Scintillation Regime Research Articles

Decoherence of high-dimensional orbital angular momentum entanglement in anisotropic turbulence

The decoherence of high-dimensional orbital angular momentum (OAM) entanglement in the weak scintillation regime has been investigated. In this study, we simulate atmospheric turbulence by utilizing a multiple-phase screen imprinted with anisotropic non-Kolmogorov turbulence. The entanglement negativity and fidelity are introduced to quantify the entanglement of a high-dimensional OAM state. The numerical evaluation results indicate that entanglement negativity and fidelity last longer for a high-dimensional OAM state when the azimuthal mode has a lower value. Additionally, the evolution of higher-dimensional OAM entanglement is significantly influenced by OAM beam parameters and turbulence parameters. Compared to isotropic atmospheric turbulence, anisotropic turbulence has a lesser influence on high-dimensional OAM entanglement.

Fourth-order moments analysis for partially coherent electromagnetic beams in random media

A theory for the characterization of the fourth-order moment of electromagnetic wave beams is presented in the case when the source is partially coherent. A Gaussian–Schell model is used for the partially coherent random source. The white-noise paraxial regime is considered, which holds when the wavelength is much smaller than the correlation radius of the source, the beam radius of the source, and the correlation length of the medium, which are themselves much smaller than the propagation distance. The complex wave amplitude field can then be described by the Itô-Schrödinger equation. This equation gives closed evolution equations for the wave field moments at all orders and here the fourth-order moment equations are considered. The general fourth-order moment equations are solved explicitly in the scintillation regime (when the correlation radius of the source is of the same order as the correlation radius of the medium, but the beam radius is much larger) and the result gives a characterization of the intensity covariance function. The form of the intensity covariance function derives from the solution of the transport equation for the Wigner distribution associated with the second-order wave moment. The fourth-order moment results for polarized waves are used in an application for imaging of partially coherent sources.

Space Diversity Mitigation Effects on Ionospheric Amplitude Scintillation With Basis on the Analysis of GNSS Experimental Data

Ionospheric density irregularities embedded in Equatorial Plasma Bubbles, with scale sizes varying from several hundred kilometers to several tens of meters, may cause amplitude and phase scintillation of transionospheric radio waves, degrading the performance and availability of space-based communication and navigation systems. A recent computer simulation study, based on ionospheric irregularities detected by the Planar Langmuir Probe onboard the Communication/ Navigation Outage Forecasting System satellite, analyzed the mitigation effects from space diversity on amplitude scintillation of transionospheric signals received on the ground. The present work, based on experimental data, will confirm and extend the previous results, indicating, in statistically quantitative terms, how space diversity, effective on uplink and downlink ground-satellite paths, particularly in the strong and saturated scintillation regimes, depends on Ionospheric Pierce Point dip-latitude and distance intervals, as well as on a well-known amplitude scintillation index.

Controlling Factors of Artificial Irregularities Triggered by Chemical Release at Low Latitude Ionosphere

AbstractThe chemical release for trigging instabilities is a challenging topic in the ionosphere since 1970s. It is a helpful tool to both recognize the physical mechanism for modeling space weather and mitigate adverse effects on technological systems. Based on the well‐developed instability simulation, the rate of change of total electron content index (ROTI) theory is employed as a diagnosis method in this paper. It is used to study the instability inducing effects or scintillation controlling effect of chemical release for the first time. The main controlling factors of the chemical release are investigated. It is found that the release amount, release altitude and the initial growth rate of the ambient ionosphere are of great importance to the instability evolution and consequent scintillation effects. First, the plasma bubble induced by chemical release will rise and penetrate to the upper ionosphere, no matter what the ambient growth rate is. However, the ambient growth rate will determine the induced scintillation regime. Second, the release altitude will significantly affect the instability evolving time and scintillation intensity. There only exist a threshold of the release amount for moderate scintillation. Finally, the release amount could dominate the bifurcation process, in which the plasma bubble will deform and extend laterally. It provides an insight into the scintillation effect induced by chemical release by using the ROTI estimation. As a result, this work is a step further for the challenging topic of chemical release, and so as to be helpful for the follow‐up active space experiments.

