湍流信道下差分索引移位键控直流偏置光OFDM

针对现有光无线正交频分复用（OFDM）索引调制均需接收端已知信道状态信息的问题，通过构建满足差分运算的时频弥散矩阵进行索引映射，提出了一种差分索引移位键控光OFDM方案。推导出了该方案在最大似然准则下的平均误码率上界，并与子载波索引移位键控直流偏置光OFDM(SISK-DCO-OFDM)系统进行了性能对比。其次，依据差分信号的特点，构造了一种适用于机器学习的差分信号特征向量，并结合径向基神经网络提出了一种多分类检测器，有效降低了译码复杂度。结果表明，所提差分索引方案有效避免了复杂的信道估计，同时在高信噪比下获得了近似SISKDCO-OFDM的误码性能。所提检测器取得了近似差分最大似然检测算法的误码性能，而且复杂度大大降低，例如当子载波块长度为2和4时，所提检测器的复杂度分别降低了16.67%和70%。     

DWDM混合光学系统中帧间和信道内四波混频效应的抑制

本文提出了光码多分址（CDMA）和光密集波分复用（DWDM）的混合系统,全面研究了四波混频（FWM）的影响。在这个系统中,主要存在两个四波混频问题：包括多址干扰（MAI）和码间干扰（ISI）的帧间四波混频和信道内四波混频。结果表明,综合考虑信道间和信道内四波混频的影响,最佳发射功率可选为18 dBm。当发射功率大于18 dBm时,混合系统的误码率（BER）将增加。基于此,本文提出了一种电光相位调制器（EOPM）模块,将其放置在波分复用器之后,通过抑制信道内四波混频的影响,同时调制所有波长信号的相位,从而增加混合系统的非线性容限,这极大地改善了基于OOK传输的光学CDMA-DWDM混合系统的性能。此外,由于多对角线（MD）结构具有零互相关特性,通过使用多对角线识别序列码可以减少多址干扰的影响。结果还表明,CDMA技术与色散相结合有助于降低信道间四波混频的影响。此外,识别序列码间隔在减轻码间干扰中起着至关重要的作用,如结果所示,当识别序列码间隔压缩至比特持续时间的25%时,可以避免码间干扰,此时所提出的混合系统的性能最佳。     

DWDM混合光学系统中帧间和信道内四波混频效应的抑制（英文）

本文提出了光码多分址(CDMA)和光密集波分复用(DWDM)的混合系统,全面研究了四波混频(FWM)的影响。在这个系统中,主要存在两个四波混频问题:包括多址干扰(MAI)和码间干扰(ISI)的帧间四波混频和信道内四波混频。结果表明,综合考虑信道间和信道内四波混频的影响,最佳发射功率可选为18 d Bm。当发射功率大于18 d Bm时,混合系统的误码率(BER)将增加。基于此,本文提出了一种电光相位调制器(EOPM)模块,将其放置在波分复用器之后,通过抑制信道内四波混频的影响,同时调制所有波长信号的相位,从而增加混合系统的非线性容限,这极大地改善了基于OOK传输的光学CDMA-DWDM混合系统的性能。此外,由于多对角线(MD)结构具有零互相关特性,通过使用多对角线识别序列码可以减少多址干扰的影响。结果还表明,CDMA技术与色散相结合有助于降低信道间四波混频的影响。此外,识别序列码间隔在减轻码间干扰中起着至关重要的作用,如结果所示,当识别序列码间隔压缩至比特持续时间的25%时,可以避免码间干扰,此时所提出的混合系统的性能最佳。     

基于水声信道传播时延排序的分层空时信号检测

基于分层空时编码的多输入多输出技术是一种极具潜力的高速水声通信技术, 但要实现这种潜力需要复杂的空时信号处理方法, 以抵消来自水声信道的多径干扰和异步到达干扰, 以及叠加在接收端的各层信号之间的干扰. 对低复杂度的空时信号处理方案进行了研究, 提出了一种基于子信道传播时延排序的有序连续干扰抵消信号检测算法, 利用子信道间的传播时延差, 实现可使差错概率最小的最佳检测排序; 给出了利用信道估计, 以极低的计算量确定排序的方法, 从而可以大幅降低信号检测的计算复杂度. 采用低复杂度的单载波频域均衡来抵消水声信道中的码间干扰和异步到达干扰. 仿真结果表明, 基于时延排序的信号处理算法可以获得检测性能的改善, 而且性能增益在高数据率时更加显著. 研究结果表明, 采用有效的信号处理方法可使水声信道中造成信号检测干扰的传播时延成为改善系统性能的有利因素.     

