Dispersion characteristics of cylindrical metal-clad opticalwaveguides

A study of the dispersion characteristics of cylindrical metal-clad optical waveguides indicates that: (1) the propagation of electromagnetic energy in metal-clad optical waveguides differs in principal from that of dielectric step-index waveguides and ideal metallic waveguides; (2) hybrid modes exist in this structure, and the cutoff frequency of the HE₁₁ mode does not equal zero; (3) TM and HE(EH) modes have attenuation higher at least by an order of magnitude than that of TE modes; and (4) the surface plasma wave can propagate in this structure. Thus, the lowest order mode is TE₀₁      

The effect of fabrication parameters on a ridge Mach-Zehnder interferometric (MZI) modulator

A study of Mach-Zehnder interferometer (MZI) modulators using unetched and etched Ti:LiNbO/sub 3/ waveguides has been made. A full vectorial finite-element-based mode solver was used, followed by a finite element-based solution of the Laplace equation to calculate the electrooptic effect and, subsequently, the half-wave voltage, V/sub /spl pi// The optical loss due to the metal electrodes was also found using the H-field finite-element method (FEM) incorporating the perturbation method. The microwave effective index, n/sub m/, and the characteristic impedance of the metal electrodes, Z/sub c/, were also found for a number of electrode thicknesses and ridge heights. A semivectorial finite-element beam propagation method (SVFEBPM) was used to estimate the radiation loss for the curved input and output (I/O) waveguides of the MZI. The device characteristics were then studied by making changes to a number of fabrication parameters, of which the two most important were found to be the etch depth of the ridge and the thickness of the SiO/sub 2/ buffer layer.     

Theory Of Optically Controlled Millimeter-wave Phase Shifters

In this paper we analyze the millimeter-wave propagation characteristics of a dielectric wavegaide containing a plasma-dominated region. Such a device presents a new method for controlfirrg millimeter-wave propagation in semiconductor waveguides via either optical or electronic means resulting in ultrafast switching and gating. We have calculated the phase shift and attenuation resulting from the presence of the plasma. Higher order modes, both TE and TM, as well as millimeter-wave frequency variation, are studied in both Si and GaAs dielectric wavegnides. We have also formulated a surface plasma model that is a good approximation to the more elaborate volume plasma model. Phase shifts are predicted to he as high as 1400°/cm for modes operating near cutoff. These modes suffer very little attenuation when the plasma region contains a sufficiently high carrier density.     

AlGaAs-based two-dimensional photonic crystal slab with defectwaveguides for planar lightwave circuit applications

An AlGaAs-based near-infrared 2-D photonic crystal (PC) with an air-bridge structure featuring defect waveguides has been developed. For the sample without defect waveguides, measurements of the optical transmission characteristics in the wavelength range from 850 nm to 1100 nm showed a deep attenuation due to a bandgap with 30-35 dB attenuation and transmittance of nearly 100% for the guided modes. Optical propagation properties of defect waveguides were obtained by two methods: measurements of transmission spectra and plan-view observations of the optical beam trace along the waveguide with an infrared-vidicon camera. 3-D finite-difference time-domain simulations for the band structure and transmission spectra in the air-bridge slab with and without defect waveguides have revealed the appearance of four defect propagation modes specific to the defect waveguide, between two slab modes for the defect-free photonic crystal slab. These defect modes were experimentally identified in the measured transmission spectra     

Impedance boundary method of moments for accurate and efficientanalysis of planar graded-index optical waveguides

An impedance boundary method of moments (IBMOM) is proposed to accurately and efficiently compute the propagation characteristics including the number of guided modes of general graded-index dielectric slab waveguide structures. The method is based on Galerkin's procedure in the method of moments and employs the exact impedance boundary condition at the interfaces between the graded-index region and constant-index cladding. Legendre polynomials are utilized in the field expansion. Computational results are shown for waveguides with various inhomogeneous refractive index profiles. The results indicate that typically five Legendre polynomials are sufficient for accurate solutions of the dominant TE and TM modes in optical waveguides having a finite region of inhomogeneous refractive index. Diffused optical waveguides with untruncated index profiles as well as coupled dielectric waveguides can be accurately analyzed using ten Legendre polynomials     

硅包层质子交换条形光波导的频率响应

退火质子交换工艺已成为一种制备低损耗铌酸锂光波导的重要技术.但目前对该类光波导传输特性的研究还不多.利用半矢量光束传播法,对硅包层X切退火质子交换铌酸锂条形光波导的频率响应特性进行了数值分析.给出了几种波导结构参数下的计算结果.结果表明,波导传输模式的衰减特性与波长相关.波导的衰减特性,可以通过调整波导表面中心处折射率增量,硅包层厚度,及缓冲层的折射率和厚度来控制.硅包层光波导可以用来制作光频滤波器.     

