阶跃光纤低阶线偏振模的M2因子分析

针对阶跃光纤输出低阶线偏振模（Linear Polarization,LP）,利用二阶距定束宽,多点拟合法计算了几个低阶LP mn模相干和非相干叠加时的M2因子.结果表明,对同一LP mn模式,光纤的归一化频率参量V减小到截止频率时,M2因子出现峰值;当V远离截止频率时M2因子趋于一定值.低阶LP mn模非相干叠加时,叠加后的较高阶模成分越大,合成光束的光束质量越差,光束在一定的条件下可能比基模更接近于理想高斯光束;而相干叠加时,合成后的M2因子可以大于参与叠加的较高阶模的光束质量因子.     

含间隔层的增益导引-折射率反导引平面波导激光器中高阶模式抑制研究

为了抑制高功率波导激光器中的高阶模式,改善其光束质量,基于增益导引-折射率反导引理论,在对称分层波导结构中添加了一层间隔层,进一步加大了高阶模式与基模之间阈值增益系数的差异,利用这种模式竞争抑制高阶模式.同时从波动方程出发,推导出各模式的本征方程,并给出了基模和高阶模的场分布.引入间隔层之后的高低阶模式损耗计算结果表明,通过合理地选择间隔层参数,可以有效地抑制高阶模式.     

Theoretical analysis of multimode fiber amplifiers and lasers containing spatial filters

By the use of the coupling theory of the modes and the multimode rate equations, the performances of the multimode fiber amplifiers or lasers containing a spatial filter have been discussed. After the rate equations applicable to amplifiers are solved, the beam quality factors of the output from a multimode fiber amplifier with and without a single mode filter have been evaluated and compared. By the use of the analytical expression of the threshold pump power applicable to the multimode fiber lasers with the spatial filter and doped with quasi two-energy-level ions, and two example fibers had been selected which may guide 4 and 9 transversal modes, respectively, the threshold pump power for each transversal fiber mode has been evaluated. The results indicate that the single mode filter, does introduce a very strong discrimination for the modes between the high order modes and the fundamental mode. This should considerably favor the operation of the fundamental mode in the lasers with a homogeneously broadened active medium.     

光纤激光模场及表征技术进展

在光纤通信、光纤激光器和光纤传感等领域的实际应用中,需要重点关注光纤中的模式问题。模分复用是提高光通信信息容量的有效方法,模间干涉是大多数光纤传感的基本方法,高功率光纤激光的光束质量控制的关键技术之一就是模式控制,因此,对光纤模式理论、模式产生及转换、模式表征技术开展研究具有重要的研究意义和实际应用价值。论文讨论了光纤的模式及光束质量,分析了多种模式发生及转换的方法,将模式表征方法归结为非相干、相干和低相干测量法。光纤模式表征是目前的研究热点,在多种表征方法中,空间和频谱成像法（S2）和双重傅里叶变换法（F2）具有显著的优越性,可不需要提前知道光纤的几何参数,就可获得模场分布、模式功率占比、群时延等特性。研究表明F2法更适合于表征高功率光纤激光的模场特性。     

高功率光纤中传输光模式与其波长相关性研究

针对高功率光纤激光模式诊断和光谱诊断的需求,研究了光纤中传输模式的波长相关性,数值计算了光纤中各个模式的模场分布随波长的变化曲线及相应的光束质量,采用双傅里叶变换F~2法实际测量了光纤模式成分与波长的关系曲线.结果表明,光纤中各个模式的模场分布随波长变化,波长越长,模场面积越大;模式的光束质量随波长变化不大,但在截止频率附近明显变差;光纤中各个模式的功率占比与波长有关.     

Beam quality factor of mixed modes emerging from a multimode step-index fiber

The beam quality factors (or M² parameter) of coherent and incoherent superposition of the several lower-order LP modes emerging from a step-index fiber have been calculated by using the second-order moment method. The results indicate that, for an individual LP mode, the M² parameter takes its maximum value when the normalized frequency V of the step-index fiber approaches the cut-off frequency, and it gradually becomes constant as V increases. In the case of incoherent superposition, the larger the fraction intensity carried by the higher-order mode, the larger the beam quality factor M². Under certain circumstances, the value of the M² parameter of the mixed mode that comprises several LP-modes contents may become even smaller and closer to the ideal Gaussian beam than that of the fundamental mode in a step-index fiber. However, in the case of the coherent superposition, the value of the M² parameter of the mixed mode may be greater than that of the higher-order constituent mode. The results reported here could be helpful for the application of the high-power fiber laser systems.     

