Bend loss calculation in single-mode graded-index fibers using variational fields

Calculation of bend loss for single-mode graded-index fibers utilizes the fundamental modal field. Using some of the single and two parameter scalar variational approximations available in the literature for this mode, the bend loss has been computed. The exact results are obtained using a well known numerical method. The accuracies of the results obtained from the use of these variational fields have been compared.     

Variational analysis of propagation characteristics in thermally diffused expanded core fibers

We present a scalar variational method for the analysis of light-propagation characteristics in thermally diffused expanded core (TEC) fibers. The method leads to simple closed-form expressions regarding the mode field diameter, numerical aperture, waveguide dispersion as well as coupling losses produced by radial, longitudinal and angular misalignments. The dependence of coupling losses on the ‘taper ratio’ and normalized frequency is investigated and previous predictions that TEC fibers transmit light more effectively in free space over the order of millimeters, is confirmed. The coupling losses between small core diameter step-index fibers typically used in most erbium-doped fiber amplifiers and TEC-fibers is also considered.     

Estimation of cut-off wavelength of rare earth doped single-mode fibers

A new empirical relation is proposed describing spectral variation of mode-field radius (MFR) as inferred from measurements in the far-field of the fiber. It is shown that using this relation, it is possible to estimate the cut-off wavelength (λ_c) of the fiber. The proposed technique is successfully tested through measurements made on two standard step index single-mode fibers, as well as on an erbium doped fiber (EDF) having λ_c falling within its strong absorption band around 980 nm. This empirical formula is more accurate than the widely used Marcuse's formula to describe spectral dependence of MFR determined through measurements made in the fiber's far-field. The proposed technique is especially suited for estimation of λ_c of doped fibers in which λ_c falls within an absorption band.     

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.     

环状光纤单模条件和远场能量分布特性

基于弱波导近似,通过数值计算得出了环状光纤保持单模状态时对芯-包层折射率差值和导光层厚度的要求,同时讨论了这些设计参数对远场能量分布的影响。计算结果表明:当环状光纤工作在单模状态时,从光纤出射的环形光束衍射至远场时形成一个中央亮斑,具有很高的能量集中度。此外,减小芯-包层折射率差值与导光层厚度有利于进一步减小远场衍射的光斑宽度。     

Surface characteristics and antistatic mechanism of plasma-treated acrylic fibers

Acrylic fibers are treated by nitrogen glow-discharge plasma to promote surface antistatic properties. The treated surfaces are characterized by scanning electron microscopy (SEM), specific surface area analysis (BET) and X-ray photoelectron spectroscopy (XPS). Plasma treatment is found to increase the surface roughness, to modify the nature and density of surface functionalities, and to drastically improve the wettability and antistatic ability of acrylic fibers.     

W形六角铁氧体BaFe18O27电子结构与导电性的第一性原理研究

采用基于第一性原理的GGA+U方法研究了BaFe₁₈O₂₇的晶体结构和基态电子结构. 以实验数据为初始结构的离子弛豫显示,由于稳定结构中离子半径的差异和2d位Fe的存在,位于BaO层中6h位的O离子脱离了实验结构中原胞的"表面"位置,产生畸变. 计算得到晶体磁矩为28 μ_B/f.u.,与实验相符. 电子态密度及能带计算表明该材料具有微弱的半金属特性,而且与c轴平行方向和垂直方向的能带色散关系有着很大不同,6g位Fe在该材料的输运特性中起着关键作用,它们形成一种"导电层",导致垂直电导率和平行电导率出现非常大的差异. 关键词： 第一性原理 W形六角铁氧体 电子结构 导电性     

Tunable properties of light propagation in photonic liquid crystal fibers

Tunable properties of light propagation in photonic crystal fibers filled with liquid crystals, called photonic liquid crystal fibers (PLCFs) are presented. The propagation properties of PLCFs strongly depend on contrast between refractive indices of the solid core (pure silica glass) and liquid crystals (LCs) filing the holes of the fiber. Due to relatively strong thermo-optical effect, we can change the refractive index of the LC by changing its temperature. Numerical analysis of light propagation in PLCF, based on two simulation methods, such as finite difference (FD) and multipole method (MM) is presented. The numerical results obtained are in good agreement with our earlier experimental results presented elsewhere [1].     

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.     

Influence of rare-earth ions on the structure and magnetic properties of barium W-type hexaferrite

Barium W-type hexaferrite with composition Ba_0.95R_0.05Mg_0.5Zn_0.5CoFe₁₆O₂₇ where R=Y, Er, Ho, Sm, Nd, Gd, and Ce ions has been prepared by the double-sintering ceramic technique. Structure of the prepared samples has been characterized by the X-ray diffraction (XRD) technique. The XRD patterns at room temperature show the presence of secondary phase with the intensity of the secondary phase increasing with increasing ionic radius of the rare earth (RE) ions. The variation of the magnetic susceptibility (χ_M) with temperature in the range 300–750 K at different magnetic field intensities (1280, 1733 and 2160 Oe) was studied by using Faraday's method. The results show that the Curie temperature (T_C) increases regularly with increasing RE ionic radius then decreases again, after which it reaches maximum value at Sm ion of radius ≈1.04 Å. This behavior was explained on the basis of the changes in Fe³⁺–O–Fe³⁺ superexchange interaction. The effective magnetic moment μ_eff. of the investigated samples was discussed in view of varying the RE element as well as the magnetization of different sublattices.     

