The possibility of reconstruction of two-dimensional random inhomogeneities in a shallow sea by frequency shifts of the spatial interference structure of the sound field

Fluctuations of frequency shifts of the spatial interference structure of the sound field in the oceanic waveguide, caused by a two-dimensional field of a random perturbation, are described. Various schemes of observation point spacing are considered. The possibility is shown to reconstruct the spatial spectrum of waveguide perturbations by measuring the spatial spectrum of the frequency shift of the interference pattern. The results of the theoretical treatment are illustrated by the examples of background internal waves and bottom roughness. The sensitivity of monitoring based on measurements of frequency shifts of the interference structure of the sound field is estimated. For medium perturbations by background internal waves, the fluctuations of liquid layer vibrations, sound speed, and temperature, which are minimum detectable by frequency shifts of the interference pattern, were estimated.     

白光频谱干涉原理及其应用

本文详细讨论了白光频谱干涉的基本原理及其在微位移测量中的应用,并将其与传统白光干涉以及时域迈克尔逊干涉进行了比较,探讨了白光频谱干涉在大学物理理论与实验教学中的重要应用前景。     

浅海矢量声场干涉结构形成机理及试验研究

浅海低频声场的微观结构特征在于具有可用波导不变量表征的 稳定空间-频率干涉结构.声场兼具标量场和矢量场, 波导条件下二者联合决定声场的全部特性. 本文研究浅海声场空频干涉结构的矢量场特征. 理论分析了声压谱、动能密度谱、声强流谱等矢量场干涉结构的形成机理, 探讨了矢量场干涉结构的波导不变量表征, 数值仿真研究了Pekeris波导中能量和能流密度的干涉特性, 进行了宽带声源辐射矢量声场干涉特性及表征的海上试验.实测结果与理论、仿真分析有较好的一致性. 研究结果表明: 中近程和中远程声场均能模态相干, 有稳定的空频干涉结构, 并且矢量声场空频干涉结构存在多种形式, 除各种能量和能流密度谱图外, 相干系数谱也呈现干涉特征, 这些形式的空频干涉结构均可用波导不变量理论有效表征. 关键词： 矢量声场 干涉结构 波导不变量 浅海低频声场     

Inter-cell spectrum efficiency improvement technology

Improvement in inter-cell spectrum efficiency is a valuable research topic in mobile communication system, which affects cell edge user experience especially. According to current research results, there are three methods to deal with inter-cell interference to improve inter-cell spectrum efficiency, including inter-cell interference randomization, inter-cell interference cancellation and inter-cell interference coordination. This paper analyzes three important inter-cell spectrum efficiency improvement technologies, soft frequency reuse (SFR), uplink power control, and downlink coordinated multi-point transmission/reception (CoMP), and relative research progress.     

Method of acoustic sweep monitoring of oceanic inhomogeneities (Review)

Theoretical grounds of the new method of monitoring of the temporal variability of oceanic inhomogeneities, which uses the data on frequency shifts of interference maxima of the sound field, were described. The method is free of limitations on both the resolution of signals coming in individual modes (rays) and the adiabatic approximation underlying the conventional methods of inhomogeneity reconstruction. The monitoring sensitivity was estimated, which allows us to estimate minimum detectable changes in the speed of sound by measurement data on frequency shifts of local maxima. Experimental data on shifts of the frequency spectrum of a broadband low-frequency signal on a stationary path in a shallow sea were presented. On their basis, the possibility of applying this method to diagnose tidal variations was shown. Within a numerical simulation, model reconstruction of the frequency spectrum of background internal waves was considered on the basis of the data on measurements of the spectrum of frequency shifts of the interference maximum. The results of the spectrum reconstruction with and without focusing of the conjugate wave field are presented. The problems of monitoring stability and efficiency with respect to the interference pattern formed by various groups of modes were discussed.     

Coupled field induced conversion between destructive and constructive quantum interference

We study the control of quantum interference in a four-level atom driven by three coherent fields forming a closed loop. The spontaneous emission spectrum shows two sets of peaks which are dramatically influenced by the fields. Due to destructive quantum interference, a dark line can be observed in the emission spectrum, and the condition of the dark line is given. We found that the conversion between destructive and constructive quantum interference can be achieved through controlling the Rabi frequency of the external fields.     

Frequency shifts of sound field maxima in oceanic waveguides

The frequency shifts of sound field maxima that are caused by the variability of the medium in an oceanic waveguide are described. The condition under which the time spectrum of the frequency shifts of interference maxima is proportional to the time spectrum of fluctuations in the dispersion characteristic of the waveguide is determined.     

