Linear theory of resonance self-modulation of an intense laserpulse in homogeneous plasma and plasma channels

The analytical solutions describing the linear stage of the intense-laser-pulse self-modulation, which results in a strong plasma wakefield excitation, are studied in terms of the paraxial approximation. The attention is focused on phase relations that were ignored in the previous studies. It is shown that the value of the phase velocity of the plasma wake wave differs from the pulse group velocity so that under some specific conditions, the relativistic factor corresponding to the phase velocity can be substantially less than that for the group velocity. This may be important for the particle acceleration in the self-modulated laser wakefield accelerator     

Achromatic doublets using group indices of refraction

One main function of short pulses is to concentrate energy in time and space [1]. The use of refractive lenses allows us to concentrate energy in a small volume of focusing around the focal point of the lens. When using refractive lenses, there are three effects that affect the concentration of energy around the focal point of the lens. These are the group velocity dispersion (GVD), the propagation time difference (PTD), and the aberrations of the lens. In this paper, we study lenses which are diffraction limited so that the monochromatic aberrations are negligible; the group velocity dispersion and the propagation time difference are the main effects affecting the spreading of the pulse at the focus. We will show that for 100-fs pulses the spatial spreading is larger than the temporal spreading of the pulse. It is already known that the effect of spatial spreading of the pulse due to PTD can be reduced by using achromatic optics. We use the theory proposed by A. Vaughan to analyze simple lenses and normal achromatic doublets, where normal means doublets that we can buy from catalogs. We then use the Vaughan theory to design achromatic doublets in phase and group, which produce no spatial spreading of the pulse, i.e., PTD = 0, when the doublet is designed for the carrier of the pulse. We compare these phase and group achromatic doublets with normal achromatic doublets. Finally, we show that apochromatic optics can give a much better correction of PTD than using normal achromatic doublets.     

Variation of the carrier-envelope phase of few-cycle laser pulses owing to the Gouy phase: a solid-state-based measurement

The carrier-envelope phase of a laser pulse has recently become an important quantity in extreme nonlinear optics. Because of the topological Gouy phase, it changes while the pulse propagates through the focus of a lens. This variation is measured by a simple solid-state-based approach. The experimental results are analyzed by comparison with simple analytical model calculations.     

Acceleration of femtosecond pulses to superluminal velocities by Gouy phase shift

The phase and the group velocities are calculated in a three-dimensional neighbourhood of the focus of an aberration-free lens illuminated by a spatially Gaussian beam. The Gouy phase shift caused by the diffraction results in superluminal pulse propagation on the optical axis within the Rayleigh range.     

High-resolution phase imaging of phase singularities in the focal region of a lens

Subwavelength-resolution phase images of phase dislocations at the focal region of a 20x , 0.4-N.A. lens have been obtained by use of an optical fiber interferometer with a tapered probe in one arm. A phase-stepping algorithm is used to determine a quantitative value of the phase at each point in the scan, clearly showing the presence of edge dislocations between the Airy rings of the diffraction pattern near the lens focus, as well as four isolated screw-type singularties caused by astigmatism in the lens.     

Contrast and phase characterization of a high-peak-power 20-fs laser pulse

We fully characterize a high-peak-power, ultrashort laser pulse in a Ti:sapphire chirped-pulse amplification laser system. The contrast, temporal, and spectral phases of the 20-fs pulse are determined by using high-dynamic-range cross-correlation and frequency-resolved optical gating techniques. Minimization of the total phase error can be achieved by balancing the phase terms of the group delay dispersion and quartic. The laser system is currently being applied to perform high-field atomic ionization experiments at 10²⁰ W/cm².     

啁啾脉冲频域相移的模拟与实验研究

在超快光学瞬态相移的测量中,结合线性啁啾脉冲与频谱干涉技术,可以获得啁啾脉冲相移随频谱的变化规律。光脉冲经过光栅色散和透镜聚焦后,不同的频率成分以空间上的不同坐标展开。当两束脉冲频谱同时在空间上展开时,相同的频谱成分会产生干涉。基于傅里叶变换频谱干涉技术,从频谱干涉图中提取相移,并对其进行了数值模拟和实验研究。结果表明,通过对不同类型频域相移进行重构所得到的结果反映出了相移随脉冲频谱变化的特性。实验结果表明了这种重构算法是有效的,与模拟结果具有较好的一致性。     

Diffraction effects accompanying focused ultrasonic pulse propagation in a medium with a thermal inhomogeneity

Propagation of a probing pulse through a thermal inhomogeneity produced in the medium as a result of heating by a focused ultrasonic beam is considered. The wave equation is used to numerically calculate the waveform of the pulse arriving at the receiver whose focus coincides with that of the radiator. Calculations are performed for a pulse focused at the center of the inhomogeneity and crossing it in the transverse direction. The results of numerical simulation are compared with experimental data. Conditions under which the diffraction effects should be taken into account in calculating the pulse delay at the receiver are formulated.     

