低差分模式时延少模光纤的有限元分析及设计

针对少模光纤展开研究, 利用Comsol软件计算其模场分布、有效折射率, 进而分析光纤的差分模式时延. 通过分析不同结构参量对模式数量和差分模式时延的影响, 设计出了两种低差分模式时延的少模光纤, 即带有外下陷的渐变式光纤和多阶少模光纤. 波长在1530–1570 nm 范围内, 带有外下陷的渐变式光纤支持四模传输, LP₁₁, LP₂₁, LP₀₂模式的差分模式时延的绝对值小于0.015 ps·m^-1; 多阶少模光纤支持两模传输, LP₁₁的差分模式时延低于0.185 ps·m^-1. 两种少模光纤均具有良好的差分模式时延特性, 适于在模分复用技术中应用.     

基于少模光纤的模分复用系统多输入多输出均衡与解调

针对光纤模分复用传输系统中模式耦合串扰问题, 设计并制备了一种新型少模光纤, 其较高的模式差分群延时保证各模式信道独立传输.在此基础上, 提出一种级联多输入多输出(MIMO)延时均衡算法, 进一步减少源于模式复用器和解复用器的模式串扰, 提高基于少模光纤的模分复用传输系统的传输距离和传输容量.与传统MIMO均衡算法相比, 级联MIMO延时均衡算法在没有显著增加计算复杂度的条件下, 能够应用于模式差分群延时很大的模分复用传输系统.对单信道传输速率为 40 Gbps的四相相移键控两模复用传输系统进行仿真, 经40 km少模光纤传输后, 采用级联MIMO均衡算法较普通MIMO均衡算法有1.7 dB的质量因子的提升. 仿真结果证明, 使用少模光纤和级联MIMO延时均衡算法能够有效地消除模分复用信号间的串扰, 有望在下一代大容量光纤传输系统中获得 推广应用. 关键词： 模分复用 少模光纤 模式差分群延时 多输入多输出均衡     

折射率不敏感的级联型单模-少模-单模光纤温度传感器

提出了一种折射率不敏感的级联型单模-少模-单模光纤温度传感器。在制作传感器的过程中,设置熔接电流为100 mA,通过将少模光纤与单模光纤进行无错位熔接,以激发稳定的传输模式,形成光纤马赫-曾德尔干涉仪。由于外界环境的改变会引起少模光纤中不同模式之间相位差的改变,从而导致干涉条纹的漂移,因此通过检测干涉条纹的漂移量就可以实现待测参数的检测。少模光纤可以传输LP₀₁,LP₁₁,LP₂₁,LP₀₂共四种模式的光。实验中对长度为81.5 mm的传感器光谱进行分析可知,发生干涉的两个模式主要是LP₀₁模和LP₁₁模。利用该长度的级联型单模-少模-单模光纤传感器进行折射率和温度传感实验,结果表明,随着传感器温度不断的增加,其传输光谱出现了明显的蓝移现象,在27.6~93.8 ℃的温度变化范围内,灵敏度高达-85.9 pm·℃-1,且具有较好的线性度;在甘油折射率为1.347 1~1.443 9变化范围内,其传输光谱没有出现明显的漂移现象,灵敏度仅为3.697 34 nm·RIU-1,具有折射率不敏感特性。因此,相对于传统的包层模干涉型与多模干涉型光纤传感器,所提出的基于FMF的传感器更易于实现对传输模式的控制与分析,且具有结构简单、易于制备、灵敏度高等优点,能够避免温度与折射率同测的交叉敏感问题,可用于电力系统、生物医学、航空航天等领域的温度检测。     

Mode-locked pulse generation from an all-FMF ring laser cavity

Experimental generation of stable mode-locked pulses and cylindrical vector beams(CVBs), from an all fewmode fiber(FMF) ring laser is first reported, to the best of our knowledge. In this laser, a section of few-mode erbium-doped fiber(FM-EDF) is used as the gain medium. The FM-EDF is pumped by 976 nm laser with LP11 mode, which is simultaneously converted and multiplexed through a homemade hybrid device, i.e., wavelength division multiplexing-mode selection coupler(WDM-MSC). All the components in our experiment are connected using FMF. The resulted CVB pulses have a spectral width of 0.33 nm with a repetition rate of 30.58 MHz under the pump power of 340 m W. Moreover, both azimuthally and radially polarized CVBs were achieved with a high purity of 95%. This mode-locked CVB fiber laser with an all FMF configuration opens the way to manipulate the transverse mode in mode-locked fiber lasers.     

