Browsing by Author "Baykal, Yahya"

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Citation Count: Cai, Y., Korotkova, O., Eyyuboğlu,H.T., Baykal, Y. (2008). Active laser radar systems with stochastic electromagnetic beams in turbulent atmosphere. Optics Express, 16(20), 15834-15846. http://dx.doi.org/10.1364/OE.16.015834
Active laser radar systems with stochastic electromagnetic beams in turbulent atmosphere
(Optical Soc Amer, 2008) Cai, Yangjian; Korotkova, Olga; Eyyuboğlu, Halil T.; Baykal, Yahya; 7688; 7812
Propagation of stochastic electromagnetic beams through paraxial ABCD optical systems operating through turbulent atmosphere is investigated with the help of the ABCD matrices and the generalized Huygens-Fresnel integral. In particular, the analytic formula is derived for the cross-spectral density matrix of an electromagnetic Gaussian Schell-model (EGSM) beam. We applied our analysis for the ABCD system with a single lens located on the propagation path, representing, in a particular case, the unfolded double-pass propagation scenario of active laser radar. Through a number of numerical examples we investigated the effect of local turbulence strength and lens' parameters on spectral, coherence and polarization properties of the EGSM beam
Citation Count: Baykal, Y. (2022). "Adaptive optics correction of beam spread in biological tissues", Journal of Quantitative Spectroscopy and Radiative Transfer, Vol.283.
Adaptive optics correction of beam spread in biological tissues
(2022) Baykal, Yahya
Beam spread in turbulent biological tissues is examined when the tissue is excited with a collimated Gaussian laser beam. Adaptive optics correction is applied to the beam spread in the form of piston only (P Only), tilt only (T Only), piston+tilt (P+T), and the reduction in the beam spread is evaluated as compared to the no adaptive optics (No AO) corrected beam spread. No AO and adaptive optics corrected beam spread are expressed for various biological tissue types, against the variations in the strength coefficient of the refractive-index fluctuations, source size, small length-scale factor of turbulence, tissue length, fractal dimension, characteristic lengths of heterogeneity and the wavelength. For the examined tissue types of liver parenchyma (mouse), intestinal epithelium (mouse), upper dermis (human) and deep dermis (mouse), No AO beam spread and the adaptive optics corrected beam spread are found to increase as the strength coefficient of the refractive-index fluctuations, tissue length, fractal dimension, the characteristic lengths of heterogeneity increase, and to decrease as the source size, small length-scale factor, wavelength increase. Reduction ratio of P+T correction is almost the same for all the evaluated cases which is 74%.
Citation Count: Baykal, Yahya, "Adaptive Optics Correction of Scintillation in Underwater Medium", Journal of Modern Optics, Vol. 67, No. 3, pp. 220-225, (2020).
Adaptive Optics Correction of Scintillation in Underwater Medium
(Taylor&Francis LTD, 2020) Baykal, Yahya; 7812
Adaptive optics correction of the scintillation index of a Gaussian laser beam in underwater turbulence is studied. To introduce the adaptive optics correction, filter functions providing the piston, tilt and astigmatism effects are adapted to promote the spectrum of underwater turbulence. The reduction of the scintillation index due to the individual piston, tilt, astigmatism effects and their sum is examined versus the ratio of temperature to salinity contributions to the refractive index spectrum, the rate of dissipation of mean squared temperature, the rate of dissipation of kinetic energy per unit mass of fluid, receiving aperture diameter, source size, link length and the wavelength. For any value of underwater turbulence parameter, the most effective adaptive optics corrections are found to be the piston, tilt and astigmatism, respectively.
Citation Count: Baykal, Yahya (2020). "Adaptive optics corrections of scintillations of Hermite-Gaussian modes in an oceanic medium", Applied Optics, Vol. 59, No. 16, pp. 4826-4832.
Adaptive optics corrections of scintillations of Hermite-Gaussian modes in an oceanic medium
(2020) Baykal, Yahya; 7812
Adaptive optics correction of the scintillation index is found when Hermite-Gaussian laser beams are used in oceanic turbulence. Adaptive optics filter functions are used to find how the tilt, focus, astigmatism, coma, and total correction will behave under high order mode excitation. Reduction of the oceanic scintillation under various oceanic turbulence and system parameters is examined under different high order modes. Also, the effects of the source size, wavelength, and link length on the total adaptive optics correction of Hermite-Gaussian modes in an oceanic medium are investigated for different modes. (C) 2020 Optical Society of America
Adaptive optics effect on performance of BPSK-SIM oceanic optical wireless communication systems with aperture averaging in weak turbulence
(2020) Gökçe, Muhsin Caner; Baykal, Yahya; Ata, Yalçın; 7812
Turbulence-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.
