Strong Oceanic Turbulence Research Articles

Partially Coherent Off-Axis Double Vortex Beam and Its Properties in Oceanic Turbulence

A partially coherent off-axis double vortex beam (PCOADVB) composed of two off-axis vortices is theoretically presented. The analytical equations of a PCOADVB in oceanic turbulence are presented, and the intensity profiles and the number of coherence vortices of the PCOADVBs are investigated based on the derived expressions. The numerical results show that the intensity profiles of PCOADVBs are determined by the initial topological charges M1 and M2 and the positions of the off-axis vortices (x1d,y1d) and (x2d,y2d). The intensity profiles of PCOADVBs will lose the off-axis ring intensity profile and acquire a Gaussian-like profile as z increases, and stronger oceanic turbulence and a smaller σ can help the PCOADVB evolve into a spot with a Gaussian-like profile faster on propagation. The number of coherence vortices of a PCOADVB in oceanic turbulence will increase on propagation. The PCOADVB may have potential applications in underwater laser sensing and wireless communications.

Propagation of a radially polarized Hermite-cos-Gaussian correlated Schell-model beam in oceanic turbulence

A new beam called radially polarized Hermite-cos-Gaussian correlated Schell-model (RPHCGSM) beam is proposed. The propagation formulae of an RPHcGSM beam in oceanic turbulence are derived, and the propagation properties of such beam are analyzed. The results show that the spectral density distribution of an RPHcGSM beam in free space can be modulated by γ as distance increases; the beamlets will overlap when an RPHcGSM beam propagating through oceanic turbulence, and the stronger oceanic turbulence can accelerate the speed of beamlets evolving into Gauss beam. Moreover, the degree of polarization can become regular distribution and degree of coherence can become Gaussian distribution due to the influence of oceanic turbulence.

Propagation properties of rotationally-symmetric power-exponent-phase vortex beam through oceanic turbulence

In this paper we explored the propagation characteristics of rotationally-symmetric power-exponent-phase vortex beam (RSPEPVB) through the oceanic turbulence. Based on the extended Huygens-Fresnel diffraction integral and the oceanic turbulence theory the theoretical model of RSPEPVBs propagating in oceanic turbulence was established. Then the propagation properties of RSPEPVBs were explored by numerical simulation and the influences of the propagation distance z the rate of dissipation of turbulence kinetic energy per unit mass of fluid ε the temperature-salinity contribution ratio ω and the dissipation rate of the mean-squared temperature χT were discussed. The results illustrated that the intensity of RSPEPVBs propagating through oceanic turbulence weakened and the size of RSPEPVBs diffused with the increasing distance and strong oceanic turbulence of larger values of parameters χT and ω and the smaller value of the parameter ε. Meanwhile the coherence of RSPEPVBs would decrease in stronger oceanic turbulence. Further an experimental setup was demonstrated to confirm that the RSPEPVBs will diffuse in strong oceanic turbulence which was strengthened with the rise of salinity and the increase of propagation distance. Contrarily with the increase of temperature the loss of RSPEPVBs mitigated and the intensity increased gradually. The obtained results may provide a potential application for optical underwater communication and imaging.

Turbulence-resistant self-focusing vortex beams

We consider recently introduced self-focusing fields that carry orbital angular momentum (OAM) [2021 Opt. Lett. 46 2384–87] and in particular, their propagation properties through a turbulent ocean. We show that this type of field is especially robust against turbulence induced degradation, when compared to a completely coherent beam. In moderately strong oceanic turbulence, the self-focusing OAM beam features over five orders of magnitude higher peak intensities at the receiver plane, an ∼80% detection probability for the signal mode, as well as an energy transmission efficiency in excess of 70% over a link of ∼100 m. Counter-intuitively, the focusing properties of such fields may be enhanced with increasing turbulence, causing the mean squared waist to become smaller with greater turbulence strength. Our results demonstrate that certain types of partial coherence may be highly desirable for optical telecommunication employing OAM.

Diffractive deep neural network based adaptive optics scheme for vortex beam in oceanic turbulence.

