大功率半导体激光泵浦固体激光器脉冲电源设计

介绍一种大功率半导体激光泵浦固体激光器(DPSSL)脉冲驱动电源的设计电路及其控制方法。根据半导体激光器的工作特性,采用前级电容充电电路与后级脉冲电流产生电路相结合的电路结构。由于LCC谐振电路具有软开关特性和抗负载短路、开路的能力,又能够实现对储能电容恒流充电的功能,因此其适合做为脉冲电源中储能电容的充电电路;后级脉冲电流产生电路选择大功率MOSFET做为主控器件,利用MOSFET饱和区的漏极电流可控性,通过栅极电压控制产生负载脉冲电流。控制部分采用模拟与数字相结合的控制方式,使脉冲电源控制更加灵活,引入脉冲电流指令给定积分器,可以更有效地控制脉冲电流上升过程,抑制电流过冲,提高控制精度,使脉冲驱动电源产生类似矩形波的大功率脉冲电流。搭建了脉冲功率为28 kW的实验平台,实验达到的指标：脉冲电流幅值80 A,脉冲电压350 V,脉冲宽度100 μs,重复频率100 Hz。     

Femtosecond electron pulse compression by using the time focusing technique in ultrafast electron diffraction

We present a new model of an electron gun for generating subrelativistic femtosecond (fs) electron pulses. The basic idea is to utilize a dc acceleration stage combined with a time focusing region, the time focusing electrode generates an electron energy chirp for bunching at the target. Without considering the space charge effects, simulations of the electron gun were carried out under the conditions of different dc voltages and various slopes of the voltage added on the time focusing electrode. Tracing and simulating large numbers of photoelectrons through Monte-Carlo and finite difference methods, the electron pulses with 1 ps can be compressed to 55 fs, which will allow significant advances in the field of ultrafast diagnosis.     

boldmath Broadband water window supercontinuum generation with a ω+3ω/2 multicycle two-colour pulse

We propose a method to generate a high-efficiency broadband water window supercontinuum with a ω+3ω/2 multicycle two-colour pulse. Our results reveal that the 3ω/2 laser pulse can simultaneously modulate the acceleration step and the ionization step, which not only broadens the bandwidth but also enhances the yield of the generated supercontinuum. An ultra-broadband supercontinuum from 290 eV to 555 eV covering the whole water window is generated. Using this method, we expect that an isolated 62-as pulse with a minor pre-pulse can be directly obtained.     

利用电光相位调制和交叉相位调制制作时间透镜的实验及仿真分析

对时间成像理论进行简要研究. 利用电光相位调制器进行光脉冲压缩实验,并分别对基于电光相位调制和交叉相位调制的时间透镜组成的时间成像系统进行了仿真和讨论. 实验结果表明,基于电光相位调制的时间透镜组成的时间成像系统可以有效压缩光脉冲,但是该系统受到了孔径限制,压缩系数较小,分辨率较低. 进一步的仿真分析结果表明,基于交叉相位调制的时间透镜组成的时间成像系统不受孔径限制,能够获得更大的压缩系数和更高的分辨率,但是该系统的实现难度较大. 关键词： 光脉冲压缩 时间透镜 电光相位调制 交叉相位调制     

核磁共振波谱仪接收机的设计与实现

介绍了液体高分辨率核磁共振波谱仪接收机的设计. 接收机以PCI-104为控制器实现网络通信与采样流程控制,通过FPGA（Field Programmable Gate Array, 现场可编程门阵列）实现数字中频接电路的控制和数据处理,简化了硬件结构;通过在接收机中引入脉冲序列生成器,产生接收机所需的采样控制信号,降低接收机与系统其他模块的耦合度. 最后组成500 MHz核磁共振波谱仪实现核磁共振信号的检测.     

利用相位啁啾单色中红外激光场产生高次谐波

本文通过求解薛定谔方程,理论探索了高次谐波及孤立阿秒脉冲在相位调制的单色中红外激光场中的产生.研究结果表明,通过加入含时相位啁啾小量,能够有效的提高高次谐波的产生效率,获得超宽平台谐波谱及超短孤立阿秒脉冲.特别是在相位为0.3π的基础上,同时加入啁啾小量βt,在β=0.3时,可得到带宽为822 e V的超连续高次谐波平台.最后通过叠加第二平台高次谐波,可得到带宽仅为2.7 as的孤立阿秒脉冲.且脉冲强度比没有加入啁啾小量的情况下显著增强.     

原子在红外激光场中产生高次谐波及阿秒脉冲随波长的变化规律

利用强场近似理论,研究了在长波红外激光场(波长800—2000 nm)驱动下氢原子产生的高次谐波,分析了在截止位置附近高次谐波的转换效率随激光波长的变化规律.发现在截止位置附近原子发射高次谐波的转换效率比平台区域的转换效率低,但获得的阿秒脉冲的宽度会随波长的增大而缩短. 关键词： 强场近似 高次谐波 阿秒脉冲     

快速数值差分递推改进算法在超宽连续谱模拟中的应用研究

对快速数值差分递推公式进行了改进,使之能够求解带有脉冲自陡峭项和脉冲内喇曼散射项的非线性薛定谔方程.通过与传统孤子解析结果及分步傅里叶方法数值结果的对比分析表明,快速数值差分递推改进算法是一种快速而准确的数值计算方法,它不仅能够同步考虑光学媒质中的群速色散作用和非线性克尔作用,而且将所有高阶非线性项对光脉冲传输的影响也...     

激光脉冲参数对冲击点火的影响

针对激光脉冲参数对冲击点火的影响进行理论分析和数值模拟.首先从冲击波的产生和碰撞过程出发,理论分析冲击脉冲峰值功率、脉宽和上升沿时间对点火的影响;然后通过数值模拟,以点火时间窗口作为评价标准,验证理论分析结果.研究表明:冲击脉冲峰值功率是点火成败的关键因素之一,脉冲宽度则需达到百皮秒以提供足够的点火能量,上升沿时间在小于600 ps的情况下不会对点火造成明显影响. 关键词： 冲击点火 激光冲击脉冲 点火时间窗口     

Fast fat-suppressed reduced field-of-view temperature mapping using 2DRF excitation pulses

The purpose of this study is to develop a fast and accurate temperature mapping method capable of both fat suppression and reduced field-of-view (rFOV) imaging, using a two-dimensional spatially-selective RF (2DRF) pulse. Temperature measurement errors caused by fat signals were assessed, through simulations. An 11×1140μs echo-planar 2DRF pulse was developed and incorporated into a gradient-echo sequence. Temperature measurements were obtained during focused ultrasound (FUS) heating of a fat-water phantom. Experiments both with and without the use of a 2DRF pulse were performed at 3T, and the accuracy of the resulting temperature measurements were compared over a range of TE values. Significant inconsistencies in terms of measured temperature values were observed when using a regular slice-selective RF excitation pulse. In contrast, the proposed 2DRF excitation pulse suppressed fat signals by more than 90%, allowing good temperature consistency regardless of TE settings. Temporal resolution was also improved, from 12 frames per minute (fpm) with the regular pulse to 28 frames per minute with the rFOV excitation. This technique appears promising toward the MR monitoring of temperature in moving adipose organs, during thermal therapies.