基于扩频码的单载波迭代频域均衡水声通信

单载波时域均衡在长时延扩展水声信道中计算量大,并对接收机参数的选择较为敏感,可靠性低,而正交频分复用信号峰均功率比高、对频率偏移敏感. 针对这些问题,提出基于扩频码的单载波块传输高速率水声通信方法和基于T/4分数间隔迭代频域均衡的接收机算法. 该接收机利用已知扩频码进行信道估计以及对由多普勒偏移引起的旋转相位进行估计,并通过一种低复杂度迭代频域均衡算法改善系统性能. 开展了湖上实验研究,结果表明在浅水1.8 km距离且复杂多径干扰条件下,利用BPSK/QPSK调制可实现10^-2–10^-4的误码率并达到1500–3000 bit/s的有效数据率. 关键词： 水声通信 单载波 频域均衡 迭代处理     

室内可见光多维CAP空间调制

针对室内可见光通信（VLC）光链路发射源（LED器件）有限的调制带宽问题,基于多维无载波幅度相位调制（CAP）技术,提出一种频带利用率高效的光空间调制方案（OSM-CAP）。OSM-CAP系统将输入信息序列分成两部分,一部分进行空域映射即空间调制,其结果用来选择激活的LED;另一部分进行信号域映射,即CAP调制。其中,CAP调制采用序列二次规划算法求取的时域正交多维CAP滤波器组实现。对于采用强度调制/直接检测（IM/DD）的VLC系统,该CAP信号还将进行单极化处理,文中提出两种相应的解决方案,即采用添加直流偏置（OSM-DCO/CAP方案）和零值位置极性编码（OSM-U/CAP方案）。基于朗伯体辐射模型,且可见光背景噪声建模为加性高斯白噪声,推导了OSM-DCO/CAP、OSM-U/CAP调制方案的误码率理论解,仿真验证了其准确性。在5 m×5 m×3 m的室内场景下,探讨了无载波幅度相位调制维度以及接收机个数、发射机间距、收发平面距离以及链路遮挡情况等信道参数对系统性能的影响。结果表明,星座调制阶数相同时,CAP调制维度越高,误码性能越好;接收机个数增多、发射机间距加大以及收发平面距离减小与系统可靠性存在正相关关系;链路存在遮挡情况时,会提高信道间的差异性,改善系统性能。     

UMFSK水声通信技术研究

在MFSK基础上,本文提出了UMFSK信道编码方法。采用窄带调频和跳频调制技术,故在一定程度上抑制了由多途效应引起的码内/码间干扰;提出了基于UMFSK的实时信道标校技术,可对信道进行稳键均衡,抗信道多途引起的码内干扰。进行了湖试,取得了试验数据并进行了数据分析。分析表明：UMFSK可抑制码元相消干涉引起的码内干扰,其效果随码元调频带宽的增大而变好;信道标校技术可对信道进行稳键均衡,并由于结构简单,可做到实时实现。基于UMFSK的误码率为10^-3（通信带宽2kHz,速率100-150bps)。     

一种光正交频分复用系统的联合相位均衡方法

正交频分复用（OFDM）技术加相干接收与数字信号处理法(DSP)的组合是超长距离光通信的理想模型。光OFDM系统对相位噪声十分敏感,必须对相位噪声进行补偿。提出一种基于正交小波基变换的光OFDM系统的联合相位均衡方案。该方案将块状导频周期性地插入OFDM信号,在接收端利用导频信息首先消除各个子载波的公共相位误差,然后采用自适应均衡方式消除每个子载波自身相位误差。仿真结果表明,对于二进制正交振幅键控(4QAM)调制信号,在采用常规的G.652光纤、100 Gb/s的相干光OFDM系统中,该联合相位补偿方法可使信号在满足传输系统的误码性能要求下,传输距离达到1000 km。     

室内可见光无载波幅度相位调制系统性能分析

针对室内可见光通信调制技术问题,提出翻转光无载波幅度相位调制和单极性光无载波幅度相位调制两种功率有效的调制方案,二者分别采用"正、负模块极性分组"以及"零值位置极性编码"方法实现信号单极性处理,以满足可见光通信"强度调制/直接检测"的要求.基于朗伯辐射模型,考虑到高斯背景光噪声的可见光直射传输信道,推导了包括直流偏置光无载波幅度相位调制在内的三种调制方案的误比特率闭式表达式,仿真验证了其准确性.在此基础上,分析比较了三者频带利用率,讨论了信道参量对光无载波幅度相位调制系统误码性能的影响,结果表明,在5m×5m×3m的室内场景下,与发射机辐射角为30°和45°相比,0°时的系统误码性能分别优于6.9dB和29.9dB;收发机距离为1m时,误码性能比2m时改善近12dB.     

自差分交流偏置超导纳米线单光子探测器

超导纳米线单光子探测器(SNSPD)因其优异的综合性能,在量子信息、激光雷达等方面有广泛的应用.通常, SNSPD工作在直流偏置下,在时域上具有自由运行探测的优点.而在卫星激光测距、单光子激光雷达等光信号到达时间有规律的应用场景中,使用交流偏置有望提升器件运行速率、有效抑制背景暗计数,却存在信号读出困难的棘手问题.本文报道了自差分读出的交流偏置SNSPD系统,该系统包含两根并行排布纳米线构成的2-pixel SNSPD器件.给两根纳米线加载相同的100 MHz交流偏置信号,并对两路输出信号差分使噪声信号相抵消,实现光子响应信号的读出.基于该方法测得,响应信号的信噪比相比差分之前提升10倍,在交流偏置下器件的暗计数降低至直流偏置下的约1/4,计数率达到直流偏置下的约1.5倍.本文为交流偏置SNSPD测试提供了一种思路,为其应用提供参考数据.     

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.