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Today, aviation has grown significantly in importance. However, the challenge of flight delays has become increasingly severe due to the need for safe separation between aircraft to mitigate wake turbulence effects. The primary emphasis of this investigation resides in elucidating the evolutionary attributes of wake vortices in homogeneous isotropy turbulence. The large eddy simulation (LES) method is used to scrutinize the dynamic evolution of wake vortices engendered by an A333 aircraft in the atmospheric milieu and assess its ramifications on the ARJ21 aircraft. The research endeavor commences by formulating an LES methodology for the evolution of aircraft wake vortices, integrating adaptive grid technology to reduce the necessary grid volume significantly. This approach enables the implementation of axial and non-axial grid adaptive refinement, leading to more accurate simulations of both axial and non-axial vortices. Numerical simulations are conducted using the LES approach to scrutinize three distinct rates of turbulence dissipation amidst the ambient atmospheric turbulence, and the results are juxtaposed with Lidar measurements (Wind3D 6000 LiDAR) of wake vortices acquired at Chengdu Shuangliu International Airport (CTU). Subsequently, the rolling moment of the following aircraft is calculated, and three-dimensional hazard zones are determined for the A333. It is found that during the approach phase, the wake turbulence separation minima for an ARJ21 (CAT-F) following an A333 (CAT-B) is 3.35 NM, which represents a reduction of approximately 33% compared to ICAO RECAT (Wake Turbulence Re-categorization). The findings validate the dependability of the fine-grained mesh used in the vortex core region, engendered through the adaptive grid method, which proficiently captures the Crow instability and the interconnected phenomena of vortices in the numerical examination of aircraft wake. The safety of wake encounters primarily depends on the magnitude of environmental turbulence and the development of structural instability in wake vortices. [ABSTRACT FROM AUTHOR]

The deployment of free-space optical (FSO) communication links is simple, fast and economical than optical fiber networks. However, it presents weakness in front of low visibility and its performance is deteriorated by atmospheric turbulence due to different weather conditions. In this paper, we have investigated an effective FSO system by integrating space division multiplexing (SDM) that can be used to enhance the information-carrying capabilities of FSO links. In addition, we have used a laser beam operating at 2 μm wavelength as an optical link in the proposed SDM-FSO system, which relates the telecom performance and the FSO link state, in terms of turbulence. In this system, information channels are transmitted concurrently over distinct Laguerre–Gaussian modes (LG) of the laser beam, where each mode is used to carrying independent 10 Gbps data stream. The availability of the LG-SDM-FSO system in dependence on weather conditions and on FSO link parameters, such as transmitted optical power, geometric loss, beam divergence, receiver aperture diameter, and link range, is discussed. The performance of our system is evaluated based on eye diagram, Q Factor and BER measurements. [ABSTRACT FROM AUTHOR]

Atmospheric turbulence and wind speed distributions are essential in determining the quality of astronomical sites and implementing adaptive optics systems. In this study, the optical turbulence with a general turbulence model and the wind speed characteristics at Lenghu are investigated by employing the 21-yr European Centre for Medium-Range Weather Forecasts' fifth set of reanalysis (ERA5) data. The vertical distributions and seasonal behaviours of the wind speed and atmospheric turbulence are analysed. The wind speed values at 200 hPa pressure level (V 200) are low in spring and summer except for June, and high in autumn and winter. The highest and lowest values of V 200 at Lenghu are 39.93 m s−1 in September and 27.48 m s−1 in April, respectively. Additionally, atmospheric conditions are relatively stable above 20 km throughout the year. The locations of the peaks in the |$C_n^2$| profiles correspond to the tropopause and the jet wind stream regions at the Lenghu site. Furthermore, the median seeing value from ERA5 is 0.72 arcsec, generally consistent with the statistically measured value of 0.75 arcsec. Moreover, the astronomical parameters (r 0, θAO, and τAO) were initially provided at the Lenghu site. The median values of r 0, ε, θAO, and τAO are 15.17 cm, 0.72 arcsec, 1.19 arcsec, and 2.83 ms, respectively, thus providing the potential reference for astronomical applications. [ABSTRACT FROM AUTHOR]

Context. With the Extremely Large Telescope (ELT) generation of telescopes come new challenges. The complexity of these telescopes' pupils creates new problems for adaptive optics (AO) that prevent the telescopes from reaching the theoretical resolutions that their size allows. In particular, the large spiders necessary to support the massive optics of these telescopes create discontinuities in the wavefront measurement. These discontinuities appear as a new phase error dubbed the "petal mode." This error is described as a differential piston between the fragment of the pupil separated by the spiders and is responsible for a strong degradation in the imaging quality, reducing the European ELT's resolution to that of a 15m telescope. Aims. The aim of this paper is to study the measurement of the petal mode by AO sensors. In particular, we want to understand why the pyramid wavefront sensor (PyWFS), the first-light wavefront sensor of any ELT-generation telescope, cannot measure this petal mode under normal conditions, and how to enable this measurement by adapting the AO control scheme and the PyWFS. Methods. To facilitate our study, we considered a simplified version of the petal mode, featuring a simpler pupil than the ELT. This allowed us to quickly simulate the properties of the petal mode and its measurement by the PyWFS. We studied specifically how a system that separates the atmospheric turbulence from the petal measurement would behave. Studying the petal mode's power spectral density, we proposed using a spatial filter to reduce the contribution of AO residuals to the benefit of petal mode contribution, eventually enabling it to be measured. Finally, we demonstrated our proposed system with end-to-end simulations. Results. A solution proposed to measure the petal mode is to use an unmodulated PyWFS (uPyWFS), but the uPyWFS does not make accurate measurements in the presence of atmospheric residuals. A spatial filtering step, consisting of a pinhole around the pyramid tip, reduces the first path residuals seen by the uPyWFS and restores its accuracy. This system was able to measure and control the petal mode during the end-to-end simulation. Conclusions. To address the petal problem, a two-path AO with a sensor dedicated to the measurement of the petal mode seems necessary. The question remains as to what could be used as the second path petalometer. Through this paper, we demonstrate that an uPyWFS can confuse the petal mode with the residuals from the first path. However, adding a spatial filter on top of said uPyWFS makes it a good petalometer candidate. This spatial filtering step makes the uPyWFS less sensitive to the first path residuals while retaining its ability to measure the petal mode. [ABSTRACT FROM AUTHOR]

