Random Phase Screen Research Articles

Investigation of the transmission characters of Ince-Gaussian beams in oceanic turbulence

Abstract We focus on the transmission characteristics of InceGaussian (IG) beams in oceanic turbulence which is simulated by the random phase screen method. The transmission characteristics of IG beams with different distances
and turbulence intensities are also shown in the experiments. The impacts of
the temperature and salinity in oceanic turbulence are discussed in detail. The
experimental results show that the IG beam maintains the intensity distribution well
after transmission. In addition, compared with the others’ structured beams, the IG
beams show better robustness in oceanic turbulence with different salinity, turbulence
intensity and suspended solids. Our work paves the way for leveraging IG beams in
underwater optical communication.

Propagation properties of a partially coherent electromagnetic hyperbolic-sine-Gaussian vortex beam through anisotropic atmospheric turbulence.

Using the extended Huygens-Fresnel principle and the Rytov approximation, the analytical formula for the propagation of a partially coherent electromagnetic hyperbolic-sine-Gaussian vortex beam (PCEShVB) in anisotropic atmospheric turbulence has been theoretically derived. Detailed studies have been conducted on the evolution characteristics of the average intensity, the degree of coherence (DOC), and the degree of polarization (DOP) of the beam in turbulence. The results show that during propagation, the intensity distribution of the beam will exhibit a spiral structure, and the overall distribution of the light spots will rotate in a direction related to the sign of the topological charge. The DOC distribution of PCEShVB will display a pattern reminiscent of beam interference fringes with an increase in propagation distance, with the number of "interference fringes" greatly impacted by the hyperbolic sine parameter. Furthermore, PCEShVB with a large initial coherent length and hyperbolic sine parameter will increase the degree of separation of the spots and yield a large DOP. Finally, for the validation of the theoretical findings, the random phase screen method was employed to simulate the propagation of PCEShVB through anisotropic atmospheric turbulence. The studies revealed a consistent alignment between the simulation results and the theoretical predictions.

A generalized and comprehensive slant channel modeling method for underwater wireless optical communication and its system performance analysis

The underwater wireless slant optical link is a more generalized type of communication links compared with vertical and horizontal links. Since several oceanic factors including temperature and salinity variations with the seawater depth, the performance of the slant link for the underwater wireless optical communication (UWOC) will be overestimated under the assumptions of constant temperature and salinity. To this end, this paper proposes a model of slant link considering the depth-dependent and inclination-angle-dependent turbulence and scattering that are the main causes of multipath fading for UWOC. Specifically, to model the turbulence-induced fluctuations of light fields, the random phase screen method is exploited, where the power spectrum of refractive index is calculated using the oceanic turbulence optical power spectrum (OTOPS) model and the light fields at the receivers are derived by the angular spectrum method. To emulate the scattering-induced divergence, the scattering phase function is applied, where the motion directions and coordinates of light spots propagating among phase screens are deduced using Rodrigues’ Rotation Formula. Simulation results demonstrate the effects of the depths and inclination angles of transmitters on the performance of UWOC by analyzing the scintillation index, channel impulse response, and BER (bit error rate).

Anisotropic ionospheric scintillation in weak scattering regime

Transionospheric radio signals might undergo random modulations of their amplitude and phase caused by scattering on irregular structures in the ionosphere. These scintillation phenomena are highly anisotropic, depend on local geomagnetic field configuaration and on the relative position of the signal receiver and scattering irregularity. We derive analytical expressions of anisotropic amplitude and phase scintillation indices using the model of a thin random phase screen. These results extend the classical derivations of [C. Rino, Radio Sci. 14, 1135 (1979)] to a larger domain of applicability including very slant propagation links of the radio wave signals. The derived generalization is based on the assumption that the ionospheric scattering layer has a spherical symmetry as opposed to the original Rino’s assumption of a plan-parallel ionospheric layer. The derived scintillation indices have the simpler analytical form, determine the regions of enhanced scintillation by taking into account the finite curvature of the Earth and of the ionospheric shell, and are divergence-free for large zenith angles of propagation links. For the illustration we discuss the geometric enhancement of scintillation for communication links via a geostationary beacon satellite over the equator.

