Communication in maritime environments presents unique challenges often requiring the secure transfer of information over long distances in a complex dynamic environment. Here a system for generating orbital angular momentum (OAM) beams, multiplexing, transmitting, and demultiplexing using a convolutional neural network (CNN) is presented. A single input from a 1550 nm seed laser is amplified, split into four separate beams that are directed and modulated by four switches, and the resulting beams directed into phase plates to create beams carrying OAM. These four beams constitute the individual channels. The beams are passed through several optical elements, coherently combined, and transmitted to a receiver at a range of 12 m. The resulting OAM beam spatial patterns are captured using a high speed short-wave infrared detector, concurrently transmitted to a workstation for storage, and processed in real-time using a trained CNN. Results from short range and quiescent environmental state show a pattern detection accuracy of <99%.
The use of Adaptive Optics (AO) to correct for aberrations in a wavefront of propagating light has become customary for Astronomical applications and is now expanding to many other areas going from medical imaging to industrial applications. However, the propagation of light underwater has remained out of the main stream AO community for a variety of reasons, not least the shear difficulty of the situation. Our group has become a program that attempts to define under which circumstances such a correction could be envisioned. We take advantage of the NRL laboratory facility in Stennis, MS, where a large Plexiglas tank of water is equipped with heating and cooling plates that allow for a well measured thermal gradient that in turn generates different degrees of turbulence that can distort a propagating laser beam. In this paper we report on the preliminary findings of this ongoing program. The paper will describe the facility and the AO test-bed, the measurements made and some of the preliminary result.
Over the last few decades, we have seen an increase in both quality and quantity of 3D data sets. These data sets primarily come in the form of discrete points that are projected onto the surface of the object (point clouds) and are often derived from either LIDAR data (in which case, the surface points are actively sensed) or stereoscopic pairs (in which case, the surface points are derived using two dimensional (2D) feature matching algorithms). As these data sets become larger and denser, they also become harder to sift through which demands methods for automatic object classification through computer vision processes. In this paper we revisit a method of recognizing objects from their surface features known as Tripod Operators.[1] More specifically, we explore how matching multiple features from an unknown object to a known shape allows us to determine the extent to which the objects are similar using the resultant Digital Elevation Model (DEM) or Surface Elevation Model (SEM) that results from manipulation of point clouds.. We apply this method to determine how to separate objects of various classes.
The use of Adaptive Optics (AO) to correct for aberrations in a wavefront of propagating light has become customary for Astronomical applications and is now expanding to many other areas going from medical imaging to industrial applications. However, the propagation of light underwater has remained out of the main stream AO community for a variety of reasons, not least the shear difficulty of the situation. Our group has become a program that attempts to define under which circumstances such a correction could be envisioned. We take advantage of the NRL laboratory facility in Stennis, MS, where a large Plexiglas tank of water is equipped with heating and cooling plates that allow for a well measured thermal gradient that in turn generates different degrees of turbulence that can distort a propagating laser beam. In this paper we report on the preliminary findings of this ongoing program. The paper will describe the facility and the AO test-bed, the measurements made and some of the preliminary result.
Characterizing atmospheric turbulence through modeling dates back to the 1960's. For decades scientists have
studied how to mitigate the effects of the atmospheric turbulence on communications and imaging systems, but learning
how to use those properties of the atmosphere instead of mitigate them raise new challenges. Due to the fact that
atmospheric turbulence is inherently a random process, it can be an ideal "key generator" for strongly secure information
transfer. The purpose of this effort is to investigate to what extent the atmospheric turbulence can be exploited as a
robust random number generator. In this paper we report the progress in characterizing the atmosphere and a random bitstream
generator.
Modeling and simulating the atmosphere in a controlled environment has been a study of interest to
scientists for decades. The development of new technologies allows scientists to perform this task in a more realistic
and controlled environment and provides a powerful tool for the study and better understanding of the propagation
of light through the atmosphere. Technologies like Free-space laser communications (FSLC) and/or studies on light
propagation through the atmosphere are areas which constantly benefit from breakthroughs in the development of
atmospheric turbulence simulators. In this paper we present the results of the implementation of a phase only spatial
light modulator (SLM) as an atmospheric turbulence simulator at the Short-Wave Infra-Red (SWIR) regime and its
use with a FSLC system.
The use of Spatial Light Modulators (SLM), Liquid Crystal Devices for atmospheric turbulence simulation in
optical system has increased in the recent years. These devices allow the development of test-beds that can be
used to simulate, analyze and improve optical components or systems in a controlled laboratory environment
before further implementation on the eld. Most research has been performed at visible wavelengths with
the use of a vast array of atmospheric turbulence simulation algorithms. We present preliminary work on an
atmospheric simulation test bed which uses an algorithm developed at NRL with a transmissive high denition
Liquid Crystal Device SLMs for applications in the short-wavelength infrared, with the main focus of interested
at 1550nm. Preliminary results are shown for the application to a high denition re
ective Liquid Crystal Device
SLM for the same wavelength.
