空-频域中自成像分析和解释

基于魏格纳变换和魏格纳分布函数,分析讨论一维物体在空域-频域空间的自成像及其形成过程。从成像过程中各衍射频谱分量的光程差出发,给出Talbot效应和Montgomery效应的统一解释。对于周期物的Talbot效应,得到了用杨氏双缝干涉解释自成像现象的理论依据。周期物的自成像是物平面上间距为2倍周期,光程差为波长的整数平方倍的各衍射频谱分量同相相干迭加的结果。Montgomery效应是物平面上间距为抛物线关系,光程差为波长整数倍的各衍射频谱分量同相相干迭加的结果。     

自成像魏格纳函数分析

在空域-频域空间,基于魏格纳变换和魏格纳分布函数,分析讨论了一维物体的自成像及其形成过程.从成像过程中各衍射频谱分量的光程差,给出了Talbot效应和Montgomery效应的统一解释.对于周期物的Talbot效应,得到了用杨氏双缝干涉解释自成像现象的理论依据.周期物的自成像是物平面上间距为两倍周期、光程差为波长的整数平方倍的各衍射频谱分量同相相干迭加的结果.Montgomery效应是物平面上间距为抛物线关系、光程差为波长整数倍的各衍射频谱分量同相相干迭加的结果.     

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。     

Hilbert-Huang变换分析THz脉冲信号的时频特性

提出一种基于Hilbert-Huang变换的THz时域光谱分析方法,将THz时域脉冲信号分解成有限数目的单分量信号之和,利用Hilbert变换求得瞬时频率来获得幅值的时频分布——Hilbert-Huang变换谱,实现了通过水蒸气的THz脉冲信号的时频分析,揭示了THz波与水蒸气相互作用的频谱时域分布特性,并与基于小波变换的时频图进行了对比分析。结果表明,该方法可以同时提高THz脉冲时频分布的时间分辨率和频率分辨率,具有局部化分析和自适应选择的特点,还能直观地表现出各频率成分之间的相对时间延迟。     

基于自适应多尺度时频熵的遥测振动信号异常检测方法

针对遥测振动信号频域成份复杂、非平稳非线性和强噪声特性,提出一种基于自适应多尺度时频熵的遥测振动信号异常检测方法;首先对采集到的遥测振动信号进行零漂修正和趋势项消除,然后采用自适应分解方法对信号进行多尺度分解,得到若干分量,利用相关系数剔除虚假分量;接下来用筛选出的分量作时频分布,对时频分布进行多层多尺度划分,计算相应尺度频段内信号的分形维数,依据分形维数的大小自适应地确定各频段的时频划分尺度;最后计算时频平面的自适应多尺度时频熵,通过时频熵的变化情况对遥测振动信号进行异常检测;实测数据验证了该方法的有效性。     

磁光光纤中光脉冲的非线性传播特性研究

根据光纤中磁光效应与非线性效应的微扰理论,推导了磁光光纤中光脉冲的非线性耦合模方程,比较了修正的分步傅里叶算法中磁光效应的时域和频域处理方案,表明了在步长足够小的情况下,两种方案的结果一致.分析了磁光效应、光纤非线性以及色散对光脉冲传输特性的影响,通过改变磁光耦合强度,不但可以灵活控制脉冲形状,还可以改变非线性引起的频率啁啾大小,有助于实现基于光脉冲展宽的动态整形功能.本文给出的理论分析方法,有助于开发可用于光纤通信、光纤传感等领域的基于非线性磁光光纤的新型磁光信息处理器件.     

磁光光纤中光脉冲的非线性传播特性研究

根据光纤中磁光效应与非线性效应的微扰理论,推导了磁光光纤中光脉冲的非线性耦合模方程,比较了修正的分步傅里叶算法中磁光效应的时域和频域处理方案,表明了在步长足够小的情况下,两种方案的结果一致.分析了磁光效应、光纤非线性以及色散对光脉冲传输特性的影响,通过改变磁光耦合强度,不但可以灵活控制脉冲形状,还可以改变非线性引起的频率啁啾大小,有助于实现基于光脉冲展宽的动态整形功能.本文给出的理论分析方法,有助于开发可用于光纤通信、光纤传感等领域的基于非线性磁光光纤的新型磁光信息处理器件.     

