The Space Development Agency (SDA) is developing a proliferated Low Earth Orbit (LEO) constellation of spacecraft. Spacecraft in this constellation will communicate with each other using optical intersatellite links. The US Naval Research Laboratory (NRL) has built and operated a laboratory testbed for investigating the interoperability of optical communication terminals in the SDA constellation. The challenges and design considerations of the testbed are discussed. The testbed’s different modes of operation, and some of the verification and validation that was done using NRL test terminals are described.
Tele-operated robots used for Explosive Ordnance Disposal (EOD) are ordinarily controlled using a radio frequency (RF) link. Use of RF links on the battlefield presents several challenges including spectrum allocation and jamming effects (both by the enemy and friendly forces). Several solutions have been attempted including electrical or fiber optic umbilicals and spread spectrum radios with varying degrees of success. Modulating Retro-reflector Free Space Optical (MRR-FSO) communications links avoid these effects entirely but are limited to line of sight operation. We have developed a system consisting of an MRR-FSO link with a tracking optical terminal, a conventional RF link and a deployable pod to provide a relay node bridging the FSO link to the operator and the RF link to the robot. The MRRFSO link provides the capability to operate the robot in the presence of jamming while the RF link allows short range non line of sight operation. The operator uses the MRR-FSO link to drive the robot to a position downrange outside the influence of the jammer or other interference. Once the robot is positioned downrange near the area of operation the pod is deployed. This allows the robot to maneuver freely including venturing beyond line of sight using the short range RF link to maintain communications between the vehicle and pod while the FSO link maintains connectivity between the pod and the operator.
Scintillation is an effect that limits the performance of many optical systems including imagers and free space optical
communication links. Scintillation can be especially severe for optical retro-reflectors. This can limit the range of links
based on modulating retro-reflectors. In this work we examine the effects of retro-reflector diversity on optical
scintillation. This technique uses multiple retro-reflectors, separated by a distance larger than the atmospheric coherence
size and illuminated by one interrogating beam, to reduce the scintillation index. We examine the dependence of
scintillation index on the number of retro-reflectors, their separation and the degree of coherent interference at the
receiver.
Data was taken in the field at three different sites: the Naval Research Laboratory's Chesapeake Bay Detachment, where
both over land and over water measurements were made, the Fort AP Hill Laser Test Range and China Lake, California.
We show that multiple retro-reflectors can reduce the scintillation index on double-pass links. We demonstrate that when
retro-reflectors are spaced too closely, coherent interference can increase the scintillation index and greatly expand its
frequency content. We also examine the effects of retro-reflector diversity on the margin needed for modulating retroreflector
data links.
The NRL Transportable Atmospheric Testing Suite (TATS) system was used to measure time resolved
scintillation over a variety of different ranges at China Lake in December 2010. In this paper an
analysis of scintillation effects on retro-reflector links is presented. Scintillation index, power spectral
density and probability distribution functions are deduced from the measured irradiance fluctuations.
Effects of aperture averaging using multiple retro-reflectors is examined as well as the contribution to
the irradiance fluctuations due to enhanced backscatter.
The spatial scale of variations of intensity imposed on a propagating beam as it transits the turbulent atmosphere
is of importance in designing free space laser communications receivers. Consecutive images of the entrance
pupil of a 125mm diameter telescope have been acquired at a rate of 328fps over a series of ranges from 2km to
24km. The images are analyzed with respect to the aperture averaging available to a given receiver system.
Optical scintillation is an effect that limits the performance of many optical systems including imagers
and free space optical communication links. The Naval Research Laboratory is undertaking a series of
measurement campaigns of optical scintillation in a variety of environments. In December of 2010
measurements were made over a one week period in the desert at China Lake, CA. The NRL TATS
system was used to measure time resolved scintillation over a variety of different ranges and terrains.
This data has been used to determine fade rate and duration as a function of weather and link margin.