A Novel Fiber Taper for Efficient Optical Interconnection

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High-speed modal decomposition of mode instabilities in high-power fiber lasers

A high-speed mode analysis technique is required to gain fundamental understanding of mode instabilities in high-power fiber laser systems. In this work a technique, purely based on the intensity profile of the beam, is demonstrated to be ideally suited to analyze fiber laser dynamics. This technique, together with a high-speed camera, has been applied to the study of the temporal dynamics of mode instabilities at high average powers with up to 20,000 frames per second. These measurements confirm that energy transfer between the fluctuating transversal modes takes place in millisecond-time-scale.     

一种新的阶跃折射率光纤本征函数表达形式

为了阐明阶跃折射率光纤的模式特性,根据电磁波辐射的能量守恒定律和经过狄拉克函数奇异性修正的亥姆霍兹方程,通过数学推导和证明得出:柱面径向行波场的本征函数是经过狄拉克函数修正的整数阶汉克尔函数,阶跃折射率光纤模式场的本征函数是零、一阶贝塞尔函数经过狄拉克函数修正的零、一阶诺埃曼函数和虚参量汉克尔函数.该结论揭示了光纤芯层和包层模式场分别是径向驻波场和倏逝波场的本质,并基此推导出新的光纤模式特征方程,模式存在条件,模式数目和符合光纤实际的基模归一化截止频率的理论值.     

长周期光纤光栅耦合常数的研究

长周期光纤光栅表现为前向传播的纤芯基模与同向传播的各阶次包层模式在特定波长时的模式耦合。由纤芯基模、一阶包层模式的场分布推得其功率分布 ,进一步得到两种模式的归一化常量。推导了纤芯基模与一阶各次包层模式 (HE1l/EH1l)的耦合常数。结果表明 ,纤芯基模与HE1l包层模式产生的耦合常数远大于与EH1l包层模式的耦合常数。次数低时 ,耦合常数随包层模式次数的升高而增大     

Fiber-optic vector vibroscope

A directional vibration sensor based on polarization-controlled cladding-to-core recoupling is demonstrated. A compact structure in which a short section of multi-mode fiber (MMF) stub containing a weakly tilted fiber Bragg grating (TFBG) is spliced to another single-mode fiber without any lateral offset. Multiple core modes of the MMF are coupled at the junction and appear as well defined resonances in reflection from the TFBG. Some of those resonances exhibit a strong polarization and bending dependence. Both the orientation and the amplitude of the vibrations can be determined unambiguously via dual-path power detection of the orthogonal-polarimetric lowest order LP(1n) modes. Meanwhile, the unwanted power fluctuations and temperature perturbations can be referenced out by monitoring the fundamental LP(01) mode resonance.     

Non-one-dimensional optical solitons and ideal fluid dynamics

Influence of transverse effects on the propagation of light beams in gradient glass fiber and on the pulsed mode of the second harmonic generation is investigated. Within the approaches under consideration the equations for the pulse or beam field envelopes are reduced to a system of hydrodynamictype equations for amplitudes and eikonals. These equations are used to describe the vortex and non-vortex beam channeling modes and the propagation of light bullets.     

新型三角芯抗弯曲大模场面积光子晶体光纤

提出了一种新型三角芯抗弯曲大模场面积光子晶体光纤.该结构采用单一尺寸的圆形空气孔, 降低了制作难度.在波长1.064 μm处, 在平直状态和弯曲半径为30 cm时, 模场面积分别为1386 μm²和1153 μm², 弯曲带来的模场面积减少量仅为16.85%. 当弯曲半径为30 cm时, 基模的损耗为0.087 dB/m, 二阶模的损耗大于1.5 dB/m, 大的损耗差有效保证了光纤单模运转.此外, 弯曲半径30 cm时, 弯曲方向角扩展至±180°, 光纤弯曲不再受弯曲方向的限制. 所设计的光纤结构具有大模场面积、小的模场面积减少量、 低的弯曲损耗以及低的弯曲方向敏感度等优势, 为小型化、集成化高功率激光传输及光纤激光器和放大器的研究奠定了基础. 关键词： 光子晶体光纤 大模场面积 抗弯曲 弯曲方向角     

