Long-cavity Nd:YAG laser used in single-shot spontaneous Raman scattering measurements

The use of a 100 ns pulse Nd:YAG laser is proposed to perform single-shot measurements by spontaneous Raman scattering. The long pulse duration is obtained thanks to a White cell placed inside the laser oscillator. This lengthens the pulse duration within the laser cavity, keeping almost the same beam quality as standard Nd:YAG lasers. The laser system is described, the advantages are compared with extra-cavity stretched lasers, and single-shot measurement of concentration in gas is demonstrated.     

Single-shot cross-correlator using a long-wavelength sampling pulse

We present a single-shot cross-correlating scheme capable of high dynamic range and large temporal window for pulse contrast characterization. By adopting a long-wavelength sampling pulse parametrically converted from the pulse under test, wavelength combination in the correlating process allows the use of a large noncollinear phase-matching angle in periodically poled lithium niobate crystal, matches the high-sensitivity detection system consisting of a fiber array and a photomultiplier tube, and favors the elimination of optical scattering noise. The prototype experiments demonstrate a detectable contrast maximum up to ～10(9), temporal window of ～50?ps, and resolution of ～1?ps, using <0.5?mJ input energy.     

Optical switching and contrast enhancement in intense laser systems by cascaded optical parametric amplification

Optical parametric chirped-pulse amplification (OPCPA) can be used to improve the prepulse contrast in chirped-pulse amplification systems by amplifying the main pulse with a total saturated OPCPA gain, while not affecting the preceding prepulses of the seed oscillator mode-locked pulse train. We show that a simple modification of a multistage OPCPA system into a cascaded optical parametric amplifier (COPA) results in an optical switch and extreme contrast enhancement that can completely eliminate the preceding and trailing oscillator pulses. Instrument-limited measurement of a prepulse contrast ratio of 1.4 x 10(11) is demonstrated from COPA at a 30 mJ level.     

Delayed pulse referencing for cancellation of polychromatic beating noise in grating spectrometry

We use a grating spectrometer with a time resolution of 17.4 μs and a spectral resolution of 5 GHz to analyze a superluminescent source operating near 1350 nm. Polychromatic beating noise is shown to dominate single-shot measurements of the source spectrum; the observed noise level is within 19% of the theoretical beating noise level. By adopting a delayed pulse referencing scheme used previously with wavelength-swept sources, the beating noise is virtually eliminated, allowing spectral measurements limited only by shot noise and camera read noise.     

Single-shot dual-energy x-ray imaging with a flat-panel sandwich detector for preclinical imaging

We describe a multi-layer (“sandwich”) configuration detector consisting of two x-ray imaging flat-panel detectors for single-shot (single-kV) dual-energy imaging. An intermediate copper filter is used to increase spectral separation between the two detectors to improve contrast at the expense of image noise. Monte Carlo and cascaded-systems analyses of the signal and noise performance are described that quantify performance characteristics. Image quality of dual-energy images obtained from a prototype sandwich-detector system is evaluated using a figure of merit (FOM), defined as the squared contrast-to-noise ratio normalized by x-ray exposure for a mouse phantom for preclinical imaging. Demonstration dual-energy bone and soft-tissues images of a postmortem mouse are obtained using the prototype system. While the FOM with the single-shot detector is lower than that achieved using a conventional dual-shot (dual-kV) method, the single-shot approach may be preferable when imaging speed or insensitivity to motion artifacts is a primary concern.     

Coherent versus incoherent Raman scattering: molecular coherence excitation and measurement

We present a comparative analysis of spontaneous and coherent Raman scattering on pyridine. The instantaneous excitation of the molecular coherence is done by a pair of ultrashort preparation pulses. Then, a long narrowband probe pulse is scattered off the molecular vibrations. The described hybrid technique allows for the single-shot acquisition of a background-free coherent Raman spectrum within the excitation band and its straightforward comparison with the spontaneous Raman measurements, performed in the same setup. We report a 10(5)-fold increase in the efficiency of the Raman scattering process due to the broadband pump-Stokes preparation. The coherence magnitude (approximately 0.5x10(-3)) is inferred experimentally, without a priori knowledge about the molecular structure.     

