Effects Of Atmospheric Turbulence Research Articles

Turbulent-degraded image restoration via improved principal component analysis method

It is challenging to removing atmospheric turbulence-induced geometric distortion and blurry degradation simultaneously. The improved principal component analysis (i-PCA) method is proposed in this paper. The image set selection strategy is implemented by the Euclidean distance between each sub-module image and its corresponding reference image to remove large geometric deformations and blurs directly. The eigenvector representing the original image data is uniquely determined by combining the maximum correlation with the original image and the maximum variance of the corresponding principal component for resolving the direction issue of the eigenvector. The algorithm is tested using synthetic and real turbulence-distorted images. The experimental results show that the i-PCA method proposed can effectively alleviate the atmospheric turbulence effects and significantly enhance visual quality, promising for its application to turbulence-distorted image.

Low-Complexity Adaptive Optics Aided Orbital Angular Momentum Based Wireless Communications

Adaptive optics (AO) has the potential to mitigate the effect of atmospheric turbulence and improve the performance of orbital angular momentum (OAM)-based optical wireless communication (OAM-OWC) links. Here, we propose a single-intensity-measurement phase retrieval algorithm (SPRA)-based AO technique of compensating for the distortion of the OAM beam. The only parameter required by the SPRA wave-front sensor is the intensity of the probe beam in the Fourier domain, which substantially simplifies the AO system. We first derive an analytical expression to characterize the expansion of probe beam in OAM-OWC links and then determine the diameter constraints as the apriori information of the SPRA required for guaranteeing a certain compensation performance. The simulation results illustrate that the SPRA-AO approach can indeed correct a distorted OAM beam both in a single-channel scenario and in multiplexed OAM-OWC systems. The bit error rate can be improved by orders of magnitude with the aid of SPRA-AO compensation. Furthermore, we establish noise models of AO-based OAM-OWC systems and analyze the robustness of the SPRA-AO technique. In a nutshell, this paper provides new insights for the applications of AO and forms the theoretical basis of employing probe beams in OAM-OWC systems.

Subaru Telescope -History, active/adaptive optics, instruments, and scientific achievements.

The Subaru Telescopea) is an 8.2 m optical/infrared telescope constructed during 1991–1999 and has been operational since 2000 on the summit area of Maunakea, Hawaii, by the National Astronomical Observatory of Japan (NAOJ). This paper reviews the history, key engineering issues, and selected scientific achievements of the Subaru Telescope. The active optics for a thin primary mirror was the design backbone of the telescope to deliver a high-imaging performance. Adaptive optics with a laser-facility to generate an artificial guide-star improved the telescope vision to its diffraction limit by cancelling any atmospheric turbulence effect in real time. Various observational instruments, especially the wide-field camera, have enabled unique observational studies. Selected scientific topics include studies on cosmic reionization, weak/strong gravitational lensing, cosmological parameters, primordial black holes, the dynamical/chemical evolution/interactions of galaxies, neutron star mergers, supernovae, exoplanets, proto-planetary disks, and outliers of the solar system. The last described are operational statistics, plans and a note concerning the culture-and-science issues in Hawaii.

Straightness metric for measurement of turbulence-induced distortion in long-range imaging.

Algorithms used for mitigation of the effects of atmospheric turbulence on video sequences often rely on a process for creating a reference image to register all of the frames. Because such a pristine image is generally not available, no-reference image quality metrics can be used to identify frames in a sequence that have minimum distortion. Here, we propose a metric that quantifies image warping by measuring image straightness based on line detection. The average length of straight lines in a frame is used to select best frames in a sequence and to generate a reference frame for a subsequent dewarping algorithm. We perform tests with this metric on simulated data that exhibits varying degrees of distortion and blur and spans normalized turbulence strengths between 0.75 and 4.5. We show, through these simulations, that the metric can differentiate between weak and moderate turbulence effects. We also show in simulations that the optical flow that uses a reference frame generated by this metric produces consistently improved image quality. This improvement is even higher when we employ the metric to guide optical flow that is applied to three real video sequences taken over a 7 km path.