Ring Pearcey vortex beam dynamics through atmospheric turbulence

The subject of free space optical communication (FSO) with an optical beam carrying orbital angular momentum (OAM) has attracted a great deal of research attention over the last two decades. Efforts to understand, model, and execute communication links through a turbulent atmosphere with OAM beams have gained particular importance. In this regard, different types of shape-preserving beams, which can withstand turbulences of varying strengths, have been proposed and studied. In this paper, we present a numerical investigation of the propagation characteristics of a ring Pearcey vortex beam (PVB) through turbulent atmosphere. The study details moderate as well as strong atmospheric turbulences. The modified von Kármán model has been relied on to model random phase screen. In moderate turbulence, the ring PVB preserved its singularity. In strong turbulence, the ring PVB preserved its singularity for short propagation distances but lost its singularity at longer propagation distances. We found that, upon increasing the value of a topological charge (l), the aperture averaged scintillation index (SI) increases. We calculated the aperture averaged SI for different truncation factors and noticed that the ring PVB with a truncation factor b=0.1 performed better in stronger turbulence. In moderate turbulence, the aperture averaged SI performed better for shorter propagation distances and relatively larger truncation factors. Further, we calculated the aperture averaged SI for a spatially chirped ring PVB, finding that aperture averaged SI improved largely for the negatively chirped ring PVB. Further, on comparing the aperture averaged SI of the ring PVB and ring Airy vortex beam (AVB), it has been noticed that, in strong turbulence, the ring PVB exhibited better aperture averaged SI. Additionally, we have calculated the beam wander for the ring PVB and ring AVB, finding that the ring PVB demonstrates better beam wander.

Investigation of Oceanic Turbulence Random Phase Screen Generation Methods for UWOC

Underwater wireless optical communication (UWOC) has recently gained great research interest due to its capability of transmitting data underwater with high data rate and low latency. However, oceanic turbulence seriously degrades the optical signal quality and hence the performance of practical UWOC systems. Establishing more accurate and efficient phase screen models is in demand for studying the oceanic turbulence effect. In this paper, techniques for generating underwater random phase screens are studied and supplemented. A promising hybrid method combining sparse spectrum and Zernike polynomials methods is proposed and investigated, which generates phase screens with improved accuracy and efficiency.

Atmospheric Turbulence Effects on the Performance of Orbital Angular Momentum Multiplexed Free-Space Optical Links Using Coherent Beam Combining

Atmospheric effects including absorption and scattering, and turbulence could introduce signal power loss and severe mode crosstalk for the orbital angular momentum (OAM)-based free-space optical communication (FSOC). Therefore, it is of great significance to simultaneously increase signal power and mitigate mode crosstalk. In this paper, for the OAM beam from a coherent laser array with a discrete vortex (CLA-DV) based on coherent beam combining, we investigate its propagation characteristics by employing theoretical derivation and the random phase screens simulation in atmospheric propagation, respectively. The probability density and OAM spectrum are given and compared for CLA-DV and Gaussian vortex beam. The results demonstrate that the Gaussian vortex beam exhibits smaller mode crosstalk under weak atmospheric turbulence conditions, while CLA-DV shows a good performance on crosstalk mitigation for strong atmospheric turbulence conditions in long-distance links. Furthermore, with a specially designed radial phase-locked Gaussian laser array composed of two orthogonal polarized coherent laser arrays carrying different OAM states, a scheme of optical communication system possessing simultaneously polarization-division multiplexing and OAM multiplexing is proposed. The normalized energy weight matrices of all 16 non-zeroth-order OAM modes are numerically calculated. To verify the feasibility of the proposed scheme, the performance of an eight-bit grayscale Lena image facing various atmosphere turbulences is evaluated. The quality of transmitted images becomes worse with the turbulence strength and transmission distance increase, which is confirmed by the trend of average optical signal error rates. This work will provide theoretical insight for improving the performance of OAM-based FSOC under scattering conditions.

Scintillation Characteristics of Gaussian Vortex Beam Propagation in Turbulent Flume

Aiming at the low sensitivity of weak turbulent wake detection of underwater vehicles, studied the laser propagation characteristics in underwater weak turbulence, and then proposed a method based on the statistical characteristics of the scintillation peak of Gaussian vortex beam for detecting the weak turbulence fluctuation. According to the refractive index power spectrum of turbulent flume, the intensity distribution images of Gaussian vortex beam propagating through turbulent flume are simulated by using the random phase screen method, and the scintillation characteristics of Gaussian vortex beam under different conditions are studied. For the simulation results, under the weak turbulence conditions with low refractive index structure coefficient, the average scintillation indexes near the peak value of spot scintillation indexes are obviously higher than those of the whole image. Under the condition that the temperature difference between hot plate and cold plate, between hot plate and water are about 6.4°C and 5.8°C, respectively, the scintillation indexes of the laser propagation intensity image under different temperature gradient (turbulence intensity) are calculated in the experiment. The results show that, the mean values of the scintillation indexes near the spot scintillation peak fluctuate around 1.02 and 1.07, and the mean values in whole image fluctuate around 0.58 and 0.68, respectively, when the temperature gradient is small, which verify the effectiveness of the statistical characteristics of the scintillation peak in detecting weak turbulence.

Analysis of atmospheric turbulence and vibration impact on the performance of satellite–ground laser communication beam

AbstractWith 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.

