A rigorous analysis of the distribution of power in plastic clad linear tapered fibers

An analytical investigation is presented of the propagation of power in a step-index plastic clad tapered optical fiber operating in the infrared region of the electromagnetic (EM) spectrum. The chosen materials are widely used in low cost optical links. It is assumed that the taper section has a linear profile. Following rigorous analytical approach, the general expressions for power in the core and the cladding sections are derived, and a study is presented of the variation of the relative power along the propagation direction in respect of different (meriodinal and skew) lower order modes. It is observed that, in general, the confinement of power is fairly high in the core section, and the confinement increases with the increase in the taper length and/or mode index. In the cladding region, the confinement decreases for higher taper lengths.     

On the transmission by liquid crystal tapered optical fibers

A three-layer liquid crystal tapered optical fiber (LCTF) is investigated with the emphasis on the power confinements by the low order TE and TM modes sustained in the different sections of LCTF. The outermost clad section is considered to be made of liquid crystal with radial anisotropy whereas the core and the inner clad are dielectric regions. Rigorous field expressions in the different LCTF sections are deduced, and the plots of power confinement factors (or the relative distributions) are ultimately made considering different fiber dimensions. The results reveal that the TE modes confine maximum amount of power in the outermost liquid crystal region, which is attributed to the radial anisotropy of the section. Such features of LCTFs attract their usefulness in the area of field coupling devices and optical sensing where evanescent field technique is primarily implemented.     

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.     

应用耦合模理论研究光在圆对称双包层光纤中的传输

为了深入理解双包层光纤的光场传输特性,应用耦合模理论将双包层光纤等效为相互耦合的单模光纤和环形芯光纤,研究了圆对称的双包层光纤的光场传输.通过计算单模光纤LP01模和环形芯光纤导模的耦合系数发现了环形芯光纤LP0n模的耦合系数远大于其它导模的耦合系数,且LP0n模中的高阶模比低阶模的耦合系数大.据此,应用耦合模理论计算得到了该双包层光纤的光传输特性,计算结果发现光场沿光纤轴向呈近似周期分布,且纤芯中光功率变化的平均周期随波长递增,但平均归一化功率与光纤参数紧的选择有关.     

应用耦合模理论研究光在圆对称双包层光纤中的传输(英文)

为了深入理解双包层光纤的光场传输特性,应用耦合模理论将双包层光纤等效为相互耦合的单模光纤和环形芯光纤,研究了圆对称的双包层光纤的光场传输.通过计算单模光纤LP01模和环形芯光纤导模的耦合系数发现了环形芯光纤LP0n模的耦合系数远大于其它导模的耦合系数,且LP0n模中的高阶模比低阶模的耦合系数大.据此,应用耦合模理论计算得到了该双包层光纤的光传输特性,计算结果发现光场沿光纤轴向呈近似周期分布,且纤芯中光功率变化的平均周期随波长递增,但平均归一化功率与光纤参数紧的选择有关.     

Comparative study of asymmetric versus symmetric planar slab dielectric optical waveguides

In this paper we have studied the asymmetric versus symmetric planar waveguide in terms of their usefulness in optical fiber communication systems. We have explored the thin waveguide versus thick waveguide first. Later on usefulness of asymmetric versus symmetric waveguide is carried out to target for WDM optical network application. All kinds of optical network components are fabricated on Si substrate with the point of view of their application. Here asymmetric planar structure may be more useful compared to symmetric waveguide in terms of their non-uniform power confinement properties. However, the symmetric waveguide structure may be more useful for their high power confinement properties. It is well known that the thin symmetric waveguide supports at least one mode. However the thick waveguide may support many even as well as odd modes. We study the power confinement properties for symmetric as well as asymmetric waveguide structure. We conclude that higher order modes show the nonlinear power variations. Mode field profile for various cases is discussed as well. Comparative study between asymmetric versus symmetric waveguide has a lot of significance in optical network area. It has been shown through analysis that in asymmetric waveguide, the power flows more through film region in the case of fundamental mode. Power confinement properties for asymmetric waveguide versus symmetric waveguide have been studied.     

