中国海洋声学研究进展

海洋声学研究声波与海洋的相互作用。它包括两方面的问题：一方面研究海洋环境对海中声场的影响；另一方面利用声波来探测海洋结构。海洋声学是一门应用范围很广、发展十分迅速的学科。本文简要介绍了海洋声学研究的某些进展，其中包括浅海与深海声场，浅海混响与低频声吸收。     

浅海混响的垂直相干性

浅海混响垂直相干特性是海洋混响研究中一个重要的课题。在射线简正波混响理论的基础上,给出了浅海混响垂直相关的一般表达式,并推导出了在均匀层浅海中混响垂直相关的简化解析表达式。同时,通过对数值计算及实验结果的讨论,研究了浅海混响垂直相关与混响时间、频率、声源深度、接收器深度、海底衰减特性及海底散射系数等参数的变化关系。研究表明,在海底散射满足可分离性的条件下,浅海海底混响的垂直相关与声源位置无关,随着混响时间和海底衰减的增加、或随频率、接收器间隔和海深的减小,混响的垂直相关增大。     

深海大接收深度海底混响研究

海洋混响对主动声纳工作性能的影响不可忽略,一直是水声学研究中的一个重要课题.在南海实验中获取了深海混响实验数据,包含近海底大接收深度的混响信号,其强度随时间变化存在锯齿形结构.为对混响数据进行深入分析,基于射线理论提出了一种深海海底混响模型,能够计算本地混响和异地混响强度,并可解释深海混响信号的产生过程.该模型首先对海底散射体进行网格式划分,然后根据每个网格内散射体产生混响信号的准确时间进行混响计算,对于传播路径丰富的深海环境比传统按圆环或椭圆环处理散射体的方式更加精确.在多种收发距离和接收深度条件下,将数值模拟结果与实验数据进行比较.结果表明:对于大接收深度,混响整体吻合较好;而靠近海面处的混响,二者的一致性下降.分析结果表明,使用的海底散射系数参数适用于该实验海区,同时验证了该散射系数模型对于小掠射角海底散射更加准确,对应深海大接收深度混响.     

采矿混响环境下超声波探测信号设计

针对深海采矿环境中混响干扰对水下微地形探测的影响,设计了一种可以抑制水下混响并提高探测精度的超声波发射信号。通过分析探测目标的接收信号模型,根据Neyman-Pearson准则得到目标回波探测模型的检测性能评价指标。基于高斯点目标最优检测理论,提出采矿混响环境下发射波形的能量谱设计方法,进而得到混响环境下超声探测信号的幅度谱。保持幅度谱分量恒定,利用信号相位谱与信号长度的关系设计了可以满足不同探测系统和探测环境需要的超声波探测信号。利用设计的超声波探测信号及常用声呐信号在模拟深海采矿环境下进行探测实验,结果表明所设计的超声波探测信号在探测精度及抗混响能力方面均优于常用声呐信号。      

基于简正波理论的浅海低频混响时间序列仿真

混响场时域信号序列的数值实现对主动声纳目标模拟器研制具有重要意义。本文所述适用于水平变化浅海环境混响时间序列的一种实现方法,基于现有的浅海简正波相干混响理论。所述方法的思路是通过相干散射场随机序列与发射信号的离散卷积来构成混响时域序列,文中以Pekeris均匀浅海环境为例,给出了相应的混响时间序列仿真结果,并对仿真结果的频谱特性、统计特性、强度衰减以及垂直相关特性等进行了系统检验。结果表明,由仿真混响时间序列所获得的混响特性与理论结果相符,可用于浅海混响序列的仿真。     

高频地波雷达多站浅海水深与海流反演

针对高频地波雷达浅海观测尝试中面临的参数反演新问题进行了研究.描述了高频地波雷达浅海探测机理,提出了多站高频地波雷达海洋参数反演方法,并推导了利用多站高频地波雷达回波多普勒谱反演浅海水深和矢量流的理论模型.通过三站探测和多站探测的反演数值模拟,表明了多站浅海探测的可行性和反演方法的有效性;通过分析苏北浅滩的三站高频地波雷达观测试验数据,表明了本文反演方法的实用性,这对于利用高频地波雷达进行浅海海洋环境探测的实际工程应用具有广阔的前景.     

