光纤锁模激光器：从单模时域耗散孤子锁模到多模时空耗散孤子锁模（特邀）

在光纤锁模激光器中,模式相位锁定产生周期短脉冲的过程称为锁模过程,产生的脉冲在广义上被称为“光耗散孤子”。光纤锁模激光器从传统的单模光纤锁模激光器发展到了多模光纤锁模激光器,锁模机理从一维（1D）时域耗散孤子锁模发展到了（3+1）维时空耗散孤子锁模。通过深入理解耗散孤子的产生机理,有望进一步推动光纤锁模激光器在科学和应用领域的发展,为更多领域带来更多创新和可能性。首先介绍单模光纤锁模激光器中的一维时域耗散孤子锁模,探讨不同色散区域中时域耗散孤子的产生机理;随后介绍多模光纤锁模激光器中时空耗散孤子的最新研究成果,讨论模间色散的补偿方法,揭示其丰富的时空锁模机理和潜在的应用场景;最后对光纤锁模激光器的发展前景进行展望。     

基于碳纳米管锁模光纤激光器的偏振锁定和偏振进动矢量孤子的研究进展

矢量孤子在现代通信、非线性光学等研究中具有重要意义。锁模光纤激光器为研究矢量光孤子特性提供了重要平台。对碳纳米管锁模的掺铒光纤激光器中矢量孤子的进动进行了综述,展示了多种矢量孤子的偏振进动轨迹。实验证明了矢量孤子的偏振进动作为非线性吸引子的存在,揭示了一种新型锁模光纤激光器的可能,并展望了其未来的应用和发展。     

基于非线性偏振旋转锁模孤子光脉冲的产生及模拟

非线性偏振旋转(NPR)技术是被动锁模光纤激光器中实现超短脉冲的一种有效方式,因其结构紧凑,可靠性高而备受关注。本文利用基于NPR锁模的掺铒光纤激光器,在1557.7 nm波段,获得了脉冲宽度为1.35 ps,基频重复率为9.49 MHz的脉冲序列输出。利用耦合的金兹堡-朗道方程,数值模拟了激光器中锁模孤子光脉冲的产生,并对锁模建立过程中孤子时域和频域演化进行了分析,模拟分析和实验观察相吻合。该结果有助于加深人们对NPR锁模光纤激光器中孤子锁模动力学特性的理解。     

时间拉伸色散傅里叶变换在被动锁模光纤激光器研究中的应用

时间拉伸色散傅里叶变换(TS-DFT)技术推动了锁模激光器中瞬态现象的研究,对于揭示复杂系统中的耗散动力学过程具有重要意义。介绍了TS-DFT技术的基本原理及其在数据采集和数据处理中存在的关键问题,总结了TS-DFT技术在被动锁模光纤激光器的类噪声脉冲与怪波、孤子爆炸、孤子束缚态、锁模自启动和矢量孤子等各类超快现象研究中的应用。     

孤子激光器动态分析

孤子激光器可以产生优良形状的超短光脉冲,一般的孤子激光器是含有均匀线宽增益介质的主激光腔与含非线性介质的光纤附加腔构成。理论分析以此三项为依据;①激光锁模,②光孤子演化,③两腔耦合。先从主腔的主动锁模激光系统获得超短脉冲,然后进入附加腔,利用光纤中相位调制与色敬互相补偿作用形成孤子。这种光孤子脉冲比主腔锁模     

