L波段微波脉冲对微型计算机的辐照效应实验

 在L波段使用1.3 GHz载波频率的微波脉冲辐照微型计算机主板，通过改变脉冲宽度、重复频率和脉冲串长度等参数，实验研究了微波脉冲辐照导致微型计算机失去响应的功率阈值的变化规律，讨论了L波段微波脉冲辐射的积累效应。实验结果表明：当微波脉冲宽度增加时，微波功率阈值下降；当微波脉冲重复频率升高时，微波功率阈值呈下降趋势；在固定重复频率的条件下，微波脉冲数目的增加也会导致微波功率阈值的下降；微波脉冲功率阈值始终小于连续波微波的功率阈值。当微波脉冲间隔时间较长或者脉冲宽度较宽时，微波功率阈值由单个微波脉冲的参数确定，与脉冲重复频率没有明显关系。利用假设的微波脉冲辐射积累效应，可以定性解释和分析微波脉冲辐照微型计算机实验中功率阈值变化的趋势。     

优化磁共振纳米医学中磁性纳米粒子的热疗效率

磁共振热疗(magnetic resonance hyperthermia)是近年来新兴的一种纳米医学治疗方法,由磁共振的硬件架构产生特定交变磁场,有效地加热磁性纳米粒子,以直接或间接地杀死癌细胞,体现诊疗一体化。提高磁性纳米粒子的加热效率是当前磁共振热疗领域亟待解决的难题之一。磁性纳米粒子的加热效率不仅与粒子本身的大小、性质以及尺寸分布有关,还和聚集状态有关。该研究利用3D Metropolis蒙特卡罗模拟方法,模拟了不同温度下磁性纳米粒子的磁共振热动力学行为及其团聚与分离现象;并通过修正过的郎之万方程,建立了相变临界温度与外加磁场频率的函数关系。模拟结果显示,磁性纳米粒子悬浮液中多聚体的相对含量随着温度的升高而降低,达到临界温度后,多聚体完全分离成单体;而提高交变磁场频率可以显著降低临界温度,且存在临界频率,高于此临界频率后临界温度不再受外加磁场频率影响,达到稳定。因而在临界频率下预热磁性纳米粒子悬浮液,使得多聚体分离成单体,可优化磁性纳米粒子的热疗效率。     

Controlling the acoustic streaming by pulsed ultrasounds

We propose a technique based on pulsed ultrasounds for controlling, reducing to a minimum observable value the acoustic streaming in closed ultrasonic standing wave fluidic resonators. By modifying the number of pulses and the repetition time it is possible to reduce the velocity of the acoustic streaming with respect to the velocity generated by the continuous ultrasound mode of operation. The acoustic streaming is observed at the nodal plane where a suspension of 800 nm latex particles was focused by primary radiation force. A mixture of 800 nm and 15 μm latex particles has been also used for showing that the acoustic streaming is hardly reduced while primary and secondary forces continue to operate. The parameter we call “pulse mode factor” i.e. the time of applied ultrasound divided by the duty cycle, is found to be the adequate parameter that controls the acoustic streaming. We demonstrate that pulsed ultrasound is more efficient for controlling the acoustic streaming than the variation of the amplitude of the standing waves.     

Thresholds for cavitation produced in water by pulsed ultrasound

The threshold for transient cavitation produced in water by pulsed ultrasound was measured as a function of pulse duration and pulse repetition frequency at both 0.98 and 2.30 MHz. The cavitation events were detected with a passive acoustic technique which relies upon the scattering of the irradiation field by the bubble clouds associated with the events. The results indicate that the threshold is independent of pulse duration and acoustic frequency for pulses longer than approximately 10 acoustic cycles. The threshold increases for shorter pulses. The cavitation events are likely to be associated with bubble clouds rather than single bubbles.     