Structural and wavelength dependence of mode power distribution for a broadband dispersion compensating microstructured fiber

Approximate empirical analysis of mode power distribution (MPD) carried by the fundamental mode is newly investigated based on a broadband dispersion compensating microstructured fiber (MF). The fraction of modal power in the core region, η, is defined with the help of extending the applicability of well-established classical optical fiber theories to MFs. In doing so, the influences of structural parameters and wavelength on MPD characteristics of the fundamental mode are systematically analyzed in detail based on simple physically consistent concepts of conventional fibers. As a result, it is shown that for the optimum MF design in Ref. [11] as a multimode fiber, when the number of guiding modes increases, the mode power ratio of the fundamental mode constantly increases from 69.4%; in addition, we find that as wavelength increases within 1.2-1.6 μm, mode power confinement ability of the fundamental mode is lessened for single-mode propagation, whereas for multimode propagation it becomes enhanced.     

Effect of multimodal coupling in imaging micro-endoscopic fiber bundle on optical coherence tomography

The effect of higher-order modes in fiber bundle imager-based optical coherence tomography (OCT) has been theoretically modeled using coupled fiber mode analysis ignoring the polarization and core size variation in order to visualize the pure effect of multimodal coupling of the imaging bundle. In this model, the optical imaging fiber couples several higher-order modes in addition to the fundamental one due to its high numerical aperture for achieving light confinement to the single core pixel. Those modes become evident in a distance domain using A-mode (depth) OCT based on a mirror sample experiment where multiple peaks are generated by the spatial convolution and coherence function of the light source. The distance between the peaks corresponding to each mode can be estimated by considering the effective indices of coupled (guided) modes obtained from numerically solving the fiber mode characteristics equations and the fiber length. The results have been compared for various types (fiber dimensions and wavelengths) and lengths of fibers, which have mode separation of 715 μm (1404 μm) and 764 μm (1527 μm) for the measurement and analysis, respectively in a 152.5 mm (305 mm)-long imaging fiber.     

零折射率手征包层光纤中导模的奇异特性

 对零折射率手征介质（介电常量和磁导率同时等于零的手征介质）作为包层的光纤中模式的奇异特性进行了理论研究.给出了导模的电磁场分布公式,推导出导模色散方程和功率的数学表达式,通过数值计算,给出了不同手征参量情形下的色散曲线,讨论了手征参量对归一化功率的影响,发现了导模的一些奇异特性,如存在表面波模、基模单模区,出现传播常量双值、模式交叉、功率储存现象等,特别是出现了纤芯和包层中的功率都为负值的完全后向波奇异模式.     

非线性包层光纤中TE0模的传播特性分析

本文对具有Kerr非线性包层和线性匀芯的光纤在不同光功率下的传播特性进行了理论分析和数值计算.结果表明,在适当光功率下,当模折射率N等于芯折射率n_f时,芯场变得相当均匀,而包层场迅速衰减.对于自聚焦包层,当从N<n_f逐渐变到N>n_f时,芯场剖面越过N=n_f从凸变凹.在拐点,芯场成为均匀的.论文给出了模场分布和光功率与模折射率的关系.指出纤芯出现匀场预期会在光纤器件、光电子器件、光子器件中得到应用.     

Electromagnetic field confined and tailored with a few air holes in a photonic-crystal fiber

Conventional and scanning near-field optical microscopy techniques are cross referenced to femtosecond nonlinear-optical measurements and finite-element numerical simulations to visualize and analyze a strong confinement of electromagnetic radiation in guided modes of a photonic-crystal fiber with only a few air holes surrounding the fiber core. A nonlinear coefficient of about 120 W⁻¹ km⁻¹ is achieved at the wavelength of 670 nm for a fused-silica fiber with a full hexagonal cycle of closely packed air holes around the fiber core. The removal of a single element from this array of air holes is shown to frustrate field confinement in guided modes, leading to mode leakage.     

Twisted clad microstructured optical fibers: revisited

The paper discusses confinements corresponding to low-order sustained hybrid modes in twisted clad microstructured optical fiber with twists in the form of conducting sheath helix structure introduced at the core–clad interface and inclined at certain angles. Dispersion relations for such fiber structures are deduced under strict electromagnetic boundary conditions. Varying the angle of pitch (of the introduced conducting sheath helix), investigations in respect of the dependence of power confinements due to the existing low-order hybrid modes have been carried out. The obtained results indicate the usefulness of such microstructured fibers for attenuation or amplification of power in the guide.     