Atmospheric turbulence profile plays an important role in designing and operating adaptive optics (AO) systems with multiple laser guide stars. To obtain representative free atmospheric profiles and resolved ground layer profiles for future AO systems at the Subaru telescope, we are conducting the Shack–Hartmann Atmospheric tuRbulence Profiling Experiment at the Subaru Telescope project. In this project, we develop a turbulence profiler comprising two Shack–Hartmann (SH) sensors to observe a pair of bright stars through the Subaru telescope with high spatial sampling by 2 cm subapertures. We perform two analyses on the SH spot data: variance analysis on the spot scintillation for free atmospheric profiles, and on the spot slope for ground layer profiles. This paper introduces the initial results of free atmospheric profiles as well as total seeing values and wind profiles obtained by the first two engineering runs. The free atmospheric profiles reconstructed by the two independent SH sensors show good agreement. The results are also consistent with simultaneous measurements by another profiler except for turbulence strength at ∼1 km, which is explained by an overestimation problem of scintillation-based profilers. Measured total seeing values are also smaller than the simultaneous measurements, possibly due to the difference in ground layer turbulence between the two sites. The wind profiles show good consistency with the direct measurements by a rawinsonde. Through this study, we establish a method to constrain the free atmospheric profile, the total seeing, and the wind profile by analysing data from a single SH sensor with fine subapertures. [ABSTRACT FROM AUTHOR]

The ambient atmospheric environment affects the growth and spread of wildland fires, whereas heat and moisture released from the fires and the reduction of the surface drag in the burned areas can significantly alter local atmospheric conditions. Observational studies on fire–atmosphere interactions have used instrumented towers to collect data during prescribed fires, but a few towers in an operational-scale burn plot (usually > 10 3 m2) have made it extremely challenging to capture the myriad of factors controlling fire–atmosphere interactions, many of which exhibit strong spatial variability. Here, we present analyses of atmospheric turbulence data collected using a 4 × 4 array of fast-response sonic anemometers during a fire experiment on a 10 m × 10 m burn plot. In addition to confirming some of the previous findings on atmospheric turbulence associated with low-intensity surface fires, our results revealed substantial heterogeneity in turbulent intensity and heat and momentum fluxes just above the combustion zone. Despite the small plot (100 m2), fire-induced atmospheric turbulence exhibited strong dependence on the downwind distance from the initial line fire and the relative position specific to the fire front as the surface fire spread through the burn plot. This result highlights the necessity for coupled atmosphere–fire behavior models to have 1–2 m grid spacing to resolve heterogeneities in fire–atmosphere interactions that operate on spatiotemporal scales relevant to atmospheric turbulence. The findings here have important implications for modeling smoke dispersion, as atmospheric dispersion characteristics in the vicinity of a wildland fire are directly affected by fire-induced turbulence. [ABSTRACT FROM AUTHOR]

The measurement of the rotational velocity using the rotational Doppler effect (RDE) of a vortex beam is easily affected by atmospheric turbulence, leading to dispersed orbital angular momentum (OAM), and reduced measurement accuracy. This study investigates the optimal ability of the Gerchberg-Saxton (GS) phase retrieval algorithm to compensate for the optical field and enhance the velocity measurement accuracy within the optimal range of intrinsic parameters, such as the number of GS iterations, and extrinsic parameters, such as the atmospheric turbulence intensity and beam properties. Through detailed theoretical and simulation analyses, we demonstrate the outstanding effectiveness of the GS algorithm in improving the velocity measurement accuracy. Simulations conducted for a system-target distance of zS-T = 500m show a 29.88% improvement in the velocity measurement accuracy and a 1.03-fold increase in the spectral signal-to-noise ratio (SSNR) within the optimal range. It showcases advantages that set it apart from other methods. This study reveals the threshold of the ability of GS algorithm to significantly enhance the rotational velocity measurement accuracy, providing valuable insights to precision measurements of rotational velocities in free-space applications. [ABSTRACT FROM AUTHOR]

A photogrammetric displacement measurement method based on machine learning was proposed to improve the robustness to environmental disturbances. (1) To reduce the target positioning error caused by environmental vibration especially atmospheric turbulence, a machine learning-based weighted location algorithm combined with an adaptive window selection strategy was developed. In an outdoor displacement table experiment, the proposed method's root mean squared error (RMSE) is 0.04 mm when the distance is 50 m, showing better accuracy and stability. (2) To complement/correct the missing or anomalous data caused by adverse external conditions, such as severe occlusion or camera shaking, a fast data self-diagnosis using a correlation vector machine was performed to make full use of the full-field measurement results obtained from the images. The applicability of the proposed method in extreme cases was demonstrated in designed experiments and an actual displacement measurement task of a long-span bridge subjected to vortex-induced vibration. [ABSTRACT FROM AUTHOR]

We present the twisted electromagnetic sinc-correlation Schell-model (EM TSSM) beam as an extension of the cylindrical sinc Schell-model beam and analyze the necessary source parameter conditions to generate a physically viable beam. Furthermore, we thoroughly investigate the propagation properties of the EM TSSM beam in atmospheric turbulence using the extended Huygens-Fresnel integral, explicitly focusing on spectral intensity, degree of polarization (DOP), and degree of coherence (DOC). It shows that the twisted phase has a noticeable impact on the intensity profiles of these beams, causing them to exhibit rotation and self-splitting while still maintaining their shape in free space. Moreover, during propagation through a turbulent atmosphere, it exhibits self-combining properties over a long range and recovers the plat-topped distribution. Compared with the sinc Schell-model beam without the twisted phase, the DOP distribution of such a beam can rotate around its distribution center. As these beams propagate through turbulent atmospheres, they can self-heal their DOP distribution within specific ranges affected by atmospheric turbulence. A twist factor causes non-unidirectional rotation of the DOC distribution in free space. The DOC gradually transforms from multi-strip profiles into a Gaussian-like distribution. Furthermore, the beam parameters play a crucial role in shaping the DOC. The results will be useful in optical trapping and optical communication. [ABSTRACT FROM AUTHOR]

Understanding the organization and dynamics of turbulence structures in the atmospheric surface layer (ASL) is important for fundamental and applied research in different fields, including weather prediction, snow settling, particle and pollutant transport, and wind energy. The main challenges associated with probing and modeling turbulence in the ASL are (i) the broad range of turbulent scales associated with the different eddies present in high Reynolds number boundary layers ranging from the viscous scale (on the order of millimeters) up to large energy-containing structures (on the order of kilometers); (ii) the nonstationarity of the wind conditions and the variability associated with the daily cycle of the atmospheric stability; and (iii) the interactions among eddies of different sizes populating different layers of the ASL, which contribute to momentum, energy, and scalar turbulent fluxes. Creative and innovative measurement techniques are required to probe near-surface turbulence by generating spatiotemporally resolved data in the proximity of the ground and, at the same time, covering the entire ASL height with large enough streamwise extent to characterize the dynamics of larger eddies evolving aloft. To this aim, the U.S. National Science Foundation sponsored the development of the Grand-scale Atmospheric Imaging Apparatus (GAIA) enabling super-large snow particle image velocimetry (SLPIV) in the near-surface region of the ASL. This inaugural version of GAIA provides a comprehensive measuring system by coupling SLPIV and two scanning Doppler lidars to probe the ASL at an unprecedented resolution. A field campaign performed in 2021-22 and its preliminary results are presented herein elucidating new research opportunities enabled by the GAIA measuring system. [ABSTRACT FROM AUTHOR]