A real-time lucky imaging algorithm based on Fourier transform and its implementation techniques

Abstract Lucky imaging is a high-angular-resolution astronomical image reconstruction technique which can effectively reduce or remove the effects of atmospheric turbulence. There have been successful real-time lucky imaging algorithms and corresponding implementations in the spatial domain. Although the results of lucky imaging algorithms in the frequency domain are better than in the spatial domain, until now there has been no corresponding real-time algorithm and implementation due to its complexity. This paper proposes a real-time lucky imaging algorithm based on Fourier transform that features real-time processing and dynamic updating and display. The algorithm is implemented on a small- or medium-scale field programmable gate array (FPGA) development board. In this system, a two-dimensional fast Fourier transform of the image is carried out by means of dimension reduction. In order to achieve real-time lucky imaging in the frequency domain, the sum of the squares of the real and imaginary parts are used to replace the amplitude for data selection, and the method of image grouping is used to speed up the data selection. Compared with other real-time lucky imaging algorithms in the spatial domain, this algorithm shows great advantages in image quality. Moreover, the system can carry out real-time lucky imaging processing in the frequency domain for more than 10000 frames of an original image of 512 × 512 pixels continuously, and dynamic display with an image of 128 × 128 pixels. The experimental results illustrate the validity of the proposed algorithm and the feasibility of the proposed implementation techniques for the FPGA-based algorithm.

Influence of Kerr nonlinearity on propagation characteristics of twisted Gaussian Schell-model beams.

The analytical propagation formulae of twisted Gaussian Schell-model (TGSM) beams through nonlinear Kerr media are derived. It is found that a TGSM beam is less sensitive to Kerr nonlinearity than a Gaussian Schell-model (GSM) beam. Furthermore, the propagation characteristics of TGSM beams with stronger twist and worse spatial coherence are less affected by Kerr nonlinearity. The self-focusing effect enhances the beam twist, but degrades the beam spatial coherence. In the atmosphere (one kind of self-focusing media), a TGSM beam has greater resistance to self-focusing effects and atmospheric turbulence effects than a GSM beam or an ideal Gaussian beam.

Design of Photoelectric Detection System Used for Atmospheric Microdroplets and Modulation Transfer Function Model of Gaussian Beam

Atmospheric corrosion is an electrochemical corrosion process of metal materials in the atmospheric environment. Microdroplet phenomenon is an experimental phenomenon in the early stage of atmospheric corrosion, which is closely related to atmospheric corrosion. Photoelectric detection of microdroplet phenomenon has become a technical means to study atmospheric corrosion. A set of microdroplet photoelectric detection systems with small size, high integration, and low detection limit is designed and constructed. First, the microdroplet optical chip is designed and fabricated. The microdroplet chip with a single function is replaced by the microdroplet chip with optical microlenses, and the miRNA is detected on the photoelectric platform of the optical chip. The optical fiber is adopted to replace the traditional objective lens to excite the optical path, reduce the volume of the spatial optical path, and realize the miniaturization of the photoelectric detection system. Based on this system, the nonlinear relationship between atmospheric turbulence effect and atmospheric optical parameters is analyzed with Gaussian beams propagating in the atmosphere. In the experiment, the detection range of the photoelectric detection system is 5 nM~100 nM, and the linear correlation is 0.9958. The signals of Cy3 and Cy5 are detected by two groups of photomultiplier tube (PMT) to realize the detection of miRNA. Microlenses enhance the intensity of the photoelectric signal and reduce the detection limit. Microdroplets are used to encapsulate miRNA, reduce the amount of detection material, and solve the degradation of detection material by RNase. The modulation transfer function model of Gaussian beam is established with the assistance of the photoelectric detection system, which is suitable for the case that cannot be ignored in Kolmogorov turbulence under the action of internal/external scale.

Design and experimental demonstration of 8-QAM coherent free-space optical communication using amplitude compensation and phase recovery.

In this paper, a new, to the best of our knowledge, scheme is proposed to mitigate the atmospheric turbulence effect in coherent free-space optical (FSO) communications with 8-quadrature amplitude modulation by employing amplitude compensation and phase recovery. The amplitude compensation and phase recovery algorithm in the scheme can significantly improve system performance without acquiring instantaneous channel state information or probability density function of a turbulence model. Numerical studies show that the bit error rate of the proposed scheme is four orders of magnitude lower than that of the system without any algorithm over a lognormal turbulence channel with normalized standard deviation of irradiance σ=0.25 and phase noise with normalized variance σϕ2=0.07, when the frequency offset fo=20MHz, combined linewidth Δf=10kHz, and average signal-to-noise ratio γ=20dB. Experiments are also carried out to investigate the performance of the scheme, and the results prove its superiority. Hence, this scheme can contribute to practical realization of the FSO system.

