We develop a model for the probability density function (pdf) of the irradiance fluctuations of an optical wave propagating through a turbulent medium. The model is a two-parameter distribution that is based on a doubly stochastic theory of scintillation that assumes that small-scale irradiance fluctuations are modulated by large-scale irradi- ance fluctuations of the propagating wave, both governed by indepen- dent gamma distributions. The resulting irradiance pdf takes the form of a generalized K distribution that we term the gamma-gamma distribution. The two parameters of the gamma-gamma pdf are determined using a recently published theory of scintillation, using only values of the refractive-index structure parameter C n (or Rytov variance) and inner scale l 0 provided with the simulation data. This enables us to directly calculate various log-irradiance moments that are necessary in the scaled plots. We make a number of comparisons with published plane wave and spherical wave simulation data over a wide range of turbu- lence conditions (weak to strong) that includes inner scale effects. The gamma-gamma pdf is found to generally provide a good fit to the simu- lation data in nearly all cases tested. © 2001 Society of Photo-Optical Instrumen-

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Mathematical model for the irradiance probability density function of a laser beam propagating through turbulent media

Over the last two decades free-space optical communication (FSO) has become more and more interest- ing as an adjunct or alternative to radio frequency commu- nication. This article gives an overview of the challenges a system designer has to consider while implementing an FSO system. Typical gains and losses along the path from the transmitter through the medium to the receiver are in- troduced in this article. Detailed discussions of these topics can be found in this special issue of the Radioengineering Journal.

An Introduction to Free-space Optical Communications

In spite of the tremendous technical advancement of available components, the major limitation of free-space laser communication (lasercom) performance is due to the atmosphere, because a portion of the atmospheric path always includes turbulence and multiple scattering effects. Starting from a fundamental understanding of the laser communications system under diverse weather conditions, this chapter provides a comprehensive treatment of the evaluation of parameters needed for analyzing system performance. The significance of higher-order statistics of probability density functions of irradiance fluctuations due to turbulence to performance analysis is explained. Starting from link analysis, the necessary expressions relating link margin, bit-error-rate, signal-to-noise-ratio, and probability of fade statistics are presented. Results for laboratory-simulated atmospheric turbulence and multiple scattering are presented. Example numerical results for simulations of lasercom systems operating under various at mospheric conditions are presented for various scenarios such as uplink-downlink (e.g., between ground and satellite, aircraft or UAV) and horizontal (terrestrial) link. Both turbulence and multiple scattering effects have been included in the analysis with both on-off keying and pulse-position modulation schemes. Statistical estimation and computation of communication parameters presented in this chapter will be useful in designing and optimizing lasercom systems that are reliable under all weather conditions.

Free-space laser communication performance in the atmospheric channel

Free-space laser communication systems have the potential to provide flexible, high-speed connectivity suitable for long-haul intersatellite and deep-space links. For these applications, power-efficient transmitter and receiver designs are essential for cost-effective implementation. State-of-the-art designs can leverage many of the recent advances in optical communication technologies that have led to global wide-band fiber-optic networks with multiple Tbit/s capacities. While spectral efficiency has long been a key design parameter in the telecommunications industry, the many THz of excess channel bandwidth in the optical regime can be used to improve receiver sensitivities where photon efficiency is a design driver. Furthermore, the combination of excess bandwidth and average-power-limited optical transmitters has led to a new paradigm in transmitter and receiver design that can extend optimized performance of a single receiver to accommodate multiple data rates.

Laser communication transmitter and receiver design

The performance of free-space optics (FSO) communication systems are affected by building sway and atmospheric interference. These adverse effects can be mitigated by appropriately adjusting the system parameters. In this paper, two optimization models are established and solved. The desirable transmitter power, transmitter wavelength, transmitter telescope gain, and receiver telescope gain can be determined accordingly. The solution feasibility relies on using the quantum cascade laser (QCL) in transmitters. Details of the numerical experiments are also reported.

Free-space optics optimization models for building sway and atmospheric interference using variable wavelength

Past, present, and future architecture and applications system design methodology optical design the communication receiver the acquisition receiver the tracking receiver key technologies configuring the system trading RF and laser communications for your system size, weight, and power trends in laser crosslinks laser communications applications.

Stephen G. Lambert

Papers

Laser Communications in Space