With satellite laser communication, advantages can be achieved in high communication rate, precise beam pointing, high confidentiality, and light‐small terminal. Satellite–ground (SG) laser links also play a crucial role as trunk links. However, the SG link is influenced by atmospheric turbulence and platform vibration, these two factors will cause unstable link tracking. This paper focuses on the viewpoint of beam energy transmission, using the low‐frequency harmonic phase compensating method to establish the random phase screen. On the basis of this model, the laser beam energy distribution and the spot mass positioning error are analyzed considering turbulence taking vibration which is not considered in previous studies. The simulation results show that the spot mass center (SMC) is impressionable in strong turbulence under platform vibration, but in a weak turbulent environment, SMC is insensitive to vibration when turbulence and vibration are at the same noise level. [ABSTRACT FROM AUTHOR]

Understanding the influence of roughness and terrain slope on stable boundary layer turbulence is challenging. This is investigated using observations collected from October to November of 2018 during the Stable Atmospheric Variability ANd Transport (SAVANT) field campaign conducted in a shallow sloping Midwestern field. We analyze the turbulence velocity scale and its variation with the mean wind speed using observations up to 10–20 m on four meteorological towers located along a shallow gully. The roughness length for momentum over this complex terrain varied with wind direction from 0.0049 m to a maximum of 0.12 m for winds coming through deciduous trees present in the field. The variation of the turbulence velocity with wind speed shows a transition from a weak wind regime to a stronger wind regime, as reported by past studies. This transition is not observed for winds coming from the tree area, where turbulence is enhanced even for weak wind speeds. For weak stratification and stronger winds, the turbulent velocity scale increased with an increase in roughness while the terrain slope is seen to have a weak influence. The sizes of the dominant turbulent eddies seen from the vertical velocity power spectra are observed to be larger for winds coming through the tree area. The turbulence enhancement by the trees is found to be strong within a fetch distance of 7 times the tree height and not observable at 16 times of the tree height. [ABSTRACT FROM AUTHOR]

A new method is proposed for the characterization of the atmospheric turbulence strength parameter ( C n 2 ) based on the principal component analysis (PCA) of the texture of the speckle intensity pattern obtained on long-range propagation of a focused charge +1 vortex beam. Employing the split-step propagation method, datasets containing instantaneous intensity images of the focused vortex beam are generated for three distinct C n 2 values, specifically 1 × 10 − 14 m − 2 / 3 , 5 × 10 − 14 m − 2 / 3 , and 1 × 10 − 13 m − 2 / 3 , representing medium to high turbulence levels for a 2 km propagation distance. The gray level co-occurrence matrix (GLCM) methodology is employed to extract key texture attributes, like contrast, correlation, homogeneity, and energy from the intensity images. These extracted texture parameters serve as inputs for training a PCA model, enabling the identification of associated C n 2 values. The PCA analysis exhibits distinct clustering of the first three principal components for each of the three C n 2 values, forming individual clusters on the PCA plot. Standard deviational ellipses are drawn to clearly demarcate these clusters on the PCA plot. The texture-based PCA classification of atmospheric turbulence was also performed for a focused Gaussian beam. The comparison of PCA plots between vortex and Gaussian beams showed that a pronounced clear separation of C n 2 values is obtained for the vortex beam. This indicates that the non-zero orbital angular momentum of the vortex beam also plays an important role in achieving the distinct separation of C n 2 values on the PCA plot. The proposed method can provide efficient real-time turbulence estimation solely on the basis of texture of the instantaneous intensity speckle with prior training and therefore may simplify the estimation of turbulence strength parameter. [ABSTRACT FROM AUTHOR]

The Perseverance rover is carrying out an original acoustic experiment on Mars: the SuperCam microphone records the spherical acoustic waves generated by laser sparks at distances from 2 m to more than 8 m. These N-shaped acoustic waves scatter from the multiple local heterogeneities of the turbulent atmosphere. Therefore, large and random fluctuations of sound travel time and intensity develop as the waves cross the medium. The variances of the travel times and the scintillation index (normalized variance of the sound intensity) are studied within the mathematical formalism of the propagation of spherical acoustic waves through thermal turbulence to infer statistical properties of the Mars atmospheric temperature fluctuation field. The comparison with the theory is made by simplifying assumptions that do not include wind fluctuations and diffraction effects. Two Earth years (about one Martian year) of observations acquired during the maximum convective period (10:00–14:00 Mars local time) show a good agreement between the dataset and the formalism: the travel time variance diverges from the linear Chernov solution exactly where the density of occurrence of the first caustic reaches its maximum. Moreover, on average, waves travel faster than the mean speed of sound due to a fast path effect, which is also observed on Earth. To account for the distribution of turbulent eddies, several power spectra are tested and the best match to observation is obtained with a generalized von Karman spectrum with a shallower slope than the Kolmogorov cascade, ϕ (k) ∝ (1 + k 2 L 2) − 4 / 3 . It is associated with an outer scale of turbulence, L, of 11 cm at 2 m above the surface and a standard deviation of 6 K over 9 s for the temperature. These near-surface atmospheric properties are consistent with a weak to moderate wave scattering regime around noon with little saturation. Overall, this study presents an innovative and promising methodology to probe the near-surface atmospheric turbulence on Mars. [ABSTRACT FROM AUTHOR]

This study leverages a multi-fan flow control instrument and a mechanized roughness element grid to simulate large- and small-scale turbulent features of atmospheric flows in a large boundary layer wind tunnel (BLWT). The flow control instrument termed the flow field modulator (FFM), is a computer-controlled 3 m × 6 m (2D) fan array located at the University of Florida (UF) Natural Hazard Engineering Research Infrastructure (NHERI) Experimental Facility. The system comprises 319 modular hexagonal aluminum cells, each equipped with shrouded three-blade corotating propellers. The FFM enables the active generation of large-scale turbulent structures by replicating user-specified velocity time signals to inject low-frequency fluctuations into BLWT flows. In the present work, the FFM operated in conjunction with a mechanized roughness element grid, called the Terraformer, located downstream of the FFM array. The Terraformer aided in the production of near-wall turbulent mixing through precise adjustment of roughness element heights. A series of BLWT velocity profile measurements were carried out at the UF BLWT test section for a range of turbulence intensity and integral length scale regimes. Input commands to the FFM and Terraformer were iteratively updated via a governing convergence algorithm (GCA) to achieve user-specified mean and turbulent flow statistics. Results demonstrate the capabilities of the FFM for significantly increasing the longitudinal integral length scales compared to conventional BLWT approaches (i.e., no active large-scale turbulence generation). The study also highlights the efficacy of the GCA scheme for attaining prescribed target mean and turbulent flow conditions at the measurement location. [ABSTRACT FROM AUTHOR]