单模光纤中二阶偏振模色散的测量及其统计特性研究

在高速光纤通信系统中,二阶偏振模色散将引起脉冲信号的展宽和畸变,严重影响系统的传输性能。应用琼斯矩阵本征分析法,测量了光纤的二阶偏振模色散,并对二阶色散的平行分量、消偏振分量、PCD和退极化率进行了详细的分析,得到了二阶PMD随波长的分布情况和统计特性。对二阶偏振模色散的补偿有一定的指导意义。     

激光多普勒振动信号的时频分析

激光多普勒雷达实际测得的振动信号绝大多数都是时变信号,而基于傅立叶变换的时频分析方法是处理时变信号的有利工具。本文针对激光多普勒雷达测得的实际振动信号,比较了魏格纳-维利分布、平滑伪魏格纳-维利分布、频谱图、波恩-约旦分布和扩展修正B分布5种形式的时频分析性能。利用激光多普勒雷达测量实际单音响产生的啁啾信号振动、双音响产生的二分量啁啾信号振动、以及成年男性心跳振动3种振动,分析了时频图的分辨率和交叉项抑制情况,并通过计算时频聚集度指数,比较了5种分布情况下振动的分析性能。实验证明,扩展修正B分布的性能优于其他4种时频分布,扩展修正B分布更适合应用于激光多普勒雷达材料共振频率探测和心跳检测领域。     

调Q激光光折变自适应光外差探测技术研究

基于光折变晶体中自泵浦与互泵浦相位共轭共存效应,利用新型光折变自适应光外差探测系统进行了调Q脉冲激光的光折变自适应光外差探测技术研究.结果表明,该系统可以用来实现单纵模调Q脉冲激光的自适应光外差探测,并可以通过提高本振光强度有效地提高外差探测灵敏度.进一步的实验和理论分析表明,对于一般的调Q脉冲激光,由于其各个纵模频谱分量之间不相关,难于实现光外差探测.     

Decentered twisted Gaussian Schell-model beams and their propagation through a misaligned first-order optical system

The propagation properties of decentered twisted Gaussian Schell-model (DTGSM) beams passing through a misaligned first-order optical system are studied. The explicit expressions for the cross-spectral density function and Wigner distribution function of the output beam are derived, which retain their form unchanged. It is shown that the DTGSM beams preserve their closed property. The second-order moments matrix and the Wigner distribution function evolve with the usual laws, whereas the first-order moments matrix varies, as if a ray passes through such system. The propagation of DTGSM beams through an aligned first-order optical system is treated as the limiting case that corresponds to the vanishing misalignment parameters.     

Bohmian and quantum phase space distribution expansions and approximations

Dias and Patra derived an expansion of the Wigner distribution and related it to the de Broglie–Bohm model. We show that the coefficients of the expansion are related to the conditional central moments and cumulants of the Wigner distribution. The even order cumulants depend only on the amplitude of the wave function and the odd order cumulants depend only on the phase. In addition, we give a different expansion of the Wigner distribution from which their expansion can be derived as a special case. Our expansion allows for different approximations for higher order terms. We also give expansions for the momentum representation. We show how the results are applicable to pulse propagation in a dispersive medium.     

Determining the time-frequency parameters of low-power bright picosecond optical pulses by using the interferometric technique

We present an approach to the characterization of low-power bright picosecond optical pulses with an internal frequency modulation simultaneously in both time and frequency domains in practically much used case of the Gaussian shape. This approach exploits the Wigner time-frequency distribution, which can be found for these bright pulses by using a novel interferometric technique under our proposal. Then, the simplest two-beam scanning Michelson interferometer is selected for shaping the field-strength auto-correlation function of low-power picosecond pulse trains. We are proposing and considering in principle the key features of a new experimental technique for accurate and reliable measurements of the train-average width as well as the value and sign of the frequency chirp of pulses in high-repetition-rate trains. This technique is founded on an ingenious algorithm for the advanced metrology, assumes using a specially designed supplementary semiconductor cell, and suggests carrying out a pair of additional measures with exploiting this semiconductor cell. Such a procedure makes possible constructing the Wigner distribution and describing the above-listed time-frequency parameters of low-power bright picosecond optical pulses. In the appendix, we follow one of possible avenues for deriving the joint Wigner time-frequency distribution via choosing the Weil's correspondence between classical functions and operators.     

