Atmospheric Structure Constant Research Articles

Fiber-Coupling Efficiency of Laser Array Beam From Turbulent Atmosphere to Fiber Link

Free-space optical communication (FSOC) systems are nowadays integrated with fiber optical components developed for fiber-optic communications. In such integrated systems, the collected portion of the incident beam on the receiver lens is coupled into a single-mode fiber. The process of coupling, however, is mostly affected by the atmospheric turbulence which distorts the coherency of the propagating beam i.e., it results in speckle over the coupling lens causing a reduction in the coupling efficiency. In this article, we aimed at investigating the fiber coupling efficiency of laser array beams propagating in a turbulent atmosphere. For this purpose, using the Huygens-Fresnel principle, mutual coherence function (MCF) for a laser array beam incidence is formulated. In this way, the average power coupled into the fiber and the average received power on the coupling lens are derived for a laser array beam incidence. It is found that the fiber coupling efficiency clearly increases with the increase in ring radius and the number of Gaussian beams in the array and rapidly decreases with increasing structure constant of atmosphere, link distance, and the number of speckles over the receiver aperture. We also demonstrate the effect of various FSOC system parameters on the coupling efficiency.

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.

Effect of partial coherence on signal-to-noise ratio performance of free space optical communication system in weak turbulence

The effect of source coherence on the average signal-to-noise ratio SNR performance of free space optical communication (FSOC) systems operating in weak atmospheric turbulence is investigated with the help of the extended Huygens–Fresnel​ principle. For an FSOC system that uses a partially coherent laser source, first, the received power at the finite-sized receiver aperture is derived. Then, the power scintillation index is evaluated that reflects the aperture averaging. Using these derived optical entities, the variations of SNR are examined versus parameters such as the degree of source coherence, wavelength, link distance, source size, structure constant of atmosphere and the receiver aperture radius. Obtained results show that a decrease in the degree of source coherence has a positive effect on SNR.

Effect of optical system and turbulent atmosphere on the average intensity of partially coherent flat-topped vortex hollow beam

Based on the extended Huygens-Fresnel diffraction integral formula, the average intensity of partially coherent flat-topped vortex hollow beam propagating through the paraxial ABCD optical system in atmosphere turbulence are investigated by using the numerical examples. The influences of optical system parameters, coherence length, parameters of turbulent atmosphere, beam order N and topological charge M of laser beam on the average intensity of partially coherent flat-topped vortex hollow beam are analyzed. Results show that the beam will evolve into Gauss-like beam in the far field with the propagation distance increasing, and the beam will evolve into Gauss-like beam more rapidly with the increasing of the structure constant of turbulent atmosphere or the decreasing of coherence length. The results of this work have the potential applications in free space optical communication.

Performance analysis of multiple-input multiple-output free-space optical systems with partially coherent Gaussian beams and finite-sized detectors

Multiple-input multiple-output (MIMO) techniques are employed in free-space optical (FSO) links to mitigate the degrading effects of atmospheric turbulence. We consider a MIMO FSO system, which consists of a radial laser array with partially coherent Gaussian beams at the transmitter and a detector array with Gaussian apertures at the receiver. The average power and the power correlation function at the finite-sized receiver apertures are formulated by using the extended Huygens–Fresnel principle in weak atmospheric turbulence. This let us further quantify the performance metrics such as the power scintillation index, the aperture averaging factor, and the average bit error rate (BER) as functions of system parameters. The derived power scintillation equation correctly reduces to the existing coherent and partially coherent Gaussian beam scintillation indices in the limiting cases. Using the performance metrics, we analyze the effect of various practical system parameters on the performance of a MIMO FSO system. Practical system parameters include the transmitter and receiver ring radius, number of beamlets, number of finite-aperture receivers, source size, degree of source coherence, receiver aperture radius, link distance, and the structure constant of atmosphere.

Scintillation index in strong oceanic turbulence

Scintillation index of spherical wave in strongly turbulent oceanic medium is evaluated. In the evaluation, modified Rytov solution and our recent formulation that expresses the oceanic turbulence parameters by the atmospheric turbulence structure constant, are employed. Variations of the scintillation index in strong oceanic turbulence are examined versus the oceanic turbulence parameters such as the rate of dissipation of kinetic energy per unit mass of fluid, the rate of dissipation of mean-squared temperature, viscosity, wavelength, the link length, and the ratio of temperature to salinity contributions to the refractive index spectrum.

