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: 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: 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: 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: 12Citation - Scopus: 11Partially 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.