Elektronik ve Haberleşme Mühendisliği Bölümü Yayın Koleksiyonu

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

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Now showing 1 - 16 of 16

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.
Citation - WoS: 5
Citation - Scopus: 10
Beams With Arbitrary Field Profiles in Turbulence - Art. No. 652209
(Spie-int Soc Optical Engineering, 2006) Baykal, Yahya
Characteristics of optical beam incidences that have arbitrary field profiles are examined when they propagate in the turbulent atmosphere. Arbitrary source field profile is introduced by decomposing the source into incremental areas and the received field in the presence of turbulence is expressed as the summation of the fields originating from each incremental area. Intensity moments such as average intensity and the scintillation index in turbulence are formulated under such excitation. Our results correctly reduce to the well established Gaussian beam wave solutions when the arbitrary source beam is taken as the Gaussian field profile. Naturally, all the beam structures such as the higher-order single-mode, multimode, off-axis Hermite-Gaussian, Hermite-sinusoidal-Gaussian, higher-order annular, flat-topped-Gaussian beams form the special cases of our derivation. Numerical results that cover the scintillations in turbulence for various types of arbitrary beam profiles are presented. Our results for the arbitrary source field profiles can be applied in atmospheric optics telecommunication links where combination of several known beams are employed as incidence in an effort to reduce the degrading effects of turbulence. Also in the problems of reflection from rough surfaces, propagation of spatially partially coherent optical beams or double passage imaging in turbulence, our formulation can be utilized.
Citation - WoS: 17
Citation - Scopus: 16
Coherence Length in Non-Kolmogorov Satellite Links
(Elsevier, 2013) Baykal, Yahya
Behavior of the coherence length in non-Kolmogorov satellite links is investigated. Equivalent structure constants for non-Kolmogorov spectra are employed in order to make relevant comparisons for different non-Kolmogorov power law exponents. Examining the coherence length versus the non-Kolmogorov power law exponent for different rms wind speeds, zenith angles, link lengths, structure constants and the wavelengths, the coherence length is found to decrease when the non-Kolmogorov power law exponent increases. At a fixed non-Kolmogorov power law exponent, the coherence length is found to decrease as the rms wind speed, the zenith angle or the structure constant increases and the wavelength decreases. As the link length increases, the coherence length decreases for power law exponent values smaller than that for the Kolmogorov case. However, an increase in the link length seems not to cause the coherence length to vary appreciably at power law exponent values larger than the Kolmogorov case power law exponent. (C) 2013 Elsevier B.V. All rights reserved.
Citation - WoS: 8
Citation - Scopus: 9
Cross Beam Scintillations in Non-Kolmogorov Medium
(Optical Soc Amer, 2014) Baykal, Yahya
For the collimated and focused cross beams, the on-axis scintillation index is evaluated when these beams propagate in weak non-Kolmogorov turbulence. In the limiting cases, our solution correctly reduces to the known Gaussian beam scintillations in Kolmogorov turbulence. For both the collimated and the focused cross beams, large power law exponent of the non-Kolmogorov turbulence is found to result in larger scintillations. Evaluating at a fixed power law exponent, the scintillation index of the collimated (focused) cross beam is higher (lower) than the collimated (focused) Gaussian beam scintillation index. When the asymmetry of the collimated (focused) cross beam increases, the scintillations increase (decrease). At a given cross beam configuration, change in the turbulence parameters varies the scintillations in the same manner for all power law exponent values. (C) 2014 Optical Society of America
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)
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.
Citation - WoS: 6
Citation - Scopus: 6
Field Correlations for Off-Axis Gaussian Laser Beams in Atmospheric Turbulence
(Amer Geophysical Union, 2014) Baykal, Yahya
The absolute field correlations in atmospheric turbulence are evaluated for the off-axis optical Gaussian beam incidence. Evaluations in the practical range of the source and the turbulent medium parameters show that an increase in the diagonal length at the receiver plane causes the absolute field correlations of the off-axis Gaussian beam to decrease. At a fixed receiver diagonal length, the off-axis Gaussian beams having smaller displacement parameters and larger source sizes exhibit larger absolute field correlations. Comparing the absolute field correlations of the off-axis Gaussian beams in atmospheric turbulence with their no turbulence counterparts, it is observed that the behavior of the absolute field correlation variations remains the same; however, the diminishing of the absolute field correlations in turbulence occurs at smaller diagonal lengths.
