单阶简正波宽带干涉特性与孤立子内波定位

为了分析孤立子内波(SIW)对声场干涉特性的影响以及由此监测SIW,我们采用抛物方程模型,分析了孤立子内波影响下简正波耦合带来的单阶简正波宽带脉冲序列的干涉特性;仿真了宽带脉冲信号穿过SIW时,单阶简正波接收脉冲序列随内波位置的变化情况,分析了它们的时空干涉特性和机理;基于耦合简正波理论,推导得到了内波位置和干涉条纹斜率之间的关系。仿真表明,这一关系式可以精确估计SIW距离,估计精度和稳定性比以往方法大大提升。此外,还根据单阶简正波干涉图样在频率上的周期性,推导了另一种估计SIW距离的方法,该方法具有更好的实时性。SIW-简正波干涉关系可用于改善声场定位,对于海洋学、海洋工程和海洋军事等方面具有重要价值。此外,该关系还可以拓展到其他的局部特性变化的探测,如金属探伤和海底地形突变定位等。      

用于超声悬浮场可视化的光学全息相位倍增方法研究

基于二次曝光全息干涉测量原理,采用四倍相位倍增光路对声悬浮声压场分布进行了可视化研究.拍摄了给定模式的超声悬浮场在不同超声换能器激励电流时的光学全息图并进行光学再现,由再现光波的全息干涉条纹图样得到了反映声压场分布的光波波前相位变化,并进行了对比分析.结果表明,与无相位倍增光路相比,采用四倍相位倍增光路可使同一声压场的全息干涉条纹数目大大增加,测量灵敏度明显提高,为进一步研究声悬浮场声压分布提供了一种有效方法.     

Effect of background internal waves on the interference structure of the sound field in a shallow sea

The results of a theoretical analysis of the effect produced by an anisotropic field of background internal waves on the localization of the interference pattern in a shallow sea are presented. The space-time variability of the interference invariant and the smearing of the observation direction of interference fringes are considered in a wide frequency range. The stability of the interference pattern formed by both the superposition of the fields of different mode groups and separate mode groups is analyzed in comparison with the unperturbed waveguide. Numerical calculations are performed for longitudinal and transverse orientations of the acoustic track relative to the propagation direction of internal waves.     

Frequency shifts of the sound field interference pattern on oceanic shelf in summer conditions

Considerable shifts Δf/ϕ ≈ 10⁻¹ of the low-frequency sound field interference pattern in the frequency domain, associated with barotropic tide and internal tidal waves, were observed in the Shallow Water’06 experiment on the New Jersey shelf in the summer of 2006. The acoustic frequency shifts appear to be strongly dependent on the modes of the sound field. By examining different modal structure, it is possible to analyze the overall interference pattern and find which part is more sensitive either to the surface tide or the internal waves. This feature can be exploited to acoustically monitor tidal waves of different kinds.     

Forces and spatial ordering of driven atoms in a resonator in the regime of fluorescence suppression

An atom in a high-Q cavity, which is coherently driven at the frequency of a cavity mode, exhibits strong suppression of fluorescence when the atomic decay rate exceeds the cavity linewidth. This effect is due to destructive interference of cavity and pump field, such that at the atomic position the total field intensity has a local minimum. For atomic ensembles the magnitude of the interference effect grows with atom number and depends on the relative atomic positions. It is strongest for a wavelength spaced array of atoms placed at the antinodes of the cavity mode. This suppresses fluorescence and enhanced collective scattering into the cavity mode. We analyze the mechanical forces in the regime where the interference condition is fulfilled. We show that the atomic pattern is mechanically stable whenever the driving frequency is red detuned with respect to the cavity frequency, irrespective of the atomic transition frequency. Hence atomic selforganization, as predicted in [6] can also occur in the parameter regime where superradiant scattering is suppressed by collective interference. PACS 32.80.Pj; 42.50.Pq; 42.50.Fx     

Measurement of interference-pattern parameters of the near acoustic field of a deep-sea noise source

We present the results of a study of the space-frequency energy distribution of the near acoustic field of noise radiation from a moving source. Dynamic spectrograms of the received signal of noise radiation in the frequency range of 0–2.0 kHz at distances of up to 1300 m are given. It is shown that the variation in the levels of spectral components of the signal due to interference modulation in the near field can attain 12 dB. The interference pattern is shown to occur over the entire range of considered distances of 100–1300m. Within the framework of ray theory, using hydrological data for C(z) in the region of measurements, we calculate the near field for a lumped noise source. The calculation results are in good agreement with the results of field experiments. It is shown that the sound velocity profile C(z) in the region of measurements has a pronounced influence on the interference pattern of the near field. Allowance for C(z) in the calculations of the near acoustic field within the framework of the existing theory allows one to predict the interference pattern and choose optimal distances for measurements under near-field conditions.Institute of Applied Physics, Russian Academy of Sciences, Nizhny Novgorod. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 38, No. 8, pp. 817–831, August, 1995.     