Decoherence of Higher Order Orbital Angular Momentum Entangled State in Non-Kolmogorov Turbulence

The decay of OAM entanglement in non-Kolmogorov turbulence has been numerically evaluated. In this work, we explore the evolution of OAM entanglement with higher-order OAM mode in the weak scintillation regime. In particular, the results of the numerical evaluation show that the OAM entanglement state with higher value of the azimuthal mode and larger radial quantum number survives over a longer distance. Meanwhile, the beam parameters and turbulence parameters usually have significant influences on OAM entanglement. In addition, it is demonstrated that the effect of turbulence on the OAM entanglement is the most serious when the generalized exponent is around 3.07.

Time and band-resolved scintillation in time projection chambers based on gaseous xenon

We present a systematic study of the time and band-resolved scintillation in xenon-based time projection chambers (TPCs), performed simultaneously for the primary (S1) and secondary (S2) components in a small, purity-controlled, setup. We explore a range of conditions of general academic interest, focusing on those of relevance to contemporary TPCs: pressure range 1–10 bar, pressure-reduced electric fields of 0–100 V/cm/bar in the drift region (S1) and up to the proportional scintillation regime in the multiplication region (S2), and wavelength bands 145–250/250–400/400–600 nm, for both alpha and beta particles. Attention is paid to the possibility of non-conventional scintillation mechanisms such as the 3rd continuum emission, recombination light from beta -electrons at high pressure (for S1), emission from high-lying excited states and neutral bremsstrahlung (for S2). Time constants and, specially, scintillation yields have been obtained as a function of electric field and pressure, the latter aided by Geant4 simulations.

Effect of atmospheric turbulence on orbital angular momentum entangled state

The entangled orbital angular momentum (OAM) photons propagating across a weakly turbulent atmosphere are investigated. Here, the paper uses the single-phase screen model based on the Kolmogorov theory of turbulence, and focuses on the influence of the backward scattering on OAM evolution. The results indicate that backward scattering plays an important role in the analysis of OAM entanglement evolution in the turbulent atmosphere. It cannot be negligible especially for higher-order OAM mode. Moreover, when OAM mode is greater than 4, entangled photon pairs composed of higher OAM modes are not more robust in turbulence within the weak scintillation regime. These results will be useful in future investigations of OAM-based optical wave propagation through turbulent atmosphere.

Dependence of the probability density function of laser radiation power on the scintillation index and the size of a receiver aperture.

Power scintillations of a Gaussian laser beam propagated through a 7 km long horizontal atmospheric path for a wide range of turbulence strengths and different sizes of the receiver aperture were studied experimentally. The probability density function (PDF) and its properties were analyzed for a wide range of scintillation conditions. It was shown that the PDF can be described by the fractional exponential distribution in the strong scintillation regime (scintillation index of power measured on aperture σPIB2>1) for apertures with diameter d < a, where a is the size of the isoplanatic region, and by the gamma distribution in the weak scintillation regime (σPIB2<1), as well as by the lognormal distribution for σPIB2≪1. More than one distribution can be considered as a good approximation for experimental data for some ranges of d/a and σPIB2, but the transition from one distribution to another as the best approximation occurs at certain characteristic values of these parameters. In the strong scintillation regime, the aperture averaging effect resulted in the transition from the fractional exponential distribution to the fractional gamma (FG) distribution when the aperture diameter is about the size of the isoplanatic region (d/a∼1). The FG distribution better approximates the experimental PDF because it accounts for the fact that the probability of zero power values becomes zero due to the averaging effect of the aperture. The fractional gamma distribution is the best approximation of the PDF for a finite aperture when σPIB2>σPIB,crit2=[0.7-0.8], while the gamma distribution is the best approximation for σPIB2<σPIB,crit2.