孤子激光器的简化模型和稳定化

采用主动锁模、被动锁模以至碰撞锁模,可使脉宽压缩到ps级,制成孤子激光器后,脉宽已可压     

锁模光纤激光器中的矢量孤子簇

为了研究锁模光纤激光器中矢量孤子的动力学特征,利用非线性偏振旋转锁模机制,实验获得了矢量孤子及矢量孤子簇。矢量孤子簇表现出来的动力学特点与抽运功率与偏振设置有关。抽运功率越高,由矢量孤子裂变所形成的矢量孤子簇对数越多。而且,孤子簇脉冲序列在时域上显示出强度周期调制现象。实验中观察到强度周期调制的4阶和7阶脉冲矢量孤子簇。结果表明,强度周期调制的矢量孤子簇输出是非线性偏振旋转锁模光纤激光器偏振调制的本征属性,孤子簇周期强度调制现象归因于腔内偏振相关隔离器对孤子簇的周期偏振调制的结果。     

自锁模飞秒钛宝石激光技术进展

给出了锁模超快钛宝石激光的实验进展,介绍了几种典型的自锁模飞秒钛宝石激光,、并讨论了自锁模钛宝石激光的克尔镜锁模理论和孤子激光器理论。     

光纤激光器中锁模孤子序列的长周期脉动

实验观察到非线性偏振旋转锁模光纤激光器中锁模孤子及孤子簇的长周期脉动现象.单孤子与孤子簇的脉动周期达103和102次脉冲周期量级.脉动周期及脉动包络形状与抽运功率及腔内偏振设置有关.孤子脉冲的长周期脉动现象归因于不严格的1阶偏振态设置.不严格的1阶偏振态设置使得偏振相关隔离器对每次腔内循环的锁模孤子脉冲进行了微弱的周期偏振滤波.积累的偏振滤波损耗导致了锁模脉冲序列出现长周期脉动.     

双孤子被动锁模光纤激光器的实验研究

利用非线性偏振旋转(NPR)效应,对被动锁模掺Er光纤激光器同时输出双波带、双孤子的现象进行了实验研究和理论分析。结果发现,激光腔内存在高的线性双折射是得到双波带、双孤子输出的关键条件,提供宽的增益带宽和较大的增益有利于得到稳定的锁模输出。通过在腔内引入较大的线性双折射,在基于NPR锁模技术的被动锁模掺Er光纤激光器中得到了重复频率为7.49 MHz、中心波长分别在1 542 nm和1 557 nm附近的双波带、双孤子锁模脉冲的同时输出。     

Mathematical proof of the validity of reciprocity in one-dimensional bipolar transistors with arbitrary base parameters

We consider the solution of the general continuity equation for the minority electrons in the base of a one-dimensional n-p-n bipolar transistor. The solution of the continuity equation can be expressed as the superposition of two linearly independent expressions. The collector current is derived as a function of these two expressions. Two cases are considered: one junction is injecting at bias VFand the other is collecting at zero bias and vice-versa. The two collector current expressions are found to be identical which confirms the reciprocity theory for the very general one-dimensional low-level injection case.     

Closed-form expressions for the prediction of microcellular mobile radio propagation in urban environments

The paper derives two closed-form expressions for predicting microcellular mobile radio signal levels in urban street-grid environments. The results demonstrate that the two expressions are faster and more accurate than existing models and methods. Benefiting from physical optics and the conservation of energy, the two expressions are alternatives to available techniques such as ray tracing. The dielectric property (i.e., permittivity) of street buildings and other obstacles is essential to the ray tracing technique. However, there are no established, widely accepted techniques for selecting the permittivity in urban mobile environments, which varies in a wide range. Establishing a natural link between the received power after a street corner to the width of the street and the angle of incidence, our expressions show the possibility of making fast and accurate propagation predictions without having to use the permittivity.     