Features of the Invariant of the Spatial–Frequency Interference Structure of Acoustic Fields in Oceanic Waveguides

The paper considers the regular patterns that can be manifested in the behavior of the invariant of the spatial–frequency interference structure of an acoustic field in oceanic waveguides that are homogeneous and inhomogeneous along a track. Using the WKB and adiabatic approximations, an analytic expression is obtained for the invariant that, when certain conditions are fulfilled, reduces to the well-known classical expression independent of the mode numbers, their parity, or the emission frequency. It has been established that the approximate classical expression for the invariant correctly describes the slope of interference lines only in ranges of variation in the grazing angles of modes where the dependence of the cycle length of their corresponding Brillouin waves on the ray parameter is quite smooth and monotonic. The paper studies the formation of the spatial–frequency interference structure of an acoustic field propagating from a shallowwater isovelocity waveguide to a relatively deep-water waveguide with a near-surface sound channel.     

声辐射模态和球谐波函数在球源声场重建中的比较

声辐射模态和球谐波函数在球源声场重建中均有应用,两者具有很多相似性,都具有相同的多极子辐射模式和"群组"分布特性。有观点认为球形声源的声辐射模态向量即为球谐波函数,但两者的内在关系一直缺乏有效的证明。为分析两种基函数的相似性及在球源声场重建领域的差异,在理论上建立了两者的内在联系,在应用上比较研究了基于两种基函数的声场重建方法的有效性和可靠性。数值算例结果表明,在中高频段时基于两种函数的声场重建效果相当,但在低频段尤其是存在干扰源的非自由场条件下的声场重建应用中,基于声辐射模态的声场重建方法能得到更高的重建精度。可见,声辐射模态具有较为明显的声场重建优越性,这为利用声辐射模态和球谐波函数进行球源声场重建提供了理论参考。      

On interference effects in the elastodynamic Green's functions G33(x, omega) of Si and GaAs

In this paper, we analyze interference effects present in the elastodynamic Green's functions G33(x,omega) of the cubic crystals Si and GaAs, which are associated with folded portions of the wave surface of the slow transverse (ST) acoustic mode. G33(x,omega) represents the three dimensional extension of the amplitude distribution imaged in the transmission acoustic microscopy of these crystals. The intensity contrast for oscillations of a particular wave vector k in the interference pattern is determined essentially by the 3D Fourier transform of G33(x,omega)G33*(x,omega). According to the Fourier autocorrelation theorem, that transform is equivalent to the autocorrelation function of the corresponding distribution G(33)(k,omega) in k-space. We show that due to the linear mapping between k-space and the slowness vector s-space, the interference phenomena discussed here are related to geometrical features of the slowness surface of the ST mode. We present calculations of these effects based on the angular spectrum technique.     

利用声场频谱频率漂移监测内波的算法改进和实验验证

声场频谱的频率漂移曲线可以用来监测内波。由于简正波幅度剧烈起伏带来的干扰,目前尚无从实验数据中提取频移曲线的有效算法.提出了一种依据实测声学传播数据提取频移曲线的算法。借助于简正波过滤技术,该算法利用相关法从简正波相角之差中提取频率漂移曲线。利用该算法提取的频移曲线与内波导致的跃层起伏具有很高的相似性,这在2011年黄海实验中得到了验证。该算法的优点是可以保留简正波相位差变化导致的频移曲线信息,同时又能有效地抑制简正波幅度起伏带来的干扰,但是需要良好的接收阵阵型来保证简正波分离。      

Coherent acoustic phonon oscillations in semiconductor multiple quantum wells with piezoelectric fields

Large coherent acoustic phonon oscillations were demonstrated using InGaN/GaN multiple quantum wells with piezoelectric fields. With UV femtosecond pulse excitation, photogenerated carriers screened the piezoelectric field and initiated the displacive coherent phonon oscillations. The specific phonon frequency was selected by the coupling between the periodic carrier distribution and the corresponding acoustic phonon mode. The induced acoustic phonon oscillation resulted in piezoelectric field modulation and then caused absorption variation through the quantum confined Franz-Keldysh effect. The wave vector uncertainty due to the finite sample width was found to determine the observed dephasing time.     