First-order statistics of intensity and phase in Laguerre-Gauss speckles.

Laguerre-Gaussian (LG) beams are characterized by an azimuthal index or topological charge (m), associated with the orbital angular momentum, and by a radial index (p), which represents the number of the rings in the intensity distribution. We present a detailed, systematic study of the first-order phase statistics of the speckle fields created when LG beams of different order interact with random phase screens with different optical roughness. The phase properties of the LG speckle fields are studied in both the Fresnel and the Fraunhofer regimes using the equiprobability density ellipse formalism such that analytical expressions can be derived for the phase statistics.

Spatiotemporal characteristics of dynamic speckle from a 3D target in atmospheric turbulence

Spatiotemporal correlation function is the basic characteristic of dynamic laser speckle and the basis of various applications. The correlation function of the speckle intensity from a 3D target in turbulent atmosphere is derived based on the model of random phase screen and Fresnel Kirchhoff Diffraction Formula, and a fast algorithm based on FFT is developed. The particularity of dynamic speckle in turbulence is numerically analyzed and discussed. The results show that the speckle intensity fluctuates at two independent scales both in space and time domain, which are affected by target size and atmospheric turbulence respectively. In particular, the time scale caused by turbulence is also affected by the target translating velocity and the wind direction. The theory and algorithm developed in this paper can play important roles in applications of laser speckle such as remote detection in atmospheric environment.

Study on Propagation Properties of Orbital Angular Momentum Modes in Plasma Sheath Turbulence for Power-Exponent-Phase Airy Beams

The theory of random phase screen is adopted to establish the propagation model of power-exponent-phase airy (PEPA) beams in plasma sheath turbulence (PST) conditions. The propagation model can determine the changes in intensity, phase, and orbital angular momentum (OAM) mode distribution of PEPA beams under PST. Simulation results reveal that PEPA beams in multiple OAM modes at the source field can better suppress PST than the conventional vortex beams because the influence of turbulence is dispersed in different OAM modes. Moreover, compared with power-exponent-phase Laguerre–Gaussian (PEPL) beams, PEPA beams are better suited for transmission under plasma turbulence because of their self-focusing and self-healing properties. The results provide a theoretical basis for studying communication problems during PST.

Bit Error Rate Analysis in Atmospheric Turbulence Channel of Synthetic Double Vortex Beam

Random phase screen method was used to simulate the light intensity and phase distribution of the synthesized double vortex beams transmitted in atmospheric turbulence given the light intensity characteristics and phase characteristics of double Laguerre–Gaussian beams (DLGB) generated by coaxial superposition. Results show that the scintillation coefficient decreases initially, and then increases with the topological charge difference of the double vortex beam, and the topological charge difference is related to the number of split spots. When the number of split spots is small, the intensity distribution is concentrated, exhibiting a good penetrating effect on atmospheric turbulence.

Modelling the intensity distribution of underwater optical wave propagation in the presence of turbulence and air bubbles

Objectives:The objective of this paper is to derive a model for the irradiance distribution of underwater optical waves propagation in the combined presence of turbulence and air bubbles. The proposed distribution is based on modelling the medium index of refraction as a shot noise process and using the Rytov solution to the stochastic wave equation. Since the oceanic turbulence spectrum is different than the atmospheric spectrum, recent studies show that the well-known probability density functions poorly describe optical wave propagation underwater in the combined presence of air bubbles and turbulence, and that a mixture of distributions is generally needed. This conclusion, however, does not follow from theoretical investigations. Moreover, the physical aspects of the phenomena, such as the bubble size distribution, are not included in these models, and as such may limit their applicability. The approach introduced in this paper allows for including the environment physical properties directly in to the medium index of refraction. Method:In this paper, intensity distribution model is derived based on Rytov theory of propagation through weak turbulence. The physical processes in effect are directly included by assuming a shot noise index of refraction model. Results:Using the beam propagation method combined with the random phase screen method, the derived distribution is validated by simulation over a large range of turbulence levels and shows excellent agreement with the simulation data. In particular, the qualitative aspect of two peak distribution is modelled by the proposed distribution.

Analysis on the characteristics of vortex beam in field turbulence

In this study, we have investigated the propagation characteristics of single-mode Laguerre–Gaussian (LG) beams under field atmospheric turbulence conditions. We constructed a 1200 meter experimental platform for representing vortex beams in the field. Accordingly, we analyzed the spot changes in an LG beam after transmitted through atmospheric turbulence. Based on the principle of Fresnel diffraction and the random phase screen method, we studied the evolution and spiral spectrum distribution of low-order LG beams transmitted by atmospheric turbulence by numerical simulation. Moreover, we carried out an experimental study regarding the LG beam propagation characteristics mainly based on field experiments. The experimental results indicate that after near-surface turbulent propagation, the LG beam gradually spreads, degree of phase singularity drift becomes larger, and spot size of the Gaussian beam grows faster than that of the LG beam with greater propagation distances. Finally, we concluded that LG beams are not significantly affected by turbulence. It also lays a foundation for the experimental study of vortex beam detection and wavefront correction at the receiving end in multiplexing communication.