Citation - WoS: 18
Citation - Scopus: 19
Field Correlations of Flat-Topped Gaussian and Annular Beams in Turbulence
(Elsevier Sci Ltd, 2011) Baykal, Yahya
Starting from the second order moment formulation for multi-beam incidence, field correlations at the receiver plane of flat-topped Gaussian and annular beams are found in turbulence. Reflecting the information on both the randomness due to turbulence and the field profile of the incident field, field correlations of the fiat-topped Gaussian beams are found to become larger at larger source size, smaller flatness parameters and smaller turbulence strengths. For the annular beam structures, field correlations are larger for thicker beams. Field correlations of larger primary beam sized annular incidences are smaller at smaller diagonal distances and larger at larger diagonal distances. As expected, annular beam field correlations are found to be larger at smaller structure constants and at smaller wavelengths. However, at large link lengths, field correlations could be larger than at smaller link lengths due to fact that for annular beams, the field at the centre of the receiver attains very small value at smaller link lengths, however, on propagation in turbulence, receiver field distribution changes to a Gaussian profile. (C) 2011 Elsevier Ltd. All rights reserved.
Citation - WoS: 27
Citation - Scopus: 28
Field Correlations of Laser Arrays in Atmospheric Turbulence
(Optical Soc Amer, 2014) Baykal, Yahya
Correlations of the fields at the receiver plane are evaluated after a symmetrical radial laser array beam incident field propagates in a turbulent atmosphere. The laser array configuration is composed of a number of the same size laser beamlets symmetrically located around a ring having a radius that determines the distance of the ring from the origin. The variations of the correlations of the received field originating from such laser array incidence versus the diagonal length starting from a receiver point are examined for various laser array parameters, turbulence parameters, and the locations of the reception points. Laser array parameters consist of the ring radius and the number and size of the beamlets. Structure constant, link length, and wavelength are the turbulence parameters whose effects on the field correlation of the laser arrays are also investigated. (C) 2014 Optical Society of America
Citation - WoS: 14
Citation - Scopus: 15
Intensity Correlations of General Type Beam in Weakly Turbulent Atmosphere
(Elsevier Sci Ltd, 2011) Baykal, Yahya
In weakly turbulent atmosphere, intensity correlations at the receiver plane are formulated for a general type optical beam. Evaluating our formula at the appropriate source parameters versus the diagonal distance, intensity correlations for cos Gaussian, cosh Gaussian, annular and flat-topped Gaussian beams are obtained. As compared to Gaussian beam, intensity correlations are found smaller for cos Gaussian and larger for cosh Gaussian beams. Intensity correlations of cos Gaussian, cosh Gaussian and flat-topped Gaussian beams become larger at large source sizes. Thicker annular beams and flatter flat-topped Gaussian beams show larger intensity correlations. Without generalizing, intensity correlations tend to increase when the link length and the structure constant decrease and the wavelength increases. Our results show that the intensity correlations are not only built up using random medium effects, but also using the diffraction pattern formed at the receiver plane for the specific incidence investigated. As a check point, for all source types and medium parameters, our evaluations indicate that intensity correlations approach zero at sufficiently large diagonal distances. (C) 2011 Elsevier Ltd. All rights reserved.
Citation - WoS: 3
Citation - Scopus: 4
Intensity Fluctuations for Source Arrays in Turbulent Atmosphere - Art. No. 630308
(Spie-int Soc Optical Engineering, 2006) Baykal, Yahya
Intensity fluctuations are formulated for source arrays in weakly turbulent horizontal atmospheric links. Source array is composed of point sources separated by variable distances in the transverse source directions. Formula yielding the on-axis scintillation index for the source array is derived by employing the Rytov solution for the structure and correlation functions in the extended Huygens Fresnel principle. Through numerical results, variations of the scintillations versus the array parameters such as the size of the array, spacing between the array elements, amplitudes and phases of the individual sources in the array are investigated. Numerically evaluated intensity fluctuations for such array parameters are compared with the well known single point source scintillations. We are interested to understand whether the use of a source array will give favorable intensity fluctuations in atmospheric communication links.
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.
