135 W average-power femtosecond pulses at 520 nm from a frequency-doubled fiber laser system

We present a high-average-power femtosecond laser system at 520 nm central wavelength. The laser system delivers sub-500 fs pulses with 135 W average power at a pulse repetition rate of 5.25 MHz. Excellent beam quality is provided by high power fiber amplifiers and maintained during frequency doubling, resulting in a beam quality factor of M2<1.2. To our knowledge, the system presented here is the highest average power green laser source generating femtosecond pulses with diffraction-limited beam quality.     

Properties of long light filaments in natural environment

The multiple filamentation of terawatt femtosecond(fs) laser pulses is experimentally studied in a natural environment.A more than 30-m long plasma filament with a millimeter diameter is formed by the collimated fs laser pulse freely propagating in an open atmosphere. This study provides the first quantitative experimental data about the electron density of a long range light filament in the atmosphere. The electron density of such a filament is quantitatively detected by using an electric method, showing that it is at the 10~(11)-cm~(-3) level.     

Intense few-cycle laser pulses from self-compression in a self-guiding filament

Sub-10-fs-pulses are generated by self-compression in a noble gas filament. Using input pulses from a Ti:sapphire amplifier system with an energy of about 1.5 mJ at a repetition rate of 3 kHz and a pulse duration of 30 fs self-compressed sub-10-fs pulses with energies of about 0.3 mJ have been generated. These pulses are characterized with spectral phase interferometry for direct electrical-field reconstruction (SPIDER). Depending on the laser parameters, we observe a significant change in the chirp of the white-light. The spectral distribution of the outcoming beam profile is measured to distinguish the white-light core from the surrounding halo.     

Pulse Compression by Filamentation in Argon with an Acoustic Optical Programmable Dispersive Filter for Predispersion Compensation

We have experimentally demonstrated pulses 0.4 mJ in duration smaller than 12 fs with an excellent spatial beam profile by self-guided propagation in argon. The original 52 fs pulses from the chirped pulsed amplification laser system are first precompressed to 32 fs by inserting an acoustic optical programmable dispersive filter instrument into the laser system for spectrum reshaping and dispersion compensation, and the pulse spectrum is subsequently broadened by filamentation in an argon cell. By using chirped mirrors for post-dispersion compensation, the pulses are successfully compressed to smaller than 12 fs.     

量子路径控制生成宽频软X射线连续辐射谱

数值研究了双色激光脉冲与一维氩原子相互作用的谐波发射功率谱.双色激光脉冲由800 nm和1600 nm的两束激光组成,脉冲持续时间为9.7 fs.计算结果表明,相对相位为π时,谐波截止频率从单色场情形的58 eV大幅度地展宽到316 eV,获得频宽为93 eV的软X射线连续辐射谱.此外,还采用短程的普薛耳－特勒势和长程的软化库仑势进行比对,数值结果表明该连续辐射谱的存在具有普适性.通过半经典的“三步”模型和时频分析小波变换方法对该连续辐射谱进行分析,发现其主要由单一的“短”量子路径生成,直接叠加频率从22 关键词： 红外双色激光脉冲 连续辐射谱 阿秒脉冲     

A 12.1-W SESAM mode-locked Yb:YAG thin disk laser

Pumped by a 940 nm fiber-coupled diode laser, a passively mode-locked Yb:YAG thin disk oscillator was demonstrated with a semiconductor saturable absorber mirror(SESAM). 12.1 W mode-locked pulses were obtained with pulse duration of 698 fs at the repetition rate of 57.43 MHz. Measurement showed that the beam quality was close to the diffraction limit.     

Ionization self-compression of intense femtosecond pulses propagating through gas-filled dielectric capillaries

A mechanism of the ionization-induced self-compression of femtosecond laser pulses propagating in a gas-filled hollow dielectric capillary has been investigated both experimentally and theoretically. In particular, the double self-compression of a laser pulse from 76 to 40 fs has been experimentally demonstrated. A theoretical model that explains the mechanism of such a self-compression and provides a good agreement with the experimental data has been developed. The model also predicts that a laser pulse shorter than 10 fs can be generated in the optimal regime with an energy efficiency exceeding the efficiency of self-compression on a filament widely discussed at present.     