Modulated resonant versus pulsed resonant photoacoustics in trace gas detection

Modulated resonant photoacoustics is a sensitive technique widely used for trace gas sensing. Generally, a continuous-wave laser is modulated at a frequency corresponding to an acoustic resonance of a photoacoustic cell. Another mode of operation—which we propose to call the pulsed resonant mode—consists in matching the frequency repetition rate of a pulsed laser to an acoustic resonance of the cell. We present a theoretical model to compare the performance of these two configurations. For a given average power of the incoming light inside the cell, the pulsed resonant mode of operation (nanosecond pulses or shorter) produces π/2 times higher photoacoustic signals than the modulated resonant scheme (the latter is optimized for a 50% duty cycle). This result agrees with experiments during which both cases were investigated at 532 nm using the same photoacoustic cell containing trace concentrations of NO₂.     

Rheo-acoustical study of the shear disruption of reversible aggregates. Ultrasound scattering from concentrated suspensions of red cell aggregates

Shear-induced disruption of reversible aggregates or clusters in a concentrated suspension is investigated by ultrasound backscattering in the low shear regime. Fractal aggregates are considered as non-Brownian scatterers much smaller than the wavelength with acoustic properties close to those of the surrounding liquid, so that the attenuation of the coherent field is weak and multiple scattering can be neglected. The concept of variance in local particle volume fraction is used to deduce a first-order expression of the ultrasound scattering cross section per unit volume for Rayleigh scatterers in a dense suspension. On the basis of a scaling law for the shear-induced disruption of aggregates, the shear stress dependence of the ultrasonic scattered intensity from a dense suspension of clusters is derived. In a second part, the shear breakup of hardened red blood cell aggregates is investigated in plane-plane flow geometry by ultrasound scattering. Rheo-acoustical experiments are analyzed within the framework of the self-consistent field approximation and the scaling laws currently used in microrheological models. Finally, the ability of ultrasonic, light reflectometry and viscometry methods to provide quantitative information about red blood cell aggregation and membrane adhesiveness is discussed.     

Digital Holographic Particle Tracking Velocimetry for 3-D Transient Flow around an Obstacle in a Narrow Channel

Digital holographic particle tracking velocimetry (PTV) is developed by single high-speed camera and single double pulsed laser with high frequency pulses. This system can directly capture 1000 hologram fringe images for 1 second through a camera computer memory. The 3-D particle location is made of the reconstruction by using a computer hologram algorithm in a personal computer. This system can successfully be applied to instantaneous 3-D velocity measurement in the water flow with a square obstacle, and can obtain an average of 300 instantaneous velocity vectors.     

Effect of time profile of laser pulse on the excitation of sound signals in a liquid with a series of laser pulses

We have studied the sound generation with high repetition rate pulsed laser. We have solved the inhomogeneous wave equation for acoustic pressure in a liquid generated by a laser, using Green’s function formalism and convolution technique. To obtain the maximum pressure of the sound waves, we found the conditions on repetition rate and on period of laser pulse of various shapes. Our analysis shows that the sound generated in a liquid with a series of laser pulses is highly affected by the time profile of the pulses besides other parameters, namely laser beam diameter, laser beam optical wavelength, repetition rate and period of laser pulse. This effect is pronounced particularly in frequency domain. We found that the noise of higher harmonics in the generated sound can be greatly removed with the proper choice of the time profile of the laser pulses. It is found that the pressure is generated around the fundamental frequency for the half-sine and rectangular pulses, with the proper choice of repetition rate and period of pulse. The application of the present analysis for underwater communication is pointed out.     

Room temperature stable single-photon source

We report on the realization of a stable solid state room temperature source for single photons. It is based on the fluorescence of a single nitrogen-vacancy (NV) color center in a diamond nanocrystal. Antibunching has been observed in the fluorescence light under both continuous and pulsed excitation. Our source delivers 2×10⁴ s^-1 single-photon pulses at an excitation repetition rate of 10 MHz. The number of two-photon pulses is reduced by a factor of five compared to strongly attenuated coherent sources. Received 1st August 2001 and Received in final form 2 October 2001     

Pulsed pumped Yb-doped fiber amplifier at low repetition rate

A pulsed pumped Yb-doped double-clad fiber (DCF) master-oscillator power amplifier (MOPA) at low repetition rate is reported. Seeded by a passive Q-switched Nd:YAG microchip laser, the fiber amplifier can generate 167-kW peak-power and 0.83-ns duration pulses at 200-Hz repetition rate. Because of the pulsed pump approach, the amplified spontaneous emission (ASE) and the spurious lasing between pulses are well avoided, and the repetition rate can be set freely from single-shot to 1 kHz. Peak power scaling limitations that arise from the fiber facet damage are discussed.     

