نشریه الکترومغناطیس کاربردی
سال نهم شماره 2 (پیاپی 23، پاییز و زمستان 1400)

Extensive applications of distribution fiber sensors in various industries have led researchers to make great efforts to improve the properties of these sensors. Spatial resolution and sensing length are considered among the most important parameters in fiber optic distribution sensor by engineers and users of these sensors. Among these fiber sensors, Rayleigh scattering-based sensors due to their very long sensing length and Brillouin scattering sensors due to their high spatial resolution, each cover a specific range of applications. Among Brillouin sensors, Brillouin dynamic grating (BDG) sensor has the highest spatial resolution, but the short sensing length of this sensor is one of its major disadvantages. For this reason, trying to increase the sensing length in this sensor is one of the priorities for researchers in this field. In this paper, using a new method called the combination of phase and frequency correlation, the maximum sensing length in the BDG sensor is simulated for spatial resolution in the range of millimeters. The simulation results show that with the help of this sensor, a spatial resolution of 9 mm over 17.7 km of the measurement fiber can be achieved.

In this study, we propose a Feynman gate based on two-dimensional photonic crystals taking into account the non-linear Kerr effects. These devices can operate at high speed, with low power consumption. The performance of the Feynman logic gate presented in this paper is based on the nonlinear effects of Kerr and the formation of identical waveguides to alternate the structure. The wavelength of this design is set in the range of 1550 nm. One of the advantages of this design is its small size of 7.54×8.55 µm2 which has been achieved due to the use of three waveguides. The minimum optical power for the case of logic 1 is 0.95 and for case of logic 0 is 0.2. So, the contrast ratio of 8.4 dB can be obtained. Hence, the Feynman optical gate provided in this article is a good option for photonic computing circuits.

In this paper, the overvoltage of the transmission lines terminated to an arrester under direct lightning strike is computed, considering the nonlinear phenomenon of ionization and the frequency dependence effect of electrical parameters of lossy soils. To this end, electromagnetic transient solvers can be used. In this scenario, the arrester is grounded via a vertical electrode. To apply the frequency dependence of soil parameters, the approximation of equivalent frequency is used in such a way that the soil parameters are replaced with equivalent constant electrical parameters, while the nonlinear phenomenon of ionization is modelled with a nonlinear resistance in the RLC equivalent circuit of the vertical electrode. The simulation results show that the induced voltages across the arrester are different in the four scenaios (neither ionization nor frequency dependence, only ionization dependance, only frequency dependence, and both ionization and frequency dependence), specifically the induced voltage across the arrester when both effects are considered is reduced compared to the situations where both effects are ignored or only one effect is considered. This reduction for highly resistive soils and for electrodes with long lengths is more pronounced. This fact plays an important role in designing lightning arresters which should be considered by power engineers..

The most important and effective part of any light detector is the photocathode. In this research, in order to improve the efficiency of the photocathode optical response, the plasmonic phenomenon is used and a new photocathode is designed and simulated based on the finite difference time domain (FDTD) method. By designing a periodic nano-grating on the surface, a structure is presented that makes it possible to couple the incoming light to an electron density wave on the surface. Plasmonic resonance is created in the desired wavelength range and the field strength is greatly increased. In this way, the metal-semiconductor becomes an excellent absorber. For this purpose, photocathodes with planar and non-planar (nano- grating) structures consisting of GaAs and Au are simulated separately. Then, the plasmonic nano- grating structure consisting of Au-GaAs composite materials is simulated and compared with the plasmonic nano- grating structure of gold, so that the absorption rate of the structure is increased by 16.1%. The plasmonic nano- grating structure consisting of Au-GaAs in the visible spectrum shows better performance in increasing the photocathode efficiency due to higher absorption. The structure provided for the photocathode has the advantage of filtering the incoming light with excellent accuracy and quality. Another important feature is the creation of multiple resonant frequencies simultaneously with the formation of repetitive geometry.

Ground penetrating radar (GPR) is a strong tool for non-destructive detection of buried targets. This radar can determine the location and shape of targets based on the received signals from the electromagnetic waves transmitted to the ground. In this paper, we intend to determine the depth and diameter of buried cylindrical conductor targets in an unknown background environment using the ground penetrating radar. Examples include the detection of water, oil or gas pipes buried under the ground. For this purpose, the SAR algorithm is used and a new method for estimating the depth and diameter of the scatterer is proposed, assuming that relative permittivity of the host environment is not known. The geometry of the problem is two-dimensional and the configuration of the antennas is similar to that of the commercial ones, which is multi-monostatic. For assessment, the proposed method is implemented on the raw data related to buried targets at the depth of 1m. The GPRMAX_2Dsoftware is used to provide raw data to reconstruct the shape of scatterer. Results show that the scatterer depth and diameter are estimated with acceptable accuracy.