Direct phase and amplitude characterization of femtosecond laser pulses undergoing filamentation in air

Measurement of the temporal (spectral) phase and amplitude of a 50 fs laser pulse approaching and exceeding the critical power for self-focusing (P(crit)) in air reveals the formation of an isolated 17 fs pulse at 3P(crit). The dynamics of self-shortening are measured directly in the filament using transient-grating cross-correlation frequency-resolved optical gating with a noble gas serving as the nonlinear medium. Our results support recent filamentary propagation simulations, suggesting that a Kerr-dominated temporal reshaping process toward the end of the filament is largely responsible for the generation of short pulses.     

基于大基模体积的10mJ飞秒钛宝石激光再生放大器

文章报导了基于大基模体积的高能量飞秒钛宝石激光再生放大器的设计与实验研究,在重复频率10 Hz、抽运能量60 mJ的激励下,得到了单脉冲能量17.4 mJ的种子脉冲放大结果,压缩后的脉冲宽度为40.6 fs,能量为13.9 mJ.借助于此大基模体积再生腔,仅增加一级多通放大,实现了峰值功率达1.9 TW飞秒激光脉冲输出.结果表明,大模体积再生放大不仅降低了后续放大对抽运能量的要求,也可以单独压缩实现再生腔直接输出10 mJ量级的飞秒激光脉冲,是大能量高峰值功率飞秒激光系统的优质前端.     

双色光场驱动产生单个阿秒脉冲过程中的宏观效应

通过数值模拟三维传播方程研究了双色场产生高次谐波过程中宏观效应的影响,重点分析了阿秒脉冲的时域形状.在6 fs/800 nm和21.3 fs/400 nm组成的双色场中,通过调节双色场间的相对延时,双色场各焦点相对于气体盒子入口的间距和双色场束腰半径和气体压强等参数,可以在平台区得到超宽带连续谱.对该连续谱中420—540 eV的谐波直接进行滤波后可以得到时间同步性很好的单个阿秒脉冲.这种性质对实验上通过双色场方案产生超短的阿秒脉冲非常有利. 关键词： 传播效应 阿秒脉冲 时间同步性     

Generation of high-energy, sub-20-fs pulses in the deep ultraviolet by using spectral broadening during filamentation in argon

Generation of sub-20-fs UV pulses with more than 300 μJ energy at 268 nm is reported. First, the UV pulses are produced by successive second-harmonic and third-harmonic (TH) generation of 805 nm pulses of a 1 kHz Ti:sapphire laser amplifier. The spectral broadening of TH pulses is realized in a filament, generated in argon. The produced pulses are compressed in a simple double-pass prism-pair compressor. Starting from 100 fs pulses, we achieve a fivefold pulse shortening.     

Spatial optimization of filaments

We demonstrate the spatial homogenization of intense laser pulses by adaptive minimization of spatial chirp of the spectrally broadened output pulses of a filament. A liquid-crystal-based two-dimensional spatial light modulator is used to control the spatial phase of the driver pulse. An evolutionary algorithm finds the optimal spatial laser phase distribution that introduces minimal distortions during filamentation and enhances the beam quality of the output pulse. A homogeneous intensity distribution favours efficient temporal compression close to the bandwidth limit without the need for spatial filtering after the filament. PACS 42.65.-k; 42.65.Re; 41.85.Ct     

超宽带近红外和蓝光飞秒激光脉冲产生的实验研究

增益介质中泵浦光束提供的软光阑效应对于实现克尔透镜稳定锁模及其超宽带光谱脉冲的产生具有非常重要的作用.实验上首先对半导体泵浦全固化钛宝石飞秒激光器的锁模动态特性进行了研究,在4 W绿光泵浦状态下获得了平均输出功率为570 mW、中心波长在794 nm～835 nm范围内调谐、光谱带宽最大可达135 nm的近红外光脉冲输出,其相应的时域变换极限脉冲宽度均小于10 fs.另外,将光束聚焦在超薄BBO晶体上,获得了中心波长在418 nm～429 nm之间调谐、光谱宽带时域变换极限小于15 fs的蓝光飞秒脉冲.     

