Elektronik ve Haberleşme Mühendisliği Bölümü Yayın Koleksiyonu

Permanent URI for this collectionhttps://hdl.handle.net/20.500.12416/260

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  • Conference Object
    Citation - WoS: 2
    Bit Error Rates for Focused General-Type Beams
    (Electromagnetics Acad, 2009) Arpali, Serap; Arpali, Serap Altay; Baykal, Yahya Kemal; Baykal, Yahya; Elektrik-Elektronik Mühendisliği; Elektronik ve Haberleşme Mühendisliği
    Using the scintillation index of general beam formulation, bit error rate (BER) is investigated for focused Gaussian, cos-Gaussian, cosh-Gaussian and annular beams in weak atmospheric turbulence. We have employed our previously derived formulation of the scintillation index of these beams by Rytov method. Rytov method scintillation theory is known to yield accurate results for focused beams on horizontal paths under certain regions. Therefore we find the scintillation indices of the mentioned beams for the valid region. Using the log-normal distributed intensity, BER values versus signal-to-noise ratio (SNR) are calculated for Gaussian, cos-Gaussian, cosh-Gaussian and annular beams. In our study, the focal lengths (radius of curvature) of all the mentioned focused beams are equal to the propagation distance. The improvement of BER is observed for variations of propagation length, source size and wavelength of operation. Based on these parameters, BER values of Gaussian, cos-Gaussian, cosh-Gaussian and annular beams are compared. BER values we found for the focused Gaussian, cos-Gaussian, cosh-Gaussian and annular beams decrease with increasing source sizes. Likewise, BER values of focused Gaussian, cos-Gaussian, cosh-Gaussian and annular beams decrease with increasing wavelength. The focused annular beam attains the lowest BER value for small source sizes and long propagation distances. Moreover, BER for focused beams is compared with their collimated counterparts. We observe that focused beams have lower BER values than the collimated beams on horizontal paths. Our formulation can easily be extended to cover corresponding higher order beams, however in this paper we concentrate mainly on the zero order beams.
  • Conference Object
    Citation - WoS: 1
    Citation - Scopus: 1
    Off-Axis Field Correlations in Turbulence
    (Ieee, 2013) Baykal, Yahya Kemal; Baykal, Yahya; Elektrik-Elektronik Mühendisliği
    The field correlations in atmospheric turbulence are evaluated for off-axis optical incidence. Within the practical range of the source and the medium parameters, increase in the diagonal length at the receiver plane is found to decrease the off-axis field correlations in turbulence. At a fixed diagonal length at the receiver plane, off-axis beams that have smaller displacement parameters and larger source sizes possess larger field correlations. When the field correlations of the off-axis beams in atmospheric turbulence are compared to their no turbulence counterparts, it is observed that the behaviour of the field correlation variations do not change, however the field correlations in turbulence diminish at smaller diagonal lengths.
  • Conference Object
    Aperture Averaging of Focused Multi-Gaussian Beams
    (Electromagnetics Acad, 2013) Baykal, Yahya Kemal; Kamacioglu, Callan; Baykal, Yahya; Yazgan, Erdem; Elektrik-Elektronik Mühendisliği
    We investigate the behavior of the power fluctuations of the focused annular and flat-topped beams when a realistic receiver possessing a finite sized aperture is employed in turbulent atmospheric optics links. Employing our previously derived formulation for the power scintillation index, the variations of the power scintillations and the receiver aperture averaging factor of the focused annular and flat-topped beams are scrutinized. Receiver aperture averaging factor is deduced from the ratio of power scintillation index detected by a finite sized aperture to that obtained by a point aperture. Influence of the receiver aperture radius, the propagation length, the structure constant, the inner and the outer beam sizes for an annular type incidence, flatness parameter for a flat-topped incidence and the focusing parameter for the multi-Gaussian beam in general, on the power scintillation and the receiver aperture averaging factor are studied. It is found that for the focused multi-Gaussian beams, the effect of the receiver aperture averaging factor increases as the aperture radius increases for larger link lengths. Additionally, for the annular incidences in turbulence, the effect of the receiver aperture averaging factor is stronger for larger inner beam source sizes. At a fixed receiver aperture radius, receiver aperture averaging becomes more effective when the structure constant becomes larger. When focused multi-Gaussian beams are compared to their collimated counterparts, it is seen that the receiver aperture averaging is more beneficial for the focused annular and focused flat-topped beams. At large link lengths, increase in the receiver aperture radius decreases the power scintillations.
