Browsing by Subject "Physical optics"
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Item Open Access Asymptotic wave-like modeling of dielectric lenses(IEEE, 2007) Yurchenko, V. B.; Altıntaş, AyhanWe propose asymptotic wave-like approximations for the accurate modeling of dielectric lenses used in quasi-optical systems of millimeter, submillimeter, and infrared wave applications. For the comparison, we obtain an exact full-wave solution of two-dimensional focusing lens problem and use it as a benchmark for testing and validation of asymptotic models being proposed.Item Open Access Effect of disorder on magnetic resonance band gap of split-ring resonator structures(Optical Society of American (OSA), 2004) Aydın, K.; Güven, K.; Katsarakis, N.; Soukoulis, C. M.; Özbay, EkmelWe investigated the influence of periodicity, misalignment, and disorder on the magnetic resonance gap of split-ring resonators (SRRs) which are essential components of left handed-metamaterials (LHMs). The resonance of a single SRR which is induced by the split is experimentally demonstrated by comparing transmission spectra of SRR and closed ring resonator. Misaligning the SRR boards do not affect the magnetic resonance gap, while destroying the periodicity results in a narrower band gap. The disorder in SRR layers cause narrower left-handed pass band and decrease the transmission level of composite metamaterials (CMMs), which may significantly affect the performance of these LHMs. © 2004 Optical Society of America.Item Open Access EFIE and MFIE, why the difference?(IEEE, 2008-07) Chew W.C.; Davis, C. P.; Warnick, K. F.; Nie, Z. P.; Hu, J.; Yan, S.; Gürel, LeventEFIE (electric field integral equation) suffers from internal resonance, and the remedy is to use MFIE (magnetic field integral equation) to come up with a CFIE (combined field integral equation) to remove the internal resonance problem. However, MFIE is fundamentally a very different integral equation from EFIE. Many questions have been raised about the differences.Item Open Access Large flat plate models in the physical optics method for RCS calculations(IEEE, 2004-09) Altıntaş, Ayhan; Çelik, AslıhanThe calculation of Radar Cross Section (RCS) of arbitrarily large perfectly conducting body is presented. The body is modelled as triangular meshes of any size by the help of graphical tools. For the calculation of scattered field, Physical Optics(PO) surface integral is analytically evaluated over each of the triangular meshes. Due to the analytical integration, there is no limitation on the size of the triangles.Item Open Access Large-scale solutions of electromagnetics problems using the multilevel fast multipole algorithm and physical optics(2015-04) Hidayetoğlu, MertIntegral equations provide full-wave (accurate) solutions of Helmholtz-type electromagnetics problems. The multilevel fast multipole algorithm (MLFMA) discretizes the equations and solves them numerically with O(NLogN) complexity, where N is the number of unknowns. For solving large-scale problems, MLFMA is parallelized on distributed-memory architectures. Despite the low complexity and parallelization, the computational requirements of MLFMA solutions grow immensely in terms of CPU time and memory when extremely-large geometries (in wavelengths) are involved. The thesis provides computational and theoretical techniques for solving large-scale electromagnetics problems with lower computational requirements. One technique is the out-of-core implementation for reducing the required memory via employing disk space for storing large data. Additionally, a pre-processing parallelization strategy, which eliminates memory bottlenecks, is presented. Another technique, MPI+OpenMP parallelization, uses distributed-memory and shared-memory schemes together in order to maintain the parallelization efficiency with high number of processes/threads. The thesis also includes the out-of-core implementation in conjunction with the MPI+OpenMP parallelization. With the applied techniques, full-wave solutions involving up to 1.3 billion unknowns are achieved with 2 TB memory. Physical optics is a high-frequency approximation, which provides fast solutions of scattering problems with O(N) complexity. A parallel physical optics algorithm is presented in order to achieve fast and approximate solutions. Finally, a hybrid integral-equation and physical-optics solution methodology is introduced.Item Open Access Memory-efficient multilevel physical optics algorithm for fast computation of scattering from three-dimensional complex targets(IEEE, 2007) Manyas, Alp; Gürel, LeventMultilevel physical optics (MLPO) algorithm provides a speed-up for computing the physical-optics integral over complex bodies for a range of aspect angles and frequencies. On the other hand, when computation of the RCS pattern as a function of θ, φ, and frequency is desired, the O N3 memory complexity of the algorithm may prevent the solution of electrically large problems. In this paper, we propose an improved version of the MLPO algorithm, for which the memory complexity is reduced to O N2 log N . The algorithm is based on the aggregation of only some portion of the scattering patterns at each aggregation step. This way, memory growth