色散缓变光纤中宽带调制不稳定性谱的产生

本文报道色散缓变光纤中调制不稳定性效应研究.从非线性Schrodinger方程出发,获得了增益谱与纵向色散变化参量和光纤传输距离的一般关系.研究发现,色散缓变光纤中调制不稳定性的增益谱较常规光纤中的宽得多.调节光纤色散纵向变化参量,可以得到较宽的增益谱.该文的研究结果为光纤中利用调制不稳定性产生超短孤子脉冲提供了理论根据.     

线性啁啾长周期光纤光栅用作EDFA增益平坦滤波器的理论研究

提出用具有特定折射率调制包络的线性啁啾长周期光纤光栅作为掺铒光纤放大器(EDFA)的增益平坦滤波器.采用龙格库塔迭代法数值求解该类型光栅耦合模方程,就特定的掺铒光纤放大器增益谱,用Nelder-Mead优化算法对光栅结构参量(光栅长度、周期、线性啁啾系数、折射率调制包络的形状等)进行优化,设计出能在C波段35 nm带宽范围内对掺铒光纤放大器进行平坦化(增益起伏在±0.5 dB之内)的平坦滤波器.     

基于自相位调制和交叉相位调制的全光开关特性研究

提出一种优化含有掺铒光纤放大器的非线性Sagnac干涉仪全光开关的新方法,建立了基于自相位调制和交叉相位调制两种解析模型,讨论了掺铒光纤放大器的小信号增益和饱和输出功率对开关性能的影响.分析表明掺铒光纤放大器的性能参量对开关所需要的Sagnac环中光子晶体光纤长度产生限制.当采用相同长度的光子晶体光纤时,基于交叉相位调制方式的全光开关与基于自相位调制方式的全光开关相比能够显著降低开关功率.采用分布傅里叶法数值求解非线性薛定谔方程,优化了开关结构,讨论了重复频率为40 GHz脉宽为5 ps的高斯型信号脉冲在开关时沿Sagnac环的传输特性.模拟结果表明,通过合理选择高非线性光子晶体光纤长度和掺铒光纤放大器的性能参量能够实现超低开关功率（＜1 mW）的开关操作.     

色散缓变光纤中的调制不稳定性分析

研究了色散缓变光纤中调制不稳定效应,得到了增益谱与光纤纵向色散参量的一般关系式.结果表明,色散缓变光纤较常规光纤具有较宽的增益谱.选取色散级变光纤的色散参量可以获得较大的增益带宽.数值模拟显示,利用色散缓变光纤中的调制不稳定性可以产生高重复率的基本孤子脉冲串.     

宽带放大器用碲基掺铒光纤结构参量的设计考虑

利用光波导理论及均匀展宽四能级模型，研究了宽带放大器用阶跃型折射率分布碲基掺铒光纤的结构参量—纤芯半径和相对折射率差的设计考虑.理论研究表明，在单模传输条件下，为获取最大信号增益，宜选择相对折射率差大的碲基掺铒光纤来构建光纤放大器.相对折射率差一定的情况下，在高泵浦功率、小信号输入或光纤长度短于各信道最佳增益长度时，选择纤芯半径大的碲基掺铒光纤可以得到大的信号增益；反之，宜选择纤芯半径小的碲基掺铒光纤.     

包含高阶色散的色散缓变光纤中交叉相位调制不稳定性分析

从包含高阶色散的广义非线性薛定谔方程出发,得到了色散缓变光纤中交叉相位调制不稳定增益谱,研究了增益谱随入射功率及光纤纵向色散参量的变化关系.结果表明：由于四阶色散的影响,在色散缓变光纤的正、反常色散区,交叉相位调制不稳定均发生在两个频谱区.反常色散区两频谱区宽度均比正常色散区宽,且反常色散区第二频谱区更靠近零点,说明色散缓变光纤中交叉相位调制不稳定更容易发生在反常色散区.增益谱宽都随两入射光波功率比值的增加而增大.色散缓变光纤中交叉相位调制不稳定增益谱宽比常规光纤的宽,且随着光纤纵向色散参量μ的增大色散缓变光纤中交叉相位调制不稳定越来越明显.     

一种增益可调掺铒光纤放大器的研究

在所设计的增益可调掺铒光纤放大器中,采用可编程控制器PC机控制可变直流电压源引发压电陶瓷的电致伸缩效应,导致光纤布拉格光栅的Bragg波长发生漂移,实现被调制参数的变化.分析了双光纤光栅受到压电陶瓷电致伸缩效应时的反射谱变化规律和增益可调掺铒光纤放大器的放大特性.由该放大器的增益与输入信号功率的关系得出小的波长间隔,增益较小,但控制范围较大;大的波长间隔,增益较大,但控制范围较小的结论.在输入宽带光源时,得到两光纤布拉格光栅在不同中心波长之差时的掺铒光纤放大器的增益特性.     

1550 nm飞秒脉冲自相似光纤放大仿真模拟EI北大核心CSCD

针对全光纤的超短脉冲掺铒光纤放大器进行了仿真模拟,对正常色散条件下掺铒光纤自相似脉冲放大过程进行了详细分析。在光纤预放大器中,使用高正色散掺铒光纤对脉冲形状进行预整形,将重复频率43 MHz、脉冲宽度600 fs、平均输出功率1.2 mW的孤子型锁模脉冲预整形为抛物线型脉冲,预整形后的脉冲通过光纤主放大器进行功率放大。经两级光纤放大后,1.2 mW的信号光功率放大为102 mW,放大增益19.3 dB。分析了掺铒光纤长度、放大功率对脉冲自相似演化过程的影响。放大后的脉冲经4.4 m长单模光纤将脉冲宽度压缩至53 fs,峰值功率为44.8 kW。     

铒镱共掺光纤放大器的高温老化输出功率

对1550 nm铒镱共掺光纤放大器不同温度下的输出功率以及经过高温老化后的输出功率和光谱进行了实验研究.通过对比高温和常温下铒镱共掺光纤放大器的输出功率随泵浦功率的变化曲线,得出铒镱共掺光纤放大器在高温环境工作可提高输出功率,且不同长度的增益光纤对温度的敏感性不同的结论.以Arrhenius模型为加速老化模型对增益光纤...     