基于矢量线阵的目标低频线谱提取方法

主动声呐系统常采用线性调频信号探测水下目标。利用分数阶傅里叶变换处理线性调频回波时,可在低信噪比和强混响背景下获得目标参数的有效估计。主动声呐系统常通过增大发射信号的时间带宽积来提升系统的距离分辨力和速度分辨力,但却带来了计算复杂度的显著增加,尤其是奈奎斯特采样下基于分数阶傅里叶变换的回波处理方法,导致数据处理时间难以满足功耗和体积受限的无人水下航行器平台的实时性要求。针对此问题,该文提出分数阶傅里叶变换的带通采样实现方法,通过对线性调频信号时频特性直线在分数阶域的投影进行修正,使利用分数阶傅里叶变换方法处理带通采样的回波数据时,可获得正确的目标参数估计。计算机仿真数据和无人水下航行器湖试数据处理结果验证了分数阶傅里叶变换的带通采样实现方法的正确性,数据处理时间能够满足无人水下航行器平台处理的实时性要求。     

压缩真空中的三能级原子的自发辐射

对压缩真空与Ｖ型三能级原子的相互作用作了理论分析,探讨了量子干涉效应对压缩真空中原子自发辐射的影响。当原子的上能级简并时强的量子干涉效应影响原子的定态和定态出射光谱,使它们对原子的初态敏感。这种情况下,压缩使谱线趋于关于压缩真空中心频率对称,量子干涉使谱线趋于不对称。     

Third-order ghost interference with thermal light

A scheme for third-order ghost interference with thermal light is proposed. The visibility and resolution of the interference fringe related to the bandwidth of the spatial frequency spectrum of the source are analysed. The results show that the visibility of the third-order ghost interference fringe is much higher than that of the second-order one.     

运动目标辐射声场干涉结构映射域特征研究

浅海低频声场呈稳定而显著的干涉特征,并蕴含声源状态和波导特性等信息.本文研究运动目标辐射声场干涉结构的简化映射以及映射域能量分布对目标运动状态的指示特征.理论分析了声压场和矢量声场空(时)频干涉谱图的二维傅里叶变换映射特征,推导了匀速运动目标时频干涉谱图映射域能量脊斜率与波导不变量以及距变率、航向角的关系式,证明了映射域脊斜率符号、脊斜率绝对值变化等对目标来袭或远离以及目标威胁程度的指示,并进行了数值仿真和海上试验研究.实测结果与理论、仿真分析有较好的一致性.研究结果表明:二维傅里叶变换可将声压场和矢量场时(空)频谱图干涉结构简化,匀速运动目标辐射声场干涉结构映射域的能谱脊斜率、距变率、航向角与波导不变量有解析关系式,声强谱、动能密度谱、声强流谱等声场干涉结构经映射后更为一致,映射域脊能简明的指示目标运动状态和威胁程度.     

全光纤干涉式结构中传感模式仿真分析

全光纤干涉式传感结构中包层模场与外界物理量作用,携带被感测信息,因此对包层模的研究是设计制作和提高该类传感器传感性能的关键.利用有限差分光束传播法获得传感光纤不同长度和不同芯径比时传感器对应的光谱,通过傅里叶变换获得其干涉频谱,计算出各主要参与干涉的包层模组的有效折射率,利用色散方程确定对应包层模.理论仿真结果显示,随着传感部分光纤长度增加,参与干涉的包层模式随之增加,并且向高阶模式变化,光谱变得稠密,是多阶包层模干涉的叠加,传感器输出干涉谱的自由光谱范围变小.随着输入光纤与传感光纤芯径比变化,会明显改变纤芯包层功率分布,同时,芯径比增大也会增加参与干涉的包层模种类和阶数.     

高重频超宽谱短电磁脉冲对GPS接收机干扰

超宽谱短电磁脉冲的频谱可覆盖GPS工作频带,对GPS接收机的正常工作存在干扰威胁。从理论分析、数值仿真和实验方面研究了高重频超宽谱短电磁脉冲对GPS接收机干扰效果与脉冲参数之间的依赖关系。结果表明：若脉冲重复频率整数倍能落入GPS接收机工作频带内,高重频超宽谱短电磁脉冲对GPS接收机干扰效果较强,且达到相同干扰效果时所需脉冲幅值随重频增大而等比例减小,即脉冲幅值与重频乘积保持为常数;若重频整数倍偏离GPS接收机工作频带,高重频超宽谱短电磁脉冲对GPS接收机干扰效果不明显。     