Laser imaging system for determination of three-dimensional scalar gradients in turbulent flames

An imaging system for the measurement of three-dimensional (3D) scalar gradients in turbulent hydrocarbon flames is described. Combined line imaging of Raman scattering, Rayleigh scattering, and CO laser-induced fluorescence (LIF) allows for simultaneous single-shot line measurements of major species, temperature, mixture fraction, and a one-dimensional surrogate of scalar dissipation rate in hydrocarbon flames, while simultaneous use of two crossed, planar LIF measurements of OH allows for determination of instantaneous flame orientation. In this manner the full 3D scalar dissipation can be estimated in some regions of a turbulent flame on a single-shot basis.     

Electro-optic technique with improved time resolution for real-time, nondestructive, single-shot measurements of femtosecond electron bunch profiles

Electro-optic detection of the Coulomb field of a relativistic electron bunch combined with single-shot cross correlation of optical pulses is used to enable single-shot measurements of the shape and length of femtosecond electron bunches. This method overcomes a fundamental time-resolution limit of previous single-shot electro-optic measurements, which arises from the inseparability of time and frequency properties of the probing optical pulse. Using this new technique we have made real-time measurements of a 50 MeV electron bunch, observing the profile of 650 fs FWHM ( approximately 275 fs rms) long bunches.     

Transport of quantum noise through random media

We present an experimental study of the propagation of quantum noise in a multiple scattering random medium. Both static and dynamic scattering measurements are performed: the total transmission of noise is related to the mean free path for scattering, while the noise frequency correlation function determines the diffusion constant. The quantum noise observables are found to scale markedly differently with scattering parameters compared to classical noise observables. The measurements are explained with a full quantum model of multiple scattering.     

Large temporal window contrast measurement using optical parametric amplification and low-sensitivity detectors

To address few-shot pulse contrast measurement, we present a correlator coupling the high gain of an optical parametric amplification scheme with large pulse tilt. This combination enables a low sensitivity charge coupled device (CCD) to observe features in the pulse intensity within a 50 ps single-shot window with inter-window dynamic range >10⁷ and <0.5 mJ input energy. We also consider use of optical densities to boost the CCD dynamic range within a single temporal window.     

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.     

Adaptive pulse compression for transform-limited 15-fs high-energy pulse generation

We demonstrate the use of a deformable-mirror pulse shaper, combined with an evolutionary optimization algorithm, to correct high-order residual phase aberrations in a 1-mJ, 1-kHz, 15-fs laser amplifier. Frequency-resolved optical gating measurements reveal that the output pulse duration of 15.2 fs is within our measurement error of the theoretical transform limit. This technique significantly reduces the pulse duration and the temporal prepulse energy of the pulse while increasing the peak intensity by 26%. It is demonstrated, for what is believed to be the first time, that the problem of pedestals in laser amplifiers can be addressed by spectral-domain correction.     

复合扫描全息术及透过强散射介质三维成像研究

应用菲涅耳波带片光学扫描全息术原理，研究强散射介质中吸收体的三维成像.针对由成像物体强散射背景产生的背景噪音，提出复合扫描全息术成像方法.实验中应用这一方法，对嵌埋在浓度为1%，深度为1.7 cm的脂肪乳剂中的吸收体成像，得到了信噪比和对比度较好的再现像.在此基础上，对复合扫描全息成像系统的信噪比，对比度及分辨率等性能进行了深入的理论分析和实验测试.研究表明，与单一模式相比，复合扫描全息术在信噪比和对比度方面有较大改善，但分辨率还需进一步提高.     