Based on the extended Huygens-Fresnel principal, we investigate the propagation behavior of a Generalized a Hermite cosh-Gaussian (GHCG) beam through random medium namely maritime atmospheric turbulence. The analytical formula for the GHCG beam propagating in the considered medium is derived. The influences of the maritime turbulence parameters and the source beam parameters on the propagation characteristics of the GHCG beam are illustrated using numerical calculations. The results show that the evolution behavior of average intensity for a GHCG beam through maritime turbulence will gradually lose its properties and its resistance to fluctuations and then evolve faster into Gaussian distribution in the far field as the turbulent constant structure increases and as the inner scale size of the turbulence decreases. The propagation of optical radiation across this medium has differing impacts compared to the atmosphere above land. The obtained results can be significantly contributed to the practical application for free-space optical communication and remote sensing. [ABSTRACT FROM AUTHOR]

The present study focuses on analysing the bit error rate (BER) of single-input single-output and multiple-input multiple-output Radio-over free space optics system under atmospheric turbulence conditions employing binary phase shift keying (BPSK) modulation. The system performance has been investigated for weak to strong scintillation conditions considering uncoded and Reed Solomon coded BPSK signals. The performance of the proposed system has also been analysed under strong turbulence effects considering varying signal to noise ratio, responsivity, Eb/No and different scattering conditions. Further, different machine learning techniques namely, artificial neural network (ANN), decision tree, and gradient boosting regression have been employed for estimation of received signal quality in terms of BER. Highest (0.98) and lowest (0.0155) values of R-squared (R2) and root mean square error respectively have been obtained in case of ANN model resulting in the best fit model. [ABSTRACT FROM AUTHOR]

The evolution properties of a Generalized Hermite cosh-Gaussian beam (GHCGB) through a turbulent oceanic medium are investigated based on the extended Huygens-Fresnel principal. The analytical formula for the average intensity of a GHCGB propagating in oceanic turbulence is derived. From the derived formula, the average intensity of the considered beam in oceanic turbulence is discussed with numerical examples. Our results show that the GHCGB propagating in stronger oceanic turbulence will lose its initial profile and rapidly evolve into a Gaussian distribution by increasing the dissipation rate of mean squared temperature and the relative strength of temperature and salinity fluctuations or by decreasing the rate of dissipation of turbulent kinetic energy per unit mass of fluid, in the far field. Meanwhile, the evolution properties of the GHCGB in the oceanic turbulence are affected by the initial beam parameters. We anticipate that our research results will be useful for practical applications in underwater optical communication and imaging systems. [ABSTRACT FROM AUTHOR]

Atmospheric turbulence is a crucial factor that affects the quality of long-distance imaging. Though current deep learning models can effectively suppress geometric displacement and spatial blurring caused by atmospheric turbulence, such models require a large number of parameters and computational resources. To tackle this problem, this paper proposed a lightweight atmospheric turbulence degraded image restoration model based on generative inversion that entailed three core mo-dules: the DeBlur module, the remove shift module, and the turbulence regeneration module. The DeBlur module used high-dimensional feature mapping blocks, detail feature extraction blocks, and feature compensation blocks to suppress image blurring caused by turbulence. The remove shift module compensated for pixel displacement caused by turbulence using two convolutional layers. The turbulence regeneration module regenerated turbulence degraded images through convolutional operations. In the DeBlur module, it designed an attention-based feature compensation module that integrated the channel attention mechanism and the spatial mixed attention mechanism to focus on essential detail information in the image during training. The proposed model achieved peak signal-to-noise ratios of 19.94 dB and 23.51 dB, and structural similarity values of 0.688 2 and 0.752 1 on publicly available dataset Heat Chamber and self-built dataset Helen, respectively. Furthermore, it reduced the number of parameters and computational resources, compared to the current state-of-the-art(SOTA) method. The experimental results demonstrate the effectiveness of this method in restoring atmospheric turbulence degraded images. [ABSTRACT FROM AUTHOR]

We present BoundaryLayerDynamics.jl, a new code for turbulence-resolving simulations of atmospheric boundary-layer flows as well as canonical turbulent flows in channel geometries. The code performs direct numerical simulation as well as large-eddy simulation using a hybrid (pseudo)spectral and finite-difference approach with explicit time advancement. Written in Julia, the code strives to be flexible and adaptable without sacrificing performance, and extensive automated tests aim to ensure that the implementation is and remains correct. We show that the simulation results are in agreement with published results and that the performance is on par with an existing Fortran implementation of the same methods. [ABSTRACT FROM AUTHOR]

We present a technique to observe stellar inteferograms, as well as the detailed analysis of those created by three bright stars: Betelgeuse, Rigel, and Sirius. It is shown that the atmospheric turbulence is responsible for the reduction of the long-exposure fringe visibility of the obtained interference patterns. By using different baselines in our interferometer, we are able to distinguish the decay of the visibility with the baseline, observe how different parameters such as the diameter or the distribution of the holes in our interferometer affect the pattern, and measure the atmospheric turbulence. This experiment can be performed by postgraduate students to gain practical experience in optical interferometry in astronomy. [ABSTRACT FROM AUTHOR]

Adaptive optics revolutionizes telescopic resolution but faces cost, complexity, and calibration hurdles. Neural network adaptive optics (NNAO) offers promise by using neural networks to tailor corrections to telescopes and atmospheric conditions, by passing calibration and sensors. This MATLAB-based study examines NNAO's impact on astronomical image quality, revealing it as a cost-efficient solution that enhances adaptive optics in astronomy. The numerical simulation results were encouraging, with a compensation rate exceeding 50% due to favorable monitoring conditions. The results indicate that the dominant factor affecting image quality is the variance of wavefront aberrations. The Strehl ratio (SR) decreases from an average of 0.548 for a variance of 0.2 to 0.020 for a variance of 0.6, while the mean squared error (MSE) increases from an average of 0.613 to 5.414. However, the effect on peak signal-to-noise ratio (PSNR) is inconclusive. Furthermore, it was found that increasing the number of neurons and training ratio does not significantly impact the results obtained, but it noticeably affects the computational time required. [ABSTRACT FROM AUTHOR]

Free space optical communication is gaining importance in the field of optical communication due to its high speed and high bandwidth applications. Free space optical communication system (FSOCS) provides many benefits as compared to traditional wireless communication system and fiber optic cables. This makes this technology the reasonable extension of metropolitan area network and also provides the quick recovery during natural disaster. This system performance is limited due to the atmospheric turbulence effect and various atmospheric losses such as rain, and fog. Gamma gamma atmospheric turbulent model is used to analyze the system performance in the presence of moderate to strong atmospheric turbulence. We have designed the FSO gamma gamma turbulent model with non-return to zero (NRZ) modulation format employing wavelength division multiplexing (WDM), spatial diversity multiple input multiple output (MIMO) (8×8) at various atmospheric turbulence levels and attenuation loss of 10 dB/km at the distance of 2-4 km. Using the proposed model, the link distance is enhanced up to 4km in the presence of turbulence and atmospheric losses with minimum laser transmitted power. [ABSTRACT FROM AUTHOR]

