Modal analysis and cutoff frequencies of a doubly clad plasma loaded waveguide having a lemniscates of Bernoulli-type core cross-section

The modal dispersion characteristics of EM waves in a new unconventional plasma loaded doubly clad waveguide having a shape of the lemniscates of Bernoulli-type core cross-section have been studied analytically. The proposed waveguide has three parts namely the core with slighter high index, the inner cladding with plasma and the outer cladding with air. Now using the necessary orthogonal coordinates for the proposed structure and imposing the boundary conditions under the weak guidance condition, the modal characteristics equation has been obtained considering variations in plasma width and plasma frequency, the dispersion curves and cutoff frequencies have been obtained and presented in this paper. It is noted that as the width of plasma layer increases, the cutoff frequency also increases considerably in all considered cases. This shows that using plasma width as a new parameter we can control any particular mode on our wish.     

Dispersion characteristics of distorted circular doubly clad optical fibers having plasma in inner cladding

The theoretical analysis of two distorted doubly clad optical waveguides having plasma in inner cladding region have been presented. By matching the electric field and its derivative on the various interfaces, the dispersion characteristic equations have been derived and hence the cutoff frequencies are obtained. It is observed that the cutoff frequency of both considered cases are the function of plasma frequency and plasma width. Also, the plasma in the inner cladding region provides the extra degree of freedom to control the dispersion characteristics and one can trail the cutoff frequencies dynamically in time by playing with the discharge parameters associated with the plasma.     

各向异性内包层对双包层光纤特性影响的分析

提出了单轴各向异性材料为内包层，且其主轴沿光纤轴线（z轴）方向的双包层光纤模型，推出了矢量模特征方程，重点研究了主轴折射率比kcl对波导色散的影响，针对从矢量模特征方程求解波导色散因表达式极为复杂而无法直接求解的困难，提出了一种求解波导色散的有效方法，研究结果发现可以在不改变光波导结构参量的条件下，通过改变kcl可有效地改变光纤的波导色散，也分析了kcl、几何参量S、光学参量R对低次模的传输和截止特性的影响。研究结果为获得更为理想的色散补偿、色散平坦光纤及设计新型无源光器件提供了重要的依据。     

Existence of negative dispersion and band gap in a helically inner cladded annular circular waveguide

Helical waveguide, a different type of waveguide structure, has been considered for analysis of modal dispersion characteristics. This paper deals with the study of a new type of annular waveguide, in which the inner cladding is made by the helical windings i.e. sheath helix and the outer cladding is as usual simple step-index cladding. Using vector approach, the general characteristic equation for the proposed waveguide has been derived. The modal dispersion characteristics for the lowest order modes for different pitch angles and thickness are determined and analyzed. We have found that there is an existence of the negative dispersion curve in the helically inner cladded annular circular waveguide. It means that the inner cladding is responsible for the negative dispersion which is generally obtained in a photonic band gap.     

A theoretical study of the modal dispersion characteristics of an annular circular waveguide with helical winding as the outer cladding

In this present communication, we introduce a new type of annular optical waveguide whose outer cladding is made of sheath helix. The sheath helix is made up of the dielectric material of lower refractive index than the core materials. Using vector approach, the general characteristic equation for the proposed waveguide has been derived. The modal dispersion characteristics for the lowest-order modes for different pitch angles are determined and analyzed. We observe that there is no effect of pitch angles on dispersion curve nearly up to 89° but drastic change is observed at 90° and above pitch angle. On comparison of helically inner cladded annular circular waveguide, we have found that there is not the existence of the negative dispersion curve and photonic band gap in the helically outer cladded annular circular waveguide. It means only the inner cladding is responsible for the negative dispersion as well as photonic band gap. There is no effect of the outer cladding.     

左手材料纤芯双包层光纤的传输特性

对纤芯为左手材料,内包层和外包层都为右手材料的双包层光纤进行了分析.在弱导情况下,且不考虑介质损耗,由亥姆霍兹方程出发,得到了HE、EH、TE和TM导模的色散方程,并在远离截止条件下,对其进行简化.所有的TE和TM模都是简并的.分别讨论了内包层折射率和厚度对该左手材料光纤最低阶导模的影响,画出了各自的色散曲线,并和右手材料光纤导模相应色散曲线进行了比较,发现左手材料光纤的低阶导模具有反常的色散特性.     