基于少模光纤的全光纤熔融模式选择耦合器的设计及实验研究

提出了三种基于少模光纤的全光纤熔融模式选择耦合器. 根据模式匹配原理采用单模光纤与少模光纤熔融连接方式, 运用耦合模理论及光束传播法模拟分析了模式选择耦合器的结构参数对模式选择及耦合特性的影响, 实现了单模光纤中基模到少模光纤中不同阶模式的转换, 以满足不同的应用需求. 实验上以2× 2熔融光纤耦合器为例, 采用对称和非对称熔融拉锥方式, 分别实现了从基模到LP₁₁, LP₂₁模式的转换. 实验结果表明所得到的LP₁₁, LP₂₁模式在1530–1560 nm的波长带宽范围内均有较高的模式纯净度, 且模式耦合效率高于80%, 与理论模拟结果基本一致.     

Observation of the first excited transverse mode in guided matter waves

In direct analogy to the textbook example of light guided in a few-mode fiber (FMF), we report the observation of the first excited mode of an optically guided atomic beam. We selectively excite the atomic analog of the LP?? optical mode by controlling the energy distribution of ultracold atoms loaded into the guide, resulting in a modal structure dominated by a 47(2)% population in the first excited transverse mode. The ability to guide lower-order modes has been essential to demonstrating optical effects such as multimode interferometry, slow light, and entanglement, and an atomic analog to a FMF may lead to similarly useful applications.     

Optical fiber curvature sensor based on few mode fiber

An optical fiber curvature sensor based on interference between LP₀₁–LP₀₂ modes of a circularly symmetric few mode fiber (FMF) is presented. The device consists of two single-mode fiber and a 10-cm FMF. The two single-mode fiber is offset-spliced to each end of the FMF. When the optical fiber is kept straight and fixed, the interference pattern appears in the transmitted spectrum. As the fiber device is bent, the visibility of the interference fringes (at 1530 nm) decreases, reaching values close to 0.3. The dynamic range of the device can be tailored by the proper selection of the length of FMF. The relationship between the fringe visibility and the curvature is linear while the curvature is between 11 m⁻¹ and 16 m⁻¹. The result indicates that the compact sensor can be used in the measurement of large curvature, which is also important in structural health monitoring.     

Optical fiber SPR biosensor with sandwich assay for the detection of prostate specific antigen

We present a new, sensitive, few mode fiber (FMF) surface plasmon resonance (SPR) biosensor with a sandwich assay for the detection of PSA. The side-polished FMF biosensor does not need a polarizer and a thin high-index overlayer. The optical sensitivity of the SPR sensor was determined as 2.5 × 10⁻⁶ RIU. In the SPR PSA sensor, the SPR signals were amplified by a factor of 6 in average over no secondary antibody, using the sandwich assay. The proposed FMF SPR biosensor has great potential for real-time analysis of immune reaction between biomolecules and the advantages of high-sensitivity and label-free detection.     

Multimode Coupling Theory for Kelvin-Helmholtz Instability in Incompressible Fluid

A weakly nonlinear model is proposed for multimode Kelvin-Helmholtz instability. The second-order mode coupling formula for Kelvin-Helmholtz instability in two-dimensional incompressible fluid is presented by expanding the perturbation velocity potential to second order. It is found that there is an important resonance in the course of the sum frequency mode coupling but the difference frequency mode coupling does not have. This resonance makes the sum frequency mode coupling process relatively complex. The sum frequency mode coupling is strongly dependent on time especially when the density of the two fluids is adjacent and the difference frequency mode coupling is not.     

