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.     

Improvement of coupling efficiency between a 0.82 μm wavelength laser diode and a VAD single-mode fibre by conical microlenses

Conical microlenses have been fabricated directly on the end faces of single-mode fibres which demonstrate −1.8 dB coupling loss to a 0.82 μm laser diode. Experimental results on coupling loss, lateral and axial offsets are also reported.     

Twin-hollow-core optical fibres

Twin-hollow-core microstructured optical fibres have been fabricated and characterised for the first time. The fibre cladding structure results in guidance by the inhibited coupling mechanism, in which there is a low overlap between the core modes and surrounding structure. This results in minimal interaction between the modes of each core in the transmission bands of the fibre and hence minimal coupling between the cores. It is shown that light is able to couple between the cores via coupling to cladding struts in the high loss wavelength bands.     

Effects of joints on mode power distribution and losses in multimode optical fibres

The transformation of mode power distribution at a joint between two different (even elliptical) optical fibres is studied. Simple analytical expressions are obtained also in the most general coupling and mismatch configuration, provided that coupling errors and mismatch parameters are small. Detailed calculations of joint losses are performed in each case, including propagation effects in both fibres. Original results are obtained particularly in the cases of misalignment, end separation between the fibres, mismatches in index profiles and coupling between two elliptical fibres. A strong dependence on the launching conditions and the amount of mode coupling is evidenced in all cases.     

Modelisation de la propagation dans les fibres optiques

This paper presents a new method for investigating the effects of mode conversion due to microbends on the transmission characteristics (attenuation, frequency response ...) of multimode optical fibres. The method applies to both step-index and graded-index fibres and takes into account any dependence of the mode coupling coefficient upon propagation angle. The general theory of propagation with mode coupling is developed and we introduce the concept of modes. The propagation problem is then simplified by using hypotheses based on exerpimental results and the solution is obtained with the help of a computer. Practical results concerning the increase of attenuation due to microbends are presented, from which the precautions to be taken when cabling fibres are deduced. It is also shown that in every practical case encountered, the prediction of the total attenuation and of the frequency response of a given fibre, when the launching conditions are specified is obtained from analytical expressions which approximate well with the exact results obtained from the computer. All these results make the method powerful and very useful for system designers when evaluating the effect of introducing an optical fibre into a transmission system.     

Modal characteristics of few-mode silica-based photonic crystal fibres

The modal characteristics of silica-based photonic crystal fibres (PCF) are examined through a scalar modal analysis combined with an effective V-parameter model using a finite-difference method. Cut off frequencies of the lowest-order modes and the normalised dispersion curves are given and are found to be quasi-independent of the hole-size-to-pitch ratio. A simple and accurate empirical relation for the cutoff wavelength is given for the first higher-order mode for fibres with hole-size-to-pitch ratio below 0.7. These results should be very useful in designing PCF's for various applications.     

Theory of power coupling between multimode optical fibres

In this paper power coupling between two multimode optical fibres is investigated. A formalism (based on a geometrical approach) suitable for analysing any coupling configuration between fibres with any index profile and any radiance distribution is presented. The model is applied to obtain coupling losses both in uniformly excited parabolic- and step-index fibres, in the presence of all kinds of coupling errors. Numerical results of extensive utility are shown and useful asymptotic expressions of coupling efficiency, valid for small values of coupling error parameters, are derived together with some practical rules for error combination. Mention is also made of some experimental results we have obtained, which corroborate some previous assertions. The work can supply a useful tool in the design of joints and connectors between single fibres or optical cables.     

Ag微米管阵列修饰的聚合物光子晶体光纤制备工艺研究

以聚合物光子晶体光纤空气孔阵列为模板,制备了Ag微米管阵列修饰的聚合物光子晶体光纤.讨论了银氨溶液和葡萄糖溶液浓度以及反应时间和温度对银微米管形貌的影响.在Ag(NH3)+2与C6H6O浓度比为2,反应时间为20 min,温度为65℃条件下光纤孔洞表面获得了规则的Ag微米管状结构,并对Ag的生长机理进行了讨论.     

矩形槽同轴布拉格结构的模式匹配分析方法及实验验证

基于模式匹配法建立了矩形槽同轴布拉格结构的全波耦合分析模型, 推导出了不同模式反射率和传输率的计算式, 并采用公开报道的实验数据验证了该理论模型. 在此基础上就本文理论与其他相关的理论方法进行了比较, 发现以前的理论近似模型由于忽略了矩形槽中的消失模而使传输率的频率响应曲线发生偏差. 本文建立的理论方法有望为矩形槽同轴布拉格结构的特性研究和工程实践提供一种理论分析手段. 关键词： 同轴布拉格结构 矩形波纹 模式匹配法 模式耦合     

Effects of stray reflections on the performance of semiconductor laser amplifiers as in-line optical repeaters

The effects of stray reflections from the ends of optical fibres on the gain and spontaneous emission of a semiconductor laser amplifier have been analysed. The transfer matrix method (TMM) has been applied to give efficient solutions to the problem. The theoretical results show that these stray reflections can seriously deteriorate the performance of a single amplifier and can degrade the overall performance of a system incorporating a cascaded chain of amplifiers as in-line repeaters.     