Effects of initial frequency chirp on the linear propagation characteristics of the exponential optical pulse

In this paper, the linear propagation characteristics of the exponential optical pulse with initial linear and nonlinear frequency chirp are numerically studied in a single mode fibre for \be₂<0. It can be found that the temporal full width at half maximum and time-bandwidth product of exponential pulse monotonically increase with the increase of propagation distance and decrease with the increase of linear chirp C for C<0.5, go through an initial decreasing stage near \zeta=1, then increase with the increase of propagation distance and linear chirp C for C\geq0.5. The broadening of pulses with negative chirp is faster than that with positive chirp. The exponential pulse with linear chirp gradually evolves into a near-Gaussian pulse. The effect of nonlinear chirp on waveform of the pulse is much greater than that of linear chirp. The temporal waveform breaking of exponential pulse with nonlinear chirp is first observed in linear propagation. Furthermore, the expressions of the spectral width and time-bandwidth product of the exponential optical pulse with the frequency chirp are given by use of the numerical analysis method.     

Dispersion-invariant features for classification

In dispersive propagation, waves from the same source will generally differ depending on how far they have traveled. Accordingly, it is desirable for classification in such environments to either account for propagation effects, or to obtain features that are invariant to such effects. The latter approach is taken in this paper, and features are derived that are unaffected by channel dispersion, per mode. A "local" view of pulse propagation in time-frequency phase space is considered. It is shown that the local duration of a wave, obtained from its time-frequency Wigner distribution, is invariant to dispersion. While higher moments of the Wigner distribution are not invariant to dispersion, the phase space considerations suggest an approach for defining "dispersion-invariant moments" (DIMs) of any order. This approach is also used to define a dispersion-invariant correlation coefficient that can be used for classification. The classification utility of the DIMs, and of the dispersion-invariant correlation coefficient, is evaluated via simulations of acoustic scattering from steel shells in a dispersive channel model (Pekeris waveguide). Receiver operating characteristic curves quantify the improved discriminability of the DIMs versus ordinary temporal moments, and of the dispersion-invariant correlation coefficient versus the ordinary correlation coefficient.     

Temporal and spectral properties of contra-propagating picosecond optical pulses in SOA

A useful analysis of contra-propagating optical pulses in semiconductor optical amplifier (SOA) is presented. Pulse temporal and spectral evolutions are investigated by resolving coupled equations describing pulse field propagation and SOA gain dynamics. With reference to the case of single pulse propagation, collision between pulses tends to maintain a good time-bandwidth product of amplified pulse and could provide a temporal compression by about 10% compared to pulse’s initial width.     

影响光孤子传输的因素分析和可能的解决方案

从光孤子传输所满足的非线性薛定谔方程出发,通过对方程中参数的分析和选择,对影响孤子传输的各种因素采用单独分析和综合分析对比方法得到了两个新的结论:(1)二阶色散参数在不太大的范围内变化只会影响脉冲的幅值,对脉冲形状影响不大,而输入脉冲的啁啾则是使脉冲发生畸变的主要原因;(2)在啁啾、三阶色散和五阶非线性同时存在的情况下,它们对脉冲都会产生很大的影响,且存在相互影响和制约关系,三者在某一参数值附近,对脉冲却存在着增益效应和"整容"作用。针对分析结果从理论上提出了改进光孤子传输特性的解决方案,这对光孤子通信的实践过程是有一定实际意义的。     

在采用集总放大器的光孤子优越传输中啁啾的作用

讨论了光孤子传输中，由脉冲包络的相位变化产生的啁啾与孤子传输的关系。数值计算表明，当孤子脉冲收缩、展宽时，啁啾总是改变符号，利用啁啾也可以定出孤子传输的光纤临界长度（即孤子传输中集总放大器的间隔），其效果比用脉宽恢复定出的临界长度要好     

圆柱对称光脉冲在负色散克尔介质中的传输

利用变分法得到了任意光脉冲传输的一般特性及高斯光脉冲压缩（或扩展）的必要条件，这说明在一定初始条件下时空非对称的光脉冲也可形成“光子弹”，在某些区域内，通过时空耦合作用，光束的空间会聚可以导至脉宽的压缩。     

Wavelet analysis of extremely short femtosecond pulse propagating in dispersion-shifted single-mode fiber

Results of the wavelet analysis of an extremely short femtosecond pulse propagating in a single-mode optical fiber with a shifted dispersion characteristic are described. The advantages of wavelet analysis as compared with Wigner analysis are shown. In particular, wavelet analysis allows improving the spectral resolution of the studied pulse in the high-frequency range.