Cladding mode coupling in long-period gratings formed in photonic crystal fibers

We extend the improved effective index method (IEIM) to analyze the cladding mode of the photonic crystal fiber (PCF) and to predict cladding mode coupling in gratings. By introducing a new step-index fiber model for the PCF, the cladding mode coupling between a LP₀₁ core mode and HE₁₁ cladding mode in long-period gratings formed in PCF is accurately predicted by the IEIM. The fiber model works well for the PCF used here not only in predicting resonant mode coupling, but also in analyzing core and low-order cladding modes.     

Three-dimensional mode coupling around a seamount

A recently developed three-dimensional normal-mode model is adopted to investigate mode coupling around a seamount in a deep water environment. As indicated by the theoretical analysis and verified by the numerical results, strong mode coupling occurs at the edge of a seamount under certain conditions. Therefore, mode coupling is critical for the investigation of the acoustic field in the presence of a seamount. In addition, the issue regarding the number of sectors assuring convergence is also presented. T...     

Comparison of quasi-symmetrical and asymmetrical mode beatings in two-section partially gain-coupled DFB lasers

The self-consistent mode-beating model is applied to a two-section partially gain-coupled distributed feedback (TS-GC DFB) laser with low coupling strengths. The model is based on introducing a fixed detuning parameter to the governing rate equations of the slowly-varying fields. The existence of quasi-symmetrical, a mix of quasi–symmetrical and asymmetrical, and asymmetrical mode beating depends on the coupling strength of the cavity. The quasi-symmetrical mode beating predicted here is similar in characteristics to the mode beating found in index-coupled self-pulsating lasers. It has a continuous and a wide tuning range. The asymmetrical mode beating is capable of producing self-pulsations over multiple self-pulsating regimes and higher self-pulsating frequencies than the quasi-symmetrical mode beating.     

Mode division multiplexing: from photonic integration to optical fiber transmission [Invited]

To overcome the capacity crunch of optical communications based on the traditional single-mode fiber(SMF), different modes in a few-mode fiber(FMF) can be employed for mode division multiplexing(MDM). MDM can also be extended to photonic integration for obtaining improved density and efficiency, as well as interconnection capacity. Therefore, MDM becomes the most promising method for maintaining the trend of "Moore's law" in photonic integration and optical fiber transmission. In this tutorial, we provide a review of MDM works and cutting-edge progresses from photonic integration to optical fiber transmission, including our recent works of MDM low-noise amplification, FMF fiber design, MDM Si photonic devices, and so on. Research and application challenges of MDM for optical communications regarding long-haul transmission and short reach interconnection are discussed as well. The content is expected to be of important value for both academic researchers and industrial engineers during the development of next-generation optical communication systems,from photonic chips to fiber links.     

2×2熔锥型单模光纤耦合器的模型

选择适当的连续函数描述了熔锥型耦合器熔锥形状的渐变特性及其相互位置关系,然后分别应用局部模式理论和变分法分析了横截面沿纵向变化的锥形区域和横截面近似不变的耦合区域内的耦合行为,得出了耦合器中任一横截面处的耦合系数和耦合功率表达式计算表明,熔锥型耦合器的总功率耦合主要发生在耦合区域,在两锥形区域,对于纤芯归一化频率小于1.08的部分,也发生了相当程度的耦合为了使耦合器的理论模型更接近于实际物理情况,锥形区域的耦合作用也是不容忽略的,从而得到了一个更精细的熔锥型光纤耦合器的模型.     

Scanning technique for investigating optical fibre dispersion

Differential mode delay measurements have been performed, both for step-index and for graded-index fibres, using a sinusoidally modulated optical source. From a differential mode delay measurement, it is possible to get all the information on the fibre baseband response with or without the effect of skew rays, and in particular of leaky rays. A theoretical model of mode coupling is developed and is used for computing the baseband response for fibres where mode coupling is not negligible.     

