Oblique incidence and polarization effects in coupled gratings

Oblique incidence and polarization orientation of the input beam have dramatic effects on the spectral response of coupled dielectric waveguide gratings. Coupled gratings with small periodic perturbations can be described as a problem of two coupled resonances at strictly normal incidence, but we find that the device involves four coupled resonances when oblique incidence and polarization effects are included in the analysis. Very small deviations from normal incidence change qualitatively the spectral response and four peaks are observed, whereas only two peaks are present at normal incidence. Polarization misalignments produce a decrease of the reflectance of the resonances at normal incidence, but a simultaneous shift of the spectral position of the peaks is observed at oblique incidence

Refractometric sensor based on whispering-gallery modes of thin capillaries

Whispering-gallery modes resonances of submicron wall thickness capillaries exhibit very large wavelength shifts as a function of the refractive index of the medium that fills the inside. The sensitivity to refractive index changes is larger than in other optical microcavities as microspheres, microdisks and microrings. The outer surface where total internal reflection takes place remains always in air, enabling the measure of refractive index values higher than the refractive index of the capillary material. The fabrication of capillaries with submicron wall thickness has required the development of a specific technique. A refractometer with a response higher than 390 nm per refractive index unit is demonstrated. These sensors are readily compatible with microfluidic systems

Filter Response of Resonant Waveguide Dielectric Gratings at Plane-Wave Conical Incidence

An accurate and efficient formulation is presented for the electromagnetic analysis of dielectric waveguide gratings under plane-wave conical incidence. An arbitrary number of dielectric bars can be placed inside each one-dimension periodic cell, including the effect of dielectric losses. The reflectance of a dielectric waveguide grating under conical incidence is compared with theoretical results presented by other authors, finding a very good agreement. A single-layer reflection filter has been designed centered at λ0=1.5 μm whose spectral and angular responses are shown. For this structure, the effect of the asymmetry of the distribution of the refraction index in the reflectance has been analyzed, observing a splitting of the reflection peak around the design wavelength. Finally it is discussed the equivalence between a volume grating and a shallow surface-relief grating, providing two examples of designing prescriptions

Surface-Impedance Formulation for Hollow-Core Waveguides Based on Subwavelength Gratings

A rigorous Surface Impedance (SI) formulation for planar waveguides is presented. This modal technique splits the modal analysis of the waveguide in two steps. First, we obtain the modes characteristic equations as a function of the SI and, second, we need to obtain the surface impedance values using either analytical or numerical methods. We validate the technique by comparison with well-known analytical cases: the parallel-plate waveguide with losses and the dielectric slab waveguide. Then, we analyze an optical hollow-core waveguide de ned by two high-contrast subwavelength gratings validating our results by comparison with reported values. Finally, we show the potential of our formulation with the analysis of a THz hollow-core waveguide de ned by two surface-relief subwavelength gratings, including material losses in our formulation

Efficient full-wave modal analysis of waveguides with arbitrary geometry defined by straight, circular and elliptical segments

Many modern waveguide devices, such as dual-mode filters and diplexers, can be modelled considering the whole circuit as a cascade of waveguide step discontinuities. Most of such discontinuities usually involve waveguides with arbitrary cross-section defined by linear, circular and/or elliptical segments. A fast and accurate procedure for the full-wave modal characterization of such arbitrarily shaped waveguides is based on the Boundary Integral— Resonant Mode Expansion (BI-RME) technique well described in [1]. In this paper, we present a very efficient implementation of a full-wave modal analysis tool for arbitrarily shaped waveguides based on the BI-RME theory.This work was supported by the European Commission under the Research and Training Networks Programme, Contract No. HPRN-CT-2000-00043

Full-wave analysis and applications of EBG waveguides periodically loaded with metal ridges

In this paper, a fast and accurate full-wave analysis tool of passive structures based on Electromagnetic-Bandgap (EBG) waveguides periodically loaded with metal ridges is proposed. For this purpose, a very efficient Integral Equation (IE) technique is followed to model the planar steps involving arbitrary waveguides. The well-known Boundary Integral - Resonant Mode Expansion (BI-RME) method is used to obtain the modal chart of the ridged waveguides. In order to show the advantages of this tool, a periodically loaded E-plane filter with improved stopband performance is analyzed and compared to standard implementation. Dispersion relations are also derived and used as guidelines for designing an EBG fifth-order low-pass filter

Multipactor effect in a parallel-plate waveguide partially filled with magnetized ferrite

The aim of this paper is the analysis of the multipactor effect in a parallel-plate waveguide when a ferrite slab, transversally magnetized by a static magnetic field parallel to the waveguide walls, is present. Employing an in-house developed code, numerical simulations are performed to predict the multipactor radio frequency voltage threshold in such a ferrite-loaded waveguide. Variations of the ferrite magnetization field strength and the ferrite slab height are analyzed. Effective electron trajectories are also shown for a better understanding of the breakdown phenomenon, finding different multipactor regimes

Wavelet-like efficient analysis of two dimensional arbitrarily shaped radomes using a surface formulation

Radomes are usually made of lossy dielectric materials, and their accurate analysis is often cumbersome because of their typical large electrical size and geometrical complexity. In real reflector antenna structures, there are always complex interactions between the radome, the reflector surfaces and the directional feeds, which are typically neglected for the sake of simplicity. In this paper we will consider all such interactions in a very accurate way, thus requiring a high number of unknowns for the numerical solution of the problem. To overcome such drawback, an integral equation formulation based on the Equivalence Principle in combination with the wavelet transform has been employed, obtaining finally a robust and accurate CAD tool for the rigorous analysis of arbitrarily shaped radomes containing continuous and discrete electromagnetic sources. It will be shown that the use of wavelet-like bases substantially improves the numerical efficiency and memory requirements of the original integral equation method. For verification purposes, the results obtained with the new technique are successfully compared with examples taken from the literature. Complex antenna structures are then discussed in order to prove the usefulness of the new method.This work has been supported by Ministerio de Educación y Ciencia, Spanish Government, under the Research Projects Ref. TEC2004/04313-C02-01 and TEC2004/04313-C02-02

Multipactor effect analysis and design rules for wedge-shaped hollow waveguides

A numerical model for predicting the multipactor breakdown effect in wedge-shaped hollow waveguides is presented in this paper. The computation of electromagnetic fields is based on the boundary integral–resonant mode expansion method, which provides the modal chart of hollow waveguides with any arbitrary cross section. The advantage of using wedge-shaped waveguides with respect to conventional rectangular ones is the deviation of the resonant paths of the electrons toward regions with lower voltages, thus reducing the probability of multipactor threshold for certain input power. To validate this method, our results have been compared with simulations from previous theoretical studies. Once the simulation tool is validated, it is used to predict the multipactor threshold of wedge-shaped waveguides with different symmetric inclination angles of their horizontal plates. Finally, susceptibility curves as the ones already available for rectangular waveguides are presented. These charts are useful for designing innovative waveguide geometries with improved multipactor-free working power ranges.This work was supported by the Ministerio de Ciencia e Innovación, Spain, under Research Project TEC2007-67630-C03-01/TCM