Browsing by Author "Gercekcioglu, Hamza"

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Citation - WoS: 36
Citation - Scopus: 43
Annular beam scintillations in strong turbulence
(Optical Soc Amer, 2010) Gercekcioglu, Hamza; Baykal, Yahya; Nakiboglu, Cem; 7812
A scintillation index formulation for annular beams in strong turbulence is developed that is also valid in moderate and weak turbulence. In our derivation, a modified Rytov solution is employed to obtain the small-scale and large-scale scintillation indices of annular beams by utilizing the amplitude spatial filtering of the atmospheric spectrum. Our solution yields only the on-axis scintillation index for the annular beam and correctly reduces to the existing strong turbulence results for the Gaussian beam-thus plane and spherical wave scintillation indices-and also correctly yields the existing weak turbulence annular beam scintillations. Compared to collimated Gaussian beam, plane, and spherical wave scintillations, collimated annular beams seem to be advantageous in the weak regime but lose this advantage in strongly turbulent atmosphere. It is observed that the contribution of annular beam scintillations comes mainly from the small-scale effects. At a fixed primary beam size, the scintillations of thinner collimated annular beams compared to thicker collimated annular beams are smaller in moderate turbulence but larger in strong turbulence; however, thinner annular beams of finite focal length have a smaller scintillation index than the thicker annular beams in strong turbulence. Decrease in the focal length decreases the annular beam scintillations in strong turbulence. Examining constant area annular beams, smaller primary sized annular structures have larger scintillations in moderate but smaller scintillations in strong turbulence. (C) 2010 Optical Society of America
Citation - WoS: 9
Citation - Scopus: 11
Application of equivalent structure constant in scintillations and BER found for non-Kolmogorov spectrum
(Elsevier Science Bv, 2014) Baykal, Yahya; Gercekcioglu, Hamza; 7812
The evaluation of system parameters in the non-Kolmogorov turbulent atmosphere involves the structure constant valid at the relevant non-Kolmogorov power law exponent. In some of the existing results, the comparisons of system parameters found under the Kolmogorov and non-Kolmogorov turbulences were made by using the same structure constant for all the power law exponents of the non-Kolmogorov spectrum. In this paper, we evaluate the scintillations and the average Bit Error Rate (< BER >) for the flat-topped and the annular beams in non-Kolmogorov turbulence, this time using the equivalent structure constant which is now different for all the power law exponents. It is observed that the scintillations and the < BER > show completely different behaviour when evaluated with the equivalent structure constant as compared to evaluations with constant structure constant. (C) 2013 Elsevier B.V. All rights reserved.
Citation - WoS: 18
Citation - Scopus: 18
Average transmittance in non-Kolmogorov turbulence
(Elsevier Science Bv, 2013) Ata, Yalcin; Baykal, Yahya; Gercekcioglu, Hamza; 7812
Average transmittance in non-Kolmogorov turbulence is evaluated. Our recently published equivalent structure constant formulation is employed in our numerical evaluations. At the fixed propagation distance and wavelength, and at the corresponding equivalent structure constant, as the power law exponent of the non-Kolmogorov spectrum increases, the on-axis transmittance is found to decrease. At the same power law exponent of the non-Kolmogorov spectrum, the off-axis transmittance is obtained to be smaller than the on-axis transmittance. Off-axis transmittance variation versus the power law exponent shows that similar to the on-axis case, increase in the power law exponent eventually causes the off-axis transmittance to decrease, however this decrease occurs at larger power law exponent for larger off-axis distance. (C) 2013 Elsevier B.V. All rights reserved.
Citation - WoS: 30
Citation - Scopus: 30
BER of annular and flat-topped beams in non-Kolmogorov weak turbulence
(Elsevier, 2013) Gercekcioglu, Hamza; Baykal, Yahya; 7812
The average bit error rate (BER) of multi-Gaussian beams in non-Kolmogorov weak turbulence is examined. For each specific incidence of annular and flat-topped optical beam, a power law of non-Kolmogorov spectrum is found which is defined as the worst power law at which the average BER attains the maximum value. Using these values of the worst power laws, it is observed that thinner collimated annular, larger focal length annular and flatter small sized collimated flat-topped structures have a slight advantage in obtaining smaller average BER. (C) 2012 Elsevier B.V. All rights reserved.
