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- Front Matter: Volume 7924
- Wave Propagation in Random Media: Theoretical Studies
- Wave Propagation in Random Media: Experimental Studies
- Mitigation of Random Media (Atmosphere and Ocean) and Applications
- Modeling and Measurements of Atmospheric Turbulence
Front Matter: Volume 7924
Front Matter: Volume 7924
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This PDF file contains the front matter associated with SPIE Proceedings Volume 7924, including the Title Page, Copyright information, Table of Contents, and Conference Committee listing.
Wave Propagation in Random Media: Theoretical Studies
Generation of various partially coherent beams and their propagation properties in turbulent atmosphere: a review
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Partially coherent beams, such as Gaussian Schell-model beam, partially coherent dark hollow beam, partially coherent
flat-topped beam and electromagnetic Gaussian Schell-model beam, have important applications in free space optical
communications, optical imaging, optical trapping, inertial confinement fusion and nonlinear optics. In this paper,
experimental generations of various partially coherent beams are introduced. Furthermore, with the help of a tensor
method, analytical formulae for such beams propagating in turbulent atmosphere are derived, and the propagation
properties of such beams in turbulent atmosphere are reviewed.
Scintillation of Airy beam arrays in atmospheric turbulence
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The scintillation properties of Airy beam arrays in atmospheric turbulence are investigated. Similar to their
propagation in free space, the average propagation paths of Airy beams are also parabolic in turbulence. By
utilizing this self-bending property, the constituent Airy beamlets propagate through relatively independent
regions of turbulence and fully overlap at the on-axis detector. Through numeric simulations, it is shown that
the scintillation of Airy beam array is significantly reduced compared to a single Airy beam.
Scintillation properties of pseudo-Bessel correlated beams in atmospheric turbulence
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We introduce the concept of pseudo-Bessel correlated beams and investigate their scintillation properties on
propagation through turbulence. In weak turbulence the scintillation index of pseudo-Bessel correlated beams
is formulated by using the Rytov approximation. The study of scintillation is extended to strong turbulence by
numeric simulations. It is shown that by choosing an appropriate coherence parameter, pseudo-Bessel correlated
beams have lower scintillation than comparable fully coherent beams in both weak and strong turbulence.
Method of evaluation of the mutual coherence function of laser beams and its application for symmetric dark hollow beams
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In our report, a method of evaluation of the mutual coherence function (MCF) of optical wave propagating in turbulent
atmosphere is proposed. The method is based on using inverse Fresnel transform and 2-D Fast Fourier Transformation
procedure (FFT) and provides a high accuracy in various propagation conditions from weak up to strong optical
turbulence regime. This technique allows significantly reducing the evaluation time of MCF. The proposed method is
suitable for optical waves with arbitrary initial distribution of amplitude and phase. Results of investigations on the
degradation of coherence of symmetric dark hollow beams (DHB) propagating in turbulent atmosphere are presented.
Analysis of evolution of MCF is brought up, and some characteristics of DHB, such as mean intensity distribution, and
moments of Wigner distribution are calculated for various kinds of profiles of structural characteristic of refractive
index. All of the evaluations for DHB are fulfilled for Kolmogorov spectrum of correlation function of refractive index
fluctuations. The comparison between mean intensity calculations within the proposed method and method based on
semi-analytical approach using of quadratic approximation of spherical wave structure function, is presented.
Wave Propagation in Random Media: Experimental Studies
Probability density function of fluctuating intensity of a laser beam propagating in marine atmospheric turbulence
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We report on the analysis of experimental data collected at the United States
Naval Academy in the Summer and Fall of 2010. A low-power red He-Ne laser
source was used to generate a Gaussian beam. The beam was directed
horizontally above the ground and over the water for 400 m during daytime. The
transverse cross-section of the beam was projected on a white screen and
imaged using a ccd camera. The histogram of the fluctuating intensity at the
center of the beam was obtained from the sequence of photographs.
Comparison was made of the histogram and probability density functions of
fluctuating intensity based on two existing analytic models, Gamma-Gamma and
Gamma-Laguerre. Also the comparison of the beam statistics above the ground
and over the water was performed.
Picosecond laser pulse propagation delay fluctuation through atmosphere
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The influence of Earth atmospheric turbulence on the propagation of a picosecond laser pulse has been investigated
from point of view detection with high temporal resolution. The results have been interpreted for optical time scale
synchronization link allowing picosecond precision and accuracy in ground-to-space time transfer on a single photon
signal levels. The details in laser beam position changes, phase wave-front deformation or beam profile changes were
not studied like in adaptive optics as the goal of time transfer link is not the imaging but time tagging. The figure of
merit of presented results is the time of propagation, its absolute delay and jitter. The correlation of the atmospheric
turbulence with the propagation delay fluctuation was measured. The physical reason of the fluctuation of propagation
time of laser pulse on picosecond level is the same, but the entirely different approach in comparison to adaptive optics
was used to describe the effect.
