超声脉冲长度与重复频率对流动微泡群稳态空化特性的影响

为了深入理解超声脉冲长度(PL)与重复频率(PRF)对流动微泡群稳态空化特性的影响,本研究自行制备仿体血管,利用注射泵推动微泡群在其中恒速流动,使用1 MHz的聚焦换能器激励微泡群产生稳态空化,另一个7.5 MHz的平面换能器接收声信号,经前置放大及高速采集后送至上位机存储。在定义临界重复频率(Critical PRF,CPRF)的基础上,通过深入的时频域分析,我们发现PL和PRF共同决定流动微泡群的稳态空化特性:当PRF低于CPRF时,流动微泡群的稳态空化剂量(Stable Cavitation Dose,SCD)与PL呈正相关,且时域分布均匀;而当PRF大于CPRF时,其稳态空化特性由PL决定,在长PL (＞100μs)下,SCD随着PL增加逐渐减小,且时域分布不均匀;在短PL (＜50μs)下,SCD和PRF无显著相关关系,且时域分布均匀。本研究结果深入阐明了流动微泡群的稳态空化特性,期望可用其获取可预测可控的生物效应,并应用于相关临床治疗。      

Exawatt-Zettawatt pulse generation and applications

A new amplification method, weaving the three basic compression techniques, Chirped Pulse Amplification (CPA), Optical Parametric Chirped Pulse Amplification (OPCPA) and Plasma Compression by Backward Raman Amplification (BRA) in plasma, is proposed. It is called C³ for Cascaded Conversion Compression. It has the capability to compress with good efficiency kilojoule to megajoule, nanosecond laser pulses into femtosecond pulses, to produce exawatt-and-beyond peak power. In the future, C³ could be used at large-scale facilities such as the National Ignition Facility (NIF) or the Laser Megajoule (LMJ) and open the way to zettawatt level pulses. The beam will be focused to a wavelength spot size with a f#1. The very small beam size, i.e. few centimeters, along with the low laser repetition rate laser system will make possible the use of inexpensive, precision, disposable optics. The resulting intensity will approach the Schwinger value, thus opening up new possibilities in fundamental physics.     

Influence of the Thomson effect on the pulse heating of high-current electrical contacts

Pulse heating of high-current contacts is notable for the presence of considerable temperature gradients in the contact area, which cause the Thomson effect—the appearance of thermoelectric currents. The amount of this effect against conventional Joule heat release is quantitatively estimated. Pulse heating of electrical contacts is numerically simulated with the use of the Comsol program package. It is demonstrated that thermoelectric currents make a negligible contribution to heating in the case of copper contacts.     

Semi-chaotic pulse effects in self-modulating Gunn oscillators

Self-modulation behaviour has been observed in GaAs Gunn oscillators operating at W-band, leading to nanosecond pulses of relatively high peak power. Pulse repetition rates and pulse-widths can be varied reproducibly as a function of bias voltage and bias line termination, and there is evidence for semi-chaotic behaviour in certain regimes.     

Nanosecond pulse propagation in a high power iodine laser amplifier

Pulse propagation in a photochemical iodine laser amplifier is investigated both experimentally and by solving the Maxwell-Schrödinger equations by computer. Pulse shortening and pulse reshaping are observed in agreement with theory. For pulse durations approaching the dephasing time, it is found that a further decrease in duration by gain saturation is hard to obtain and requires high power levels or high initial inversion. The numerical analysis includes reservoir effects due to tightly coupled hyperfine levels in the iodine groundstate and phase modulation (chirping) of the input pulse. Comparison of the iodine parameters with those of Nd and CO₂ lasers shows that for nanosecond pulses pulse shortening occurs more readily in the latter two systems in agreement with experiments.     

19-fs pulse generated by supercontinuum compression

Supercontinuum generation and compression in a length of 50-mm photonic crystal fiber with pulse of nanojoule energy are investigated theoretically and experimentally. Chirped mirror pair is used for dispersion compensation and pulse compression. Pulse characteristics are measured by frequency-resolved optical gating. And 19-fs pulse is generated.     

Piezoelectric multilayer transducers for ultrasonic pulse compression

This paper describes piezoelectric multilayer transducers for application in ultrasonic pulse compression systems. The transducers are constructed in such a way as to produce binary pulse sequences, in particular Barker-coded sequences. This is achieved by the polarization pattern of the active layers. Pulse compression is effected without any electronic circuitry by using a receiver transducer with a pattern corresponding to that of the transmitter.     

Femtosecond laser pulse energy self-stabilization

A photoconductive switch driving a Pockels cell ensures energy self-stabilization of ultra-fast laser pulses from single shot to hundreds of kHz. In this kind of stabilization, the pulse corrects itself. In a first experiment, the self-stabilization of a 20-Hz, 800-nm femtosecond Chirped Pulse Amplification laser shows a reduction of the energy fluctuations from 7% rms to 0.64% rms. PACS 42.60.Lh; 42.60.Mi; 42.65.Re     

Shaping ability of all fiber coherent pulse stacker

Pulse stacking is an effective method to generate a long-shaped pulse from short pulses. In this paper, we study all-fiber coherent pulse stacking systematically; we show that the time delays and phase differences between the short pulses are the key parameters of the stacked pulse. The permitted variation of the time delay and phase difference are obtained. The ability of the stacker to produce arbitrary pulse shapes is discussed.     