Using Machine Learning Algorithms for Accurate Received Optical Power Prediction of an FSO Link over a Maritime Environment

The performance prediction of an optical communications link over maritime environments has been extensively researched over the last two decades. The various atmospheric phenomena and turbulence effects have been thoroughly explored, and long-term measurements have allowed for the construction of simple empirical models. The aim of this work is to demonstrate the prediction accuracy of various machine learning (ML) algorithms for a free-space optical communication (FSO) link performance, with respect to real time, non-linear atmospheric conditions. A large data set of received signal strength indicators (RSSI) for a laser communications link has been collected and analyzed against seven local atmospheric parameters (i.e., wind speed, pressure, temperature, humidity, dew point, solar flux and air-sea temperature difference). The k-nearest-neighbors (KNN), tree-based methods-decision trees, random forest and gradient boosting- and artificial neural networks (ANN) have been employed and compared among each other using the root mean square error (RMSE) and the coefficient of determination (R2) of each model as the primary performance indices. The regression analysis revealed an excellent fit for all ML models, indicative of their ability to offer a significant improvement in FSO performance modeling as compared to traditional regression models. The best-performing R2 model found to be the ANN approach (0.94867), while random forests achieved the most optimal RMSE result (7.37).

On secrecy analysis of wireless-powered relaying FSO-RF systems

We investigate the physical-layer security performance for a wireless-powered relaying mixed free-space optical-radio frequency (FSO-RF) system. In this system, a source transmits optical information to a relay (R), which decodes the received signal with a decode-and-forward protocol, converts it into an RF signal, and forwards it to a destination in the presence of an eavesdropper. Furthermore, we also assume that R has no extra power supply, and it only relies on harvesting energy from a power beacon to forward the information. Assuming that the FSO and all of the RF channels experience gamma-gamma and Rayleigh fading, the closed-form analytical expressions for secrecy outage probability and the probability of strictly positive secrecy capacity are derived. In addition, the effects of atmospheric turbulence, pointing errors, and detection techniques on the system performance are analyzed. Finally, we perform Monte Carlo simulations to verify the accuracy of the derived expressions.

Capacity and BER analysis of BCH and LDPC coded FSO communication system for different channel conditions

Free space optical (FSO) communication is of paramount importance for the design of next-generation 5G+ wireless networks. These are, however, susceptible to degrading effects of atmospheric turbulence. The use of error correcting codes (ECCs) can mitigate the effects of fading caused by atmospheric turbulence. We evaluate the performance of uncoded and coded FSO communication system, where we use Bose Chaudhuri Hocquenghem (BCH) and low-density parity-check codes for the coded system. Both the systems under consideration are based on intensity-modulation and direct detection. We investigate the performance of a coded communication system under various channel conditions, viz., slow fading and fast fading gamma–gamma channels for different turbulence regimes and subsequently, obtain the coding gain. We derive the expressions of channel capacity for a fast fading channel and outage capacity for a slow fading channel. We conclude that ECCs deteriorate the bit error rate performance in a slow fading channel. However, for this kind of channel, the combination of ECCs with an interleaver reduces the burst errors and improves the performance which leads to positive coding gain. In a fast fading channel, coding gain is not much and it remains almost same in different turbulence conditions. The analytical results are supported by simulation study.

Method for Measuring the Second-Order Moment of Atmospheric Turbulence

The turbulence moment of order m (μm) is defined as the refractive index structure constant Cn2 integrated over the whole path z with path-weighting function zm. Optical effects of atmospheric turbulence are directly related to turbulence moments. To evaluate the optical effects of atmospheric turbulence, it is necessary to measure the turbulence moment. It is well known that zero-order moments of turbulence (μ0) and five-thirds-order moments of turbulence (μ5/3), which correspond to the seeing and the isoplanatic angles, respectively, have been monitored as routine parameters in astronomical site testing. However, the direct measurement of second-order moments of turbulence (μ2) of the whole layer atmosphere has not been reported. Using a star as the light source, it has been found that μ2 can be measured through the covariance of the irradiance in two receiver apertures with suitable aperture size and aperture separation. Numerical results show that the theoretical error of this novel method is negligible in all the typical turbulence models. This method enabled us to monitor μ2 as a routine parameter in astronomical site testing, which is helpful to understand the characteristics of atmospheric turbulence better combined with μ0 and μ5/3.