Citation - WoS: 25
Citation - Scopus: 25
Scintillations of Higher-Order Laser Beams in Non-Kolmogorov Medium
(Optical Soc Amer, 2014) Baykal, Yahya
In an atmospheric medium that shows a non-Kolmogorov turbulence behavior, the variation of the on-axis scintillation index is evaluated when higher-order laser modes are used as the excitation. The Rytov method is employed together with the equivalent structure constant, which makes our results valid in weak turbulence. In the limiting case, our solution correctly reduces to the known scintillation index of the Gaussian beam in Kolmogorov turbulence. For all the higher-order even modes, increase in the power law exponent, a of the non-Kolmogorov spectrum is found to increase the scintillations. When the source size of the higher-order modes is large, higher-order even modes attain almost the same scintillation index values for all a. However, for small source sizes, being valid for any realization of the non-Kolmogorov spectrum, the scintillations decrease as the mode order becomes large. The changes in the propagation distance, structure constant, and the wavelength do not vary these trends. (C) 2014 Optical Society of America
Citation - WoS: 16
Citation - Scopus: 17
Sinusoidal Gaussian Beam Field Correlations
(Iop Publishing Ltd, 2012) Baykal, Yahya
Field correlations of sinusoidal Gaussian beams are formulated in turbulence, and specifically cos Gaussian (cG) and cosh Gaussian (chG) beam field correlations are evaluated versus the diagonal length at the receiver plane. The effects of the displacement parameters, the coordinates of the first receiver point and the source sizes on the field correlations of monochromatic light sources having cG and chG field distributions are investigated when such beams traverse turbulent media. Such parameters affect spatial heterodyne measurement. Field correlations found at the receiver plane reflect the combined variations of diffraction patterns and turbulence effects. To differentiate the diffraction patterns and the turbulence effects, field correlations of cG and chG beams in turbulence and in the absence of turbulence are compared. For cG beams, the oscillatory behaviour of the field correlations versus the diagonal length at the receiver plane in the absence of turbulence becomes smoother in the presence of turbulence. The received fields of cG and chG beams become decorrelated at shorter diagonal distances in turbulence.
Citation - WoS: 1
Citation - Scopus: 1
Speckle Contrast for Flat-Topped Field Profiles
(Elsevier Sci Ltd, 2013) Baykal, Yahya
Speckle contrast is evaluated for flat-topped field profiles where the characterization and evaluation of speckle contrast are based on mathematical formulation and theoretical parameter variation. The speckle field which is represented by the random phase shift and the random phase tilt is multiplied by the flat-topped shaped field. The increase in the source size of the flat-topped field first causes the speckle contrast to increase, then reaches saturation. Increase in the variance of the phase shift of a flat-topped field causes an appreciable increase in the speckle contrast. Depending on the diffraction characteristics of the finite sized flat-topped field distribution, upon changing the variance of the phase shift, the speckle contrast exhibits different behavior at short and long propagation distances. However, especially for small variances of phase tilts, the speckle contrast seems to monotonically increase as the propagation distance becomes larger. The effect of the order of flatness on the speckle contrast is small at short propagation distances which can be appreciable at long propagation distances. (C) 2012 Elsevier Ltd. All rights reserved.
Citation - WoS: 10
Citation - Scopus: 9
Structure Functions in Turbulence for Incidence With Arbitrary-Field Distribution
(Optical Soc Amer, 2007) Baykal, Yahya
In a turbulent atmosphere, the log-amplitude, phase-correlation, and structure functions are formulated for light sources exhibiting arbitrary-field distribution. This is done by extending the formulations of the correlation and structure functions for the known general-type beam sources to cover any type of source having arbitrary-field distribution. To introduce the arbitrary field, the source is presented by an incidence that is a function of transverse source coordinates. The received field in the random medium is found by employing the Rytov method, which is a single-scattering solution obtained by the first-order approximation; thus our results are valid in weak turbulence. All the existing results can be correctly reproduced from our formulations when the corresponding source-field distributions are inserted into our presented correlation and structure-function expressions. Our results for the arbitrary-source field profiles can be utilized in finding the scintillation index and the angle-of-arrival fluctuations of any type of incidence in optical atmospheric links. Additionally, our formulations can find applications in reflection from rough surfaces and imaging in turbulence. (C) 2007 Optical Society of America

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