色散缓变光纤中飞秒光脉冲的调制不稳定性研究

研究了色散缓变光纤中飞秒光脉冲的调制不稳定性 ,发现当色散缓变光纤的色散参量满足一定关系式时 ,增益谱的谱宽最宽 ,获得了增益谱的表达式 ;三阶色散对调制不稳定性不起作用 ;自变陡效应使增益谱的谱宽变窄 ,振幅的增长速度减漫 ;拉曼效应改变了调制不稳定性的产生区域。     

Self-organization of five species in a cyclic competition game

Cyclic competition game models, particularly the “rock–paper–scissors” model, play important roles in exploring the problem of multi-species coexistence in spatially ecological systems. We propose an extended “rock–paper–scissors” game to model cyclic interactions among five species, and find that two of the five can coexistent when biodiversity disappears, which is different from the “rock–paper–scissors” game. As the number of fingers is five, we named the new model the “fingers” game, where the thumb, forefinger, middle finger, ring finger, and little finger cyclically dominate their subsequent species and are dominated by their former species. We investigate the “fingers” model in two ways: direct simulations and nonlinear partial differential equations. An important finding is that the number of species in a cyclic competition game has an influence on the emergence of biodiversity. To be specific, the “rock–paper–scissors” model is in favor of maintaining biodiversity in comparison with the “fingers” model when the variables (population size, reproduction rate, selection rate, and migration rate) are the same. It is also shown that the mobility and reproduction rate can promote or jeopardize biodiversity.     

Contrastive analysis on tone between Putonghua and Taiwan Mandarin

The differences of the pitch and duration of Chinese syllables between Putonghua （PTH） and Taiwan Mandarin （TM） were studied. The speech materials to be used are not only isolated syllables, but also sentences. The results reveal that： For the isolated syllables, T1 and T2 in TM are influenced by Minnan dialect, therefore their pitch are lower than those in PTH. T3 is fall-rise in PTH, while it is fall in TM. Moreover, the syllable duration sequence for different tone is T3〉T2〉T1〉T4 in PTH, while it is T1〉T2〉T3〉T4 in TM. For the syllables in sentences, T2 is mid-rise in PTH, while it is mid-level in TM. And the T3 is longer than T4 but shorter than T1 or T2 in PTH, while it is the shortest in TM. Furthermore the effects of prosodic phrase boundary on duration for different tones are almost the same in PTH, but the lengthening part of T1 or T2 is longer than that of T3 or T4 in TM.     

Experimental study on downstep in Chinese intonation

Downstep in pitch contour of Chinese Putonghua is examined using subtly designed sentences by controlling tone combinations. The results show both automatic and nonautomatic downstep phenomena exist in Chinese. In non-automatic downstep, low tones compress downwards the pitch range of the following syllables. and the main influence of downstep is on topline. Low tone not only lower the topline behind it, but also raise the high tones before it, the effects are compatible with each other. In automatic downstep, the topline of pitch contour in intonational phrase is presented as a linear downtrend, but it differs among speakers due to the effect of personal stress practice. In comparison with downstep phenomenon in other tone or non-tone languages, the downstep ratio in Chinese is not constant, and the domain of downstep is not limited within the adjacent tones.     

Abelian-Higgs phase of SU(2) QCD and glueball energy

It is shown that SU(2) QCD admits an dual Abelian-Higgs phase, with a Higgs vacuum of a type-Ⅱ superconductor. This is done by using a connection decomposition for the gluon field and the random-direction approximation. Using a bag picture with soft wall, we presented a calculational procedure for the glueball energy based on the recent proof for wall-vortices [Nucl. Phys. B 741(2006)1].     

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.     

Signal generation in an isotropic medium in scanning electron acoustic microscope

Based on the research in Ref. [5][Materials Science and Engineering, 1989; A122： 57 63], an improved model of heat source is set up, the different modes of Lamb wave in an isotropic sample generated by a chopped electron beam at frequency f are obtained with integral transform and normal function expansion method, and the output signal of PZT coupled at the back surface of the sample is found out. The generation mechanism of SEAM （Scanning Electron Acoustic Microscopy） signal is discussed. It shows that the SEAM is a near field imaging technique with high spatial resolution and its best lateral spatial resolution is about 2√2α （α is the radius of the focused electron beam）. Some of experimental results of SEAM images are presented in the paper and it shows that the spatial resolution of SEAM is better than 0.5 μm and smaller than the thermal diffusion length of the sample. Therefore the character of near field imaging in SEAM is also proved experimentally.     

High efficiency filtering using two-dimensional photonic crystal coupled cavity structures

Transmission spectra of coupled cavity structures (CCSs) in two-dimensional (2D) photonic crystals (PCs) are investigated using a coupled mode theory, and an optical filter based on CCS is proposed. The performance of the filter is investigated using finite-difference time-domain (FDTD) method, and the results show that within a very short coupling distance of about 3λ, where λ is the wavelength of signal in vacuum, the incident signals with different frequencies are separated into different channels with a contrast ratio of 20 dB. The advantages of this kind of filter are small size and easily tunable operation frequencies.