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.
In this work, a free-space infrared communications system is described. The system has the capability of using
previously captured scintillation data and introducing the effects onto the bench-top system. This effectively acts as
a scintillation simulator which emulates an optical link that is effected by the weather and various physical
conditions at the time of transmission. The method used for scintillation simulation is described. The transmission
method of the system is a hybrid combination of traditional frequency modulation (FM) and optical amplitude
modulation (OAM) combined with Multiple Quantum Well (MQW) Modulating Retroreflector (MRR) technology.
The result has produced a robust, low power system that is capable of transmitting real-time audio information with
high clarity along a channel that accurately simulates the atmospheric effects of scintillation. The system is capable
of transmitting along a link of several kilometers, depending specifically on the characteristics of the interrogator
and sensor components chosen for the system.
Free space optical (FSO) communication has enjoyed a renewal of interest in the past decade driven by
increasing data rate requirements and decreasing amounts of radio frequency spectrum. These needs exist in
both the commercial and military sectors. However military communications requirements differ in other
ways. At the U.S. Naval Research Laboratory (NRL) we have been conducting research on FSO
communications for over ten years with an emphasis on tactical applications. NRL's FSO research has
covered propagation studies in the maritime domain, new component development, and systems
demonstrations. In addition NRL has developed both conventional, direct, laser communications systems and
retro-reflecting systems. In this paper we review some of this work and discuss possible future applications of
FSO communications.
Characterizing the fundamental response and operational parameters of a deformable mirror is a critical first step in the
design of an adaptive optics system. This paper describes the characterization of the influence function and training of a
piezoelectric deformable mirror (PDM) at 632 nm. We scale the results to 1550 nm for low to mid order aberration
correction for free space laser communications applications in the Short Wave Infrared (SWIR). A modified
Twyman-Green interferometer was used to measure the influence functions and to characterize the mirror. The data was analyzed
using commercial and customized software.
KEYWORDS: Frequency modulation, Fermium, Video, Modulation, Free space optics, Signal to noise ratio, Receivers, Analog electronics, Sensors, Modulators
The Naval Research Laboratory (NRL) in collaboration with the Defence Science and Technology Organisation (DSTO)
of Australia has performed long distance experiments with analog modulated free space optical communication links
across the Chesapeake Bay. Results will be presented on estimating the probability density functions of the RF
parameters of gain, noise factor, and linearity after propagating an RF modulated, 1550nm laser beam over a 32km
distance (folded round-trip across Chesapeake Bay). In addition, results from the transmission of video using analog FM
modulation of a 1550nm laser beam over the link will be presented.
KEYWORDS: Scintillation, Free space optics, Switching, Data transmission, Radio optics, Switches, Hybrid optics, Signal attenuation, Fiber optics tests, Fiber optics
We describe characterization of a hybrid Free-Space Optical (FSO) and Radio Frequency (RF) link for efficient
switching between the two links. The monitoring and switching are controlled by a program that checks the FSO
connection health using echo packets. The switching program was tested using a fiber optic link that can simulate
atmospheric attenuation and scintillation effects by using an optical modulator. The sensitivity to connection quality
degradation and momentary connection outages can be optimized for a given system. Connection quality dependent
switching rather than continuous operation of both FSO and RF links is desired in situations where RF use is to be
minimized, while maintaining high reliability.
Atmospheric effects deleteriously impact free space laser communications. Beam wander, distortion and beam bending
can affect pointing and tracking in particular. Mirages are an example of these effects. In June 2006, a campaign was
conducted across the Chesapeake Bay by the Naval Research Laboratory to quantify effects of mirages at the marine
layer. We imaged a series of lights positioned strategically on a tower across the bay, at Tilghman Island, approximately
ten miles away from NRL's Chesapeake Bay Detachment (NRL-CBD). Recorded images were subject to displacement
and distortion as functions of temperature, humidity, dew point, and other meteorological parameters. Results from the
experiment will be presented and phenomenology discussed.
Measurements of Cn2 time series using unattended commericial
scintillometers over long time intervals inevitably lead to data drop-outs
or degraded signals. We present a method using Principal Component Analysis
(also known as Karhunen-Loève decomposition) that seeks to correct for
these event-induced and mechanically-induced signal degradations.
We report on the quality of the correction by examining
the Intrinsic Mode Functions generated by Empirical Mode Decomposition.
