Weak Intensity Fluctuations Research Articles

Experimental indoor validation of weak intensity fluctuations over free-space optical communication link

In this paper, we present experimental results for characterizing turbulence phenomena in free-space optical (FSO) communication under laboratory controlled conditions. Collected data is used for statistical determination of the probability density function (PDF) of irradiance signal fluctuations. In literature, there are a number of empirical models for variety of turbulence regimes. Thus, obtained PDF is compared with selected models from the bibliography in order to describe properly statistical properties of real turbulent channel and help improving of FSO system performance calculation. In overall, analysis has shown that the results obtained under given constraints fit well with the exponentiated Weibull turbulence model.

Influence of laser intensity noise on gated CW-STED microscopy

Stimulated emission depletion (STED) microscopy provides previously unobtainable spatial resolution. But the complexity and cost of the early implementations based on pulsed mode-locked lasers have limited its wide dissemination. The combination of time-gated detection with STED microscopy significantly helped to overcome this limitation. Indeed, a cheap, easy-to-implement and efficient implementation of STED microscopy can be obtained using also continuous-wave (CW) lasers, the so-called gated CW-STED implementation. Here we show that the performance of this system substantially depends on the intensity noise characteristic of the CW laser. Weak intensity fluctuations reduce the average intensity needed for obtaining a certain resolution, which is a key condition to reduce photodamage on the sample. We simulate the influence of intensity noise on the performance of a gated CW-STED microsope and we validate the results comparing two lasers with different intensity noise properties. Finally, we show that the excellent noise characteristics of the optical-pumped-semiconductor lasers allow implementation of a gated CW-STED microscope which provides ∼45 nm resolution in a biological sample at moderate (average and peak) intensity light.

Structure of the spatiotemporal spectrum of a laser beam in the atmosphere under strong turbulence

In this paper, possibilities of spatiotemporal filtering of the correlation function of laser beam intensity in a turbulent atmosphere in conditions of strong fluctuations are analyzed. It is shown that despite significant distortions in the high-frequency band of the spatiotemporal spectrum in comparison with weak intensity fluctuations, an increase in the time delay between the frames allows separation of contributions of different optical path segments. The characteristic scales of spectral responses differ significantly for different path points. This allows extracting information about the wind velocity profile.

Evaluating a linearized Euler equations model for strong turbulence effects on sound propagation

Sound propagation outdoors is strongly affected by atmospheric turbulence. Under strongly perturbed conditions or long propagation paths, the sound fluctuations reach their asymptotic behavior, e.g., the intensity variance progressively saturates. The present study evaluates the ability of a numerical propagation model based on the finite-difference time-domain solving of the linearized Euler equations in quantitatively reproducing the wave statistics under strong and saturated intensity fluctuations. It is the continuation of a previous study where weak intensity fluctuations were considered. The numerical propagation model is presented and tested with two-dimensional harmonic sound propagation over long paths and strong atmospheric perturbations. The results are compared to quantitative theoretical or numerical predictions available on the wave statistics, including the log-amplitude variance and the probability density functions of the complex acoustic pressure. The match is excellent for the evaluated source frequencies and all sound fluctuations strengths. Hence, this model captures these many aspects of strong atmospheric turbulence effects on sound propagation. Finally, the model results for the intensity probability density function are compared with a standard fit by a generalized gamma function.

A novel technique to measure intensity fluctuations in EUV images and to detect coronal sound waves nearby active regions

