WoS İndeksli Yayınlar Koleksiyonu

Permanent URI for this collectionhttps://hdl.handle.net/20.500.12416/8653

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Subject: search.filters.subject.Atmospheric TurbulenceWoS Q: search.filters.wosQuartile.Q4

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Now showing 1 - 7 of 7
  • Article
    Citation - WoS: 8
    Citation - Scopus: 8
    M-Ary Pulse Position Modulation Performance With Adaptive Optics Corrections in Atmospheric Turbulence
    (Taylor & Francis Ltd, 2020) Gokce, Muhsin C.; Baykal, Yahya; Ata, Yalcin
    The performance of M-ary pulse position modulated (PPM) optical wireless communication (OWC) systems in atmospheric weak turbulence medium is evaluated by using adaptive optics corrections. Piston, tilt, defocus and coma components of adaptive optics corrections are applied to the avalanche photodetector (APD) type of receiver and the results are obtained depending on various turbulence and receiver parameters. The lognormal channel distribution is used to model the weak atmospheric turbulence conditions. Adaptive optics correction increases the bit-error-rate (BER) performance of an OWC system operating in atmospheric turbulence conditions. Piston component yields the highest BER performance, followed by the tilt, defocus and coma adaptive optics correction components respectively.
  • Article
    Citation - WoS: 10
    Citation - Scopus: 7
    Analysis of Laser Multimode Content on the Angle-Of Fluctuations in Free-Space Optics Access Systems
    (Spie-soc Photo-optical instrumentation Engineers, 2005) Eyyuboglu, HT; Baykal, Y
    The effects of laser beam multimode content on the angle-of-arrival fluctuations are examined for free-space optics (FSO) access systems. Multimode excitation is represented by coherent addition of Hermite-Gaussian modes. Mean square angle-of-arrival fluctuations are formulated using our previously reported multimode phase structure function. Numerical evaluations are obtained for practical FSO links operating at 1.55- and 0.85-mu m wavelengths with link spans of up to 5 km. Mode content is arranged by sequentially grouping all possible mode combinations starting from the single fundamental mode (TEM00) up to a certain higher order (n,m). Angle-of-arrival fluctuations are found to be of the order of several tens of microradians, except for the cases when the mode group terminates with an odd mode or when the extremely higher order modes are present. In these instances, the fluctuations will rise to the radian level. From these results, it is concluded that the performance of a practical FSO receiver, having a field of view of several milliradians, will not be substantially affected by the angle-of-arrival fluctuations due to multimode excitation, provided that the mode content of source excitation is confined to mode indices below 20. (c) 2005 Society of Photo-Optical Instrumentation Engineers.
  • Article
    Citation - WoS: 13
    Citation - Scopus: 13
    Angle-Of Fluctuations for General-Type Beams
    (Spie-soc Photo-optical instrumentation Engineers, 2007) Eyyuboglu, Halil T.; Baykal, Yahya
    Starting with the recently introduced phase correlation function of a general-type beam, the angle-of-arrival fluctuations are derived and evaluated in atmospheric turbulence for lowest-order Gaussian, cos-Gaussian, cosh-Gaussian, annular, and flat-topped beams. Our motivation is to examine the improvement in the performance of optical atmospheric links when such beams are employed. For these beams, the dependence of the angle-of-arrival fluctuations on the propagation length, displacement and focusing parameters, source size, inner and outer scales of turbulence, and receiver radius is investigated. It is found that in the majority of the cases examined, the angle- of- arrival fluctuations remain small and hence are not expected to adversely affect the operation of free-space atmospheric links. It is observed that amongst the beams considered, the cos-Gaussian beam offers the least amount of angle-of-arrival fluctuations, while the worst behavior is exhibited by the cosh-Gaussian beam. This situation is reversed, however, if focused beams are used. (C) 2007 Society of Photo-Optical Instrumentation Engineers.
  • Article
    Citation - WoS: 2
    Citation - Scopus: 2
    Bit-Error Performance of Optical Wireless System Using Higher Order Mode Laser in Anisotropic Non-Kolmogorov Turbulence
    (Spie-soc Photo-optical instrumentation Engineers, 2018) Ata, Yalcin; Baykal, Yahya