Vortex beam carrying orbital angular momentum (OAM) is disturbed by oceanic turbulence (OT) when propagating in underwater wireless optical communication (UWOC) system. Adaptive optics (AO) is a powerful technique used to compensate for distortion and improve the performance of the UWOC system. In this work, we propose a diffractive deep neural network (DDNN) based AO scheme to compensate for the distortion caused by OT, where the DDNN is trained to obtain the mapping between the distortion intensity distribution of the vortex beam and its corresponding phase screen representing OT. In the experiment, the distorted vortex beam is input into the DDNN model where the diffractive layers are solidified and fabricated, and the intensity distribution of the modulated light field of the vortex beam can be recorded. The experiment results show that the proposed scheme can extract quickly the characteristics of the intensity pattern of the distorted vortex beam, and the predicted compensation phase screen can correct the distortion caused by OT in time. The mode purity of the compensated vortex beam is significantly improved, even with a strong OT. Our scheme may provide a new avenue for AO techniques, and is expected to promote the communication quality of UWOC system immediately.

Propagation Properties of Multi-Hyperbolic Sine-Correlated Beams in Oceanic Turbulence

As a new partially coherent beam, the propagation properties of the multi-hyperbolic sine-correlated (MHSC) beams in turbulent atmospheres have been studied. But as another important medium, the propagation properties of MHSC beams in oceanic turbulence. This paper has studied these questions in detail. The analytical formulas of spectral density and degree of coherence for the propagation are derived and the numerical simulations are represented. It is found that the intensity patterns of MHSC beams will evolve from dark-hollow profiles into Gaussian profiles caused by oceanic turbulence and will degenerate more rapidly with stronger oceanic turbulence. In addition, the coherence region becomes larger with decreasing in the dissipation rate of turbulence kinetic energy in unit mass liquid or increasing in the relative intensity of temperature and salinity fluctuations, mean square temperature dissipation rate. We also find that the degree of coherence of MHSC beams with a higher-order N will decrease more slowly than those of hyperbolic sine-correlated (HSC) beams.

Effects of adaptive optics on bit error rate of M-ary PPM oceanic optical wireless communication systems with aperture averaging in strong turbulence

Scintillation is the result of oceanic turbulence reducing the bit error rate (BER) performance of oceanic optical wireless communication (OWC) systems. The scintillation, also known as intensity fluctuations, occurs due to the turbulence-induced wavefront deformations. The correction of deformations by adaptive optics (AO) reduces the scintillation effect of turbulence and results in improved BER performance. In this paper, an oceanic OWC (OOWC) system that has a Gaussian laser beam at the transmitter, finite-sized circular aperture at the receiver, employing M-ary pulse position modulation (PPM) and operating in strong oceanic turbulence, is considered. Improvement in the BER performance of the OOWC system is examined with the implementation of AO correction. Comparison of BER performances between the AO and non-adaptive optics OOWC systems is shown by calculating the metric defined. BER of M-ary PPM OOWC links is evaluated over gamma–gamma fading channels. The modified Rytov theory together with the Zernike filter functions is used to find the AO corrected aperture averaged scintillation index where extended Huygens–Fresnel technique is used to obtain the average received signal power.

Aperture Averaged Scintillation of Gaussian Beam in Strong Oceanic Turbulence

Oceanic turbulence caused by salinity and temperature fluctuations underwater affects the characteristics of a transmit laser beam resulting in turbulence-induced intensity fluctuations (i.e., scintillation) at the receiver. Oceanic optical wireless communication (OOWC) systems employ a receiver lens of aperture to focus the collected light onto a photodetector. This way, aperture averaging takes place and the scintillation on the detector is reduced. Using the modified Rytov theory, aperture averaged scintillation of the Gaussian beam passing through strong oceanic turbulence is examined in this study. Effects of Gaussian beam parameters and the turbulence parameters on the aperture averaged scintillation and the aperture averaging factor are illustrated. The scintillation behaviors of the limiting cases of a spherical wave and a plane wave are also reported. Results show that aperture averaged scintillation decreases with increasing the size of the receiver aperture for any turbulence level. The effect of Gaussian size on the aperture averaged scintillation varies depends on the turbulence level. It is also shown that there is a close match between the point scintillation index values obtained from the modified Rytov theory and the conventional Rytov theory in low levels of turbulence.

Average BER performance of rectangular QAM-UWOC over strong oceanic turbulence channels with pointing error

We theoretically analyze the average bit error rate (ABER) performance of underwater wireless optical communication (UWOC) systems using rectangular quadrature-amplitude modulation (QAM) over strong oceanic turbulence channels with pointing error. The strong oceanic turbulence channel is assumed to be a gamma-gamma model. The close-form expression for the ABER is derived by using the Meijer’s G-function. Based on this expression, various effects of system parameters, including QAM scheme, link distance, diameter aperture, ratio of source beam width to aperture radius, and four oceanic turbulence parameters on ABER performance are taken into account. Simulation results show that applying a smaller rectangular QAM dimension and link distance, or a larger aperture diameter and the ratio of beam width to aperture radius can improve the UWOC quality significantly. The contributions of four oceanic turbulence parameters also need to be considered in practical UWOC applications. The results reported in this paper will be benefit to design UWOC systems.

