WoS İndeksli Yayınlar Koleksiyonu
Permanent URI for this collectionhttps://hdl.handle.net/20.500.12416/8653
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Conference Object Off-Axis Gaussian Beams With Random Displacement in Atmospheric Turbulence(int inst informatics & Systemics, 2005) Baykal, Yahya Kemal; Baykal, Yahya K.; Eyyuboğlu, Halil Tanyer; Eyyuboglu, Halil T.; Yenice, Yusuf E.; Elektronik ve Haberleşme Mühendisliği; Elektrik-Elektronik MühendisliğiOur recent work in which we study the propagation of the general Hermite-sinusoidal-Gaussian laser beams in wireless broadband access telecommunication systems is elaborated in this paper to cover the special case of an off-axis Gaussian beam. We mainly investigate the propagation characteristics in atmospheric turbulence of an off-axis Gaussian beam possessing Gaussian distributed random displacement parameters. our interest is to search for different types of laser beams that will improve the performance of a wireless broadband access system when atmospheric turbulence is considered. Our formulation is based on the basic solution of the second order mutual coherence function evaluated at the receiver plane. For fixed turbulence strength, the coherence length calculated at the receiver plane is found to decrease as the variance of the random displacement is increased. It is shown that as the turbulence becomes stronger, coherence lengths due to off-axis Gaussian beams tend to approach the same value, irrespective of the variance of the random displacement. As expected, the beam spreading is found to be pronounced for larger variance of displacement parameter. Average intensity profiles when atmospheric turbulence is present are plotted for different values of the variance of the random displacement parameter of the off-axis Gaussian beam.Conference Object Citation - WoS: 1Citation - Scopus: 7Beam 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, YangjianIn 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.Conference Object Citation - WoS: 2Citation - Scopus: 5Intensity Fluctuations of Focused General-Type Beams in Atmospheric Optics Links(Spie-int Soc Optical Engineering, 2007) Baykal, Yahya; Eyyuboglu, Halil T.The intensity fluctuations of focused general-type beams in weakly turbulent atmospheric links are formulated and numerically evaluated. Focused general-type beams in general cover very large range of beams, however in this work we concentrate specifically on the focused sinusoidal-Gaussian, annular and flat-topped beams. The behavior of the scintillations for these beams is examined versus the focusing distance, wavelength of operation and the source size. In our formulation, atmospheric turbulence is introduced through the Rytov method where the free space field (i.e., in the absence of turbulence) at the receiver plane for the general-type focused beam is found by utilizing the Huygens-Fresnel principle. Figures are presented showing the scintillation index for focused general-type beams and collimated general-type beams. To find out the source and medium parameters that will yield favourable scintillation levels, the intensity fluctuations of the focused sinusoidal-Gaussian, annular and flat-topped beams are compared. Within the range of selected source and medium parameters, our observations indicate that the intensity fluctuations in weak turbulence tend to be the smallest for the focused flat-topped Gaussian beams and the largest for the focused cos-Gaussian beams. Gaussian, cosh-Gaussian and annular beams experience interim level fluctuations. The comparison of the scintillation levels for the mentioned types of focused beams follow the same tendency for all the propagation distances. Also, the intensity fluctuations of the focused general-type beams in turbulence are compared with their collimated counterparts. Such a comparison reveals that for all the beams at a selected source size, the scintillations are nearly the same for both the focused and the collimated cases at all the propagation distances, except for the flat-topped Gaussian beams. When focused flat-topped Gaussian beams are employed, the intensity fluctuations seem to be lower as compared to the equivalent collimated flat-topped Gaussian beam at shorter link lengths.Article Citation - WoS: 13Citation - Scopus: 13Angle-Of Fluctuations for General-Type Beams(Spie-soc Photo-optical instrumentation Engineers, 2007) Eyyuboglu, Halil T.; Baykal, YahyaStarting 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: 41Citation - Scopus: 46Transmittance of Partially Coherent Cosh-Gaussian, Cos-Gaussian and Annular Beams in Turbulence(Elsevier Science Bv, 2007) Eyyuboglu, Halil T.; Baykal, YahyaAverage relative power transmittance is evaluated, by incorporating atmospheric turbulence, for partially coherent cosh-Gaussian, cos-Gaussian, Gaussian and annular beams. For all the collimated versions of these beams, against the increasing propagation length, there is a typical trend of the decrease in the relative average power transmittance with incremental drop being much less for partially coherent cos-Gaussian beams. The