A new method for measuring the pulse-front distortion of arbitrary shapes in high-power ultrashort laser systems

We present a new method that can be used to calculate pulse-front distortion by measuring the spectral interference of two point-diffraction fields in their overlapped district. We demonstrate, for the first time, the measurement of the pulse-front distortion of the pulse from a complex multi-pass amplification system, which exists in almost all high-power laser systems, and obtain the irregular pulse-front distribution. The method presented does not need any reference light or assumption about the pulse-front distribution, and has an accuracy of several femtoseconds.     

大型短脉冲激光装置中脉冲前沿畸变效应的研究

阐述了短脉冲激光装置中脉冲前沿畸变效应的产生机理和可能的畸变源.利用Fourier光学方法,建立了一个基于单程压缩构型的研究脉冲前沿畸变效应的理论模型,分析不同种脉冲前沿畸变效应对于压缩器输出脉冲的近场时空特性的影响.基于此,对一个在建的大口径（300 mm）短脉冲装置中的各种畸变源（包括可编程声光光谱色散滤波器、透镜色差以及劈板等元件）进行了数值计算.研究表明：近场输出脉冲宽度显著加宽,输出脉冲前沿形状不同于输入时的畸变形状,只有中心部分区域的脉冲前沿形状与输入类似,该区域的大小由系统带宽决定. 关键词： 脉冲前沿畸变 角色散 单程压缩 透镜     

Distortion of ultrashort laser pulses in a spatial filter

By means of Collins diffraction integral, the propagation expression of an ultrashort laser pulse passing through a spatial filter is derived. The effects of the magnification of spatial filter on pulse broadening and distortion of pulse-front for the ultrashort optical pulse are analytically deduced and numerically simulated. It is found that pulse broadening and propagation time difference of a laser pulse getting through spatial filter is proportional to the magnification. As a conclusion, in an ultrashort pulse laser system with a large aperture, the effect of pulse broadening and distortion of pulse-front should be considered.     

Adjusting pulse-front tilt and pulse duration by use of a single-shot autocorrelator

We present a method of adjusting the pulse duration and eliminating the pulse-front tilt of an ultrashort pulse in real time by use of a specially configured single-shot autocorrelator. Pulse-front tilt, or a temporal delay across the pulse front, is a common ultrashort-pulse phenomenon when dispersive elements are being used. We show the design of an autocorrelator that can be used to eliminate the pulse-front tilt and simultaneously adjust the pulse duration in real time by adjustment of the pulse compressor of a chirped-pulse amplified laser system.     

Temporal characterization of short-pulse third-harmonic generation in an atomic gas by a transmission-grating Michelson interferometer

By use of a transmission-grating-based Michelson interferometer, second-order interferometric as well as intensity autocorrelation traces of the third harmonic of a Ti:sapphire 50-fs laser beam produced in Ar have been measured. The duration of the harmonic is found to be that expected from lowest-order perturbation theory. At this wavelength, the performance of the interferometer with respect to pulse-front distortion and dispersion is found to be satisfactory. This result is a first step toward the use of the interferometer for the temporal characterization of higher harmonics or harmonic superposition forming attosecond pulse trains.     

Ultra-broadband optical parametric chirped-pulse amplification by matching of both group-velocity and pulse-front

Ultra-broadband optical parametric chirped-pulse amplification is analyzed based the compensation of phase-mismatch, which is achieved by matching of both group-velocity and pulse-front between signal and idler by the combination of the noncollinear-phase-match and pulse-front-tilt. The results show exactly matching of both group-velocity and pulse-front is the important criterion for constructing an UBOPCPA. Its general model is developed, in which the group velocities, noncollinear angles, spatial walk-off angles, linear angular spectral dispersion coefficients and pulse-front tilted angles are suitably linked to each other. Finally, specific numerical calculations and simulations are presented for β-barium borate OPCPA with type-I noncollinear angularly dispersed geometry.     