  • Conference Object
    Citation - WoS: 2
    Diffraction Properties of Partially Coherent Elegant High-Order Beam
    (Electromagnetics Acad, 2010) Eyyuboğlu, Halil Tanyer; Wang, Fei; Baykal, Yahya Kemal; Cai, Yangjian; Eyyuboglu, Halil T.; Baykal, Yahya; Elektronik ve Haberleşme Mühendisliği; Elektrik-Elektronik Mühendisliği
    The diffraction properties of a partially coherent elegant higher-order beam in free space are investigated in detail. It is found that the diffraction properties of a partially coherent elegant higher-order beam upon propagation are closely related to its initial coherence. A partially coherent elegant higher-order beam spreads more slowly than a partially coherent standard higher-order beam, which has potential application in free-space optical communications.
  • Conference Object
    Effects of Focusing on Scintillations of Higher Order Laser Modes in Non-Kolmogorov Turbulence
    (Electromagnetics Acad, 2014) Baykal, Yahya Kemal; Baykal, Yahya; Elektrik-Elektronik Mühendisliği
    The scintillation index of focused higher order laser beam propagating in non-Kolmogorov atmospheric turbulence is formulated by employing the Rytov method and the equivalence of the structure constant. Our evaluations are performed for even modes. The equivalence formula for the structure constant is extracted from our earlier work in which the equivalence is obtained by equating the scintillation indices found in the Kolmogorov and the non-Kolmogorov turbulence. If not specified otherwise, the focused beam is defined when the focal length is equal to the link length. For the focused higher order laser beams, as the power law exponent of the non-Kolmogorov spectrum decreases, the scintillations decrease. At any power law exponent, the scintillations tend to become larger when the mode order of the focused beam becomes larger, i.e., the focused Gaussian beam is advantageous over the focused higher order laser beams for any realization of the non-Kolmogorov turbulence. Again being valid for any power law exponent, increase in the source size is found to decrease the intensity fluctuations of all the focused higher order mode scintillations. Especially for the larger order beams, focusing the higher order beam at a distance smaller than the link length results in a change in the behaviour of the scintillation index versus the power law exponent. In such cases, the scintillations are observed to increase. Comparison of the focused higher order beam scintillations with the previously obtained collimated higher order beam scintillations yields that the focused higher order beam scintillations are lower. Collimated higher order beams exhibit lower scintillations than the collimated Gaussian beams whereas this is reversed in the focused case. Another observation in such comparison shows that the difference of the intensity fluctuations between the Gaussian and the higher order beams are much larger in the focused case, especially at larger power law exponent values.
  • Article
    Citation - WoS: 8
    Citation - Scopus: 7
    Bit Error Rate of a Gaussian Beam Propagating Through Biological Tissue
    (Taylor & Francis Ltd, 2020) Arpali, Serap Altay; Arpali, Caglar; Baykal, Yahya
    The scintillation index and bit error rate (BER) of a Gaussian beam propagating in a weakly turbulent soft tissue are formulated and analysed numerically. The scintillation indices are plotted against half of the measured slope in the range of power-law scaling at different tissue parameters, such as the random variations in the refractive index of the tissue, outer scale of the tissue turbulence and the tissue length between the optical source and the detector. Moreover, BERs of Gaussian beams against the signal to noise ratio (SNR) are examined for different tissue parameters. Our graphical results show that the scintillation index and BER increase with larger outer scales, longer tissue lengths and larger random variations in the refractive index of the tissue. In comparison with the spherical wave propagation, it was found that Gaussian beam yields larger scintillation index and BER values.
