Elektronik ve Haberleşme Mühendisliği Bölümü Yayın Koleksiyonu

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

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Citation - WoS: 34
Citation - Scopus: 35
Average Transmittance in Turbulence for Partially Coherent Sources
(Elsevier, 2004) Baykal, Y
Average intensity and power-transmittance in turbulence are formulated for a source with arbitrary degree of coherence (both spatial and temporal). Average power-transmittance is shown to reduce to the average intensity-transmittance if the receiver dimension is much less than the beam size. The average transmittance is found to attain its minimum value when the source is on the order of the Fresnel zone, irrespective of the degree of coherence of the source. We obtain the correct average intensity when the source is coherent, partially coherent and incoherent. Average transmittances due to turbulence are found for practical FSO (Free Space Optics) communication links. (C) 2003 Elsevier B.V. All rights reserved.
Citation - WoS: 103
Citation - Scopus: 104
Formulation of Correlations for General-Type Beams in Atmospheric Turbulence
(Optical Soc Amer, 2006) Baykal, Y
Log-amplitude and phase correlations of general-type beams are formulated in atmospheric turbulence. A general beam is described as the superposition of many sets of multimode contents, each mode being off-axis Hermite-Gaussian. Since the Rytov solution is utilized, the formulas are valid in the weakly turbulent regime. The results are presented in integral forms that should be numerically evaluated for the specific beam type of interest. Our general beam results correctly reduce to the existing solutions for the correlations of limiting-case beams such as higher-order single-mode, multimode, off-axis Hermite-Gaussian, Hermite-sinusoidal-Gaussian, higher-order-annular, flat-topped-Gaussian, and thus naturally fundamental mode, plane, and spherical waves. Scintillation index and phase fluctuations in atmospheric optical links employing such special beams will be examined in future using the results reported here. (c) 2006 Optical Society of America.
Citation - WoS: 48
Citation - Scopus: 55
Log-Amplitude and Phase Fluctuations of Higher-Order Annular Laser Beams in a Turbulent Medium
(Optical Soc Amer, 2005) Baykal, Y
Log-amplitude and phase-correlation and structure functions of higher-order annular laser beams in a turbulent atmosphere are derived. A higher-order annular beam source is defined as the superposition of two different higher-order Hermite-Gaussian beams. A special case of such an excitation is the annular Gaussian beam in which two beams operate at fundamental modes of different Gaussian beam sizes, yielding a doughnut-shaped (annular) beam when the second beam is subtracted from the first beam. Our formulation utilizes Rytov approximation, which makes it applicable in the weak-turbulence regime, especially for log-amplitude fluctuations. Limiting cases of our formulations correctly match with known higher-order-mode solutions that in turn reduce to the Gaussian-beam-wave (TEM00-mode) results. Our results can be applied to determine the scintillation index and the phase fluctuations in free-space optics links under higher-order annular laser beam excitation. Except for the numerical evaluation of a specific example covering an annular Gaussian beam, the results in general are left in integral form and need to be numerically evaluated in detail to obtain quantitative results. (c) 2005 Optical Society of America.
Rate Averaging in Free Space Optics Systems Using Incoherent Sources
(Spie-int Soc Optical Engineering, 2004) Baykal, Y
Effect 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.

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