大气湍流对空间光耦合至单模光纤的影响

 根据弱湍理论,考虑湍流强度闪烁、孔径平均效应、湍流波前畸变及耦合系统跟踪误差的影响,导出了空间光到单模光纤的平均耦合效率及耦合功率起伏方差模型。根据Hufnagel-Valley 5/7折射率结构模型,对下行连接及上行连接的平均耦合效率及耦合功率起伏进行了研究。仿真结果表明,对于下行连接,随着地面接收天线孔径的增大,相对功率起伏先是由于孔径平均效应而减小,而后由于湍流波前畸变的影响而逐渐增大;对于上行连接,可忽略湍流波前畸变及孔径平均效应的影响。     

Effect of non-kolmogorov turbulence on angle-of-arrival fluctuations of starlight

We revised a non-Kolmogorov turbulent power spectrum for the refractive-index fluctuations based on the consistency between the structure function and its power spectrum and the experimental data of recent lidar measurements. We investigate the joint influence of Kolmogorov turbulence from the ground up to 6 km and non-Kolmogorov turbulence above 6 km on the fluctuations in the angle of arrival (AOA) of starlight. WE show that the AOA fluctuations of starlight are mainly determined by Kolmogorov turbulence nearby the receiver. Non-Kolmogorov turbulence is responsible for 20–40% of the total AOA fluctuations for different apertures of the receiver. In addition, the AOA fluctuations induced by non-Kolmogorov turbulence depend on the receiver aperture, outer scale, and intensity of non-Kolmogorov turbulence.     

湍流大气中光波闪烁的圆环孔径平均因子

利用弱起伏条件下球面波在大气湍流中传输的光强起伏(闪烁)理论和圆环孔径滤波函数,获得了圆环孔径平均因子的精确表达式。将圆环孔径平均因子关于一个无量纲参量(孔径半径与菲涅耳尺度的比值)拟合成2阶多项式,再找出多项式系数与孔径内外径之比的函数关系,获得了圆环孔径平均因子关于该参量和孔径内外径之比的函数关系式。通过误差分析确定了拟合关系式的适用范围,在此范围内,拟合式与精确式的相对误差小于25%。分别用Tatarskii谱和修正Hill谱分析了湍流内尺度对圆环孔径平均因子的影响,结果显示:在其他条件不变的情况下,内尺度越大,孔径平均效应相对越小。     

Numerical modelling of the pump-to-signal relative intensity noise transfer in two-pump fibre optical parametric amplifiers

An accurate numerical model to investigate the pump-to-signal relative intensity noise (RIN) transfer in two-pump fibre optical parametric amplifiers (2-P FOPAs) for low modulation frequencies is presented. Compared to other models in the field, this model takes into account the fibre loss, pump depletion as well as the gain saturation. As a result, the model allows to include a wide range of practical circumstances, both in and beyond the undepleted pump regime, related to different applications of FOPAs. In the small-signal or undepleted pump regime, the model predicts the ripples of the RIN spectrum very well and yields better results than those of other models. It is shown that beyond the small-signal regime, pump power variations do not remain unchanged over the length of the amplifier and for high signal powers, when the FOPA saturates, minimum pump-to-signal RIN transfer occurs. The results of the model are also compared with the available experimental data in the field and a very good agreement can be seen.     

Reconstruction of ultrasonic fields by deconvolving the hydrophone aperture effects. II. Experiment

The correction of the averaging effect of finite-size hydrophones is at present realized by various methods which are based on idealized models for the sound field and thus are valid only to a limited extent. In the present article three methods for inverting the aperture effects of the hydrophone viz. the Wiener method, the power spectral equalization (PSE) method and the maximum a posteriori (MAP) method, are experimentally tested and their efficiency is compared. It is shown that both for a rectangular and for a circular hydrophone aperture all three methods are suitable to reconstruct complex sound field structures from spatially averaged data. The results of this paper confirm the finding of Part I [Ultrasonics 39 (2002) 603], i.e. the quality of reconstruction depends on the spatial frequency bandwidth of the sound field investigated. Good quality reconstructions are obtained when the effective dimensions of the hydrophone's sensing area are directly derived from the spatial frequency spectrum of the measurement signal. The Wiener method and the MAP method are superior to the PSE method, since the PSE filter does not take phase information into account. For the Wiener and the MAP method normalized correlation coefficients, r(pp), between the reconstructed sound pressure field and that of a reference measurement of typically 0.97 are obtained provided that the axial distance is not too small. Although the working frequency used for the experiments is rather low, it can be expected that the results obtained are applicable at higher frequencies provided that the complex spatial transfer function is determined with the necessary high precision.     

