Two-dimensional microwave imaging by a numerical inverse scatteringsolution

A numerical approach that aims to detect, by means of interrogating microwaves, the locations and the dielectric permittivities of unknown inhomogeneous dielectric cylindrical objects of arbitrary cross sections that might be present inside a fixed area of interest is proposed. An illumination is assumed with the electric field vector polarized along the cylindrical axis. The two-dimensional Lippman-Schwinger integral equation of electromagnetic scattering is transformed into matrix form by the moment method. The system obtained is solved by using a pseudoinversion algorithm to overcome ill-conditioning problems. The first-order Born approximation is also applied when the dielectric inhomogeneities are weakly scattering. Computer simulations have been performed by means of a numerical program. Results show the capabilities and limitations of the proposed approach     

Perturbations in hyperthermia temperature distributions associated with counter-current flow: numerical simulations and empirical verification

Two numerical techniques are used to calculate the effect of large vessel counter-current flow on hyperthermic temperature distributions. One is based on the Navier-Stokes equation for steady-state flow, and the second employs a convective-type boundary condition at the interface of the vessel walls. Steady-state temperature fields were calculated for two energy absorption rate distributions (ARD) in a cylindrical tissue model having two pairs of counter-current vessels (one pair with equal diameter vessels and another pair with unequal diameters). The first assumed a uniform ARD throughout cylinder; the second ARD was calculated for a tissue cylinder inside an existing four antenna Radiofrequency (RF) array. A tissue equivalent phantom was constructed to verify the numerical calculations. Temperatures induced with the RF array were measured using a noninvasive magnetic resonance imaging technique based on the chemical shift of water. Temperatures calculated using the two numerical techniques are in good agreement with the measured data. The results show: 1) the convective-type boundary condition technique reduces computation time by a factor of ten when compared to the fully conjugated method with little quantitative difference (approximately 0.3 degree C) in the numerical accuracy and 2) the use of noninvasive magnetic resonance imaging (thermal imaging) to quantitatively access the temperature perturbations near large vessels is feasible using the chemical shift technique.     

Emissivity simulations in passive microwave remote sensing with 3-D numerical solutions of Maxwell equations

In the numerical Maxwell-equation model (NMM3D) of rough-surface scattering, we solve Maxwell equations in three dimensions to calculate emissivities for applications in passive microwave remote sensing of soil and ocean surfaces. The difficult cases for soil surfaces are with exponential correlation functions when the surfaces have fine-scale structures of large slopes. The difficulty for ocean surfaces is that because the emissivities are close to that of a flat surface, the emissivities have to be calculated accurately to correctly assess the rough-surface effects. In this paper, the accuracies of emissivity calculations are improved by using Rao-Wilton-Glisson basis functions. We further use sparse matrix canonical method to solve the matrix equation of Poggio-Miller-Chang-Harrington-Wu integral equations. Energy conservation checks are provided for the simulations. Comparisons are made with results from the pulse basis function. Numerical results are illustrated for soil and ocean surfaces respectively with exponential correlation function and ocean spectrum. The emissivities of soil are illustrated at both L- and C-bands and at multiple incidence angles for the same physical roughness parameters. The brightness temperatures for ocean surfaces are illustrated for cases with various wind speeds. We compare results with those from the sparse matrix methods. Comparisons are also made with experimental emissivity measurements of soil surfaces. Parallel computation is also implemented. Lookup tables of emissivities based on NMM3D are provided.     

An active microwave imaging system for reconstruction of 2-Delectrical property distributions

The goal of this work is to develop a microwave-based imaging system for hyperthermia treatment monitoring and assessment. Toward this end, a 4-transmit channel and 4-receive channel hardware device and concomitant image reconstruction algorithm have been realized. The hardware is designed to measure electric fields (i.e., amplitude and phase) at various locations in a phantom tank with and without the presence of various heterogeneities using standard heterodyning principles. Particular attention has been paid to designing a receiver with better than 115 dB of linear dynamic range which is necessary for imaging biological tissue which often has very high conductivity, especially for tissues with high water content. A calibration procedure has been developed to compensate for signal loss due to 3-dimensional radiation in the measured data, since the reconstruction process is only 2-dimensional at the present time. Results are shown which demonstrate the stability and accuracy of the measurement system, the extent to which the forward computational model agrees with the measured field distribution when the electrical properties are known, and image reconstructions of electrically unknown targets of varying diameter. In the latter case, images of both the reactive and resistive component of the electrical property distribution have been recoverable. Quantitative information on object location, size, and electrical properties results when the target is approximately one-half wavelength in size. Images of smaller objects lack the same level of quantitative information, but remain qualitatively correct     