change in the transmittance versus the propagation length will be similar to the corresponding collimated cases, when these beams are focused at a certain focal length. Also partially coherent beams are less sensitive to propagation length changes, except for cos-Gaussian case. Partially coherent cosh-Gaussian beams exhibit a drop in the transmittance as the displacement parameter of the beam is made larger, whereas this trend is just the opposite for partially coherent cos-Gaussian beams. When examined versus the source size, for all the four types of beams, the transmittance has a similar behavior, i.e., it becomes high at small source sizes, falling with increasing source size, and following a dip, it starts to rise, eventually approaching the plane wave limit of unity. The occurrence of the dip coincides with the smallest source size for cosh-Gaussian, with the largest for cos-Gaussian, and about the same source size for Gaussian and annular beams. In general, the average relative power transmittance of coherent beam is affected much more than the partially coherent beams against the variations in source properties. (c) 2007 Elsevier B.V. All rights reserved.Conference Object Citation - WoS: 5Citation - Scopus: 9Incoherent Sinusoidal-Gaussian and Annular Beam Scintillations(Spie-int Soc Optical Engineering, 2008) Baykal, Yahya; Eyyuboglu, Halil T.; Cai, YangjianThe scintillation index is evaluated in horizontal turbulent atmospheric optical links for incidences of incoherent cosh-Gaussian (IChG), cos-Gaussian (ICG) and annular (IA) beams. Weak turbulence solution is obtained for a slow detector. Dependence of the intensity fluctuations of the IChG, ICG and IA beams on the link length, source size, wavelength and the structure constant are examined. At all the link lengths, fixed size IChG and ICG beams exhibit lower scintillations for larger absolute displacement parameters. At a fixed link length, IChG beam yields lower fluctuations than the ICG beam having the same absolute displacement parameter. For the same size primary beams, IA beam with narrower ring scintillates less than the IA beam with wider ring, and this holds to be valid for each link length. Investigation of the scintillation versus the source size reveals that increase in the source size lowers the scintillations for all types of the incoherent beams. At the same source size and the same absolute displacement parameter, IChG beams have lower fluctuations than the ICG beams, larger absolute displacement parameters exhibiting lower scintillations at the same source size for both beams. For IA beams, as the size of the primary beam is increased, the scintillations are reduced for all ring sizes, the reduction being pronounced for narrower rings. As the wavelength increases, the scintillations of IChG and ICG beams first increase, then at around the wavelength forming the Fresnel zone, the scintillations start to decrease and eventually for all types of IChG and ICG beams, the scintillation indices merge towards a certain value. Similar behaviour of the scintillations versus the wavelength is observed for IA beams as well. As long as the structure constant is kept within the range of interest to remain in the weak turbulence, raising the structure constant first increases the intensity fluctuations of beams for all the mentioned beam types where further rises in the structure constant result in the same level of scintillation index. Comparison of the scintillations of IChG, ICG and IA beams with their coherent counterparts and the coherent Gaussian beam shows that the IChG, ICG and IA beams are favorable for large sized sources.Conference Object Citation - WoS: 20Citation - Scopus: 33Effect of Beam Types on the Scintillations: a Review(Spie-int Soc Optical Engineering, 2009) Eyyuboglu, Halil T.; Cai, Yangjian; Baykal, YahyaWhen different incidences are launched in atmospheric turbulence, it is known that the intensity fluctuations exhibit different characteristics. In this paper we review our work done in the evaluations of the scintillation index of general beam types when such optical beams propagate in horizontal atmospheric links in the weak fluctuations regime. Variation of scintillation indices versus the source and medium parameters are examined for flat-topped-Gaussian, cosh-Gaussian, cos-Gaussian, annular, elliptical Gaussian, circular (i.e., stigmatic) and elliptical (i.e., astigmatic) dark hollow, lowest order Bessel-Gaussian and laser array beams. For flat-topped-Gaussian beam, scintillation is larger than the single Gaussian beam scintillation, when the source sizes are much less than the Fresnel zone but becomes smaller for source sizes much larger than the Fresnel zone. Cosh-Gaussian beam has lower on-axis scintillations at smaller source sizes and longer propagation distances as compared to Gaussian beams where focusing imposes more reduction on the cosh-Gaussian beam scintillations than that of the Gaussian beam. Intensity fluctuations of a cos-Gaussian beam show favorable behaviour against a Gaussian beam at lower propagation lengths. At longer propagation lengths, annular beam