浅海近程混响的振荡现象*

研究浅海近程混响特性对于评估和提高主动声纳性能具有重要意义。多次浅海混响实验显示,近程混响强度存在稳定的振荡现象,脉宽基本对振荡的幅度和周期没有影响。为解释这一现象,本文基于射线理论和小斜率近似给出了浅海近程混响模型,仿真与实测数据结果基本吻合。数值仿真结果表明：海底反射声场对单站声纳接收到回声信号的贡献远小于海底近垂向大掠射角散射声场的作用;混响强度振荡现象是海底近程散射声场的多途现象造成的,并由此给出了振荡周期与海深及收发深度的关系。     

浅海混响建模的声束跟踪理论

研究建立了基于声束跟踪理论的浅海混响强度计算方法和混响时间序列仿真方法。给出了混响强度计算的简要理论推导,并进行了模型计算值与实验值的比较。建立了一种混响时间序列仿真模型,给出了其实现框架和方法。结合实验数据与文献研究结果,进行了混响序列相关特性的检验与分析。结果表明:建立的混响强度计算模型能很好地进行浅海混响强度的预报,混响序列仿真模型能仿真具有不同包络分布的混响序列,且其相关特性符合实验与文献研究结果。      

浅海平均声场角度谱分析法与回波混响比的距离、深度结构

浅海平均声场角度谱分析法以Ray-mode理论为基础。引进平均声场角度谱的概念以后,可使浅海传播衰减、远程混响和空间相关等平均特性的一些计算变得较为简单。用该法在浅海负梯度和均匀层情况下,计算了远程混响和回波、混响比的距离、深度结构。理论证明,在非均匀层浅海远程探测中,深度可变声呐比固定声呐有较为明显的优越性。本方法和结果将有助于分析小掠角海底(面)散射的角度、频率依赖关系。     

浅海海底界面混响平均强度特性仿真

海底粗糙界面是产生混响的主要因素之一。本文通过理论分析和数值仿真的方法,根据浅海全波动混响模型对不同海底粗糙界面所引起浅海混响平均强度特性进行研究,主要考虑Goff-Jordan谱、Gauss谱和指数谱三种不同粗糙界面条件下的海底反向散射强度和混响平均强度特性。计算结果表明:海底粗糙界面会引起海底反向散射强度的频率特性的差异,进而导致海底平均混响强度的频率特性的差异,但随入射角度的变化不大。即使界面起伏的方差和相关长度相同,不同的粗糙度谱也会引起平均混响强度的差异。     

Theoretical analysis and simulation of conjugate heights for dual-conjugate AO system in lidar

A multi-conjugate adaptive optics （MCAO） can offer a possibility of widening field of view （FOV） characterized by the isoplanatic angle, and the choose of conjugate height becomes a basic problem for MCAO, which influences the size of iosplanatic angle. Considering the application of lidar, the isoplanatic angle＇s expressions of two deformable mirrors （DMs） MCAO for uplink and downlink are deduced. The effects of conjugate heights for dual-conjugate AO are thoughtfully discussed, and the isoplanatic angles are further analyzed. The results show that the isopanatic angle varies with the conjugate height and reaches the maximum as the conjugate height is at the optimal altitude. Moreover, the optimal conjugate height changes with the propagation distance.     

Quick single-photon detector with many avalanche photo diodes working on the time division

Due to the limit of response speed of the present single-photon detector, the code rate is still too low to come into practical use for the present quantum key distribution (QKD) system.A new idea is put up to design a quick single-photon detector.This quick single-photon detector is composed of a multi-port optic-fiber splitter and many avalanche photo diodes (APDs).Au of the ports with APDs work on the time division and cooperate with a logic discriminating and deciding unit driven by the clock signal.The operation frequency lies on the number N of ports, and can reach N times of the conventional single-photon detector.The single-photon prompt detection can come true for high repetition-rate pulses.The applying of this detector will largely raise the code rate of the QKD, and boost the commercial use.     

Characterization of PolyA and PolyC mismatches by Raman spectroscopy

A.C mismatches are studied by Raman spectral characterization of PolyA, PolyC, and their equimolar complex in solution of 0.14 mol/L Na ,pH7.0.Experimental results show that A·C mismatches occur to be A/B (mainly A) conformers, and unlike Watson-Crick base pairing, this kind of mismatches is stabilized by only one hydrogen bond involving cytosine N4H2 and adenine N7.The formation of A·C complex makes the base stacking interactions much stronger, and conformation of the backbone more ordered, which leads to obvious Raman hypochromic effect with some shifts in corresponding bands.     

