Elektrik Elektronik Mühendisliği Bölümü Yayın Koleksiyonu
Permanent URI for this collectionhttps://hdl.handle.net/20.500.12416/411
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Article Citation - WoS: 11Citation - Scopus: 9Multimode Beam Propagation Through Atmospheric Turbulence(Pergamon-elsevier Science Ltd, 2024) Baykal, Yahya; Ata, Yalcin; Gercekcioglu, Hamza; Gokce, Muhsin CanerThe investigation focuses on studying the propagation characteristics of multimode lasers in the turbulent amosphere. By employing the Huygens-Fresnel integral, we develop analytical formulations for various propagation parameters. These include the average intensity distribution, kurtosis parameter, beam spread, and the average transmittance of multimode beams in turbulent atmosphere. Our findings reveal that as the propagation distance or the structure constant of the atmosphere increases, i.e., turbulence becomes stronger, the kurtosis parameter and the beam spread increase. The multimode beam exhibits a Gaussian like intensity profile when the propagation distance is significantly increased or when the structure constant becomes sufficiently large. For the case of the Gaussian beam, the kurtosis parameter is found to be 3. The multimode beam's kurtosis parameter rises as the turbulence becomes stronger and eventually approaches 3. Raising the mode content leads to a rise in the average transmittance; however, it leads to a decline in the Kurtosis parameter and the beam spread.Article Citation - WoS: 5Citation - Scopus: 6Underwater Turbulence Effect on Optical Imaging(Iop Publishing Ltd, 2022) Gokce, Muhsin Caner; Baykal, Yahya; Ata, YalcinModulation transfer function (MTF) of oceanic turbulence plays an essential role in the design and quality of underwater image sensing systems capturing optical signals. MTF gives clues about the characteristics of turbulence which can help image reconstruction where the image resolution can be increased in this way. In the paper, under the conditions of weak turbulence and Gaussian beam propagation, we derive the modulation transfer function for short-exposure and long-exposure images based on the recently developed turbulence spectrum model: Oceanic turbulence optical power spectrum (OTOPS). With the aid of the OTOPS model, the effect of measurable turbulence parameters, namely average temperature, average salinity concentration, and temperature-salinity gradient ratios, as well as imaging system parameters, namely receiver aperture radius and wavelength of the laser source on the MTF are reported. Obtained results indicate that MTF rapidly decreases with increasing relative spatial frequency and turbulence strength. Turbulence becomes stronger with the increase in the average temperature, average salinity concentration, energy dissipation rate, temperature-salinity gradient ratio and with the decrease in the temperature dissipation rate, wavelength.Article Citation - WoS: 3Citation - Scopus: 3Performance Evaluation of Aeronautical Uplink/Downlink Free-Space Optical Communication System With Adaptive Optics Over Gamma-Gamma Turbulence Channel(Iop Publishing Ltd, 2022) Baykal, Yahya; Gokce, Muhsin Caner; Ata, YalcinIn this study, we analyze the effect of adaptive optics corrections on the performance of an aeronautical free-space optical (FSO) system with bidirectional slant path uplink and downlink communication channels. The aeronautical FSO communication (FSOC) system operates in a gamma-gamma atmospheric turbulence channel and employs adaptive optics corrections for the distorted wave front of the Gaussian beam wave. The modulation type of the aeronautical FSOC system is chosen to be M-ary phase-shift-keying-subcarrier intensity modulation and the type of the employed photodetector is positive-intrinsic-negative. In analysis, the effect of system parameters such as zenith angle, the height of transmitter/receiver on the ground, M-ary level, filter bandwidth, link distance, and the adaptive optics correction modes on bit-error-rate are demonstrated.Article Citation - WoS: 7Citation - Scopus: 7Intensity Fluctuations in Biological Tissues at Any Turbulence Strength(Iop Publishing Ltd, 2022) Baykal, Yahya; Ata, Yalcin; Gokce, Muhsin CanerThis study investigates the intensity fluctuations of the optical plane and spherical waves in biological tissue that experience any strength of turbulence. Biological tissue is a random and complex medium for optical wave propagation, having a power spectrum reflecting the turbulent characteristics that depend on the structural parameters. It is important to accurately determine the strength of turbulence and classify turbulence regimes for the correct modeling of the behavior of the optical wave propagation. To classify weak, moderate and strong turbulent regimes, closed-form expressions of modified Rytov variances are obtained. Based on the modified Rytov variance that involves the large-scale and small-scale variations, the intensity fluctuations specified by the metric of scintillation index, are calculated versus various parameters such as the propagation distance, refractive index, characteristic length of heterogeneity, small length-scale factor, wavelength, fractal dimension and strength of the refractive index fluctuations. Behavior of optical plane and spherical waves in different turbulent regimes and the comparison of intensity fluctuations in different specimens of human and animal tissues are shown.Article Citation - WoS: 7Citation - Scopus: 5Tissue Turbulence and Its Effects on Optical Waves: a Review(Elsevier, 2023) Ata, Yalcin; Baykal, Yahya; Gokce, Muhsin CanerTissue turbulence and the effects of tissue turbulence on the propagation of optical waves are reviewed. After the introduction of a survey on the reported research in this area, various topics are elaborated. These topics include the spectrum of tissue turbulence, propagation of light in the tissue, average intensity, beam spread that occur at the receiver plane in the tissue. Other entities examined are the signal to noise ratio (SNR), intensity correlation, beam wander, mutual coherence