Turbo 码在无信道状态信息的Nakagami 衰落信道中的性能分析

讨论了Turbo码在无法获取信道状态信息的移动通信系统中的纠错性能联合上界，提出无信道状态信息时，Nakagami慢衰落信道中成对差错概率的2个精确计算式和2个近似计算式。数值分析结果表明，成对差错概率的2个精确计算式的数值积分曲线完全吻合；封闭形式近似计算式的计算结果与精确值相近。仿真结果说明，理论联合上界在高信噪比处逼近仿真性能。所提出的性能分析式可快速有效地评估无信道状态信息的移动通信系统中Turbo码的性能。     

Uniform physical theory of diffraction equivalent edge currents fortruncated wedge strips

New uniform closed-form expressions for physical theory of diffraction equivalent edge currents are derived for truncated incremental wedge strips. In contrast to previously reported expressions, the new expressions are well behaved for all directions of incidence and observation and take a finite value for zero strip length. This means that the expressions are well suited for implementation in general computer codes. The new expressions are expressed as the difference between two terms. The first term is obtained by integrating the exact fringe wave current on a wedge along an untruncated incremental strip extending from the leading edge of the structure under consideration. The second term is calculated from an integration of the asymptotic fringe wave (FW) current along another untruncated incremental strip extending from the trailing edge of the structure. The new expressions are tested numerically on a triangular cylinder and the results are compared with those obtained using the method of moments and the previously reported expressions     

A comparison between near-field shielding-effectivenessmeasurements based on coaxial dipoles and electrically small apertures

The near-field shielding effectiveness (SE) of a material can be measured by placing it between two closely spaced dipoles (electric or magnetic) and noting the resulting insertion loss. An alternative approach is to cover an electrically small aperture with the test material and measure the resulting loaded aperture polarizability (electric or magnetic), as is done in a dual TEM (transverse electromagnetic) cell. The author develops approximations leading to simple closed-form SE expressions by developing expressions that relate these two configurations. Small-aperture theory is discussed first, followed by a development of approximate dipole coupling expressions. The results are then compared with an experimental example     

两种正交曲线坐标系单位矢量间的一般表达式

本文采用不同的分析思路,导出了曲线坐标系与直解坐标系单位矢量间简明的解析关系,并推广到更一般的情况－－任何两种正交曲线坐标系单位矢量间的关系,只要一种正交曲线坐标系与直解坐标系或另一种正交曲线坐标系坐标间的简值关系已知,利用这些关系式即可得到正交曲线坐标系与直解坐标系或另一种正交曲线坐标系单位矢量间的关系,利用文献上已有的正交曲线坐标系坐标间的单值关系,文中提供了正交曲线坐标系与直角坐标系及圆柱坐标系单位矢量间的变换矩阵,进而可得任何两种正交曲线坐标系单位矢量间的关系。     

Simple expressions for worst and best case Cramer-Rao bounds foramplitude and phase estimation of closely spaced cisoids

Simple expressions are presented for the worst and best case Cramer-Rao bounds for the amplitude and phase estimation of two cisoids in the presence of complex white Gaussian noise. The expressions are valid in the sub-Rayleigh region where the difference between the critical values of the bounds becomes important     

Simple expressions for the directivity of a circular microstrip antenna

We present two sets of simple, closed-form expressions for accurately computing the directivity of a circular microstrip antenna on thin and thick substrates of any relative permittivity. These expressions are useful for computation by a hand calculator and, therefore, useful to a practical antenna designer.     

Multicellular Alamouti scheme performance in Rayleigh and shadow fading

In this paper, we study the performance of two downlink multicellular systems: a multiple inputs single output (MISO) system using the Alamouti code and a multiple inputs multiple outputs (MIMO) system using the Alamouti code at the transmitter side and a maximum ratio combining (MRC) as a receiver, in terms of outage probability. The channel model includes path-loss, shadowing, and fast fading, and the system is considered interference-limited. Two cases are distinguished: constant shadowing and log-normally distributed shadowing. In the first case, closed form expressions of the outage probability are proposed. For a log-normally distributed shadowing, we derive easily computable expressions of the outage probability. The proposed expressions allow for fast and simple performance evaluation for the two multicellular wireless systems: MISO Alamouti and MIMO Alamouti with MRC receiver. We use a fluid model approach to provide simpler outage probability expressions depending only on the distance between the considered user and its serving base station.