Temporal correlation of fades has also been calculated. This data allows simulation of a variety of
communication protocols and the effects of those protocols on link throughput. In this paper we
present a comparison of different protocols for both direct and retroreflector links.
Experiments have been carried out over a series of ranges at China Lake in which the irradiance fluctuations,
angle-of-arrival variance and images of the scintillation patterns were recorded, together with meteorological
parameters. The decrease in scintillation index associated with the lower turbulence conditions at sunset was
observed for the shorter ranges but not for the longer ranges of 19 and 24km where the propagation was in the
saturated regime.
As military sensors and systems become more sophisticated, tactical situations will require reliable, high data rate
communications. The current RF communication systems are increasingly competing for the limited amount of RF
spectrum and bandwidth. One possible way to augment the current RF communication systems is by the use of free
space lasercomm in tactical networks for links in which direct line of sight is possible. Free space lasercomm has
been demonstrated over horizontal distances greater than 10 nautical miles and at data rates greater than 1
gigabit/sec. Lasercomm links do not require any RF frequency allocation, nor do they have an RF signature. They
are inherently low probability of intercept and detection and they are very difficult to jam due to the very narrow
divergence of the communication beams and the very narrow acceptance angle of the receivers.1-6 The U.S. Naval Research Laboratory has demonstrated the use of free space lasercomm in tactical networks at
Trident Spectre 2009 and Empire Challenge 2010. This paper will discuss these lasercomm demonstrations and
present packet error rate test data captured at both.
The Naval Research Laboratory has developed a system to optically "playback" recorded atmospheric
scintillation data files. Scintillation data files typically consist of voltage versus time data taken during
various outdoor field tests. Optical playback of these files involves modulating laser light in an optical
fiber such that the output mimics the fluctuations observed in the outdoor field tests. This capability allows
greatly simplified testing of lasercomm hardware and software in a laboratory environment without the
need for more expensive and complicated outdoor field tests. This system also enables repetitive testing
with the same fluctuations which is extremely useful for optimization of lasercomm system performance.
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.
Small robots are finding increasing use for operations in areas that may be dangerous to humans.
These robots often have needs for high bandwidth communications to return video and other data.
While radio frequency (RF) links can be used in may cases, in some circumstances they may be
impractical due to frequency congestion, reflections off surfaces, jamming or other RF noise. In
these cases an optical link may be advantageous, particularly when a clear line of sight exists.
However, a conventional optical link has limitations for this application. For example, a
conventional optical link operating at rates of megabits per second at ranges of 1 Km requires
about a 1 degree pointing accuracy. This implies a need for active pointing and tracking, which
maybe be unacceptable for a small platform. We explored an optical modulating retroreflector
(MRR) link for these cases. An array of 6 MRRs and photodetectors with a field of view of 180 degrees (azimuth)x 30 degrees (elevation) was constructed and mounted a small robot, the iRobot
PackbotTM. An Ethernet modem designed to work with MRR links was also part of the system.
Using a tracking laser interrogator at the other end of the link, a 1.5 Mbps free space optical
Ethernet link was established that completely replaced the normal RF Ethernet link. The link was
demonstrated out to ranges of 1 Km down a road, exceeding the range of the RF link. Design
issues and measurements of performance will be described.
Parameters characterizing the atmospheric turbulence in a 16 km maritime optical link were
measured over a 6 month period in 2007, on a continuous basis as conditions allowed. Both the
scintillation index and the atmospheric structure constant were found to have a strong dependence on
the air - water temperature difference. Temporal-frequency spectra were also generated from the
received intensity fluctuations. In this paper the frequency components of the laser irradiance are
studied in order to ascertain the degree of correlation with prevailing meteorological conditions. In
addition, the high frequency behavior of the power spectra is documented and shows an extensive
range of values for the power-law scaling index.
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.