In this paper, we propose, design, and evaluate a new hybrid terrestrial–underwater optical communication link for providing high-speed connectivity between land and underwater systems. A device based on an amplify-and-forward strategy is considered and used for the hybrid optical link. A performance analysis of the proposed hybrid system is then carried out, taking into account both the atmospheric and underwater channels and their respective degradation sources. Different networking scenarios and conditions are evaluated. To this end, the channel model of the terrestrial free-space optical (FSO) link is modeled using the Gamma–Gamma distribution, while the underwater optical link is modeled using the Weibull distribution. The former takes into account atmospheric and turbulence attenuation, geometric spread and pointing errors, while the latter takes into account underwater and turbulence attenuation and geometric spread. Accordingly, a new analytical closed-form expression for the bit error rate (BER), which depends on the cumulative distribution function of the holistic hybrid system, is derived. Analytical results show that pointing errors as well as atmospheric and oceanic turbulence seriously degrade the performance of the hybrid system. In addition, ocean turbulence leads to the occurrence of a BER floor in some scenarios. This is the first time that such a network is proposed and modeled under the assumption of critical channel impairments. [ABSTRACT FROM AUTHOR]

This paper proposes an adaptive bit-interleaved polar coded modulation (A-BIPCM) method based on minimum logarithmic upper bound weight (MLUW). It is designed to reduce the fading effects and long string of bit error interference caused by atmospheric turbulence in free-space optical communications (FSOC). To assess the effectiveness of this method across turbulent channels of varying intensities, we conducted an evaluation of the bit error rate (BER) performance of polar codes in turbulent channels. The results demonstrate significant performance improvements provided by the A-BIPCM method compared to conventional polar code encoding and decoding. Specifically, under weak, moderate, and strong turbulence conditions, the A-BIPCM method achieves performance gains of 0.96 dB, 1.66 dB, and 1.35 dB, respectively. Additionally, an experimental verification platform for FSOC employing intensity modulation direct detection (IM/DD) with an atmospheric turbulence simulation channel, is established in this work. When the optical power of the detector is −16 dBm, the traditional polar code encoding and decoding performance at BER = 2.36 × 10−5, whereas the A-BIPCM scheme exhibits a significantly higher performance at BER = 2.11 × 10−6. The BER has been improved by representing an order of magnitude. [ABSTRACT FROM AUTHOR]

In general, wireless relays are used for connecting wireless communication links, but relays can bring security and deployment issues. This paper investigates the performance of air-to-water communication systems without relay nodes. The probability density function (PDF) of the RF link is obtained by unifying α − μ , the extended generalized - K (EGK) and Fisher Snedecor (ℱ) distributions of atmospheric turbulence. We obtained the joint PDF of the RF/UWOC system for atmospheric turbulence and ocean turbulence with nonzero mean single-sided pointing error as well as air–water interface under foggy conditions. Based on statistical results, exact expressions for outage probability (OP), bit error rate (BER), and channel capacity (CC) were derived. Finally, we validated the analytical expression using Monte Carlo simulation. The simulation results show that the fading parameters of the channel have a significant impact on the performance of a without-relay-assisted RF/UWOC system. [ABSTRACT FROM AUTHOR]

The wind profiler radar (WPR) system requires a dual-polarized antenna with multiple low-sidelobe and high-gain beams to facilitate the detection of weak signals reflected by atmospheric turbulence. This paper proposes a dual-polarized continuous transverse stub (CTS) K-band antenna with four reconfigurable beams, which comprises a series-fed CTS array and four 1-to-14 power dividers as line source generators (LSGs) to generate a high-quality quasi-TEM wave. The CTS element incorporates a stepped transition radiation stub design and employs a short cutoff stub on the upper surface of the series-fed parallel plate waveguide (PPW) to achieve optimal impedance matching. The entire antenna is an all-metal structure with remarkably low loss, and low-cost standard fabrication processes are employed for the prototype, which achieves fast reconfigurable four-beam scanning to 15°, with a gain of 31.09 dBi and sidelobe levels below −17.6 dB. Measurement results in an anechoic chamber agree well with simulations, demonstrating the antenna's ease of manufacture, stability, and suitability for wind profile radar applications. [ABSTRACT FROM AUTHOR]

This study emphasizes the importance of conducting a comprehensive analysis considering the C n 2 atmospheric parameter for a reliable assessment of the seeing value. It highlights the use of the ECMWF ERA5 model to simulate pressure, temperature, and humidity profiles, enabling the evaluation of optical degradation. Accurate modeling and monitoring of temperature and humidity variables are emphasized for precise data in optical communication and astronomical observations. It also discusses the significance of optical turbulence models in understanding the impact of atmospheric turbulence on optical system performance. The validation of the selected C n 2 models is thoroughly analyzed. To evaluate the impact of the boundary layer on seeing conditions, three factors are considered. First, ERA5 data is used to simulate surface C n 2 values using the PAMELA model. Second, typical C n 2 values for specific dates in Redu are obtained. Finally, the thickness of the boundary layer, denoted as hbl, is calculated. This finding highlights the significant influence of the boundary layer on atmospheric seeing conditions and by incorporating Ibl into the models, a more accurate representation of the effects of the boundary layer on seeing quality is achieved. [ABSTRACT FROM AUTHOR]

Free Space Optical (FSO) communication systems are gaining popularity due to its tremendous speed, advanced capacity, cost effectiveness, secure and easy to deploy wireless networks. This technology has proven an effective choice for last mile applications and hard to reach areas where deployment of optical fibre links is not feasible. But FSO link is highly weather dependent and as signal passes through the atmospheric channel, the main impairments are the atmospheric turbulence, which induce fading and deteriorate the system performance. Delhi has a great potential for FSO communication because of its clear skies. Since there is no analysis for weather condition found in Delhi, this work provides analysis of typical Delhi weather condition ranging from heavy to light rain, fog and clear sky. The performance of FSO link is analysed in terms of attenuation and link length margin under different weather condition. The results are concluded to identify which atmospheric condition influences more on FSO link performance. [ABSTRACT FROM AUTHOR]

Summary: With the development of free space optical links (FSO) for space‐ground communications comes the need to mitigate the effects of the atmospheric turbulence to guarantee a lossless connection. By having a network of addressable ground stations, we want to guarantee to always target a point where the link is available. Assuming atmospheric transmission is managed thanks to site diversity, we focus only on the influence of atmospheric turbulence on the signal injected into a single mode fiber on the downlink. The use of adaptive optics (AO) is assumed to avoid turbulence‐induced signal disruptions and enable a sufficiently high level of received signal for data transmission. Up to now, AO performance adequate assessment required the knowledge of high‐resolution Cn2 and wind profiles. With the advent of integrated atmospheric parameters measurement instruments, we investigate here the possibility to estimate AO‐corrected performance from a limited number of integrated parameters. In this paper, we propose to use a Gaussian process metamodel to assess the statistics of the received optical power after an AO correction. Taking as input only four integrated parameters of the turbulence profile and associated wind profile, which can be measured with simple instruments, the estimation error on the value of the 1% quantile of the received optical power is inferior to 0.7dB. We also demonstrate the possibility to estimate the half correlation time of the received optical power using the same integrated parameters. [ABSTRACT FROM AUTHOR]

