A simple but accurate method for prediction of splice loss in single-mode dispersion shifted trapezoidal as well as dispersion flattened graded and step W fibers

We present simple and accurate analytical expressions for transmission coefficient at the splice for both angular offset and transverse offset in case of single-mode dispersion shifted trapezoidal as well as dispersion flattened graded and step W fibers. Here, we employ the recently developed simple series expression for fundamental modal field for such fibers. The evaluation of the concerned parameters based on our formalism requires very little computations. We show that our estimations match excellently with the exact numerical results in case of the said fibers. Further, splices being highly tolerant with respect to longitudinal separation, we restrict our investigations to the cases of transverse and angular mismatches only. The present study should find application in all optical technology involving said kinds of fiber.     

Evaluation of splice loss in single-mode graded index fibres by a simple technique

We present simple and accurate analytical expressions for the evaluation of splice loss in single-mode graded index fibres. Our analysis is based on simple series expression for fundamental mode of such fibres which we have developed recently. Since splices are highly tolerant for longitudinal separation, we restrict our analysis to cases of transverse offset and angular tilt only. The concerned calculations require much less computations. With examples of step and parabolic index fibres, we show that our predictions agree excellently with the exact results. A useful collection of integration and differentiation formulae involving Bessel functions is presented in the appendix for ready reference to engineering problems of similar computational context.     

光子晶体光纤接续损耗的理论分析

基于超格子构造法,采用全矢量模型计算了光子晶体光纤的模场半径,由此出发理论分析了光子晶体光纤与普通单模光纤之间接续损耗分别受横向偏移、轴向倾斜以及模场不匹配的影响,给出了光子晶体光纤在部分常用结构参量区域{Λ,d/Λ}内与SMF-28接续损耗的理论值,讨论了光子晶体光纤各结构参量与接续损耗之间的关系。并简要分析了不同结构光子晶体光纤之间的接续损耗。结果表明,接续损耗对横向偏移和轴向倾斜都非常敏感;孔距是决定接续损耗大小最主要的因素;与普通单模光纤接续,当光子晶体光纤的孔距比该单模光纤纤芯半径大一些时,接续损耗比较小;两种不同结构光子晶体光纤之间的接续损耗大小最主要取决于它们孔距的差异。     

A simple but accurate technique of predicting radial variation of pump and signal intensities in erbium-doped graded index fiber amplifier for propagation of first higher order mode

An optically pumped doped fiber amplifier has emerged as an important device for providing broadband amplification in case of multi-channel optical signals in long-haul communication systems. In this context, dual mode graded index fiber is finding application in all optical technology. So it is needed to prescribe accurately the first order modal field in the doped fiber, both for the pump and the signal as well and it is also equally important to study their variations with the distance. Employing the simple but accurate power series expression of first higher order modal field for graded index fiber, we predict the variation of modal intensity with the radial distance from the axis of the fiber, both for pump and signal fields. Taking step and parabolic index fibers of some typical V values as examples, we show that our estimation matches excellently with the exact results. Our formalism involves little computations in the process of evaluation of the concerned parameters. Such simple but accurate analysis will be beneficial for the system engineers.     

Evaluation of dimensionless vector and scalar propagation constants for single-mode graded index fibers by a simple technique

We present simple but accurate analytical expressions for prediction of scalar and vector propagation constants for single-mode graded index fibers. Our formulation is based on the recently developed Chebyshev power series expression for the fundamental mode of graded index fibers. Taking parabolic index fiber as example, we show that our estimations agree excellently with the exact results. The concerned calculations involve very little computations.     

A simple method for prediction of first-order modal field and cladding decay parameter in graded index fiber

Based on a series expansion method involving the Chebyshev technique, we present analytical expressions for first higher order modal field as well as cladding decay parameter in case of graded index fiber. This method utilizes the formulation of a linear relation of the ratio of first- and zero-order modified Bessel functions, with reciprocal of Cladding decay parameter. This leads to the evaluation of the Cladding decay parameter from a simple equation involving a third-order determinant and the first higher order modal field is thereafter found from two simple equations. Thus the paper presents the technique of avoiding complicated calculations of Chebyshev coefficients and therefore, the present formalism will be extremely important for practical engineering problems. Taking step and parabolic index fibers as examples, we show that our estimations of the said propagation characteristics agree excellently with the available exact results. The concerned calculations involve very little computations.     

An analytical model for studying two-mode fibers: Splice loss prediction

In this paper we present a simple analytical model for the modal fields of an arbitrary graded index core two mode fiber and use it to study loss and mode conversion at a splice. The model is simple enough to obtain the modal field distributions and predict splice loss across jointed two-mode fiber links within a few percent accuracy on a pocket programable calculator.     

Mismatch considerations in laser diode to monomode fiber excitation via a hemispherical lens on the elliptical core fiber tip

We present what, as per our knowledge, is the first theoretical investigation of the coupling efficiency of a laser diode to a single-mode fiber via a hemispherical microlens on the elliptical core fiber tip in presence of possible transverse and angular misalignments. Using the ABCD matrix for refraction by a hemispherical lens on the tip of an elliptical core single-mode fiber, we present analytical expressions of such coupling efficiencies and investigate the corresponding losses due to possible transverse and angular misalignments. The concerned calculations involve little computations. Our analysis takes into consideration the limited aperture provided by the hemispherical lens. The results should be useful in optimum launch optics for designing suitable hemispherical microlenses on the tip of elliptical core fibers or circular core fibers which suffer from noncircularity.     