Matching of both group-velocity and pulse-front for ultrabroadband three-wave-mixing with noncollinear angularly dispersed geometry

Ultrabroadband three-wave-mixing is analyzed using the concept of pulse-front-tilt, which is achieved by means of angular dispersion. The results show exactly matching of both group-velocity and pulse-front is the important criterion for constructing an ultrabroadband TWM. A theoretical model is developed, in which the group velocities, noncollinear angles, spatial walk-off angles, linear angular spectral dispersion coefficients and pulse-front tilted angles are suitably linked to each other. Expressions for finding ideal phase matching angle, noncollinear angle and linear angular spectral dispersion coefficient are obtained. Specific numerical results are presented for crystals BBO and LBO with type-I noncollinear angularly dispersed geometry. The numerical simulations verify that our model is valid for designing a practicable ultrabroadband parametric process. PACS 42.65.Yj; 42.65.Lm; 42.65.Ky; 42.60.By     

The Measurement of Ultrashort Light Pulses—Simple Devices,Complex Pulses

We review the state of the art of ultrashort-light-pulse measurement using frequency-resolved-optical-gating (FROG). Recent developments have extended the state of the art considerably. FROG devices for measuring the intensity and phase of ultrashort laser pulses have become so simple that almost no alignment is required. In addition, such devices not only operate single shot, but they also yield the two most important spatio-temporal distortions, spatial chirp and pulse-front tilt. With other FROG variations, it is now possible to measure more general ultrashort light pulses (i.e., pulses much more complex than common laser pulses), with time-bandwidth products as large as several thousand and as weak as a few hundred photons, and despite other difficulties such as random absolute phase and poor spatial coherence.     

Group-velocity-matched three-wave mixing in birefringent crystals

It is shown that the combination of pulse-front slant, kappa -vector tilt, and crystal birefringence often permits exact matching of both phase and group velocities for three-wave mixing in birefringent crystals. This exact match makes possible more-efficient mixing of short light pulses, and it permits efficient mixing of chirped or broadband light. I analyze this process and present examples.     

Traveling-wave Thomson scattering and optical undulators for high-yield EUV and X-ray sources

We present a novel high-yield Thomson scattering geometry that takes advantage of compact electron bunches, as available in advanced, low-emittance linear accelerators or laser wakefield accelerators. In order to avoid the restrictions on the X-ray photon yield imposed by the Rayleigh limit, we use ultrashort, pulse-front tilted laser pulses in a side-scattering geometry. Such a traveling-wave setup allows an overlap of electron and laser beams, even after propagating over distances much longer than the Rayleigh length. Experimental designs are discussed and optimized for different scattering angles. Specifically, to minimize group delay dispersion at large scattering angles >10°, we propose the use of varied-line spacing (VLS) gratings for spatio-temporal laser pulse shaping. Compared to head-on (180°) Thomson scattering, interaction lengths are in the centimeter to meter range and photon numbers for ultrashort X-ray pulses can increase by several orders of magnitudes.     

Pulse-front tilt created in misaligned dispersionless optical systems and correct interferometric autocorrelation

In this work we present a matrix analysis of the sensitivity of otherwise dispersionless 4f and 2f-2f optical systems to misalignments resulting in spatio-temporal distortions of ultrashort laser pulses. Special attention is given to the possible creation of a pulse-front tilt (PFT). The exact analytical expression for the second-harmonic interferometric autocorrelation signal of an inverted-field autocorrelator is calculated. It confirms the effective broadening of the ultrashort pulses in the presence of an arbitrary PFT.     

A calibration method of lens distortion for line scan cameras

We present a practical method to calibrate the lens distortion in line scan cameras. The distortion correction software based on this method has been designed and implemented in the digital protection of cultural relics. In this paper, a lens distortion model which applies to line scan cameras is derived from the widely used array camera distortion model. Then, a new calibration method which utilizes the imaging characteristic of equidistant collinear feature points is proposed to calibrate the model. Experiment results show the proposed calibration method is stable and effective.     

Digital reconstruction of grating-distorted images through virtual-optics imaging systems

We develop a technique to correct the distortion generated by the presence of a grating that prevents correct imaging of a test object in an optical system. We use the design concept of virtual-optics imaging systems to implement a setup and a corresponding filter to remove the distortion. Results are presented showing the potential of the method.     

