Effect Of Weak Turbulence Research Articles

Turbulence effects mitigation for twin-photons generated by high order transverse modes

Turbulence presents challenges for free-space light-based protocols. In this work, we investigated the effect of turbulence on the correlations of twin photons prepared in Spontaneous Parametric Down Conversion (SPDC) by a pump beam with different transverse modes. We demonstrate that the strategy of coordinate inversion for mitigating turbulence-induced distortions on transverse coincidence profiles is effective for twin photons generated by HG/LG modes. The conservation of orbital angular momentum in SPDC enables us to encode information in the transverse degrees of freedom of the down-converted photon pairs. This information can have its correlation protected by the coordinate inversion technique against weak turbulence effects in free-space links.

Joint optimization of spatial correlation for space-constrained MIMO underwater optical wireless communication system in weak turbulence

For the space-constrained underwater installations, spatial correlation is an essential factor affecting the reliability of the underwater optical wireless communication (UOWC) system with multiple-input multiple-output (MIMO) technology. In this paper, a joint optimization method for the layout and transmit power allocation of the light-emitting diode (LED)-based UOWC system is proposed and demonstrated under the weak turbulent channel. To analysis the transmission performance of the MIMO UOWC system, a composite channel model incorporating absorption, scattering and weak turbulence effects is established, and the channel spatial correlation characteristics with various system parameters is analyzed based on this model. Among these parameters, we take a 9 × 9 MIMO UOWC system as an example to simulate the communication performance of the optimization method under different turbulence intensities. The simulation results show the optimized system exhibits a channel capacity improvement of 13bps/Hz compared to the original system at the signal-to-noise ratio (SNR) of 20 dB and the same turbulence intensity. Moreover, the SNR of jointly optimized system can achieve a gain of 5 dB at the same bit error rate (BER). By extending the joint optimization algorithm to a 16 × 16 MIMO UOWC system, the optimized SNR can also obtain a gain of 7.5 dB which effectively demonstrates the universality of the proposed joint optimization algorithm.

Weak turbulence effects on different beams carrying orbital angular momentum.

The study of beams carrying orbital angular momentum (OAM) has been of interest for its use in free-space optical communications (FSOC), directed energy applications, and remote sensing (RS). For FSOC and RS, it is necessary to measure the wavefront of the beam to recover transmitted or environmental information, respectively. In this computational study, common OAM beams such as the Laguerre-Gaussian (LG), Bessel-Gaussian (BG), and Bessel beams are propagated through atmospheric turbulence and compared to their Gaussian beam counterpart. The turbulence is simulated using multiple phase screens within the framework of a split-step method. Beam metrics used to quantify beam propagation will include the spatial coherence radius, OAM spectrum, on-axis intensity, spot size, divergence, and on-axis scintillation. Atmospheric turbulence along the path is limited to the weak scintillation limit, where beam parameters can be predicted analytically using the Rytov approximation. The results show that BG beams and multiplexed BG beams retain more OAM information than their LG and Bessel beam counterparts. The LG beam on-axis intensity and on-axis scintillation are seen to be independent of OAM mode. The scintillation of the LG beam is less than a BG, Bessel, and Gaussian beam across low- and high-order OAM modes. Insight into these results is discussed through studying the beam divergence, while the initial spot sizes of the Gaussian, LG, and BG beams are limited to the same spatial extent.

Experimental performance evaluation of weak turbulence channel models for FSO links

Outdoor wireless communication is subject to several outdoor environment conditions. Among them, atmospheric turbulence which happens on a daily basis, introduces signal fading and degrades the system performance. Many channel models have been proposed in the literature to model the turbulence fading in free space optic (FSO) communication systems. In this work, we aim to evaluate the performance of some key proposed channel models for FSO systems under weak turbulence condition. First, an outdoor experimental setup is utilized to measure the turbulence fading and build a dataset. The setup is all-optical, which gives us the advantage of isolating the solar radiation noise during measurements. Second, the obtained dataset is used to investigate the performance of five key FSO channel models claimed to perform well under weak turbulence conditions. The results show that for very weak turbulence with scintillation index (SI) <10−3, four models that claim to work well under such condition failed to fit the dataset. For weak turbulence with SI>10−3, all the models fit the dataset except I-K model. Third, we exploited the obtained dataset to propose a new empirical channel model that has good performance under weak turbulence conditions. Using this model, we studied the outage probability of FSO system under the joint effect of weak turbulence and fog attenuation.