Optically controlled seeding of Raman forward scattering and injection of electrons in a self-modulated laser-wakefield accelerator

Optical seeding of plasma waves and the injection of electrons are key issues in self-modulated laser-wakefield accelerators. By implementing a copropagating laser prepulse with proper timing, we are able to control the growth of Raman forward scattering and the production of accelerated electrons. The dependence of the Raman intensity on prepulse timing indicates that the seeding of Raman forward scattering is dominated by the ionization-induced wakefield, and the dependence of the divergence and number of accelerated electrons further reveals that the stimulated Raman backward scattering of the prepulse plays the essential role of injecting hot electrons into the fast plasma wave driven by the main pulse.     

Aberrations theory of a time dependent magnetic field lens

We analyze new forms of aberrations affecting a time dependent magnetic field lens recently proposed. The lens consist of an axially symmetric ellipsoidal solenoid producing a spatially uniform but time pulsating homogeneous field. The dynamics of electrons emitted by some object and injected into this coil shows that this system is capable of focusing a parallel beam or produce images free of geometrical aberrations. The cardinal elements of this lens change periodically in positions and therefore to obtain a stationary image it is necessary to employ a chopped electron beam. Two forms of aberrations originate from the finite duration of the electron pulse and the energy dispersion of the incoming beam (chromatic aberration). Their image degradation effects are quite similar and result from recording different axially displaced images at some fixed stationary plane. We show how, by adjusting the parameters characterizing the magnetic pulse and introducing small apertures that increase the depth of focus, their effects can be reduced or in some cases eliminated. Other forms of instrumental aberrations characteristic of this time dependent magnetic field lens are briefly discussed.     

Aberrations theory of a time dependent magnetic field lens

We analyze new forms of aberrations affecting a time dependent magnetic field lens recently proposed. The lens consist of an axially symmetric ellipsoidal solenoid producing a spatially uniform but time pulsating homogeneous field. The dynamics of electrons emitted by some object and injected into this coil shows that this system is capable of focusing a parallel beam or produce images free of geometrical aberrations. The cardinal elements of this lens change periodically in positions and therefore to obtain a stationary image it is necessary to employ a chopped electron beam. Two forms of aberrations originate from the finite duration of the electron pulse and the energy dispersion of the incoming beam (chromatic aberration). Their image degradation effects are quite similar and result from recording different axially displaced images at some fixed stationary plane. We show how, by adjusting the parameters characterizing the magnetic pulse and introducing small apertures that increase the depth of focus, their effects can be reduced or in some cases eliminated. Other forms of instrumental aberrations characteristic of this time dependent magnetic field lens are briefly discussed.     

Graded-index time-lens implementation

The propagation of a laser pulse in a graded-index (GRIN) space lens and in an electro-optic crystal with an appropriate refractive index modulation is studied. It is shown that the crystal functions as a GRIN time lens if the propagation's second-order dispersion is included and if the phase velocity of the modulating wave equals the group velocity of the laser pulse.     

GW级纳秒脉冲源激光触发同步实验

结合GW级纳秒脉冲源建立了实验平台,研究了激光触发开关的延迟时间及其抖动与激光能量、工作电压、激光波长、透镜焦距等参数的关系。结果表明:开关延迟与抖动随激光脉冲能量的上升、工作电压的上升而逐渐减小;采用紫外光得到的开关延迟与抖动小于相同条件下红外光得到的结果;在高气压小间隙开关条件下,采用小的透镜焦距可以得到更小的开关延迟与抖动。开关在自击穿电压220 kV、欠压比为80%、采用7 mJ,266 nm 波长激光时的抖动为0.3 ns,在此条件下进行了两台脉冲源的同步实验,得到其同步偏差85%置信率小于0.3 ns。     

Nonlinear propagation acoustics of dual-frequency wide-band excitation pulses in a focused ultrasound system

In this article, acoustic propagation effects of dual-frequency wide-band excitation pulses in a focused ultrasound system are demonstrated in vitro. A designed and manufactured dual-frequency band annular array capable of transmitting 0.9/7.5 MHz center frequency wide-band pulses was used for this purpose. The dual-frequency band annular array, has been designed using a bi-layer piezo-electric stack. Water tank measurements demonstrate the function of the array by activating the low- and high-frequency layers individually and simultaneously. The results show that the array works as intended. Activating the low- and high-frequency layers individually, results in less than -50 dB signal level from the high- and low-frequency layers respectively. Activating both layers simultaneously, produce a well defined dual-frequency pulse. The presence of the low-frequency pulse leads to compression, expansion, and a time delay of the high-frequency pulse. There is a phase shift between the low- and high-frequency pulse as it propagates from the array to the focus. This makes the latter described effects also dependent on the array configuration. By varying the low-frequency pressure, a shift of up to 0.5 MHz in center frequency of a 8.0 MHz transmitted high-frequency pulse is observed at the array focus. The results demonstrate the high propagation complexity of dual-frequency pulses.     

