Propagation characteristics of the vortex beam array through anisotropic non-Kolmogorov maritime atmospheric turbulence

This study employs the power spectrum inversion method to generate the phase screens of atmospheric turbulence over the sea, the theoretical model of the vortex beam array emitted by a focusing system through anisotropic non-Kolmogorov maritime atmospheric turbulence was built. The propagation characteristics of the vortex beam array were investigated theoretically and compared to those of the Gaussian beam array. The results show that the vortex beam array is less affected by turbulence and exhibits stronger resistance to the turbulence. On this basis, using the evaluation parameters including the relative beam width and the beam wander, the influences of various parameters of the vortex beam array and atmospheric turbulence, such as topological charge, beamlet width, number of beamlets, anisotropy factor, and power law, on its propagation characteristics through anisotropic non-Kolmogorov maritime atmospheric turbulence were studied extensively. The results can provide a useful reference for the applications of the vortex beam array in optical communication through the maritime atmosphere

Free-space optical system performance for laser beam propagation through non-Kolmogorov turbulence

It is well know that free-space laser system performance is limited by atmospheric turbulence. Most theoretical treatments have been described for many years by Kolmogorov\u27s power spectral density model because of its simplicity. Unfortunately, several experiments have been reported recently that show that the Kolmogorov theory is sometimes incomplete to describe atmospheric statistics properly, in particular, in portions of the troposphere and stratosphere. We present a non-Kolmogorov power spectrum that uses a generalized exponent instead of constant standard exponent value 11/3, and a generalized amplitude factor instead of constant value 0.033. Using this new spectrum in weak turbulence, we carry out, for a horizontal path, an analysis of long-term beam spread, scintillation index, probability of fade, mean signal-to-noise ratio (SNR), and mean bit error rate (BER) as variation of the spectrum exponent. Our theoretical results show that for alpha values lower than alpha=11/3, but not for alpha close to alpha=3, there is a remarkable increase of scintillation and consequently a major penalty on the system performance. However, when alpha assumes a value close to alpha=3 or for alpha values higher than alpha=11/3, scintillation decreases, leading to an improvement on the system performance

Free space optical system performance for a Gaussian beam propagating through non Kolmogorov weak turbulence

Atmospheric turbulence has been described for many years by Kolmogorov's power spectral density model because of its simplicity. Unfortunately several experiments have been reported recently that show Kolmogorov theory is sometimes incomplete to describe atmospheric statistics properly, in particular in portions of the troposphere and stratosphere. It is known that free space laser system performance is limited by atmospheric turbulence. In this paper we use a non-Kolmogorov power spectrum which uses a generalized exponent instead of constant standard exponent value 11/3 and a generalized amplitude factor instead of constant value 0.033. Using this spectrum in weak turbulence, we carry out, for a Gaussian beam propagating along a horizontal path, analysis of long term beam spread, scintillation, probability of fade, mean signal to noise ratio and mean bit error rate as variation of the spectrum exponent. Our theoretical results show that for alpha values lower than 11/3 , but not for alpha close to 3 , there is a remarkable increase of scintillation and consequently a major penalty on the system performance. However when alpha assumes values close to 3 or for alpha values higher than 11/3 scintillation decreases leading to an improvement on the system performanc

Accurate seeing measurements with MASS and DIMM

Astronomical seeing is quantified by a single parameter, turbulence integral, in the framework of the Kolmogorov turbulence model. This parameter can be routinely measured by a Differential Image Motion Monitor, DIMM. A new instrument, Multi-Aperture Scintillation Sensor (MASS), permits to measure the seeing in the free atmosphere above ~0.5km and, together with a DIMM, to estimate the ground-layer seeing. The absolute accuracy of both methods is studied here using analytical theory, numerical simulation, and experiments. A modification of the MASS data processing to compensate for partially saturated scintillation is developed. We find that the DIMM can be severely biased by optical aberrations (e.g. defocus) and propagation. Seeing measurements with DIMM and MASS can reach absolute accuracy of ~10% when their biases are carefully controlled. Pushing this limit to 1% appears unrealistic because the seeing itself is just a model-dependent parameter of a non-stationary random process.Comment: 13 pages, 14 figures. Accepted for publication in MNRA

Simulating thick atmospheric turbulence in the lab with application to orbital angular momentum communication

We describe a procedure by which a long ($\gtrsim 1\,\mathrm{km}$) optical path through atmospheric turbulence can be experimentally simulated in a controlled fashion and scaled down to distances easily accessible in a laboratory setting. This procedure is then used to simulate a 1-km-long free-space communication link in which information is encoded in orbital angular momentum (OAM) spatial modes. We also demonstrate that standard adaptive optics methods can be used to mitigate many of the effects of thick atmospheric turbulence.Comment: Rewritten abstract and introductory section to emphasize the importance of the work and to make it accessible to a more general audience. Section 2 was expanded to include some background on the physics of turbulence to allow the paper to be self-contained and understood by nonspecialist

Astroparticle yield and transport from extragalactic jet terminal shocks

The present paper deals with the yield and transport of high-energy particle within extragalactic jet terminal shocks, also known as hotspots. We investigate in some details the cosmic ray, neutrinos and high-energy photons yield in hotspots of powerful FRII radio-galaxies by scanning all known spatial transport regimes, adiabatic and radiative losses as well as Fermi acceleration process. Since both electrons and cosmic rays are prone to the same type of acceleration, we derive analytical estimates of the maximal cosmic ray energy attainable in both toroidal and poloidal magnetic field dominated shock structures by using observational data on synchrotron emission coming from various hot-spots. One of our main conclusions is that the best hot-spot candidates for high energy astroparticle production is the extended ($L_{HS}\geq 1kpc$), strongly magnetized ($B> 0.1mG$) terminal shock displaying synchrotron emission cut-off lying at least in the optical band. We found only one object (3C273A) over the six objects in our sample being capable to produce cosmic rays up to $10^{20}$ eV. Secondly, we investigate the astroparticle spectra produced by two characteric hot-spots (Cygnus A and 3C273 A) by applying a multi-scale MHD-kinetic scheme, coupling MHD simulations to kinetic computations using stochastic differential equations. We show that 3C273 A, matching the previous properties, may produce protons up to $10^{20}$ eV in a Kolmogorov type turbulence by both computing electron and cosmic ray acceleration. We also calculate the high-energy neutrino and gamma-ray fluxes on Earth produced through p-$\gamma$ and p-p processes and compare them to the most sensitive astroparticle experiments.Comment: To be published in Astroparticle Physic