Wave Propagation and Attenuation in the General Class of Circular Hollow Waveguides with Uniform Curvature

A general method has been developed to evaluate the propagation constant in oversized circular hollow-core waveguides characterized by a surface impedance and admittance due to a uniform bend. Completely different formulas are obtained for the attenuation constants of the modes in metallic or dielectric hollow waveguides from those obtained by Marcatili and Schmeltzer. Electric-field lines are also presented for several lower order modes in bent waveguides.     

Finite-element solution of planar arbitrarily anisotropic diffused optical waveguides

The finite-element method for propagation in planar anisotropic diffused optical waveguides with arbitrary permittivity tensor is presented. A Galerkin procedure has been introduced to the finite-element formulation, to study both the nonleaky and leaky surface waves. The complex propagation constants are determined as a function of frequency for possible modes of propagation. The accuracy of the method has been checked by calculating the nonleaky and leaky surface waves of Ti-diffused LiNbO3waveguides with Gaussian index profiles. The numerical results of Ti-diffused LiNbO3waveguides with dielectric overlays are also presented and the effects of dielectric overlays on the propagation characteristics for the nonleaky and leaky surface waves are examined.     

Spectral Domain Analysis of Frequency Dependent Propagation Characteristics of Planar Structures on Uniaxial Medium

The propagation characteristics of single and multilayered uniaxial dielectric waveguides and planar structures on uniaxial medium can be determined by utilizing Hertzian potentials along the optical axis. The electric and magnetic Hertzian potentials, having components along the optical axis only, lead to TM and TE modes, respectively, with respect to that axis. The dyadic Green's function in Fourier transform domain (immittance matrix) required to solve for the propagation characteristics of planar structures on uniaxial medium are derived for all three orientations of the optical axis. The immittance matrix for all three cases is in the same form as that for the isotropic medium and hence the known Galerkin's method can be used to solve for the propagation characteristics of the structure.     

Modal analysis of strip-loaded diffused optical waveguides by a variational method

The propagating modes supported by strip-loaded three-dimensional diffused optical waveguides are analyzed in detail theoretically by using a variational method. Some typical numerical examples of the propagation constants and the field distributions are illustrated. The dependence of propagation characteristics on the geometrical parameters of the loading dielectric strip is discussed.     

Nonuniform Layer Model of a Millimeter-Wave Phase Shifter

The electromagnetic wave propagation of millimeter waves in dielectric waveguides with thin surface plasma layers is characterized. The phase and attenuation of a 94-GHz wave are computed for various surface plasma layer thicknesses as a function of earner density levels. The electron/hole pairs generated in the vicinity of the dielectric waveguide surface by photo excitation are assumed to have an exponential profile due to either carrier diffusion or the exponential absorption of the optical field. Field computations made for a uniform plasma layer are compared with those of the nonuniform plasma to illustrate the effects of the exponential tails of the carrier profiles on both the phase and attenuation of the millimeter wave. The thin plasma layers slightly affect the field profile of the transverse electric modes (fields polarized parallel to the plasma layer). The transverse magnetic fields are highly distorted at plasma densities greater than 10/sup 16/ cm/sup -3/.     

Leaky mode propagation in planar multilayer birefringentwaveguides: longitudinal dielectric tensor configuration

In this paper we examine leaky mode propagation in a general five-layered c-rotated optical structure with longitudinal dielectric tenser configuration that can be considered a useful pattern for many actual waveguides. The dependence of the leaky mode propagation on the longitudinal angle φ (between the optical c-axis and laboratory axis) is shown and the dispersion characteristics for different types and thicknesses of buffer and metal layer are reported. The guided mode losses at the wavelength λ=0.633 μm assume the lowest values (about 1 dB/cm) for an Ag layer and for φ=0°. Furthermore, we investigate the variation in the propagation characteristics of the leaky and guided modes with respect to the source wavelength. We obtain the transition wavelength from (G) guided modes to lowest order (L¹ ) leaky mode, having the ordinary component that leaks into the substrate; the transition wavelength to a higher order (L²) leaky mode, which has both ordinary and extraordinary leaky components and the leaky cutoff wavelength. As an example, for φ=10° and an Ag metal layer, the first-order G₁₁ mode transforms from guided to leaky L₁₁¹ at λ_gl≃0.9 μm. The losses exhibit a change of several dB near the wavelength transition from guided to leaky mode (e.g. The attenuation constant of the G₁₁ mode changes from 0.26×10² dB/cm at λ=0.633 μm to 0.18×10 ⁵ dB/cm at λ=0.95 μm where its ordinary component is a leaky one). A similar change is found near the transition wavelength from a lowest-order mode to the highest-order leaky mode     

Loss/gain characterization of optical waveguides

Finite element analysis employing the vector H-field formulation, with the aid of the perturbation technique, is used to calculate modal loss or gain for several different types of optical waveguides. Further, the complex propagation constant of an optical waveguide is obtained from the solution of the complex transcendental equation and the use of the effective index approach. Results obtained by both methods for different optical waveguides are found to be in good agreement for a wide range of gain/loss values. The accuracy limit for modal loss or gain calculation using the perturbation technique is also examined     

Field configurations and propagation constants of modes in hollow rectangular dielectric waveguides

The field configurations and propagation constants of linearly polarized low-loss modes in hollow dielectric waveguides with rectangular cross section are computed analytically under the assumption that the dimensions of the rectangular cross section are much larger than the free-space wavelength. The attenuation constant is then equal to the sum of the corresponding TE- and TM-mode attenuation constants in parallel-plate waveguides.     