    The average bit-error-rate, (BER), of optical wireless system using higher order mode laser beam is investigated when atmospheric turbulence shows anisotropic and non-Kolmogorov characteristics. Results reveal that increase in anisotropy in both x- and y-directions positively affects the optical wireless systems performance. Increase in the beam order results in an increase in (BER) for any anisotropy level, and thus, higher order beams adversely affect the optical wireless systems performance. Larger structure constant, beam source size, and propagation distance result in larger (BER), but larger wavelength, inner scale length, and signal-tonoise ratio tend to reduce (BER). Increase in the power-law exponent of non-Kolmogorov turbulent spectrum first increases the (BER) until a certain value, and then (BER) starts to decrease when the power-law exponent is further increased. Adverse effect of higher order laser beam holds to be valid for any power-law exponent of non-Kolimogorov turbulence. (C) 2018 Society of Photo-Optical Instrumentation Engineers (SPIE)
  • Article
    Citation - WoS: 11
    Citation - Scopus: 10
    Receiver-Aperture Averaging of Annular Beams Propagating Through Turbulent Atmosphere
    (Spie-soc Photo-optical instrumentation Engineers, 2013) Kamacioglu, Canan; Baykal, Yahya; Yazgan, Erdem
    For an annular beam incidence, the power scintillation index in a weakly turbulent atmosphere is derived at the receiver plane, which has a Gaussian aperture. Employing this derivation, the receiver-aperture averaging factor is evaluated. Annular beams are found more advantageous than the Gaussian beams when compared on receiver-aperture averaging basis. The analyses indicate that the effect of the aperture averaging increases as the propagation length increases. Increase in the structure constant and the secondary beam source size is observed to strengthen the effect of the aperture averaging when the primary beam source size is fixed. (C) 2013 Society of Photo-Optical Instrumentation Engineers (SPIE)
  • Article
    Citation - WoS: 8
    Citation - Scopus: 8
    Crossbeam Intensity Fluctuations in Turbulence
    (Spie-soc Photo-optical instrumentation Engineers, 2014) Baykal, Yahya
    Intensity fluctuations of a crossbeam are evaluated in weak atmospheric turbulence. A crossbeam is defined as two asymmetrical Gaussian beams oriented perpendicular to each other, and one of these beams is wider along the x-axis whereas the other beam is wider along the y-axis. Our results indicate that in terms of the intensity fluctuations in weak turbulence, focused crossbeams offer favorable results when compared to the corresponding focused Gaussian beam intensity fluctuations. However, for collimated crossbeams, such a comparison is in favor of the collimated Gaussian beam. (C) 2014 Society of Photo-Optical Instrumentation Engineers (SPIE)
  • Article
    Citation - WoS: 12
    Citation - Scopus: 11
    Partially Coherent Off-Axis Gaussian Beam Scintillations
    (Taylor & Francis Ltd, 2010) Baykal, Yahya; Eyyuboglu, Halil T.; Cai, Yangjian; Eyyubolu, Halil T.
    The scintillation index at the receiver origin is formulated for a partially coherent off-axis Gaussian beam in atmospheric turbulence by employing the extended Huygens-Fresnel principle. Our formula correctly reduces to the existing coherent and partially coherent Gaussian beam scintillation indices in the limiting cases. For off-axis Gaussian beams with imaginary, real and complex displacement parameters, the scintillation index reduces when the incidence becomes more incoherent. When the source size of the off-axis Gaussian beam increases, the scintillations increase for partially coherent sources and decrease for incoherent sources, the tendency being observed for imaginary, real and complex displacement parameters. For the fully coherent off-axis Gaussian beams, increase in the source size first causes an increase in the scintillations, eventually reaching saturation at large source sizes, the increase is not monotonic and may exhibit a peak around the Fresnel zone sized off-axis Gaussian sources. For all degrees of partial coherence, off-axis beams possessing imaginary displacement parameters exhibit larger scintillations when the displacement parameter increases for large sized incidences, however, for small sized incidences, scintillations stay at the same level when the imaginary displacement parameter increases, the fixed scintillation value being lower for more incoherent sources. For off-axis Gaussian sources possessing real displacement parameters, this behavior is reciprocal with respect to the source size, i.e. for all degrees of partial coherence, off-axis beams possessing real displacement parameters exhibit larger scintillations when the displacement parameter increases for small sized incidences, and for large sized incidences, scintillations stay at the same level when the real displacement parameter increases, the fixed scintillation value again being lower for more incoherent sources. For all degrees of partial coherence, off-axis Gaussian beams possessing imaginary displacement parameters exhibit larger scintillations than the off-axis Gaussian beams possessing real displacement parameters when the absolute value of displacement parameter increases for large sized incidences.