What does FRB light-curve variability tell us about the emission mechanism?

ABSTRACT A few fast radio bursts’ (FRBs) light curves have exhibited large intrinsic modulations of their flux on extremely short ($t_{\rm r}\sim 10\, \mu$s) time-scales, compared to pulse durations (tFRB ∼ 1 ms). Light-curve variability time-scales, the small ratio of rise time of the flux to pulse duration, and the spectro-temporal correlations in the data constrain the compactness of the source and the mechanism responsible for the powerful radio emission. The constraints are strongest when radiation is produced far (≳1010 cm) from the compact object. We describe different physical set-ups that can account for the observed tr/tFRB ≪ 1 despite having large emission radii. The result is either a significant reduction in the radio production efficiency or distinct light-curve features that could be searched for in observed data. For the same class of models, we also show that due to high-latitude emission, if a flux f1(ν1) is observed at t1 then at a lower frequency ν2 &amp;lt; ν1 the flux should be at least (ν2/ν1)2f1 at a slightly later time (t2 = t1ν1/ν2) independent of the duration and spectrum of the emission in the comoving frame. These features can be tested, once light-curve modulations due to scintillation are accounted for. We provide the time-scales and coherence bandwidths of the latter for a range of possibilities regarding the physical screens and the scintillation regime. Finally, if future highly resolved FRB light curves are shown to have intrinsic variability extending down to ${\sim}\mu$s time-scales, this will provide strong evidence in favour of magnetospheric models.

Entanglement protection of high-dimensional states by adaptive optics

We theoretically study the potential of adaptive optics (AO) to protect entanglement of high-dimensional photonic orbital-angular-momentum (OAM) states against turbulence-induced phase distortions. We demonstrate that AO is able to reduce crosstalk among the OAM modes and, consequently, the entanglement decay as well as photon losses. A test of the AO-stabilized output state against high-dimensional Bell inequalities shows that the transmitted entanglement allows for secure communication, even in the strong scintillation regime.

Strehl ratio and spread rate of Gaussian–Schell model beams in the marine–atmosphere link of weak to strong scintillations

We present the models of the Strehl ratio (SR) and the spread rate of Gaussian–Schell model (GSM) beams in the long slant path of marine–atmosphere turbulence. The models are developed based on the extended Rytov theory which permits us extending the results of weak scintillation into the moderate-to-strong scintillation regimes. Our results show that there exists a minimum of SR for selected waist radiuses and the minimum SR is dependent on the transverse coherent width of light source. The spread rate increases with increasing the light wavelength but it decreases with increasing the waist radius and the inner scale. The effects of turbulence on the peak received irradiance can be reduced by selecting larger waist radius of transmitting aperture and longer light wavelength.

Decoherence of orbital angular momentum tangled photons in non-Kolmogorov turbulence.

Orbital angular momentum (OAM) entangled photons propagating through non-Kolmogorov turbulence are studied by numerical simulations. Here, the paper uses the multiphase screen model, especially focusing on the influences of the azimuthal mode and the turbulence parameters (i.e., the generalized exponent, the outer scale of turbulence, and the inner scale of turbulence) on entanglement evolution in the weak scintillation regime. The results indicate that the azimuthal mode, the generalized exponent, and the outer scale of turbulence have obvious influences on OAM entanglement. However, the influence of the turbulence inner scale on OAM entanglement can be ignored.

Parameter dependence in the atmospheric decoherence of modally entangled photon pairs

When a pair of photons that are entangled in terms of their transverse modes, such as an orbital angular momentum (OAM) basis, propagates through atmospheric turbulence, the scintillation causes a decay of the entanglement. Here, we use numerical simulations to study how this decoherence process depends on the various dimension parameters of the system. The relevant dimension parameters are the propagation distance, the wavelength, the beam radius, and the refractive index structure constant, indicating the strength of the turbulence. We show that beyond the weak scintillation regime, the entanglement evolution cannot be accurately modeled by a single phase screen that is specified by a single dimensionless parameter. Two dimensionless parameters are necessary to describe the OAM entanglement evolution. Furthermore, it is found that higher OAM modes are not more robust in turbulence beyond the weak scintillation regime.

