Jennifer C. Ricklin

Atmospheric channel effects on free-space laser communication

AbstractFree-space laser communication offers an attractive alternative for transferring high-bandwidth data when fiber optic cable is neither practical nor feasible. However, there are a variety of deleterious features of the atmospheric channel that may lead to serious signal fading, and even the complete loss of signal altogether. Physical obstructions—such as birds, insects, tree limbs, or other factors—can temporarily or permanently block the laser line-of-sight. Platform/building motion due to wind, differential heating and cooling, or ground motion over time can result in serious misalignment of fixed-position laser communication systems. But most importantly of all, absorption and scattering due to particulate matter in the atmosphere may significantly decrease the transmitted optical signal, while random atmospheric distortions due to optical turbulence can severely degrade the wave-front quality of a signal-carrying laser beam, causing intensity fading and random signal losses at the receiver.

Effects of the atmospheric channel on free-space laser communications

In this paper we provide an overview of how the presence of atmospheric turbulence and scattering/absorbing media in the atmospheric channel degrade high-data-rate free-space laser communication performance. The impact of the atmospheric channel on overall link budget performance is discussed. Fog, rain, dust, snow, smoke, molecular absorption, and aerosol particulate matter all attenuate the signal-carrying laser beam, and to a certain extent can be compensated for by increasing the signal gain or by appropriate selection of the optical wavelength. In contrast, random fluctuations in the atmospheres refractive index severely degrade the wave-front quality of a signal-carrying laser beam, causing intensity fading and random signal losses at the receiver. This results in increased system bit error rates, especially along horizontal propagation paths. Atmospheric turbulence-induced signal losses increase as the distance between the transmitter and receiver is increased, and there is no wavelength window where these effects can be avoided, although longer wavelengths are less affected. With atmospheric turbulence, increasing the signal gain will not necessarily improve laser beam quality. For many cases of practical interest, the limiting factor in robust free-space optical communication link performance can be the presence of clear-air atmospheric turbulence in the optical channel. Various proposed probability density functions of laser intensity fluctuations through atmospheric turbulence will be discussed as they relate to laser communications performance and reliability under different weather conditions. Results from numerical simulations are presented for analyzing communications performance for various scenarios: Downlink, Uplink and Terrestrial (Horizontal) link.

Atmospheric turbulence measurements over desert site using ground-based instruments, kite/tethered-blimp platform, and aircraft relevant to optical communications and imaging systems: preliminary results

New results of the (temperature) refractive index structure parameter (CT2), Cn2 are presented from fast response sensor observations near the ground and also using a kite/tethered blimp platform and an aircraft, at the Edward Air Force Base in Mojave Desert, California. Additional optical measurements include near-ground scintillation observations over horizontal paths. Atmospheric turbidity were also calculated from direct beam solar radiation measurements using pyrheliometer. Comparisons were made of the observed profiles of refractive index structure parameters (Cn2) with theoretical modeled profiles, and two derived quantities such as transverse coherence length (r0) and isoplanatic angle (θ0) for a slant path are discussed. All of these parameters are the major indicators of turbulence and are important to design an aircraft or space-craft-based free-space laser communication and high resolution optical synthetic-aperture imaging systems. Non-isotropic turbulence observations from some of the data will be pointed out. Probability density functions (PDF) of the distribution of Cn2 will be described using histograms. Fundamental limits imposed by atmospheric effects in high data rate communication and optical synthetic-aperture imaging systems will be discussed.

Nondimensional beam parameters and the wave structure function

Two pairs of nondimensional beam parameters are identified, either of which combined with wavelength and path length completely characterizes the diffractive propagation environment of a Gaussian beam. One pair is associated with the beam radius and radius of curvature at the transmitter while the other pair is associated with these same quantities at the receiver. The fundamental nature of the receiver parameters is revealed in the simple analytic forms for the wave structure function derived under the assumption of a modified Kolmogorov spectral model for refractive index fluctuations.

Irradiance variance for convergent Gaussian beams using nondimensional beam parameters

The longitudinal and radial irradiance variance for a convergent Gaussian beam is examined in terms of two pairs of nondimensional beam parameters, the first pair (Omega) equals (lambda) L/(pi) Wo2 and (Omega) o equals 1 - L/Ro associated with the beam when transmitted, and the second pair given by (Lambda) equals (lambda) L/(pi) W2 and (Theta) equals 1 + L/R associated with the received beam. Here, (lambda) is wavelength, L is path length, Ro and R are radii of curvature of the phase front at the transmitter and receiver, respectively, and Wo and W are the beam radii at the transmitter and receiver. With the addition of (lambda) and L, either of these beam parameter pairs completely characterizes the diffractive propagation environment for a lowest- order paraxial Gaussian beam, and is fundamental in the analytic expression of the irradiance variance. Special attention is paid to differences between the perfectly focused beam and the nearly focused beam. We also show that every beam has a convergent counterpart with identical diffractive irradiance behavior at the receiver, but decreased irradiance variance.

Academician Vladimir Evseevich Zuev—Scientist, Teacher, and Organizer

This PDF file contains the editorial “Editorial: Academician Vladimir Evseevich Zuev—Scientist, Teacher, and Organizer” for OE Vol. 44 Issue 07

Description Of Optical Turbulence Effects On Propagation In The Atmospheric Surface Boundary Layer

A methodology is presented to characterize propagation through clear air turbulence in the surface boundary layer of the atmosphere, based on readily obtained environmental observables. Obukhov similarity theory is employed to define a consistent set of flux profile relationships, and the Kolmogorov principle of universal equilibrium to estimate a profile for the refractive index structure parameter. This profile serves as input to a propagation model based on weak perturbation theory, allowing estimates of log-amplitude variance, receiver coherence diameter, isoplanatic effective path length, and scintillation averaging length, as well as a variety of subsidiary imaging statistics.