Citation Count: Baykal, Yahya; Uysal, Murat, "Analysis of Field Correlations in Atmospheric Optical MIMO Systems", 3rd International Workshop in Optical Wireless Communications (IWOW), pp. 30-34, (2014).
Analysis of Field Correlations in Atmospheric Optical MIMO Systems
(IEEE, 2014) Baykal, Yahya; Uysal, Murat
In this paper, we evaluate the absolute field correlations for a free space optical (FSO) multiple-input multiple-output (MIMO) link operating in a turbulent atmosphere. Such knowledge is needed in heterodyne reception to keep all the optical detectors within the region in which a predefined correlation value is fulfilled. The variations of the absolute field correlations versus the longest distance between the detectors are found for different array, medium and receiver parameters such as the number of beamlets, the radius of the radial array, beamlet size, link length, wavelength, structure constant and the position of the detectors.
Citation Count: Eyyuboğlu, Halil T.; Baykal, Yahya, "Analysis of laser multimode content on the angle-of-arrival fluctuations in free-space optics access systems", Optical Engineering, Vol.44, No.5, (2005).
Analysis of laser multimode content on the angle-of-arrival fluctuations in free-space optics access systems
(Spie-Soc Photo-Optical Instrumentation Engineers, 2005) Eyyuboğlu, Halil T.; Baykal, Yahya; 7688; 7812
The effects of laser beam multimode content on the angle-of-arrival fluctuations are examined for free-space optics (FSO) access systems. Multimode excitation is represented by coherent addition of Hermite-Gaussian modes. Mean square angle-of-arrival fluctuations are formulated using our previously reported multimode phase structure function. Numerical evaluations are obtained for practical FSO links operating at 1.55- and 0.85-mu m wavelengths with link spans of up to 5 km. Mode content is arranged by sequentially grouping all possible mode combinations starting from the single fundamental mode (TEM00) up to a certain higher order (n,m). Angle-of-arrival fluctuations are found to be of the order of several tens of microradians, except for the cases when the mode group terminates with an odd mode or when the extremely higher order modes are present. In these instances, the fluctuations will rise to the radian level. From these results, it is concluded that the performance of a practical FSO receiver, having a field of view of several milliradians, will not be substantially affected by the angle-of-arrival fluctuations due to multimode excitation, provided that the mode content of source excitation is confined to mode indices below 20.
Citation Count: Eyyuboğlu, Halil T.; Baykal, Y., "Analysis of reciprocity of cos-Gaussian and cosh-Gaussian laser beams in a turbulent atmosphere", Optics, Express, Vol.12, No.20, pp.4659-4674, (2004).
Analysis of reciprocity of cos-Gaussian and cosh-Gaussian laser beams in a turbulent atmosphere
(Optical Soc Amer, 2004) Eyyuboğlu, Halil T.; Baykal, Yahya; 7688; 7812
In a turbulent atmosphere, starting with a cos-Gaussian excitation at the source plane, the average intensity profile at the receiver plane is formulated. This average intensity profile is evaluated against the variations of link lengths, turbulence levels, two frequently used free-space optics wavelengths, and beam displacement parameters. We show that a cos-Gaussian beam, following a natural diffraction, is eventually transformed into a cosh-Gaussian beam. Combining our earlier results with the current findings, we conclude that cos-Gaussian and cosh-Gaussian beams act in a reciprocal manner after propagation in turbulence. The rates (paces) of conversion in the two directions are not the same. Although the conversion of cos-Gaussian beams to cosh-Gaussian beams can happen over a wide range of turbulence levels (low to moderate to high), the conversion of cosh-Gaussian beams to cos-Gaussian beams is pronounced under relatively stronger turbulence conditions. Source and propagation parameters that affect this reciprocity have been analyzed.
Citation Count: Ata, Yalçın; Baykal, Yahya; Gökçe, Muhsin C. (2022). "Analysis of wander and spreading of an optical beam by using the oceanic turbulence optical power spectrum", Journal of the Optical Society of America B: Optical Physics, Vol.39, No.8, pp.2129-2137.