Analysis of waveguide dispersion characteristics of WI- and WII-type triple-clad single-mode fibers

Waveguide dispersion characteristics of two kinds of triple-clad single-mode fibers named WI and WII were examined in this paper. In view of the impossibility to deduce the expression of waveguide dispersion directly due to the complexity of the characteristic equations, a feasible approach to calculate waveguide dispersion was established. The calculated results indicate that the adjustable range of the zero-dispersion point of the triple-clad WI- and WII-type single-mode fibers is obviously wider than that of the conventional doubly clad W-type single-mode fibers in the same conditions. The influences made by the geometric parameters and optical parameters on the waveguide dispersion and cutoff characteristics of low-order modes for WI- and WII-type fibers were analyzed. The method presented to calculate waveguide dispersion can be applied to multi-clad fibers.     

Bending losses of a doubly clad slab waveguide

This communication considers the bending losses of a doubly clad slab waveguide in which the core has the largest refractive index and the inner cladding has the lowest. It is found that the bending losses are drastically reduced by inserting a low-index inner cladding between the core and the outer cladding.     

Investigation on characteristics of W-type fiber with an inner cladding made of negative refractive index materials

A doubly clad optical fiber with an inner cladding made of negative refractive index material was proposed and the modes distribution, dispersion and time delay were investigated by fully vectorial numerical method. The results indicate that HE₁₂ mode can be operated as a single mode, the cut-off frequency of modes shifts to low normalized frequency with increasing thickness of inner cladding made of negative index materials, large negative dispersion and time delay can be obtained and tailored by the inner cladding. These results provide theoretical basis for designing novel fibers or optical devices.     

Analysis of Planar Dielectric Waveguide with Chiral Cladding

The propagation characteristics of a planar waveguide with chiral media filled in the cladding are presented. The effect of chirality on cutoff, dispersion and field distribution of the waveguide are studied. The results show that the cladding chirality may lead to a single-mode operation in the planar waveguide.     

Analysis of Planar Dielectric Waveguide with Mirror-Conjugate Chiralities in the Core and the Cladding

The propagation characteristics of a planar waveguide with mirror-conjugate chiralities filled in the core and the cladding (mirror-conjugate chirality waveguide) are presented. The effects of chirality on cutoff, dispersion and field distribution characteristics of the waveguide are studied. The results show that the waveguide with desired features can be made by appropriately selecting the chiralities of the core and the cladding, because the core chirality and cladding chirality have different influences on waveguide propagation characteristics.     

Computing Eigen value equation and modal dispersion characteristics of an elliptical Bragg waveguide

Using the method of separation of variables in the elliptical coordinate system, a recursive formula for the electromagnetic fields in a new type of Bragg waveguide having elliptical core cross section with multilayered cladding is derived. The eigen equation is written in the form of Mathieu and the modified Mathieu functions and a dispersion relation is obtained for various modes supported by the proposed Bragg waveguide. The cutoff frequencies for several lower order even-odd modes have been calculated and their propagation characteristics are plotted. The results show that the dispersion curves are discontinuous and modes can exist only in particular wavelength bands. The effects of elliptical eccentricity on the mode cutoff values and mode transmission are addressed. Finally, the modal birefringence in the said waveguide is also estimated.     

Analysis of a Metal-Coated Planar Dielectric Waveguide with Chiral Cladding

The propagation characteristics of a metal-coated planar waveguide with chiral media filled in the cladding are presented. The effects of chirality on cutoff, dispersion characteristics and field distributions of the waveguide are studied. The results show that cladding chirality brings phase chopping to the transverse electric field component of those modes with odd mode numbers.     

Analysis of Circular Dielectric Waveguide with Chiral Cladding

The propagation characteristics of a circular dielectric waveguide with chiral media filled in the cladding are investigated. The relation between the cutoff frequency and the chiral admittance, the dispersion curves for some modes with different angular mode numbers and the transverse distribution of fields are presented. Some new features different from the ordinary circular dielectric waveguide are resulted. The results show that the cladding chirality may lead to a single-mode operation in the circular dielectric waveguide.     

Effect of plasma on modal dispersion characteristics of elliptical Bragg waveguide

In this paper, the dispersion characteristics of a plasma filled elliptical Bragg waveguide is investigated. The modal characteristic equations of the proposed Bragg waveguide for both ω > ω_p and ω < ω_p are derived. The effects of plasma frequency, numbers of cladding layers and the eccentricity of elliptical Bragg waveguide on the dispersion characteristics are studied. The analysis shows that the introduction of plasma in the proposed waveguide allows to control the propagation of modes.     

