1kHz飞秒激光脉冲在空气中传输成丝的演化过程

利用高重复频率(１kHz)、低能量的飞秒激光脉冲研究激光脉冲在空气中传输成丝的演化过程.采用成像的方法观测飞秒激光脉冲在大气传输过程中光束截面上光强分布变化，以及大气等离子体通道对光强分布的反作用.实验结果表明，激光脉冲在传输过程中先形成双丝结构，再逐渐合并成单丝结构.实验结果与利用非线性薛定谔方程耦合多光子电离的数值模拟结果基本一致. 关键词： 飞秒激光脉冲 等离子体通道 成像     

整形飞秒激光脉冲的成丝超连续辐射控制

飞秒激光成丝超连续辐射具有高强度和高时空相干性等优点,作为一种超宽带光源在很多领域都具有广泛的应用前景.本文提出一种结合微透镜阵列的空间调制和基于液晶空间光调制器的时域整形的飞秒激光脉冲整形方式,利用基于遗传算法的反馈优化控制,实现了飞秒激光在熔融石英中成丝产生的超连续辐射强度的调制,得到了在一定范围内光谱强度可控的超连续辐射光谱;光谱的能量密度可以从0.03μJ/nm调制到0.09μJ/nm,其能量密度变化达到了初始值的3倍.计算了典型迭代代数对应的整形脉冲时域包络,分析了超连续光谱随迭代代数的演化趋势,结果表明,脉冲包络的峰值强度和波形分布是影响超连续光谱展宽和强度的主要物理原因.     

高强度飞秒激光脉冲在空气中的自压缩

利用高重复频率（1kHz）、吉瓦级飞秒激光脉冲实验验证了高强度飞秒脉冲在空气中的自 压缩现象，研究了入射脉冲在不同初始啁啾情况下经空气中聚焦成丝后，时域及频域特性随 入射脉冲能量的变化规律.实验结果表明，在无需后继色散补偿情况下，高强度飞秒脉冲仅 通过在空气中的非线性传输过程就可以实现脉冲压缩；在入射脉冲为负啁啾情况下，实验观 察到脉冲光谱及时域宽度同时得到压缩，并可获得比激光源所能提供的更短的近双曲正割型 变换限脉冲. 关键词： 高强度飞秒激光脉冲 自压缩 自聚焦     

Optical breakdown versus filamentation in fused silica by use of femtosecond infrared laser pulses

The competition between optical breakdown (OB) and laser-pulse filamentation (FL) in bulk fused silica is investigated by using a 1-kHz femtosecond infrared laser. We measure input powers corresponding to the threshold of OB and FL in terms of external focusing conditions. The results demonstrate that OB precedes FL for tight focusing, whereas for sufficiently long focal lengths FL takes places at a lower power than OB does.     

Multiple filamentation of intense femtosecond laser pulses in air

The propagation of focused femtosecond laser pulses with supercritical peak powers in air has been investigated by the methods of optical visualization, profilometry, and calorimetry. Laser pulses with supercritical powers create a bundle of submillimeter filaments with a diameter of about 5 μm ahead of the lens focus; the maximum number of filaments in the beam cross section and their length increase linearly and sublinearly, respectively, with the radiation peak power. The optical visualization and calorimetry indicate that the plasma channels of filaments are optical contrast (a plasma density of 10¹⁸–10¹⁹ cm⁻³), ensuring the refraction of laser radiation incident on them.     