选择性激光烧结聚苯乙烯粉末成形温度场的数值模拟研究

本文以聚苯乙烯材料为例，对SLS成形三维瞬态温度场的分布变化进行数值模拟计算。计算模型中，考虑了材料的热物性参数随温度变化的情况和激光光强分布不均匀的因素。计算结果表明：烧结过程可以分为两个阶段：第一阶段是光照期的热量输入加热阶段，在该阶段内，光照表面的温度迅速升高，使材料处于过热状态，甚至超过材料的分解点，这一阶段完成粉末表面的致密化，但粉床内部的致密效果不是特别明显；第二阶段是光照结束之后的热量传输阶段，在该阶段内，表面材料吸收的热量向四周扩散，一部分通过对流和辐射换热方式散失到周围环境中，另一部分通过热传导的方式向粉床内部传输，促进粉床内部的烧结，这一阶段粉床内部的致密程度显著提高。     

A comparative study of temperature sensitivity of InGaAsP andAlGaAs MQW lasers using numerical simulations

We used numerical simulation to compare the temperature sensitivity of an InGaAsP MQW laser emitting at 1.55 μm and an AlGaAs MQW laser at 0.82 μm. By artificially changing the InGaAsP laser gradually into a structure similar to the AlGaAs laser, we gained quantitative insight into how each material or structural parameter causes the relatively low T₀ of the InGaAsP MQW laser. Using a typical MQW structure we demonstrated the relative importance of parameters involving Auger recombination, current leakage over the quantum barrier, optical confinement and band offset. We found that if these parameters were made the same as the AlGaAs laser, the T₀ of the InGaAsP laser was even better than that of the AlGaAs laser. Our numerical simulation confirmed that the Auger recombination is the main cause of low T₀ in MQW InGaAsP lasers. We also discovered that thermal current leakage over the barrier and Auger recombinations are correlated with each other and both factors must be improved to increase the T₀ of InGaAsP lasers to that of AlGaAs lasers     

Nonlinear electron properties of an InGaAs/InAlAs-based ballistic deflection transistor: Room temperature DC experiments and numerical simulations

We present a detailed experimental and numerical study of a novel device so-called ballistic deflection transistor (BDT). Based on InGaAs-InAlAs heterostructure on InP substrate, BDT utilizes a two dimensional electron gas (2DEG) supported by a gated microstructure to achieve nonlinear electron transport at room temperature. BDT channel is larger than the mean free path implying that electron transport is not purely ballistic in nature. However, the asymmetric geometrical deflection combined with the electron steering caused by the applied differential gate voltages ultimately results in an attractive nonlinear behavior of the BDT useful for the working of the large scale devices at room temperature. Device performance was studied by analyzing the effects of several modifications of the BDT geometry and biasing conditions, both experimentally and by numerical simulations.     

Active microwave imaging: a model-based approach

A model-based approach that improves the spatial resolution of a buried inhomogeneous object using an active microwave imaging system is discussed. Algorithms are developed for quasi-real-time processing using a microwave camera specifically constructed for civil engineering applications using reflection mode imaging. The model is generated by numerically solving an integral equation representing the ideal scattered field of the object. The model is used to develop an optimal spatial filter to process the measured field. Finally, the inverse problem is solved using an imaging algorithm based on diffraction tomography and Fourier transforms. Experimental results indicate a reasonable performance of the approach     

Dispersion-managed breathing-mode semiconductor mode-locked ring laser: experimental characterization and numerical simulations

A dispersion-managed breathing-mode mode-locked semiconductor ring laser is studied. The working regime and pulse evolution at the key cavity points are experimentally characterized and numerically simulated. Linearly chirped, asymmetric exponential pulses are generated and externally compressed to 274 fs, which is within 10% of the bandwidth limit. The close agreement between the simulated and the measured results verifies our ability to control the physical mechanisms involved in pulse formation and shaping within the ring cavity.     