becomes advantageous. In focused cases, the scintillation index of annular beam is lower than the scintillation index of Gaussian and cos-Gaussian beams starting at earlier propagation distances. Cos-Gaussian beams are advantages at relatively large source sizes while the reverse is valid for annular beams. Scintillations of a stigmatic or astigmatic dark hollow beam can be smaller when compared to stigmatic or astigmatic Gaussian, annular and flat-topped beams under conditions that are closely related to the beam parameters. Intensity fluctuation of an elliptical Gaussian beam can also be smaller than a circular Gaussian beam depending on the propagation length and the ratio of the beam waist size along the long axis to that along the short axis (i.e., astigmatism). Comparing against the fundamental Gaussian beam on equal source size and equal power basis, it is observed that the scintillation index of the lowest order Bessel-Gaussian beam is lower at large source sizes and large width parameters. However, for excessively large width parameters and beyond certain propagation lengths, the advantage of the lowest order Bessel-Gaussian beam seems to be lost. Compared to Gaussian beam, laser array beam exhibits less scintillations at long propagation ranges and at some midrange radial displacement parameters. When compared among themselves, laser array beams tend to have reduced scintillations for larger number of beamlets, longer wavelengths, midrange radial displacement parameters, intermediate Gaussian source sizes, larger inner scales and smaller outer scales of turbulence. The number of beamlets used does not seem to be so effective in this improvement of the scintillations.Article Citation - WoS: 1Citation - Scopus: 1Mode Coupling in Vortex Beams(Elsevier Sci Ltd, 2018) Eyyuboglu, Halil T.We examine the mode coupling in vortex beams. Mode coupling also known as the crosstalk takes place due to turbulent characteristics of the atmospheric communication medium. This way, the transmitted intrinsic mode of the vortex beam leaks power to other extrinsic modes, thus preventing the correct detection of the transmitted symbol which is usually encoded into the mode index or the orbital angular momentum state of the vortex beam. Here we investigate the normalized power mode coupling ratios of several types of vortex beams, namely, Gaussian vortex beam, Bessel Gaussian beam, hypergeometric Gaussian beam and Laguerre Gaussian beam. It is found that smaller mode numbers lead to less mode coupling. The same is partially observed for increasing source sizes. Comparing the vortex beams amongst themselves, it is seen that hypergeometric Gaussian beam is the one retaining the most power in intrinsic mode during propagation, but only at lowest mode index of unity. At higher mode indices this advantage passes over to the Gaussian vortex beam. (C) 2017 Elsevier Ltd. All rights reserved.Article Citation - WoS: 8Citation - Scopus: 8Intensity Fluctuations of Partially Coherent Cos Gaussian and Cosh Gaussian Beams in Atmospheric Turbulence(Iop Publishing Ltd, 2011) Eyyuboglu, Halil T.; Cil, Celal Z.; Cai, Yangjian; Korotkova, Olga; Baykal, YahyaOn-axis and off-axis scintillation indices of partially coherent collimated cos Gaussian and cosh Gaussian beams are evaluated on their propagation in a weakly turbulent atmosphere. In the coherent limits, our results are in quite good agreement with those for the existing Rytov scintillation index of coherent cos Gaussian and cosh Gaussian beams, especially for very small and very large sized beams and at weaker turbulence levels. The on-axis scintillation index becomes lower as the degree of source coherence decreases for both cos Gaussian and cosh Gaussian beams. Comparing to the partially coherent Gaussian beam on-axis scintillations, partially coherent cos Gaussian beam scintillations are generally lower and partially coherent cosh Gaussian beam scintillations are higher. Scintillations of Gaussian beams are very close to the scintillations of equal-power cos Gaussian and are quite similar to the scintillations of equal-power cosh Gaussian beams. Off-axis evaluations yield that the scintillations of both cos Gaussian and cosh Gaussian beams increase as the diagonal distance from the origin increases, being valid for all degrees of partial coherence.Article Citation - WoS: 3Citation - Scopus: 5Irradiance Fluctuations of Partially Coherent Super Lorentz Gaussian Beams(Elsevier, 2011) Eyyuboglu, Halil T.; Gokce, Muhsin C.By using the semi-analytic approach introduced earlier, we formulate and subsequently evaluate the irradiance fluctuations of partially coherent super Lorentz Gaussian beams for orders of 10 and 11. Within the range of examined source and propagation conditions, our calculations show that there will be less fluctuations at short propagation distances as the Lorentzian property is increased. But the reverse will be applicable, if the longer propagation distances are considered. The use of focusing will cause reductions, particularly for beams with increased Lorentzian property. (C) 2011 Elsevier B.V. All rights reserved.