Compact low-threshold Q-switched intracavity optical parametric oscillator

We report a singly resonant pulsed intracavity KTiOPO>(4) optical parametric oscillator that uses a semi-monolithic microchip laser design. The compact (50-mm-long), low-threshold (1.3-W) cavity uses a novel quadrupole deflector Q switch to give 4-microJ pulses at 1.064 microm and 0.4-microJ signal pulses of 5.6-ns duration at 1.53 microm with a repetition frequency of 5 kHz when it is pumped with a 2-W laser diode. The signal pulses are diffraction limited and single frequency.     

A frequency-doubled laser system producing ns pulses for rubidium manipulation

We have constructed a pulsed laser system for the manipulation of cold ⁸⁷Rb atoms. The system combines optical telecommunications components and frequency doubling to generate light at 780 nm. Using a fast, fibre-coupled intensity modulator, we sliced output from a continuous laser diode into pulses with a length between 1.3 and 6.1 ns and a repetition frequency of 5 MHz. These pulses are amplified using an erbium-doped fibre amplifier, and frequency-doubled in a periodically poled lithium niobate crystal, yielding a peak power up to 12 W. Using the resulting light at 780 nm, we demonstrate Rabi oscillations on the F=2,m_F=+2↔F^′=3, m^′ _F=+3-transition of a single ⁸⁷Rb atom. PACS 32.80.Qk; 39.25.+k; 42.55.-f     

Pulse-resolved measurement of quadrature phase amplitudes of squeezed pulse trains at a repetition rate of 76 MHz

We report pulsed homodyne detection of squeezed pulses at a repetition rate of 76 MHz. Measurement of individual pulses, which were interrogated by the correlation coefficient between adjacent pulses, was realized. A homodyne detector was constructed using a low-noise, high-speed operational amplifier; it had a usable bandwidth of 250 MHz. We observed 2.3 dB of squeezing in both the time and frequency domains.     

Versatile Raman fiber laser for sodium laser guide star

Robust high‐power narrow‐linewidth lasers at 589 nm are required for sodium laser guide star adaptive optics in astronomy. A high‐power 589 nm laser based on Raman fiber amplifier is reported here, which works in both continuous‐wave and pulsed formats. In the continuous‐wave case, the laser produces more than 50 W output. In the pulsed case, the same laser produces square‐shaped pulses with tunable repetition rate (500 Hz to 10 kHz) and duration (1 ms to 30 μs). The peak power is as high as 84 W and remains constant during the tuning. The laser also emits an adjustable sideband at 1.71 GHz away from the main laser frequency for better sodium excitation. The versatility of the laser offers much flexibility in laser guide star application.     

Theoretical analysis of the electromagnetic field inside an anomalous-dispersion microresonator under synthetical pump

We study the spatiotemporal evolution of the electromagnetic field inside a microresonator showing an anomalous dispersion at the pump wavelength by using the normalized Lugiato–Lefever equation. Unlike the traditional single continuous wave(CW) pumping, an additional pump source consisting of periodical pulse train with variable repetition rate is adopted.The influences of the microresonator properties and the pump parameters on the field evolution and the electromagnetic field profile are analyzed. The simulation results indicate that, in the anomalous dispersion regime, both increases of the input pulse amplitude and the repetition frequency can result in the field profiles consisting of multiple peaks. A series of equidistant pulses can also be obtained by increasing the CW pump power. In addition, we find that a large physical detuning between the pump laser carrier and the cavity resonance frequency also causes the splitting of the inside field.     

An investigation into the effect of pulsed ultrasound on the brain

The effect of pulsed ultrasound on the human brain, for longer times than that administered in diagnostic work, in five adult patients and one child was studied by clinical evaluation, electro-encephalography, biochemical and histopathological studies.

