基于光折变效应提升啁啾脉冲信噪比的光谱扫描滤波法

基于光谱扫描滤波原理,提出了一种利用光折变效应来提高啁啾脉冲信噪比的扫描滤波方法,并进行了理论分析。针对内置光折变晶体的Fabry-Perot (F-P)标准具光谱扫描滤波装置,定量分析了其透射谱特性,详细讨论了Fabry-Perot(F-P)标准具两平板的反射率、透射谱窗口带宽、外加场的控制参数以及信号光啁啾率变化等对啁啾脉冲信噪比的提升及其滤波透射率的影响。结果表明,F-P两平板的反射率越高,则透射谱越尖锐,透射窗口越窄,滤波效果也就越好。为了得到有效的信噪比提升,一般要求其反射率大于0.99;外加场的控制和信号光啁啾率的变化对扫描滤波的影响较大,在实际工作中,应尽量保证外加场和啁啾率的精确控制,以确保信号光和滤波函数的同步匹配。针对中心波长800 nm的啁啾信号脉冲,采用提出的光谱扫描滤波方法,对放大自发辐射(ASE)的随机噪声与预脉冲进行滤波,可得到近3个量级的信噪比提升效果。     

低成本啁啾脉冲光源用于光码分多址系统的优化改进

低成本脉冲光源应用于光码分多址系统,有助于光码分多址系统的实用化进程。对实用化的啁啾脉冲光源应用于光码分多址系统进行了优化改进。研究啁啾脉冲光源本身啁啾的消除即压缩啁啾脉冲对编解码性能的提升作用。针对通常实用的啁啾脉冲光源的谱宽相对较窄而不能充分覆盖编解码频谱的现实,研究利用编解码器频谱的半边带进行编解码以改善解码性能的效果。仿真和实测数据表明,对啁啾脉冲进行消啁啾、压缩脉宽有助于提高编解码性能;利用编解码频谱的半边带进行编解码能够更加充分利用光源频谱,编解码性能进一步得到优化。研究结果表明消啁啾和半边带能够改进啁啾脉冲光源用于光码分多址系统的有效性。     

增益饱和在啁啾脉冲放大中的影响

研究了增益饱和在啁啾脉冲放大中的影响，发现由于增益饱和在存在，压缩后脉冲的信噪比会下降，而且展宽器附加的色散也会降低输出的信噪比，这种降低对应着一定的光谱畸变。通过对展宽／压缩器作用的分析，提示了啁啾脉冲放大中光谱与光强的内在联系，提出利用非共振放大来修正光谱畸变提高放大压缩后的信噪比。     

负五阶非线性下预啁啾高斯光脉冲的啁啾和频谱

为探究光纤负五阶非线性和光脉冲预啁啾对高斯光脉冲频率啁啾和频谱的综合影响,在忽略光纤色散的情况下,利用包含三五阶非线性的扩展非线性薛定谔方程,计算和分析了啁啾高斯脉冲的频谱和总啁啾。结果表明,脉冲无预啁啾时,负五阶非线性的存在可使脉冲频率啁啾的形状由单“S”形变为双“S”形,并使脉冲的中心附近由正啁啾变为负啁啾。此外,负五阶非线性的存在使预啁啾对脉冲频谱的影响发生了较大的变化。预啁啾与非线性所致啁啾叠加后对脉冲频谱的谱峰数目影响较小,而对能量在各个谱峰上的分布影响较大,在某些最大非线性相移下,甚至会出现总啁啾减小使谱峰值数目增加的情况。该研究对光开关、光脉冲压缩以及光纤通信系统等领域的深入发展具有一定的指导意义。     

高功率钕玻璃啁啾脉冲放大系统光谱整形

增益窄化效应限制了啁啾脉冲放大系统输出的最短脉冲宽度,需要对注入放大系统的种子啁啾脉冲进行光谱整形,以保证放大后的啁啾脉冲光具有足够的带宽并满足特定的光谱分布.理论上假定放大后啁啾脉冲的光谱分布为高斯分布或超高斯分布,对"百焦耳级钕玻璃啁啾脉冲放大系统"进行逆运算.研究发现,所需种子啁啾光的能量、带宽以及压缩后脉冲的信噪比都与该分布密切相关.通过光谱整形,使放大后的啁啾脉冲光的光谱分布为二阶超高斯分布.能够获得小于400 fs高信噪比的超短脉冲输出,此时所需种子啁啾脉冲能量仅为33μJ,带宽14 nm,在装置前级注入能力的范围之内.为百焦耳装置光谱整形实验的开展提供了理论指导.     

线性扫频光脉冲功率谱的理论分析

本文给出了线性扫频(啁啾)的高斯型光脉冲的功率谱表达式,它等效于有调制的、脉宽压窄的无啁啾光脉冲的功率谱.理论分析表明:啁啾可由脉冲的功率谱并借助强度自相关方法确定.     