Waveform preservation of the backscattered stimulated Brillouin scattering wave by using a prepulse injection

We have found that it is possible to preserve the temporal waveform of the reflected wave generated from stimulated Brillouin scattering (SBS) by using a prepulse technique. The waveform of the SBS wave usually shows a steep rising edge in the ordinary SBS process. It has been found that the waveform of the reflected wave depends on both the prepulse energy and the time delay between the main pulse and the prepulses. A prepulse energy of 5 mJ and a time delay of 5 ns have been measured to be the optimum values under the experimental conditions. This prepulse method is useful in developing a multistage system employing several SBS cells in series for high-power laser applications.     

一种超快时间分辨速度干涉仪的设计和理论研究

在单发、超快时间分辨的测量技术中,线性啁啾脉冲和光谱仪的结合使用是一种常见方法.本文以成像型光谱仪为记录系统,利用线性啁啾脉冲的频-时对应关系,设计了一种应用于自由面速度超快测量的速度干涉仪.理论研究了其工作原理和适用范围,该速度干涉仪可以实现皮秒量级的时间分辨,且速度灵敏度可调、易调,大大拓宽了速度干涉仪的应用范围.数值模拟了系统的测速过程,结果表明,重建速度信号与给定物体速度符合得很好,进一步验证了系统的可行性.     

Fiber-array-based detection scheme for single-shot pulse contrast characterization

We propose and demonstrate a fiber-array-based detection scheme for single-shot pulse contrast characterization. The parallel to serial mapping using the fiber array allows the use of a high-sensitivity photomultiplier tube and eliminates the external neutral density filter, both resulting in significantly improved dynamic range. The proof-of-principle experiments show a dynamic range of 2x10(7):1. The demonstrated technique can be readily applied to existing instruments for single-shot pulse characterization.     

Pedestal Measurement of Laser Pulses

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Direct determination of diffusion properties of random media from speckle contrast

We present a simple scheme to determine the diffusion properties of a thin slab of strongly scattering material by measuring the speckle contrast resulting from the transmission of a femtosecond pulse with controlled bandwidth. In contrast with previous methods, our scheme does not require time measurements nor interferometry. It is well adapted to the characterization of samples for pulse shaping, nonlinear excitation through scattering media, and biological imaging.     

利用相关技术测量激光脉冲的对比度

研究了在放大激光脉冲的噪声测量中，二阶相关技术和三阶相关技术的差别。理论分析显示，三阶相关技术不仅可以显示噪声脉冲的正确位置，而且具有比二阶相关技术更高的测量精度，数值计算也验证了这一点。     

Single-shot two-dimensional measurement of nanoparticles in turbulent jet-diffusion flames using phase-selective laser-induced breakdown spectroscopy

In this work, we demonstrate a single-shot two-dimensional measurement of vanadic oxide (V₂O₅) nanoparticles in a turbulent jet-diffusion flame based on phase-selective laser-induced breakdown spectroscopy (PS-LIBS). By collecting the atomic spectra of vanadium near 439?nm from nano-sized plasmas within the ～30?ns lifetime, particle information can be revealed without interference from elastic scattering or Bremsstrahlung emissions. As the laser intensity increases, the signal intensity first increases and then saturates when the laser intensity reaches 0.5 GW/cm². In the saturation regime, a proportional correlation is established between the signal intensity and the particle volume fraction. Based on the parametric study in the laminar condition, we image the instantaneous distribution of particle volume fraction in the turbulent condition using single-shot PS-LIBS measurements. The snapshots show that vanadic oxide nanoparticles concentrate near the diffusion flame surface, which may be caused by the rapid formation of particles on the oxidizer/precursor side and quick dilution across the reaction layer. The Proper Orthogonal Decomposition (POD) analysis further indicates that the fluctuations of the particle volume fraction originate from the unsteady flame surface at the upstream positions and large-scale mixing at the downstream positions. The single-shot PS-LIBS measurement shows promising potential for resolving complex processes of particle formation in turbulent flame synthesis.