Intersymbol interference in underwater acoustic communications using time-reversal signal processing

Coherent underwater communication is hampered by the time spread inherent to acoustic propagation in the ocean. Because time-reversal signal processing produces pulse compression, communications has been suggested as a natural application of the technique. Passive versions of time-reversal processing use a receive-only array to do combined temporal and spatial matched filtering. It can be shown, however, that the pulse compression it achieves is not perfect and that an equalizer that relies solely on time-reversal processing will have an error floor caused by uncompensated intersymbol interference (ISI). In the present paper, a physics-based model is developed for the uncompensated ISI in a passive time-reversal equalizer. The model makes use of a normal-mode expansion for the acoustic field. The matched-filtering integral is approximated and the intermediate result interpreted using the waveguide invariant. After combining across the array and sampling, formal statistical averages of the soft demodulation output are calculated. The results show how performance scales with bandwidth, with the number and position of array elements, and with the length of the finite impulse response matched filters. Good agreement is obtained between the predicted scaling and that observed in field experiments.     

Effect of acoustic field parameters on arc acoustic binding during ultrasonic wave-assisted arc welding

As a newly developed arc welding method, power ultrasound has been successfully introduced into arc and weld pool during ultrasonic wave-assisted arc welding process. The advanced process for molten metals can be realized by utilizing additional ultrasonic field. Under the action of the acoustic wave, the plasma arc as weld heat source is regulated and its characteristics make an obvious change. Compared with the conventional arc, the ultrasonic wave-assisted arc plasma is bound significantly and becomes brighter. To reveal the dependence of the acoustic binding force on acoustic field parameters, a two-dimensional acoustic field model for ultrasonic wave-assisted arc welding device is established. The influences of the radiator height, the central pore radius, the radiator radius, and curvature radius or depth of concave radiator surface are discussed using the boundary element method. Then the authors analyze the resonant mode by this relationship curve between acoustic radiation power and radiator height. Furthermore, the best acoustic binding ability is obtained by optimizing the geometric parameters of acoustic radiator. In addition, three concave radiator surfaces including spherical cap surface, paraboloid of revolution, and rotating single curved surface are investigated systematically. Finally, both the calculation and experiment suggest that, to obtain the best acoustic binding ability, the ultrasonic wave-assisted arc welding setup should be operated under the first resonant mode using a radiator with a spherical cap surface, a small central pore, a large section radius and an appropriate curvature radius.     

利用矢量海洋环境噪声提取声场格林函数

考虑到矢量水听器在垂直方向上具有8字形指向性,能够有效抑制远方非平稳噪声源的干扰,提出了一种矢量环境噪声相关函数(NCF)提取声场时域格林函数(TDGF)的方法。基于简正波理论建立了声压和垂直振速垂直相关性模型。在此基础上,给出了声压和垂直振速相关函数提取声场纵向格林函数的过程.数值仿真对比和实验数据分析表明,相对于声压提取方法,垂直振速提取方法能够有效消除直达波前出现的亮纹与亮区干扰。此外,对于同等时间长度噪声序列,声压提取方法只提取到直达波路径,而垂直振速提取方法还提取到了我们更为关心的海底反射路径。利用直达波与海底反射波到达时延差估计的海深与实测海深吻合较好。      

利用宽带声场频率—掠射角干涉结构的深海直达声区目标深度估计方法

针对典型深海环境中宽带声源的深度分辨问题,通过研究深海声场随频率起伏的干涉结构与垂直线阵频域波束输出图中的干涉结构,给出一种直达声区内可区分多水下目标的宽带声源深度估计方法.该方法以近水面目标的射线声场模型为基础,推导出近海面宽带声源接收声场的波束输出表达式,阐明了频域波束输出图中干涉结构与声源深度的对应关系.然后利用...     