Large-pitch kagome-structured hollow-core photonic crystal fiber

We report the fabrication and characterization of a new type of hollow-core photonic crystal fiber based on large-pitch (approximately 12 microm) kagome lattice cladding. The optical characteristics of the 19-cell, 7-cell, and single-cell core defect fibers include broad optical transmission bands covering the visible and near-IR parts of the spectrum with relatively low loss and low chromatic dispersion, no detectable surface modes and high confinement of light in the core. Various applications of such a novel fiber are also discussed, including gas sensing, quantum optics, and high harmonic generation.     

大芯径大数值孔径阶跃光纤传导模特征方程求解

导出了相邻阶第二类变型贝塞尔函数的比值在小宗量的逼近形式,利用Lentz—Thompson方法计算相邻阶第一类贝塞尔函数的比值在小宗量的值,计算精度为浮点数系统的最低有效位。从而解决了求解大芯径、大数值孔径阶跃光纤的特征方程时．贝塞尔函数溢出双精度浮点数表示范围的问题。分别对多模石英光纤和大芯径大数值孔径的聚合物光纤的传导模特征方程进行了求解,石英光纤的传导模特征值计算结果与Optiwave公司的软件一致;对于聚合物光纤,算法给出了所有模式的计算结果,其中模式角向序数小于70的计算结果与Optiwave公司的软件一致,而Optiwave公司的软件不能计算角向序数大于70的模式。     

双包层手性光纤中导模的光功率特性

在对W型阶跃双包层手性光纤进行解析求解的基础上,对几个低阶导模的功率特性进行了仔细的研究.对典型的内包层厚度,计算了几个低阶模留在纤芯中的光功率随纤芯和内外包层的手性参数的变化关系曲线,讨论了纤芯和内外包层的手性参量以及内包层厚度对不同符号模式的功率特性的影响.内包层厚度越大,功率越被限制在纤芯中.随着纤芯中手性参量的增大, HE－11模和HE－21模（HE01模）限制在纤芯中的光功率缓慢增加,而HE11模和HE21模（HE02模）限制在纤芯中的光功率则逐渐减小.包层手性参量的作用正好与此相反.     

多叶应力区扭转光纤的耦合模理论

从线性极化模的耦合模理论出发,分析了双叶应力区扭转光纤,即扭转保偏光纤,和四叶应力区扭转光纤.推导应力引起的在光纤芯区各向异性的介质张量表达式;利用该表达式得到线性极化模在有转角时的多叶应力区光纤中传输的耦合模方程;从而推得多叶应力区扭转光纤中的超本地模,由超本地模的相位常数来确定该光纤的双折射.在双叶、四叶应力区理想对称的条件下,前者可以得到较高的椭圆双折射,而后者则无任何双折射.同样的结论可推广到8叶或16叶的扭转光纤,都无任何双折射.     

端面光斑对阶跃光纤中轴对称导模功率的影响

由波动方程的积分解导出了光纤中各轴对称传导模式传输功率的解析表达式, 该式包含了入射光对模式功率的影响, 同时也反映了不同阶次模式间的差别. 在此基础上, 以高斯光斑模拟光纤端面的实际光斑, 讨论了聚焦情况对各轴对称模式间功率分配情况的影响. 结果表明, 把工作频率和聚焦情况综合起来加以调整, 可达到压制某些模式而提升另一些模式的目的.     

左手材料阶跃型光纤的模场特性

从Maxwell方程组出发,讨论纤芯为左手材料,包层为右手材料阶跃型光纤,推导了矢量解的场方程.经过数学计算,求得了左手材料阶跃型光纤矢量解的特征方程.根据矢量模的分类,找到了TE模、TM模、HE模和EH模的特征方程.根据各模的特征方程,并且与右手材料光纤相关的特征方程,在靠近截止和远离截止两种情况下进行了比较,得到了左手材料光纤的某些奇异特性.     

椭圆芯手征光纤的基模特性研究

对椭圆芯手征光纤进行了严格的求解，得到了模式场的解析解；推了了模式特征方程；并对基模的偶模和奇模特征方程进行数值求解，研究了手征参数对不同椭圆比下基模的色散曲线，双折射，模间色散等特性的影响。     

Calculation of nonlinear waves guided by optical fibers with an inhomogeneous nonlinear core

A circular optical fiber consisting of an inhomogeneous nonlinear core bounded by a linear cladding medium is considered here. In this case, the calculation of the nonlinear modes is of practical importance because of the joint effects of nonlinearity and inhomogeneity on the field concentration and the achievable optical confinement. Numerical results indicate that the combination of this type of inhomogeneity along with the characteristics of nonlinearity may play a significant role towards the optical confinement in the core.