Enhancement of single mode operation in coaxial optical waveguide using DB boundary conditions

In this study, a competent numerical strategy to compute the dispersion of optical waveguides is presented and propagation of electromagnetic waves in a coaxial optical waveguide with DB boundary conditions is instigated. For this intend, cylindrical coordinates are here being used to derive the DB boundary conditions and to obtain field components for the modes. The propagation constant for the waveguide to be studied is determined by solving the Bessel and the modified Bessel functions. The cutoff frequencies for various lower order modes have been calculated and their dispersion characteristics are plotted correspondingly. The behavior of the coaxial optical waveguide under DB boundary conditions is shown to be significantly different from that of coaxial optical waveguide and conventional optical waveguide under traditional or tangential boundary conditions. Finally, the effect of waveguide dimensions on the mode cutoff frequencies and fabrication issues are also addressed.     

Coaxial laser cladding on an inclined substrate

This paper describes an experimental and theoretical study of the cladding mode of coaxial laser cladding on an inclined substrate. Based on the image analysis of the powder stream and clad profile measurements in coaxial laser cladding, it was found that irregular clad profiles always formed on an inclined surface and the location of the peak profile shifted away from the clad center. This phenomenon is caused by uneven distributions of powder concentration and laser beam intensity. A modified Gaussian mode for powder stream and laser beam was proposed to estimate the clad profiles on an inclined plane under laser beam irradiation. The effects of the inclined steel substrate on the CO₂ laser beam absorption and stainless-steel powder catchment were examined experimentally. The results show that both the laser absorption and the powder catchment on the mild steel decrease with increasing the cladding angle. From the analysis of laser beam mode, the clad width is equivalent to the beam spot size on the inclined substrate. However, the clad height correlates well with the distribution of the powder concentration. The results show that the Gaussian cladding mode could be adopted in various laser cladding applications such as rapid prototyping and butt welding to predict the clad profiles precisely.     

Perturbation analysis of finitely clad optical waveguides and couplers

In calculating the modal characteristics of a finitely clad optical waveguide, if the wavelength is much shorter than the core of the waveguide, then the cladding of the waveguide can be assumed to be infinite. On the other hand, if the wavelength is much longer than the core, the core can be ignored. These two approximations, although simple and accurate in the two respective extreme cases, do not provide adequate information in the transition region, where many tapered devices are operated. To overcome this a novel perturbation method is devised, in which these two approximations are combined in such a way that a result that is accurate in all cases can be obtained. Analytical expressions based on this method for describing the dispersion characteristics of finitely clad slab waveguides and circular step-index fibres are derived and compared with other conventional approximations. The combined use of this method and the coupled-mode theory for the analysis of optical couplers is also described.     

Effect of Distortions on an Annular Optical Fiber Waveguide

We present the modal analysis and dispersion curves of some asymmetric annular optical-fiber waveguides. The proposed cross-sections can be conceived as having the shape of a distorted annular circular waveguide and leads to a tolerance study of deformation in an annular optical-fiber waveguide. We obtain mathematical expressions for the modes of the proposed waveguides using boundary matching techniques under some logical assumptions. We observe that the proposed distortions are able to tune the cutoff values of annular circular waveguide at desired levels.     

等离子体辅助平板波导的传输特性及应用研究

如何灵活地控制和操纵太赫波是目前研究的热点. 根据电磁波传输理论, 导出了等离子体辅助平板波导的场分布和色散关系表达式, 计算了其传输特性, 并通过全波仿真进行了证实. 结果表明, 等离子体辅助平板波导具有带阻特性, 上边带截止频率等于等离子体频率, 等离子体层越薄, 下边带截止频率越高, 带宽越窄; 阻带内存在两种不同的物理机理, 一种与等离子体和中间媒质的谐振耦合有关, 另一种与表面波的形成有关. 此外, 本文还研究了等离子体频率及碰撞频率对传输特性的影响, 提出了通过改变等离子体频率调谐平板波导滤波器特性的方法. 同时, 采用褶皱金属结构实现了等离子体层, 设计了平板波导传感模型, 通过改变凹槽内的材料的介电常数仿真了其传感特性, 结果表明当材料的介电常数变化0.1%时, 平均截止频率变化1.8 GHz; 通过检测截止频率的变化, 传感器能明显分辨氮、汽油、液态石蜡、甘油和水, 证实了其优良的太赫传感特性. 这项工作对研究太赫波的传输及太赫器件的设计和制备具有指导意义.