The U.S. Naval Research Laboratory (NRL) established a one-way Gigabit Ethernet lasercomm link during the
Seahawk exercise in August, 2007 to transfer data ~8 miles across the inlet of San Diego Bay from Point Loma to
the Imperial Beach base camp. The data transferred over the link was from an NRL developed, wide field of view
(90 degrees), high resolution, mid-wave infrared camera operating at 30 frames per second. Details of the high
speed link will be presented as well as packet error rate data and atmospheric propagation data taken during the two
week long exercise.
The use of 1.55μm lasers for free space optical links has become well established. While one expects there to be
advantages to operating at 3.6μm, namely higher transmission through water vapor and reduced scintillation, the
availability of both lasers and detectors at the mid IR wavelength is not as mature as those available at the telecomm
choice of 1.55μm. However, there are potential schemes for using frequency conversion to probe the atmosphere in the
mid-ir but to detect back in the near-ir.
A sequence of experiments has been conducted, over a 16km one-way link across the Chesapeake Bay, to directly
compare the intensity variances and the power spectrum imposed by the atmosphere at the two wavelengths. An
interband cascade laser was used which operated at a wavelength of 3.6μm and had an output power of 100mW. The
1.55μm system used standard telecomm parts. Data were recorded simultaneously from both systems at 4kHz and were
digitized using a 16-bit card. A telescope measured the angle-of-arrival variance of the 1.55mm beam in order to give a
measure of the atmospheric structure constant Cn2. In addition a visibility monitor and weather station were usually
operational at each end of the link.
The U.S. Naval Research Laboratory has been examining propagation of a 1550nm laser at its free-space lasercomm test
facility at Chesapeake Bay Detachment (NRL-CBD). NRL-CBD offers a ten mile free-space optical laser
communication (FSO lasercomm) path over water. Atmospheric propagation data and as well as bit error rate and packet
error rate data has been collected along the one-way ten mile link and a round-trip twenty mile link using passive retroreflectors.
Long term 24/7 data collection on the one-way range at the lasercomm test facility (LCTF) provides insight
into availability and packet error rates of maritime FSO lasercomm. Results from this study will be presented.
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.
The US Naval Research Laboratory has undertaken a multi-year research effort in free
space optical communications. The goals of this research are to understand and quantify
the propagation of optical beams through the maritime atmosphere, assess the global
maritime availability and performance of Naval lasercomm, and research methods to
improve the robustness of Naval lasercomm links. In support of these goals, NRL has
continued to add monitoring and testing capabilities to the Lasercomm Test Facility
(LCTF) at NRL-Chesapeake Bay Detachment (CBD). The LCTF has provided volumes
of information about maritime laser propagation and atmospheric turbulence. Highlights
of recent research collected at the LCTF are presented in this paper.
An experimental study has been made on the contribution to the effective scintillation index due to aperture averaging and interference effects when using multiple retroreflectors in free-space optical links. These studies are of relevance to asymmetric data links where modulating retroreflectors are used at the remote end of the free-space optical link. For closely spaced retroreflectors the effect of coherent interference at the receiver is seen to increase the effective variance of the received signal, whereas spatial averaging is apparent for more widely spaced retroreflectors. The scintillation index, probability density functions, and fade rates are all affected by the interference. Experiments were conducted over both a short-range (500 to 800 m) and a long-range (16 km) link. The long-range link used three retroreflectors having a spacing that was large enough that interference effects contributed minimally to the observed variance. However, the variance clearly depended on the number of retroreflectors. In addition to measuring the scintillation index, images of the returned light were recorded using a fast framing camera. These experiments were conducted over land and mixed water-land terrains in the case of the short-range links, and exclusively over water in the case of the 16-km link from Chesapeake Beach to Tilghman Island.
