基于FPGA的低场核磁共振谱仪脉冲编程器设计与实现

脉冲编程器在核磁共振(NMR) 谱仪中非常重要,具有产生脉冲序列所需的射频脉冲,控制回波信号的同步接收等功能. 基于FPGA的同步性能和时序控制功能,设计了低场NMR谱仪的脉冲编程器,可产生任意脉冲序列,实现脉冲频率、相位、幅度的灵活调节. 利用设计的脉冲编程器在0.540 Tesla均匀磁场下进行了CuSO₄溶液的T₁和T₂测量,得到高信噪比的NMR回波串.     

多脉冲序列可编程辅助控制器及其应用

本文描述了一种以单板机为基础的多脉冲序列可编程辅助控制器(PCMP),在常规液体NMR谱仪上实现多脉冲实验。应用PCMP获得了固体的J-耦合化学位移各向同性谱、化学位移缩窄谱,同时也可以完成时间平均旋进频率交叉极化实验以及组合脉冲宽带去耦实验,进一步开拓了常规液体谱仪的功能。     

基于数字调制技术的核磁共振射频脉冲发生器

在核磁共振（NMR）领域,射频脉冲信号的质量、形状对NMR性能及应用有着重要影响.本文基于现场可编程门阵列（FPGA）和直接数字频率合成（DDS）芯片AD9910设计了一种硬件结构更为简单的NMR射频脉冲发生器,实现了射频脉冲各项参数的数字化调制.其频率、相位、振幅的控制精度分别达到了32位、16位和14位,脉冲调制的时间精度为0.01 μs,可灵活生成持续时间不小于0.1 μs、载波频率不高于400 MHz的各类软脉冲和硬脉冲.同时,针对脉冲序列的特点建立了"脉冲+延时"的基础模型,提出了一种通用性更强的列表式脉冲序列控制方案,精简了对上级控制单元的控制需求.此外,对射频脉冲信号的频谱特性进行了理论分析,并采用Hanning窗对软脉冲的包络波形进行了优化处理,仿真和实验结果表明,Hanning窗可以有效抑制软脉冲的频谱泄漏问题.     

针对磁共振射频脉冲的时频域分析方法比较研究

射频脉冲可实现样本自旋体系的精确操控,进而产生预期的核磁共振(NMR)信号,在NMR信号产生过程中扮演重要角色.该文分别采用短时傅里叶变换(STFT)、连续小波变换(CWT)和维格纳-威利分布(WVD)几种时频域分析方法对射频脉冲(优化形状脉冲)进行特性分析和比较.结果表明,三种方法各自具有优缺点,结合各自优势对射频脉冲进行各种方法分析,可以更好地理解复杂脉冲的幅度、相位特性在时频域的分布情况.该文的研究方法将为直观理解复杂射频脉冲对自旋体系的作用机制提供参考.     

基于数值计算模拟的仿真核磁共振波谱仪开发

核磁共振（NMR）波谱是研究有机分子结构的重要工具之一，其设备昂贵、仪器数量有限、科研机时安排紧凑、教学实验机时有限；若学生的NMR理论和实验基础较为薄弱，则易发生误操作损坏设备或降低设备性能；另外，NMR实验涉及流程较多、教学耗时较长.因此，针对学生的NMR实验教学难以广泛开展.为将虚拟技术更好地用于辅助NMR实验教学，本文基于数值模拟仿真技术，开发了具备真实NMR波谱仪基本功能的仿真NMR波谱仪——VMRS1.0软件.VMRS1.0软件可实现真实NMR实验须进行的调谐匹配、匀场、锁场、射频脉冲调节、原始数据采集等操作步骤，以及快速傅里叶变换、相位校正、自动寻峰、积分、测距、化学位移校正等基本数据处理功能；除简单的1D ¹H和¹³C NMR波谱采集和数据处理外，还可模拟去偶、DEPT、HSQC等相对复杂的实验；另外，该软件可自行虚拟编辑¹H或¹³C NMR实验样品.VMRS1.0软件摆脱硬件条件的限制，实现了与真实NMR实验类似的教学效果，可以让学生通过反复操作调试，更充分地掌握NMR基本原理和实验技能，再配合在真实NMR谱仪上的少量实际操作，可达到更加令人满意的实验教学效果.     