Photoionization microscopy of Rydberg hydrogen atom in a non-uniform electrical field

In this paper,we investigate the photoionization microscopy of the Rydberg hydrogen atom in a gradient electric field for the first time.The observed oscillatory patterns in the photoionization microscopy are explained within the framework of the semiclassical theory,which can be considered as a manifestation of interference between various electron trajectories arriving at a given point on the detector plane.In contrast with the photoionization microscopy in the uniform electric field,the trajectories of the ionized electron in the gradient electric field will become chaotic.An infinite set of different electron trajectories can arrive at a given point on the detector plane,which makes the interference pattern of the electron probability density distribution extremely complicated.Our calculation results suggest that the oscillatory pattern in the electron probability density distribution depends sensitively on the electric field gradient,the scaled energy and the position of the detector plane.Through our research,we predict that the interference pattern in the electron probability density distribution can be observed in an actual photoionization microscopy experiment once the external electric field strength and the position of the electron detector plane are reasonable.This study provides some references for the future experimental research on the photoionization microscopy of the Rydberg atom in the non-uniform external fields.     

Frequency shifts of the sound field interference pattern in shallow water because of second-mode soliton-like internal waves

Numerical simulation is carried out to analyze the effect of an internal soliton of the second gravity mode on low-frequency sound propagation in an oceanic shelf region. The simulation is performed using the data of a full-scale experiment performed on the shelf of the South China Sea near Dongsha atoll, where the aforementioned solitons had been detected by stationary vertical thermistor arrays. The calculations take into account the effect of horizontal refraction of sound waves. It is assumed that a stationary acoustic track is oriented across the predominant propagation direction of internal waves. The results of simulation show that, when the soliton crosses the stationary track, some of the sound field modes are focused, whereas other modes are defocused. It is demonstrated that the soliton parameters can be adequately determined from the frequency shifts of the sound field interference pattern. However, such an estimate of the soliton parameters is only possible for a limited length of the stationary track for which the effect of horizontal refraction is sufficiently weak.     

利用线谱起伏实现目标测距

常见水中目标辐射噪声的线谱会随目标运动而出现强弱起伏。不同号简正波之间的干涉是目标声强随距离起伏的主要原因,利用简正波水平波数差和波导不变量之间的关系,推导了不同线谱随距离变化起伏的理论关系式。基于此关系式,提出利用两条或多条线谱声强起伏进行声源测距的方法。针对两条线谱起伏测距方法存在的旁瓣问题,解释其形成原因,并提出用多条线谱声强起伏抑制旁瓣。数值仿真验证了方法的可行性,利用多条线谱声强起伏测距的方法可有效抑制旁瓣。海试数据分析表明,当连续谱部分非常微弱无法有效测距时,利用多条线谱声强起伏测距的方法仍能稳定的估计渔船目标距离,估计距离的平均相对误差为12%。      

深海海底反射区声场的相干结构及其有源声呐估距

有源声呐在探测深海海底反射区的目标时,由于声线大掠射角弯曲且声速沿声线传播路径不断变化,造成了常规估距方法产生较大的误差。有效声速法是减小常规估距方法误差的有效途径,但由于需要预先计算空间每一位置点的"声线时延有效声速"对,复杂度高,实时性差。针对有效声速法的实时性问题,本文基于深海海底反射区声场的相干结构,提出一种改进的有效声速估距方法.首先指出深海声场能量沿声源出射角强弱相间变化及其引起的海底反射区离散声呐可探测区现象,并利用深海近水面声源的声线干涉效应解释了该现象的物理机理,建立了声呐可探测区与高能量声线的量化关系。在此基础上,计算声呐可探测区边界位置的"声线时延-有效声速"对,并线性拟合出可探测区所有位置点对应的值。经仿真验证,该方法与传统的有效声速法均可实现对常规估距方法估距误差的有效校正。虽然该方法估距精度较传统的有效声速法略有增大,但计算复杂度和计算时间显著减小,实时性好,具有良好的工程应用前景。      

基于时空滤波理论的低频声场干涉结构研究

基于时空滤波理论, 采用相干函数、功率响应函数、扩展分布函数、双频函数四个系统函数较波导不变量更全面地描述了低频声场的相干结构. 理论分析、仿真研究和海试数据处理都验证了低频声场存在稳定的干涉结构, 当目标由远及近、又由近及远做匀速直线运动时, 相干函数(LOFAR图)的干涉结构为一簇类双曲线; 功率响应函数的干涉图表征了简正波群时延的差分, 体现了其频散特性; 扩展分布函数能反映LOFAR图上干涉条纹的斜率大小; 而双频函数能体现相慢度的差分. 各个系统函数均能特别突出干涉结构的某方面的特征, 各有特色.