长周期光纤光栅模式与耦合的研究

长周期光纤光栅是在光纤纤芯中沿轴向形成折射率周期性调制的带阻型光纤器件。与光纤布拉格光栅不同，它表现为前向传播的纤芯基模与同向传播的各阶次包层模式在特定波长的耦合。研究了纤芯基模、包层模式（HE1t/EH1t）及其有效折射率随波长的变化，研究了纤芯基模与包层模式（HE1t/EH1t）的耦合系数。研究表明，纤芯基模与一阶低次包层模式的有效折射率随波长增大而减小，纤芯基模与HE1t产生的耦合系数远大于与EH1t的耦合系数，并且包层模式次数较低时，耦合系数随次数的升高而增加。这与以前的研究结果有所不同。     

1 550 nm单模锥形光纤模场的演化特性

根据光纤模式传输理论,结合单模锥形光纤的结构,使用有限差分光束传播法(FD-BPM)仿真了1 550 nm波长下光纤模式的传输,分析包层内模场和纤芯内模场之间的相互影响。仿真结果表明:因为包层模的存在,随着包层内模式数的减少和纤芯对模场约束状态的变化,锥形光纤纤芯内不同区域模场分布变化明显,模场的均方根宽度在不同区域有不同程度的起伏。通过数值计算得到了光纤内模场能量的分布,发现因为包层模的存在,纤芯内模场能量减小速度比没有包层模时的速度要慢。     

Single-mode propagation in triangular tube lattice hollow-core terahertz fibers

The mode properties of hollow-core fibers with a cladding formed by a periodic arrangement of dielectric tubes in a triangular lattice for THz applications are numerically investigated. The fiber supports a high number of modes. Effectively single-mode operation can be obtain by reducing the core size, but at the expense of high fundamental mode propagation loss. Single-mode propagation can be obtained by exciting the fiber with a linear polarized gaussian beam with proper spot size.     

Mode competition in a high harmonic tunable gyrotron

The nature of the competition between a fundamental mode and a second harmonic mode depends on whether their azimuthal mode numbers are the same or different. If they are the same, the second harmonic may aid the excitation of the fundamental and eventually, at high beam currents, be suppressed by the fundamental, if the azimuthal mode numbers are different, the two modes may grow together.     

Variational analysis of Er:Ti:LiNbO3 waveguide lasers based on Gaussian/two-half Gaussian mode field distribution

Starting with two dimensional, scalar wave equation, a variational equation was established for the fundamental TE and TM modes guided in Ti:LiNbO₃ waveguides on the basis of assuming a symmetric Gaussian mode field function in the width direction and two-half Gaussian trial functions in the depth direction. The controllable waveguide fabrication parameters, including channel width, diffusion temperature, initial Ti-strip thickness and diffusion time, dependent of fundamental mode size, effective pump area, coupling efficiency between pump and laser modes, and the coupling loss between a Ti:LiNbO₃ waveguide and a fiber were numerically calculated for Z-cut Er:Ti:LiNbO₃ channel waveguide lasers at three possible emission wavelengths 1532,1563 and 1576 nm and two possible pump wavelengths 1480 and 980 nm. The calculated results were compared with those of Gaussian/Hermite–Gaussian mode field distribution in detail.     

单模光纤端面衍射光束参量与其结构参数关系研究

本文采用指数函数拟合光纤基模端面衍射场光束参量与光纤基本参数的关系，分别得到了各个光束参量与光纤归一化频率V和光纤芯层半径a关系的解析表达式。把曲线拟合得到的基于V和a表达的解析函数的计算结果，与基于光纤芯层参量U、包层衰减参量W和光纤芯层半径a表达的解析函数的计算结果相比，当1.5 相似文献   

Starting energy and current for a large diameter finite lengthbackward wave oscillator operated at the fundamental mode

We study the starting conditions for a large diameter (diameter/wavelength=4.8) finite length backward wave oscillator designed for 24-GHz operation at the fundamental TM₀₁ mode. This geometry is very promising for high power handling capability. We analyze two separate threshold conditions. First, finite length effects give rise to a threshold in electron beam energy below which oscillations cannot be sustained at any beam current. The second is the more familiar current threshold known as a start current. It is also found that the growth rate for the fundamental mode can be much larger than those of other higher order modes thus leading to coherent operation of large diameter sources free from mode competition