Measurement of fiber propagation constants using a prism-fiber coupler

We report on the use of a prism-fiber coupler to determine the propagation constants for the scalar modes in an optical fiber by measuring, respectively, the incident angle, θ_i, and the position of the m-lines coupled out of the fiber by the prism coupler. The position is related to the value of the corresponding propagation constant. The degeneracy of the modes is shown on the m-line photographs. Good agreement is obtained between measured and calculated values of the propagation constants.     

熔锥型熊猫光纤耦合器的传输特性分析

熔锥型保偏耦合器的传输特性决定了其光学原理与功能的实现。为了从理论上分析各参量对保偏耦合器传输特性的影响,基于热-结构-电磁多物理场耦合理论,建立了熔锥型熊猫光纤耦合器双锥模型,应用有限元法,分析了各参量对熊猫光纤耦合器传输特性的影响。结果表明应力区与包层折射率差影响耦合区纵向电磁场的分布,折射率差越大,纵向电磁场分布的变形也越大;HE1x1模和HEy11模的传播常量对偏振主轴角度差不敏感,偏振主轴角度差是通过耦合系数进而影响保偏光纤耦合器的消光比的;HE1x1模的传播常量对熔锥的熔锥区横截面椭圆率比较敏感,横截面椭圆率变化6.67%时Δβx11变化0.14%,计算结果表明当熔锥区横截面椭圆率为0.56时可获得较高性能的耦合器。     

2×6熔融拉锥型单模光纤耦合器的特性分析

以线性耦合模方程为基础,推导了在弱熔、弱耦条件下2×6熔融拉锥型单模光纤耦合器的传播常量、本征模以及各端口的功率分布情况,并利用“熔融拉锥”法研制成功了两层排列结构的2×6单模光纤耦合器,实验得到的功率分布与理论分析结果基本一致。     

Simulation and experimental research on polymer fiber mode selection polished coupler

Multimode dispersion is the main obstacle for high bandwidth in multimode optical fiber （MMF） communication system. Mode selection is an effective method to oppress multimode dispersion. We propose and investigate a kind of polymer optical fiber polished coupler. Beam propagation method （BPM） is employed to calculate the coupling coefficient of transmission modes in MMF coupler, and an output pattern from coupling branch is obtained. Analysis and experiment show that this coupler can select certain modes by changing polished depth, contact area, and intersection angle of two branches, which means that the device can be employed both as a mode selector and a sensor. In addition, simulation shows that five times bandwidth enhancement may be realized by selecting modes with the polymer fiber polished coupler.     

微纳光纤环形腔海水温度的谐振光谱分析

微纳光纤环形腔作为传感器件,具有灵敏度高、响应快的优点,因而越来越广泛地应用在传感领域。环形腔的谐振光谱直接反映外界环境的变化,因此谐振光谱分析对探测环境参数至关重要。从理论和实验研究了微纳光纤环形腔谐振光谱与海水温度的关系。首先,数值计算了基模(HE₁₁)两个垂直偏振态的传播常数随光纤直径和探测波长的变化关系,计算结果表明传播常数随光纤直径增大而增大,随波长增大而减小,同时计算了传播常数随海水温度的变化,海水温度越高,传播常数越大,表明海水温度的变化会影响到模式的传播常数,因而可以通过测量谐振光谱的变化得知海水温度的变化。其次,搭建了海水温度传感实验系统,获得了微纳光纤环形腔海水温度的谐振光谱,实验发现同时存在两套谐振峰,分别对应基模的TE模和TM模,两个偏振模式的传感灵敏度分别为5.54和5.24 pm·℃-1。最后,探讨了基模两个偏振态谐振光谱的产生原因,由于结型耦合区的扭曲耦合使得两个偏振态分离,并对两个偏振模式的谐振强度进行了分析。两个模式的谐振强度不同,而且随着波长的增加,一个模的谐振强度不断增强,另一个模的谐振强度逐渐减弱。这是由于不同耦合态的耦合系数和衰减决定的,而且他们随着波长而改变。实验结果与理论计算相吻合。     

Optical vortices in anisotropic optical fibers with torsional stress

Analytical expressions for the higher-order modes with the azimuthal number equal to unity and for corresponding propagation constants of optical fibers with linear anisotropy of the fiber material and a circular anisotropy induced by torsional mechanical stress been obtained at practically important relationships between fiber parameters. The possibility of stable propagation of optical vortices in these fibers and the dependence of characteristics of sustained optical vortices on the fiber parameters are demonstrated.     