宽带脉冲DC SQUID核磁共振谱仪的研制

DC SQUID NMR在研究凝聚态体系 ,尤其在研究低温下量子有序态等方面是一项非常有价值的技术。文中描述了在带有宽带电子设备的商业 DC SQUID基础上建立起宽带脉冲谱仪 ,以及马鞍形激发线圈、采集线圈和低温装置的设计 ,并测量了铂样品在 4.2 Kz温度下的 NMR信号。     

低场核磁共振短死时间射频线圈与射频开关的设计

本文以核磁共振（NMR）射频线圈振铃信号产生原理为对象进行分析研究,提出了一种适用于低场环境下由环状间隙腔线圈与螺线管线圈构成的收发分离式短死时间射频线圈设计方案,采用优化调谐匹配网络提高发射效率;根据射频线圈方案需求设计了快速切换的射频开关及驱动．在此基础上依据仿真结果制作了短死时间射频线圈,并应用于自主研制的低场9.51 MHz便携式NMR谱仪系统,进行NMR实验,结果显示可将收发切换时间缩短至10 μs以内,验证了该设计方案的可行性．     

动态核极化谱仪的技术改造

介绍了动态核极化(DNP)谱仪数字系统的改造和一套的射频发射和接收系统的建立.谱仪射频通道的90°脉冲宽度为5μs,能完全满足固体交叉极化和大功率去耦的实验要求.实现了DNP增强曲线的自动测量     

双Z脉冲实现多量子相干选择检测

本文提出一种改进的多量子相干选择检测(滤波)方法。滤波Z脉冲分别在准备期和混合期末。它能有效地防止单量子相干的"漏进"并避免使用数字相移器。在一般谱仪上容易实现,用密度算符理论对AMX体系进行理论计算,实验结果与理论预期相符。     

核磁共振波谱仪用射频功率放大器的设计与实现

介绍了一种应用于核磁共振（Nuclear Magnetic Resonance,NMR）波谱仪的宽频带射频功率放大器的设计与实现,提出了射频功率放大器门控信号的设计,探讨了利用传输线变压器构建功率MOSFET输入输出阻抗匹配网络的方法. 该射频功率放大器采用多级驱动结合功率放大的结构,包含2个工作模块和基于宽频带定向耦合器的辅助电路,支持高带和宽带通道的相关实验. 将该射频功率放大器应用在自主研发的500 MHz高分辨率液体NMR谱仪上,测量射频功放的上升、下降时间及输出线性度,并对比测试¹H和¹³C NMR标样的信噪比,实验结果证明了该设计的可行性.     

Partial-coherence method to model experimental free-electron laser pulse statistics

A general numerical approach is described that allows obtaining model sets of temporal pulse shapes of free-electron lasers (FELs) operating in the self-amplified spontaneous emission mode. Based on a random partial-coherence approach, sets of pulse shapes can be calculated that satisfy statistical criteria of FEL light predicted by established FEL theory. Importantly, the numerically retrieved sets of pulses reproduce the experimentally accessible FEL light characteristics as measured at the Free-electron LASer at Hamburg (FLASH), such as the average spectrum, single-shot spectral shape, and pulse duration. The high-precision agreement with the experimental average spectral shape, without further knowledge of FEL machine parameters, makes this approach a convenient tool for the analysis and theoretical modeling of nonlinear optical or pump-probe experiments with FEL light.     

Active long pulse shaping technique of pulsed CO2 laser using multi-pulse discharge control

We describe the pulse forming of pulsed CO₂ laser using multi-pulse superposition technique. Various pulse shapes, high duty cycle pulse forming network (PFN) are constructed by time sequence. This study shows a technology that makes it possible to make various long pulse shapes by activating SCRs of three PFN modules consecutively at a desirable delay time with the aid of a PIC one-chip microprocessor. The power supply for this experiment consists of three PFN modules. Each PFN module uses a capacitor, a pulse forming inductor, a SCR, a high voltage pulse transformer, and a bridge rectifier on each transformer secondary. The PFN modules operate at low voltage by driving the primary of HV pulse transformer. The secondary of the transformer has a full-wave rectifier, which passes the pulse energy to the load in a continuous sequence.We investigated various long pulse shapes as different trigger time intervals of SCRs among three PFN modules. As a result, we could obtain laser beam with various pulse shapes and durations from about 250 to 1000 μs.     

PMD effect on pulse shapes and power penalty in optical communication systems

We present an analytical expression relating the output state of polarization and the first-order polarization mode dispersion (PMD) vector in terms of the angle of precession of the output state of polarization around the PMD vector. We derive, incorporating for the first time this angle of precession, a general relation to study the effect of first-order PMD on pulses of arbitrary shapes, and expressions for pulse shape, pulse broadening and power penalty taking into account both PMD and chromatic dispersion. Measured experiment results are presented for NRZ, RZ, NRZ-DPSK, and RZ-DPSK modulation formats.     

Demonstration of radiation pulse shaping with nested-tungsten-wire-array pinches for high-yield inertial confinement fusion

Nested wire-array pinches are shown to generate soft x-ray radiation pulse shapes required for three-shock isentropic compression and hot-spot ignition of high-yield inertial confinement fusion capsules. We demonstrate a reproducible and tunable foot pulse (first shock) produced by interaction of the outer and inner arrays. A first-step pulse (second shock) is produced by inner array collision with a central CH2 foam target. Stagnation of the inner array at the axis produces the third shock. Capsules optimized for several of these shapes produce 290-900 MJ fusion yields in 1D simulations.     

