We introduce a realistic model for the impact of atmospheric phase and amplitude fluctuations on free-space links using
either synchronous or nonsynchronous detection. We compare options for atmospheric compensation, including active
modal methods and diversity combining techniques. We consider the effects of log-normal amplitude fluctuations and
Gaussian phase fluctuations, in addition to local oscillator shot noise. We study the effect of various parameters,
including the ratio of receiver aperture diameter to wavefront coherence diameter, the scintillation index, the number of
modes compensated, and the number of independent diversity branches combined at the receiver. We analyze outage
Shannon capacity, placing upper bounds on the achievable spectral efficiency and enabling the performance of specific
system designs to be predicted.
Free-space optical communications systems provide the opportunity to take advantage of higher data transfer
rates and lower probability of intercept compared to radio-frequency communications. However, propagation
through atmospheric turbulence, such as for airborne laser communication over long paths, results in intensity
variations at the receiver and a corresponding degradation in bit error rate (BER) performance. Previous
literature has shown that two transmitters, when separated sufficiently, can effectively average out the intensity
varying effects of the atmospheric turbulence at the receiver. This research explores the impacts of adding more
transmitters and the marginal reduction in the probability of signal fades while minimizing the overall transmitter
footprint, an important design factor when considering an airborne communications system. Analytical results
for the cumulative distribution function are obtained for tilt-only results, while wave-optics simulations are used
to simulate the effects of scintillation. These models show that the probability of signal fade is reduced as the
number of transmitters is increased.
We consider theoretically and numerically the suppression of fluctuations (scintillations) of a laser
beam propagating through turbulent atmospheres by applying a phase modulator. Both spatial and
temporal phase variations introduced by this phase modulator are analyzed. The explicit
dependences of the scintillation index on the initial correlation length and finite-time phase
variations for long propagation paths are obtained. Results of modeling and numerical simulations
are presented. We demonstrate that an appropriately chosen phase modulator can significantly
suppress the scintillations of the laser beam caused by turbulent atmospheres.
The dynamic features of the adaptive optical systems are researched. It is executed development of the traditional
adaptive systems, having in its composition wave-front sensor, active mirrors and guide source. The efficiency of the
using algorithms of phase and amplitude-phase corrections are analyzed. The schemes full and partial correcting of the
phase distortion are considered. It is entered in analysis the new class adaptive systems, including in itself, the "system
constant delay", as well as "speed" system, "forecasting" adaptive system and systems, using idea of "frozen"
fluctuations in optical wave. For development of this ideas I offer to use the differential optical measurements in channel
of guide source.
A hybrid centroiding technique involving Iteratively Weighted Center of Gravity (IWCoG) algorithm and correlation
technique for a Laser Guide Star (LGS) based Shack Hartmann wavefront sensor is proposed. A simple
method for simulating LGS elongated spots with photon noise and read out noise is demonstrated. The problems
associated with IWCoG are addressed (a) Error saturation is minimized by adding random numbers iteratively
to centroid positions, (b) non uniform convergence of Centroid Estimation Error (CEE) is reduced by using
the hypothesis that the iteration number with maximum correlation between the weighting function and the
actual spot image function is the iteration with minimum error, (c) convergence rate is improved by shifting the
weighting function to the point of maximum intensity in first iteration. The novelty of the algorithm is tested
by comparing with other centroiding algorithms.
In this paper we report the results of the analysis and experimental modeling of the target-in-the-loop (TIL) approach
that is used to form a localized beacon for a laser beam propagating through turbulent atmosphere. The analogy between
the TIL system and the laser cavity has been used here to simulate the process shaping the laser beacon on a remote
image-resolved target with rough surface. The TIL breadboard was integrated and used for laboratory modeling of the
proposed approach. This breadboard allowed to simulate the TIL arrangement with a rough-surface target and laser beam
propagation through the turbulent atmospheric layer. Here we present the initial results of the performed studies.
At present, system design usually assumes the Kolmogorov model of refractive index fluctuation spectra in the
atmosphere. However, experimental data indicates that in the atmospheric boundary layer and at higher altitudes the
turbulence can be different from Kolmogorov's type.
In optical communications, analytical models of mean irradiance and scintillation index have been developed for a
traditional Kolmogorov spectrum and must be revised for non-Kolmogorov turbulence.
The image quality (resolution, MTF, etc.) is essentially dependent on the properties of turbulent media. Turbulence MTF
must be generalized to include non-Kolmogorov statistics. The change in fluctuation correlations of the refractive index
can lead to a considerable change in both the MTF form and the resolution value.
In this work, on the basis of experimental observations and modeling, generalized atmospheric turbulence statistics
including both Kolmogorov and non-Kolmogorov path components are discussed, and their influence on imaging and
communications through the atmosphere estimated for different scenarios of vertical and slant-path propagation. The
atmospheric model of an arbitrary (non-Kolmogorov) spectrum is applied to estimate the statistical quantities associated
with optical communication links (e.g., scintillation and fading statistics) and imaging systems.