On the analytical investigation of fields and power patterns in coaxial omniguiding Bragg fibers

Plots of the electric fields in the different sections of multilayered coaxial omniguiding Bragg fibers are presented. Investigations are made for the different dimensional values of the fiber, and a comparative study is made. It is found that the fields match very smoothly at the layer interfaces, and decay with the increase in radial distance. Particularly, field patterns are best matched in the case of large core Bragg fibers. For illustration, plots of power are also presented for two-layer and four-layer (core-clad) sections of the fiber. Power peaks are observed in both the types of fiber sections, and in this case also, it is observed that the power patterns match smoothly at the layer boundaries. Smooth match of fields as well as power patterns justifies the validity of the analytical procedure that implements the theory of Bloch formulation in omniguiding fibers.     

Kagome结构空芯光子晶体光纤纤芯参数对传输特性的影响

应用全矢量有限元方法,研究大间距Kagome结构空芯光子晶体光纤中纤芯的大小、形状与壁厚对光纤传输损耗谱的影响。结果表明,某些纤芯尺寸会造成包层中的结构缺陷,易使纤芯基模、表面模及包层模之间发生能量耦合,产生较大损耗。而纤芯形状与壁厚的改变会引起表面模式的变化,从而影响发生在基模与表面模之间反向耦合的位置和强度,使光纤传输频带变窄和损耗变大。据此,提出Kagome结构光纤的纤芯设计思路,即纤芯的大小应使包层保持完整的微结构,纤芯形状应与包层中的单元微结构相楔合,纤芯壁厚应与包层中玻璃支柱的宽度相同。     

On the validity of the variational approximation in discrete nonlinear Schrödinger equations

The variational approximation is a well known tool to approximate localized states in nonlinear systems. In the context of a discrete nonlinear Schrödinger equation with a small coupling constant, we prove error estimates for the variational approximations of site-symmetric, bond-symmetric, and twisted discrete solitons. This is shown for various trial configurations, which become increasingly more accurate as more parameters are taken. It is also shown that the variational approximation yields the correct spectral stability result and controls the oscillatory dynamics of stable discrete solitons for long but finite time intervals.     

Study of bending loss and mode field diameter in depressed inner core triple-clad single-mode optical fibers

In this paper, the bending loss and the mode field diameter (MFD) of the R-type depressed inner core triple clad single-mode optical fibers are investigated. The effects of the optical and geometrical parameters on the bending loss and the MFD are examined in these fibers. The simulation results indicate that with increasing of the core radius (a), which is desired from manufacturing point of view, the bending loss and MFD coefficients are decreased. Consequently, the large core radius can be used to optimize the bending loss in the foregoing fibers. In the meantime, simulation outcomes show that the Δ and Q have considerable impact on the bending loss in the RI and RII fibers, respectively. The MFD and bending loss is decreased with increasing of Δ, but the case is inversed for Q. Based on the presented simulations, it is found out that the bending loss strongly depends on the distribution profile of the electric field in the cladding region for a given MFD. In other words, the field amplitude and damping rate in the cladding region determine the fiber bending loss.     

Interesting effect of optical Kerr nonlinearity in expanding single-mode regime of optical fibers using dispersion-flattened profiles

An interesting effect of optical Kerr nonlinearity is reported from theoretical investigation on the fundamental and first higher mode cutoff frequencies in single-mode fibers doped with highly nonlinear materials like CdS_xSe_1−x with W-type dispersion-flattened profiles. It is observed that whereas the single-mode regime decreases absolutely for dispersion-shifted profiles due to reduction of the first higher order mode cutoff frequency as is evident from earlier studies, it increases for W-type profiles for such nonlinearity taking its effect on the nonzero fundamental mode cutoff frequency into consideration. Thus, one can go further down below the fundamental mode cutoff frequency while using W-fibers in single mode nonlinear fiber devices involving evanescent field coupling in the low V region.     

Numerical calculation of cutoff frequencies of optical fibers by a variational technique

A numerical method for calculating cutoff frequencies of modes of weakly guiding optical fibers is described. Starting from a properly constructed field representation, the method follows a variational approach to obtain an eigenvalue problem for the cutoff frequencies which can be easily solved by a standard numerical routine. The field representation uses an expansion in terms of the Laguerre–Gauss functions in the fiber core and limiting form for small arguments of modified Bessel functions in the fiber cladding. Fibers with power-law refractive index profiles and fibers with a profile showing a dip on the axis are analyzed. Results obtained for modes of arbitrary order (both azimuthal and radial) show that our method is very efficient and accurate.     

Polarization properties of Rayleigh backscattering with a high degree of coherence in single-mode fibers

The state and degree of polarization (SOP, DOP) of Rayleigh backscattering (RB) are analyzed by using Jones calculus for the case the source coherent length is larger than the fiber length. It shows that the SOP of RB is the same as the SOP of input light, and the fiber disturbance does not influence the SOP of RB. The DOP of RB is also the same as that of input light. The corresponding experiments have been carried out, and the results agree well with all the predictions.     

Determination of propagation constants in a Ti:LiNbO3 optical waveguide by using finite element and variational methods

The finite element and variational methods are used to determine the propagation constants in a titanium indiffused lithium niobate waveguide with the reconstructed refractive index profile (in depth) from the near field measurements. A subsequent second diffusion of magnesium affects the shape of the calculated electric field and we have defined an effective depth of the profile.     

Breaking the limit of maximum effective area for robust single-mode propagation in optical fibers

We propose and demonstrate a novel approach in optical fiber design in which the optical waveguide is formed by a ring of large air holes surrounding a solid silica core. With an appropriate choice of the geometrical configuration, robust single-transverse-mode propagation with a record effective area of 1417 microm2, verified by various methods, was demonstrated. A breakthrough was made toward the development of practical ultra-high-power fiber lasers as we observed negligible loss of the fiber at bending diameters as small as 15 cm.