Broadly tunable multiwavelength Brillouin-erbium fiber laser using a twin-core fiber coupler

A tunable multiwavelength Brillouin-erbium fiber laser (MW-BEFL) using a twin-core fiber (TCF) coupler is proposed and demonstrated. The TCF coupler is formed by splicing a section of TCF between two single-mode fibers. By simply applying bending curvature on the TCF coupler, the peak net gain is shifted close to the Brillouin pump (BP), which has advantage for suppressing self-lasing cavity modes with low-BP-power injection. In this work, the dependency of the Stokes signals tuning range on the free spectral range (FSR) of TCF coupler is studied. It is also found that the tuning range of MW-BEFL can exceed the FSR of TCF coupler by adopting proper BP power and 980-nm pump power. Up to 40 nm tuning range of MW-BEFL in the absence of self-lasing cavity modes is achieved.     

Matching of the propagation constants in an asymmetric single-mode fused fiber coupler for core pumping thulium-doped fiber at 795 nm

We developed a fused fiber coupler (FFC) capable of multiplexing wavelengths in the range of 795 nm and 2 μm. A simple 2D simulation model to calculate the pretaper length for matching the propagation constants in the coupling region was established. Based on the numerical data, we fabricated an asymmetric FFC consisting of two different fibers with single-mode guidance for the respective wavelength, achieving a transmission of 90% in the signal fiber for both wavelengths. In order to demonstrate the application, we integrated the FFC into a core pumped thulium-doped fiber amplifier.     

Design of a single-polarization single-mode photonic crystal fiber double-core coupler

A simple single-polarization single-mode (SPSM) photonic crystal fiber (PCF) coupler with two cores is introduced. The full-vector finite-element method (FEM) is applied to analyze the modal interference phenomenon of the even and odd modes of two orthogonal polarizations and the power propagation within the two cores. Meanwhile, the SPSM coupling wavelength range and its corresponding coupling length for different structure parameters are numerically analyzed. The numerical results show that SPSM coupling can be realized with a broad range of wavelength, and the coupling length can be of millimeter magnitude. Moreover, the SPSM coupling wavelength range and the coupling length can be optimized by designing proper mirco-structure parameters of the coupler.     

低功耗聚合物波导可调耦合器的优化设计

提出并研究了一种带隔热槽的马赫-曾德尔干涉结构聚合物波导可调耦合器。仿真分析了热光电极作用下有、无隔热槽的耦合器波导温度分布,表明设置隔热槽可以降低热光调谐所需的功耗。研究了铝电极作用下不同上包层厚度对光传输损耗的影响,确定了上包层厚度。根据隔热槽与电极之间距离对热光调谐功耗的影响,结合隔热槽对导波光模式的影响及波导制备工艺,确定了隔热槽的位置。最终设计的带隔热槽聚合物波导可调耦合器实现1～0的分束比所需功耗为2.20mW,是无隔热槽的47.4%。     

Prism coupling method to excite and analyze Floquet-Bloch modes in linear and nonlinear waveguide arrays

We demonstrate an experimental method for the direct measurement of band structures of one-dimensional periodic optical media. With the help of a prism coupler that is used in a retroreflective scheme, the allowed bands of a waveguide array in lithium niobate are determined and the results are compared with numerical calculations. Furthermore, we demonstrate the suitability of this method to measure propagation constants of extended nonlinear modes inside the forbidden gap.     

Field Solution and Characteristics of Cladding Modes of Optical Fibers

In this article, we give out the formulations of the fields in a three-layer fiber. For a typical step-index single-mode fiber, we calculate propagation constants and mode field profiles of the cladding modes in it. Results show that the hybrid cladding modes, whose azimuthal quantum number is not zero, can also be rigorously divided into HE and EH modes. Comparison with the results of the two-layer geometry approximation indicates that when solving the cladding modes in a fiber the simplified model is unsuitable.     

Mode attenuation in microstructured optical fibers with absorbing coating

The characteristics of leaky modes in microstructured fibers with finite-thickness absorbing coatings are studied theoretically and experimentally. It is shown that, if the thickness of the coating is constant, the fiber has a quasi-periodic transmission spectrum. The period and depth of modulation of the spectrum are determined by resonance coupling between the modes of the fiber and coating. Since the constants of mode propagation in the coating are very sensitive to environmental parameters, the discovered effect seems to be promising for sensor applications.     