Flexible,Low-loss Waveguide Designs for Efficient Coupling to Quantum Cascade Lasers in the Far-infrared

We coupled linearly polarized and azimuthally polarized Terahertz quantum cascade lasers (QCLs) to the low-loss optical modes of hollow core waveguides having a sequence of different metallic or dielectric inner coatings. The latter waveguides have been specifically designed to force the propagation of a dominant optical mode once the thickness (d) of the inner dielectric coating is properly chosen. Our results demonstrate that both the TE01 and the TE11 modes can be easily converted to a hybrid one when d > 6 μm allowing the propagation of THz QCL beams with transmission losses as low as 1.5 dB/m, bending losses < 1.1 dB and reasonably high coupling efficiencies (87%).     

Metal-Clad Multilayer Waveguide: Circle-Chain Convergent Method and Two Perturbation Methods

Computer modeling studies for the calculation of the propagation constants and attenuation coefficients of TE mode in an asymmetric metal/ dielectric/dielectric layers structure are presented. Two different perturbation methods, circle chain convergent method and Downhill method are used. The last two methods can be used easily for the waveguides consist of any number of layers with complex refractive index due to gain and loss. The developed programs were run on a personal computer and numerical results are presented.     

Dielectric Loaded Elliptical Waveguides

Wave propagation in a metallic elliptic waveguide loaded with a dielectric rod or a dielectric lining is investigated theoretically. The mode spectrum for both slow and fast hybrid modes is obtained by numerical solution of the characteristic equations. Correspondence is established between the modes of the loaded and unloaded elliptical waveguides. Typical field plots for /sub e/HE/sub 01/ and /sub 0/EH/sub 01/ modes are presented. Power flow, power loss, and attenuation are obtained using a perturbation method.     

Finite-Element Analysis of Optical and Microwave Waveguide Problems

A vector H-field formulation is developed for electromagnetic wave propagation for a wide range of guided-wave problems. It is capable of solving microwave or optical waveguide problems with arbitrarily anisotropic materials. We have introduced infinite elements to extend the region of explicit field representation to infirdly, to consider open-type waveguides more accurately. Computed results are given for a variety of optical planar guides, image lines, and waveguides containing skew anisotropic dielectic.     

Some effects of anisotropy on planar antiresonant reflectingoptical waveguides

In this paper, propagation characteristics of some planar antiresonant reflecting optical waveguides (ARROW's) comprised of anisotropic media are studied using an integral equation approach. The integral equation method is rigorous and general, with the added advantage that multiple layers of crystalline material with arbitrary anisotropy can be accommodated in a straightforward manner. The integral equation method is applied to study basic propagation characteristics of the ARROW structure where one or more dielectric layers are allowed to be anisotropic. Practically, the presence of anisotropy may be unintentional, due to material fabrication or processing techniques, or it may be intentionally utilized to allow integration of anisotropy-based devices and waveguiding structures on a single semiconducting substrate. Propagation characteristics and field distributions are shown for a uniaxially anisotropic ARROW where the material's optic axis is rotated in each of the three principal geometrical planes of the structure. It Is found that even moderately large levels of anisotropy do not significantly affect the propagation characteristics of the ARROW if either the optic axis of the material is aligned with one of the geometrical axes of the waveguide, or if the optic axis is rotated in the equatorial plane. In these cases, pure TE ₀ modes can propagate, resulting in a low-loss structure. In the event of misalignment between the geometrical axes and the material's optic axis in the transverse or polar planes, the influeuce of even small levels of anisotropy is quite pronounced. In this case, pure TE₀ modes do not exist, and attenuation loss increases significantly due to the hybrid nature of the fundamental mode     

Helix Leaky Waveguide

The propagation characteristics of leaky waves in a helix waveguide covered with a slitted cylinder are presented by a method of transverse network representation. The main interest is in helix waveguides with small pitch angles, characterized by a hybrid mode consisting of TE/sub 01/, and a small amount of TM/sub 01/ modes. The leaky wave discussed in this paper may then be regarded as a perturbation of the TM/sub 01/ wave by the slitted cylinder outside the helix. The radiation, metal, and dielectric losses are calculated numerically at a frequency of 50 GHz. The relation between the radiation loss and aperture angle of slit is very different from that of an ordinary leaky waveguide composed of a slitted cylinder without helix, especially when the distance between the helix and shield cylinder is about a quarter of the radial wavelength. The metal and dielectric losses are the same order as radiation loss, however the dielectric loss decreases as the power factor /spl epsiv/"/ /spl epsiv/' increases. The measured total attenuation constant averages about 5 dB/km, almost twice the theoretical value.