高重频超宽谱短电磁脉冲对GPS接收机干扰

超宽谱短电磁脉冲的频谱可覆盖GPS工作频带,对GPS接收机的正常工作存在干扰威胁。从理论分析、数值仿真和实验方面研究了高重频超宽谱短电磁脉冲对GPS接收机干扰效果与脉冲参数之间的依赖关系。结果表明:若脉冲重复频率整数倍能落入GPS接收机工作频带内,高重频超宽谱短电磁脉冲对GPS接收机干扰效果较强,且达到相同干扰效果时所需脉冲幅值随重频增大而等比例减小,即脉冲幅值与重频乘积保持为常数;若重频整数倍偏离GPS接收机工作频带,高重频超宽谱短电磁脉冲对GPS接收机干扰效果不明显。     

Eliminate the influence of zero-order diffraction for wavefront reconstruction with selectable magnification in digital holographic

The objective surface is considered as the scattering plane; frequency spectrum of a digital hologram transmission light is studied when the reference light and reconstruction wave are spherical waves. The relation between relevant parameters and the object light, conjugate object light and zero-order diffraction wave frequency spectrum distribution. The research results show that: each frequency spectrum of the diffracted wave broadens to various degrees as the radius of the reconstruction wavefront decreases, and frequency aliasing is generated under certain conditions. Based on the research results, a new method that carries out high-pass filtering processing without zero-order diffraction interference upon digital hologram is proposed and the reconstruction experimental proof for eliminating the interferential changeable magnification wave is given.     

空间外差光谱仪干涉图非均匀性校正研究

空间外差光谱仪在研制加工过程中,由于加工误差及胶合误差会使CCD接收到的干涉图存在光强分布不均匀现象,降低了变换光谱的准确性。基于对空间外差光谱仪干涉图光强非均匀性的产生机制与特点分析基础上,提出了一种干涉图非均匀性校正方法,该方法通过对实际干涉图进行单调分解、分段归一化及重新组合过程求解出光强分布函数,然后将变换光谱与光强分布函数倒数的傅里叶变换结果进行卷积来获得非均匀性校正后的光谱,最后将校正光谱进行逆傅里叶变换从而实现干涉图的非均匀性校正。将此方法应用于空间外差试验仪的近红外实测单色光干涉图的非均匀性校正,结果显示,该方法可以有效改善干涉图光强分布的非均匀性,抑制变换光谱的边频信号,通过与仿真的理想光谱对比,1 571和1 572 nm光谱校正前后噪声的减小率分别达到40.7%和24%,提高了光谱信噪比和准确性。     

利用Doppler-warping变换的水下对空运动声源的线谱探测EI北大核心CSCD

提出了一种利用Doppler-warping变换的水下对空运动声源的线谱探测方法。该方法首先利用Doppler-warping变换线性化目标线谱的多普勒相位,再提取线性化后的目标线谱点,结合波束形成算法实现对目标的探测。仿真和海试数据处理结果表明,与直接提取目标线谱实现探测的方法相比,所提出方法有以下几个优势:(1)将目标线谱多普勒相位线性化,线谱跟踪更为稳定连续;(2)将目标线谱频带展宽的能量重新集中到原频率附近,在低信噪比环境下能更好地提取目标线谱;(3)将干扰线谱瞬时频率非线性化,能更好地实现干扰线谱的分离。海试数据处理结果表明,实现了对远至16.8 km处空中运动目标的水下有效探测,探测能力与抗干扰能力明显增强。     

Two-photon interference in the presence of absorption

Experiments on two-photon interference are discussed in the case when there is absorption of all the modes participating in the process of spontaneous parametric down-conversion (SPDC) of light. The objects of investigation are 10-to 80-Å-thick ultrathin gold films deposited on fused-silica substrates. Conditions are determined under which the effect of absorption of the signal and pump waves on the interference pattern is small. It is shown that, under these conditions, the visibility of the interference pattern and the shape of the frequency-angular spectrum at the signal frequency are determined by the optical parameters of the medium at the idler frequency, which belongs to the near-infrared region.     

Angular spectrum redistribution from a real image of a light as a secondary source

Coherence theory predicts that the correlation in the fluctuations of a source distribution can cause frequency shifts in the spectrum of the emitted radiation, even when the source is at rest relative to the observer. Recently we measured angular spectrum redistribution, or frequency shifts from a real image of a point source, and further verified the coherent interference effect of a finite-band source.     

Acoustic monitoring of background internal waves with the use of the correlation method for measuring the frequency shifts of interference maxima

A numerical experiment is carried out to demonstrate the reconstruction of the frequency spectrum of background internal waves with the use of the correlation method for measuring the frequency shifts of interference maxima. The method is based on monitoring the behavior of the frequency shift for the maximum of the cross-correlation function of signal spectra received at different instants of time. The noise immunity of the correlation method is analyzed in comparison with the direct method based on monitoring the frequency shift of a chosen maximum of the interference pattern.