Implications can be significant for optical communication, imaging through the atmosphere, and remote sensing.
The DARPA Optical RF Communications Adjunct (ORCA) program was created to bring high data rate networking to
the warfighter via airborne platforms. Recent testing of the ORCA system was conducted by the Northrop Grumman
Corporation (NGC) at the Nevada Test and Training Range (NTTR) at the Nellis Air Force Range near Tonopah, NV.
The University of Central Florida (UCF) conducted a parallel test to measure path-averaged values of the refractiveindex
structure parameter, the inner scale of turbulence, and the outer scale of turbulence along the ORCA propagation
path from an airborne platform to the ground at Antelope Peak. In addition, weather instrumentation was set up at
ground level on Antelope Peak to measure local conditions on the mountain top. This paper presents background
information on expected atmospheric conditions for the channel, models that were used by UCF for the measurements,
path-averaged values of the three atmospheric parameters, and a Cn2 profile model as a function of altitude.
Real-time measurement of effective wind speed helps in continuous monitoring of the temporal bandwidth in
adaptive optics systems. In this paper, we propose a simple and efficient method for estimating wind speed
from wavefront sensor data. A small portion from the center of a wavefront arriving at time 't' is mapped at all
possible positions on phase screens coming later than 't' by a time n × Δtint (Δtint-integration time, n-integer)
to form correlation maps. Wind speed and direction is estimated statistically via the calculation of the vectors
formed by joining the central portion and the position of maximum correlation on the correlation maps. Number
of such realizations to be considered to arrive at an accurate estimate of wind speed was optimized to reduce
computing time and increase accuracy.
Electromagnetic and electro-optical (EM/EO) propagation and scattering in the ocean is of interest for a wide range of
science problems. For example, the biological productivity of ocean waters through photochemical processes is governed
by the vertical attenuation of solar radiation. Also, EO scattering theory is the primary basis for determining
biogeochemical parameters (e.g. phytoplankton, suspended sediments, and dissolved matter) from the water leaving
optical radiance. In addition, EO scattering from suspended sediments and bubbles is the limiting factor for active lidar
systems used to map the sea bottom. This work will review specific applications of EO/EM scattering theory with regard
to the influence of bubbles and droplets on remote sensing in the nearshore ocean. The current state of understanding
concerning models and applications for optical scattering from bubbles in the water column as well as microwave
scattering from water droplets produced by breaking waves at the ocean surface will be discussed as well as future
Author(s): Fabrice R. A. Onofri; Mariusz Krzysiek; Séverine Barbosa; Mariusz Wozniak; Janusz Mroczka; Yijia Yuan; Kuan-Fang Ren
Under real flow conditions, the critical-angle scattering of a spherical bubble is too noisy to obtain directly the bubble
diameter and refractive index. To solve this problem and to limit the drawback of any counting technique, the critical
angle refractometry and sizing (CARS) technique has been extended as a collective ensemble technique. As an inverse
method it allows to get the size distribution and composition of a cloud of bubbles. In this paper we review the principle,
the advantages and limits of this new optical particle characterization method.
Author(s): Gergely Dolgos; J. Vanderlei Martins; Lorraine A. Remer; Alexandre L. Correia; Manfredo Tabacniks; Adriana R. Lima
Characterization of aerosol scattering and absorption properties is essential to accurate radiative transfer calculations in
the atmosphere. Applications of this work include remote sensing of aerosols, corrections for aerosol distortions in
satellite imagery of the surface, global climate models, and atmospheric beam propagation. Here we demonstrate
successful instrument development at the Laboratory for Aerosols, Clouds and Optics at UMBC that better characterizes
aerosol scattering phase matrix using an imaging polar nephelometer (LACO-I-Neph) and enables measurement of
spectral aerosol absorption from 200 nm to 2500 nm. The LACO-I-Neph measures the scattering phase function from
1.5° to 178.5° scattering angle with sufficient sensitivity to match theoretical expectations of Rayleigh scattering of
various gases. Previous measurements either lack a sufficiently wide range of measured scattering angles or their
sensitivity is too low and therefore the required sample amount is prohibitively high for in situ measurements. The
LACO-I-Neph also returns expected characterization of the linear polarization signal of Rayleigh scattering. Previous
work demonstrated the ability of measuring spectral absorption of aerosol particles using a reflectance technique
characterization of aerosol samples collected on Nuclepore filters. This first generation methodology yielded absorption
measurements from 350 nm to 2500 nm. Here we demonstrate the possibility of extending this wavelength range into the
deep UV, to 200 nm. This extended UV region holds much promise in identifying and characterizing aerosol types and
species. The second generation, deep UV, procedure requires careful choice of filter substrates. Here the choice of
substrates is explored and preliminary results are provided.