Exposure for one hour to pulsed ultrasound, through scalp and bone, with a 2MHz frequency crystal, emitting 430 pulses/sec, with each pulse lasting for 1 μs, was found harmless. The pulse strength of the apparatus used in one microsecond is about 10 watts, resulting in a mean strength of about 5 milliwatts and an ultrasonic dose of 1 milliwatt per centimetre square.     

Two-photon fluorescence bioimaging with an all-semiconductor laser picosecond pulse source

We have demonstrated successful two-photon excitation fluorescence bioimaging using a high-power pulsed all-semiconductor laser. Toward this purpose, we developed a pulsed light source consisting of a mode-locked laser diode and a two-stage diode laser amplifier. This pulsed light source provided optical pulses of 5 ps duration and having a maximum peak power of over 100 W at a wavelength of 800 nm and a repetition frequency of 500 MHz.     

Enhancing sonochemical activity in aqueous media using power-modulated pulsed ultrasound: an initial study

Relatively little is known about the effects of pulsed ultrasound on the facilitation of chemical reactivity. Previous studies have indicated that sonochemistry using pulses is generally less effective than continuous ultrasonic irradiation. However, the pulse trains employed were such that the peak power of the pulses was the same as the maximum power used in continuous irradiation. As a result, less acoustic energy was transmitted to the solutions over the same period of time. The effectiveness of ultrasound when the pulse is adjusted so that the same amount of acoustic energy is input compared to continuous irradiation over a given time has not been previously explored. In this study we have embarked on an examination of the efficacy of power-modulated pulsed (PMP) sonochemistry. Specifically, we have explored the effects of pulse type and pulse frequency on the oxidation of potassium iodide and the degradation of acid orange, a common industrial colorant. A rate increase by a factor of three was observed compared with continuous irradiation under conditions of equivalent acoustic input power.     

Schlieren visualization of ultrasonic wave fields with high spatial resolution

The visualization of ultrasonic wave fields in optically transparent liquids using the acousto-optic interaction is a well proven tool for the experimental investigation of wave propagation including wave field interaction effects with certain discontinuities and obstacles like reflection, refraction, and diffraction effects as well as for transducer testing and design. For high resolution visualization of wave fields including pulsed waveforms, pulsed light sources and sensitive optical imaging sensors with certain specifications are needed. In this paper the technical requirements of optical and electronic components for high resolution visualization of ultrasound wave fields will be presented. Also, specifications and operation results of a new designed, inexpensive Schlieren optical system will be presented, which is capable of pulsed wave field visualization in the MHz frequency range. The spatial resolution is high enough, not only for accurate beam shape and wave pattern visualization, but also for a gray-scaled display of wave amplitudes including amplitude zero crossings in ultrasound pulses. Consequently, ultrasonic wavelengths can be visualized quantitatively as well as wavelength changes of the ultrasound pulses while traveling through transparent media with different sound velocities. Results to be presented will include 2 MHz and 10 MHz experiments using single transducers as well as linear arrays of commercial medical scanners during their standard operation showing the system beamforming characteristics.     

Mechanistic modeling of destratification in cryogenic storage tanks using ultrasonics

Stratification is one of the main causes for vaporization of cryogens and increase of tank pressure during cryogenic storage. This leads subsequent problems such as cavitation in cryo-pumps, reduced length of storage time. Hence, it is vital to prevent stratification to improve the cost efficiency of storage systems. If stratified layers exist inside the tank, they have to be removed by suitable methods without venting the vapor. Sonication is one such method capable of keeping fluid layers mixed. In the present work, a mechanistic model for ultrasonic destratification is proposed and validated with destratification experiments done in water. Then, the same model is used to predict the destratification characteristics of cryogenic liquids such as liquid nitrogen (LN₂), liquid hydrogen (LH₂) and liquid ammonia (LNH₃). The destratification parameters are analysed for different frequencies of ultrasound and storage pressures by considering continuous and pulsed modes of ultrasonic operation. From the results, it is determined that use of high frequency ultrasound (low-power/continuous; high-power/pulsing) or low frequency ultrasound (continuous operation with moderate power) can both be effective in removing stratification.