Context. In the past years, evidence for the existence of outward-moving (Doppler blue-shifted) plasma and slow-mode magnetoacoustic propagating waves in various magnetic field structures (loops in particular) in the solar corona has been found in ultraviolet images and spectra. Yet their origin and possible connection to and importance for the mass and energy supply to the corona and solar wind is still unclear. There has been increasing interest in this problem thanks to the high-resolution observations available from the extreme ultraviolet (EUV) imagers on the Solar TErrestrial RElationships Observatory (STEREO) and the EUV spectrometer on the Hinode mission. Aims. Flows and waves exist in the corona, and their signatures appear in EUV imaging observations but are extremely difficult to analyse quantitatively because of their weak intensity. Hence, such information is currently available mostly from spectroscopic observations that are restricted in their spatial and temporal coverage. To understand the nature and origin of these fluctuations, imaging observations are essential. Here, we present measurements of the speed of intensity fluctuations observed along apparently open field lines with the Extreme UltraViolet Imagers (EUVI) onboard the STEREO mission. One aim of our paper is to demonstrate that we can make reliable kinematic measurements from these EUV images, thereby complementing and extending the spectroscopic measurements and opening up the full corona for such an analysis. Another aim is to examine the assumptions that lead to flow versus wave interpretation for these fluctuations. Methods. We have developed a novel image-processing method by fusing well established techniques for the kinematic analysis of coronal mass ejections (CME) with standard wavelet analysis. The power of our method lies with its ability to recover weak intensity fluctuations along individual magnetic structures at any orientation, anywhere within the full solar disk, and using standard synoptic observing sequences (cadence <3 min) without the need for special observation plans. Results. Using information from both EUVI imagers, we obtained wave phase speeds with values on the order of 60−90 km s −1 , compatible with those obtained by other previous measurements. Moreover, we studied the periodicity of the observed fluctuations and established a predominance of a 16-min period, and other periods that seem to be multiples of an underlying 8-min period. Conclusions. The validation of our analysis technique opens up new possibilities for the study of coronal flows and waves, by extending it to the full disk and to a larger number of coronal structures than has been possible previously. It opens up a new scientific capability for the EUV observations from the recently launched Solar Dynamics Observatory. Here we clearly establish the ubiquitous existence of sound waves which continuously propagate along apparently open magnetic field lines.

Reconstruction of the wind velocity profile from the intensity fluctuations of a reflected spherical wave in the turbulent atmosphere

The problem of reconstructing the wind velocity profile from the spatiotemporal statistics of turbulent reflected optical radiation intensity fluctuations is considered in the article. Expressions for the spatiotemporal correlation function and the spectrum of weak intensity fluctuations of the wave scattered on a diffusive screen are derived. An algorithm for reconstructing the wind velocity profile from the spatiotemporal spectra of the intensity of a reflected spherical wave in the turbulent atmosphere is suggested. The results of closed numerical experiments are presented that confirm the efficiency of the suggested algorithm.

Reconstruction of the wind velocity profile by the intensity fluctuations of a scattered wave in a receiving telescope

The problem of the wind velocity profile reconstruction from random intensity fluctuations of a laser beam diffusely scattered by a screen and received by a telescope is considered. Expressions for a spatiotemporal correlation function and spectrum of weak intensity fluctuations of the reflected beam are presented. An algorithm is proposed for the reconstruction of the profile and direction of the wind velocity from the intensity fluctuation spectra of reflected radiation received by the telescope. The efficiency of the algorithm is confirmed by the results of numerical experiments.

Wind profiling based on the optical beam intensity statistics in a turbulent atmosphere

Reconstruction of the wind profile from the statistics of intensity fluctuations of an optical beam propagating in a turbulent atmosphere is considered. The equations for the spatiotemporal correlation function and the spectrum of weak intensity fluctuations of a Gaussian beam are obtained. The algorithms of wind profile retrieval from the spatiotemporal intensity spectrum are described and the results of end-to-end computer experiments on wind profiling based on the developed algorithms are presented. It is shown that the developed algorithms allow retrieval of the wind profile from the turbulent optical beam intensity fluctuations with acceptable accuracy in many practically feasible laser measurements set up in the atmosphere.

Wind velocity profile reconstruction from intensity fluctuations of a plane wave propagating in a turbulent atmosphere

Reconstruction of a wind profile based on the statistics of plane-wave intensity fluctuations in a turbulent atmosphere is considered. The algorithm for wind profile retrieval from the spatiotemporal spectrum of plane-wave weak intensity fluctuations is described, and the results of end-to-end computer experiments on wind profiling based on the developed algorithm are presented. It is shown that the reconstructing algorithm allows retrieval of a wind profile from turbulent plane-wave intensity fluctuations with acceptable accuracy.