强光束局部小尺度调制致多路成丝现象研究

基于非线性Schrdinger方程模拟了Kerr介质中强光束局部小尺度调制致光束多路成丝现象，发现在一定的B积分范围内，光束局部小尺度调制基本上只从该局部本底场中吸收能量获得增长，从而将该局部光束分裂成细丝.当光束不同部分的小尺度调制相隔较远时，各 细丝的演化基本上不相关. 关键词： 自聚焦 多路成丝 非线性Schrdinger方程     

Interference of transverse rings in multifilamentation of powerful femtosecond laser pulses in air

We observe multiple filaments and interference of their ring structures in the propagation of 14 mJ, 45 fs infrared laser pulse in air. We suggest a simple physical model describing the formation and the interference of rings as the result of superposition of the background field of the whole beam and the fields that diverge from the filaments due to the defocusing in the laser-produced plasma. The size and the number of maxima in the interference pattern depend on the position of the filament formation along the direction of propagation. The simulated picture of the ring structure interference is in qualitative agreement with the one obtained from the experiment.     

飞秒脉冲激光1维强度调制的传输特性

实验研究了飞秒激光干涉条纹在CS2介质中传输的增长率与条纹间隔以及增长率与中心峰值强度的关系。结果发现,在非线性介质中传输的１维干涉调制特别是当其调制频率位于中心光强对应的最快增长频率附近时,随着传输距离的增加调制场迅速获得增长,将高斯光束分裂成多根峰值强度很高的带状细丝。相应的数值模拟与实验结果具有较好的一致性。     

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.     

3D Raman bullet formed under filamentation of femtosecond laser pulses in air and nitrogen

Complex experimental study of spectral, spatial and temporal behaviors of the IR shifted component observed under filamentation of the collimated femtosecond laser beam (80 GW, 50 fs, 805 nm) in molecular gases showed that this component behaves like a Raman soliton. Namely, it is confined in all domains: (a) it propagates within the filament core, (b) it has a stable duration of 30 fs along the filament, and (c) its spectrum shifts as a whole from 820 to 870 nm on the distance of 2 m from the filament start. A simple model explaining the origin of anomalous group velocity dispersion in the plasma channel of a filament is suggested.     

温度对飞秒激光脉冲在NaCl溶液中成丝产生的超连续谱的影响

利用飞秒激光脉冲在NaCl溶液中成丝,研究了溶液温度对飞秒激光成丝过程中产生的超连续谱的影响.发现在激光脉冲能量较低时,溶液温度对超连续谱的影响几乎可以忽略;而在激光脉冲能量较高时,随着NaCl溶液温度的升高,超连续谱呈现出被压缩的趋势.此外,在激光脉冲能量较高的情况下, NaCl中会产生大量的气泡.通过分析,得出了飞秒激光在溶液中成丝产生的气泡是影响超连续谱发射的主要因素.     

Dynamic spatial replenishment of femtosecond pulses propagating in air

We present numerical simulations of nonlinear pulse propagation in air whereby an initial pulse is formed, absorbed by plasma generation, and subsequently replenished by power from the trailing edge of the pulse. This process can occur more than once for high-power input pulses and produce the illusion of long-distance propagation of one self-guided pulse.     

Multiple filamentation of terawatt laser pulses in air

The filamentation of femtosecond light pulses in air is numerically and experimentally investigated for beam powers reaching several TW. Beam propagation is shown to be driven by the interplay between intense, robust spikes created by the defects of the input beam and random nucleation of light cells. Evolution of the filament patterns can be qualitatively reproduced by an averaged-in-time (2D+1)-dimensional model derived from the propagation equations for ultrashort pulses.     

Bonding of glass with femtosecond laser pulses at high repetition rates

We report on the welding of fused silica with ultrashort laser pulses at high repetition rates. Femtosecond laser pulses were focused at the interface of two optically contacted fused silica samples. Due to the nonlinear absorption in the focal volume and heat accumulation of successive pulses, the laser acts as a localized heat source at the focus position. Here, we analyze the influence of the laser and processing parameters on the amount of molten material. Moreover, we determine the achievable breaking stress by a three point bending test. With optimized parameters up to 75% of the breaking stress of the bulk material have been obtained.     