利用交叉相位调制和四波混频制作的时间透镜的仿真分析

利用高非线性光纤中的交叉相位调制和四波混频分别在仿真中实现了时间透镜. 对基于交叉相位调制的时间透镜中的高非线性光纤中的非线性过程进行了仿真分析. 仿真结果表明, 该时间透镜的主要影响因素为色散、自相位调制与四波混频; 通过采用带有一定色散斜率的高非线性光纤可同时消除色散、自相位调制和四波混频的影响; 另外, 该高非线性光纤的色散零点最好选在信号脉冲和抽运脉冲波长的中心附近. 然后对基于四波混频的时间透镜的实现进行了仿真分析. 仿真结果表明, 该时间透镜的主要影响因素为色散、 自相位调制和其他的四波混频; 通过设定合适大小的信号脉冲和抽运脉冲的功率可消除自相位调制和其他的四波混频的影响; 另外, 通过在高非线性光纤中引入一定的色散可进一步提高信号脉冲和抽运脉冲的功率, 从而获得更高功率的输出脉冲. 最后对两种时间透镜系统做出了比较. 关键词： 光脉冲压缩 时间透镜 交叉相位调制 四波混频     

Validity of extracting photoionization time delay from the first moment of streaking spectrogram

Photoionization time delays have been studied in many streaking experiments in which an attosecond pulse is used to ionize the atomic or solid state target in the presence of a dressing infrared laser field. Among the methods of extracting the time delay from the streaking spectrogram, the simplest one is to calculate the first moment of the spectrogram and to measure its offset relative to the vector potential of the infrared field. The first moment method has been used in many theoretical simulations and analysis of experimental data, but the meaning of this offset needs to be investigated. We simulate the spectrograms and compare the extracted time delay from the first moment with the input Wigner delay. In this study, we show that the first moment method is valid only when the group delay dispersions corresponding to both the spectral phase of the attosecond pulse and the phase of the single-photon transition dipole matrix element of the target are small. Under such circumstance, the electron wave packet behaves like a classical particle and the extracted time delay can be related to a group delay in the photoionization process. To avoid ambiguity and confusion, we also suggest that the photoionization time delay be replaced by photoionization group delay and the Wigner time delay be replaced by Wigner group delay.     

Optical pulse compression using the temporal Radon-Wigner transform

The temporal Radon-Wigner transform (RWT), which is the squared modulus of the fractional Fourier transform (FRT) for a varying fractional order p, is here employed as a tool for pulse compression applications. To synthesize the compressed pulse, a selected FRT irradiance is optically produced employing a photonic device that combines phase modulation and dispersive transmission. For analysis purposes, the complete numerical generation of the RWT with 0 < p < 1 is proposed to select the value of p required for pulse compression. To this end, the amplitude and phase of the signal to be processed should be known. In order to obtain this information we use a method based on the recording of two different FRT irradiances of the pulse. The amplitude and phase errors of the recovered signal, which are inherent to the recording process, are discussed in connection with the RWT production. Numerical simulations were performed to illustrate the implementation of the proposed method. The technique is applied to compress signals commonly found in fiber optic transmission systems, such as chirped gaussian pulses, pulses distorted by second and third-order dispersion and nonlinear self-modulated pulses.     

光纤激光陀螺镜在超短脉冲测量中的应用研究

介绍了采用光纤激光陀螺镜测量超短脉冲宽度、形状、相位时的应用理论．以及相关群速度色散的补偿办法：利用可以自行调节旋转角速度的光纤陀螺镜作为光学延迟线实现旋转角速度的任意连续调整．并在使用归一化的Gerchberg-Saxlon算法中实现过程自动控制、克服了原有测量方法中因时延一定．只能作一次自相关处理．及把时延测量变为路径测量时准确性差和测量精度有限的缺点。利用高精度单光子计数系统对相干光包络进行测量．极大地提高了测量精度．进一步拓宽了超短脉冲在物理、生物、化学以及超高速光纤通信等领域的应用范围.     

Phase velocity and particle injection in a self-modulated proton-driven plasma wakefield accelerator

It is demonstrated that the performance of the self-modulated proton driver plasma wakefield accelerator is strongly affected by the reduced phase velocity of the plasma wave. Using analytical theory and particle-in-cell simulations, we show that the reduction is largest during the linear stage of self-modulation. As the instability nonlinearly saturates, the phase velocity approaches that of the driver. The deleterious effects of the wake's dynamics on the maximum energy gain of accelerated electrons can be avoided using side-injections of electrons, or by controlling the wake's phase velocity by smooth plasma density gradients.     

Starch-based second-harmonic-generated collinear frequency-resolved optical gating pulse characterization at the focal plane of a high-numerical-aperture lens

We report the use of starch as an ideal nonlinear medium with which to perform collinear frequency-resolved optical gating measurements of ultrashort pulses at the focal plane of a high-numerical-aperture (NA) lens. We achieved these measurements by simply sandwiching starch granules (suspended in water) between two coverslips and placing them within the focal plane of a high-NA lens. The natural nonlinear characteristics of starch allow the correct phase matching of pulses at the focal plane of a high-NA lens at different wavelengths. This elegant arrangement overcomes all the complexity and problems that were previously associated with pulse characterization within a multiphoton microscope.