Citation - WoS: 17
Citation - Scopus: 15
BER of annular and flat-topped beams in strong turbulence
(Elsevier, 2013) Gercekcioglu, Hamza; Baykal, Yahya; 7812
The average bit error rate (< BER >) of annular and flat-topped beams are evaluated in strong turbulence. In this respect, our earlier results on the scintillation indices obtained by the unified Rytov method are employed and the intensity is taken to be gamma-gamma distributed. For comparison purposes, < BER > for the log-normal intensity distribution is also evaluated. It is found that for the annular beams, the ones that are thinner, possessing smaller ratio of primary to secondary beam size, and smaller focal lengths will have smaller average BER in strong turbulence. For the flat-topped beams, the ones that are flatter and possessing large source sizes have smaller average BER in strong turbulence. Large average SNR substantially reduces the average BER in weak and moderate turbulence, whereas in strong turbulence, the average BER stays at the same value no matter what the average SNR is. Comparison of the log-normal and the gamma-gamma statistics for the intensity shows that the average BER will be higher for the log-normal case when the average SNR is small and the reverse relationship holds at large average SNR. For both the gamma-gamma and the log-normal intensity distributions, < BER > obtained for the annular and the, flat-topped beams in strong turbulence is advantageous over the Gaussian beam < BER > values. (C) 2013 Elsevier B.V. All rights reserved.
Citation - WoS: 3
Citation - Scopus: 4
Bit Error Rate of M-Pulse Position Modulated Laser Beams for Vertical Links Operating in Weak Oceanic Turbulence
(Iop Publishing Ltd, 2024) Gercekcioglu, Hamza; Baykal, Yahya
The on-axis scintillation index of laser beams is investigated by employing the Rytov method in a weakly turbulent oceanic medium for up/downlink coupling of laser communication between any underwater vehicles or divers. For vertical links, the formulation of the on-axis scintillation index of laser beams is derived analytically and evaluated for plane, collimated Gaussian and spherical beams in specific mediums, including the Atlantic Ocean at mid and low latitudes associating temperature and salinity changes at low latitudes, at mid latitude-summer and at mid latitude-winter. Using the scintillation index, bit error rate (BER) performance of M-pulse position modulation is investigated for these types of laser beams. The variations of the scintillation index against the uplink/downlink propagation distances, source size and zenith angle are examined, and BER variations versus the Kolmogorov microscale and the symbol orders, and results are compared. It is noted that the behavior of the scintillation index that depends on the relative strength of temperature and salinity fluctuations which changes in depth, is different for uplink/downlink and for each latitude due to its distinct characteristics. The source size that minimizes the scintillation index values is in the range of about 0.1 cm-0.2 cm for all latitudes.
Citation - WoS: 4
Citation - Scopus: 5
Correlation of multimode fields in atmospheric turbulence
(Optica Publishing Group, 2023) Baykal, Yahya; Gokce, Muhsin Caner; Ata, Yalcin; Gercekcioglu, Hamza; 7812; Elektronik ve Haberleşme Mühendisliği
Multimode field correlations are evaluated in atmospheric turbulence. High order field correlations are special cases of the results that we obtained in this paper. Field correlations are presented for various numbers of mul- timodes, various multimode contents of the same number of modes, and various high order modes versus the diagonal distance from various receiver points, source size, link length, structure constant, and the wavelength. Our results will be of help especially in the design of heterodyne systems operating in turbulent atmosphere and fiber coupling efficiency in systems employing multimode excitation.(c) 2023 Optica Publishing Group
Citation - WoS: 2
Citation - Scopus: 4
Depth dependence of oceanic turbulence optical power spectrum under any temperature and salinity concentration
(Iop Publishing Ltd, 2024) Gercekcioglu, Hamza; Baykal, Yahya; 7812
The Oceanic Turbulence Optical Power Spectrum (OTOPS) with depth variations is acquired under any temperature and salinity concentration. It is supposed that specific medium is the Atlantic Ocean at high latitude and the Pacific Ocean at high, mid and low latitudes. For the OTOPS model, a depth-varying functions that include low-latitude, high- and mid-latitude-summer and mid-latitude-winter salinity and temperature changes are found. With the help of the equations for the temperature and salinity changes, figures are obtained for the eddy diffusivity ratio depth of seawater and OTOPS model against the depth and kappa at these media. In the ocean, downlink (uplink) is defined as the optical wireless communication link where the receiver (transmitter) is located at a deeper point than the transmitter (receiver), i.e., in the downlink, optical signal proceeds from a point close to ocean surface to deeper ocean and in the uplink, optical signal proceeds from deeper ocean to a point close to ocean surface. In this paper, the OTOPS model is investigated on how its properties change in the underwater environment in downlink and uplink. Different behavior of the OTOPS model is exhibited.