Scintillation in the White-Noise Paraxial Regime

In this paper the white-noise paraxial wave model is considered. This model describes for instance the propagation of laser beams in the atmosphere in some typical scaling regimes. The closed-form equations for the second- and fourth-order moments of the field are solved in two particular situations. The first situation corresponds to a random medium with a transverse correlation radius smaller than the beam radius. This is the spot-dancing regime: the beam shape spreads out as in a homogeneous medium and its center is randomly shifted according to a Gaussian process whose variance grows like the third power of the propagation distance. The second situation corresponds to a plane-wave initial condition, a small amplitude for the medium fluctuations, and a large propagation distance. This is the scintillation regime: the normalized variance of the intensity converges to one exponentially with the propagation distance, corresponding to strong intensity fluctuations and in agreement with the conjecture that the statistics of the field becomes complex Gaussian.

Wave Backscattering by Point Scatterers in the Random Paraxial Regime

When waves penetrate a medium without coherent reflectors, but with some fine scale medium heterogeneities, the backscattered wave is incoherent without any specific arrival time or the like. In this paper we consider a distributed field of weak microscatterers, like aerosols in the atmosphere, which coexists with microstructured clutter in the medium, like the fluctuations of the index of refraction of the turbulent atmosphere. We analyze the Wigner transform or the angularly resolved intensity profile of the backscattered wave when the incident wave is a beam in the paraxial regime and when the Born approximation is valid for the microscatterers. An enhanced backscattering phenomenon is proved, and the properties of the enhanced backscattering cone (relative amplitude and profile) are shown to depend on the statistical parameters of the microstructure but not on the microscatterers. These results are based on a multiscale analysis of the fourth-order moment of the fundamental solution of the white-noise paraxial wave equation. They pave the way for an estimation method of the statistical parameters of the microstructure from the observation of the enhanced backscattering cone. In our scaling argument we differentiate the two important canonical scaling regimes, which are the scintillation regime and the spot dancing regime

Enhancement of the laser return mean power at the strong optical scintillation regime in a turbulent atmosphere

The results of the analysis of the backscattered radiation mean power enhancement in a turbulent atmosphere are presented. The above enhancement arises because of the correlation of the forward and backward waves in a random media. The analysis was performed for the strong optical scintillation regime in a turbulent atmosphere. It is shown that the laser return mean power in the turbulent atmosphere can two times exceed that in a homogeneous medium because of the correlation of forward and backscattered waves. With an increase in the optical turbulence strength, the enhancement effect decreases according to the power law.

Diffractive and refractive timescales at 4.8 GHz in PSR B0329+54

We present the results of flux density monitoring of PSR B0329+54 at the frequency of 4.8 GHz using the 32-meter TCfA radiotelescope. The observations were conducted between 2002 and 2005. The main goal of the project was to find interstellar scintillation (ISS) parameters for the pulsar at the frequency at which it was never studied in detail. To achieve this the 20 observing sessions consisted of 3-minute integrations which on average lasted 24 hours. Flux density time series obtained for each session were analysed using structure functions. For some of the individual sessions as well as for the general average structure function we were able to identify two distinctive timescales present, the timescales of diffractive and refractive scintillations. To the best of our knowledge, this is the first case when both scintillation timescales, t_DISS=42.7 minutes and t_RISS=305 minutes, were observed simultaneously in a uniform data set and estimated using the same method. The obtained values of the ISS parameters combined with the data found in the literature allowed us to study the frequency dependence of these parameters over a wide range of observing frequencies, which is crucial for understanding the ISM turbulence. We found that the Kolmogorov spectrum is not best suited for describing the density fluctuations of the ISM, and a power-law spectrum with beta =4 seems to fit better with our results. We were also able to estimate the transition frequency (transition from strong to weak scintillation regimes) as 10.1 GHz, much higher than was previously predicted. We were also able to estimate the strength of scattering parameter u=2.67$ and the Fresnel scale as 6.7x10^8 meters.

