Self-induced heterodyne: The interaction of a frequency-swept pulse with a resonant system

The behavior of a pulse whose frequency sweeps through the resonance of a linear system, although first studied in spin resonance, is widely applicable to many wave phenomena. A frequency-swept optical pulse transmitted through a resonant medium develops intensity oscillations, which are the result of heterodyning the input pulse with the field radiated by the resonant system. This self-induced heterodyne signal allows the determination of the relative phase between the input pulse and the single frequency field radiated by the resonant system. A simple approximate model which very accurately describes the time evolution of the output pulse is given a physical and mathematical basis. Results for the optically thin case are extended to include the effects of propagation, revealing phenomena which have no spin-resonance analogy.     

Transient interactions of an EM pulse with a dielectric sphericalshell

The authors study the scattering interaction of short electromagnetic pulses with a spherical target. The target is assumed penetrable and they model it as an air-filled dielectric shell. The radar cross-section (RCS) of such a target is obtained and its resonance features are analyzed. A dielectric composition makes the resonance features become very prominent compared with the case of a perfectly conducting sphere. When the interrogating waveform is a pulse of short duration, the resonance features of the RCS can be extracted within the frequency band of the spectrum of the incident pulse. To verify their theoretical predictions they illuminate spherical targets with short, broadband pulses using an impulse radar system. The actual shape of the pulse that is incident on the targets is theoretically modeled using a digital filter design technique together with pulse returns from a reference target. They verify that the shape of the predicted, backscattered pulse that results from their design method agrees well with the experimental findings using three additional targets of different sizes and materials. They investigate in the combined time-frequency domain the development in time of the various frequency features of the spectra of backscattered pulses using time-windowed Fourier transforms. The methodology developed can handle broadband pulses of any sufficiently smooth spectrum, interacting with (lossy or lossless) dielectric scatterers, and can extract resonance features within the frequency band of the spectrum of the transmitted pulse. Accordingly, this method could be also used for assessing the performance of high-power impulse radar systems     

Propagation of an ultrashort radio pulse in a resonance-absorbing atmosphere

The problem on propagation of an ultrashort radio pulse is analytically solved. The pulse has an envelope of a general shape and a carrier frequency lying within or near the frequency interval of an isolated spectral line of a resonance-absorbing component of a gaseous medium. The solution is used to analyze the character of dispersion distortions of a biexponential quasi-monochromatic pulse and a biexponential chirp pulse that propagate along near-surface and elevated atmospheric paths. The analysis is performed for a single strong line of water vapor in the millimeter wavelength band with a resonance frequency of 183.36 GHz. It is found that the most pronounced feature of distortions of the quasi-monochromatic pulse is gradual transformation of its one-peak envelope into a set of field maxima of various heights. The number of maxima is determined by the optical depth of a path, and, in all of the considered cases, the precursor of a propagating pulse is not formed.     

Dispersion-Induced Distortions of a Rectangular Radio Pulse with Linear Chirp in a Resonantly Absorbing Gas Medium

Existing analytical results on the propagation of electromagnetic pulse in resonantly absorbing gas medium are used to analyze possible character and features of the dispersion-induced distortions of an ultrashort frequency-modulated radio pulse with the initial rectangular shape and the carrier frequency that is close to the resonance frequency of a single strong absorption band of water vapor in the millimeter-wavelength range (183.36 GHz).     

时变源作用下介观ＬＣ电路系统量子态的演化

采用Wei-Norman方法,求出含时变电压源的介观LC电路随时间演化的精确解,应用wigner函数研究了时变电源作用下介观LC电路相干态的量子特性,结果表明wigner函数是一个二维运动的Gauss波包。研究了时变电源在不同频率的正弦信号对系统量子态的影响,结果表明,当所加正弦信号的频率远离共振频率信号时,对介观LC电路没有影响;当所加信号的频率为共振频率时,系统由低能态向高能跃迁,使处于高能态的几率增大;研究了输入电压源为单矩形脉冲的特例。     

核电磁脉冲与开孔金属腔体耦合特性研究

利用基于时域有限差分法(FDTD)的电磁仿真软件XFDTD,研究了核电磁脉冲与开孔腔体的耦合规律。分析了不同极化方向的核电磁脉冲入射时,矩形孔洞长宽比对耦合特性的影响;讨论了核电磁脉冲照射下,腔体谐振和孔缝与腔体的耦合谐振现象;给出了腔体内部电场分布的截面图,讨论了核电磁脉冲入射时腔体内外电场的空间分布情况。研究结果表明:当核电磁脉冲的极化方向与开孔矩形短边平行时,耦合的电场强度比正方形开孔时的要大,且长宽比越大,耦合的电场强度越大;当核电磁脉冲的极化方向与开孔矩形长边平行时,耦合的电场强度比正方形开孔时的小,且长宽比越大,耦合的电场强度越小。极化方向与矩形开孔短边平行时,腔体内产生了腔体谐振和孔缝与腔体的耦合谐振,开孔尺寸的变化会引起谐振频率的偏移;极化方向平行于长边时无明显电磁谐振发生。核电磁脉冲对屏蔽体内的影响主要局限于开孔附近。     

Features of the performance of a transient voltage suppressor in the pulsed mode

It is experimentally shown that the pulse withstand capacity of transient voltage suppressors, independent of power rating, decreases by the same law with increasing pulse duration, which indicates their optimum design parameters. The interrelation between transition times (turned on and off) and the characteristic parameters of the transient voltage suppressor structure is shown. The possibility of emitting a power fraction at the resonance frequency is an additional stimulus to increasing the withstand power.     