Analysis of wander and spreading of an optical beam by using the oceanic turbulence optical power spectrum
(2022) Ata, Yalçın; Baykal, Yahya; Gökçe, Muhsin Caner
Variance of beam displacement and short-term and long-term spreading of a Gaussian beam propagating in the presence of underwater turbulence are examined by using the oceanic turbulence optical power spectrum (OTOPS). Analytical expressions for both beam wander displacement variance and beam spreading are presented. Results show that the underwater turbulent channel causes deflection from the on-axis mean irradiance and brings significant wander and spreading effects to the propagating Gaussian beam wave. The variations of beam wander and short- and long-term spreading are obtained depending on the underwater medium parameters such as the average temperature, average salinity concentration, temperature-salinity gradient ratio, and temperature and energy dissipation rates. In particular, the real values of the average temperature and salinity concentration of turbulent water are used to obtain the results. In addition, the effects of propagation distance, Gaussian beam source size, and wavelength are shown. The results demonstrate that the underwater turbulent channel brings displacements in the centroid and spreading of the optical beam.
Citation Count: Eyyuboğlu, Halil Tanyer; Baykal, Yahya, "Angle-of-arrival fluctuations for general-type beams", Optical Engineering, Vol.46, No.9, (2007).
Angle-of-arrival fluctuations for general-type beams
(Spie-Soc Photo-Optical Instrumentation Engineers, 2007) Eyyuboğlu, Halil T.; Baykal, Yahya; 7688; 7812
Starting with the recently introduced phase correlation function of a general-type beam, the angle-of-arrival fluctuations are derived and evaluated in atmospheric turbulence for lowest-order Gaussian, cos-Gaussian, cosh-Gaussian, annular, and flat-topped beams. Our motivation is to examine the improvement in the performance of optical atmospheric links when such beams are employed. For these beams, the dependence of the angle-of-arrival fluctuations on the propagation length, displacement and focusing parameters, source size, inner and outer scales of turbulence, and receiver radius is investigated. It is found that in the majority of the cases examined, the angle- of- arrival fluctuations remain small and hence are not expected to adversely affect the operation of free-space atmospheric links. It is observed that amongst the beams considered, the cos-Gaussian beam offers the least amount of angle-of-arrival fluctuations, while the worst behavior is exhibited by the cosh-Gaussian beam. This situation is reversed, however, if focused beams are used.
Citation Count: Ata, Yalçın; Baykal, Yahya (2020). "Anisotropic non-Kolmogorov turbulence effect on transmittance of multi-Gaussian beam", Waves in Random and Complex Media, Vol. 30, No. 1, pp. 118-129.
Anisotropic non-Kolmogorov turbulence effect on transmittance of multi-Gaussian beam
(2020) Ata, Yalçın; Baykal, Yahya; 7812
The effect of anisotropic non-Kolmogorov turbulent atmosphere on multi-Gaussian beam is investigated and the results are presented against different beam and medium parameters. Results show that anisotropy increases the power efficiency of wireless communication systems. Besides anisotropy, turbulence effects on the multi-Gaussian optical beam are represented versus the beam source size, beam flatness order, propagation distance, inner scale length, turbulence strength, non-Kolmogorov turbulence power law exponent, wavelength, inner and outer source sizes for flat-topped and annular beams.
Citation Count: Ata, Y., Baykal, y. (2018). Anisotropy effect on multi-Gaussian beam propagation in turbulent ocean. Chinese Laser Press, 16(8). http://dx.doi.org/10.3788/COL201816.080102
Anisotropy effect on multi-Gaussian beam propagation in turbulent ocean
(Chinese Laser Press, 2018) Ata, Yalçın; Baykal, Yahya; 7812
Average transmittance of multi-Gaussian (flat-topped and annular) optical beams in an anisotropic turbulent ocean is examined analytically based on the extended Huygens-Fresnel principle. Transmittance variations depending on the link length, anisotropy factor, salinity and temperature contribution factor, source size, beam flatness order of flat-topped beam, Kolmogorov microscale length, rate of dissipation of turbulent kinetic energy, rate of dissipation of the mean squared temperature, and thickness of annular beam are examined. Results show that all these parameters have effects in various forms on the average transmittance in an anisotropic turbulent ocean. Hence, the performance of optical wireless communication systems can be improved by taking into account the variation of average transmittance versus the above parameters.
Citation Count: Baykal, Yahya, "Anisotropy effect on performance of PPM optical wireless oceanic communication links", Journal of Quantitative Spectroscopy & Radiative Transfer, Vol. 228, pp. 73-78, (2019).
Anisotropy effect on performance of PPM optical wireless oceanic communication links
(Pergamon-Elsevier Science LTD, 2019) Baykal, Yahya; 7812
The performance, quantified by the bit-error-rate (BER), of M-ary pulse position modulated (PPM) optical wireless oceanic communication (OWOC) link is investigated when such a link operates in anisotropic weak oceanic turbulence. For this purpose, formulations of the average received power and the scintillation index of collimated Gaussian optical beam detected by a point detector are developed for anisotropic weak oceanic turbulence, which in turn are employed in the BER expression of the PPM OWOC links. BER is evaluated under various turbulence parameters of anisotropic oceanic turbulence, M of M-ary PPM, data bit rate, average current gain of avalanche photodiode (APD). For any investigated parameter, it is found that the BER performance of M-ary PPM OWOC links is improved as the ocean becomes more anisotropic. (C) 2019 Elsevier Ltd. All rights reserved.