The bidirectional mode expansion method for two-dimensional waveguides: the TM case

The mode expansion propagation method is a modelling technique for a large variety of waveguide components. Until now only the case with TE modes has been reported; we give details on the mode expansion propagation method for TM modes. Instead of one type of overlap integral in the TE case, a TM analysis requires two kinds of overlap integrals. The inclusion of radiation modes in this method is discussed. The modifications in the algorithm to include waveguides with gain or loss structures are also considered.As an example, the method was used for analysing a grating-assisted codirectional coupler. In particular, the radiation loss is calculated and compared with calculations when TE modes, instead of TM modes, propagate through the codirectional coupler. The radiation losses are found to be much higher for TM modes.     

Fiber coupler for generating orbital angular momentum modes

We propose a fiber coupler consisting of a central ring and four external cores to generate up to ten orbital angular momentum (OAM) modes. Four coherent input lights are launched into the external cores and then coupled into the central ring waveguide to generate OAM modes. By changing the size of the external cores, one can selectively excite a high-order OAM mode. The quality of the generated OAM modes can be enhanced by adjusting the polarization state and the phase of input lights. We show the generation of OAM modes with odd charge numbers of -9 to +9 (i.e., 10 modes totally) with mode purity of >99% using <2?mm long fiber. This fiber coupler design can be extended to enable all-fiber spatial-mode (de)multiplexing.     

Optical vortex solitons and soliton clusters in photonic crystal fibres

We study higher-order nonlinear modes in the form of vortex solitons and soliton clusters propagating in the waveguides created in photonic crystal fibers made of a material with the focusing Kerr nonlinearity. We find numerically different families of such nonlinear modes with a nontrivial topology and study their bifurcations. We also study the soliton stability to propagation. We demonstrate that waveguides in photonic crystal fibers may support a variety of soliton clusters with the symmetries that may differ from the lattice symmetry. We also discuss briefly the case of a dual-core coupler created by two neighboring cores in a photonic crystal fiber and find numerically the profiles of symmetric and asymmetric nonlinear modes.     

Wave-matching method for mode analysis of dielectric waveguides

Frequently the cross-section of a longitudinally homogeneous dielectric waveguide may be decomposed into rectangles with constant permittivity. For points inside these rectangles the wave equation for modal fields is solved analytically by expanding into functions with harmonic or exponential dependence on the transverse coordinates. Minimization of a least-squares expression for the remaining misfit on the boundary lines allows us to determine propagation constants and fields for guided modes. Semivectorial calculations for two sets of rib waveguides and the centre sections of a directional coupler and an MMI device show very good agreement with results found in the literature. This revised version was published online in November 2006 with corrections to the Cover Date.     

三芯非线性光纤耦合器中的短脉冲光开关

利用变分法研究三芯非线性光纤耦合器中的短脉冲光开关,解析分析线性三耦合非线性Schr?dinger方程的结果与数值模拟符合得很好.并且得到在非线性光纤耦合器中孤子的耦合长度和开关阈值,与连续波情况和两芯光纤耦合器的结果不同. 关键词： 光纤耦合器 光开关 耦合长度 开关阈值     

阶跃型椭圆光纤的色散特性与几何双折射

本文采用计算介质波导色散关系的叠代矩量法计算阶跃型椭圆光纤两个基本偏振模式的传播常数、几何双折射及群折射率差.本方法简便、有效,能给出与精确数值计算吻合的结果.     

Chirofibers with helical windings - An analytical investigation

In this communication, the propagation characteristic of a chiral fiber having helical windings at the core-cladding boundary is analyzed in respect of the relative distribution of power over the sustained modes. The core and the cladding sections of the chirofiber are assumed to have different chirality admittance values while the helical winding pitch angle is considered to attain two particular values -0^° and 90°. It is observed that the introduction of helical structure leaves a dominant effect on the relative power distribution of the chiral fiber as the variations of the confinement factor with the allowed values of the propagation constants corresponding to different azimuthal modes exhibit much difference in nature for different values of the angle of pitch. It is noticed that the power confinement in the core is much higher when the pitch angle of the helix is effectively introduced by keeping the windings perpendicular to the propagation direction. Further, the property of mode degeneracy in chirofibers is also observed, which is reported to be of much significance when the helical structure remains parallel to the direction of propagation. A more profound mode degeneration characteristic is found for the chiral fiber with larger core dimension.