Effect of anisotropy on performance of M-ary phase shift keying subcarrier intensity-modulated optical wireless communication links operating in strong oceanic turbulence

In strong oceanic turbulence, the performance of M-ary phase shift keying subcarrier intensity-modulated optical wireless communication (OWC) links is investigated in terms of the bit-error-rate (BER) by considering the effect of anisotropy of the oceanic channel. To calculate the BER of the OWC link, a gamma–gamma statistical channel model is adopted. The extended Huygens–Fresnel principle and the asymptotic Rytov theory are used to obtain the received optical power and the large-scale and small-scale log-intensity variances, respectively. Our graphical illustrations include the BER versus anisotropic factor for various system parameters such as the modulation order, filter bandwidth, link distance, peak amplitude of each subcarrier and the oceanic turbulence parameters.

Performance Investigation of OAMSK Modulated Wireless Optical System Over Turbulent Ocean Using Convolutional Neural Networks

The adaptive demodulators based on convolutional neural networks (CNNs) are proposed for an M-ary orbital angular momentum-shift keying (OAMSK) modulated underwater wireless optical communication (UWOC) system. The impacts of different modulation orders, oceanic turbulence strengths, seawater types, signal to noise ratios (SNRs), transmission distance, and scales of CNNs are investigated. For the proposed system model, oceanic turbulence and pass loss caused by absorption and scattering are emulated by Monte Carlo (MC) methods. By comparing four CNNs, AlexNet and GoogLeNet inception v4 are adopted as the key part of M -ary OAMSK adaptive demodulating modules. After being trained by 100 000 OAM intensity specimens under unknown levels of oceanic turbulence, the suitable network parameters are achieved. Results show that the average bit error rate performances of both CNN-based demodulators outperform the traditional conjugate demodulator by several orders of magnitude. Moreover, deeper and more complex CNN would be more effective in enhancing the ABER performance under the strong oceanic turbulence condition. Furthermore, for both CNN-based and conjugate demodulators, the ABER of the system with noise will approach the saturation values when the instantaneous SNR is approximately 26 dB larger than the pass loss. This work benefits the design of OAMSK modulation-based UWOC system.

Gamma Gamma强海洋湍流和瞄准误差下水下无线光通信系统的性能研究

Gamma Gamma强海洋湍流和瞄准误差下水下无线光通信系统的性能研究

Average capacity analysis of underwater optical wireless communication links over anisotropic strong oceanic turbulence channels.

The effect of anisotropy on the channel capacity of underwater optical wireless communication (OWC) links operating in strong oceanic turbulence is investigated. We consider a Gaussian beam wave propagating through a turbulent oceanic fading channel whose statistical distribution is modelled by a gamma-gamma function. To numerically calculate the channel capacity of the OWC system, related entities of the propagating beam such as coherence length, received signal intensity, and the scintillation index are formulated. Further, in this way, the received signal-to-noise ratio and fading distribution of the channel are obtained. The channel capacity examinations analyzed in this paper depend on the oceanic turbulence parameters, especially for the anisotropic factor of oceanic turbulence, and also depend on the other system parameters such as wavelength, link distance, noise variance, and the quantum efficiency of the photodetector.

Influence of eddy diffusivity ratio on Gaussian beam under weak to strong oceanic turbulence

ABSTRACTThe oceanic turbulence refraction power spectrum model with a function of the eddy diffusivity ratio is adopted. By using the Rytov method, the scintillation index (SI) is derived using the aperture averaging technique for Gaussian beam under weak to strong oceanic turbulence. Variations of the SI in weak to strong oceanic turbulence are evaluated versus link length and the oceanic turbulence parameters such as the ratio of temperature to salinity contributions to the refractive index spectrum, the rate dissipation of the mean squared temperature, and the rate of dissipation of kinetic energy per unit mass of fluid. Considering the eddy diffusivity ratio of salinity to temperature, the effects of the ratio of temperature to salinity contributions to the refractive index spectrum on the performance of underwater wireless optical communication (UWOC) systems in weak to strong turbulence are analysed.