in each step is prevented, and a significant amount of saving is achieved.Item Open Access Memory-efficient multilevel physical optics algorithm for the solution of electromagnetic scattering problems(2007) Manyas, Kaplan AlpFor the computation of electromagnetic scattering from electrically large targets, physical optics (PO) technique can provide approximate but very fast solutions. Moreover, higher order approximations, such as physical theory of diffraction (PTD) including the diffraction from the edges or sharp corners can also be added to the PO solution in order to enhance the accuracy of the PO. On the other hand, in real-life radar applications, where the computation of the scattering pattern over a range of frequencies and/or angles with sufficient number of samples is desired, further acceleration may be needed. Multilevel physical optics (MLPO) algorithm can be used for such applications, in which a remarkable speed-up can be achieved by evaluating the PO integral in a multilevel fashion. As the correction terms like PTD are evaluated independently just on the edges or sharp corners, whereas the PO integration is carried out on the entire target surface, PO integration is the dominant factor in the computational time of such higher order approximations. Therefore accelerating the PO integration will also reduce the computational time of such higher order approximations. In this thesis, we propose two different improvements on the MLPO algorithm.First improvement is the modification of the algorithm that enables the solution of the scattering problems involving nonuniform triangulations, thus decreasing the CPU time. Second improvement is the memory-efficient version, in which the O (N3 ) memory requirement is decreased to O (N2 log N). Efficiency of the two proposed improvements are demonstrated in numerical examples including a reallife scattering problem, with which the scattering pattern of a three-dimensional stealth target is evaluated as a function of elevation angle, azimuth angle, and frequency.Item Open Access Multilevel physical optics algorithm for fast solution of scattering problems involving nonuniform triangulations(IEEE, 2007) Gürel, Levent; Manyas, AlpThis paper shows the computational efficiency of the multilevel physical optics (MLPO) algorithm can be further increased by employing nonuniform triangulations of the target surface so that the triangle size is not nearly uniform, but instead, is determined by the surface curvature.Item Open Access Multilevel PO algorithm for non-uniform triangulations(ESA Publications, 2006) Manyas, Alp; Gürel, LeventFast physical optics (FPO) algorithm provides a speedup for computing the physical optics (PO) integral over complex bodies for a range of aspect angles and frequencies. In this paper, this algorithm is further developed in order to compute the scattered field from nonuniform triangulations of complex bodies. In the original "uniform" FPO algorithm, only the radiation patterns of the smallest subdomains in the bottom level are directly computed and the radiation patterns of the larger subdomains in the upper levels are computed via interpolation and aggregation. In this modified "nonuniform" algorithm, the radiation patterns of the larger triangles, which are too large to fit in the bottom-level subdomains, are directly computed and incorporated in the appropriate aggregation levels. It is also shown that, by applying different interpolation methods, accuracy of the FPO algorithm can be improved without any computational cost.Item Open Access Physical optics modeling of 2D dielectric lenses(Optical Society of America, 2009-01-27) Yurchenko, V. B.; Altintas, A.We propose an advanced physical optics formulation for the accurate modeling of dielectric lenses used in quasi-optical systems of millimeter, submillimeter, and infrared wave applications. For comparison, we obtain an exact full-wave solution of a two-dimensional lens problem and use it as a benchmark for testing and validation of asymptotic models being considered.Item Open Access Structural analysis of an InGaN/GaN based light emitting diode by X-ray diffraction(Springer, 2009-04-18) Öztürk, M. K.; Hongbo, Y.; SarIkavak, B.; Korçak, S.; Özçelik, S.; Özbay, EkmelThe important structural characteristics of hexagonal GaN in an InGaN/GaN multi quantum well, which was aimed to make a light emitted diode and was grown by metalorganic chemical vapor deposition on c-plain sapphire, are determined by using nondestructive high-resolution X-ray diffraction in detail. The distorted GaN layers were described as mosaic crystals characterized by vertical and lateral coherence lengths, a mean tilt, twist, screw and edge type threading dislocation densities. The rocking curves of symmetric (00.l) reflections were used to determine the tilt angle, while the twist angle was an extrapolated grown ω-scan for an asymmetric (hk.l) Bragg reflection with an h or k nonzero. Moreover, it is an important result that the mosaic structure was analyzed from a different (10.l) crystal direction that was the angular inclined plane to the z-axis. The mosaic structure parameters were obtained in an approximately defined ratio depending on the inclination or polar angle of the sample.