新型缓变锥型PCF结构接口耦合损耗特性

提出一种新型的缓变锥型光纤接口器件。该接口器件是一个长度为150mm左右的缓变锥型光子晶体光纤。利用有限元方法对模场直径差别较大的不同光纤进行拼接时的损耗问题进行研究。研究结果表明:在模场直径分别为9μm和10.4μm的光纤之间加入锥型光纤后可以有效降低耦合损耗,并且在不同波长下耦合损耗都维持在一个较低水平。因此,该接口器件能够实现光纤拼接时物理结构的过渡和不同模场直径的转换,从而使拼接损耗降到最低。     

Applicability of classical optical fiber theories to holey fibers

We discuss the applicability of well-established classical optical fiber theories to holey fibers. By appropriately defining the V parameter, we can easily estimate the fundamental properties of holey fibers, such as effective index, group-velocity dispersion, mode field diameter, beam divergence, and splice loss, through simple empirical expressions without the need for heavy numerical computations. We confirm the validity of the V parameter defined here by comparing the calculated results with the earlier experimental and numerical results.     

Spin-orbit interaction and evolution of optical eddies in perturbed weakly directing optical fibers

The transformation of the angular momentum of an optical eddy in a weakly directing perturbed optical fiber is analyzed within the spin-orbit operator representation. The case of fibers with anisotropy of the core and cladding materials and the case of fibers with an elliptic cross section are considered. The spectrum of polarization corrections to the scalar propagation constant is determined for fibers of two types. For both the strongly anisotropic and elliptic fibers, the spin angular momentum of the linearly polarized LV eddy is suppressed and the orbital angular momentum is characterized by simple oscillations with a beating length dependent only on the spin-orbit parameter of an unperturbed fiber. The orbital and spin angular momenta of the circularly polarized CV eddy in the anisotropic fiber interchange in the elliptic fiber. The orbital angular momentum can be completely restored in the strongly anisotropic fiber, whereas only the spin angular momentum is completely restored in the elliptic fiber.     

Analysis of circular fibers with an arbitrary index profile by the Galerkin method

We propose a full-vectorial Galerkin method for the analysis of circular symmetric fibers with arbitrary index profiles. A set of orthogonal Laguerre-Gauss functions is used to calculate the dispersion relation and mode fields of TE and TM modes. Examples are given for both standard step-index fibers and Bragg fibers. For standard step-index fiber with low or high index contrast, the Galerkin method agrees well with the analytical results. In the case of the TE mode of a Bragg fiber it agrees well with the asymptotic results.     

基于数字全息图的光纤折射率测量研究

利用数字全息显微层析技术构建了光纤折射率测量的测试系统, 包括光路系统与软件系统. 以物光波的相位分布曲线为依据由CCD记录最优的数字全息图; 在优化频谱图的基础之上对全息图做频域滤波, 准确选取物光波所对应的频谱范围; 使用基于角谱理论的再现算法提取出物光波的相位分布信息, 并通过模拟全息图验证相位提取的准确性; 由单幅全息图提取的相位分布信息, 结合光纤的多层模型, 还原出光纤断层面沿直径方向的折射率分布. 以单模、多模光纤为实验样品进行了测量, 测量结果与S14折射率测量仪的测量结果吻合, 精度可达10^-4. 实验对比结果表明本文方法可简单、快速、准确地测量光纤内部的折射率. 本文还对特种光纤的折射率进行了测量研究.     

Characteristic of microstructured optical fibers: an analytical approach

We have recently developed a simple analytical model to study propagation characteristics of microstructured optical fibers (MOFs). We have used this model to study splice losses between two MOFs and also between an MOF and a conventional fiber. We have also presented here the results on evolution of fundamental mode from near-field to far-field, the effective mode area and the beam divergence using this model. Comparisons with available experimental and modeling results have also been included.     

Investigation of refractive index profile and mode field distribution of optical fibers using digital holographic phase shifting interferometric method

In this paper a digital holographic phase shifting interferometric method is used to characterize the graded index (GR-IN) optical fibers. Off-axis digital holographic Mach–Zehnder like arrangement in aid of phase shifting tool is applied. Numerical reconstruction of phase shifted holograms is applied to calculate the optical phase difference distribution in the image plane, and then the optical phase differences across GR-IN fibers are extracted considering image enhancement and noise reduction steps. Also, a simple algorithm is presented to modify the position of the extracted optical phase differences across the fibers to be perpendicular to x-axis, so that it becomes easy to calculate the mean optical phase differences along the fiber. The mean optical phase differences in combination with multilayer model are applied to calculate the refractive index profiles across GR-IN optical fibers. An analytical model is presented to predict the mode distribution associated with the effective indices and the propagation coefficient of the parabolic refractive index profile GR-IN fiber.