Electromagnetic waves scattering by cylindrical dielectric structures: Application to microwave devices design

A method which allows us to analyze the electromagnetic scattering characteristics of multiple discontinuities in shielded dielectric waveguides is presented. There are not restrictions both geometry of the cross section and electrical parameters of the dielectrics which are assumed to be linear, inhomogeneous, isotropic and free from losses. Each discontinuity is analyzed combining a modal matching technique with a generalized telegraphist's equations formulation; in this way, we obtain its scattering matrix. By using the concept of the generalized scattering matrix of two discontinuities, the equivalent generalized scattering matrix (EGSM) of the cascaded set is calculated. Theoretical and experimental results were obtained for different dielectric structures such as dielectric posts, isolated and coupled, as well as dielectric waveguides with circular cross section connected by means of abrupt and gradual transitions. The experimental values for the scattering properties show a good agrement with the theoretical ones. This study has shown the possibility of using cylindrical dielectric structures to design microwave devices such as: resonators, power-dividers and filters.     

Cylindrical geometry: a further step in active microwave tomography

A prototype imaging system for active microwave tomography using cylindrical geometry has been developed, making it possible to obtain images of the dielectric properties of biological targets at 2.45 GHz. This configuration allows a fast exploration of body slices placed along the array axis, in a way similar to that of present X-ray scanners. The electromagnetic compatibility (EMC) of this approach is critical because the strongly attenuated received fields are measured on the same array which is being used to emit a high-level illuminating signal. Therefore, carefully designed high-frequency architectures and detection techniques are necessary. The system requires no mechanical movements to illuminate the body from multiple directions (views) and measure the scattered fields. In this way, a complete data set consisting of 64 views is acquired in 3 s using low-power illumination. The system is described, and images obtained with biological phantoms and actual bodies are presented     

Slab delivery of incoherent pump light to double-clad fiber amplifiers: numerical simulations

Compact fiber amplifiers with tapered slab delivery of the pump are analyzed numerically. A procedure for the selection of the structural parameters of this device is presented together with a simple asymptotic estimate for the efficiency. Simulations based on the scalar paraxial beam propagation equation demonstrate the validity of this estimate.     

固体激光器新型冷却热沉的设计和CFD数值研究

为固体激光器设计了一种新型内部结构扰流柱结构的冷却热沉,采用计算流体力学（CFD）方法对此水冷热沉的三种典型设计方案以及传统的空腔结构和等截面小通道结构热沉分别进行了数值模拟,据此研究了冷却水流量对各种方案的增益介质最高温度、冷却面温度分布以及热沉的压力损失等特性的影响。在相同传热量和相同冷却水流量前提下,等截面小通道热沉和扰流柱热沉的传热特性都明显优于空腔结构热沉。与等截面小通道水冷热沉相比较,扰流柱热沉传热热阻更小,而流动压力损失较大。数值模拟结果表明扰流柱热沉传热性能优于传统的两种热沉（空腔结构和等截面小通道结构）设计方案,具有更好的冷却效果。在较高流量下工作时,扰流柱热沉传热性能略优于等截面小通道热沉,在较低流量下工作时则显著优于等截面小通道热沉。     

Airborne active and passive microwave observations of Super TyphoonFlo

Airborne microwave measurements of precipitation associated with Super Typhoon Flo in the western North Pacific were conducted during September 16-18, 1990. The sensor package aboard the NASA DC-8 aircraft included a dual-frequency precipitation radar at 10 GHz and 34 GHz and a host of radiometers operating at 10 GHz, 18 GHz, 19 GHz, 34 GHz, and 92 GHz, as well as three frequencies near the strong water vapor absorption line of 183.3 GHz. The measurements were made during a few passes over the storm center, and active and passive microwave signatures of the rainbands were detected with a fine spatial resolution. The relationship between the measured brightness temperature and radar-estimated rain rate is examined at the frequencies between 10-92 GHz. At both 34 and 92 GHz this relationship is analyzed with the 10 GHz radar reflectivity factor measured at altitudes above the freezing layer as a further constraint. The results show that frozen hydrometeors strongly scatter radiation at these frequencies, especially at 92 GHz. It was shown from a close examination of both active and passive microwave signatures that a significant scattering of radiation at frequencies 118 GHz occurred in the inner eyewall at altitudes of 3-8 km. This scattering of microwave radiation by hydrometeors in both liquid and frozen forms is discussed under the authors' current understanding of the scattering mechanism     