Stable generation of high-order harmonics of femtosecond laser radiation from laser produced plasma plumes at 1 kHz pulse repetition rate

We present a method for the creation of stable weakly ionized plasmas from laser ablation of solid targets using a 1 kHz pulse repetition rate laser, which can be used for stable high-order harmonic generation from plasma plumes. The plasma plumes were generated from cylindrical rotating targets. Without target rotation the intensity of harmonics in the 40-80 nm range drops by more than one order of magnitude during less than 10(3) shots, while, with rotation of the target at typically 30 revolutions per minute, stable emission of high-order harmonics from aluminum plasma plumes with variation of less than 10% was maintained for >10(6) laser shots.     

High power supercontinuum generation by dual-color femtosecond laser pulses in fused silica

High power supercontinuum (SC) is generated by focusing 800 nm and 400 nm femtosecond laser pulses in fused silica with a microlens array. It is found that the spectrum of the SC is getting broader compared with the case of a single laser pulse, and the spectral energy density between the two fundamental laser wavelengths is getting significantly higher by optimizing the phase matching angle of the BBO. It exceeds μJ/nm over 490 nm range which is from 380 nm to 870 nm, overcoming the disadvantage of relative lower power in the ranges far from the fundamental wavelength.     

Competition between multiphoton/tunnel ionization and filamentation induced by powerful femtosecond laser pulses in air

In this work we present experiments by focusing 42 femtosecond laser pulses in air using three differentfocal length lenses: f=100, 30 and 5 cm. For the longest focal length, only the filament, which is aweak plasma column,is observed. When the shorter focal length lens is used, a high density plasma isgenerated near the geometrical focus and coexists with a weak plasma channel of the filamemt. Under thetightest focusing condition, filamentation is prevented and only a strong plasma volume appears at tehgeometrical focus.     

各阶克尔非线性折射率对强激光大气传输的影响

用数值方法研究了各阶克尔非线性折射系数对强激光传输的影响,结果显示第7阶克尔非线性系数和第9阶克尔非线性系数协同影响激光传输,当第7阶克尔效应起作用时,忽略第9阶克尔效应的作用会导致光强随传输距离震荡剧烈,传输非常不稳定。要得到稳定传输需要第7阶克尔非线性系数和第9阶克尔非线性系数同时起作用或同时不起作用。在自聚焦阶段第3阶非线性折射率对折射率改变起主要作用,在稳定光丝传输阶段第7阶克尔非线性系数,第9阶克尔非线性系数起主要作用。第5阶克尔系数在整个传输过程中对折射率的改变一直比较小。     

基于高重复频率飞秒激光直写技术制作衍射光栅的研究

基于由光子晶体光纤飞秒激光器产生的高重复频率飞秒激光搭建了飞秒激光微加工系统,利用LabVIEW编写程序精密控制三维移动平台的移动路径和高速快门,在加工功率为1.5W、移动速率为1mm/s时,利用直写技术在无胶铬版上得到了微米量级的任意形状的平面衍射光栅。利用CCD相机对光栅制备过程进行实时监视,保证了光栅的加工质量。制备的光栅在显微镜下观察,条纹边缘清晰、线纹粗细均匀。使用He-Ne激光照射加工的光栅,可以获得清晰、稳定、与理论符合很好的夫琅和费(Frauhofer)衍射图样,显示了所加工的光栅具有良好的光学性能。实验结果也表明了光子晶体光纤飞秒激光在光栅制作过程中所具备的良好的加工性能。     

Highly efficient synchronous amplification of a picosecond dye laser at a kHz repetition rate

A short pulse, high energy, high repetition rate dye amplifier with superior conversion efficiency is reported. The dye amplifier is composed of three single-pass dye cells, longitudinally pumped by a frequency doubled 1 kHz Nd: YLF regenerative amplifier. The dye amplifier yields 3.5 ps, 150 J pulses at 595 nm, corresponding to a 12% conversion efficiency. The ASE is 1% or less, and the transverse mode quality is near-Gaussian.In absentia from the Department of Chemistry, Princeton University, Princeton, NJ 08544, USA