KrF激光脉冲整形研究

 实验研究了脉冲堆积法对KrF激光的整形能力。利用受激布里渊散射(SBS)压缩脉冲获得了脉宽为3~5 ns的短脉冲，用4束激光堆积获得了脉宽为10~15 ns、平顶宽度为5~10 ns的整形脉冲。利用实测SBS脉冲数据，对脉冲堆积法进行了数值模拟，模拟的脉冲宽度为12.7 ns，平顶宽度为7.7 ns，该结果与实验相符。对脉冲堆积法的任意整形能力进行了模拟，讨论了脉冲宽度变化对整形的影响，发现脉宽变化在5%以内时，对整形脉冲的影响较小。对整形脉冲的放大进行了定性研究，结果表明：平顶脉冲通过放大器后，前沿部分放大较多，由于饱和作用，后沿放大较少；如果前端整形脉冲前沿为斜坡状，放大后可以得到近平顶的脉冲。     

激光测距用半导体脉冲激光测头的研究

研究了激光测距用半导体脉冲激光测头。通过对基于脉冲激光测距原理的半导体脉冲激光器的参数测量分析,研究了半导体脉冲激光的发射和接收。通过优化光学镜头组合,对半导体激光分散光束进行了平行准直。通过开发脉冲激光驱动电路、光电探测接收放大电路和半导体脉冲激光测距实验装置,研制了满足中距离激光测距要求的半导体脉冲激光测头。     

Variable square pulse vs conventional PFN pumping of Er:YAG laser

The influence of the flashlamp pump current pulse shape on Er:YAG laser efficiency and laser rod thermal focusing was studied theoretically and experimentally. Two pulse shapes, PFN (Pulse Forming Network) and VSP (Variable Square Pulse), were considered. Theoretical modeling and experimental measurements show that the pump pulse shape itself does not have a significant influence on the Er:YAG laser efficiency or thermal focusing. Instead, the major parameter influencing Er:YAG laser efficiency and thermal focusing was found to be the overall pulse duration. For PFN pulses, rise and fall times directly define the overall pulse duration, and therefore do have influence on thermal focusing. By contrast, VSP pulse duration is defined by the externally controlled on-time of the switching transistor. For square shaped pulses, short rise and fall times do not have a direct beneficial influence on thermal lensing.     

Picosecond pulse shaping in a semiconductor-coated D-shaped fiber

Amorphous films of alpha-SiC:H and alpha-Si:H were deposited upon the flat surfaces of D-shaped fibers. Interaction of light with the semiconductor films occurred through evanescent coupling. Pulse shaping on a picosecond time scale was observed in 3.5-cm-long fibers with optical powers of <10 W, enhancing the effective nonlinearity by a factor of 10(3) .     

35 J broadband femtosecond optical parametric chirped pulse amplification system

We report on what is believed to be the first large-aperture and high-energy optical parametric chirped pulse amplification system. The system, based on a three-stage amplifier, shows 25% pump-to-signal conversion efficiency and amplification of the full 70 nm width of the seed spectrum. Pulse compression to 84 fs achieved after amplification indicates a potential of 300 TW pulse power for 35 J amplified pulse energy.     

High-gain free-electron laser pulse amplifiers with variable wigglers

Calculations are presented for pulse amplification in high-gain free-electron lasers with variable wiggler magnets. Better than 90% peak electron trapping and overall peak energy extraction efficiencies of over 60% are demonstrated for 1 μm radiation. Pulse reshaping and phase modulation effects are investigated.     

Spectral phase control and temporal superresolution toward the single-cycle pulse

The concept of temporal superresolution is applied to optical few-cycle laser pulses for the first time to our knowledge. Pulse durations of as little as to 3.7 fs, well below the Fourier limit, are achieved by pulse shaping of an octave-spanning Ti:sapphire oscillator spectrum. Our prism-based pulse shaper also enables us to generate a manifold of well-controlled pulse sequences that are important for coherent control applications on a femtosecond time scale.     

受激布里渊放大光脉冲波形的研究

利用瞬态包含抽运抽空的SBS理论模型对布里渊放大器进行了数值模拟，给出了脉冲波形随 抽运光功率密度和种子光与抽运光能量比的变化规律，并用实验进行了验证，理论与实验符合得较好. 关键词： 布里渊放大器 脉冲波形 脉宽     

Decay time analysis in pulse thermography

Pulse Thermography is being used since several years for rapid detection of subsurface defects. This method is well suited for industrial applications because no physical contact to the test object is needed. Within a short measurement time robust results are obtained. Besides phase evaluation the decay time of surface temperature can be analyzed. In this paper the idea was to investigate the temperature half-life period of each surface point. The sample is assumed to be composed of small elements having the same surface area but different lengths. In a simple model, heat propagation in between these elements is neglected (local one dimensional model). Only changes in mass, heat capacity, and volume are considered. If the material has homogeneous thermal properties, the length (e.g. thickness) has a strong impact on the cooling down process. The concept takes the decay time for each element as a parameter for imaging.     

Femtosecond single-bunched linac for pulse radiolysis based onlaser wakefield acceleration

A conceptual design of a linac for pulse radiolysis is presented based on laser wakefield acceleration. Pulse radiolysis spectroscopically studies the initial stage of chemical reactions induced by electron beams. Single-bunched beams with a bunch-length on the order of a femtosecond are ideal for this purpose. The present design gives pure 20 pC single-bunches with an RMS bunch length of less than 10 fs. It accelerates and compresses only the head part of a high-current beam from a photocathode. Some practical problems concerning the design are also presented     

Experimental investigation of a pulse line ion accelerator in Lanzhou

In order to put the Pulse Line Ion Accelerator (PLIA) concept to its practical application, a small experimental platform was built. It was found that the actual axial electric field is smaller than the theoretical calculation, so the accelerated ions will enter into the deceleration zone before leaving the helix, which will seriously affect the acceleration process. Based on the improved parameters, the He⁺ with 24 keV is accelerated to 55 keV, and the proof-of-principle experiment is completed on this platform.