This paper investigates the performance analysis of radio frequency/free space optical-visible light communication (RF/FSO-VLC) system, where a hybrid RF/FSO link is connected to an indoor VLC subsystem using decode-and-forward (DF) relay. The selection combining (SC) technique is used at the DF relay. We have considered Nakagami-m fading, Double Generalized Gamma (DGG) distributed atmospheric turbulence with Rayleigh distributed pointing errors and Lambertian pattern of the light emitting diode (LED) for the RF, FSO, and VLC links, respectively. To analyze the performance of the considered co-operative system, different performance metrics such as outage probability, bit-error-rate (BER), and ergodic capacity of the system are taken into consideration. For deriving the performance metrics, novel expressions for cumulative distribution function and probability density function of the instantaneous signal-to-noise-ratio (SNR) of hybrid RF/FSO-VLC system in terms of Meijer-G function are obtained and used. The numerical results show that the outage probability of the system is more in the strong turbulence conditions and intensity modulation/direct detection (IM/DD) technique in comparison to weak turbulence and heterodyne detection scenario. It is also seen from the results, that the BER of the system is better in case of coherent binary modulation techniques in comparison with non-coherent binary and M-ary modulation techniques. Moreover, the capacity of the system increases as the number of LEDs increase and as the height of the LEDs reduce. The asymptotic and simulation results match with the derived analytical results. [ABSTRACT FROM AUTHOR]

In this article, a novel method for removing atmospheric turbulence from a sequence of turbulent images and restoring a high-quality image is presented. Turbulence is modeled using two factors: the geometric transformation of pixel locations represents the distortion, and the varying pixel brightness represents spatiotemporal varying blur. The main framework of the proposed method involves the utilization of lowrank matrix factorization, which achieves the modeling of both the geometric transformation of pixels and the spatiotemporal varying blur through an iterative process. In the proposed method, the initial step involves the selection of a subset of images using the random sample consensus method. Subsequently, estimation of the mixture of Gaussian noise parameters takes place. Following this, a window is chosen around each pixel based on the entropy of the surrounding region. Within this window, the transformation matrix is locally estimated. Lastly, by considering both the noise and the estimated geometric transformations of the selected images, an estimation of a low-rank matrix is conducted. This estimation process leads to the production of a turbulence-free image. The experimental results were obtained from both real and simulated datasets. These results demonstrated the efficacy of the proposed method in mitigating substantial geometrical distortions. Furthermore, the method showcased the ability to improve spatiotemporal varying blur and effectively restore the details present in the original image. [ABSTRACT FROM AUTHOR]

Turbulence-free ghost imaging is a promising solution for the degraded image caused by atmospheric turbulence. However, turbulence-free ghost imaging requires two harsh conditions. To solve this problem, the temporal statistical property of the photon number fluctuations introduced by space-time fluctuating atmospheric turbulence is investigated. We find that the photons collected by the detectors is a superposition of the fluctuating photons introduced by space-time fluctuating atmospheric turbulence and the nonfluctuating photons with probabilities inversely proportional to the beam diameter. The fluctuating photons, which cause image degradation, lead to the absence of stable temporal photon number correlations, while the nonfluctuating photons have the opposite effect. Thus, fluctuating photons are eliminated by measuring the temporal correlations of the signal light and reference light separately. The results show that turbulence-free ghost images can be reconstructed even if the typical conditions are not met. This work does not negate the results of Meyers et al. and provides a method to address the harsh constraints of turbulence-free ghost imaging. [ABSTRACT FROM AUTHOR]

Understanding surface temperature is important for habitability. Recent work on Mars has found that the dependence of surface temperature on elevation (surface lapse rate) converges to zero in the limit of a thin CO2 atmosphere. However, the mechanisms that control the surface lapse rate are still not fully understood. It remains unclear how the surface lapse rate depends on both greenhouse effect and surface pressure. Here, we use climate models to study when and why "mountaintops are cold." We find the tropical surface lapse rate increases with the greenhouse effect and with surface pressure. The greenhouse effect dominates the surface lapse rate transition and is robust across latitudes. The pressure effect is important at low latitudes in moderately opaque (τ ∼ 0.1) atmospheres. A simple model provides insights into the mechanisms of the transition. Our results suggest that topographic cold‐trapping may be important for the climate of arid planets. Plain Language Summary: Understanding surface temperature on a planet is important for life on Earth and beyond. On Earth, we know "mountaintops are cold," which means that surface temperature decreases with elevation. However, this idea does not apply on present‐day Mars. Here, we investigate when and why the Earth‐based understanding holds for planets with different types of atmospheres. Using a global climate model, we show that both the greenhouse effect (atmospheric infrared opacity) and the pressure effect (atmospheric turbulence) are important. The weaker the greenhouse effect, or the thinner the atmosphere, the slower the surface cools with elevation. The greenhouse effect plays the dominant role, but in moderately opaque atmospheres, the pressure effect becomes important as well. Our work reveals a novel connection between climate and geomorphology. For example, on a planet with a relative thin pure O2 or N2 atmosphere, we do not expect that "mountaintops are cold." Key Points: Surface lapse rate robustly increases with atmospheric longwave optical thickness (greenhouse effect) in a general circulation modelIncreased pressure further contributes to a tropical surface lapse rate increase in moderately opaque atmospheresA simple model, assuming weak temperature gradient and highland convective adjustment, provides insight into the mechanisms [ABSTRACT FROM AUTHOR]

Turbulence effects can cause performance degradation for underwater optical wireless communication (UOWC) system. In this paper, a hybrid modulation (modulation–modulation, M–M) is presented to resist turbulence‐induced fading and improve the transmission performance of UOWC systems. The proposed differential phase shift keying‐multiple pulse position modulation (DPSK‐mPPM) is adopted as hybrid modulation to improve the bit error rate (BER) performance over UOWC turbulence channel of Gamma–Gamma distribution. Subsequently, the closed‐form average BER expressions of mPPM and hybrid DPSK‐mPPM modulation are derived and investigated, which are related to the oceanic turbulence parameters. The influence of the variation of anisotropy factor of ocean turbulence is analysed for the UOWC system over strong Gamma–Gamma turbulent channels. Finally, the numerical simulation results demonstrate the effectiveness of proposed DPSK‐mPPM to improve the BER performance compared to the traditional single modulation over UOWC channel under the same conditions. Furthermore, the work of this paper improves the practicality of the proposed DPSK‐mPPM scheme for underwater communication in complex oceanic environments. [ABSTRACT FROM AUTHOR]

