximal asymmetry in lower intensities varies with the CE phase visibly while at higher intensities not; thirdly, the partial ionization probabilities equal to each other for some special CE phases. For lower pulse intensities, the corresponding value of CE phase is always 0.5π and 1.5π, while for higher pulse intensities, the corresponding value varies with the pulse intensity. Similar phenomena were observed in a recent experiment using few-cycle radio-frequency (RF) pulses.     

Strong field ionization by ultrashort laser pulses: Application of the Keldysh theory

Analytical expressions and numerical results describing ionization of atoms by intense linearly polarized ultrashort laser pulses are obtained in the frame of the Keldysh approach. Photoelectron spectra and total ionization probabilities are presented for several analytical models of a single-cycle laser pulse. In particular, strong left-right asymmetry of the spectra is shown for the case of odd pulses.     

C. 超短激光光源

本文对超短激光脉冲的产生、压缩技术的最新发展做了详细评述。     

Analysis of saturable absorbers,interacting with gaussian pulses

Mode-locked lasers, Q-switch systems and discrimination amplifiers make use of saturable absorbers. The interaction of those absorbers with light pulses is well known, either for very long or very short pulses, but not, if the pulse width is comparable with the absorber life time, which occurs very often in experiments. Moreover, the spatial structure of the pulse is mostly not considered. In this paper, we investigate in detail the interaction of gaussian pulses (spatial and temporal) with the two-level-absorber. Energy transmission, pulse shortening and pulse asymmetry are calculated as functions of pulse width and small signal transmission of the absorber.     

不同激光等离子体条件下的阿秒光脉冲产生

通过一维粒子模拟研究了利用相对论少周期强激光与固体密度等离子体表面相互作用实现单个孤立阿秒光脉冲产生的参数条件。主要研究描述相互作用的多维参数,如激光强度、入射角和等离子体标尺长度等,对相对论高次谐波能量转换效率和孤立阿秒光脉冲分离度的影响。研究发现,虽然激光等离子体参数对阿秒光脉冲产生的影响是复杂的,但是存在着能够实现大能量孤立阿秒光脉冲的最佳等离子体标尺长度和最佳入射角。当其他相互作用条件确定时,使用中等强度的相对论强激光可以在较宽的参数范围内实现孤立的阿秒光脉冲。大角度入射时,孤立阿秒光脉冲的分离度较高,能够实现孤立阿秒光脉冲的相互作用参数范围也较宽。     

Talbot effect under illumination of double femtosecond laser pulses

The Talbot effect under illumination of double femtosecond laser pulses has been reported. Spectrums of double femtosecond laser pulses with phase differences are quite different from that of one single femtosecond laser pulse. Therefore, the Talbot images of the double femtosecond laser pulses with phase differences are different from that of one single femtosecond laser pulse. Specifically, for the phase difference corresponding to π, the Talbot image shows the largest difference from that of one single pulse. Experimental results are in good agreement with the theoretical analysis. The behaviors of Talbot images under double femtosecond laser pulses illumination cannot be obtained under one femtosecond laser pulse, monochromatic or polychromatic light illumination. Therefore, it is a new interesting optical phenomenon for the Talbot effect which should have potential applications.     

Electron-positron pair production in a strong asymmetric laser electric field

By solving the quantum Vlasov equation, electron positron pair production in a strong electric field with asymmetric laser pulses has been investigated. We consider three different situations of subcycle, cycle and supercycle laser pulses. It is found that in asymmetric laser pulse field, i.e.. when the pulse length of one rising or falling side is fixed while the pulse length of the other side is changed, the pair production rate and mnnber density can be significantly modified comparable to symmetric situation. For each ca,se of these three different cycle pulses, when one side pulse length is constant and the other side pulse length becomes shorter, i.e., the whole pulse is compressed, the more pairs can be produced than that in tile vice versa case, i.e., the whole pulse is elongated. In compressed pulse case there exists an optimum pulse length ratio of asylnmetric pulse lengths which makes the pair number density maximunn. Moreover, the created maximum pair number density by subcycle pulse is larger than that by cycle or/and supercycle pulse. In elongated pulse case, however, only for supercycle laser pulse the created pairs is enhanced and there exists also an optimum asymmetric pulse length ratio that maximizes the pair number density. On the other hand. surprisingly, in both cases of subcycle and cycle elongated laser pulses, the pair number density is monotonically decreasing as the asymmetry of pulse increases.     

All-fiber passive mode-locked laser to generate ps pulses based in a symmetrical NOLM

We experimentally investigate an all-fiber passively mode-locked laser generating ps pulses. The experimental setup is a figure eight fiber laser configuration, including a power-symmetric Nonlinear Optical Loop Mirror (NOLM) with highly twisted low-birefringence fiber in the loop. NOLM switching is achieved by polarization asymmetry between the counter-propagating beams in the loop. We used a Quarter-Wave Retarder in the loop to break the polarization symmetry. Using a polarizer beam-splitter cube as the NOLM output we got the best quality output pulses from the laser. At this output, we are monitoring the output pulse polarization component which is parallel to the input NOLM component. We achieved stable generation of ～25 ps pulses at the repetition frequency of 0.78 MHz with milliwatts average output power. The mode-locked laser ran in stable operation for hours.