三五阶非线性下啁啾超高斯脉冲的啁啾和频谱

利用三五阶非线性效应下的扩展非线性薛定谔方程,在忽略光纤色散的情况下,计算模拟了以二阶情形为例的啁啾超高斯光脉冲的啁啾和频谱。结果表明,脉冲无预啁啾时,正五阶非线性增大啁啾量,负五阶非线性在减小啁啾量的同时还改变啁啾曲线的形状。当预啁啾与五阶非线性系数同(异)号时,总啁啾增加(减小)。随着超高斯脉冲阶次的增大,总啁啾量增大,脉冲中心附近无啁啾的范围变宽,整个有啁啾的范围变小,总啁啾中预啁啾所占比重增大。脉冲无预啁啾时,正负五阶非线性分别可以增大和减小频谱展宽。预啁啾若增强非线性所致啁啾,则可能使频谱峰值结构加强,谱峰数目增多。当超高斯光脉冲的阶次或最大非线性相移增大时,在某些宽大的频谱峰上还可能出现许多精细谱峰。     

飞秒脉冲初始脉宽和啁啾对其自相似演化及压缩的影响

采用自相似抛物脉冲放大器对飞秒脉冲直接进行放大,并以低非线性系数的光子带隙光纤作为脉冲压缩器对放大的脉冲进行压缩.通过分步傅里叶方法研究了飞秒脉冲的初始宽度和啁啾对其自相似演化和压缩的影响.研究结果表明脉冲宽度越宽,脉冲自相似演化程度越差,演化脉冲的压缩质量也越差,压缩脉冲宽度较大,且有基座;脉冲的初始啁啾会严重阻碍其自相似演化,并最终影响脉冲的压缩质量;啁啾越大,脉冲的自相似演化结果越差,脉冲的压缩质量也越差;正的啁啾对脉冲的自相似演化的影响比负啁啾的要大.     

利用Mach-Zehnder干涉仪提升啁啾脉冲信噪比

基于内置电光晶体的Mach-Zehnder干涉仪,利用Mach-Zehnder干涉仪双光束干涉原理和晶体的线性电光效应,提出了一种新的能有效提高啁啾脉冲信噪比的扫描滤波方法。定量分析了其扫描滤波谱特性,探讨了预脉冲噪声延时对信噪比提升效果的影响,并讨论了电光晶体的电压控制参数对信噪比提升效果的影响。研究结果表明:预脉冲噪声的延时和控制电压的变化会对滤波效果产生较大影响,利用该扫描滤波方法可将啁啾脉冲激光信噪比提升约2个量级,且信号光透过率高达95%以上。     

基于光电振荡器和孤子压缩的双波长、高稳光窄脉冲

提出了一种在波长双环路光电振荡器(OEO)中,利用啁啾压缩和孤子压缩效应产生光窄脉冲的有效方案。此方案能够同时产生双波长光窄脉冲。将双环路OEO产生的高质量微波信号调制到系统中的直调激光器和相位调制器上,产生光脉冲和光啁啾。调节直调激光器和相位调制器之间的延时量,从而获得最大的啁啾率。利用色散补偿光纤和高非线性光纤对光脉冲进行啁啾压缩和光孤子压缩,使得光脉冲最终压窄至1.2ps。采用锁相环技术对系统腔长进行有效控制,使光脉冲的长期稳定性得到改善。     

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.     

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.     

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.     

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].     

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.     

Real-time restoration of rotational blurred image using gradient-loading

The key to the restoration of rotational motion blurred image is how to restore the image under a low cost and to correct the irreversibility of the degradation function matrix.Based on the special qualities of degradation function matrix and precise deduction in space-domain, we present a new approach using gradient-loading for restoration of rotational blurred image.By easily adding a gradient operator, the irreversibility of the original matrix is corrected and can be applied for inverse filtering then.Gradientloading is the optimized approach which combines the advantages of both the approaches using constrained least square filtering and traditional diagonal-loading.Compared with the approach using least square filtering, its peak signal-to-noise ratio (PSNR) is improved from 3.18 to 6.46dB, while the computing time is reduced to 1/2-1/3.Experimental results demonstrate the effectiveness, noise-resistibility, robustness, and low complexity of this approach, which make it more suitable for real-time environment.     

All-Optical Logic Device Using Bent Nonlinear Tapered Y-Junction Waveguide Structure

We propose an all-optical logic device made of a bent tapered Y-junction waveguide with a Kerr-type nonlinear interface. It could provide an AND gate, OR gate, and exclusive-OR (XOR) gate. We could obtain different transmission results by adjusting the bending angle. The numerical simulation results show that the device functions as AND, OR, and XOR gates.     

Real-time, continuous-wave terahertz imaging by a pyroelectric camera

Real-time, continuous-wave terahertz (THz) imaging is demonstrated. A 1.89-THz optically-pumped farinfrared laser is used as the illumination source, and a 124 × 124 element room-temperature pyroelectric camera is adopted as the detector. With this setup, THz images through various wrapping materials are shown. The results show that this imaging system has the potential applications in real-time mail and security inspection.