Ξ型介质中相对载波包络相位对脉冲特性的影响

利用数值模拟方法,研究了单光子共振及完全非共振情况下相对载波包络相位(RCEP)φ对在Ξ型三能级原子介质中传播的双色超短脉冲波形及频谱特性的影响。结果表明:在单光子共振情况中,RCEP对双色脉冲的传播形式及频谱特性有显著的控制作用。RCEP的大小可以决定脉冲振幅大小、传播速度快慢、脉冲分裂与否及子脉冲数目多少。当φ=π时,在传播过程中,脉冲不再发生分裂,而且脉冲的形状和强度基本保持不变。在完全非共振条件下,脉冲分裂不再发生,比单光子共振情况的频率范围大得多的超连续谱可以被得到。虽然RCEP对脉冲形式和频谱特性的影响不如单光子共振时那样强烈,但改变值φ仍可以使频谱范围及不同频率的强度分布有明显的改变。当φ=0时,可得到频谱更宽、连续性更好、高频分量强度更大的超连续谱。     

Cyclotron maser oscillator experiments in a periodically loadedwaveguide

The periodic-waveguide cyclotron maser is a device in which orbiting, nonrelativistic electrons interact with traveling waves in a metallic, periodically loaded waveguide. In this paper, we describe a table-top oscillator experiment that operates in the microwave regime (9.4 GHz) with a low-energy (8 keV), low-current (0.2 A) electron beam pulse (1 ms pulse width). The frequency modulation (chirping) observed is used in the determination of cyclotron resonance condition for this interaction. The results show that the cyclotron interaction occurs with a backward-propagating wave in the periodically loaded waveguide. Microwave power of ⩾0.3 kW and efficiency of ⩾20% are measured by an external loading of the oscillator     

Deemphasizing low frequency defects in GTD analysis of pulsed signal scattering by a perfectly conducting flat strip

Geometrical theory of diffraction (GTD) based wavefront, resonance and hybrid formulations of transient scattering by targets, while adequate to generate resonance frequencies, yield incorrect late time scattered fields because of the early time (high-frequency) nature of GTD asymptotics. Thus, impulsive signals are improperly treated in this manner. However, for signals with low frequency cutoff, deemphasizing the low frequency range may eliminate the troublesome portions of the GTD algorithm. This is confirmed here for plane wave scattering by a perfectly conducting flat strip when the exciting pulse has a raised cosine shape. The solution is now stabilized and compares very well with an exact solution obtained by eigenfunction expansion. This suggests that the GTD method can be employed with confidence for transient scattering responses for a class of input pulses with sufficiently weak low frequency content.     

Influence of the medium nonlinearity on the reflection of monochromatic and pulsed radiation

The features of Gaussian pulse reflection from a nonlinear medium with the resonance frequency dependence of permittivity are investigated. The results of numerical analysis of the equations describing the light-field distribution in reflected pulses with s and p polarizations are presented. The influence of the intensity, incidence angle, carrier frequency, and incident-pulse duration on the reflected pulse envelope shape is discussed.     

Effect of detuning on the few-cycle laser pulse propagation in the three-level atomic medium

The effect of detuning on the few-cycle laser pulse propagation in the ladder-type three-level atomic medium is investigated by using the numerical solution from the Maxwell-Bloch equations without the slowly varying envelope and rotating-wave approximations.The results show that in the resonance case,the obvious variation of the pulse form,including the carrierenvelope phase,the pulse duration,the oscillation amplitude and frequency,even the pulse splitting will occur in the propagation,and the output puls...     

微带宽缝天线的谐振频率和带宽特性

借助时域有限差分法(FDTD)对微带宽缝天线的频率和带宽特性进行了分析讨论.采用高斯脉冲激励,对计算空间采用非均匀网格划分,通过FDTD,一次计算就可得到天线谐振频率和带宽的参数.研究了微带宽缝天线的频率和带宽特性随着缝宽变化的规律.发现当缝宽从1mm增加到30mm时,天线的谐振频率从3.4GHz降低到2.3GHz,天线的10dB带宽可从9%增加到28%.     