Modulating retroreflectors (MRRs) couple passive optical retroreflectors with electro-optic modulators to allow free-space optical communication with a laser and pointing-acquisition-tracking system required on only one end of the link. Recently, MRR using multiple quantum well (MQW) modulators have been demonstrated using a large-area MQW placed in front of the aperture of a corner cube. For a MQW modulator, the maximum modulation rate can range into the gigahertz, limited only by the RC time constant of the device. Most MRR systems have used corner-cube retroreflectors with apertures of about 1 cm, which require large, and hence high-capacitance, modulators. Thus data rates exceeding a few megabits per second are not possible. We describe a new kind of MQW MRR that uses a cat's-eye retroreflector with the MQW in the focal plane of the cat's-eye. This system decouples the size of the modulator from the size of the optical aperture and allows much higher data rates. A 45-Mbit/s free space link over a range of 7 km is demonstrated.
KEYWORDS: Signal to noise ratio, Discrete wavelet transforms, Fermium, Frequency modulation, Wavelets, Scintillation, Free space optics, Free space optical communications, Demodulation, Interference (communication)
Atmospheric noise signals are fundamental limitation of free-space optical communications, as the decrease in signal-to-noise ratio reduces the range and/or bandwidth of the link. In this paper we consider the limitations that this imposes, and investigate the use of discrete wavelet transformation (DWT) to overcome them. Simulations are performed to validate the use of the DWT in the demodulation of the analog data in the presence of noise. Results of the experiments are presented.
The U. S. Naval Research Laboratory (NRL) and OptoGration, Inc. have collaborated in the development
and testing of large area, high speed InGaAs avalanche photodiode (APD) receivers for use in free-space
lasercom systems. A 200 micron diameter InGaAs APD receiver has been tested in a free-space lasercom
testbed and has demonstrated sensitivities of -42.4 dBm at 622 Mbps and -44.8 dBm at 155 Mbps. A 100
micron diameter receiver has been tested with a resulting sensitivity of -35.75 dBm at 2.4883 Gbps. These
receivers are made possible due to OptoGration's capability to manufacture a large area, high speed InGaAs APD with an effective ionization ratio of < 0.2 and by matching the APD device with an appropriate transimpedance amplifier and limiting amplifier. Development and testing of the APD receivers will be described below.
Current mathematical models describing laser propagation through the atmosphere were developed for terrestrial
environments. An atmospheric index of refraction power spectrum specifically tailored to the marine environment has
been created and applied to scintillation theory. Optical measurements of a diverge laser beam propagating in a marine
environment, in combination with scintillation theory and a numerical scheme, were used to infer the refractive index
structure parameter, Cn2, along the propagation paths. The analysis was repeated for both marine and terrestrial
theoretical scintillation expressions, each resulting in one set of inferred Cn2-values. In the moderate-to-strong
fluctuation regime, the inferred Cn2-values based on marine theory were about 20% smaller than those based on
terrestrial theory, but a minimal difference was observed in the weak fluctuation regime.
The US Naval Research Laboratory has an ongoing research effort in the continuous
observation of the maritime environment for free space optical communications. One of
the goals of our research program is to characterize the behavior of the maritime
environment for lasercomm systems, and use the data gathered from the Lasercomm Test
Facility (LCTF) at NRL-Chesapeake Bay Detachment (CBD) to develop a method of
predicting the global availability of maritime lasercomm. The LCTF has provided
volumes of information about maritime laser propagation and atmospheric turbulence.
Highlights of the work on the characterization of the maritime atmosphere are provided
in this paper.
The U.S. Naval Research Laboratory, Chesapeake Bay Detachment (NRL-CBD), has a ten mile free-space optical laser
communication (FSO lasercom) maritime testbed. Over the past year, a comparison study between packet error rates and
bit error rates has been performed. These are the two most common methods to characterize the quality of an FSO
lasercom link. Bit error rate (BER) testing and packet error rate (PER) testing are measured in a variety of atmospheric
conditions on the one-way range at the lasercom test facility (LCTF). Results from this study will be presented.
An experimental study has been made on the contribution to the effective scintillation index due to two retroreflectors, as a function of retroreflector spacing. For closely spaced retroreflectors the effect of coherent interference at the receiver is seen to increase the effective variance of the received signal, whereas spatial averaging is apparent for more widely spaced retroreflectors. The scintillation index, probability density functions, power spectral densities and fade rates are all affected by the interference.