Citation Count: Baykal, Yahya; Gokce, Muhsin C.; Ata, Yalcin, "Anisotropy effect on performance of subcarrier intensity modulated binary phase shift keying optical wireless communication links in weakly turbulent underwater channel", Journal of Modern Optics, (2019).
Anisotropy effect on performance of subcarrier intensity modulated binary phase shift keying optical wireless communication links in weakly turbulent underwater channel
(Taylor&Francis LTD, 2019) Baykal, Yahya; Gökçe, Muhsin Caner; Ata, Yalçın; 7812
The effect of the anisotropy on the bit-error-rate (BER) performance of subcarrier intensity modulated (SIM) binary phase shift keying (BPSK) optical wireless communication (OWC) links operating in weakly turbulent underwater channels is examined. BER variations versus the anisotropic factor are examined when the bandwidth, photodetector responsivity, load resistor and the underwater turbulence parameters are varied. As anisotropy in the underwater channel becomes larger, SIM BPSK OWC links have better BER performance at any link and turbulence parameter.
Citation Count: Gerçekçioğlu, H., Baykal, Y. (2012). Annular beam scintillations in non-Kolmogorov weak turbulence. Applied Physisc B-Lasers And Optics, 106(4), 933-937. http://dx.doi.org/10.1007/s00340-012-4921-0
Annular beam scintillations in non-Kolmogorov weak turbulence
(Springer, 2012) Gerçekçioğlu, Hamza; Baykal, Yahya; 7812
In a weakly turbulent atmosphere governed by the non-Kolmogorov spectrum, the on-axis scintillation index is formulated and evaluated when the incidence is an annular Gaussian type. When the power law of the non-Kolmogorov spectrum is varied, the scintillation index first increases, and reaches a peak value, then starts to decrease, and eventually approaches zero. The general trend is that when turbulence has a non-Kolmogorov spectrum with power law larger than the Kolmogorov power law, the scintillation index values become smaller. For all power laws, collimated annular Gaussian beams exhibit smaller scintillations when compared to pure Gaussian beams of the same size. Intensity fluctuations at a fixed propagation distance diminish for the non-Kolmogorov spectrum with a very large power law, irrespective of the focal length and the thickness of optical annular Gaussian sources
Citation Count: Gerçekçioğlu, H., Baykal, Y., Nakiboğlu, C. (2010). Annular beam scintillations in strong turbulence. Journal of the Optical Society of America A-Optics Image Science and Vision, 27(8), 1834-1839.
Annular beam scintillations in strong turbulence
(Optical Soc Amer, 2010) Gerçekçioğlu, Hamza; Baykal, Yahya; Nakiboğlu, Cem; 7812
A scintillation index formulation for annular beams in strong turbulence is developed that is also valid in moderate and weak turbulence. In our derivation, a modified Rytov solution is employed to obtain the small-scale and large-scale scintillation indices of annular beams by utilizing the amplitude spatial filtering of the atmospheric spectrum. Our solution yields only the on-axis scintillation index for the annular beam and correctly reduces to the existing strong turbulence results for the Gaussian beam-thus plane and spherical wave scintillation indices-and also correctly yields the existing weak turbulence annular beam scintillations. Compared to collimated Gaussian beam, plane, and spherical wave scintillations, collimated annular beams seem to be advantageous in the weak regime but lose this advantage in strongly turbulent atmosphere. It is observed that the contribution of annular beam scintillations comes mainly from the small-scale effects. At a fixed primary beam size, the scintillations of thinner collimated annular beams compared to thicker collimated annular beams are smaller in moderate turbulence but larger in strong turbulence; however, thinner annular beams of finite focal length have a smaller scintillation index than the thicker annular beams in strong turbulence. Decrease in the focal length decreases the annular beam scintillations in strong turbulence. Examining constant area annular beams, smaller primary sized annular structures have larger scintillations in moderate but smaller scintillations in strong turbulence. (C) 2010 Optical Society of America
Citation Count: Gökçe, M.C., Baykal, Y.K. (2018). Aperture averaging and BER for Gaussian beam in underwater oceanic turbulence. Optic Communications, 410, 830-835. http://dx.doi.org/10.1016/j.optcom.2017.11.049
Aperture averaging and BER for Gaussian beam in underwater oceanic turbulence
(Elsevier Science BV, 2018) Gökçe, Muhsin Caner; Baykal, Yahya; 28643; 7812
In an underwater wireless optical communication (UWOC) link, power fluctuations over finite-sized collecting lens are investigated for a horizontally propagating Gaussian beam wave. The power scintillation index, also known as the irradiance flux variance, for the received irradiance is evaluated in weak oceanic turbulence by using the Rytov method. This lets us further quantify the associated performance indicators, namely, the aperture averaging factor and the average bit-error rate (). The effects on the UWOC link performance of 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, Kolmogorov microscale, the ratio of temperature to salinity contributions to the refractive index spectrum as well as system parameters, i.e., the receiver aperture diameter, Gaussian source size, laser wavelength and the link distance are investigated.