Ghost imaging in the presence of turbulent atmosphere in both arms of the arrangement is
predicted to show the improvement if the propagation distances from the source are
sufficiently large. For intermediate distances from the source ghost images are very weak or
Intensity fluctuations of incoherently superposed Gaussian beams are formulated in weak turbulence by employing the
extended Huygens-Fresnel principle. Each individual beam superposed is taken to be fully incoherent. The scintillation
index evaluated for different number of beams indicates that as the number of beams increase, scintillations decrease.
Incoherent superposition of smaller sized Gaussian sources exhibits smaller fluctuations. Comparing the scintillation
index arising from incoherently superposed Gaussian beams to the scintillation index of coherently superposed Gaussian
beams of the same structure shows that incoherent superposition yields lower intensity fluctuations, thus can be
advantageous in atmospheric optical communication links.
Propagation of elegant higher-order Gaussian beams in turbulent atmosphere is studied in detail. Analytical propagation
formulae of elegant higher-order Gaussian beams in turbulent atmosphere are derived based on extended
Huygens-Fresnel integral. The intensity and spreading properties of elegant higher-order Gaussian beams and standard
higher-order Gaussian beams in turbulent atmosphere are studied numerically and comparatively. It is found that the
propagation properties of elegant higher-order Gaussian beams and standard higher-order Gaussian beams are much
different from their properties in free space The standard higher-order Gaussian beams spread more rapidly than the
elegant higher-order Gaussian beams in turbulent atmosphere.
The interest for free space laser beam propagation has recently increased due to several experiments. These experiments
have shown that optical links through the atmosphere can be effectively established, regardless of the several deleterious
negative effects of refractive index fluctuations. The most deleterious effect is the scintillation which, consequently, has
been widely investigated. However, it has not been reported yet what influence a ground profile has on scintillation
values. In this paper we show theoretical results of the ground impact on scintillation, for a laser link between aircraft
and mountain top; Antelope peak Nevada. The link was investigated for a specific mountainous profile of the ground for
path lengths of 50 km and 200 km. The theoretical analysis shows that, if a low value is assumed for the Rytov variance
(weak turbulence condition), then the presence of high peaks or mountains along the propagation path could have a
remarkable impact on scintillation because the scintillation saturation effects do not occur; if the Rytov variance assumes
medium-high values (moderate-to-strong turbulence conditions), then the scintillation index will be close to the
saturation regime. Therefore the scintillation will be slightly affected by the mountains along the path, both for the plane
wave model and the Gaussian beam wave model.
In this paper we review our work done in the evaluations of the root mean square (rms) beam wander characteristics of
the flat-topped, dark hollow, cos-and cosh Gaussian, J0-Bessel Gaussian and the I0-Bessel Gaussian beams in
atmospheric turbulence. Our formulation is based on the wave-treatment approach, where not only the beam sizes but the
source beam profiles are taken into account as well. In this approach the first and the second statistical moments are
obtained from the Rytov series under weak atmospheric turbulence conditions and the beam size are determined as a
function of the propagation distance. It is found that after propagating in atmospheric turbulence, under certain
conditions, the collimated flat-topped, dark hollow, cos- and cosh Gaussian, J0-Bessel Gaussian and the I0-Bessel
Gaussian beams have smaller rms beam wander compared to that of the Gaussian beam. The beam wander of these
beams are analyzed against the propagation distance, source spot sizes, and against specific beam parameters related to
the individual beam such as the relative amplitude factors of the constituent beams, the flatness parameters, the beam
orders, the displacement parameters, the width parameters, and are compared against the corresponding Gaussian beam.
Analytical propagation formula for the propagation factor of a phase locked radial laser array beam is derived based on
the extended Huygens-Fresnel integral and the Wigner distribution function. Evolution properties of the propagation
factor in turbulent atmosphere are studied numerically. It is found that unlike its propagation invariant properties in free
space, the propagation factor of a radial laser array beam increases on propagation in turbulent atmosphere, and is closely
related to the parameters of initial beam and the atmosphere.
The propagation of partially coherent vortex beams through atmospheric turbulence in weak-to-strong fluctuation
regimes is investigated. Irradiance profiles from wave optics simulations and analytical theory compare favorably for a
variety of link parameters. Simulation results indicate that partially coherent vortex beams can reduce scintillation index
values relative to comparable classic Gaussian Schell model beams when turbulence conditions are mediate to strong.
However, the overall propagation performance of partially coherent vortex beams, as measured by the metric Δ, tends to
be poorer than classic Gaussian Schell model beams because of larger inherent beam spread.
We find that on propagation in turbulent ocean with power
spectrum being a combination of power spectra of temperature and salinity
fluctuations a stochastic beam can exhibit polarization changes which are qualitatively
very similar to those which occur on propagation in atmospheric turbulence.
However, pronounced quantitative difference between polarization
changes in atmosphere and ocean should be noted. The analytical development
is supported by numerical examples.