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Principles Of Optics Electromagnetic Theory Of Propagation Interference And Diffraction Of Light

The scintillation index of plane and spherical light waves as well as of a Gaussian beam, propagating in the clear-water weakly turbulent ocean, is revealed. The results are of utmost importance for underwater optical communications and sensing. An analysis of the threshold between the weak and strong regimes of oceanic turbulence is made, with the accent on the contribution from salinity-induced turbulence. It is found that strong oceanic turbulence can occur at distances as short as several meters, in striking contrast with atmospheric studies for which the typical distances are on the order of a kilometer.

Light scintillation in oceanic turbulence

In a recent publication [Opt. Lett.37, 2970 (2012)10.1364/OL.37.002970], a novel class of planar stochastic sources, generating far fields with flat intensity profiles, was introduced. In this paper we examine the behavior of the spectral density and the state of coherence of beamlike fields generated by such sources on propagation in free space and linear isotropic random media. In particular, we find that at sufficiently large distances from the source, the medium destroys the flat intensity profile, even if it remains such for intermediate distances from the source.

Multi-Gaussian Schell-model beams

Atmospheric turbulence induces significant variation on the angle-of-arrival of laser beams used in free space laser
communication. Angle-of-arrival fluctuations of an optical wave in the plane of the receiver aperture can be described
in terms of the phase structure function that already has been calculated by Kolmogorov's power spectral density model.
Unfortunately several experiments showed that Kolmogorov theory is sometimes incomplete to describe atmospheric
statistics properly. In this paper, for horizontal path and weak turbulence, we carry out analysis of angle-of-arrival
fluctuations using a non Kolmogorov power spectrum which uses a generalized exponent factor instead of constant
standard exponent value 11/3 and a generalized amplitude factor instead of constant value 0.033. Also our non
Kolmogorov spectrum includes both inner scale and outer scale effects.

Angle of arrival fluctuations for free space laser beam propagation through non kolmogorov turbulence

In clean ocean water, the performance of a underwater optical communication system is limited mainly by oceanic turbulence, which is defined as the fluctuations in the index of refraction resulting from temperature and salinity fluctuations. In this paper, using the refractive index spectrum of oceanic turbulence under weak turbulence conditions, we carry out, for a horizontally propagating plane wave and spherical wave, analysis of the aperture-averaged scintillation index, the associated probability of fade, mean signal-to-noise ratio, and mean bit error rate. Our theoretical results show that for various values of the rate of dissipation of mean squared temperature and the temperature-salinity balance parameter, the large-aperture receiver leads to a remarkable decrease of scintillation and consequently a significant improvement on the system performance. Such an effect is more noticeable in the plane wave case than in the spherical wave case.

Underwater optical communication performance for laser beam propagation through weak oceanic 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's 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 =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 a value close to =3 or for alpha values higher than =11/3, scintillation decreases, leading to an improvement on the system performance.

/pdf/free-space-optical-system-performance-for-laser-beam-1hqn2qrcug.pdf

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

A wealth of experimental data has shown that atmospheric turbulence can be anisotropic; in this case, a Kolmogorov spectrum does not describe well the atmospheric turbulence statistics. In this paper, we show a quantitative analysis of anisotropic turbulence by using a non-Kolmogorov power spectrum with an anisotropic coefficient. The spectrum we use does not include the inner and outer scales, it is valid only inside the inertial subrange, and it has a power-law slope that can be different from a Kolmogorov one. Using this power spectrum, in the weak turbulence condition, we analyze the impact of the power-law variations α on the long-term beam spread and scintillation index for several anisotropic coefficient values ς. We consider only horizontal propagation across the turbulence cells, assuming circular symmetry is maintained on the orthogonal plane to the propagation direction. We conclude that the anisotropic coefficient influences both the long-term beam spread and the scintillation index by the factor ς2−α.

Light propagation through anisotropic turbulence.

In this paper, the concept of anisotropy at different atmospheric turbulence scales is introduced. A power spectrum and its associated structure function with inner and outer scale effects and anisotropy are also shown. The power spectrum includes an effective anisotropic parameter ζeff to describe anisotropy, which is useful for modeling optical turbulence when a non-Kolmogorov power law and anisotropy along the direction of propagation are present.

Introducing the concept of anisotropy at different scales for modeling optical turbulence.

Free space laser system performance is limited by atmospheric turbulence that 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. In this paper we present 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 new spectrum in weak turbulence, we carry out, for horizontal path, analysis
of Long Term Beam Spread, Scintillation index, Probability of fade, mean SNR and mean BER as variation of the
spectrum exponent.

/pdf/free-space-optical-system-performance-for-laser-beam-1uaap2ae84.pdf

Italo Toselli

Papers

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

Angle of arrival fluctuations for free space laser beam propagation through non kolmogorov turbulence

Light propagation through anisotropic turbulence.

Introducing the concept of anisotropy at different scales for modeling optical turbulence.

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