Citation Count: Gökçe, M.C., Baykal, Y., Uysal, M. (2016). Aperture averaging in multiple-input single-output free-space optical systems. Journal Of The Optical Society Of America A-Optics İmage Science And Vision, 33(6), 1041-1048. http://dx.doi.org/10.1364/JOSAA.33.001041
Aperture averaging in multiple-input single-output free-space optical systems
(Optical Soc Amer, 2016) Gökçe, Muhsin Caner; Baykal, Yahya; Uysal, Murat; 28643; 7812; 124615
Multiple-input single-output (MISO) techniques are employed in free-space optical (FSO) links to mitigate the degrading effects of atmospheric turbulence. In this paper, for the MISO FSO system, a partially coherent radial array and a finite-sized receiver aperture are used at the transmitter and the receiver, respectively. Using the extended Huygens - Fresnel principle, we formulate the average power and the power correlation at the finite-sized slow detector in weak atmospheric turbulence. System performance indicators such as the power scintillation index and the aperture averaging factor are determined. Effects of the source size, ring radius, receiver aperture radius, link distance, and structure constant and the degree of source coherence are analyzed on the performance of the MISO FSO system. In the limiting cases, the numerical results are found to be the same when compared to the existing coherent and partially coherent Gaussian beam scintillation indices.
Citation Count: Gökçe, M.C...et al. (2015). Aperture averaging in multiple-input single-output free-space optical systems. Optical Engineering, 54(6). http://dx.doi.org/10.1117/1.OE.54.6.066103
Aperture averaging in multiple-input single-output free-space optical systems
(Spie-Soc Photo-Optical Instrumentation Engineers, 2015) Gökçe, Muhsin Caner; Baykal, Yahya; Kamacıoğlu, Canan; Uysal, Murat; 28643; 7812; 124615
Multiple-input single-output systems are employed in free-space optical links to mitigate the degrading effects of atmospheric turbulence. We formulate the power scintillation as a function of transmitter and receiver coordinates in the presence of weak atmospheric turbulence by using the extended Huygens Fresnel principle. Then the effect of the receiver aperture averaging is quantified. To get consistent results, parameters are chosen within the range of validity of the wave structure functions. Radial array beams and a Gaussian weighting aperture function are used at the transmitter and the receiver, respectively. It is observed that the power scintillation decreases when the source size, the ring radius, the receiver aperture radius, and the number of array beamlet increase. However, increasing the number of array beamlets to more than three seems to have negligible effect on the power scintillation. It is further observed that the aperture averaging effect is stronger when radial array beams are employed instead of a single Gaussian beam.
Citation Count: Gökçe, M.C., Baykal, y. (2018). Aperture averaging in strong oceanic turbulence. Optics Communications, 413, 196-199. http://dx.doi.org/ 10.1016/j.optcom.2017.12.059
Aperture averaging in strong oceanic turbulence
(Elsevier Science BV, 2018) Gökçe, Muhsin Caner; Baykal, Yahya; 28643; 7812
Receiver aperture averaging technique is employed in underwater wireless optical communication (UWOC) systems to mitigate the effects of oceanic turbulence, thus to improve the system performance. The irradiance flux variance is a measure of the intensity fluctuations on a lens of the receiver aperture. Using the modified Rytov theory which uses the small-scale and large-scale spatial filters, and our previously presented expression that shows the atmospheric structure constant in terms of oceanic turbulence parameters, we evaluate the irradiance flux variance and the aperture averaging factor of a spherical wave in strong oceanic turbulence. Irradiance flux variance variations are examined versus the oceanic turbulence parameters and the receiver aperture diameter are examined in strong oceanic turbulence. Also, the effect of the receiver aperture diameter on the aperture averaging factor is presented in strong oceanic turbulence.