飞秒激光脉冲在正色散固体材料中的自压缩

实验研究了正色散固体介质中的激光脉冲自压缩现象,证明了无需任何外加色散补偿情况下，固体透明介质中的自聚焦传输过程可使高功率飞秒激光脉冲实现时域脉冲压缩，并详细研究了输出脉冲的时域和频域特性随入射脉冲强度的演化规律.实验结果表明脉冲自压缩量随入射脉冲强度的增加呈递增趋势，然而当入射光强增大到足以引起超连续谱及锥形辐射产生时，脉冲时域形状会发生分裂.此外还发现发散光束入射情况下同样可以观察到脉冲自压缩现象. 关键词： 超短激光脉冲 脉冲压缩 非线性传输     

Pulse self-compression to the single-cycle limit by filamentation in a gas with a pressure gradient

We calculate pulse self-compression of a 30 fs laser pulse traversing gas with different pressure gradients. We show that an appropriate density profile brings significant improvement to the self-compression by filamentation. Under an optimal pressure gradient, the pulse duration is reduced to the single optical cycle limit over a long distance, allowing easy extraction into an interaction chamber.     

Optimization of a femtosecond pulse self-compression region along a filament in air

We have identified the pulse self-compression region in a filament produced by 55 fs, 4 mJ, 805 nm radiation propagating in air without geometrical focusing. In our experiment the pulse self-compression region is attained by the propagation distance, where the shortest wavelength in the supercontinuum blue wing reaches a minimum, and the growing conversion efficiency to white light has a large gradient. Numerical tracking of the pulse along the filament shows a single-peak 9 fs pulse with a flat spectral phase at the optimum compression distance. PACS 42.65.Jx; 42.65.Re; 42.65.Ky     

激光脉冲在等离子体中的压缩分裂

通过数值求解一维非线性薛定谔方程,研究了圆偏振入射激光脉冲在初始密度范围为1/4到略低于1倍临界密度的等离子体中的自压缩和分裂现象. 提高等离子体密度和入射激光强度以及减小脉冲宽度可以在更短的传输距离获得有效的激光脉冲压缩,压缩后的脉冲半高宽度可达到初始脉冲半高宽度的1/35,甚至更小. 这种压缩是激光脉冲在等离子体中形成高阶孤子的过程中产生的,可以获得比在稀薄等离子体中更好的压缩比例. 数值计算的结果给出了该情况下激光脉冲在等离子体中自压缩后形成的高阶孤子分裂. 利用一维粒子数值模拟程序(particle-in-cell,PIC)也观察到了脉冲的压缩和分裂现象,得到了与数值计算一致的结果. 关键词： 非线性薛定谔方程 自压缩 脉冲分裂 粒子模拟     

圆偏振光和线偏振光在正常色散材料中的时空自压缩

实验研究了圆偏振和线偏振高强度飞秒激光脉冲在正常色散材料中传输时的时空自压缩现象。实验中利用BK7玻璃作为正色散材料,比较研究了不同偏振入射情况下脉冲波形及频谱的变化规律。圆偏振光入射时,可以获得更短脉冲宽度的压缩脉冲和更窄的光谱宽度。在圆偏振光入射条件下,50 fs入射脉冲成功地自压缩到了19 fs,获得了大于2.5倍的压缩倍率。所以利用圆偏振光可以获得更短压缩脉冲,更大能量,更好光束质量的激光。     

Frequency shifting of sub-100 fs laser pulses by stimulated Raman scattering in a capillary filled with pressurized gas

The technique of Raman conversion of sub-100 fs laser pulses based on excitation of active medium by two orthogonally polarized pulses has been developed for Raman lasers with a glass capillary. 52 fs Stokes pulse at the wavelength of 1200 nm has been generated by stimulated Raman scattering of 48 fs Ti:sapphire laser pulse at the wavelength of 800 nm in hydrogen. 13% energy conversion efficiency has been achieved at pulse repetition rate up to 2 kHz.     