Probability density of aperture-averaged irradiance fluctuations for long range free space optical communication links

The probability density function (PDF) of aperture-averaged irradiance fluctuations is calculated from wave-optics simulations of a laser after propagating through atmospheric turbulence to investigate the evolution of the distribution as the aperture diameter is increased. The simulation data distribution is compared to theoretical gamma-gamma and lognormal PDF models under a variety of scintillation regimes from weak to strong. Results show that under weak scintillation conditions both the gamma-gamma and lognormal PDF models provide a good fit to the simulation data for all aperture sizes studied. Our results indicate that in moderate scintillation the gamma-gamma PDF provides a better fit to the simulation data than the lognormal PDF for all aperture sizes studied. In the strong scintillation regime, the simulation data distribution is gamma gamma for aperture sizes much smaller than the coherence radius rho0 and lognormal for aperture sizes on the order of rho0 and larger. Examples of how these results affect the bit-error rate of an on-off keyed free space optical communication link are presented.

Gaussian beam weak scintillation: low-order turbulence effects and applicability of the Rytov method

A generally applicable and computationally efficient description of random irradiance fluctuations induced by single scattering from distributed low-order turbulence (LOT) phase fluctuations is developed for Gaussian beams in the weak scintillation regime. The LOT solution describes irradiance statistics resulting from coarse beam irradiance fluctuations such as beam wander and beam breathing and will generally underestimate the true scintillation owing to the neglect of higher orders. For a subset of beam and turbulence settings that naturally result in non-log-normal irradiance behavior in the weak regime, the LOT solution closely approaches the exact solution and accurately describes the irradiance statistics for any point on the observation plane. For the same settings, beam-wave scintillation theory derived from the Rytov perturbation method yields inaccurate predictions owing to an inherent confinement to log-normal behavior. Examples that naturally exhibit non-log-normal irradiance behavior include focused beams on horizontal paths and collimated beams on ground-to-space paths. The complementary nature of the two scintillation theories (LOT and Rytov) enables a hybrid combination that yields accurate and convenient scintillation predictions for any case exhibiting weak scintillation regardless of irradiance behavior. Comparison of hybrid model predictions with wave optics simulation data reveals excellent agreement.

Microstructural correlations between reaction medium and combustion wave propagation in heterogeneous systems

The combustion synthesis (CS) of materials is an advanced approach in powder metallurgy. Our previous studies of various CS systems have shown that at small time scales ( 10 - 3 – 10 - 4 s ), the behavior of self-propagating high-temperature reaction waves in heterogeneous mixtures follows two modes, classified according to their microstructures: quasihomogeneous and scintillating reaction waves. The latter is a novel phenomenon where the reaction propagates in a form of short and high-temperature flashes, so-called scintillations, which randomly arise in vicinity of the combustion front and promote its local movement. Using Ti–Si mixture as an example, this work is aimed to find quantitative relationship between reactant particle size and microstructural characteristics of the reaction front, including scintillation size and its distribution, as well as lifetime. It was shown that, for coarse powders ( > 40 μ m ) , specific number of Ti particles approximately equals the number of hot spots observed during the combustion process. This means that essentially every Ti particle forms a hot spot. However, for finer Ti particles, average hot spot is formed by several (3–5) such particles. Since size of scintillation decreases to a lesser extent than Ti particle size, the scintillation regime does not vanish when using finer Ti powders. These results confirm the general assumption of the relay-race model that the limiting factor of combustion wave propagation is heat transfer between reaction cells rather than reaction within the cell itself.