In this paper, we describe the details of different detection strategies of a communication system using Gaussian vortex beams. These are listed as (a) simultaneous transmission of actual data symbol and reference signals (no multiplexing), (b) transmission of data symbol and reference signals in a wavelength division multiplexed manner, and (c) transmission of data symbol and reference signals in a time-division multiplexed manner. The performance of each one is evaluated for strong turbulence regimes with the help of an appropriately arranged random phase screen setup. It is found that the first two detection strategies work error-free within the source and propagation parameters chosen. In the last detection strategy, performance depends on the transverse wind velocity.

We investigate a topological charge (TC) detection schema for an optical communication system employing Gaussian vortex beam (GVB). In this scenario, the transmitter maps the electrical message symbols to the TCs of GVBs. Thus obtained optical signal propagates in turbulent atmosphere arriving at the receiver, where a detection process is implemented to determine the TC of GVB by correlating the imaginary part of the mutual coherence function (MCF) of the incoming beam against the stored profiles. The feasibility of such a schema is firstly established by examining and comparing the analytical formulation of MCF and that of random phase screen setup. The latter is then used to explore the success rate and boundaries of this particular detection schema. Our results show that the proposed detection schema can operate with a error rate of 5% at a link length of 5.5 km and atmospheric turbulence structure constant of 10 −14 .

Nowadays, the most critical agriculture-related problem is the harm caused in fruit, vegetable, nut, and flower crops by harmful pests, particularly the Mediterranean fruit fly, Ceratitis capitata, named in short as Medfly. Medfly existence in agricultural fields must be monitored systematically for effective combat against it. Special traps are utilized in the field to catch Medflies which will reveal their presence, and applying pesticides at the right time will help reduce their population. A technologically supported automated remote monitoring system should eliminate frequent site visits as a more economical solution. In this paper, a machine learning system that can detect Medfly images on a picture and count their numbers is developed. A special trap equipped with an integrated camera that can take photos of the sticky band where Medflies are caught daily is utilized. Obtained pictures are then transmitted by an electronic circuit containing a SIM card to the central server where the object detection algorithm runs. This study employs a faster region-based convolutional neural network (Faster R-CNN) model in identifying trapped Medflies. When Medflies or other insects stick on the sticky band of the trap, they continue to spend extraordinary effort trying to release themselves in a panic until they die. Therefore, their shape is badly distorted as their bodies, wings, and legs are all buckled. The challenge here is that the machine learning system should detect these Medflies of distorted shape with high accuracy. Therefore, it is crucial to utilize pictures that contain trapped Medfly images that possess distorted shapes for training and validation. In this paper, the success rate in identifying Medflies when other insects are also present is approximately 94% that is achieved by the machine learning system training process, owing to the considerable amount of purpose-specific photographic data. This rate may be seen as quite favorable when compared to the success rates provided in the literature.

We undertake the aperture averaged scintillation derivation of Gaussian vortex beam using Rytov approximation. For this purpose, point-like scintillation is formulated initially and is then converted into the aperture averaging form via overlap procedure. The numeric values delivered by this aperture averaged derivation are plotted side by side against the results obtained from random phase screen simulation. There it is seen that similar to point-like scintillation, the range of validity of the aperture averaged scintillation of Rytov approximation is also limited to weak turbulence range. Incorporating the effective spectrum of spherical wave into the aperture averaged expression demonstrates that it is possible to extend the validity of the aperture averaged derivation based on Rytov approximation to moderate and strong turbulence regimes. Our tests show that additional source beam specific terms are needed to fully adapt the effective spectrum.

Phase fluctuations of vortex beams are examined using the random phase screen setup. By taking Gaussian, Bessel Gaussian and Laguerre Gaussian vortex beams, with the main focus being on Gaussian vortex beam, we investigate the phase fluctuations in a region around on-axis. These phase fluctuations are quantified in terms of phase variance and phase structure function. Variations of phase fluctuations against some of the source parameters, propagation distance and receiver plane radial position are plotted. Our analysis shows that the studied vortex beams have more phase fluctuations on and around the on-axis region in comparison to Gaussian beam. This is reflected into the phase variance of the vortex beams starting approximately aroundπfollowed by a fall. Similar trend applies to the phase structure function which exhibits a rapid increase just past the on-axis point. These observations change however at longer propagation distances where strong turbulence characteristics prevail. Here we observe saturation regimes, whereby all phase variances and structure functions settle around π and 2π respectively.

We describe the details of an optical communication system using Gaussian vortex beams (GVBs). Our main focus will be on the detection strategy. The transmitter encodes the message symbols into the topological charges of the GVBs. Then the receiver implements a detection strategy based on the orthogonality of the GVBs. The graphical results obtained from the related theoretical derivation indicate that this detection strategy has almost no bounds. Thus, it is concluded that the performance of such a system can only be practically limited.