Properties of partially coherent elegant Laguerre-Gaussian beam in free space and oceanic turbulence

The analytical expression of partially coherent elegant Laguerre-Gaussian (PCELG) beam generated by anisotropic Gaussian Schell-model sources is given. The analytical formulas of CSD for a PCELG beam propagating in free space and oceanic turbulence are derived. The characteristics of average intensity and spectral degree of coherence of a PCELG beam propagating in free space and oceanic turbulence are investigated by using numerical simulations. It is shown that the stronger oceanic turbulence will accelerate the evolution of a PCELG beam propagating in oceanic turbulence, and the influences of coherence length on the spectral degree of coherence will disappear as the propagation distance increases. The findings may be useful in underwater optical communications.

Effect of aperture averaging on mean bit error rate for UWOC system over moderate to strong oceanic turbulence

In moderate to strong oceanic turbulence, a differential phase-shift-keying (DPSK) modulated underwater wireless optical communication (UWOC) system with an aperture receiver is presented. The analytical expressions for mean bit error rate (BER) of UWOC system over moderate to strong oceanic turbulence are derived based on Gamma–Gamma channel model by the help of the Whittaker function. The variations of mean BER are investigated versus modulation methods, beam types, aperture diameters as well as four oceanic turbulence parameters, i.e., the rate of dissipation of kinetic energy per unit mass of fluid, the rate of dissipation of mean-squared temperature, the ratio of temperature to salinity contributions to the refractive index spectrum and the kinetic viscosity.

M-ary phase shift keying-subcarrier intensity modulation performance in strong oceanic turbulence

In strong oceanic turbulence, we investigate the bit error rate (BER) performance of underwater wireless optical communication links by employing phase shift keying subcarrier intensity modulated Gaussian laser beam at the transmitter and positive-intrinsic-negative photodetector having finite sized aperture at the receiver. Using the extended Huygens–Fresnel principle, which is conventionally used to analyze the optical beam propagation through turbulence, we evaluate the optical intensity and corresponding signal power over the receiver aperture. Gamma–gamma statistical model for the received intensity is adopted due to strong oceanic turbulence and the required aperture averaged scintillation for this model is obtained by the use of asymptotic Rytov theory. In our performance investigation, we consider the effects of various oceanic turbulences, modulation, receiver noise type, and the photodetector parameters on the BER performance.

BER of Underwater Wireless Optical Communication Systems with SIMO Detection over Strong Oceanic Turbulence

Abstract A spherical wave propagates through the strong underwater turbulence media is modeled as gamma–gamma random variable in the underwater wireless optical communication (UWOC) systems. To mitigate turbulence-induced fading, spatial diversity over UWOC links is proposed. Furthermore, the exact bit error rate (BER) expressions for both single-input single-output (SISO) and single-input multiple-output (SIMO) UWOC systems with optimal combining based on on–off keying (OOK) modulation are analytically derived. Then the system performance is simulated with various variations of the underwater turbulence, i.e. the rate of dissipation of kinetic energy per unit mass of fluid, the ratio of temperature to salinity contributions to the refractive index spectrum, and the UWOC system link length. The results show that the analytical expressions for describing the system performance are valid and spatial diversity can considerably improve the system performance.

Effect of oceanic turbulence on the visibility of underwater ghost imaging.

We carry out a detailed study on underwater ghost imaging (GI) in oceanic turbulence. We set up a physical model of GI through oceanic turbulence, which includes light-field transmission, and interaction between light field and oceanic turbulence without considering the effects of water absorption and scattering of light. We obtain theoretical expressions for the impulse response function and the visibility of GI in oceanic turbulence based on the power spectrum of the turbulence and the extended Huygens-Fresnel integral. The results show that the quality of GI under the effects of oceanic turbulence is related to the intensity of turbulence and the propagation distance of light. The quality of GI could be maintained at a relatively small distance in strong oceanic turbulence, whereas the quality is degraded dramatically at a relatively long distance in strong oceanic turbulence. We further analyze the quality of GI under various turbulence conditions and over different propagation distances by numerical calculation. Our results provide guidance for the realization of adaptive underwater optical GI over different length scales under the effect of oceanic turbulence.

Properties of a random electromagnetic multi-Gaussian Schell-model vortex beam in oceanic turbulence

A new kind beam called the random electromagnetic multi-Gaussian Schell-model (REMGSM) vortex beam has been introduced. Based on the Huygens–Fresnel principle, the elements of the cross-spectral density matrix of the REMGSM vortex beam propagation in oceanic turbulence have been derived. The average intensity and spectral degree of polarization properties of the REMGSM vortex beam propagating in oceanic turbulence are illustrated and analyzed using numerical examples. The results show that the REMGSM vortex beam propagating in stronger oceanic turbulence will evolve into flat-topped beam and Gaussian-like beam more rapidly as the propagation distance increases.