The range, under which these experiments were conducted, was typically 500 - 800m over mixed water/land terrains. The interrogator used a monostatic, 1550nm laser probe beam with a divergence of 0.4 mrad and had a 50mm diameter receiver aperture. Data sets of received power were recorded for durations of 10s each, using a photodiode with a bandwidth of 100kHz. For comparison, the received power from a single retroreflector at various radial positions in the probe beam was recorded. Knowledge of the fade rates and fade durations is of practical importance in considerations regarding the optimal transmission of data packets.
Modulating retro-reflectors (MRR) allow free space optical links with no need for pointing, tracking or a laser on one end of the link. They work by coupling a passive optical retro-reflector with an optical modulator. The most common kind of MRR uses a corner cube retro-reflector. These devices must have a modulator whose active area is as large as the area of the corner cube. This limits the ability to close longer range high speed links because the large aperture need to return sufficient light implies a large modulator capacitance.
To overcome this limitation we developed the concept of a cat's eye MRR. Cat's eye MRRs place the modulator in the focal plane of a lens system designed to passively retro-reflect light. Because the light focuses onto the modulator, a small, low capacitance, modulator can be used with a large optical aperture. However, the position of the focal spot varies with the angle of incidence so an array of modulators must be placed in the focal plane, In addition, to avoid having to drive all the modulator pixels, an angle of arrival sensor must be used.
We discuss several cat's eye MRR systems with near diffraction limited performance and bandwidths of 45 Mbps. We also discuss a link to a cat's eye MRR over a 7 Km range.
April 2005, a laser propagation experiment was conducted over a 470m horizontal maritime path. Scintillation
measurements of a divergent Gaussian beam wave were taken simultaneously for different receiver aperture sizes.
Terrestrial scintillation theory combined with a numerical algorithm was used to infer the atmospheric parameters Cn2
and lo from the optical maritime scintillation measurements. This paper presents the initial results.
The Chesapeake Bay Detachment of the Naval Research Laboratory (NRL-CBD) provides an ideal environment for characterizing the effects of the marine atmosphere on free space optical communication links. The site has recently been converted to an operational 10 mile (16.2 km) one-way test range to collect information on propagation statistics in a variety of atmospheric conditions. The results presented here compare the contributions of thermal gradients across the bay to the variations in intensity scintillations across the bay.
NRL's Chesapeake Bay lasercom test facility (LCTF) offers a variety of ranges for researching free-space optical laser communication (FSO lasercom) links in a maritime environment. This paper discusses link performance over the 16 km one-way range at the LCTF. There are several methods to determine the link quality in FSO lasercom. Bit-error-rate (BER) testing and packet testing are two possible methods. Since errors generally tend to occur in bursts in FSO channels, packet testing may offer a better indication of the quality of service (QoS) rather than BER testing. Link performance measured via packet testing is being investigated in a variety of atmospheric conditions. Results of these experiments will be presented.
It is important to be able to characterize and compensate for refraction effects in free-space optical laser communication (FSO lasercom). The refractive index depends on various properties of the propagation medium such as temperature, pressure, and moisture, with temperature having the largest affect. Very strong but slow-varying thermal gradients have been observed at the NRL Chesapeake Bay lasercom testbed, which offers a 16 km one-way (32 km round-trip) FSO lasercom link over water. Thermal gradients affect the elevation-pointing angle, and their magnitudes are a function of the time of day and year and also the weather conditions. These vertical refraction errors are corrected by the use of a fiber positioner controlled by a position-sensing detector (PSD). This system is implemented into the receiver at the NRL Chesapeake Bay lasercom testbed. System test results will be presented.
In this paper, we summarize progress in free space laser propagation research at the U.S. Naval Research Laboratory, specifically in the context of propagating and detecting signals through the atmosphere in a maritime environment. Transmission through the atmosphere over large bodies of water presents different challenges than transmission through the atmosphere over land. Our paper reports some of these findings as well as progress in our collaborative efforts to mitigate turbulence to enhance our data links.
