中性碳原子基态关联效应的多体理论计算

应用原子多体理论对开壳层原子中电子关联效应的有效哈密顿量及波算符的Goldston图进行了分析,在此基础上得到了计算有效哈密顿量的简单方法;以HFS模型为零级近似对中性碳原子基态的关联效应进行了计算并通过与实验结果及其他文献基于HF模型进行的计算相比较,得到了令人满意的结论。     

生物流体中声参量效应的研究

本文利用Fenlon理论分析了流体中低频泵浦波与高频弱声波非线性相互作用而产生的参量效应,计算了和差频边带声的声压表式,并由此导出计及媒质衰减时的非线性参量B/A的公式。理论计算表明和差频边带声压幅值随泵浦声压线性地增加,由此提供了一个测量非线性参量的方法,用此方法测量了若干生物流体和混合物的B/A值。     

金属纳米线阵列的光学非线性增强因子的分析和计算

以平行排列的无限长金属圆柱体为模型,基于金属颗粒与电磁场相互作用的平均场理论和非线性光学的基本理论,计算了局域场增强和表面等离子体共振引起金属纳米线阵列的光学非线性增强,通过理论计算得到了银纳米线阵列的局域场增强因子和有效三阶非线性极化率的增强因子,并分析了不同参量和非参量非线性光学效应总的场增强因子.     

倒Y型四能级系统中自发辐射诱导相干对弱探测场的色散和吸收特性的影响

本文研究了倒Y型四能级系统中自发辐射诱导相干对探测场的色散和吸收特性的影响。在稳态条件下利用密度矩阵微扰理论推导出了密度矩阵的迭代解。数值分析了自发辐射诱导相干对线性和非线性折射率和吸收系数的影响,分析了泵浦场和耦合场偏离双光子共振对弱探测场色散和吸收特性的影响。研究发现自发辐射诱导相干使弱探测光的线性和非线性折射率增强,同时形成一个较宽的透明窗。在没有自发辐射诱导相干的情况下,泵浦场和耦合场偏离双光子共振对探测场的线性和非线性折射率影响明显,在有自发辐射诱导相干的情况下,泵浦场和耦合场偏离双光子共振使得探测场的线性透明窗变窄,非线性吸收增加。     

泵浦光强对激光相位共轭波非线性效应的影响

分析了泵浦光强对激光相位共轭谐振腔中探测光场的作用。基于稳定性定理,通过确定系统的Lyapunov指数,数值计算了在不同泵浦光强下的激光相位共轭波的非线性效应。结果显示,泵浦光强是系统的控制参量。调节泵浦光强的数值,系统的动力学性质发生了改变,可以呈现一周期、二周期、四周期以及倍周期分岔、周期并缩和混沌现象。     

电子的相对论平均场理论与一阶、二阶Rashba效应

用多体平均场意义下电子的Dirac方程讨论了电子自旋动力学及其相关问题. 在大分量Dirac方程的非相对论展开中讨论了电子自旋动力学的高阶效应,并且在二维情形下得到了包括一阶和二阶Rashba效应的电子自旋动力学哈密顿量,求出了相应的包括二阶Rashba效应的哈密顿量的能量和波函数的本征值解,由此讨论了二阶Rashba效应修正的物理含义和大小. 关键词： 二阶Rashba效应 自旋电子学 Dirac方程 相对论平均场理论     

占空比对光子晶体光纤激光器性能的影响

 在传统光纤激光器工作原理的基础上,考虑光子晶体光纤(PCF)模场分布特征,给出了连续泵浦情况下单模PCF激光器的速率方程和功率传输方程。利用该方程对掺镱单模PCF激光器的性能进行了数值模拟研究。结果表明：虽然空气占空比大小对PCF激光器的输出功率、泵浦阈值和斜率效率等影响不显著,但对拉曼非线性阈值影响却很大。当泵浦功率小于拉曼非线性阈值时,激光器主要输出信号激光;当超过拉曼阈值且在较宽的范围内,激光器同时输出功率相近的信号激光和拉曼光。基于这种效应,提出一种由泵浦功率控制的双波长光纤激光器的新思路。考虑到PCF非线性系数的可调控特性,采用不同光纤有可能在较宽的功率范围获得双波长激光振荡。     

用于10 kW级高光束质量激光输出的国产部分掺杂光纤北大核心CSCD

当前光纤激光功率提升受限于模式不稳定效应及非线性效应,为了克服上述功率提升限制因素,自主设计并研制了大模场部分掺杂光纤。最终,采用自研部分掺杂光纤及后向级联泵浦方案,成功实现了10.1 kW的光纤激光输出,对应的光束质量因子(M2)为2.16。     

真空度对强紫外激光在空气中长程传输的影响

 采用瞬态受激旋转拉曼散射(SRRS)模型，并充分考虑非线性介质分子密度、增益系数、泵浦光的衰减、斯托克斯光的非线性放大以及初始的随机斯托克斯散射场等因素，研究了空气真空度对强紫外激光在空气长程传输过程中产生的SRRS效应的影响。结果表明，真空度对SRRS效应的阈值传输距离和Stokes光的转换效率的影响较大，随着真空度的提高，Stokes光的转换效率将降低，产生SRRS效应的阈值传输距离将增大。因此，在实际工作中，可以采用真空度较高的激光传输环境来抑制SRRS效应的产生，以减小入射泵浦光能量的衰减并提高光束质量。     

随路遥泵系统二阶泵浦方式泵浦性能分析与优化

为了解决随路遥泵系统采用传统二阶泵浦方式时出现自激的问题,提出了基于滤波隔离技术的降低多径干扰方案和基于混合型光纤技术的降低非线性系数方案.滤波隔离技术通过将滤波隔离单元放置于光纤线路中来有效改善多径干扰现象;混合型光纤技术通过采用大的有效面积光纤和普通光纤混合结构降低非线性效应.理论分析表明减少泵浦光的多次反射可减弱多径干涉;增加光纤有效面积可以降低非线性系数.实验结果表明:采用由波分复用器和隔离器组成的滤波隔离单元,进入远程增益单元的泵浦功率可提高1.2dB;采用20km EX2000和80km G.652混合型光纤结构,进入远程增益单元的泵浦功率可提高1dB.该研究对降低自激影响的随路二阶泵浦的研制和应用具有一定意义.     

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.