Free space optical communication (FSO) is thriving with greater pace nowadays due to its ability to support high speed, greater capacity, improved security, ease of installation, not having of Electromagnetic interference (EMI) and unlicensed spectrum. Atmospheric turbulence like Rain, Fog, and Haze is a prominent issue in FSO system that adversely affects the performance. This paper presents the demonstration of a multibeam FSO system operating at 1.25 Gbps in several haze types, including hazardous, very unhealthy, sensitive, good, moderate, and unhealthy. Additionally, BER and Q factor comparisons are made across three distinct pulse formats: carrier suppressed RZ (CSRZ), return to zero (RZ), and non-return to zero (NRZ). Utmost goal for the deployment of multibeam FSO is to get high received power and to compensate attenuation arising from haze effects. Results revealed that CSRZ performed exceptionally well as compared to NRZ and RZ. [ABSTRACT FROM AUTHOR]

Recently, different research articles have given more emphasis to the design of high-speed free space optical (FSO) links using various multiplexing techniques to achieve optimum long reach under adverse weather conditions. This research paper presents a spectral-efficient, robust design of a novel, 2 × 120 Gb/s, hybrid multiplexed FSO system based on orthogonal frequency division multiplexing (OFDM). The 16-QAM modulated multiplexed data is sent over the helical structure of the orthogonal laser modes to increase the optical link's capacity, whereas OFDM is used to mitigate the fading induced by the atmospheric turbulence. The in-phase (I) and quadrature phase (Q) based optical modulators, along with the digital signal processing (DSP) unit, are used at the receiver to take care the high-speed data below the acceptable FEC limit. The system performance is evaluated in terms of scatter plot, link distance, optical signal-to-noise ratio (OSNR), and received optical power under strong turbulence, heavy rain and snow conditions to achieve the maximum transmission range of 2.5 km for an acceptable threshold bit error rate of ≤ 2 × 10−3. The simulation results illustrate the receiver sensitivity of − 21.71 dBm and OSNR requirements of 32.98 dB for the B2B case under clear weather conditions. [ABSTRACT FROM AUTHOR]

Atmospheric turbulence parameters, such as turbulent kinetic energy and dissipation rate, are of great significance in weather prediction, meteorological disasters, and forecasting. Due to the lack of ideal direct detection methods, traditional structure function methods are mainly based on Kolmogorov's assumption of local isotropic turbulence and the well-known -5/3 power law within the inertial subrange, which limits their application. Here, we propose a method for directly measuring atmospheric turbulence parameters using coherent Doppler wind lidar, which can directly obtain atmospheric turbulence parameters and vertical structural features, breaking the limitations of traditional methods. The first published spatiotemporal distribution map of the power-law exponent of the inertial subrange is provided in this study, which indicates the heterogeneity of atmospheric turbulence at different altitudes, and also indicates that the power-law exponent at high altitudes does not fully comply with the -5/3 power law, proving the superiority of our method. We analyze the results under different weather conditions, indicating that the method still holds. The turbulent kinetic energy and power-law index obtained by this method are continuously compared with the results obtained with an ultrasonic anemometer for a monthlong period. The results of the two have high consistency and correlation, verifying the accuracy and applicability of the proposed method. The proposed method has great significance in studying the vertical structural characteristics of atmospheric turbulence. [ABSTRACT FROM AUTHOR]

Results of numerical simulation of the return flux from a sodium laser guide star for atmospheric conditions of astronomical observatories of the North Caucasus are presented. The simulation is performed in accordance with modern concepts of the physics of the interaction of polarized laser radiation with mesospheric sodium atoms. Two cases of producing an artificial reference source by laser radiation, with circular and linear polarizations, are considered. The limitations of the photon flux due to the saturation effect are estimated. [ABSTRACT FROM AUTHOR]

The paper studies the performance of a free space optical communication link, which consists of a Gaussian Laser beam employing direct-current-biased optical frequency division multiplexing (DCO-OFDM) modulation, to carry information. While optical multi-carrier modulation is capable of alleviating multi-path fading, higher transmit power by virtue of dc-bias, helps the optical beam in partially off-setting signal intensity loss due to beam divergence in free space channel. In order to reduce the overall transmitted power apart from avoiding nonlinearities due to high peak-to-average-power-ratio (PAPR), we utilize a PAPR-dependent dc-bias, to generate a DCO OFDM light beam from a pre-clipped electrical baseband OFDM signal. The radiated optical beam encounters turbulence-induced impairments like beam-spreading, beam-wander, etc., which affect the signal-to-noise ratio (SNR) of the received data symbols. Further signal pre-clipping and optical source linearity generate clipping noise components resulting in compromised receiver performance or reduction of span length, to achieve target bit rate error (BER). In this paper, we derive an analytical model for evaluating the performance of the free-space optical (FSO) link, by modeling various impairments as noise variances and characterizing atmospheric turbulence by refractive index structure parameter under weak- and strong-turbulent conditions. Numerical results are obtained by varying system design parameters in the model, which graphically provide useful insight on the power penalties and various trade-offs involved in operating the link on a longer span, at a higher data rate, and with reduced transmitted power through pre-clipping while ensuring desired signal BER. [ABSTRACT FROM AUTHOR]

In this paper, radio-over-free-space optic (RoFSO) transmission is investigated for the next-generation wireless networks. Here, the 60 GHz based radio frequency (RF) system is demonstrated using 16-quadrature amplitude modulation with orthogonal frequency division multiplexing scheme. System performance for FSO link is evaluated by modeling the channel with the gamma-gamma fading. The optical spectrum and constellation diagram of the received signals are analyzed. The improvement reported for error vector magnitude is 3.8–0.6% and symbol error rate of 0.03–0.005 for free-space range of 0.2–1 km. Proposed work claims that orthogonal frequency division multiplexing can be helpful for the FSO systems by providing robustness to adverse atmospheric turbulence. [ABSTRACT FROM AUTHOR]

Free space optical (FSO) communication refers to a line of sight technology, which comprises optical source and detector to create a link without the use of physical connections. Similar to other wireless communication links, these are severely affected by losses that emerged due to atmospheric turbulence and lead to deteriorated intensity of the optical signal at the receiver. This impairment can be compensated easily by enhancing the transmitter power. However, increasing the transmitter power has some limitations as per radiation regulations. The requirement of high transmit power can be reduced by employing diversity methods. This paper presents, a wavelength-based diversity method with equal gain combining receiver, an effective technique to provide matching performance to single input single output at a comparatively low transmit power. [ABSTRACT FROM AUTHOR]

In this paper, a hybrid-subcarrier-intensity-modulation (hybrid-SIM) technique for the performance improvement of free-space-optical (FSO) communication system has been proposed. Subsequently, for further error performance improvement, avalanche photodiode (APD) based receiver is used in the proposed system. The system performance is analyzed at various atmospheric turbulence levels over weak and strong turbulence channels. The bit error rate (BER) is theoretically derived using Gauss–Hermite approximation and Meijer-G function and it is simulated in the MATLAB environment. The simulation result shows that the BER performance of hybrid-SIM is better than BPSK-SIM technique irrespective of the channel types and also the significant BER performance improvement is observed by APD receiver. [ABSTRACT FROM AUTHOR]