Experimental demonstration of OAM-based transmitter mode diversity data transmission under atmosphere turbulence.

Twisted light carrying orbital angular momentum (OAM), which features helical phase front, has shown its potential applications in diverse areas, especially in optical communications. For OAM-based free-space optical (FSO) links, a significant challenge is the power fading induced by atmospheric turbulence. In this paper, we experimentally demonstrate the mitigation of atmospheric turbulence effects with an OAM-based transmitter mode diversity scheme. By designing multi-OAM phase patterns, we successfully generate multiple OAM modes (OAM-1,0,1, OAM+2,+3,+4, OAM+5,+6,+7) carrying the same data stream for transmitter diversity without adding system complexity. An intensity-modulated direct-detection (IM-DD) system with 39.06 Gbit/s discrete multi-tone (DMT) signal is employed to confirm the feasibility of the OAM-based transmitter mode diversity scheme under atmosphere turbulence. The obtained experimental results show that the received power fluctuation and average bit-error rate (BER) are decreased under moderate to strong turbulence compared to the traditional single OAM mode transmission. In addition, the required transmitted power at 10% interruption probability is relaxed by nearly 2 dB under moderate to strong turbulence.

Performance of Vehicular Visible Light Communications under the Effects of Atmospheric Turbulence with Aperture Averaging.

In this paper, we investigate the performance of a vehicular visible light communications (VVLC) link with a non-collimated and incoherent light source (a light-emitting diode) as the transmitter (Tx), and two different optical receiver (Rx) types (a camera and photodiode (PD)) under atmospheric turbulence (AT) conditions with aperture averaging (AA). First, we present simulation results indicating performance improvements in the signal-to-noise ratio (SNR) under AT with AA with increasing size of the optical concentrator. Experimental investigations demonstrate the potency of AA in mitigating the induced signal fading due to the weak to moderate AT regimes in a VVLC system. The experimental results obtained with AA show that the link’s performance was stable in terms of the average SNR and the peak SNR for the PD and camera-based Rx links, respectively with <1 dB SNR penalty for both Rxs, as the strength of AT increases compared with the link with no AT.

RSSI Probability Density Functions Comparison Using Jensen-Shannon Divergence and Pearson Distribution

The performance of a free-space optical (FSO) communications link suffers from the deleterious effects of weather conditions and atmospheric turbulence. In order to better estimate the reliability and availability of an FSO link, a suitable distribution needs to be employed. The accuracy of this model depends strongly on the atmospheric turbulence strength which causes the scintillation effect. To this end, a variety of probability density functions were utilized to model the optical channel according to the strength of the refractive index structure parameter. Although many theoretical models have shown satisfactory performance, in reality they can significantly differ. This work employs an information theoretic method, namely the so-called Jensen–Shannon divergence, a symmetrization of the Kullback–Leibler divergence, to measure the similarity between different probability distributions. In doing so, a large experimental dataset of received signal strength measurements from a real FSO link is utilized. Additionally, the Pearson family of continuous probability distributions is also employed to determine the best fit according to the mean, standard deviation, skewness and kurtosis of the modeled data.

Developing certification procedures for quiet supersonic aircraft using shaped sonic boom predictions through atmospheric turbulence

New sonic boom noise certification procedures for quiet supersonic aircraft are in development under the International Civil Aviation Organization for a proposed standard that could enable supersonic overland flight. Although these low-boom aircraft do not exist yet, the procedures can be developed and evaluated using existing empirical and simulation datasets. While prior analyses used N-wave datasets to exercise various proposed procedures, these evaluations were limited because the effects of atmospheric turbulence on N-waves do not necessarily generalize to shaped sonic booms. The analysis is therefore extended to include shaped sonic boom predictions through several atmospheric turbulence conditions, such as different atmospheric boundary layer heights, using three different numerical methods. Six noise metrics (PL, ASEL, BSEL, DSEL, ESEL, and ISBAP) are calculated for the ground waveform predictions and compared to metrics for predictions performed under quiescent standard day conditions. Methods for validating prediction tools or correcting for the atmospheric effects are discussed, and ideas for robust methods for implementation in certification procedures are proposed.