Unified statistical performance of FSO link due to the combined effect of weak turbulence and generalized pointing error with HD and IM/DD

In this paper, we set up the statistical channel model of ship-to-ship (or ship-to-shore) free space optical links considering generalized pointing error and weak turbulence. We combine various pointing error models with weak turbulence and derive the composite probability density functions (PDF) for each case of pointing error model. Also, using the similarity of the composite PDFs, we obtain a unified expression for the composite PDF. Furthermore, we conduct error rate analysis based on both intensity modulation and direct detection (IM/DD) and heterodyne detection (HD). At last, the numerical results confirm that the derived average error rate gives precise prediction on error rate.

Long Term Irradiance Statistics for Optical GEO Satellite Feeder Links: Validation Against Experimental Data

In this paper, optical feeder links to support high throuput satellite communication systems are studied. A methodology for the generation of the received irradiance/power time series for GEO optical feeder links (ground-to-GEO), that reproduces the long term irradiance/power statistics, is presented. In the proposed synthesizer, the atmospheric phenomena under clear sky that degrade the optical signal and the weak turbulence effects, i.e. scintillation, beam wandering and beam spreading, are considered. The proposed methodology takes advantage of the use of stochastic differential equations driven by fractional Brownian motion, for the time series synthesis. The methodology refers to the transmission of a single collimated beam. For the modelling of turbulence effects, Kolmogorov turbulence is assumed and the model is based on Rytov theory under weak fluctuations. Additionally, the proposed model is validated in terms of first order statistics with experimental data from the bi-directional optical GEO ARTEMIS satellite link experiment showing good agreement. Finally, the synthesizer is employed for the generation of fist order statistics for different transmission scenarios.

Impact of Link Parameters and Channel Correlation on the Performance of FSO Systems With the Differential Signaling Technique

We investigate the effects of link parameters and the channel correlation coefficient on the detection threshold, Q-factor, and bit-error-rate (BER) of a free-space optical system employing a differential signaling scheme. In systems employing differential signaling schemes, the mean value of the signal is used as the detection threshold level, provided that differential links are identical or highly correlated. However, in reality, the underlying links are not essentially identical and have a low level of correlation. To show the significance of the link parameters as well as the correlation coefficient, we derive analytical relations describing the effect of weak turbulence and we determine the improvement of Q-factor with the channel correlation. Further, for the same signal-to-noise ratio, we demonstrate that a link with a higher extinction ratio offers improved performance. We also propose a closed-form expression of the system BER. We present experimental results showing improved Q-factor for the correlated channel case compared to the uncorrelated channel.

DUCTING AND TURBULENCE EFFECTS ON RADIO-WAVE PROPAGATION IN AN ATMOSPHERIC BOUNDARY LAYER

The split-step Fourier (SSF) algorithm is applied to simulate the propagation of radio waves in an atmospheric duct. The refractive-index fluctuation in the ducts is assumed to follow a two- dimensional Kolmogorov power spectrum, which is derived from its three-dimensional counterpart via the Wiener-Khinchin theorem. The measured profiles of temperature, humidity and wind speed in the Gulf area on April 28, 1996, are used to derive the average refractive index and the scaling parameters in order to estimate the outer scale and the structure constant of turbulence in the atmospheric boundary layer (ABL). Simulation results show significant turbulence effects above sea in daytime, under stable conditions, which are attributed to the presence of atmospheric ducts. Weak turbulence effects are observed over lands in daytime, under unstable conditions, in which the high surface temperature prevents the formation of ducts.