Investigation of optimum pulse shape for 112 Gbps DP-DQPSK in DWDM transmission

This paper reports the simulative analyses to investigate the impact of different pulse shapes on DP-DQPSK modulation for high-spectral efficient DWDM transmission at 112 Gbps per channel. In the simulation model we have considered three different pulse shapes viz. NRZ, 50% duty cycle RZ (RZ50) and 67% duty cycle RZ (RZ67) in symbol-aligned and symbol-interleaved format. The analysis has been carried out for various linear and nonlinear system impairments where the pulse is subjected to degradation under the influence of amplified spontaneous emission (ASE) noise, group velocity dispersion (GVD), cross-phase modulation (XPM) and polarization mode dispersion (PMD) and thus, limiting the system performance. Results show that the RZ50 pulse shape for DP-DQPSK with symbol interleaving shows the maximum tolerance for the various system degradations in long haul DWDM transmission. Even the system based on the NRZ pulse shape can gain significant improvement from the symbol-interleaving.     

不同波形脉冲激光的时域误差对相干合成的影响

相干合成技术是实现高功率、高亮度光纤激光系统的重要技术途径.然而, 脉冲激光阵列中常常存在时域误差,这将影响脉冲激光的相干合成效果. 建立了脉冲激光存在时域误差时的相干合成理论模型,并在不同波形(方波、三角波、正弦波) 的脉冲激光存在时域误差时,对相干合成光束在远场的脉冲波形、峰值功率、 光强分布和桶中功率(PIB)等特性进行了数值计算和对比分析.计算结果表明: 方波脉冲激光相干合成光束的脉冲波形受时域误差影响严重,光强分布和PIB随着时域误差 的增大发生线性变化;三角波脉冲激光相干合成光束的脉冲波形和峰值功率受时域误差 影响严重,光强分布和PIB在时域误差较大时随着时域误差的增大发生较为剧烈的变化; 正弦波脉冲激光相干合成光束具有较好的输出特性,在两路正弦波脉冲激光相干合成中, 将两脉冲之间的时延控制在脉冲持续时间的10%以内,就能取得良好的合成效果.     

The effect of pulse to pulse variation on ultrashort pulsewidth measurements

An alternate interpretation is presented for the exponential shapes of the second harmonic autocorrelation functions obtained from synchronously pumped mode locked dye laser systems. The shape is reinterpreted as being due to a weighted average of pulses of varying widths and smoothly varying pulse shapes in time. Such pulse width measurements in the past have averaged over a large number of pulses. Several pulse shapes and pulse width distributions are examined all leading to exponential shaped autocorrelation functions. This interpretation implies that the currently determined pulsewidths are shorter, possibly many times shorter than the actual average pulsewidths. A method for selectively descriminating the shortest pulses from the train to increase the time resolution of current laser systems is presented.     

Parametrically shaped femtosecond pulses in the nonlinear regime obtained by reverse propagation in an optical fiber

We present the experimental realization of a method to generate predetermined, arbitrary pulse shapes after transmission through an optical fiber in the nonlinear regime. The method is based on simulating the reverse propagation of the desired pulse shape in the fiber. First, linear and nonlinear parameters of a single-mode step-index fiber required for the simulation are determined. The calculated pulse shapes are then generated in a pulse shaper.     

红外激光场中共振结构原子对极紫外光脉冲的压缩效应

通过数值求解一维原子的含时薛定谔方程, 研究了具有共振结构的原子在双色场(红外激光(IR)+极紫外光(XUV)) 驱动下发射高次谐波的特征. 研究结果表明, 具有共振结构的原子所发射的高次谐波与无共振结构原子(简称为一般原子)发射的高次谐波有明显不同, 共振结构的原子除了在某一能量附近(原子的共振能量+电离能)高次谐波的强度有很大提高外, 它还对XUV光的响应较一般原子表现得更为敏感, 即使XUV光的强度较弱, 也能够明显提高XUV光脉冲中心频率附近的谐波强度, 更重要的是通过调节双色场的时间延迟, 能使输入的XUV光的脉宽得到明显的压缩, 通过时间-频率分析给出了发生这种现象的原因. 由此提出了通过滤波-连续反馈的方式可使XUV光的脉冲从200 as压缩至120 as左右.     

Erbium-doped fiber laser passively Q-switched by an InGaAs/InP multiple quantum well saturable absorber

We report on the behavior of an erbium-doped fiber laser passively Q-switched by an InGaAs/InP multiple quantum well semiconductor saturable absorber. The fiber is moderately doped in erbium ions. The saturable absorber consists of 40 periods of InGaAs (8.5 nm)/InP (10 nm) grown on an InP substrate. The laser efficiency is nonlinear under Q-switch regime and unusual temporal pulse shapes with rising times lower than decay times are observed around threshold. Pulse duration is around the microsecond range. Simple arguments based on a rate-equations approach are proposed in order to explain our results. They are justified a posteriori via numerical integration of the equations. The variation of threshold level versus spectroscopic parameters has been determined and that makes a guideline for generalizing our discussion.