Optical repeater using PFM at 1 GHz

We report the first realization of PFM optical communication with a repeater station, using double heterostructure GaAs/GaAlAs semiconductor lasers. The system uses the property of semiconductor lasers for which the optical pulse rate can be locked by an external modulation of the injection current in the vicinity of the resonance frequency of the laser. This modulation scheme has the advantages of a high information rate capability by using the "spiking" resonance of the laser and, more importantly, of a simple repeater station in which another pulsing laser acts as a regenerative pulse amplifier.     

Resonance frequency tracking control for piezoelectric transformerDC-DC converter

A control strategy for piezoelectric transformer DC-DC converters based on a phase and frequency detector circuit is presented. The control always tracks transformer resonance frequency, independently of differences between devices of the same type and temperature variation. Voltage regulation is achieved by adopting the pulse width modulation technique. A prototype confirmed the expected results     

Effect of switching a magnetoplasma medium on the duration of a monochromatic pulse

The main effect of switching a magnetoplasma medium is to split the source wave into new waves whose frequencies are different from the source wave. In addition, if the source is a monochromatic pulse, the duration of the pulse is altered. Analytical expressions for the pulse duration of the various characteristic waves in a magnetoplasma are derived. The variations of the pulse duration with the source frequency, the plasma frequency and the cyclotron frequency are illustrated. The principle of the change in the pulse duration may be used to diagnose dynamically the time — varying parameters of a magnetoplasma medium.     

Oscillation properties of AlGaAs DH lasers with an external grating

External control of the longitudinal oscillation mode of AlGaAs double heterostructure lasers using a diffraction grating is presented. Dependence of threshold current on selected mode wavelength is observed experimentally and is compared with a theory based on multilongitudinal mode rate equations. Baseband frequency responses for various bias currents above threshold are estimated by small-signal analysis and coincide well with experimental observations. Resonance frequency shifts to lower frequency and the resonance width is broadened by the optical feedback. Theoretical and experimental step pulse responses are also studied. Single longitudinal mode builds up rapidly. Relaxation oscillation observed after the oscillation builds up is less damped when mode selective feedback is provided. Output fluctuation frequency corresponds to the resonance frequency.     

饱和情况下微通道板短脉冲信号放大特性研究

采用传输线方法研究了微通道板的短脉冲信号放大特性。分析了单脉冲信号的波形畸变以及微通道增益的变化情况。当信号饱和参数大于1时，脉冲前沿消耗的电荷会对脉冲后沿的放大形成影响，造成脉冲后沿增益下降。研究了多脉冲间的互扰问题。若前一脉冲放大消耗的电荷得不到及时补充，将影响后续脉冲的放大过程。分析了信号频率对微通道板输出特性的影响。信号单脉冲电荷量恒定的情况下，提高信号频率将使得微通道板增益显著下降；信号平均电流恒定时，通道增益以及输出电流基本不随信号频率发生变化。通过脉冲激光对像增强器的辐照实验，验证了上述分析结果的正确性。     

Methodology for design of a vibration operated valve for abrasive viscous fluids

In this paper a new ultrasonic operated valve is presented. The ultrasonic valve design was analyzed and the valve was prototyped and tested for flow control of abrasive viscous fluid. This innovative valve concept is based on controlling the friction of material by employing several friction elements along the flow direction. Abrasive particles in the viscous fluid are stopped by the force of friction when coming into contact with the friction elements. Friction is neutralized by use of vibration to break away the abrasive particles from the friction element surfaces. Several factors were considered in designing the piezoelectric valve. Factor identification was done by conducting experiments and analyzing the resulting data. Some important factors that affect the valve design were recognized to be pumping pressure, size of friction blades along the direction of flow, density of material, viscosity, amplitude of vibration, frequency of vibration, and proportion of abrasive particles in the mix. First, a method was designed for measuring the friction coefficient of the given viscous materials. A design of experiment approach was pursued in order to identify the significant parameters. A piezoelectric transducer was used, which vibrated at the resonance frequency of 20 kHz. FEM modeling was used at that stage to ensure that the resonance frequency of the designed valve matched the resonance frequency of the transducer and booster assembly that provided vibration. In order to perform proportional flow control pulse width modulation was used to control the duty cycle of the ultrasonic power transferred to the valve. A study was performed to find the best vibration characteristics.     

微波脉冲对电容近炸引信腔体耦合特性分析*

利用有限积分法仿真分析了微波脉冲对电容近炸引信腔体的耦合过程,分别分析了微波入射方向、 电场极化方向和微波频率对引信腔体耦合特性的影响。结果表明:微波入射角度为30°、电场极化方向与弹轴夹角 为30°时,耦合系数最大;引信探测电路位置处的微波场最强;随着微波频率增大,耦合系数存在明显减小的趋势,同 时存在多个共振增强的频率点。分析表明,对于环状孔缝而言,其共振频率与矩形孔缝不同,且数值与环状孔缝的 圆周长成反比。