Phase control of a focused laser beam: the comparison of the efficiency of methods

The adaptive phase correction of distortions of spatially limited laser beams and a plane wave transmitted through a turbulent atmosphere layer is considered. The requirements to the size of the mirror element and the bandwidth of the correction system are determined, which proved to be the same as in the case of weak intensity fluctuations.

Statistical theory of the propagation of optical radiation in turbulent media

The problem of the propagation of a plane light wave in a turbulent medium is studied on the basis of the ideas of statistical topography. A cluster (caustic) structure of the intensity of the wave field in a plane perpendicular to the direction of propagation of the wave is analyzed both in the region of weak intensity fluctuations and in the region of saturated fluctuations. The specific (per unit area) values of the total area of the regions where the intensity is greater than a fixed level, the fraction of the power confined in these regions, and the total perimeter and average number of such regions are estimated. It is shown that estimates of this kind can be made on the basis of a knowledge of the joint one-point probability distribution of the intensity and transverse gradient of the wave field.

Scintillation of a radio source observed through the tail of comet Halley

On 29 March 1986 between 16:00 and 23:00 UT we observed the strong compact radio source 1827-360 as the plasma tail of comet Halley swept across the source. We noted the appearance of weak intensity fluctuations during the interval 17:00 to 19:00 UT when the Sun, the comet's nucleus and the radio source were accurately aligned. We interpret this behaviour as the result of diffraction of radio waves by electron-density irregularities in the plasma tail. At a distance 5.4 × 106 km downstream from the cometary nucleus the central part of the plasma tail contained electron density irregularities about 14 times as strong as those in the undisturbed solar wind.

Thermal self-interaction of laser radiation in a turbulent medium

The results are given of an experimental investigation of fluctuations in the intensity of a laser beam passing through a turbulent absorbing liquid medium (ethyl alcohol). On increasing the beam power, the intensity fluctuations were found to weaken. A qualitative explanation of this phenomenon is given. Conclusions following from calculations of weak intensity fluctuations, in which the temperature variation of the medium was described by a semi-empirical turbulent heat-conduction equation, were experimentally verified.

Spectra of temporal intensity fluctuations of laser radiation traveling in a turbulent atmosphere

The Huygens-Kirchhoff principle is generalized to continuously inhomogeneous media and used in a study of the temporal frequency spectra of the intensity fluctuations in laser beams traveling in a turbulent atmosphere. The propagation of an optical wave in a medium which is randomly inhomogeneous in time and space is considered in a quasistationary approximation using the turbulence hypothesis. Approximate analytic expressions are obtained for the temporal correlation coefficient in the limiting cases of strong and weak intensity fluctuations. It is shown that, in the case of weak intensity fluctuations, the main contribution to the temporal spectrum is made by frequencies corresponding to the transit time of frozen inhomogeneities across the transmitting aperture. The spectrum shifts toward higher frequencies when the length of the path is increased or the intensity of turbulent pulsations of the refractive index becomes greater. In the case of strong fluctuations, the form of the spectrum is governed entirely by meteorological turbulent conditions and is independent of the diffraction size of the transmitting aperture or the focusing of the radiation. The results of the calculations are compared with the experimental data.

Interstellar scintillation of extragalactic radio sources

Observations of 12 compact extragalactic sources were made at 2695 and 8085 MHz in order to detect weak intensity fluctuations caused by interstellar scintillation. Pulsar data are used to estimate the parameters of the interstellar medium needed to interpret the measured upper limits in terms of source angular diameters which are much larger than the scintillation cutoff diameter. It is shown that the observed source rms brightness temperatures are less than 10 to the 15th K and 10 to the 14th K at 2695 and 8085 MHz, respectively, making self-absorbed proton-synchroton radiation and high-brightness coherent mechanisms unlikely. If the sources are composed of 'point' components, each source can contain no fewer than 10,000 such components.