Study of optical phase change measurement using the Hilbert Transform and interferometric techniques
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Advances in the fields of optics and optical communications have created a demand for effectively measuring
relative phase changes along an optical path or within an optical system. We present a method for obtaining these
measurements using an interferometric setup with processing involving Empirical Mode Decomposition and the
Hilbert Transform. In this work, the Hilbert Transform algorithm is justified by accurately measuring the phase
changes in software generated signals. Progress and improvements are shown regarding the ongoing design and
implementation of an experimental benchtop setup. This testbed will prove the method in applications such as
measuring and recording phase changes caused by propagating light through a turbulent freespace channel.
Mitigation of Random Media (Atmosphere and Ocean) and Applications
Optical wireless communication through random media
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The growing need for high data-rate communication both through the atmosphere and the ocean
(sub-sea) has stimulated considerable interest in optical wireless communication (OWC) technologies. The
main advantages of OWC as compared with RF communication in the atmosphere and with acoustic
communication in sub-sea applications are a) high achievable data-rate, b) small size of equipment and c)
low power-consumption. On the other hand the characteristics of the communication channel in both
scenarios are stochastic with high values of variance, which severely degrades OWC communication
system performance. In this paper we present a tutorial discussing the effects of random media on OWC
and expand on two examples: Monte-Carlo simulation for sub-sea communication and mathematical
synthesis using Meijer G-function for OWC through atmospheric turbulence. These two examples
demonstrate that it is possible to gain significant insights on the effects of the random channel on system
performance. The results of the different analysis methods could also indicate solutions for the
improvement of performance using adaptive solutions or for extending the communication range by
applying a multi-hop concept. We summarize the paper with a brief review of two emerging research fields
that could, surprisingly, benefit from the characteristics of light propagation through random media and its
effect on the communication system performance. The first research field is trans-cutaneous OWC and the
second is an unguided optical communication bus for next-generation computers.
Wavefront sensing and adaptive control in phased array of fiber collimators
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A new wavefront control approach for mitigation of atmospheric turbulence-induced wavefront phase aberrations in
coherent fiber-array-based laser beam projection systems is introduced and analyzed. This approach is based on
integration of wavefront sensing capabilities directly into the fiber-array transmitter aperture. In the coherent fiber array
considered, we assume that each fiber collimator (subaperture) of the array is capable of precompensation of local (onsubaperture)
wavefront phase tip and tilt aberrations using controllable rapid displacement of the tip of the delivery fiber
at the collimating lens focal plane. In the technique proposed, this tip and tilt phase aberration control is based on
maximization of the optical power received through the same fiber collimator using the stochastic parallel gradient
descent (SPGD) technique. The coordinates of the fiber tip after the local tip and tilt aberrations are mitigated
correspond to the coordinates of the focal-spot centroid of the optical wave backscattered off the target. Similar to a
conventional Shack-Hartmann wavefront sensor, phase function over the entire fiber-array aperture can then be retrieved
using the coordinates obtained. The piston phases that are required for coherent combining (phase locking) of the
outgoing beams at the target plane can be further calculated from the reconstructed wavefront phase. Results of analysis
and numerical simulations are presented. Performance of adaptive precompensation of phase aberrations in this laser
beam projection system type is compared for various system configurations characterized by the number of fiber
collimators and atmospheric turbulence conditions. The wavefront control concept presented can be effectively applied
for long-range laser beam projection scenarios for which the time delay related with the double-pass laser beam
propagation to the target and back is compared or even exceeds the characteristic time of the atmospheric turbulence
change - scenarios when conventional target-in-the-loop phase-locking techniques fail.
Scintillation reduction for combined Gaussian-vortex beam propagating through turbulent atmosphere
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We numerically examine the spatial evolution of the structure of coherent and partially coherent laser beams (PCBs),
including the optical vortices, propagating in turbulent atmospheres. The influence of beam fragmentation and
wandering relative to the axis of propagation (z-axis) on the value of the scintillation index (SI) of the signal at the
detector is analyzed. A method for significantly reducing the SI, by averaging the signal at the detector over a set of
PCBs, is described. This novel method is to generate the PCBs by combining two laser beams - Gaussian and vortex
beams, with different frequencies (the difference between these two frequencies being significantly smaller than the
frequencies themselves). In this case, the SI is effectively suppressed without any high-frequency modulators.