First of all, the beam propagation of Super Lorentz Gaussian (SLG) profile is propagated via space, the recent research dealt extensively with the investigation of the propagation of SLG in a level of specified atmospheric. In a turbulent atmosphere of intensity and receiver field, models were derived from a new mathematical expression of intensity and analyzed. Also, to find the power scintillation indicators for the SLG beam in a random turbulence of receiver plane. The equations are obtained for the average receiver-aperture. The new beam of SLG systems generated a modified model when compared with the receiver-aperture averaging. When we revisions the parameters, firstly is started the factor source size, this affected the profile for the power propagation and the analysis proved that the average of the aperture is affected by increasing the distance of propagation length. The enhancement of the average power of the aperture effect reliably with the source size of the initial beam source depends on several factors, including the structure constant, the beam order and static value of source size. Finally, the target of this article is detected a novel of mathematical expression of the receiver intensity is applied in the system of optical communications. [ABSTRACT FROM AUTHOR]

An analytical approach is developed in this paper to evaluate the bit error rate (BER) performance of an optical wireless (OW) communication system with multiplexing of the RF orthogonal frequency division (OFDM) over turbulent condition taking into account the effect of pointing error. The received signal is detected through direct detection receiver followed by RF synchronous demodulation including the effect of OW channel and different form of noises such as receiver thermal noise, background channel noise and photo detector shot noise. Analysis is developed for an OFDM system over the OW channel, taking into account the effect of pointing error between the transmitter and the receiver in turbulent condition and the analysis reveals that the OFDM OW system is less affected by pointing error with deference to the major power penalty at BER performance. For instance, power penalty at BER 10−9 is found to be 3 dB for 256 OFDM subcarriers with 9 millidegree displacement angle at a data rate of 10 Gbps under turbulent condition. It is found that the system is more influenced by the atmospheric turbulence at a higher data rate. [ABSTRACT FROM AUTHOR]

Due to atmospheric turbulence, the reliability and security of free space optical (FSO) communication will be affected seriously. In this paper, we propose a novel scheme of quasisynchronous spatial diversity free-space optical code division multiple access (FSO-CDMA) wiretap channel based on gamma-gamma distribution. Considering multiple access interference, shot noise, thermal noise and background noise, performances of bit error rate (BER) and physical-layer security are analyzed theoretically. The physical-layer security of quasisynchronous spatial diversity FSO-CDMA wiretap channel is evaluated by secrecy capacity and interception probability. Closed-form expression of interception probability is obtained. In order to compare the performances between conventional spatial diversity FSO-CDMA and the proposed scheme, Optisystem simulation is carried out. The results indicate that quasisynchronous spatial diversity FSO-CDMA scheme can improve the reliability and physical-layer security simultaneously. [ABSTRACT FROM AUTHOR]

Free-space optical (FSO) communication system provides several advantages over radio frequency (RF) system offering high bandwidth, low cost, small space requirements and more secure transmission which is free from Electromagnetic Interference (EMI). However, when the transmitted light signal passes through the atmosphere it experiences attenuation and fluctuations due to atmospheric turbulence. This paper analyzes the bit error rate (BER) performance of FSO communication systems under strong atmospheric turbulence for on-off keying (OOK), binary phase-shift keying (BPSK), differential phase shift keying (DPSK), quadrature phase shift keying (QPSK) and 8-phase shift keying (8-PSK) for link distances of 500, 1,000, 1,500 and 2,000 m. The probability density function (pdf) of the received irradiance is modelled using the gamma-gamma distribution model. It is found that the system exhibits the best BER performance and compensates the lowest power penalty at BER of 10 - 9 for BPSK modulation compared with other modulation techniques which makes BPSK more appropriate to be used with FSO turbulent system. [ABSTRACT FROM AUTHOR]

The Log Normal model for weak turbulences and Gamma-Gamma model for moderate to strong turbulences are being used for calculating the path losses for engineering Free Space Optical (FSO) communication link. Since FSO has been an attractive substitute to overcome the issues of RF spectrum license, high infrastructure cost and difficult terrains to ensure the availability of a network. Their application in mountainous and high density built up areas needs to be exploited. Atmospheric turbulence degrades the FSO communication performance arising due to variations in C n 2 (Refractive index structure parameter). Both these models have the limitation of NOT considering Rainfall parameter while calculating C n 2 . In this present paper we have included effect of Rainfall and presented the modified analytical model by re modeling C n 2 . To validate the impact, comparison of existing model and the modified path loss model has been presented using Meteorological (MET) data for a designed FSO communication link between geographically known mountainous locations of North eastern region of Arunachal Pradesh. The results emphasize that, the modified model is essential to ensure reliable and fail safe FSO link. [ABSTRACT FROM AUTHOR]

Vortex beams carrying orbital angular momentum (OAM) have gained much interest in optical communications because they can be used to expand the number of multiplexing channels and greatly improve the transmission capacity. However, the number of states used for OAM-based communication is generally limited by the imperfect OAM generation, transmission, and demultiplexing methods. In this work, we proposed a dense space-division multiplexing (DSDM) scheme to further increase the transmission capacity and transmission capacity density of free space optical communications with a small range of OAM modes exploiting a multi-ring perfect vortex (MRPV). The proposed MRPV is generated using a pixel checkerboard complex amplitude modulation method that simultaneously encodes amplitude and phase information in a phase-only hologram. The four rings of the MRPV are mutually independent channels that transmit OAM beams under the condition of occupying only one spatial position, and the OAM mode transmitted in these spatial channels can be efficiently demodulated using a multilayer annular aperture. The effect of atmospheric turbulence on the MRPV was also analyzed, and the results showed that the four channels of the MRPV can be effectively separated under weak turbulence conditions. Under the condition of limited available space and OAM states, the proposed DSDM strategy exploiting MRPV might inspire wide optical communication applications exploiting the space dimension of light beams. [ABSTRACT FROM AUTHOR]

Laser heterodyne detection (LHD) is a key velocimetry technique that provides better accuracy and sensitivity than direct laser detection. However, random phase noise can be introduced by the surface topography of the moving target undulation or atmospheric turbulence during transmission. The random phase noise causes the target echo to undergo decoherence, resulting in degradation of the signal-to-noise ratio (SNR). Here, we propose a novel LHD method based on photon time–domain differential detection. It can infer the heterodyne spectrum of the target echo and the local oscillator light from the time intervals of the photon arrival. The time interval statistic is a relative quantity, which can effectively avoid the effect of random phase noise in LHD. With our method, the SNR of LHD can be improved in application scenarios where the target echo is decoherent. We developed a complete solution model for acquiring the heterodyne spectrum based on photon time–domain differential detection and performed proof-of-principle experiments. The experimental results show that in the presence of random phase noise, the SNR and velocity measurement error of our method are significantly better than that of the conventional method, and the larger the phase noise is, the more the SNR and velocity measurement error of our method are improved. Moreover, along with the increase in phase noise, the SNR of our method is basically unchanged, which also indicates that our method is not affected by random phase noise. This advantage is significant for photon-level weak echoes that require long detection times to be detected. [ABSTRACT FROM AUTHOR]