Citation - WoS: 66
Citation - Scopus: 70
Equivalence of structure constants in non-Kolmogorov and Kolmogorov spectra
(Optical Soc Amer, 2011) Baykal, Yahya; Gercekcioglu, Hamza; 7812
We find the equivalence of the structure constants in non-Kolmogorov and Kolmogorov spectra in a turbulent atmosphere. As the reference point, the spherical wave scintillation index in a non-Kolmogorov medium is used. Relations of the structure constants are found to be functions of the power law of the turbulence spectrum and the Fresnel zone. It will be useful to employ the equivalence of the structure constants in making performance comparisons found with non-Kolmogorov and Kolmogorov spectra. (C) 2011 Optical Society of America
Citation - WoS: 0
Citation - Scopus: 1
Field Correlations of Multimode Optical Beams in Underwater Turbulence
(Optica Publishing Group, 2024) Baykal, Yahya; Gokce, Muhsin C.; Ata, Yalcin; Gercekcioglu, Hamza
For multimode optical beams, field correlations at the receiver plane are found in underwater turbulence. Field correlations of single high order beams in underwater turbulence are special cases of our formulation. Variations of field correlations against the underwater turbulence parameters and the diagonal length from various receiver points are examined for different multimode and single high order beams. Stronger underwater turbulence is found to reduce the field correlations of multimode and single high order optical beams. The results will be of help in heterodyne detection analysis and fiber coupling efficiency in an underwater medium experiencing turbulence. (c) 2024 Optica Publishing Group
Citation - WoS: 4
Citation - Scopus: 6
Field correlations of partially coherent optical beams in underwater turbulence
(Optica Publishing Group, 2022) Baykal, Yahya; Gercekcioglu, Hamza
Field correlations of partially coherent optical beams at the receiver plane are formulated and evaluated in underwater turbulence. Variations of the field correlations are examined against changes in the degree of source coherence, diagonal length from the receiver point, receiver point, propagation distance, source size, ratio of temperature to salinity contributions to the refractive index spectrum, rate of dissipation of mean-squared temperature, and rate of dissipation of kinetic energy per unit mass of fluid. Under any underwater turbulence and link conditions, it is found that field correlations at the receiver plane reduce when the optical source becomes less coherent. (c) 2022 Optica Publishing Group
Citation - WoS: 31
Citation - Scopus: 34
Hermite Gaussian beam scintillations in weak atmospheric turbulence for aerial vehicle laser communications
(Elsevier, 2020) Sayan, Omer F.; Gercekcioglu, Hamza; Baykal, Yahya; 7812
Scintillation index of Hermite Gaussian beams used for air vehicle communication systems in vertical paths of weak atmospheric turbulent medium are investigated by employing the modified Rytov method. By evaluating the on-axis scintillation index, variations of the scintillation indices of these beams are examined against the changes in the Gaussian beam size of the Hermite Gaussian beam mode, propagation distances and the zenith angles at the realistic propagation distances involved in uplink and downlink configurations. In the atmospheric environment, for uplink, the Hermite Gaussian beam modes have no advantage over the Gaussian beams at short propagation distances like L = 20 km,as well as at long propagation distances like L = 700 km. However, for downlink, although Hermite Gaussian beam modes are disadvantageous over the Gaussian beams at short propagation distances like L = 20 km, they are found advantageous over the Gaussian beams at long propagation distances like L = 700 km. The results of this study may encourage to use Hermite Gaussian beams, especially in the air vehicle laser communication links, and can be used in the design of an optical wireless communication link utilizing the vertical atmospheric medium.
