Federico Dios

Scintillation and beam-wander analysis in an optical ground station-satellite uplink

In an optical communication link between an optical ground station and a geostationary satellite the main problems appear in the uplink and are due to beam wander and to scintillation. Reliable methods for modeling both effects simultaneously are needed to provide an accurate tool with which the robustness of the communication channel can be tested. Numerical tools, especially the split-step method (also referred to as the fast-Fourier-transform beam propagation method), have demonstrated their ability to deal with problems of optical propagation during atmospheric turbulence. However, obtaining statistically significant results with this technique is computationally intensive. We present an analytical-numerical hybrid technique that provides good information on the variance in optical irradiance with an important saving of time and computational resources.

Exponentiated Weibull distribution family under aperture averaging for Gaussian beam waves

Nowadays, the search for a distribution capable of modeling the probability density function (PDF) of irradiance data under all conditions of atmospheric turbulence in the presence of aperture averaging still continues. Here, a family of PDFs alternative to the widely accepted Log-Normal and Gamma-Gamma distributions is proposed to model the PDF of the received optical power in free-space optical communications, namely, the Weibull and the exponentiated Weibull (EW) distribution. Particularly, it is shown how the proposed EW distribution offers an excellent fit to simulation and experimental data under all aperture averaging conditions, under weak and moderate turbulence conditions, as well as for point-like apertures. Another very attractive property of these distributions is the simple closed form expression of their respective PDF and cumulative distribution function.

Probability of fade and BER performance of FSO links over the exponentiated Weibull fading channel under aperture averaging

Recently a new proposal to model the fading channel in free-space optical links, namely, the exponentiated Weibull (EW) distribution, has been made. It has been suggested that the EW distribution can model the probability density function (PDF) of the irradiance under weak-to-strong conditions in the presence of aperture averaging. Here, we carry out an analysis of probability of fade and bit error-rate (BER) performance using simulation results and experimental data. The BER analysis assumes intensity modulation/direct detection with on-off keying, and new expressions are derived. Data is modeled following the statistics of the EW fading channel model, and compared with the Gamma-Gamma and Lognormal distributions, as the most widely accepted models nowadays. It is shown how the proposed EW model is valid in all the tested conditions, and even outperforms the GG and LN distributions, that are only valid under certain scenarios.

Temporal statistics of the beam-wander contribution to scintillation in ground-to-satellite optical links : an analytical approach

The beam-wander contribution to the scintillation in a ground-to-satellite free-space optical link is one of major importance. An analytical model, based on the duality between beam wander and angle-of-arrival fluctuations, is proposed for the temporal statistics. The expression of the probability density function of the log-amplitude fluctuations is first obtained. Then, the expressions of the spatial and temporal autocovariances are also obtained. We present plots of the beam-wander contribution to the log-amplitude variance, as a function of the transmitter aperture size and the turbulence accumulated in the propagation path. We also present the angular fluctuation and log-amplitude scintillation spectrum plots for some selected cases.

Self-consistent solution for general nonlinear slab waveguides

Abstract Nonlinear waveguides formed of a saturable intensity dependent refractive index medium bounded by linear materials are studied by means of a simple, versatile and exact numerical method, based on the transfer matrix approach in addition of the self-consistent nature of the field. In this work we have studied the TE propagation in a realistic nonlinear non-Kerr type waveguide. The application of the method to TM waves would be quite straightforward.

Temporal analysis of laser beam propagation in the atmosphere using computer-generated long phase screens.

Temporal analysis of the irradiance at the detector plane is intended as the first step in the study of the mean fade time in a free optical communication system. In the present work this analysis has been performed for a Gaussian laser beam propagating in the atmospheric turbulence by means of computer simulation. To this end, we have adapted a previously known numerical method to the generation of long phase screens. The screens are displaced in a transverse direction as the wave is propagated, in order to simulate the wind effect. The amplitude of the temporal covariance and its power spectrum have been obtained at the optical axis, at the beam centroid and at a certain distance from these two points. Results have been worked out for weak, moderate and strong turbulence regimes and when possible they have been compared with theoretical models. These results show a significant contribution of beam wander to the temporal behaviour of the irradiance, even in the case of weak turbulence. We have also found that the spectral bandwidth of the covariance is hardly dependent on the Rytov variance.

Accurate calculation of phase screens for the modelling of laser beam propagation through atmospheric turbulence

In this work, simulation of beam propagation through atmospheric turbulence is made by means of the split-step method, including spatially separated two-dimensional phase screens, which represent the existing turbulence. These phase screens may be generated mainly by two techniques, namely fractal interpolation or in the spatial frequency domain. We report some important considerations to take into account in order to generate phase screens with enough accuracy to properly reproduce the structure function of the turbulence. It is shown that slight deviations from the theoretical structure function in the set of screens used along the propagation may increase in an appreciable way the statistical error inherent to any particular realization. Some comparisons are made with analytical results based on the second order Rytov approximation. One of the conclusions that may be clearly drawn from these comparisons is that beam wander effects are not included in these theories.

Modeling of power fluctuations induced by refractive turbulence in a multiple-beam ground-to-satellite optical uplink

As a support for the experimental activities related to the operation of bi-directional optical links between the European Space Agency (ESA) ARTEMIS geostationary satellite and the Optical Ground Station (OGS) in Teide Observatory (Tenerife island, Canary Islands, Spain), carried out by ESA and Instituto de Astrofisica de Canarias (IAC), calculations on the behavior of a multi-beam ground-to-satellite link have been performed. The goal is to assess the impact of refractive turbulence on the uplink (deemed to be more critical than the downlink because of beam-wander effects) and the mitigation effect on the power fluctuations in the satellite receiver achievable with such a space diversity approach, that involves several mutually time-incoherent beams in the uplink. Results from the multiple-beam uplink model and comparison with experiments are presented.

Exponentiated Weibull fading model for free-space optical links with partially coherent beams under aperture averaging

Abstract. The recently proposed exponentiated Weibull (EW) fading channel model is assessed with partially coherent beams (PCBs) using the Gaussian Schell-model (GSM) theory. The use of a PCB in free-space optical links has been suggested as an atmospheric turbulence mitigation technique, called transmitter aperture averaging. A wave optics code based on the GSM theory is used to generate a PCB propagating in atmospheric turbulence, and the probability distribution function (PDF) is obtained from the simulated irradiance data. This study on the PDF includes the new proposed EW model in the presence of aperture averaging, in weak-to-strong turbulence regime. Point receiver conditions are also analyzed. For completeness of the study, the Lognormal and the Gamma-Gamma models have also been included in the analysis, allowing for a direct comparison to the EW distribution. Using a least-squares curve fitting algorithm, the EW model always has the best fit to simulated PDF data. Thus, the EW fading channel model proves to be valid for PCBs.

Intensity-modulated linear-frequency-modulated continuous-wave lidar for distributed media: fundamentals of technique

We analyze the intensity-modulation frequency-modulated continuous-wave (FMCW) technique for lidar remote sensing in the context of its application to distributed media. The goal of the technique is the reproduction of the sounded-medium profile along the emission path. A conceptual analysis is carried out to show the problems the basic version of the method presents for this application. The principal point is the appearance of a bandpass filtering effect, which seems to hinder its use in this context. A modified version of the technique is proposed to overcome this problem. A number of computer simulations confirm the ability of the modified FMCW technique to sound distributed media.