Improvement of four-wave mixing-based wavelength conversion efficiency in dispersion shifted fiber by 40-GHz clock pumping

40-GHz clock modulated signal as a pump to improve the efficiency of four-wave mixing (FWM)-based wavelength conversion in a 26.5-km dispersion shifted fiber (DSF) is investigated. The experimental results demonstrate that the conjugated FWM component has higher intensity with the clock pumping than that with the continuous-wave (CW) light pumping. The improvement of FWM-based wavelength conversion efficiency is negligible when the pump power is less than Brillouin threshold. But when the pump power is greater than Brillouin threshold, the improvement becomes significant and increases with the increment of pump power. The improvement can increase up to 9 dB if pump power reaches 17 dBm.     

Multi-pulse operation of a Kerr-lens mode-locked femtosecond laser

Our experimental results show that the presence of a proper amount of negative group velocity dispersion is essential to multi-pulse operation of a Kerr-lens mode-locked femtosecond laser. We demonstrate that the pulse separations and the number of pulses contained within a cavity round trip are strongly dependent on the initial perturbations. The results allow us to get a better understanding on the influences of the convoluted self-phase modulation and intra-cavity dispersions on the stable multi-pulse oscillation in a Kerr-lens mode-locked femtosecond laser.     

Propagation of Helmholtz-Gauss beams in weak turbulent atmosphere

Based on the Rytov approximation of light propagation in weak turbulent atmosphere,the closed-form expressions of field and average irradiance of each one of the four fundamental families of Helmholtz-Gauss (HzG)beams:cosine-Gauss beams,stationary Mathieu-Gauss beams,stationary parabolic-Gauss beams,and Bessel-Gauss beams,which are propagating in weak turbulent atmosphere,are obtained.The results show that the field and average irradiance can be written as the product of four factors:complex amplitude depending on the z-coordinate only,a Ganssian beam.a factor of complex phase perturbation induced by atmospheric turbulence,and a complex scaled version of the transverse shape of the non-diffracting beam.The effect of weak atmospheric turbulence on irradiance distribution of the HzG beam can be ignored.     

An effective method for reducing speckle noise in digital holography

An effective method for reducing the speckle noise in digital holography is proposed in this paper.Different from the methods based on classical filtering technique,it utilizes the multiple holograms which are generated by rotating the illuminating light continuously.The intensity images reconstructed by a series of holograms generated by rotating the illuminating light possess different speckle patterns.Hence by properly averaging the reconstructed intensity fields,the speckle noises can be reduced greatly.Experimental results show that the proposed method is simple and effective to reduce speckle noise in digital holography.     

Influence of purity of HfO2 on reflectance of ultraviolet multilayer

The impurities in two kinds of HfO2 materials and in their corresponding single layer thin films were determined through glow discharge mass spectrum technology and secondary ion mass spectrometry(SIMS) equipment respectively.It was found that ZrO2 was the main impurity in the two kinds of HfO2 either in the original HfO2 materials or in the electron beam deposited films.In addition,the difference of Zr content in the two kinds of HfO2 single laver films was much laxger than that of the other impurities such as Ti and Fe.which showed that it was just ZrO2 that made the difference between the optical performance of the film products including the two kinds of HfO2.With these two kinds of HfO2 and the same kind of SiO2.we deposited HfO2/SiO2 multilayer reflective coatings at the wavelength of 266 nm.Experimental results showed that the reflectances of these two mirrors were about 99.85% and 99.15% respectively,which agreed well with the designed results what were based on the optical constants obtained from the corresponding single layer thin films.     

Speech enhancement by array crosstalk resistant ANC and spectrum subtraction

Microphone array-based speech enhancement has great importance for speech communications and speech recognition. To reduce the aperture of the microphone array and to increase the effect of the speech enhancement will greatly broaden the application areas of the microphone array. An array crosstalk resistant adaptive noise cancellation method is therefore presented. And then an improved spectral subtraction algorithm is further cascaded to obtain better enhancement results. Theoretic analysis and experiments indicate that the proposed scheme needs only a very small microphone array while it simultaneously achieves a higher SNR improvement. Besides, the proposed scheme can be used in many noisy environments and is easy for real-time implementation.