Mutual coherence function based topological charge detection in a Gaussian vortex beam optical communication system

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.

Inverse synthetic aperture lidar imaging and compensation in slant atmospheric turbulence with phase gradient algorithm compensation

To systematically study the influence of slant atmospheric turbulence on the inverse synthetic aperture lidar (ISAL) imaging, the atmospheric random phase screen with the modified von Kármán spectrum was numerically simulated via the fast Fourier transform spectral inversion method. Intensity phase distribution and brightness images of the Gaussian beam slant transmission were simulated under different turbulence intensities. Using the Kirchhoff approximation and convolution back-projection algorithm, ISAL images of rough surface (rough square and rough circular plates), rough graphics and rough body with different roughnesses under various turbulence intensities were obtained. The analyses showed that these ISAL images of rough targets can be resolved under weak turbulence but not under moderately strong turbulence. Aiming at the reduction of the resolution of the rough target ISAL images caused by atmospheric turbulence, the phase gradient autofocus (PGA) algorithm was used to improve image focusing and distinguish the shapes of rough targets.

Laser Gaussian beam analysis of structure constant depends on Kolmogorov in turbulent atmosphere for a variable angle of wave propagation

Laser Gaussian beam propagation is important in optical communications; the challenges of the measured constant parameters are varied angle wave propagation, the receiver plan in the slant path of laser Gaussian beam (G) in Kolmogorov gauges, and the effect of the laser in a random medium. At this point, atmospheric turbulence is reputable based on laser Gaussian beam propagation. Moreover, the space propagation of Gaussian beam waves of higher-order Gaussian beam modes has been expanded from the effects of the scintillation index and with different zenith angles between the laser propagator and the detector (receiver) in the ramp path. Furthermore, in the Cartesian coordinate system, the scintillation index in the Kolmogorov standard is evaluated, and it employs two methods; first, Huygens–Fresnel fundamental and, second, random phase screen for work synchronization. Additionally, to analyze the contour parameters of constant construction, the intensity and receiver field is found to compute the high turbulent modes in the atmospheric deposition, which depends on propagation distances. These result in significant reserves in the matlab computation period and serve the imitation for enhancement of the link transmission; therefore, the zenith angle and scintillation index are pretentious by slant beam, and they enhance the performance that makes the comparisons found with fixed structure constant parameters C2n and the propagation distance against the scintillation index.

Research on Orbital Angular Momentum Different Modes Networking Method in Wireless Communication

Orbital angular momentum (OAM) has been a popular topic due to the natural orthogonality of OAM waves with different modes. It is expected to greatly improve the channel capacity of wireless communication systems. From the perspective of networking, this letter proposes an OAM-based different modes networking method (OAM DMN) that sets adjacent cells with OAM carriers of different modes, so as to improve the signal quality and spectrum efficiency of wireless network and better adapt to the high-density base station distribution scenario. This letter firstly analyzes the characteristics of atmospheric channel on OAM waves by regarding turbulent as random phase screen based on Kolmogorov turbulence theory. Combining with stochastic geometry, we get the outage probability function of wireless cells. Then two types of OAM DMN schemes are proposed to test the performance of the network under different channel conditions. Simulation shows that OAM DMN wireless network has lower outage probability compared with other OAM-based and conventional networks and better performance on high-density base station distribution.

Stable propagation of Ince–Gaussian vector beams through atmospheric turbulence

In this paper, we investigate the propagation properties of the Ince–Gaussian​ vector beams through atmospheric turbulence. For this type of generalized fundamental vector beams, the spatial structures of intensity and polarization are demonstrated to have the stability of transmission over kilometers. Analysis of scintillation index and spot centroid drift also supports this view. The von Kármán type index power spectrum and the resulting random phase screens are used to simulate the atmospheric turbulence model. Owing to the strong quantum-like correlation between polarization and the spatial components, the Ince–Gaussian vector beams exhibit stable photon entanglement and high inseparability. In particular, the increasing complexity of light field structure will enhance this stability of structure transmission. Higher-order Ince–Gaussian vector beams have the longer decoherence time and longer identified distance. The fourth-order Ince–Gaussian vector beams have stronger transmission stability than the third-order Ince–Gaussian vector beams. Our work contributes to the understanding of the stable propagation of the vectorial structured light and improves imaging and free-space optical communication in perturbing media.