光纤纤芯及包层模有效折射率计算及仿真

光纤结构设计、模间色散求解、光纤光栅模式耦合等问题的研究, 都需要对光纤纤芯及包层模的有效折射率进行精确计算. 本文以光纤三层结构模型为基础, 结合该模型下的模式本征方程, 使用截弦法求解了纤芯模有效折射率, 并将计算结果与COMSOL软件模拟的对应纤芯模的传输光场进行对比, 验证了计算结果正确.使用区间遍历算法对包层模有效折射率进行了求解, 与已有的传统方法相比, 该方法可以有效防止求解过程中根的遗漏、避免特征方程产生的奇点, 并能保证模式的正规性.本文采用Mathematica软件对求解过程进行仿真, 获得了纤芯模和包层模有效折射率与波长关系曲线. 关键词： 光纤传输模式 有效折射率 截弦法 区间遍历算法     

Temperature dependence of the refractive index of optical fibers

Many experimental investigations on the temperature dependence of the refractive index of optical fibers have been reported previously, however a satisfying theoretical explanation for it is still absent. In this paper, a theoretical model about the temperature dependence of the refractive index of optical fibers is presented and it is in agreement with the previous experimental results. This work is a significant reference for the research and development of temperature sensors based on optical fiber delay lines.     

Fabrication and study of microstructured optical fibers with elliptical holes

We report the fabrication of what are believed to be the first microstructured optical fibers with uniformly oriented elliptical holes. A high degree of hole ellipticity is achieved with a simple technique that relies on hole deformation during fiber draw. Both form and stress-optic birefringence are characterized over a broad wavelength range. These measurements are in excellent agreement with numerical modeling and demonstrate a birefringence as high as 1.0 x 10(-4) at a wavelength of 850 nm.     

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.     

Single parameter scalar variational analysis for single-mode fibers with smoothed-out and composite refractive index profiles: A comparative study

A number of single parameter variational approximations for single-mode graded-index fibers have been reported in the literature. One of these approximations was also proposed by us some time ago which describes the propagation characteristics of such fibers having power-law profile with a high degree of accuracy. Using these approximations we have computed the propagation constant and the normalized dispersion for single-mode fibers with smoothed-out and composite refractive index profiles which are of great current interest. We have shown that the single parameter approximation proposed by us earlier computes these propagation characteristics with a better degree of accuracy than all other single parameter approximations even for these special types of profiles.     

Method of integral equations in the theory of microstructured optical fibers

A method for designing microstructured optical fibers that is based on exact integral equations for the transverse components of the magnetic field of the mode is proposed. A solution to the vector waveguide problem for fibers with a finite number of circular capillaries in the round cavity of the cladding can be refined successively. Quartz fibers with hexagonal capillary rings are also studied.     

Simultaneous determination of particle size and refractive index by time-resolved Mie scattering

With this numerical study we have investigated the pulse-induced and time-resolved Mie scattering with the aim of determining the size and the refractive index of transparent spherical particles simultaneously. The temporal interval between the scattering light signals of two different orders of scattered light allows only particle sizing. But if it is possible to detect three different orders of scattered light, then we have two independent time intervals. This situation is given if the detector has a position about θ=90°. With these scattering angle signals of reflection and refraction of 1st and 3rd order appear with approximately the same intensity. Then the numerical quotient of the two temporal intervals between these scattering orders is a function of the refractive index only. We have calculated these specific quotients by models of geometric optics for 1.1≤m≤1.6 and 75°≤θ≤90° and have seen that there is a very high agreement with the results of time-resolved Mie calculations.     

Spectroturbidimetric determination of the size,concentration, and refractive index of silica nanoparticles

A method for determining the refractive index, size, and concentration of silica nanoparticles used as cores in synthesis of gold nanoshells is described. The average refractive index of silica nanoparticles, n = 1.475 ± 0.005, is determined by a modified immersion method, which involves spectroturbidimetry data in immersion media (dimethyl sulfoxide + ethanol). Working calibrations are obtained, which allow one to determine the size and concentration of silica particles and, correspondingly, the concentration of gold nanoshells in final preparations from measured values of the wavelength exponent and optical density.     

Polaron effects on the optical refractive index changes in asymmetrical quantum wells

The optical refractive index changes considering polaron effects in asymmetrical quantum wells are theoretically studied. It is shown that a pronounced optical refractive index changes dependence of the quantum well parameters can be obtained. Moreover, the theoretical values of the optical refractive index changes obviously increase when considering the electron-LO-phonon interaction.     

On determination of refractive index profile in optical waveguides formed by ion implantation

We report a refinement of the Fermi function approximation to compute the Refractive Index Profile of optical waveguides formed by ion implantation. The computation is carried out at an exact value of the phase step at the upper boundary of a planar optical waveguide with allowances for the perturbative influence of the prism coupling element. It is shown that under real experimental conditions the method can be simplified and is applicable to the low-mode waveguides.     