飞秒激光相干场诱导材料功能微结构

飞秒激光在整个脉冲宽度内具有极好的相干性，因而当从同一光束分出的两束或两束以上的光束时间与空间上实现相互叠加时将会形成强度周期性调制的电磁场．这种周期调制的电磁场与材料产生相互作用，能诱导出相应的周期微结构．最近通过两束或两束以上飞秒激光干涉诱导功能微结构得到了广泛研究．文章综合了国内外飞秒激光研究小组在干涉诱导微结构方面的一些最新成果以及作者在这方面开展的工作，对飞秒激光干涉技术的原理及其在诱导微结构方面的应用进行了介绍．     

Conical Emission Patterns by Femtosecond Pulses with Different Spectral Bandwidths

Different conical emission （CE） patterns are obtained experimentally at various incident powers and beam sizes of pump laser pulses with pulse durations of 7fs, 44fs and 100fs. The results show that it is the incident power but not the incident power density that determines a certain CE pattern. In addition, the critical powers for similar CE patterns are nearly the same for the laser pulses with the same spectral bandwidth. Furthermore, as far as a certain CE pattern is concerned, the wider the spectral bandwidth of pump laser pulse is, the higher the critical power is. This will hopefully provide new insights for the generation of CE pattern in optical medium.     

Systematic study of highly efficient white light generation in transparent materials using intense femtosecond laser pulses

We report the results of a systematic study of white light generation in different high band-gap optical media (BaF₂, acrylic, water and BK-7 glass) using ultrashort (45 fs) laser pulses. We have investigated the influence of different parameters, such as focal position of the incident laser light within the medium, the polarization state of the incident laser radiation and the pulse duration of the incident laser beam on the white light generation. Our results indicate that for intense, ultrashort pulses, the position of physical focus inside the media is crucial in the generation, with high efficiency, of white light spectra over the wavelength range 400–1100 nm. Linearly polarized incident laser light generates white light with higher intensity in the blue region than circularly polarized light. Ultrashort (45 fs) pulses generate a flatter spectrum with higher white light conversion efficiency than longer (300 fs) pulses of the same laser power. We believe that a flat response over a wide range of wavelengths in the continuum may be efficiently compressed for generation of sub-10 fs pulses. PACS 52.38.Hb; 42.65.Jx; 42.65.Tg; 33.80.Wz; 52.35.Mw     

飞秒强激光经氩气和氩团簇的传播

通过数值求解三维电场传播方程,理论模拟了飞秒强激光脉冲（50 fs,1016 w/cm2）在氩气和中等尺寸氩团簇中的传播效应.结果表明,飞秒强激光脉冲经氩气传播将发生频谱蓝移展宽和光束发散;而经中等尺寸氩团簇传播,则存在一定程度的自聚焦效应.     

Compression of the pulses of a Ti:sapphire laser system to 5 femtoseconds at 0.2 terawatt level

High intensity contrast ratio, pre-pulse free pulses with pulse duration of 5 fs and pulse energy of 1 mJ were generated at a repetition rate of 1 kHz by compressing the output pulses of a multi-stage Ti:sapphire laser amplifier system in a Ne-gas-filled hollow fibre. The spatial and temporal properties of the output laser beam were fully characterised. PACS 42.60.Jf; 42.65.Re; 42.81.Qb     

光纤激光器直接输出的高功率贝塞尔超短脉冲

构建了一种能够直接输出高功率贝塞尔超短脉冲的光纤激光放大器. 该方案基于在光纤端面特殊设计和制备的微型负轴锥镜, 针对常规超短脉冲光纤激光放大系统所设计, 不需要引入其他分立整形器件, 避免了目前基于轴锥透镜产生贝塞尔光束的通用方法所带来的额外烦琐准直工作, 极大简化了产生贝塞尔光束的方法. 其中的微型负轴锥镜由聚焦粒子束刻蚀法在一段掺镱大模场光子晶体光纤的端面制备, 它和光纤激光系统中的固有准直透镜构成了集成化的光束整形器件. 基于数值模拟结果成功搭建的系统与理论设计一致, 直接输出了在米量级具有高度准直无衍射特性的啁啾皮秒贝塞尔超短脉冲波包, 平均功率高达10.1 W, 对应脉冲能量178 nJ, 经过光栅对压缩后脉冲宽度可达140 fs. 关键词： 衍射 超短脉冲产生 光纤器件 光纤激光器     

Arsenic beam from a MBE source characterized by femtosecond laser spectroscopy

4 are the constituents of a molecular beam. Of importance are parameters such as laser intensity, pulse duration, and laser wavelength for a quantitative characterization and thereby improvement of MBE sources. This would help to optimize the epitaxial growth process. We demonstrate that nonresonant multiphoton ionization (MPI) by fs laser pulses leads to detection of the parent molecule, whereas nonresonant MPI with ns laser pulses leads to strong fragmentation. Received: 8 August 1997/Revised version: 3 December 1997