  • Article
    Citation - WoS: 15
    Citation - Scopus: 16
    Binary Phase Shift Keying-Subcarrier Intensity Modulation Performance in Weak Oceanic Turbulence
    (Elsevier, 2019) Baykal, Yahya; Ata, Yalcin; Gokce, Muhsin Caner
    The performance analysis of underwater wireless optical communication (UWOC) system that employs binary phase shift keying-subcarrier intensity modulation (BPSK-SIM) is investigated in weak oceanic turbulence. BPSK-SIM Gaussian beam and finite sized positive-intrinsic-negative (PIN) photodetector are employed at the transmitter and at the receiver, respectively. Bit error rate (BER) is taken as the performance indicator and in the evaluation of the BER, the required signal power and the aperture averaged scintillation index are obtained respectively by the use of the extended Huygens-Fresnel principle and the Rytov theory. Considering various noise types in underwater turbulence, BER variations are examined versus the oceanic turbulence parameters and the photodetector parameters, namely receiver aperture diameter, PIN-responsivity, noise factor, load resistor, and the electronic bandwidth. (c) 2019 Elsevier B.V. All rights reserved.
  • Article
    Citation - WoS: 16
    Citation - Scopus: 18
    Adaptive Optics Correction of Scintillation in Underwater Medium
    (Taylor & Francis Ltd, 2020) Baykal, Yahya
    Adaptive optics correction of the scintillation index of a Gaussian laser beam in underwater turbulence is studied. To introduce the adaptive optics correction, filter functions providing the piston, tilt and astigmatism effects are adapted to promote the spectrum of underwater turbulence. The reduction of the scintillation index due to the individual piston, tilt, astigmatism effects and their sum is examined versus the ratio of temperature to salinity contributions to the refractive index spectrum, the rate of dissipation of mean squared temperature, the rate of dissipation of kinetic energy per unit mass of fluid, receiving aperture diameter, source size, link length and the wavelength. For any value of underwater turbulence parameter, the most effective adaptive optics corrections are found to be the piston, tilt and astigmatism, respectively.
  • Conference Object
    Citation - Scopus: 2
    Propagation of Elegant Higher-Order Gaussian Beams in Turbulent Atmosphere
    (Spie-int Soc Optical Engineering, 2010) Wang, Fei; Cai, Yangjian; Eyyuboglu, Halil T.; Baykal, Yahya
    Propagation of elegant higher-order Gaussian beams in turbulent atmosphere is studied in detail. Analytical propagation formulae of elegant higher-order Gaussian beams in turbulent atmosphere are derived based on extended Huygens-Fresnel integral. The intensity and spreading properties of elegant higher-order Gaussian beams and standard higher-order Gaussian beams in turbulent atmosphere are studied numerically and comparatively. It is found that the propagation properties of elegant higher-order Gaussian beams and standard higher-order Gaussian beams are much different from their properties in free space The standard higher-order Gaussian beams spread more rapidly than the elegant higher-order Gaussian beams in turbulent atmosphere.
  • Conference Object
    Citation - WoS: 1
    Citation - Scopus: 7
    Beam Wander Characteristics of Flat-Topped, Dark Hollow, Cos and Cosh-Gaussian, J0- and I0- Bessel Gaussian Beams Propagating in Turbulent Atmosphere: a Review
    (Spie-int Soc Optical Engineering, 2010) Eyyuboglu, Halil T.; Baykal, Yahya; Cil, Celal Z.; Korotkova, Olga; Cai, Yangjian
    In this paper we review our work done in the evaluations of the root mean square (rms) beam wander characteristics of the flat-topped, dark hollow, cos-and cosh Gaussian, J(0)-Bessel Gaussian and the I-0-Bessel Gaussian beams in atmospheric turbulence. Our formulation is based on the wave-treatment approach, where not only the beam sizes but the source beam profiles are taken into account as well. In this approach the first and the second statistical moments are obtained from the Rytov series under weak atmospheric turbulence conditions and the beam size are determined as a function of the propagation distance. It is found that after propagating in atmospheric turbulence, under certain conditions, the collimated flat-topped, dark hollow, cos-and cosh Gaussian, J(0)-Bessel Gaussian and the I-0-Bessel Gaussian beams have smaller rms beam wander compared to that of the Gaussian beam. The beam wander of these beams are analyzed against the propagation distance, source spot sizes, and against specific beam parameters related to the individual beam such as the relative amplitude factors of the constituent beams, the flatness parameters, the beam orders, the displacement parameters, the width parameters, and are compared against the corresponding Gaussian beam.