Generation of femtosecond pulses by two-photon pumping supercontinuum-seeded collinear traveling wave amplification in a dye solution

Visible femtosecond (fs) laser pulses have been obtained in a dye solution with a very simple traveling wave collinear configuration. A femtosecond Ti:sapphire laser (790 nm) pumps the dye solution by two-photon absorption and simultaneously generates supercontinuum, which seeds a light-amplification mechanism. Cross-correlation frequency-resolved optical gating measurements reveal a chirped structure in the dye pulse. The shortest pulse duration achieved is 170 fs and the overall energy efficiency of the process is typically 25%. Received: 22 March 2001 / Revised version: 4 July 2001 / Published online: 19 September 2001     

Direct carrier–envelope phase stabilization of a soliton-effect compressed sub-two-cycle pulse source through nonlinear mixing of solitonic and dispersive waves

We present a carrier–envelope phase (CEP) stabilized sub-two-cycle 5.2 fs pulse source based on soliton-effect self-compression of femtosecond laser pulses in millimetre-long highly nonlinear photonic crystal fibres. We employ a simple and efficient scheme to generate a strong (40–60 dB, configuration dependent) CEP beat signal directly from the pulse source via f-to-2f interferometry where the second harmonic of the main soliton pulse is mixed with the isolated dispersive blue/green radiation peak that is also generated in the compression process, obviating the need for additional spectral broadening mechanisms.     

High-order harmonics generation by few-cycle and multi-cycle femtosecond laser pulses

This paper investigates experimentally high-order harmonic generation(HHG) of neon gas with 5-fs and 25-fs driving laser pulses.It has been demonstrated that the cutoff energy of the harmonic extreme ultraviolet photons is extended to 131 eV and the HHG spectrum near the cutoff region becomes continuum as the driving laser pulse duration is 5 fs;whereas much lower cutoff photon energy and discrete harmonic spectrum near the cutoff region are presented as the laser pulse duration is 25 fs.The results can be explained by the fact that neutral atoms can be exposed to more intense laser field before they are depleted by ionization because of the extremely short rising time of the few-cycle pulse.The 5-fs driving laser pulse paves the way of generation of coherent x-ray in the water window and single attosecond pulse.     

Pulse self-compression in normally dispersive bulk media

We experimentally demonstrate that high-power femtosecond pulses can be compressed during the nonlinear propagation in the normally dispersive solid bulk medium. The self-compression behavior was detailedly investigated under a variety of experimental conditions, and the temporal and spectral characteristics of resulted pulses were found to be significantly affected by the input pulse intensity, with higher intensity corresponding to shorter compressed pulses. By passing through a piece of BK7 glass, a self-compression from 50 to 20 fs was achieved, with a compression factor of about 2.5. However, the output pulse was observed to be split into two peaks when the input intensity is high enough to generate supercontinuum and conical emission.     

Observation of laser-pulse shortening in nonlinear plasma waves

We have measured the temporal shortening of an ultraintense laser pulse interacting with an underdense plasma. When interacting with strongly nonlinear plasma waves, the laser pulse is shortened from 38 +/- 2 fs to the 10-14 fs level, with a 20% energy efficiency. The laser ponderomotive force excites a wakefield, which, along with relativistic self-phase modulation, broadens the laser spectrum and subsequently compresses the pulse. This mechanism is confirmed by 3D particle in cell simulations.     

双波长飞秒激光器量子理论研究

本文建立了一个不涉及具体光腔结构的双波长固体飞秒激光器模型,分析了群达弥散(GVD)、自相位调制(SPM)、交叉耦合、损耗调制和增益放大的动态平衡过程.从场算符的定义出发,导出了双波长飞秒激光器的量子锁模方程组,并且将该量子销模方程组化为经典锁模方程组.结果表明,该方程组和非线性薛定谔方程有着类似的结构,方程组存在稳定的脉冲解,因而从理论上证明了双波长飞秒激光器可以实现双波长同时锁模.     