We have measured the optical turbulence structure parameter, C2n, in two extremely different locations: the first being the littoral region on the southwest coast of Puerto Rico. The second location is over the dry desert in central New Mexico. In both cases, the horizontal beam paths are approximately 0.6 km long, within 2 meters of the local surface (Puerto Rico) and varying between 2 to 100 meters (New Mexico). We present our findings from the two datasets.
We present new optical turbulence structure parameter measurements, C2n, over sea water between La Parguera
and Magueyes Island (17.6N 67W) on the southwest coast of Puerto Rico. The 600 meter horizontal paths were
located approximately 1.5 m and 10 m above sea level. No data of this type has ever been made available in the
literature. Based on the data, we show that the C2n
measurements are about 7 times less compared to equivalent
land data. This strong evidence reinforces our previous argument1-4 that humidity must be accounted for to
better ascertain the near surface atmospheric turbulence effects, which current visible / near infrared C2n
bulk
models fail to do. We also explore the generalised fractal dimension of this littoral data and compare it to our
reference land data. We find cases that exhibit monofractal characteristics, that is to say, the effect of rising
temperatures during the daylight hours upon turbulence are counterbalanced by humidity, leading to a single
characteristic scale for the measurements. In other words, significant moisture changes in the measurement
volume cancels optical turbulence increases due to temperature rises.
We have recently shown the refractive index structure constant C2n in the visible and near infrared to be a strong function of humidity in the absence of solar insolation effects, in stark contrast to the commonly held assumption that the humidity contribution can be ignored in that waveband. We expand our analysis of the effects of humidity on C2n as measured across a 100-m long horizontal beam path to include temperature. Also we present a new technique for extracting information on changes in the parameter space of C2n and local weather variables, which we term Hilbert Phase Analysis (HPA). This methodology, based on extracting the phase of the analytic signal via Hilbert transforms, reveals a wealth of detail that conventional analysis techniques cannot determine. The HPA provides additional confirmation that C2n is strongly influenced by local humidity in the visible region. We have also found that HPA provides a clear demonstration that humidity competes with temperature in affecting the value of C2n.
KEYWORDS: Monte Carlo methods, Turbulence, Signal analyzers, Solar energy, Transmitters, Physics, Analytical research, Optical calibration, Calibration, Geometrical optics
The Hilbert Huang Transform is a new technique for the analysis of non-stationary signals. It comprises two distinct parts: Empirical Mode Decomposition (EMD) and the Hilbert Transform of each of the modes found from the first step to produce a Hilbert Spectrum. The EMD is an adaptive decomposition of the data, which results in the extraction of Intrinsic Mode Functions (IMFs). We discuss the application of the EMD to the calibration of two optical scintillometers that have been used to measure Cn2 over horizontal paths on a building rooftop, and discuss the advantage of using the Marginal Hilbert Spectrum over the traditional Fourier Power Spectrum.
Humidity and C2n data collected from the Chesapeake Bay area during the 2003/2004 period have been analyzed. We demonstrate that there is an unequivocal correlation between the data during the same time periods, in the absence of solar insolation. This correlation manifests itself as an inverse relationship. We suggest that C2n in the infrared region is also function of humidity, in addition to temperature and pressure.
We present the current status and developments of a horizontal beam path laser propagation experiment over the sea performed off the coast of Puerto Rico. Atmospheric turbulence effects have been measured by a Shack-Hartmann wavefront sensor with a Dalsa CCD camera and by a scintillometer from Optical Scientific, Inc* (OSI). We present preliminary scintillation measurements for an approximate period of two days from the two optical systems during the month of July 2005, also suggestions for improvement in the software, data acquisition protocol and hardware are presented.
We describe the status and initial results of a long term campaign to measure the effects of atmospheric turbulence upon a horizontally directed laser beam, whose altitude above the sea surface is 2 metres. The measurements are made with a video rate Shack-Hartmann wavefront sensor. Currently the source-receiver distance is 110 metres and additional sites have been identified to extend this single pass geometry to around 1 km. To date we have sampled over 70 hours of data, from December 2003 to April 2004. The preliminary scintillation power spectra and phase structure functions have been determined for some of the data sets, which we present here.
We report on the current status of a horizontal path length laser propagation campaign that is being performed over the sea, just off the coast of Puerto Rico. The effects of atmospheric turbulence in a tranquil marine environment have been measured with a video rate Shack--Hartmann wavefront sensor. The small perturbations in the wavefront phase and the degree of scintillation are presently being determined in a single pass from source to receiver, over a trial distance of 110 meters. Additional sites have been identified which allow for single pass measurements up to approximately 1 kilometer. Over 70 hours of data have been sampled to date, between December 2003 and April 2004.
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