Theoretical analysis of widely tunable external cavity semiconductor laser with sampled fiber grating

Combination of equivalent reflection cavity model and transmission line theory is applied in theoretical analysis of external cavity semiconductor lasers with sampled fiber Bragg grating. The effects of device parameters on the resonant cavity mode and the side mode suppression ratio have been discussed. For a perfect anti-coated facet and the high coupling efficiency, it is found that the Vernier effect between combed reflectivity peaks and F-P cavity modes can be destroyed and the laser will work on multi-wavelength. The high coupling efficiency plays the same role as the lower anti-reflection coating reflectivity in the side mode suppression ratio and the mode resonance. Finally, the side mode suppression ratio is investigated in detail depending on the anti-coating reflectivity, the coupling efficiency and the high-reflection coating reflectivity.     

双层非线性耦合反应扩散系统中复杂Turing斑图

采用双层耦合的Brusselator模型, 研究了两个子系统非线性耦合时Turing 模对斑图的影响, 发现两子系统Turing 模的波数比和耦合系数的大小对斑图的形成起着重要作用. 模拟结果表明: 斑图类型随波数比值的增加, 从简单斑图发展到复杂斑图; 非线性耦合项系数在0–0.1时, 系统1中短波模在系统2失稳模的影响下不仅可形成简单六边形、四边形和条纹斑图, 两模共振耦合还可以形成蜂窝六边形、超六边形和复杂的黑眼斑图等超点阵图形, 首次在一定范围内调整控制参量观察到由简单正四边形向超六边形斑图的转化过程; 耦合系数在0.1–1时, 系统1中短波模与系统2失稳模未发生共振耦合仅观察到与系统2相同形状的简单六边形、四边形和条纹斑图. 关键词： Brusselator模型 非线性耦合 Turing模     

Mode energy transformation between two connected multimode general square-law-index optical waveguides

A coherent states method is used to investigate the mode coupling coefficients in the most general buttjoint problem for two multimode general square-law-index optical waveguides, when two different waveguides with elliptical cross-sections are to be butt-jointed with axis displacement, tilt and some turning of symmetry axes with respect to each other. The explicit expressions for the mode coupling coefficients and recurrence relations for the mode overlap integrals are obtained; the sum rules for the mode coupling coefficients are formulated and a method of experimental data analysis is proposed for this general problem. The method of calculating mode coupling coefficients is proposed for the case of the butt-joint problem when the waveguide cross-sections are only partially overlapping and then it is used to investigate the influence of misalignment on mode power distribution and pulse dispersion in the second waveguide. The experimental data are compared and it is shown that the coupling coefficients obtained in square-law-index model can be used in dispersion calculations forα-profile fibres in the case when the valueα is not far from the valueα=2.     

Exactly solvable toy model that mimics the mode coupling theory of supercooled liquid and glass transition

A toy model is proposed which incorporates the reversible mode coupling mechanism responsible for ergodic-nonergodic transition of the mode coupling theory (MCT) of structural glass transition with trivial Hamiltonian. The model can be analyzed without relying on uncontrolled approximations inevitable in the current MCT. The strength of hopping processes can be easily tuned and the ideal glass transition is reproduced only in a certain range of the strength. On the basis of the analyses of our model, we discuss about a sharp ergodic-nonergodic transition and its smearing out by "hopping."     

水平变化波导中的简正波耦合与能量转移

针对海底地形水平变化对声场能量传播和声场干涉结构的影响,对简正波之间的耦合和能量转移进行了研究.建立了一种二维大步长格式的耦合简正波模型和三维楔形波导耦合简正波模型,以便快速有效地分析简正波之间的耦合和能量转移.基于耦合简正波模型,阐述了前向声场能量在水平变化波导中传播时的转移过程.并根据射线简正波理论,解释了海底地形变化对声场能量分布的影响机理.水平变化波导中声场的仿真计算表明,当本征值虚部发生剧烈变化时声场存在着较强的简正波耦合和能量转移,且海底地形变化将导致声场能量的水平传播方向偏转至海水深度增加的方向.在声场能量转移和传播方向变化中,声场的能量趋于保留在波导中而不向海底泄漏.同时,声场能量分布受到类似于压缩或稀疏的作用,从而形成椭圆状的干涉结构.