Evaluation of the uncertainty of element determination using the energy‐dispersive x‐ray fluorescence technique and the emission–transmission method

A detailed study of the uncertainty quantification procedure for the determination of element mass fractions in a soil matrix by using energy‐dispersive x‐ray fluorescence spectrometry and the emission–transmission method was carried out. The particular case of secondary target excitation with an Mo target and Mo anode x‐ray tube was addressed. The effects of simplification of the theoretical model on the determined mass fractions of elements were studied. The main contributions to the uncertainty were identified and the ways of reducing their magnitude were indicated. The approach was found to be helpful in designing an optimum analytical strategy. It can also be used as guidance for assessing the uncertainty in other modes of x‐ray fluorescence analysis. Copyright © 2003 John Wiley & Sons, Ltd.     

Dimensional crossover of a Rabi-coupled two-component Bose–Einstein condensate in an optical lattice

Dimensionality is a central concept in developing the theory of low-dimensional physics.However,previous research on dimensional crossover in the context of a Bose-Einstein condensate(BEC) has focused on the single-component BEC.To our best knowledge,further consideration of the two-component internal degrees of freedom on the effects of dimensional crossover is still lacking.In this work,we are motivated to investigate the dimensional crossover in a three-dimensional(3D) Rabi-coupled two-compon...     

X‐ray spectromicroscopy in soil and environmental sciences

X‐ray microscopy is capable of imaging particles in the nanometer size range directly with sub‐micrometer spatial resolution and can be combined with high spectral resolution for spectromicroscopy studies. Two types of microscopes are common in X‐ray microscopy: the transmission X‐ray microscope and the scanning transmission X‐ray microscope; their set‐ups are explained in this paper. While the former takes high‐resolution images from an object with exposure times of seconds or faster, the latter is very well suited as an analytical instrument for spectromicroscopy. The morphology of clusters or particles from soil and sediment samples has been visualized using a transmission X‐ray microscope. Images are shown from a cryo‐tomography experiment based on X‐ray microscopy images to obtain information about the three‐dimensional structure of clusters of humic substances. The analysis of a stack of images taken with a scanning transmission X‐ray microscope to combine morphology and chemistry within a soil sample is shown. X‐ray fluorescence is a method ideally applicable to the study of elemental distributions and binding states of elements even on a trace level using X‐ray energies above 1 keV.     

Experimental investigation of fibre-optical confocal scanning microscopy: Including a comparison with pinhole detection

Reported in this paper are the experimental results on fibre-optical confocal scanning microscopy (FOCSM). We have performed various measurements in the FOCSM involving axial responses, edge responses, signal levels and interference axial responses in order to demonstrate the differences in confocal scanning imaging between using optical fibres and using pinholes. Experimentally measured results are compared with our theoretical predictions, revealing good agreements. It is shown convincingly that the FOCSM is a purely coherent imaging system as predicted in our previous theory.     

纤维光锥有效透过率的理论分析

光锥的有效透过率不仅影响光锥与CCD的耦合效率,而且会影响到耦合器件的信噪比.本文主要从理论上讨论了影响光锥有效透过率性能的因素,从纤维光学元件的实际内部结构推导出了其实际有效填充率,从光线在光锥中的传输特征推导出了光线在光锥中传播时产生的衰减损耗,最后提出了提高光锥有效透过率的方法和途径.     

Theory of dispersion in lossless multimode optical fibres

Using the ray propagation model generalized expressions for the impulse response of multimode fibres have been derived. The analysis has been applied to lossless fibres for pulses having both gaussian and lambertian spatial distributions as well as impulse and gaussian temporal distributions. Detailed results are given for output pulse shapes and fibre dispersions for various configurations.     

Circularly birefringent optical fibres: new proposals

This paper deals with a family of azimuthally inhomogeneous fibres whose guided modes have been shown, in a previous paper, to have analytical expressions. It elaborates on the per-unit-length torque-induced circular birefringence as a function of the geometrical parameters and of the refractive indices. Recipes are given for enhancing the birefringence above that of an azimuthally homogeneous fibre. The coupling loss, which is unavoidable when such a fibre is fed by a circularly symmetrical fibre or beam, is shown not to be a major concern.     

Radiation properties of a uniaxial chiral quadratic inhomogeneous slab under oblique incidence

In this paper, the focussing of the transmitted electromagnetic field through a quadric inhomogeneous slab of lossless uniaxial chiral medium is derived using transmission coefficient under oblique incidence. The inhomogeneity in the uniaxial chiral slab has been incorporated through permittivity parameter. Asymptotic ray theory provides valid field everywhere except at focal point where it gives infinite value. Singularity of the field at focal point is addressed using Maslov's method. The derived analytical field expressions at caustic or focal point of uniaxial quadratic inhomogeneous slab have been solved numerically using MATHEMATICA. The effects of chirality parameter, axial permittivity, transvers permittivity, angle of incidence on the refracted field are discussed and the effects of Brewster angle on the focussed field are also discussed. The results obtained using Maslov's method are compared with Huygens–Kirchhoff's integral which are in good agreement.