The main purpose of this paper is to modify the holographic technique relationship (formula) and to present an algorithm for the design of multiple frequency compact leaky-wave antennas with broadside pencil beam radiation. In the design process, the following three software HFSS, MATLAB and CST, have been utilized. As a prototype, a triple frequency planar leaky-wave antenna is designed with the SLL less than -15 dB at E and H planes and broadside pencil radiation gain of 17.9, 18.5 and 19.2 dB at 15, 16 and 17 GHz, respectively. The unit cell forming the impedance surface of the hologram is selected as a regular hexagon to have the most isotropic behavior in response to the surface wave. Modification of the holographic relationship leads to the possibility of using a surface wave feed with a simple structure for proper matching as well as controlling the SLL on the E and H planes of the antenna radiation pattern at the designed frequencies. Eventually, by constructing and testing the proposed antenna, the accuracy of the proposed design process has been verified. In measuring the radiation pattern of the E and H planes of the different frequencies in the antenna room for each frequency, the launcher corresponding to that frequency receives the excitation and the other two launchers are perfectly matched. Finally, a good agreement is observed between the simulation and measurement results.

In this paper, we have introduced a measurement scheme for the wedge angle and bending of the faces of the laser disk active medium. To increase the efficiency of the laser, the front face of the medium has been coated with an antireflection coating. In addition, a small wedge angle between the two faces of the medium has been considered to avoid the etalon effect inside the medium. Pumping the active medium causes thermodynamic effects and deformation of the faces of the medium. Characterization of this deformation is necessary for designing an adaptive optics and retrieving it in the resonator. For this purpose, a visible laser has been utilized which unlike the pumping beam has multi reflections from the faces of medium and contains the information of both wedge and bending. We have demonstrated that with a single shot interference pattern the information of the wedge and front and back faces of the medium can be measured.

In this research, we obtain the resonance frequency of the toroid cold plasma, by calculating the related electric potential in two different states of polarization of the incident electric field. In the first case, we consider the incident electric field parallel to the axis of the torus and in the second case, we consider it perpendicular to the axis of the torus. By solving the Laplace equation and using the appropriate boundary conditions in the toroidal coordinates, we calculate the electrical potential inside and outside the torus and the resonance frequency of the toroidal plasma. Furthermore, the electrical field inside the torus is calculated. Then, we plot the resonance frequency and the amplitude of the potential in different sta.

In this article an efficient multifunctional method is presented to reduce mutual coupling and cross polarization, simultaneously in a 2-element H-plane array antenna in a compact format. In this approach, a pair of parasitic microstrip elements is placed near the radiating edges in two radiation patches, creating a new field coupling path opposite to the main coupling, leading to a significant increase in isolation. In addition, due to the structure and position of these parasitic strips, the near field resulting from cross currents on the patches is significantly neutralized, leading to the elimination of cross polarization and thus improving the polarization purity. Furthermore, this method can prevent the increase of the cross polarization due to the error of probe position on the patches up to 8 dB. In order to validate the proposed method, an optimal sample is fabricated and measured and the results are compared to those of simulations. The measurements show more than 32dB isolation, over 30dB impedance matching, and about 38dB (reduced by at least 10dB) cross polarization, which are in good agreement with the simulations. The advantages of this design are that the gain and radiation efficiency are not decreased and the reduction of the resonance frequency is about 150 MHz which can decrease the overall size of the array. Finally, a comparison with novel designs and the related discussions are presented.

Microwave interferometry which is being used in various tokamaks in the world, is considered as one of the most reliable techniques for electron density measurement of tokamaks. The simplest type of homodyne microwave interferometers is the single channel or central-chord interferometer which measures the line integrated electron density along the longest chord of the plasma cross section of the tokamak. One of the restrictions of central-chord interferometers is that they cannot determine the spatial distribution of electron density. In this paper a semi-analytic approach based on phase difference analysis, is proposed for qualitative evaluation of electron distribution in tokamaks with central-chord interferometer. According to the suggested model, several density profiles are proposed to describe the electron distribution. It is found that in high frequency ranges the trapezoidal distribution can provide a better qualitative estimation of the spatial distribution of electron density in tokomaks. The proposed model can be used as a primary analysis of the spatial distribution of electron density along with complimentary techniques such as the microwave reflectometry and multi-channel interferometers.

This article presents a new method for high voltage capacitor design that utilizes barium titanate which has a high permittivity specification value, instead of common insulators. The main purpose is to define a full electrical model which considers the effects of temperature and electric field values on tanδ and permittivity calculations. The main challenge is that the dielectric constant varies during the process and a hysteresis loop exists in which the dielectric constant and dissipation factors change with temperature variations; a detailed analysis of which is presented in this article. This investigation also leads to realizing the application of new ferroelectric insulators in modern high voltage capacitors.

A digital comparator is a logic circuit used to compare two binary numbers. So far, various designs have been proposed using logic gates, which are mainly based on electrical signals and lack the desired high speed. This paper presents a new design of an ultra-fast all-optical single-bit comparator based on photonic crystal ring resonators consisting of highly nonlinear glass. The structure of this comparator consists of two inputs, four ring resonators with a number of waveguides for comparison and three outputs for displaying the result, all created in a photon crystal bed. Using the plane wave expansion method, its band structure is calculated and the results show that the fundamental photonic crystal has a photonic band gap in the polarized TM mode in S, C and L bands that is a suitable tool for telecommunication applications. To solve the Maxwell's equations, the  finite-difference time-domain method is used, which aims to investigate the light propagation inside the final structure. The results of numerical studies show that the designed structure has a very short response time of 3 ps, making it faster than all the comparators designed so far, including electronic, electro-optical and all-optical comparators. Also, its relatively small area of 826 μm2 makes it applicable in the design of photonic integrated circuits.