Daily Variation Analysis of Atmospheric Turbulence from Inland to Open Sea

Random fluctuation of turbulence brings random fluctuation of refractive index, which makes atmosphere become a random fluctuation medium and destroys the coherence of light wave especially laser transferring in it. Exploration of atmospheric turbulence is essentially investigation of atmospheric refractive index. The atmospheric structure constant of refractive index is a basic parameter of expressing atmospheric turbulence, and was measured using HTP-2 micro-thermal meter at different areas from inland to open sea. It is analysed that the relation of atmospheric structure constant of refractive index with corresponding temperature and wind speed. The conclusion of turbulence and main influencing factors is to deepen the research in atmospheric optical transmission, and to provide data support for the siting of ship board photoelectric systems.

Real-time compensation of phase distortions by digital phase conjugation using CCD and liquid crystal panel.

A video-rate optical phase conjugator using an image sensor and a liquid crystal panel that operated at 30 Hz was developed and its temporal characteristic was studied. Strong air turbulence of atmospheric structure constant, Cn=6.3×10(-7) [m(-1/3)], was successfully compensated, reducing beam dancing by a factor of 2. To our knowledge, this is the first report demonstrating real-time wavefront correction by digital phase conjugation using commercial devices.

Propagation properties of a radial phased-locked partially coherent anomalous hollow beam array in turbulent atmosphere

Abstract The propagation properties of a phase-locked partially coherent anomalous hollow beam (AHB) array in turbulent atmosphere is investigated in detail. The analytical expressions for the average intensity of a radial phase-locked partially coherent AHB array in turbulent atmosphere are derived based on the extended Huygens–Fresnel principle. By using the analytical expressions, the average intensity distribution and spreading properties of a radial phase-locked partially coherent AHB array in turbulent atmosphere and in free space are numerically studied. The results show that the propagation properties of a radial phase-locked partially coherent AHB array in turbulent atmosphere are significantly affected by initial beam parameters and the structure constant of turbulent atmosphere.

Profile and Character of Atmospheric Structure Constant of Refractive Index Cn2

Random fluctuations of turbulence bring random fluctuations of the refractive index, making the atmosphere a random fluctuation medium that destroys the coherence of light-waves. Research in atmospheric turbulence is actually the investigation of the atmospheric refractive index. The atmospheric structure constant of refractive index, Cn2, is an important parameter denoting atmospheric turbulence. In this paper, Cn2 is measured during the day and at night and in all four seasons using a high sensitivity micro-thermal meter QHTP-2. The vertical profile of Cn2 in Hefei (0–30 km) is investigated by the analysis of experimental data. The average profile of Cn2 in Hefei exhibits conspicuous day and night differences with increased altitude. The distribution of log(Cn2) is nearly normal and has conspicuous seasonal differences.

Propagation properties of radially polarized partially coherent beam in turbulent atmosphere

On the basis of the extended Huygens–Fresnel integral formula, analytical propagation formulae for radially polarized partially coherent beam propagating in turbulent atmosphere are derived, and some analyses are illustrated by numerical examples. It is shown that the structure constant of turbulent atmosphere and the initial coherence length have significant influence on the degree of polarization and coherence for radially polarized partially coherent beam.

The propagation of radially polarized partially coherent beam through an optical system in turbulent atmosphere

The propagation of a radially polarized partially coherent beam through an optical system in turbulent atmosphere is investigated. Analytical propagation formulae for the radially polarized partially coherent beam through an optical system in turbulent atmosphere are derived, and some analyses are illustrated by numerical examples related to propagation properties of the radially polarized partially coherent beam through a double-lens system in turbulent atmosphere. It is shown that the optical system, structure constant of turbulent atmosphere and the initial coherent length have significant influence on intensity, degree of polarization and degree of coherence for radially polarized partially coherent beam.

LOG-AMPLITUDE VARIANCE OF LASER BEAM PROPAGATION ON THE SLANT PATH THROUGH THE TURBULENT ATMOSPHERE

Based on the altitude-dependent model of ITU-R slant atmospheric turbulence structure constant, the log-amplitude variance of laser beam propagation on the slant path through turbulent atmosphere is obtained with transmitter and receiver parameters and can be degenerated to the result of the horizontal path with atmospheric structure constant as a flxed value. These expressions are convenient tools for beam-wave analysis. Finally, we apply the ITU-R turbulence structure constant model to calculate collimated, divergent and convergent beam log-amplitude variance. The numerical conclusions indicate the log-amplitude variance of laser beam propagation on slant path is generally smaller than those on horizontal path.