Citation - WoS: 0
Citation - Scopus: 0
Intensity and Degree of Coherence of Vortex Beams in Atmospheric Turbulence
(Ieee-inst Electrical Electronics Engineers inc, 2024) Gokce, Muhsin Caner; Baykal, Yahya; Gercekcioglu, Hamza; Ata, Yalcin
We utilize the Huygens-Fresnel principle to derive the mutual coherence function (MCF) for a vortex beam, which is the main focus of our investigation. Then, we examine the intensity and modulus of the complex degree of coherence (DOC) characteristics of vortex beams in atmospheric turbulence. Our results indicate that as the topological charge increases, the intensity distribution of the vortex beam becomes less affected by atmospheric turbulence. However, the modulus of the complex DOC decreases.
Citation - WoS: 34
Citation - Scopus: 39
Intensity fluctuations of flat-topped beam in non-Kolmogorov weak turbulence
(Optical Soc Amer, 2012) Gercekcioglu, Hamza; Baykal, Yahya; 7812
Results obtained on the intensity fluctuations of flat-topped Gaussian beams in weakly turbulent non-Kolmogorov horizontal atmospheric optics links are represented. Effects on the scintillation index of the power law a that describes the non-Kolmogorov spectrum are examined. Our results correctly reduce to the existing intensity fluctuations of flat-topped beams in Kolmogorov turbulence. Variation of the scintillation index against non-Kolmogorov power law alpha exhibits a peak at the worst power law alpha(w), which happens to be smaller than the Kolmogorov power law of 11/3. If the power law is smaller (larger) than alpha(w), increase in alpha will increase (decrease) the intensity fluctuations. Evaluation of the scintillation index at the worst power law results in smaller fluctuations for a Gaussian beam at short propagation distances; however, at long propagation distances flatter beams happen to possess smaller fluctuations. The scintillation change versus the source size follows a similar trend regardless whether the flat-topped beam propagates in a Kolmogorov or non-Kolmogorov medium. (C) 2012 Optical Society of America
Citation - WoS: 4
Citation - Scopus: 1
Intensity Fluctuations of Flat-Topped Beam in Non-Kolmogorov Weak Turbulence: Reply
(Optical Soc Amer, 2012) Gercekcioglu, Hamza; Baykal, Yahya
In our recent publication, we have examined the intensity fluctuations of flat-topped beam in non-Kolmogorov weak turbulence [J. Opt. Soc. Am. A 29, 169 (2012)] in which our comparison of the scintillation indices in the Kolmogorov and in various non-Kolmogorov turbulences was based on the same structure constant, no matter what power law the non-Kolmogorov spectrum takes. In such choice of the fixed structure constant, which is also being used by many researchers in the field [Opt. Express 18, 451 (2010); Proc. SPIE 6747, 67470B (2007); Opt. Commun. 285, 880 (2012)], we have found that the variation of the scintillation index against non-Kolmogorov power law exhibits a peak at the worst power law, which happens to be smaller than the Kolmogorov power law of 11/3. Charnotskii commented [J. Opt. Soc. Am. A. 29, 1838 (2012)] on our paper. In this paper, in our to reply to Charnotskii's comment, we have re-evaluated the scintillation index of flat-topped beam in non-Kolmogorov weak turbulence by employing our recently reported equivalent structure constant [Opt. Lett. 36, 4554 (2011)] and re-compared the intensity fluctuations in Kolmogorov and in non-Kolmogorov turbulences. As the result of such re-comparison, the worst power law is observed to disappear. c 2012 Optical Society of America
Citation - WoS: 12
Citation - Scopus: 12
Intensity fluctuations of laser array beams in non-kolmogorov turbulence
(Ieee-inst Electrical Electronics Engineers inc, 2015) Gercekcioglu, Hamza; Baykal, Yahya; 7812
On-axis intensity fluctuations of laser array beams are evaluated when they are used in a weakly turbulent non-Kolmogorov atmosphere. Our formulation of the scintillation index is based on the Rytov method, which in the limiting case, correctly reduces to the known Gaussian beam scintillation index in weak Kolmogorov turbulence. When the radius of the ring (on which the array beamlets are placed), number of beamlets forming the laser array, source size of the beamlets, propagation distance or the wavelength is fixed, a decrease in the power law exponent of the non-Kolmogorov spectrum is found to decrease the scintillation index. Examining for any realization of the non-Kolmogorov spectrum, it is observed that an increase in the ring radius, number of beamlets, wavelength, and decrease in the source size of the beamlet, propagation distance reduces the intensity fluctuations.
