斜辐照激光等离子体辐射X光子特性

 在神光Ⅱ高功率激光装置上,实验研究了激光斜辐照形成的激光等离子体辐射X射线光子的特性及真空喷射热等离子体流的方向。采用针孔X射线相机测量了钕玻璃激光（基频1.053 μm）辐照铝靶形成的激光铝等离子体辐射的X射线光子的空间分布,并针对正入射和入射激光斜辐照情况下测得的X射线光子量及特性进行了分析和比较。结果发现：入射激光斜辐照固体平面靶产生的向真空喷射热等离子体流的方向是垂直靶面（即法线方向）;正入射和斜入射激光叠加驱动靶时,一定程度上能改善激光辐照的均匀性,但等离子体源辐射的X射线光子数并未发现显著地增加;当激光斜辐照与靶相互作用时,激光能量被等离子体吸收下降。     

基于离散小波包变换的规整化波前解卷积

 直接波前解卷积可以有效地克服大气湍流对天文观测的影响,但解卷积问题具有病态特性,必须进行规整化。基于奇异值分解的规整化方法在处理图像边缘等高频部分不够理想;而WVD方法仅适用某些解卷积问题。在提出一种基于小波包变换的新规整化方法的基础上,将此方法应用于室内模拟点源实验中,并与基于奇异值分解规整化的维纳逆滤波进行了对比,实验结果表明：该规整化方法可以有效地解决解卷积问题的病态特性,应用该规整化方法所恢复的图像质量明显提高。     

Avoiding temporal distortions in tilted pulses

Tilted femtosecond laser pulses, having an intensity front with an angle to the propagation direction, can be generated by a dispersive element and a lens or mirror for imaging. Here we show that conventional geometries, for example with a grating at Littrow's condition, produce significant temporal distortions over the beam profile. The aberrations are the result of a mismatch between the grating's surface and the object plane of the imaging system. This changes the chirp of the pulses over the beam profile and lengthens the pulses to picoseconds for millimeter-sized beams. The distortions can be avoided by choosing a geometry in which the propagation direction of the tilted pulses is perpendicular to the grating's surface.     

Angular chirp and tilted light pulses in CPA lasers

We investigate analytically and experimentally various aspects of the angular chirp of ultrashort laser pulses. This type of chirp is easily produced by slight misalignment of standard pulse stretcher and/or compressor setups. Angular chirp leads to tilted pulse fronts in the near field and to a strong reduction of intensity in the focus. The effect is rather difficult to observe with standard diagnostic techniques. We present a method that is based on interferometric field autocorrelation and allows us to measure the angular chirp reliably. Suggestions on how to avoid this effect are outlined as well.     

The mode structure and spectral properties of supercontinuum emission from microstructure fibers

The mode structure and spectral properties of supercontinuum emission generated by femtosecond pulses of Ti: sapphire laser radiation in microstructure fibers are studied. The long-wavelength (720–900 nm) and visible (400–600 nm) parts of supercontinuum emission are shown to be spatially separated in microstructure-fiber modes, which can be isolated with an appropriate spectral filtering. The spatial modes thus isolated in spectrally sliced supercontinuum emission possess a spatial quality sufficient for further efficient frequency conversion. The possibility of achieving a high spectral quality of supercontinuum emission is also demonstrated. We explore the ways to control the spectrum of supercontinuum emission by matching parameters of the pump pulse with the parameters of a microstructure fiber and by tuning the initial chirp of the pump pulse. The results of our studies show that supercontinua produced in microstructure fibers offer new approaches to designing a new generation of optical parametric amplifiers and broadband radiation sources for spectroscopic, metrological, and biomedical applications.     

Numerical analysis of spatial distortions in a single-grating chirped pulse amplification system

A numerical study of spatial distortions of a laser beam after propagation through an optical single-grating chirp pulse amplification (CPA) system was developed. This study is based on numerical simulation using the ray-tracing model from Rayica module of MATHEMATICA and it relates the behavior of the aberrated beam in terms of spatial distortions (eg. spatial chirp and pulse front tilt) in case of grating incident angle variation. The results are relevant for different applications which use CPA systems with needs of high quality laser beam profile.     

Second-harmonic generation with traveling-wave pulses

We experimentally studied properties of traveling-wave excitation and traveling-wave pulse compression for a Nd : glass laser system with a streak camera. These properties were extended to second-harmonic generation with a type-11 KDP crystal. We found that the second-harmonic pulse generated from the chirped-fundamental pulse also has a good chirp linearity, but the spectral bandwidth and temporal duration were reduced. We demonstrated the effective subpulse suppression in the compressed pulse with the second-harmonic generation. The pulse with tilted wave front, short wavelength and high contrast ratio can be obtained by the second-harmonic generation of traveling-wave pulse.     