Simulation of a Kolmogorov phase screen

Two new methods for modelling Kolmogorov phase fluctuations over a finite aperture are described. The first method relies on the incorporation of subharmonics in order to model accurately the low frequencies of the Kolmogorov spectrum. The second method provides a less accurate, but much faster method for simulating the Kolmogorov spectrum by using a midpoint displacement algorithm used in computer graphics.     

Speckle interferometry through small multiple apertures: Michelson stellar interferometer and aperture synthesis in optics

The use of a Michelson stellar interferometer or multiple-hole interferometers is considered as a special case of speckle interferometry. Formulas are given for the power spectrum of the interference pattern produced by the multiple apertures. The limitation in stellar magnitude is shown to increase with the number of apertures, while it is independent of the telescope size when a large single aperture is used.     

开孔对封闭空间共振特性的影响

研究了开孔对封闭空间声场的影响。通过将孔内振动空气等效为点源,用模态展开法建立了开孔封闭空间的声场模型,计算了开孔封闭空间高阶共振频率和在共振频率激励下的声压分布。结果显示:开孔等效于孔处声质量减小,一般使得开孔封闭空间的共振频率增加;但当孔位于某模态节点时,由于该阶模态与任一模态在开孔处未发生耦合,该模态共振频率不变;由于在开孔区域对应于激励频率的模态声压和其余各阶模态声压之和的相位相反,高阶共振频率激励下靠近小孔位置的声压减小。因此,开孔对封闭空间声场有影响,其影响程度与开孔位置和开孔尺寸有关。      

Simulation of a Kolmogorov phase screen

Abstract

Two new methods for modelling Kolmogorov phase fluctuations over a finite aperture are described. The first method relies on the incorporation of subharmonics in order to model accurately the low frequencies of the Kolmogorov spectrum. The second method provides a less accurate, but much faster method for simulating the Kolmogorov spectrum by using a midpoint displacement algorithm used in computer graphics.     

Role of shape in middle-infrared transmission enhancement through periodically perforated metal films

We report experimental results on the role of aperture shape in middle-infrared enhanced transmission through a metal film perforated with a periodic aperture array. The transmission spectrum is highly dependent on the aperture shape. When the aperture shape changes from circular to cross-dipole or the cross-dipole arm length increases, the transmission spectrum exhibits a large redshift. The enhanced transmission is proposed to be an interplay of surface plasmon polaritions at the metal surface and localized surface plasmons (LSPs) inside the apertures. The shift of the transmission bands is proposed to be due to the redshift of the LSP resonance spectrum of the individual apertures. The role of shape in enhanced transmission in the cascaded structure is also studied.     

Beamforming using spatial matched filtering with annular arrays (L)

A linear array beamforming method for ultrasonic B-mode imaging using spatial matched filtering (SMF) and a rectangular aperture geometry was recently proposed Kim et al., [J. Acoust. Soc. Am. 120, 852-861 (2006)]. This letter extends those results to include circularly symmetric apertures. SMF applied to annular arrays can improve the lateral resolution and echo signal-to-noise ratio as compared with conventional dynamic-receive delay-sum beamforming. At high frequencies, where delay and sum beamforming is problematic, SMF showed greatly improved target contrast over an extended field of view.     

不同形状孔阵屏蔽效应的分析

 屏蔽效应的研究对于防御电磁干扰，保证电子系统的正常工作是非常重要的。计算了屏蔽腔上开有不同形状的孔阵时对屏蔽效应的影响，分析比较了各种不同孔间距对屏蔽效应的影响。计算结果表明，圆形孔阵的穿透系数最小，方形孔阵次之，蜂巢形孔阵较大，但频谱分布基本一致。而对于哑铃形孔阵，由于孔面形状差异较大，孔面处磁流分布变化也较大，不仅穿透系数比前三者都大，而且高频时穿透系数的频率分布也发生较大改变。另外，随着孔阵中孔间距变大，孔之间的互耦减弱，耦合进屏蔽腔内的场强也变小。     

湍流特征光学测试对比

在10℃～230℃温差下,对大气相干长度r0分别采用夏克-哈特曼的到达角起伏法、差分像运动法、波面法三种测量法和四象限探测器进行了测试和对比;对折射率结构常量Cn2及闪烁功率谱分别采用夏克-哈特曼和光电倍增管进行对比.实验结果表明:对于r0,在强湍流时四象限探测器比夏克-哈特曼的稳定性明显降低,且对夏克-哈特曼三种方法,差分像运动法可克服设备抖动等问题,但引入了方向上不一致的问题,波面法可有效避免该问题;对于Cn2,夏克-哈特曼比光电倍增管测量更稳定,拟合相关系数高达0.96;对于闪烁功率谱,由于噪音影响,在200℃时夏克-哈特曼比光电倍增管测得的最大频率高15Hz;最后,通过对夏克-哈特曼子孔径的闪烁功率谱分析得出,若同一子孔径入射光强不在CCD响应的线性区间时无法准确测量闪烁功率谱,否则可通过不同子孔径可完成湍流均匀性的测量.这将为湍流池提供最优的测试方法及理论依据.     