On fading probability density functions of fast-tracked and untracked free-space optical communication channels
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Free-space optical (FSO) communication systems suffer from average power loss and instantaneous power fading
due to the atmospheric turbulence. The channel fading probability density function (pdf) is of critical importance
for FSO communication system design and evaluation. The performance and reliability of FSO communication
systems can be greatly enhanced if fast-tacking devices are employed at the transmitter in order to compensate
laser beam wander at the receiver aperture. The fast-tracking method is especially effective when communication
distance is long. This paper studies the fading probability density functions of both fast-tracked and untracked
FSO communication channels. Large-scale wave-optics simulations are conducted for both tracked and untracked
lasers. In the simulations, the Kolmogorov spectrum is adopted, and it is assumed that the outer scale is infinitely
large and the inner scale is negligibly small. The fading pdfs of both fast-tracked and untracked FSO channels are
obtained from the simulations. Results show that the fast-tracked channel fading can be accurately modeled as
gamma-distributed if receiver aperture size is smaller than the coherence radius. An analytical method is given
for calculating the untracked fading pdfs of both point-like and finite-size receiver apertures from the fast-tracked
fading pdf. For point-like apertures, the analytical method gives pdfs close to the well-known gamma-gamma pdfs
if off-axis effects are omitted in the formulation. When off-axis effects are taken into consideration, the untracked
pdfs obtained using the analytical method fit the simulation pdfs better than gamma-gamma distributions for
point-like apertures, and closely fit the simulation pdfs for finite-size apertures where gamma-gamma pdfs deviate
from those of the simulations significantly.
RF-modulated pulsed fiber optic lidar transmitter for improved underwater imaging and communications
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We present results on the design, development and initial testing of a fiber-optic based RF-modulated lidar transmitter
operating at 532nm, for underwater imaging application in littoral waters. The design implementation is based on using
state-of-the-art high-speed FPGAs, thereby producing optical waveforms with arbitrary digital-RF-modulated pulse
patterns with carrier frequencies ≥ 3GHz, with a repetition rate of 0.5-1MHz, and with average powers ≥5W (at 532nm).
Use of RF-modulated bursts above 500MHz, instead of single optical pulse lidar detection, reduces the effect of
volumetric backscatter for underwater imaging application, leading to an improved signal-to-noise-ratio (SNR) and
contrast, for a given range. Initial underwater target detection tests conducted at Patuxent River Naval Air Station, MD,
in a large water-tank facility, validates the advantages of this hybrid-lidar-radar (HLR) approach for improved
underwater imaging, over a wide range of turbidity levels and both white and black targets. The compact, robust and
power-efficient fiber laser architecture lends very well to lidar sensor integration on unmanned-underwater-vehicle
(UUV) platforms. HLR transmitters can also provide similar advantages in active-sensing situations dominated by
continuous backscatter, e.g. underwater communications, imaging through smoke and fire environment, rotor-craft
landing in degraded visual environment, and pointing-tracking of active-EO sensors through fog.
Modeling and Measurements of Atmospheric Turbulence
A tunable diode laser absorption system for long path atmospheric transmission and high energy laser applications
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An open-path Tunable Diode Laser Absorption Spectroscopy (TDLAS) system composed of narrow band (~300 kHz)
diodes fiber coupled to a 12" Ritchey-Chrétien transmit telescope has been developed to study atmospheric transmission
of key High Energy Laser wavelengths. The ruggedized system has been field deployed and tested for propagation
distances of greater than 1 km. Initial experiments were performed in the vicinity of molecular oxygen X3Σ-g to b1Σ+gelectronic transition lines near 760 nm. The potassium version of the Diode Pumped Alkali Laser (DPAL) operates in
between two of the sharp oxygen rotational features in the PP and the PQ branches. By scanning across many laser free
spectral ranges and monitoring the laser frequency with a very precise wavemeter, the full structure of the oxygen
molecular feature is observed. The device can also be used to observe rotational temperatures, oxygen concentrations,
and total atmospheric pressure.
Atmospheric absorption spectroscopy using Tm: fiber sources around two microns
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We report on a thulium doped silica fiber ASE source for absorption spectroscopy of CO2. The average spectral power
of this source was 2.3-6.1 μW/nm. This low spectral power of this source posed limitation in the sensitivity of the
system which was overcome by using an ultrashort pulsed Raman amplifier system with 50-125 μW/nm average spectral
power. This system produced CO2 sensitivity better than 300 ppm making measurement of CO2 possible at standard
atmospheric concentrations.
Hybrid technique for propagation and scattering from random medium containing random distribution of particles
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On the basis of the angular spectrum representation of stochastic, statistically stationary
scalar fields, the Rytov's perturbation technique for propagation in weakly fluctuating
media and the first Born approximation for weak scattering, we develop a technique for
transmission of stochastic fields through turbulence containing randomly distributed
particles. Results for transmission of the deterministic (laser) field may be obtained
from our general results as a limiting case. We show how our technique can be applied
specifically for the atmospheric turbulence, but in general can also be of interest for
propagation in oceanic turbulence and biological tissues.