Performance enhancement of Brillouin optical correlation domain analysis based on frequency chirp magnification

We propose a method to enhance the performance of Brillouin optical correlation domain analysis(BOCDA)with a broad chirp span of optical sources based on the frequency chirp magnification technique. In BOCDA systems, the number of effective sensing points is proportional to the chirp span of the light source, which is normally limited by the characteristics of the laser diode. We demonstrate a chirp span of 126 GHz with the proposed method, to double the effective sensing points of BOCDA. By combining with differential measurement schemes, a spatial resolution of 10 cm over a 1 km range is achieved.     

Intense THz-radiation from semiconductors under magnetic field irradiated with femtosecond laser pulses

The THz-radiation power from InAs reaches sub-mW level in a 1.7-T magnetic field irradiated with femtosecond laser pulses of 1.5-W average power. The THz-radiation power is related almost quadratically both to the magnetic field and to the excitation laser power. Furthermore, the THz-radiation spectrum is found to be controlled by the excitation pulsewidth, chirp direction of the excitation pulse, and the magnetic field.     

Effect of interface anisotropy on tilted growth of eutectics: A phase field study

Interfacial energy anisotropy plays an important role in tilted growth of eutectics. However, previous studies mainly focused on the solid—solid interface energy anisotropy, and whether the solid—liquid interface energy anisotropy can significantly affect the tilted growth of eutectics still remains unclear. In this study, a multi-phase field model is employed to investigate both the effect of solid—liquid interfacial energy anisotropy and the effect of solid—solid interfacial energy anisotropy on tilted growth of eutectics. The findings reveal that both the solid—liquid interfacial energy anisotropy and the solid—solid interfacial energy anisotropy can induce the tilted growth of eutectics. The results also demonstrate that when the rotation angle is within a range of 30°—60°, the growth of tilted eutectics is governed jointly by the solid—solid interfacial energy anisotropy and the solid—liquid interfacial energy anisotropy; otherwise, it is mainly controlled by the solid—solid interfacial energy anisotropy. Further analysis shows that the unequal pinning angle at triple point caused by the adjustment of the force balance results in different solute-diffusion rates on both sides of triple point. This will further induce an asymmetrical concentration distribution along the pulling direction near the solid—liquid interface and the tilted growth of eutectics. Our findings not only shed light on the formation mechanism of tilted eutectics but also provide theoretical guidance for controlling the microstructure evolution during eutectic solidification.     

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     

Simulation of a chirped femtosecond relativistic laser pulse interaction with underdense plasma by using a hydrodynamic approach

A chirped laser pulse indicates that the laser frequency changes over the duration of the pulse: a positively (negatively) chirped pulse implies that the laser frequency increases (decreases) with time. In this paper, we use a simplified, fully relativistic hydrodynamic approach to simulate the influence of chirp on the propagation of a femtosecond relativistic laser pulse in underdense plasma. Based on this simplified cold‐fluid model, the influence of chirp on the main dynamics of the laser pulse, such as self‐steepening, red‐shift in the leading edge, variation of the frequency chirp, and the generated wakefields can be studied self‐consistently. The simulation results show that a pulse with a positive chirp results in a larger increment in the intensity parameter a₀ when propagating a certain distance into an underdense plasma compared with an un‐chirped and a negatively chirped pulse, which is largely because of a much greater forward shift of the peak amplitude and more severe pulse self‐steepening effect due to the frequency red‐shift at the leading edge when exciting a plasma wave. The ponderomotive force, which relates to the first‐order differential of the laser pulse intensity envelope, is expected to be stronger for a positively chirped pulse because of its steeper leading edge and larger intensity parameter a₀. As a result, the wakefield driven by the positively chirped laser pulse is more intense than that driven by an un‐chirped and a negatively chirped laser pulse, which is confirmed by our self‐consistent hydrodynamic simulation.     