function and the spectral degree of coherence. Furthermore, spectral change, cross spectral density, spectral correlation function, scintillation, bit error rate (BER), coupling efficiency in tissue turbulence are investigated. The refractive index structure of tissues, imaging in the presence of turbulence in the tissue, scattering, absorption and polarization aspects in tissues are reported. Finally, optical coherence tomography applications in turbulent tissues are reviewed.Article Citation - WoS: 4Citation - Scopus: 4Coupling Efficiency of Multimode Beam To Fiber in Atmospheric Turbulence(Pergamon-elsevier Science Ltd, 2023) Baykal, Yahya; Ata, Yalcin; Gokce, Muhsin CanerAtmospheric turbulence causes wavefront distortions in the propagated laser beam. By the beam shaping of the transmitted laser, wavefront distortions can be mitigated effectively. In this paper, we consider a shaped laser modeled by multimode beams at the transmitter and investigate the coupling of the light wave to a single-mode fiber under the atmospheric turbulence effect. We derive the light power on the coupling lens and light power accepted by the fiber core using the extended Huygens-Fresnel princi-ple. Then, the fiber coupling efficiency (FCE) is scrutinized for different system parameters such as the number of modes of the transmit laser, link distance, structure constant of atmosphere, focal length and radius of the coupling lens, wavelength, source size, and the number of speckles (NOS).(c) 2023 Elsevier Ltd. All rights reserved.Article Citation - WoS: 15Citation - Scopus: 19Adaptive Optics Effect on Performance of Bpsk-Sim Oceanic Optical Wireless Communication Systems With Aperture Averaging in Weak Turbulence(Pergamon-elsevier Science Ltd, 2020) Baykal, Yahya; Ata, Yalcin; Gokce, Muhsin CanerTurbulence-induced wavefront deformations cause the irradiance of an optical signal to fluctuate resulting a in serious degradation in the bit-error-rate (BER) performance of optical wireless communication (OWC) system. Adaptive optics is an effective technique to compensate for the wavefront aberrations to reduce the fluctuations in the received intensity. In this paper, we investigate how the adaptive optics technique affects the BER performance of an oceanic OWC (OOWC) system employing binary phase shift keying-subcarrier intensity modulation (BPSK-SIM) and aperture averaging. To evaluate BER performance in weak oceanic turbulence, the required entities such as the received optical power captured by a circular aperture and the aperture averaged scintillation index measuring the fluctuations in the received irradiance are derived. The effect of adaptive optics correction of various wavefront aberrations (i.e., tilt, defocus, astigmatism and the coma) on the BER performance is illustrated and the performance of the adaptive optics-OOWC system is compared to that of a non-adaptive optics OOWC system by the metric defined. (C) 2020 Elsevier Ltd. All rights reserved.Article Citation - WoS: 12Citation - Scopus: 11Error Performance of Optical Wireless Communication Systems Exercising Bpsk Subcarrier Intensity Modulation in Non-Kolmogorov Turbulent Atmosphere(Elsevier Science Bv, 2019) Baykal, Yahya; Gokce, Muhsin Caner; Ata, YalcinSubcarrier intensity modulation (SIM) scheme is preferred due to efficient bandwidth usage superiority over other modulation techniques such as on-off keying (OOK), pulse position modulation (PPM). In this paper, we investigate the bit error rate (BER) performance of optical wireless communication (OWC) system using binary phase shift keying (BPSK) SIM in non-Kolmogorov turbulent atmosphere. We pay attention to the weak turbulence conditions by using Rytov approximation and considering that the receiver is a PIN photodetector. Propagating beam type is Gaussian. It is seen that BER performance of the BPSK SIM OWC is significantly affected from non-Kolmogorov power law exponent, load resistor, responsivity of the PIN photodetector, bandwidth, beam source size, turbulence strength and noise factor.Article Citation - WoS: 24Citation - Scopus: 26Average Channel Capacity in Anisotropic Atmospheric Non-Kolmogorov Turbulent Medium(Elsevier, 2019) Baykal, Yahya; Gokce, Muhsin Caner; Ata, YalcinThe average channel capacity of a free space optical (FSO) communication system running an intensity modulated Gaussian beam is examined in anisotropic non-Kolmogorov atmospheric weak turbulence based on Rytov variance. Results are obtained by employing the log-normal distribution of irradiance fluctuations corresponding to weak turbulence regime. Our results show that average channel capacity increases together with the increase in anisotropy factor in x and y direction, non-Kolmogorov power law exponent, quantum efficiency of photo detector, Gaussian beam source size and the inner scale length. However, the average channel capacity is found to decrease when turbulence strength, link length and noise variance increase.Article Citation - WoS: 32Citation - Scopus: 40Performance Analysis of M-Ary Pulse Position Modulation in Strong Oceanic Turbulence(Elsevier Science Bv, 2018) Baykal, Yahya; Ata, Yalcin; Gokce, Muhsin CanerIn this paper, we consider an underwater wireless optical communication (UWOC) system which consists of an M-ary pulse position modulated (PPM) Gaussian optical beam at the transmitter and an avalanche photodiode (APD) at the receiver. In strong oceanic turbulence, we aimed at investigating the system performance in terms of bit error rate (BER) by the help of gamma-gamma channel model. For this purpose, the average power and the aperture averaged scintillation at the finite sized detector are derived by using the extended Huygens-Fresnel principle and the asymptotic Rytov theory, respectively. BER variations are examined versus the average APD gain, modulation order, bit rate as well as the oceanic turbulence parameters, i.e., the rate of dissipation of kinetic energy per unit mass of fluid, the rate of dissipation of mean-squared temperature and the ratio of temperature to salinity contributions to the refractive index spectrum.