Beatriz Garcia-Banos

Numerical analysis of all-optical switching based on a 2-D nonlinear photonic crystal directional coupler

A photonic crystal (PhC) all-optical switch based on a nonlinear directional coupler is presented and discussed. The coupling strength of the structure may be tuned by a high optical power control signal that propagates along the coupling region. Thus, the power ratio between the output ports of the device can be tuned and, hence, switching is accomplished. The study of the present structure is performed with a numerical tool based on the finite-element method. This tool allows to calculate the nonlinear band diagram of a two-dimensional PhC. The implementation and characteristics of this method are thoroughly addressed.

Dynamic Measurement of Dielectric Properties of Materials at High Temperature During Microwave Heating in a Dual Mode Cylindrical Cavity

A microwave cavity and heating system for microwave processing and in situ dynamic measurements of the complex permittivity of dielectric materials at high temperatures ( ~ 1000 °C) has been developed. The method is based on a dual-mode cylindrical cavity where heating and testing are performed by two different swept frequency microwave sources. A cross-coupling filter isolates the signals coming from both sources. By adjusting the frequency bandwidth of the heating source and the level of coupling to the cavity, an automatic procedure allows for the establishment of a desirable level of heating rate to the dielectric sample to reach high temperatures in short cycles. Dielectric properties of materials as a function of temperature are calculated by an improved cavity perturbation method during heating. Accuracy of complex permittivity results has been evaluated and an error lower than 5% with respect to a rigorous analysis of the cavity has been achieved. The functionality of the microwave dielectric measurement system has been demonstrated by heating and measuring glass and ceramic samples up to 1000 °C. The correlation of the complex permittivity with the heating rate, temperature, absorbed power, and other processing parameters can help to better understand the interactions that take place during microwave heating of materials at high temperatures compared to conventional heating.

Enhancement of Sensitivity of Microwave Planar Sensors With EBG Structures

A strategy to improve microstrip sensor performance for monitoring dielectric properties of materials is proposed. The method relies on the reduction of the wave group velocity to induce higher interaction between the sensor and the material. This is achieved by the design of periodic patterns in the sensor ground plane, which exhibit electromagnetic band gap (EBG) effects. The presence of these EBG structures turns out to be highly effective, inducing a noticeable decrease of the wave velocity. The sensitivity is defined and measured for different sensor configurations in order to quantify the improvements obtained. It is observed that, with the EBG structures, the residence time of the wave in the material under test is longer, and a substantial increase of the sensor sensitivity is obtained. EDICS Category-MICR

Design rules for the optimization of the sensitivity of open-ended coaxial microwave sensors for monitoring changes in dielectric materials

Open-ended coaxial probes are widely used for non-destructive measurement of dielectric properties of materials, and also as microwave sensors for industrial processes and quality control applications. The main design parameters of these sensors are the coaxial radii and working frequency. In this paper, the influence of these variables on the final sensitivity of the coaxial sensor when monitoring dielectric materials is analysed, and a novel expression for this parameter selection is proposed. Moreover, a method to select the optimum parameters of experimental configurations is provided. Measurements demonstrate that high discrimination can be achieved with this method when monitoring changes in the dielectric properties of materials.

A nondestructive method of measuring the dielectric and magnetic properties of laminate materials in open cavities

A complete analysis of split-cylinder resonators is presented by means of circuital techniques for nondestructive measurement of dielectric and magnetic properties of laminate materials.

Compensating intermodal dispersion in photonic crystal directional couplers

Intermodal dispersion between the supermodes of a directional coupler may induce undesirable pulse breakup in a sufficiently large device. When this happens the device will no longer exchange power between its arms, and the extinction ratio is completely canceled. It is shown that, by carefully designing the coupling area of the directional coupler, one may compensate for intermodal dispersion. The compensating device should accomplish three basic requirements: inverse intermodal dispersion, balanced coupling of each supermode, and maximum power transfer while preserving the sign of the slope of the coupling coefficient with frequency for multiplexing-demultiplexing applications. This structure is designed and optimized with a genetic algorithm.

Circuital analysis of a coaxial re-entrant cavity for performing dielectric measurement

We apply circuital analysis to the re-entrant cavity method for measuring the relative permittivity and loss tangent of dielectric material. With this new model, we are able to make use of the higher-order resonant modes of the re-entrant cavity, and are therefore able to broaden the frequency range over which we can measure the dielectric properties of the sample. We compare this new model with others in the literature and validate its capabilities through a comparison with other dielectric measurement techniques.

Noninvasive Monitoring of Polymer Curing Reactions by Dielectrometry

A microwave sensor system for the noninvasive monitoring of the curing process of a thermoset material placed inside a metallic mold is described. The microwave sensor is designed as an open-ended coaxial resonator with a curved surface adapted to the mold inner shape. The analysis of the microwave resonator comprises a recently developed method for deembedding the effect of coupling network in overcoupled resonators, so the range of permitted measurements encompass both low and high dielectric losses of polymeric materials. Results show that noninvasive, continuous monitoring of the microwave dielectric properties of the thermoset material can be performed in real time, allowing one to check initial conditions and to verify the evolution of the cure process.

Improvement in the Accuracy of Dielectric Measurement of Open-Ended Coaxial Resonators by an Enhanced De-Embedding of the Coupling Network

An improvement of the accuracy of dielectric measurements of the open-ended coaxial resonator method is described. The technique is based on an empirical technique for de-embedding the coupling network excited by electric probes. By this procedure, the influence of the coupling structure on the resonance can be precisely eliminated independently of the coupling conditions, which guarantees a high accuracy in the permittivity determination of materials by open-ended coaxial resonators. The technique is applicable to materials with a wide range of dielectric constants and losses. The results of dielectric measurements are compared with those obtained using other standard methods.

Frequency Deviation Due to a Sample Insertion Hole in a Cylindrical Cavity by Circuital Analysis

One of the most widely used techniques for dielectric material characterization is the partially filled circular cavity. The exact analysis of this structure is well known in the bibliography. But in practice when the measured material is introduced through a circular pipe below its cut off frequency a problem arises. Traditionally the tube effect is neglected, but then some errors appear, whose magnitude depends on the material properties and the cavity and tube dimensions. In some cases, the errors introduced are noticeable and the tube effect should not be neglected. Some authors have tried to evaluate in a simple way this effect, but their equations are valid only in a restricted range of practical configurations. In this paper we present a model of this effect based on a circuital analysis of the full structure. A simple monomode equation valid for any material property and cavity dimensions is given and a saturation effect, not reported before, is analyzed.