AVERAGE INTENSITY AND SPREADING OF PARTIALLY COHERENT FOUR-PETAL GAUSSIAN BEAMS IN TURBULENT ATMOSPHERE

The concept of partially coherent four-petal Gaussian (PCFPG) beam is introduced and described in analytical forms. Based on the Huygens-Fresnel integral formula, average intensity and beam spreading in turbulent atmosphere are derived in analytical expressions. Efiects of beam parameters and atmospheric structure constant on intensity distributions and efiective beam sizes are investigated in detail, respectively. Results show that PCFPG beams carrying larger coherence lengths or higher beam orders would be less afiected by turbulence. It is also indicated that, when the propagation distance increases, the PCFPG beam would convert into the Gauss- like proflle sooner or later, but this degradation can be reduced by modulating beam parameters. Results in this paper may provide potential applications in free-space optical communications.

STUDY ON SCINTILLATION CONSIDERING INNER- AND OUTER-SCALES FOR LASER BEAM PROPAGATION ON THE SLANT PATH THROUGH THE ATMOSPHERIC TURBULENCE

Based on both the modified Rytov method and the altitude-dependent model of the ITU-R slant atmospheric turbulence structure constant, the uniform model of scintillation index considering inner- and outer-scales is derived form weak to strong fluctuation regions with Gaussian beam propagation on the slant path, and can be degenerated to the result of the horizontal path with atmospheric structure constant is a fixed value. The numerical conclusions indicate the smaller wavelength, the inner-scale has a stronger impact on scintillation than outer-scale. But, in strong fluctuation, the outer- scale effect is prominence. Finally, the numerical results are compared and verified with the experimental data.

Application of moiré technique to the measurement of the atmospheric turbulence parameters related to the angle of arrival fluctuations

There are several methods for measuring ground-level atmospheric turbulence parameters, such as the refractive index profile and its fluctuations, correlations of the fluctuations in space and time, and the atmospheric refractive-index structure constant. These methods are based mainly on the measurement of fluctuations in intensity and location of an image formed by light propagating in the turbulent atmosphere or the fluctuations in impinging points of narrow light beams traversing the ground-level atmosphere. Exploiting the moiré technique, we suggest a high-precision approach for determining fluctuaions in the angle of arrival. When a low-frequency grating (carrier grating) is installed at a suitable distance from a telescope, its image, practically, forms on the focal plane of the telescope objective. Superimposing a physical grating (probe grating) of the same pitch as the image grating on the image forms the moiré pattern. The atmospheric turbulence distorts the image grating. Processing the fluctuations of successive moiré fringes can yield the mentioned parameters across a rather large cross section of the atmosphere with high accuracy, because of the moiré technique's magnifying character and access to a large volume of data, and does so in a comparatively simple and reliable manner.

Remote sensing of atmospheric turbulence and transverse winds from wave-front slope measurements from crossed optical paths

In the theory of atmospheric turbulence, the strength of the spatial variations of the index of refraction n is proportional to a parameter known as the atmospheric-structure constant. The atmosphericstructure constant is denoted C(2)(n)(z) and is a function of position along the optical path z. The characteristics of the temporal variations of the index of refraction are related to both C(2)(n)(z) and to the transverse wind velocity V(z). Current optical techniques for remotely sensing C(2)(n)(z) and V(z) rely primarily on the spatial or temporal cross-correlation properties of the intensity of the optical field. In the remote-sensing technique proposed here, we exploit the correlation properties of the wave-front slope measured from two point sources to obtain profiles of C(2)(n)(z) and V(z). The two sources are arranged to give crossed optical paths. The geometry of the crossed paths and the characteristics of the wave-front slope sensor determine the achievable resolution. The signal-to-noise ratio calculationsindicate the need for multiple measurements to obtain useful estimates of the desired quantities.

Comparison of scintillometer and microthermometer measurements of CN2

The atmospheric structure constant, CN2, has been measured by use of three instruments, one optical and two thermal. The data show very good correlation (ρxy = 0.88), but are related by a consistent scale factor: CN (optical) = 0.34 × 10−8 cm−2/3 + 1.45CN (thermal). Several effects are discussed-that may explain these results.