周期性啁啾脉冲色散衍射实现近场强度匀滑

 对惯性约束聚变（ICF）激光驱动器中的周期性啁啾脉冲光谱色散后的自由空间衍射传输和近场强度匀滑特性进行了理论分析和数值模拟,并计算了色循环数对两者的影响。计算结果表明：远场光斑随色循环数增加而变大,强度调制加剧;近场强度匀滑效果随色循环数和传输距离增加而增强,但空间形状变化加剧。通过匀滑效果对比发现,基于周期性啁啾脉冲的光谱色散平滑系统的近场强度匀滑效果优于传统的正弦调频光束。     

Microstructure fibers as frequency-tunable sources of ultrashort chirped pulses for coherent nonlinear spectroscopy

Microstructure fibers are shown to allow the creation of new tunable sources for femtosecond nonlinear spectroscopy. These fibers provide a high efficiency of frequency upconversion of regeneratively amplified femtosecond pulses of a Cr:forsterite laser, permitting the generation of subpicosecond anti-Stokes pulses with a smooth temporal envelope and a linear positive chirp. These pulses from a microstructure fiber were used to measure the spectra of coherent anti-Stokes Raman scattering (CARS) of toluene solution by cross-correlating these pulses with the femtosecond second-harmonic output of the Cr:forsterite laser in boxcars geometry (XFROG CARS). PACS 42.65.Wi; 42.81.Qb     

To measure chirped ultrashort laser pulse by the Talbot effect of a grating

We study the Fresnel diffraction of a phase grating under the illumination of a chirped femtosecond laser pulse. Using spatial spectrum theory, first we extract the central wavelength signal from the diffraction wave of the chirped pulse through a diaphragm. Then the central wavelength waves are directed to interfere in remote regions by two reflectors. The interference stripes at different distance will be exactly the same in interference region. Since the intensity distribution of the stripes is related to the chirp parameter of the femtosecond pulse, through measuring the stripe intensity distribution, we can indirectly detect the chirp parameter of the femtosecond pulse.     

Control and characterization of multiple circularly polarized femtosecond filaments in argon

We provide what is believed to be the first experimental evidence of spatial control on multiple filamentation (MF) using circularly polarized femtosecond laser pulses. The exceptional shot-to-shot reproducibility of the MF pattern allowed complete characterization of the two copropagating high-energy filaments, revealing for the first time temporal (self-) compression of circularly polarized filamenting pulses to a fifth of the initial 57 fs laser pulse duration without any sophisticated chirp control. Compared with LP MF, an enhancement in spatial stability and an increase in energy throughput are reported for circular input polarization.     

带调谐光纤F-P腔的光纤环腔半导体激光器的频偏特性研究

本文由速率方程出发分析了带调谐光纤F-P腔的光纤环形腔半导体激光器的频偏特性,结果表明在低频时的频偏功率比自由运转激光器低,而在高频时则趋于一致。改变调制频率和F-P腔的腔长可以改变激光器的频偏功率比,选择适当的调制频率和F-P腔参数就能获得不同的频偏特性。     

Irregular frequency chirp in radiation of an injection laser in high-velocity code modulation

The effect of the shape of the excitation pulses on the magnitude of the frequency chirp in an injection laser is investigated. It is established that as the degree of rectangularity of the pulses becomes higher, the frequency chirp increases. Simultaneous consideration of the effect of residual charge in the active region of the laser, the delay of stimulated radiation, and the frequency chirp indicates that in the case of high-velocity code modulation of an injection laser, in the latter an irregular frequency chirp appears that can exert a noticeable influence on the distortion of code trains of pulses having a super-Gaussian shape, when they propagate in media with a dispersion of 100 p.s./nm/km and above. Belarusian State University, 4, F. Skorina Ave., Minsk, 220050, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 1, pp. 45–49, January–February, 1997.     

激光啁啾对激光等离子体细丝传输的影响

对不同啁啾状态下的飞秒激光在大气中的成丝过程进行了研究.实验研究发现成丝状况及其演化与初始激光脉冲的啁啾状态密切相关，适当的负啁啾有利于激光的成丝传输.当初始激光脉冲的啁啾量不断变大时，激光脉冲在大气中的成丝起点位置会逐渐变远.还研究了初始脉冲啁啾量对使用聚焦透镜产生的细丝的影响，发现其与自由传输情况具有相似的变化规律. 关键词： 飞秒激光等离子体通道 脉冲啁啾     

High-order harmonic generation by self-guided femtosecond laser pulses in argon

High-order harmonic generation (HHG) with 40 fs laser pulse in a quasistatic gas cell filled with argon gas was studied. Experiment results in several aspects were given and analyzed. As a result of balancing diffraction and plasma defocusing, and a large harmonic-generation cross-section, the spatial quality of the 23rd harmonic is successfully optimized with a divergence angle of 0.38 mrad. The modulation of spectral spatial divergence angle by the gas pressure was given, which was explained by the formation of plasma channel. The effect of initial laser pulse chirp on the harmonic generation and the harmonic spectral blueshift under different gas pressure were given and analyzed.