Optimization and Representativeness of Atmospheric Chemical Sampling by Hovering Unmanned Aerial Vehicles Over Tropical Forests

AbstractAtmospheric chemical species play critical roles in ecosystem functioning and climate, but spatially resolving near‐surface concentrations has been challenging. In this regard, hovering unmanned aerial vehicles (UAVs) represent an emerging technology. The study herein provides guidance for optimized atmospheric sampling by hovering copter‐type UAVs. Large‐eddy simulations are conducted for species having chemical lifetimes ranging from reactive (i.e., 102 s) to long‐lived (i.e., 108 s). The case study of fair‐weather conditions over an equatorial tropical forest is used because of previous UAV deployments in this region. A framework is developed of influence length and horizontal shift of upwind surface emissions. The framework quantifies the length scale of the contribution of upwind forest emissions to species concentrations sampled by the downwind hovering UAV. Main findings include the following: (1) sampling within an altitude that is no more than 200 m above the canopy is recommended for both high‐ and intermediate‐reactivity species because of the strong decrease in species concentration even in a highly turbulent atmosphere; (2) sampling durations of at least 5 and 10 min are recommended for intermediate‐ and high‐reactivity species, respectively, because of the effects of atmospheric turbulence; and (3) in the case of heterogeneity of emissions across the underlying landscape, maximum recommended altitudes are presented for horizontal sampling strategies that can resolve the variability in the landscape emissions. The coupled effects of emission rate, wind speed, species lifetime, turbulence, and UAV sampling duration on influence length must all be considered for optimized and representative sampling over forests.

Improved field performance of Fourier telescopy in real atmosphere turbulence using new phase closure iteration for object spatial spectrum reconstruction

Fourier telescopy is a remote imaging method that generates several interfering fringes from a coherent laser transmitter array. In this technique, the spatial spectrum of an object is acquired by sweeping fringes of different spatial frequencies and directions on the object. Most traditional triplet phase closure iterations are accomplished by three beams and are used to alleviate the phase aberration induced by atmospheric effects. For reducing the effect of scintillation, which is an atmospheric turbulence effect, we propose a phase closure method wherein the complex amplitude of four beams are combined in iteration. We evaluated the performance of the proposed method both theoretically and experimentally. We conducted the field experiments at 200 m and 1.2 km. The results demonstrated that the proposed phase closure method performs well in enhancing the image quality in a multibeam array when there is scintillation in the atmosphere.

Outage Performance for Optical Feeder Link in Satellite Communications With Diversity Combining

This letter investigates the outage performance of an optical feeder link in satellite communications, where the gateway is equipped with multiple apertures and employs diversity combining schemes to mitigate the effect of atmosphere turbulence. In particular, by assuming that the optical link undergoes the Málaga fading, we first derive a closed-form expression for the outage probability (OP) of the considered system with selection combining (SC). Subsequently, when equal gain combining (EGC) is utilized, we present an approximate yet accurate probability density function expression for the sum of independent and identically distributed Málaga random variables. Next, we derive an OP expression for the considered system with EGC. To gain more insights, we further obtain diversity order for both SC and EGC schemes. It is found that the diversity order for both schemes is related to the number of receive apertures and the effective number of large-scale cells of the scattering process.

Machine-Learning Based Equalizers for Mitigating the Interference in Asynchronous MIMO OWC Systems

The error performance of the optical wireless communication (OWC) link suffers from the effects of atmospheric turbulence and pointing errors. The multi-input-multi-output (MIMO) system can combat the damage by transmitting diverse replicas of symbols to the receivers, i.e. the spatial diversity. However, different delays between the transceivers can introduce inter-symbol interference (ISI) which degrades the system performance. The delays during transmission are mainly caused by placing locations and optical path differences. This article proposes algorithms to mitigate the ISI based on machine learning techniques, including both neural networks and the genetic algorithm. In the case of multi-input-single-output (MISO) system, we propose an algorithm based on a bidirectional long short-term memory (LSTM) recurrent neural network (RNN), which works as an equalizer. In the case of single-input-multiple-output (SIMO) system, additional delayers are utilized to align the signals in different apertures. The problem of deducing the values of the delayers is considered as seeking the minimum value in a high-dimensional space. With the help of the genetic algorithm, optimal values of the delayers are maintained, which is named as GAD (genetic algorithm-based delayers). In a more general MIMO case, the GAD and LSTM equalizers are further combined to deal with the ISI issue in the asynchronous MIMO OWC systems, (i.e. GAD-LSTM). Both experimental and simulation results show the remarkable performance improvement of the proposed method over conventional methods.