Analysis of non-Kolmogorov weak turbulence effects on infrared imaging by atmospheric turbulence MTF

The atmospheric turbulence modulation transfer functions (MTF) can be used to describe the degrading effects of atmospheric turbulence on an optical imaging system. In this study, new expressions of long exposure atmospheric turbulence MTF are derived with the generalized atmospheric spectral model for optical waves propagating through non-Kolmogorov weak turbulence with horizontal path. They consider the finite turbulence inner and outer scales, and have a general spectral power law value in the range of 3 to 4 instead of the standard power law value of 11/3. Numerical calculations are conducted to analyze the influence of non-Kolmogorov weak turbulence on the infrared imaging in term of atmospheric turbulence MTF, and results show that the non-Kolmogorov weak turbulence produces less effect on the far infrared imaging.

On Modulational Interaction of the Lower‐Hybrid Drift Oscillations

AbstractThe general nonlinear equation of the third order in field strength for the lower‐hybrid drift waves in inhomogeneous plasma is obtained on the basis of kinetic theory. This equation enables us to describe strong turbulence effects (modulational instability, soliton‐like solutions, etc.) as well as weak turbulence effects (decays, scattering). The investigation of the modulational instability of the lower‐hybrid drift waves is carried out. It is demonstrated that the development of the lower‐hybrid drift wave modulational instability is possible only when the wavevector of the modulational perturbations is less or of the order of the wavevector of the pump wave. The condition on the wave vectors, when the nonlinear response defining the character of the modulational instability is determined by the inhomogeneity effects, is obtained.

On modulational interaction of lower-hybrid waves

The general nonlinear equation of the third order in field strength for the lower-hybrid (LH-)waves is obtained on the basis of kinetic theory. This equation enables us to describe strong turbulence effects [modulational instability (MI), soliton-like solutions, etc.] as well as weak turbulence effects (decays, scattering). The MI of the LH-waves propagating at the angles Θ0 with respect to the external magnetic field such as 1 ≫ cos Θ0 ⩾ (m/M)1/2, where m/M is the electron to ion mass ratio, is investigated. The situation when the MI prevails over the induced scattering and decay effects is shown to be possible even for the case of sufficiently low density of the pump wave energy. The condition on the wave-vectors is obtained for the pump wave of sufficiently low intensity; it is shown that if this condition takes place then the character of the "short-scale" MI (∣Δk ∣ ≫ ∣k0∣ where Δk, k0 are the wavevectors of the density modulation and the pump wave respectively) differs from the character of the parametric instability (∣k0∣ = 0) essentially. The influence of the LH-wave MI on the current drive generation is considered.

Parabolic approximations for coherent low-frequency acoustic propagation through atmospheric turbulence.

A parabolic equation (PE) method for the prediction of coherent low-frequency acoustic propagation through small-scale atmospheric turbulence is presented. Frequency constraints on the applicability of stochastic parabolic approximations are avoided by first averaging the stochastic Helmholtz equation and then applying a parabolic approximation to the resulting deterministic equation. Turbulence effects are incorporated by means of spatially varying effective wave numbers. Comparison of exact solutions in the case of infinite-space propagation demonstrates the advantages and limitations of this approach. A uniform asymptotic expression for the effective wave-number profile in the case of isotropic turbulence is used to develop a half-space PE formulation that is valid in the limit of low-frequency, small-scale inhomogeneity. For anisotropic turbulence that is correlated more strongly in range than height, a modified mean-value theorem for the 2-D Helmholtz operator is used to find the effective wave number. Numerical examples demonstrate that excess attenuation due to the imaginary component of the effective wave number is the primary effect of weak turbulence on coherent low-frequency propagation. [Work supported by the U.S. Army Atmospheric Sciences Laboratory through the United States Military Academy and by NASA.]