Tunable visible and near-infrared pulse generator in a 5 fs regime

Transform-limited (TL) visible pulses with as short a duration as 4.7 fs with a 5 7J pulse energy have been generated for the first time from a novel noncollinear optical parametric amplifier (NOPA). Both signal-idler group-velocity matching and pulse-front matching are essential to generate coherent down-converted pulses compressible to the TL pulse with more than a 150-THz bandwidth. Tunable operation with bandwidth-limited sub-10-fs pulses in the visible (550-700 nm) and near-infrared (900-1300 nm) ranges, achieved by increasing the seed chirp, is also a remarkable property. The NOPA is believed to be on useful light source for ultrafast spectroscopy on an extremely short time-scale.     

有限元程序网格沙漏变形的分析与控制

分析二维拉氏四边形四节点等参有限元程序存在的网格沙漏变形问题,认为单元刚度矩阵的秩缺陷是沙漏变形的原因,给出一种消除单元刚度矩阵秩缺陷且正交于线性位移场的沙漏控制方法,不仅有效地控制了沙漏变形,而且通过了分片检验.把此种沙漏粘性加入程序后计算多种算例,证明了此控制方法的有效性.     

Horizontal parallax distortion in toed-in camera with wide-angle lens for mobile device

An effect of the toed-in camera configuration is keystone distortion, which causes vertical and horizontal parallax in the stereoscopic image. The horizontal parallax is the source of the depth plane curvature. However, if the stereoscopic image captured by the toed-in camera system with fish-eye lens is displayed on mobile device, it is uncomfortable to view because the horizontal parallax contains horizontal parallax distortion occurred by the wide field of view of the lenses. Therefore, in this paper, we propose a novel correction method of the horizontal parallax distortion, which is one of the keystone distortions, in a toed-in camera with fish-eye lenses for mobile device. We have experimented to attest the proposed method. In experiment, we used two charge coupled device cameras, whose field of view is 48.48°. In addition, the captured stereoscopic image was corrected for the barrel distortion and the horizontal parallax distortion. Therefore, the proposed method provides correcting of the horizontal parallax distortion from a toed-in camera system in order that the users can enjoy three-dimensional effects without the visual fatigue. Additionally, the proposed method is able to apply to toed-in camera consisting of any kinds of cameras.     

Convergence test for inversion of frequency-resolved optical gating spectrograms

We describe a new and simple method to aid in the analysis of retrieved pulses from inverted frequency-resolved optical gating (FROG) traces. The analysis can separate noise from distortion and shows that distortion is more deleterious to the retrieved pulse than is pure noise. The analysis relies on the fact that FROG traces can be constructed from a single outer product of two vectors, whereas distortion and noise require the sum of a series of outer products.     

基于畸变率的图像几何校正

大视场成像光学系统中的畸变会降低图像质量，必须预以校正。提出一种新的校正方法，即根据畸变率的定义推导出畸变校正公式。根据公式，在镜头畸变率已知的情况下可以很容易地校正畸变。对于畸变率未知的情况，给出了建立畸变模型的方法，通过畸变模型可近似计算畸变率。得出通过控制畸变模型中某一个形状的参数可以控制畸变量大小的结论。提出的方法已经在实际工程中采用。实践证明，这种模型可以满足大多数镜头的畸变校正要求。     

电场描绘实验中等位线平移畸变及消除方法的研究

通过分析电场描绘实验中等位线平移时产生畸变的原因,给出了消除等位线平移畸变的原理,并由此对平移装置和描绘方法进行了适当的改进,使之能够将模拟电场中的电位分布完整地平移到坐标纸上,消除等位线平移装置变形和描绘方法不尽完善所引起的等位线畸变和测量误差．     

Camera lens radial distortion correction using two-view projective invariants

This Letter proposes a new method for automatically removing radial lens distortion from image feature point correspondences of two views. Based on the projective geometric relationship between two views of a planar scene, we have derived a system of algebraic equations that relates the invariants to the distortion parameters to be found. We then propose a noniterative procedure to solve the equations system, and a kernel-voting scheme to select the best root. Being a noniterative approach, our method overcomes many problems with the conventional iterative approach. It also largely decouples the estimation of the distortion from the estimation of other camera parameters and, therefore, delivers more reliable results. Experiments on both synthetic data and real images have provided satisfactory results.