Image interpretation and prediction in microwave diversity imaging

The microwave image of a metallic object is interpreted from a point of view based on the understanding of the interconnection between the scattering mechanisms, the data acquisition system, and the image reconstruction algorithm. From this understanding it is possible to interpret and predict microwave images reconstructed from data collected over specified and angular windows. The connection between a special scattering mechanism, edge diffraction, and its reconstructed image is established. The microwave image of an edge is two bright points whose locations correspond to the end points of the edge if the normal aspect angle is not included in the angular windows; otherwise a line joining the two end points and representing the edge will appear in the image. Experimental images of a trihedral reflector constructed from data collected over different angular windows support this approach to image interpretation and prediction     

Bistatic frequency-swept microwave imaging: Principle, methodologyand experimental results

The basic principle, methodology, and experimental results for frequency-swept microwave imaging of continuous shape conducting and discrete line objects in a bistatic scattering arrangement are presented. The theoretical analysis is developed under the assumptions of plane wave illumination and physical optics approximation. The measurement system and calibration procedures are implemented based on plane wave spectrum analysis. Images of three different types of scattering objects reconstructed from the experimental data obtained in the frequency range 7.5-12.5 GHz are shown to be in good agreement with the scattering object geometries. The results demonstrate that this bistatic frequency-swept microwave imaging system has potential as a cost-effective tool for remote sensing, imaging radar, and nondestructive evaluation     

Analysis of current collapse effect in AlGaN/GaN HEMT: Experiments and numerical simulations

In this work, current collapse effects in AlGaN/GaN HEMTs are investigated by means of measurements and two-dimensional physical simulations. According to pulsed measurements, the used devices exhibit a significant gate-lag and a less pronounced drain-lag ascribed to the presence of surface/barrier and buffer traps, respectively. As a matter of fact, two trap levels (0.45 eV and 0.78 eV) were extracted by trapping analysis based on isothermal current transient. On the other hand, 2D physical simulations suggest that the kink effect can be explained by electron trapping into barrier traps and a consequent electron emission after a certain electric-field is reached.     

Low-grazing-angle microwave scattering from a three-dimensional spilling breaker crest: a numerical investigation

The microwave backscattering from a three-dimensional (3-D) target approximating the rough crest of a gently spilling water wave at low grazing angle illumination has been numerically examined. The target surfaces were synthesized from the direct two-dimensional (2-D) measurement of the time evolution of the upwave-downwave cross-section of a wave-tank breaker. The reference scattering was found using the multilevel fast multipole algorithm implemented with impedance boundary conditions and resistive surface loading to suppress nonphysical edge diffraction. The scattering was compared with the predictions of the two-scale model and a synthesis of the 3-D backscattering from individual 2-D calculations. Specular reflection from a bulge feature that appeared on the crest prior to breaking dominated the backscattering at both polarizations, overwhelming even the strong vertical polarization Bragg scattering that appeared in the corresponding scattering from the individual 2-D profiles used to synthesize the 3-D target. The scattering from the surface including the bulge could be accurately modeled using a coherent addition of the scattering from the 2-D profiles. The two-scale model performed poorly whenever there are steep sections on the surface that provide significant quasi-specular back-reflection. Accuracy improved when the specular points were eliminated and the dominant scattering roughness was fully illuminated, but was still sensitive to the surface-roughness scale-separation threshold used in its application.     

Time-domain reconstruction algorithms and numerical simulations for thermoacoustic tomography in various geometries

In this paper, we present time-domain reconstruction algorithms for the thermoacoustic imaging of biological tissues. The algorithm for a spherical measurement configuration has recently been reported in another paper. Here, we extend the reconstruction algorithms to planar and cylindrical measurement configurations. First, we generalize the rigorous reconstruction formulas by employing Green's function technique. Then, in order to detect small (compared with the measurement geometry) but deeply buried objects, we can simplify the formulas when two practical conditions exist: 1) that the high-frequency components of the thermoacoustic signals contribute more to the spatial resolution than the low-frequency ones, and 2) that the detecting distances between the thermoacoustic sources and the detecting transducers are much greater than the wavelengths of the high-frequency thermoacoustic signals (i.e., those that are useful for imaging). The simplified formulas are computed with temporal back projections and coherent summations over spherical surfaces using certain spatial weighting factors. We refer to these reconstruction formulas as modified back projections. Numerical results are given to illustrate the validity of these algorithms.