We analyze the scintillation properties of a flat-topped Gaussian vortex beam propagating in a weakly turbulent atmosphere. We utilize a random phase screen model to perform this analysis. We evaluate the scintillation against the changes of topological charge, beam order, operating wavelength, receiver aperture side length, and source size parameters. The results show that using a flat-topped Gaussian vortex beam with a high topological charge causes a reduction in scintillation. Increasing the receiver aperture side length reduces scintillation level. As the source size increases, scintillation decreases. Our results also indicate that a flat-topped Gaussian vortex beam with high topological charges has less scintillation than the fundamental Gaussian beam. We believe that our findings will be useful in improving the performance of next-generation optical communication links and networks. (C) 2019 Society of Photo-Optical Instrumentation Engineers (SPIE)

We study the propagation properties of optical bottle beams in turbulent atmosphere. By allowing the mathematical expression of source plane to cover both the symmetric and asymmetric forms, the beam is propagated through turbulence using random phase screens. On the source plane, the intensity profile of the bottle beam resembles a dark hollow beam with an outside ring for symmetric cases, whereas it becomes divided into two or more separate lobes for the asymmetric cases. During propagation, both symmetric and asymmetric beams concentrate the intensity toward the center, eventually assuming a Gaussian shape, where this process seems to be more rapid for the former beam types. The rising trend of the kurtosis parameter of bottle beams changes to a falling trend when the refractive index structure constant is reduced from 10(-14) to 10(-13) m(-2/3). (C) 2019 Society of Photo-Optical Instrumentation Engineers (SPIE)

We examine the mode coupling in vortex beams. Mode coupling also known as the crosstalk takes place due to turbulent characteristics of the atmospheric communication medium. This way, the transmitted intrinsic mode of the vortex beam leaks power to other extrinsic modes, thus preventing the correct detection of the transmitted symbol which is usually encoded into the mode index or the orbital angular momentum state of the vortex beam. Here we investigate the normalized power mode coupling ratios of several types of vortex beams, namely, Gaussian vortex beam, Bessel Gaussian beam, hypergeometric Gaussian beam and Laguerre Gaussian beam. It is found that smaller mode numbers lead to less mode coupling. The same is partially observed for increasing source sizes. Comparing the vortex beams amongst themselves, it is seen that hypergeometric Gaussian beam is the one retaining the most power in intrinsic mode during propagation, but only at lowest mode index of unity. At higher mode indices this advantage passes over to the Gaussian vortex beam. (C) 2017 Elsevier Ltd. All rights reserved.

In this paper, the probability of error is evaluated for an optical link using the phase distribution correlation of Gaussian vortex beam. This evaluation is implemented in a computer environment, where the turbulent atmosphere is modelled as a series of random phase screens. The transmitted message signal consists of eight Mary levels and is encoded into the topological charge of the source vortex beam. The detection technique employed is based on correlating the aperture confined and topological charge related phase distribution of the received beam with those of the free space equivalents. It is seen that this technique performs well in weak turbulence, degrading as moderate turbulence levels are approached. It is anticipated that the obtained results will be beneficial to optical links incorporating the use of Gaussian vortex beams.

We assess the performance bounds of an optical communication system that uses irradiance profile modulation. This modulation is based on the four different orders of vortex beams. To this end, we find the turbulence induced average irradiance profiles of Gaussian, Bessel-Gaussian and modified Bessel-Gaussian vortex beams on the receiver plane. Each one is then cross correlated against the free space equivalents. Plotting the cross-correlation coefficients, it becomes possible to identify the borders of correct decision and error regions, thus, deduce the performance bounds of such a system. When measured in terms of structure constant, i.e. the turbulence strength and the propagation length being fixed to 3km, it is seen that the error region extends beyond the structure constant values of 10(-13) m(-2/3) and higher. There seem to be some variations with the beam type and the order of the vortex beam. The performance of Bessel-Gaussian vortex beam comes out to be slightly better than the rest.

We study the scintillation behaviour of vectorial vortex beams in strong turbulence region. For this purpose, a list of vortex source beams is prepared. Then their scintillation performances are analysed one by one using the random phase screen approach. The results indicate that there will always be scintillation reductions with increasing values of topological charge, although its effect will diminish as we go towards higher values of the topological charge. The increases in the other specific beam parameters seem to have opposite effect. For the vectorial Hermite Gaussian beam, the use of higher orders will also aid scintillation reductions. It is foreseen that the outcome of this study will be useful for long haul optical communication links.

We investigate the propagation properties of cylindrical sinc Gaussian beam in turbulent atmosphere. Since an analytic solution is hardly derivable, the study is carried out with the aid of random phase screens. Evolutions of the beam intensity profile, beam size and kurtosis parameter are analysed. It is found that on the source plane, cylindrical sinc Gaussian beam has a dark hollow appearance, where the side lobes also start to emerge with increase in width parameter and Gaussian source size. During propagation, beams with small width and Gaussian source size exhibit off-axis behaviour, losing the dark hollow shape, accumulating the intensity asymmetrically on one side, whereas those with large width and Gaussian source size retain dark hollow appearance even at long propagation distances. It is seen that the beams with large widths expand more in beam size than the ones with small widths. The structure constant values chosen do not seem to alter this situation. The kurtosis parameters of the beams having small widths are seen to be larger than the ones with the small widths. Again the choice of the structure constant does not change this trend.

In this paper the results of simulation are presented of multichannel radiation propagation in the atmosphere, and correction for turbulent distortion on the base of the beam phase control is considered. The results demonstrate dependence of correction effectiveness on number of channels and on precision of a reference beam phase reconstruction. Additionally increase of effectiveness is possible with adjustment of amplification in the channels of the optical system, i.e., with the use of amplitude-phase control of radiation.