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: 6Citation - Scopus: 4Propagation of Cross Beams Through Atmospheric Turbulence(Spie-int Soc Optical Engineering, 2005) Yenice, YE; Eyyuboglu, HT; Baykal, Y; Venice, Yusuf E.Propagation properties of cross beam in turbulent medium are studied. A cross beam is constructed by the sum of two highly asymmetric Gaussian beams placed along transverse axes. It is known that such beams, when propagating in free space, will exhibit contrasting diffraction behaviours; they expand widely in one axis, while they are almost nondiffracting in the other axis within useful link lengths. This behaviour allows detecting the two components and a sum component if desired separately with a practical multiaperture receiver. Bearing in mind that this property can be exploited for a diversity scheme, our present work focuses on the propagation of such beams in turbulent atmosphere. To this end, starting with a source field expression of the cross beam, the second order mutual coherence function is formulated at the receiver plane. Intensity plots describing the dependence on the source and propagation parameters on the receiver plane are provided. The results tend to confirm the applicability of the concept provided the design parameters are appropriately chosen. For a decisive assessment, however, turbulence-induced beam wander must also be examined.Conference Object Rate Averaging in Free Space Optics Systems Using Incoherent Sources(Spie-int Soc Optical Engineering, 2004) Baykal, YEffect of the information rate on the scintillation index is examined for free space optical (FSO) broadband access applications that use spatially incoherent sources. For this purpose, intensity fluctuations are formulated indicating the effect of the rate on the scintillation index in the presence of the atmospheric turbulence. The bandwidth of modulation of the incoherent source is taken to be much smaller than the carrier frequency, i.e., narrowband approximation is employed. Rate averaging factor for spatially incoherent source is derived as to represent the averaging in weak atmospheric turbulence due to rate of modulation of the intensity. It is found that the scintillations decrease as the rate of transmission through atmospheric turbulence increases. This decrease is independent of the carrier wavelength of the FSO system but depends on the outer scale of turbulence. Up to 10 Gbps, the decrease is negligible for realistic outer scale values. When extremely large eddies are present in the formation of turbulence, rate can be effective in the reduction of the scintillations even at rates up to 10 Gbps. In the limit when the information rate is taken as zero, our results correctly reduce to the known scintillations for spatially incoherent monochromatic excitation.Conference Object Citation - WoS: 7Citation - Scopus: 7Cosine-Gaussian Laser Beam Intensity in Turbulent Atmosphere(Spie-int Soc Optical Engineering, 2004) Eyyuboglu, HT; Baykal, YThe effects of turbulent atmosphere on cosine-Gaussian laser beams are examined. To this end, a cosine-Gaussian excitation is taken at the source plane, and subsequently the average intensity profile at the receiver plane is formulated. Our formulation correctly reduces to the known Gaussian beam wave result in turbulence and the cosine-Gaussian beam solution in free space (in the absence of turbulence). Variation of the average intensity profile of the receiver plane is evaluated and plotted against the variations of link length, turbulence levels, two frequently used free space optics (FSO) wavelenaths and beam displacement parameters. From these results, it is seen that cosine-Gaussian beam, following the natural diffraction, is eventually transformed into a hyperbolic-cosine Gaussian beam. Hence, the beam energy becomes concentrated around two main lobes at the receiver plane. Combining our earlier result with the findings of this paper, we conclude that cosine-Gaussian and hyperbolic-cosine-Gaussian beam act in a reciprocal manner after having C, propagated. This rneans, starting with a cosine-Gaussian beam excitation, we obtain hyperbolic-cosine-Gaussian distribution at the receiver plane, whereas hyperbolic-cosine-Gaussian beam excitation will yield a cosine-Gaussian distribution. This reciprocity is applicable both in free space and in turbulence.