Citation - WoS: 3
Citation - Scopus: 3
Laser beam scintillations of LIDAR operating in weak oceanic turbulence
(Optica Publishing Group, 2022) Gercekcioglu, Hamza; Baykal, Yahya; 7812
The formulation of light detection and ranging (LIDAR) systems is derived and examined for the scintillation index, evaluated on-axis, of laser beams in horizontal links in the ocean with weak turbulence by utilizing the Rytov method. These scintillation indices, obtained for the Gaussian beam which is collimated, the limits of plane and spherical waves, are depicted versus the source size, target size, and parameter of the normalized reflector size. It is found that the source size, target size, and normalized reflector size parameter, lessening the scintilla-tion index evaluated on-axis, are approximately 0.44 cm, 56 x 10-4 cm, and 2.2, respectively. Additionally, by using these values that minimize the scintillation index, the variation of the scintillations is shown against the propagation distance, radius of reflector, temperature and salinity fluctuation effects, mean squared temperature, and turbulent kinetic energy dissipation rate per unit mass of fluid at various selected source size and radius of reflector values. (c) 2022 Optica Publishing Group
Citation - WoS: 4
Minimization effects on scintillations of sinusoidal Gaussian beams in strong turbulence
(Iop Publishing Ltd, 2011) Gercekcioglu, Hamza; Baykal, Yahya; 7812
Minimization effects on the on-axis scintillation index of cos Gaussian (cG) and cosh Gaussian (chG) beams are studied in strong turbulence. In our formulation, the unified solution of the Rytov method, which imposes spatial filtering to extend the solution to the strong turbulence regime, is applied. Our solution correctly reduces to the weak turbulence sinusoidal beam scintillations and the strong turbulence Gaussian beam scintillations. The conditions to minimize the scintillations are found to be focused chG beams. Small scale scintillations mainly determine the overall scintillations of cG and chG beams in strong turbulence. In strong turbulence, increase in the source size decreases the scintillations of collimated cG beams but does not change the scintillations of focused cG beams. Collimated cG beams having larger displacement parameters and large focal lengths show smaller scintillations in the strong regime. Change in the displacement parameters for collimated and focused chG beams and the focal length of focused chG beams do not considerably vary their scintillations in strong turbulence.
Citation - WoS: 17
Citation - Scopus: 17
Minimization of scintillation index against displacement parameters
(Elsevier Science Bv, 2008) Eyyuboglu, Halil T.; Gercekcioglu, Hamza; Baykal, Yahya; 7688; 7812
For sinusoidal beams, minimization of scintillation index is carried out against the displacements parameters. It is found-that x-y asymmetric cosh-Gaussian beam fulfills the requirements of such optimum beam. Our minimization procedure reveals that the optimum beam is achieved by continually focusing it at the chosen propagation length and by further adjusting displacements parameters to be propagation distance dependent. Scintillation index of thus constructed optimum beam is formulated and numerically evaluated., Our graphical comparisons entailing collimated and focused versions of cos-, cosh-Gaussian, annular-Gaussian and Gaussian beams show that the optimum beam yields the lowest scintillations provided that Propagation range is less than or equal to the focusing distance. (C) 2008 Elsevier B.V. All rights reserved.
Citation - WoS: 4
Citation - Scopus: 4
Minimization of the scintillation index of sinusoidal Gaussian beams in weak turbulence for aerial vehicle-satellite laser communications
(Optical Soc Amer, 2021) Gercekcioglu, Hamza; Baykal, Yahya; 7812
Minimization of the on-axis scintillation index of sinusoidal Gaussian beams is investigated by using the modified Rytov method in weak atmospheric turbulence for uplink/downlink of aerial vehicle-satellite laser communications. Among the focused cosh-Gaussian (cosh-G), cos-Gaussian (cos-G), annular, and Gaussian beams, a suitable displacement parameter for a cosh-G beam is determined that will minimize the scintillation index in uplink and downlink configurations. Then, for both uplink and downlink, the variations of the scintillation index against the propagation distance, source size, and zenith angle are examined and compared among themselves to show the optimum beam that possesses the minimum scintillation index. Sinusoidal Gaussian beams that are focused at the receiver and obtained by employing the appropriate displacement parameter, which we name the optimum beams, are recommended to obtain smaller intensity fluctuations in atmospheric wireless optical communication systems operating in vertical links in weak turbulence. (C) 2021 Optical Society of America