Elektronik ve Haberleşme Mühendisliği Bölümü Yayın Koleksiyonu
Permanent URI for this collectionhttps://hdl.handle.net/20.500.12416/260
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Article Citation - WoS: 19Citation - Scopus: 19Global Minima for Free Ptn Clusters (n=22-56): a Comparison Between the Searches With a Molecular Dynamics Approach and a Basin-Hopping Algorithm(Springer, 2004) Sebetci, A; Güvenç, ZBUsing molecular dynamics and thermal quenching simulation techniques, and the basin-hopping Monte Carlo algorithm we have studied the global minima and energetics of free Pt-N clusters in the size range of N = 22-56. The clusters have been described by the Voter and Chen version of an embedded-atom model, which is derived by fitting to experimental data of both the diatomic molecule and bulk platinum simultaneously. A comparison between the two search techniques has been performed and it is found that the basin-hopping algorithm is more efficient than a molecular dynamics minimization approach in the investigation of the global minima. The results show that the global minima of the Pt clusters have structures based on either octahedral, decahedral or icosahedral packing. Some of the icosahedral global minima do not have a central atom. The 54-atom icosahedron without a central atom is found to be more stable than the 55-atom complete icosahedron. The resulting structures have been compared with the previous theoretical calculations.Article Citation - WoS: 60Citation - Scopus: 65Degree of Polarization for Partially Coherent General Beams in Turbulent Atmosphere(Springer, 2007) Eyyuboglu, H. T.; Baykal, Y.; Cai, Y.The degree of polarization is found for optical excitations of cosh-Gaussian, cos-Gaussian and annular-Gaussian beams in a turbulent atmosphere. The related formulation is based on the beam coherence polarization matrix. The self and mutual coherence functions appearing in the beam coherence polarization matrix are evaluated, when the above mentioned excitations exhibit partial source coherence for self and cross fields. Plots showing the variation of the degree of polarization are provided versus the propagation length when the source size, displacement parameter, structure constant and the degree of source coherence for self and cross fields change.Conference Object Citation - WoS: 3Citation - Scopus: 5Local Decision Making and Decision Fusion in Hierarchical Levels(Springer, 2009) Leblebicioglu, Kemal; Beldek, UlasHierarchical problem solving is preferred when the problem is overwhelmingly complicated. In such a case, the problem should better be analyzed in hierarchical levels. At each level, some temporary solutions are obtained; then a suitable decision fusion technique is used to merge the temporary solutions for the next level. The hierarchical framework proposed in this study depends on reutilization or elimination of previous level local agents that together perform the decisions due to a decision-fusion technique: a performance criterion is set for local agents. The criterion checks the success of agents in their local regions. An agent satisfying this criterion is reutilized in the next level, whereas an agent not successful enough is removed from the agent pool in the next level. In place of a removed agent, a number of new local agents are developed. This framework is applied on a fault detection problem.Article Citation - WoS: 57Citation - Scopus: 64Intensity Fluctuations in J-Bessel Beams of All Orders Propagating in Turbulent Atmosphere(Springer, 2008) Sermutlu, E.; Baykal, Y.; Cai, Y.; Korotkova, O.; Eyyuboglu, H. T.The scintillation index of a J (n) -Bessel-Gaussian beam of any order propagating in turbulent atmosphere is derived and numerically evaluated at transverse cross-sections with the aid of a specially designed triple integral routine. The graphical outputs indicate that, just like the previously investigated J (0)-Bessel-Gaussian beam, higher-order members of the family also offer favorable scintillation characteristics at large source sizes. This advantage is maintained against rising beam orders. Viewed along the propagation axis, beams with lower orders and smaller widths exhibit smaller values of the scintillation index at shorter propagation distances and large values at longer propagation distances. Further, it is shown that the scintillation index of the J (n) -Bessel-Gaussian beams (n > 0) is larger than that of the fundamental Gaussian and the J (0)-Bessel-Gaussian beams only near the on-axis points, while remaining smaller towards the edges of the beam.Article Citation - WoS: 10Citation - Scopus: 12Concept of Area Scintillation(Springer, 2009) Eyyuboglu, H. T.Stemming from the results of our earlier investigations, the concept of area scintillation is introduced, which takes into account the intensity distribution over the receiver plane. In this context, the area scintillation of fundamental Gaussian and annular beams is formulated, numerically evaluated and graphically illustrated. From the comparison, it is seen that, under the same source power conditions, annular Gaussian beams provide much less scintillations than the fundamental Gaussian beams at small source size. At large source sizes and at shorter propagation distances, annular beams are still favorable, but, as the propagation range is extended, the reverse becomes true. A review of previous findings leading up to the newly introduced concept is also presented.Article Citation - WoS: 26Citation - Scopus: 26Beam Wander Characteristics of Cos and Cosh-Gaussian Beams(Springer, 2009) Eyyuboglu, H. T.; Cai, Y.; Baykal, Y.; Cil, C. Z.Within the context of a general beam formulation, beam wander characteristics of cos and cosh-Gaussian beams are derived and numerically evaluated. In our graphs, the fundamental Gaussian beam is used as a benchmark for comparisons. The associated plots reveal that at small source sizes, a cos-Gaussian beam has the lowest beam wander, while this property is enhanced with increasing values of the displacement parameter. At large source sizes however, this advantage is taken over by cosh-Gaussian beam. Joint examination against the changing source sizes and propagation lengths shows that the range of source sizes, where the beam wander of cos-Gaussian beam remains lower, is enlarged as we go toward higher propagation lengths. Asymmetric beams tend to exhibit higher beam wanders both at small and large source sizes, but for the intermediate source size ranges, the beam wanders of asymmetric beams will fall below those of the symmetric beams. Explanations concerning these behaviors are offered. A historical account of beam wander formulation is also included.Article Citation - WoS: 13Citation - Scopus: 13Comparison of Wave Structure Functions for Intensity Profiles(Springer, 2009) Eyyuboglu, H. T.We give a list of available wave structure functions (WSFs) of a spherical wave, simultaneously providing some derivation details. The aim is to assess the impact of these WSFs on intensity profiles of various beams propagating in turbulence. For comparisons, coherent and partially coherent fundamental Gaussian, hyperbolic, sinusoidal and annular Gaussian beams are chosen. Comparisons are made by calculating the difference between the intensity profiles of the analytic solution that uses the normalized WSF with quadratic approximation and the intensity profiles obtained by numerically solving the quadruple extended Huygens-Fresnel integral containing other WSFs. The graphical results show that in general the differences arising from the use of different WSFs are not substantial. Such differences become much greater however toward the complete incoherence limit and at relatively higher structure constant values. Even then, at these extremes, the receiver intensity levels are much reduced, making such big differences immaterial.Article Citation - WoS: 12Citation - Scopus: 13Generalized Tensor Abcd Law for an Elliptical Gaussian Beam Passing Through an Astigmatic Optical System in Turbulent Atmosphere(Springer, 2009) Cai, Yangjian; Lin, Q.; Eyyuboglu, H. T.; Baykal, Y.The propagation of an elliptical Gaussian beam (EGB) through an astigmatic ABCD optical system in a turbulent atmosphere is investigated. An analytical formula for the average intensity of an EGB and a generalized tensor ABCD law for the generalized complex curvature tensor are derived. As an application example, we derived an analytical formula for the average intensity of an elliptical flat-topped beam propagating through an astigmatic ABCD optical system in a turbulent atmosphere. As a numerical example, the focusing properties of an EGB focused by a thin lens in a turbulent atmosphere are studied. It is found that the focused beam at the focal plane becomes a circular Gaussian beam when the atmospheric turbulence is strong enough, and the beam width of the circular Gaussian beam is determined by atmospheric turbulence strength, focal length of the thin lens, and wavelength of the initial beam but is independent of the initial beam widths (i.e., initial intensity distribution).Article Citation - WoS: 6Citation - Scopus: 8Propagation Aspects of Mathieu-Gaussian Beams in Turbulence(Springer, 2008) Eyyuboglu, H. T.Based on our recent source plane formulation, the propagation characteristics of Mathieu-Gaussian beams in turbulent atmosphere are investigated. In this connection, the average receiver plane intensity expression is deduced using the Huygens-Fresnel integral. Our results offered in the form of graphical illustrations reveal that, for some settings of source and propagation parameters, the center of the source beam is evacuated after propagation, while the initially smaller side lobes begin to grow. In a parallel development, the angular distribution of the beam also changes. At small Gaussian source sizes and transverse components of the wave vector, the source beam profile remains almost invariant throughout the propagation. The larger refractive index structure constant values cause the final Gaussian beam profile to be attained at earlier propagation distances. Smaller refractive index structure constants, on the other hand, do not change the beam profile substantially from that of free space.Article Citation - WoS: 4Citation - Scopus: 5A Study of Source Plane Mathieu Beams(Springer, 2008) Eyyuboglu, H. T.We derive the source field expressions of different Mathieu beams. In particular, Mathieu beams consisting of the infinite summations of J-type Bessel functions and their Gaussian counterparts are studied. Mathieu beams based on the summation of I-type Bessel functions are introduced as well. By plotting the source intensities of such beams, the variations of the related profiles are examined against the changes in the source parameters. It is found that, via the adjustment of these parameters, it is possible to obtain completely new beam configurations and also those similar to the existing beams of the present literature.