Quadruple Gaussian Laser Beam Profile Dynamics in Collisionless Magnetized Plasma

This paper presents an investigation of self-focusing of a quadruple Gaussian laser beam in collisionless magnetized plasma. The nonlinearity due to ponderomotive force which arises on account of nonuniform intensity distribution of the laser beam is considered. The nonlinear partial differential equation governing the evaluation of complex envelope in the slowly varying envelope approximation is solved using a paraxial formalism. The self-focusing mechanism in magnetized plasma, in the presence of self-compression mechanism will be analyzed in contrast to the case in which it is absent. It can be observed that, in case of ponderomotive nonlinearity, the self-compression mechanism obstructs the pulse self-focusing above a certain intensity value. The effect of an external magnetic field is to generate pulses with smaller spot size and shorter compression length. The lateral separation parameter and the initial intensity of the laser beam play a crucial role on focusing and compression parameters. Also, the three-dimensional analysis of pulse propagation is presented by coupling the self-focusing equation with the self-compression one.     

Few-cycle pulse compression through cascade of bulk media and hollow-core fiber

We experimentally demonstrated a new few-cycle pulse compression technique through the cascade of bulk media and hollow-core fiber (HCF) and this compression system has been intensively studied. The pulses with the duration of ∼5 fs and the energy of 0.33 mJ near 800 nm have been generated by compressing the ∼40 fs input pulse from a commercial laser system. In principle, this technique allows compression of pulses with duration of picoseconds to a few cycles (sub-7 fs) and the output can be above 1 mJ.     

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     

Quadruple Gaussian Laser Beam Profile Dynamics in Collisionless Magnetized Plasma

This paper presents an investigation of self-focusing of a quadruple Gaussian laser beam in collisionless magnetized plasma. The nonlinearity due to ponderomotive force which arises on account of nonuniform intensity distribution of the laser beam is considered. The nonlinear partial differential equation governing the evaluation of complex envelope in the slowly varying envelope approximation is solved using a paraxial formalism. The self-focusing mechanism in magnetized plasma, in the presence of self-compression mechanism will be analyzed in contrast to the case in which it is absent. It can be observed that, in case of ponderomotive nonlinearity, the self-compression mechanism obstructs the pulse self-focusing above a certain intensity value. The effect of an external magnetic field is to generate pulses with smaller spot size and shorter compression length. The lateral separation parameter and the initial intensity of the laser beam play a crucial role on focusing and compression parameters. Also, the three-dimensional analysis of pulse propagation is presented by coupling the self-focusing equation with the self-compression one.     

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.     

在相对论性激光-等离子体系统中初始物理参数对激光脉冲的影响

基于相对论性激光-等离子体动力学理论，研究了相对论性激光-等离子体系统中圆偏振入射脉冲激光和等离子体相互作用对激光脉冲宽度的影响. 具体分析了在不同初始物理参数下脉冲激光的脉冲宽度在等离子体传播过程中的变化情况，重点分析了激光脉冲在等离子中压缩. 计算结果表明增加入射激光的强度和入射脉冲宽度以及减小等离子体的初始密度，能够有效地实现脉冲宽度在等离子体中压缩；当激光脉冲的初始参数a₀=0.12和τ=70以及等离子体密度n₀=0.3时，脉冲宽度相对压缩T/τ接近于1/10，从而给出了激光压缩的理论优化参数. 关键词： 相对论性激光-等离子体 激光脉冲宽度 等离子体密度 自压缩     

脉宽对超快激光平衡光学互相关测距影响的研究

采用基于平衡光学互相关的超快激光绝对距离测量方法对大气中一段15m的空间距离进行了测量。仿真了不同脉冲宽度、不同光强损耗对平衡互相关信号的影响,理论分析表明脉宽展宽对平衡互相关信号影响较大。采用中心波长为1037nm、重复频率为260MHz的超快激光器光源,搭建了测距系统。实验验证了在脉宽分别为1.44ps、132fs时,对平衡互相关信号的影响,实验结果符合理论分析。研究了不同脉冲宽度对测量分辨力的影响,结果显示,采样时间为5ms,脉宽为1.44ps,测量分辨力不低于50μm;而当脉宽为132fs时,测量分辨力优于5μm。     

飞秒强激光脉冲在等离子体中的自压缩

文章从激光等离子体相互作用的非线性薛定谔方程出发,理论研究了飞秒强激光脉冲在等离子体中的自压缩行为.结果表明在一定范围内随着激光脉冲宽度、激光强度的增大以及等离子体密度的减小,飞秒强激光脉冲在等离子体中传播的自压缩现象越明显.另外通过适当设定参量得到了近似稳定传播的基孤子.