Splicing of index-guiding microstructured optical fibers and single-mode fibers by controlled air-hole collapse: an analytical approach

Low-loss splicing of index-guiding microstructured optical fibers (MOFs) and conventional single-mode fibers (SMFs) can be achieved by enlarging the mode field diameter of MOFs, which leads to an optimum mode field match at the joint interface. We study analytically the low-loss fusion splicing between an MOF and an SMF by expanding the modal field of MOF, using the controlled air-hole collapse method. Our method is simple and offers analytical solution for light coupling between MOFs and SMFs. Comparisons with available experimental and simulation results have also been included.     

In situ determination of refractive index and size of Bacillus spores by light transmission

Light-extinction measurements in the wavelength range of 400 to 1000 nm are performed in situ on Bacillus subtilis endospores during heat-shock-induced activation. Simultaneous information on particle size and refractive indices during activation is calculated from the transmission spectra by use of the Gaussian ray approximation of anomalous diffraction theory. During activation the refractive index of the core decreases from 1.51 to 1.39, and the size increases from 0.38 to 0.6 microm.     

Laser radiation effects on optical absorptions and refractive index in a quantum dot

An investigation of the laser radiation effects of a hydrogenic impurity in a quantum dot has been performed by using the matrix diagonalization method. We find that the laser field amplitude has an important influence on the linear, third-order nonlinear, and total absorption coefficients as well as the refractive index changes.     

近地面折射率结构常数的长期测量和统计分析

 统计分析了1997～2000年合肥地区一复杂地形处近地面折射率结构常数的测量数据。给出了不同下垫面、不同季节、不同高度上的统计结果。白天强弱依次为6月、5月、4月、9月、1月。白天水面上比草地上小，夜间比草地上大。不论白天还是夜晚，近地面结构常数都随高度递减，且3～10m高的结构常数递减较慢，10～15m高的结构常数递减较快。结构常数随月份的日变化与模式计算结果基本一致。     

Analysis of backscattered light from step-index optical fibers: Refractive index and radius ratio determination

A method has been suggested for the determination of the relative size of the core and cladding,c=a/b, and of the refractive indices of the core and cladding,n ₁ andn ₂, of step-index circular fibers by using the analysis of backscattered light. The method is suitable for fibers withc (1–1·5). Comparison with the methods used so far shows that the method extends the range of magnitude ofc for large-core fibers and raises the precision of determination ofc, n ₁ andn ₂.Paper presented at the 16th Microsymposium IUPAC, Prague July 12–16, 1976.I am thankful to Dr. B. Sedlacek for valuable comments and stimulating discussions.     

Femtosecond laser writing of optical waveguides with controllable core size in high refractive index glass

Optical waveguides have been produced by femtosecond-laser writing in a high linear and nonlinear refractive index glass (SF57). Light guiding occurs nearby a central damaged zone due to the collateral densification caused by shockwaves generated in the focal volume. High pulse energies are required to induce a modified region capable of supporting a guided mode. An alternative processing method, based on using multiple structures, has been successfully used for the production of waveguides with controllable core size at low energies. PACS 42.82.Et; 42.65.Re; 42.70.Ce; 81.20.–n     

Neutron refractive index of liquid mixtures in the critical region

Size effects in neutron optics of mesoscale liquid mixtures in the critical region are studied. It is shown that the Schrödinger equation for the neutron wave function can be presented as the corresponding electrodynamical wave equation with the Fermi pseudopotential for a binary liquid mixture taken into account. Formulae for the neutron refractive index of bulk and confined liquid mixtures are derived and discussed. As an example, numerical calculations for a mixture of ethane and carbon dioxide C₂H₆-CO₂ are presented, which illustrates a good agreement between experimental and theoretical data.     

Negative refractive index, perfect lenses and checkerboards: Trapping and imaging effects in folded optical spaces

Newly discovered metamaterials have opened new vistas for better control of light via negative refraction, whereby light refracts in the “wrong” manner. These are dielectric and metallic composite materials structured at subwavelength lengthscales. Their building blocks consist of local resonators such as conducting thin bars and split rings driving the material parameters such as the dielectric permittivity and magnetic permeability to negative (complex) values. Combined together, these structural elements can bring about a (complex valued) negative effective refractive index for the Snell–Descartes law and result in negative refraction of radiation. Negative refractive index materials can support a host of surface plasmon states for both polarizations of light. This makes possible unique effects such as imaging with subwavelength image resolution through the Pendry–Veselago slab lens. Other geometries have also been investigated, such as cylindrical or spherical lenses that enable a magnification of images with subwavelength resolution. Superlenses of three-fold (equilateral triangle), four-fold (square) and six-fold (hexagonal) geometry allow for multiple images, respectively two, three, and five. Generalization to rectangular and triangular checkerboards consisting of alternating cells of positive and negative refractive index represents a very singular situation in which the density of modes diverges at the corners, with an infinity of images. Sine-cosecant anisotropic heterogeneous square and triangular checkerboards can be respectively mapped onto three-dimensional cubic and icosahedral corner lenses consisting of alternating positive and negative refractive regions. All such systems with corners between negative and positive refractive media display very singular behavior with the local density of states becoming infinitely large at the corner, in the limit of no dissipation. We investigate all of these, using the unifying viewpoint of transformation optics. To cite this article: S. Guenneau, S.A. Ramakrishna, C. R. Physique 10 (2009).