Low-frequency intensity fluctuations of radiation from a distant source in the infrared region of the spectrum. II

The results are considered of signal-fluctuation measurements of a thermal source in the infrared region of the spectrum on an extended surface path. Experimental averaging functions are obtained for receiver-transmitting apertures in the case of large limits of variation of the latter. The dependence of the dispersion of the fluctuations on the wavelength of the radiation is investigated over the range 1.06 to 3.9 .Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 111–115, February, 1975.     

抑制孔径间距误差影响的相干场成像质量提升方法研究

相干场成像基于激光发射孔径间距相等、频谱相等的基本假设, 迭代计算频谱重建高分辨图像, 实际应用中不可避免的激光发射孔径间距误差是影响成像质量的重要因素. 针对发射孔径间距误差造成的成像质量下降问题, 提出一种抑制孔径间距误差影响的成像质量提升方法. 首先分析了孔径间距误差对激光回波频谱和成像质量的影响机理; 推导得到了频谱误差迭代模型; 理论上构建了孔径间距误差对信号频谱和成像质量的零影响条件方程; 提出一种线性规划方法求解成像质量零影响条件方程, 得到孔径间距误差最优化分布矩阵; 实际应用中基于该最优化误差矩阵合理分配各孔径间距误差, 就可抑制孔径误差对成像质量的影响. 实验验证了该方法的正确性和有效性. 结果表明: 该方法可提升成像质量评价指标斯特列尔比近1倍; 所提方法可便捷有效地抑制孔径误差对成像质量的影响. 该研究为实际相干场系统成像质量的提升和发射阵列孔径间距精度设计提供了理论指导.     

Aperture averaging effects in stellar scintillation

Stellar scintillation has been measured for apertures as low as 0.012 m by using photoncounting techniques. The scintillation measurements have been made for various aperture sizes in quick succession in order to determine experimentally the effects of aperture averaging. The results of the experiment are compared with the predictions of a model proposed earlier in the literature to quantify the phenomenon.     

近场衍射图样的抽样定理处理方法

提出近场衍射图样的抽样定理处理方法,在近场衍射中,孔径和衍射场分布为或近似为空间有界物,在其频域中可以比较精确地恢复衍射的分布。衍射场频谱带宽的近似值由频谱的幅值大于中心频谱极大幅值的１％的频率范围所确定。     

Low Earth orbit satellite-to-ground optical scintillation: comparison of experimental observations and theoretical predictions

Scintillation measurements of a 1064 nm laser at a 5 kHz sampling rate were made by an optical ground station at the European Space Agency observatory in Tenerife, Spain while tracking a low Earth orbit satellite during the spring and summer of 2010. The scintillation index (SI), the variance of irradiance normalized to the square of the mean, and power spectra measurements were compared to theoretical predictions based on the Kolmogorov spectrum, the Maui3 nighttime turbulence profile, weak scintillation finite-beam wave theory, included receiver, and source aperture averaging with no free-fitting parameters. Good agreement was obtained, not only for the magnitude of the observed fluctuations, but also for the corresponding elevation angle dependence and shape of the power spectra. Little variation was seen for the SI between daytime and nighttime links. For all elevation angles, ascending and descending, the observed scintillation over extensive regions of the atmosphere is consistent with log-normal statistics. Additionally, it appears from the results presented here that the nighttime turbulence profile for the atmosphere above the observatory in Tenerife is similar to that above Haleakala in Maui, Hawaii.     

Particle Size Analysis by Transmission Fluctuation Spectrometry: Experimental Results Obtained with a Gaussian Beam and Analog Signal Processing

Transmission fluctuation spectrometry (TFS) is a method for the analysis of particle size distributions based on the statistical fluctuations of a transmission signal. Complete information on the PSD and particle concentration can be retrieved by a special transformation of the transmission signal, whereby the expectancy of the transmission square (ETS) is determined after the signal has been subjected to a procedure of spatial and temporal averaging. By varying the averaging parameters over a wide range, a spectrum of ETSs is obtained and introduced into a linear equation system, which yields the PSD. In the experimental realization presented here, variable temporal averaging is realized in the frequency domain with a series of low pass filters at different cutoff frequencies while spatial averaging inevitably occurs as the particles pass through a focused Gaussian beam of finite cross section. Experimental results on spherical particles (glass beads) and non‐spherical particles (SiC) are presented. The PSDs are resolved in 30 intervals within a particle size range from 1–1000 μm, employing a modified Chahine inversion algorithm. So far, the measurements are limited to moderate particle concentrations. Some influences affecting the measurements, especially for higher particle concentrations, are discussed in detail.