Yalçın Ata

Scintillations of optical plane and spherical waves in underwater turbulence

The scintillation indices of optical plane and spherical waves propagating in underwater turbulent media are evaluated by using the Rytov method, and the variations in the scintillation indices are investigated when the rate of dissipation of mean squared temperature, the temperature and salinity fluctuations, the propagation distance, the wavelength, the Kolmogorov microscale length, and the rate of dissipation of the turbulent kinetic energy are varied. Results show that as in the atmosphere, also in underwater media the plane wave is more affected by turbulence as compared to the spherical wave. The underwater turbulence effect becomes significant at 5-10 m for a plane wave and at 20-25 m for a spherical wave. The turbulence effect is relatively small in deep water and is large at the surface of the water. Salinity-induced turbulence strongly dominates the scintillations compared to temperature-induced turbulence.

Structure functions for optical wave propagation in underwater medium

The features of the wave structure function (WSF) derived for spherical excitation in turbulent water are investigated. It is found that as the rate of the dissipation of turbulent kinetic energy ɛ decreases, WSF increases. The rate of dissipation of the mean-squared temperature XT is observed to be proportional to the WSF value. Deviation from the source and the receiver axis reveals greater turbulence effect. Salinity driven turbulence gives greater WSF values compared to the temperature driven turbulence. As expected, WSF is found to increase as the propagation distance increases.

Turbulence effect on transmittance of atmospheric optics telecommunication system using dense wavelength division multiplexing

The effect of atmospheric turbulence on atmospheric optics telecommunication links employing dense wavelength division multiplexing (DWDM) systems is examined. For this purpose, transmittance obtained by using the MODTRAN (MODerate resolution atmospheric TRANsmission) code is modified by the transmittance due to turbulence when the incidence is a partially coherent optical beam. The spanned wavelengths cover the range employed in a practical DWDM system operating in the range of 1550 nm. The effect of turbulence is manifest as a decrease in the transmittance calculated by MODTRAN, being more effective when the incidence becomes less coherent.

Flat-topped beam transmittance in anisotropic non-Kolmogorov turbulent marine atmosphere

Abstract. Turbulence affects optical propagation, and, as a result, the intensity is attenuated along the path of propagation. The attenuation becomes significant when the turbulence becomes stronger. Transmittance is a measure indicating how much power is collected at the receiver after the optical wave propagates in the turbulent medium. The on-axis transmittance is formulated when a flat-topped optical beam propagates in a marine atmosphere experiencing anisotropic non-Kolmogorov turbulence. Variations in the transmittance are evaluated versus the beam source size, beam number, link distance, power law exponent, anisotropy factor, and structure constant. It is found that larger beam source sizes and beam numbers yield higher transmittance values; however, as the link distance, power law exponent, anisotropy factor, or structure constant increase, transmittance values are lowered. Our results will help in the performance evaluations of optical wireless communication and optical imaging systems operating in a marine atmosphere.

Transmittance of Multi Gaussian Optical Beams for Uplink Applications in Atmospheric Turbulence

On-axis slant path uplink transmittance (used in short as transmittance throughout the text) for multi Gaussian optical beam in Kolmogorov atmospheric turbulent medium is investigated. It is observed that for both the flat-topped and the annular beams, as the propagation distance, wind speed and the zenith angle increase, the transmittance decreases. The transmittance of flat-topped beams increases when the number of beams, source size or the wavelength increases. For the annular beam, when the outer/inner beam size ratio is kept constant, larger source sizes yield larger transmittance values. Transmittance of the thicker annular beams is found to be larger than the transmittance of the thinner annular beams.

Average transmittance of flat-topped beam in non-Kolmogorov medium

The average transmittance of flat-topped beam in non-Kolmogorov turbulence is investigated at the receiver origin. The formulation is based on the extended Huygens–Fresnel principle and the evaluations are made by numerical integration. It is found that as the power-law exponent, structure constant, propagation distance, source size and the degree of flatness of the flat-topped beam increase, the on-axis transmittance decreases. Transmittance decrease is substantial for high power-law exponent values and in strong turbulent regime. Increase in the wavelength of the flat-topped beam causes the average transmittance to increase.

Field correlation for flat-topped beam in non-Kolmogorov turbulent medium

Absolute field correlation of flat-topped beam in non-Kolmogorov turbulent medium is examined at the receiver plane. The power law exponent increase affects absolute field correlation inversely. It is found that the absolute field correlation decreases when the propagation distance, deviation from the receiver axis, diagonal transverse distance from the receiver point and turbulence strength increase. Beam flatness order increase yields smaller absolute field correlation. For the employed parameters, the flat-topped beam attains higher absolute field correlation when the wavelength and the source size increase.

Transmittance for Dense Wavelength Division Multiplexing system in non-Kolmogorov turbulence

Transmittance for a Dense Wavelength Division Multiplexing (DWDM) system operating in non-Kolmogorov medium is evaluated. MODTRAN (MODerate resolution atmospheric TRANsmission) is also used to include the absorption and scattering effects due to atmospheric gases, aerosols and molecules. Increase in the power law of the non-Kolmogorov spectrum is found to decrease the transmittance at all the wavelengths employed in DWDM. Being valid for all the DWDM wavelengths and for all the power law exponent values of non-Kolmogorov spectra, as the turbulence falls into stronger regimes, the transmittance levels decrease.

Field correlation of flat-topped beams in anisotropic non-Kolmogorov turbulent atmosphere

ABSTRACT Field correlation of flat-topped beams in anisotropic non-Kolmogorov turbulent atmosphere is formulated and evaluated. Larger anisotropic factor causes higher field correlations. Smaller field correlations are seen when the transverse distance at the receiver plane increases. Smaller field correlations are observed at large off-axis transverse receiver points, which are valid for any anisotropic factor and for any power-law exponent of non-Kolmogorov turbulence. When the flat-topped beam is composed of large number of Gaussian beams, the field correlation becomes smaller. In anisotropic non-Kolmogorov turbulence, longer propagation distances, larger structure constants, smaller inner scales and smaller source sizes decrease the field correlation. Larger power law exponent of non-Kolmogorov turbulence increases the field correlations at any anisotropic factor.