The optimum detection threshold for optical communication receivers with large signal-dependant noise components can be derived from a Bayes' Likelihood Ratio Test; however, the bit level statistics must be known a priori. In free-space communication systems, atmospheric conditions cause variations in optical transmission and subsequently in the bit level means and variances. These bit parameters must be tracked, estimated, and predicted, in order to update the detection threshold at a rate greater than the frequency of atmospheric changes. A laboratory implementation of an adaptive thresholding system is being implemented at the U.S. Naval Research Laboratory's Chesapeake Bay Free-space Lasercom Testbed. Early results of experiments underway and initial design of the system will be presented.
NRL has established a 20 mile round trip laser communication test facility across the Chesapeake Bay for investigating lasercomm performance in a maritime environment. Experiments at this facility have successfully demonstrated links at data rates up to 2.5 Gbps and at lower rates in light rain and fog. This facility is currently being upgraded to allow long term monitoring of a one-way 10 mile link across the Bay. Parameters monitored will include BER, turbulence conditions, atmospheric transmission, and meteorological conditions. A summary of past results, the design/status of the upgrade to the test facility, and recent results will be presented.
The Naval Research Lab (NRL) is currently operating a lasercom test facility (LCTF) across the Chesaepeake Bay between NRL's Chesapeake Bay Detachment (NRL-CBD) and NRL-Tilghman Island. This lasercom test facility has successfully demonstrated 32 km retro-reflected links at data rates up to 2.5 Gbps. Along with lasercom link studies, atmospheric characterization of the NRL-CBD to Tilghman Island optical path has been investigated. These studies range from passive optical turbulence monitoring based on angle-of-arrival measurements of a spotlight's apparent motion, to intensity and angle-of-arrival measurements of a retro-reflected laser beam. Currently the LCTF is being upgraded from a retro-reflected link to a direct one-way link from NRL-CBD to NRL-Tilghman Island. Initial measurements of atmospheric turbulence effects in this one-way configuration have recently been performed. Results of these past and current atmospheric turbulence studies are presented.
The Naval Center for Space Technology at the Naval Research Laboratory reports the latest results from the long-range, maritime, free-space lasercom test facility located between Chesapeake Beach, MD and Tilghman Island, MD. The two sections of the facility are separated by 16.2 km of the Chesapeake Bay. Using a new OC-48 receiver developed by NRL’s Optical Science Division with a sensitivity of -33dBm for 10-9 bit error rate at 2.5 Gbps, we have closed a 32.4 km maritime lasercom link (round trip across the Chesapeake Bay) and performed bit error rate testing while transmitting 1.13 Terabytes of data. Bit error rate testing was also performed at lower data rates when atmospheric conditions were not favorable for high speed (2.5 Gbps), including testing at 150 Mbps through light fog and rain. In addition, we have set up a system for digitizing and transmitting full-color, uncompressed, video along with six audio channels and three RS-232 data channels over the maritime link. The digital link operated at 311 Mbps and could be maintained indefinitely, depending on atmospheric conditions. Several complete videos were transmitted in entirety or in part as well as live video from a handheld camcorder to test the system operation and robustness. The transmitter and receiver were co-located on the western shore of the bay at the NRL Chesapeake Bay Detachment. The data for both the bit error rate testing and the video was transmitted across the bay and returned from an array of retroreflectors located on a tower at Tilghman Island on the eastern shore. The lasercom links were closed with static pointing and with no active atmospheric aberration mitigation such as adaptive optics or fast steering mirrors on the receiver optics.
A 1550-nm eye-safe, free-space optical communications link is demonstrated at rates up to 5 Mbits/s over a distance of 2 km in the Chesapeake Bay, using quantum-well-based modulating retroreflectors. Tests are conducted under various atmospheric conditions over a time period of about a year. The experimental and theoretical link budgets are compared and statistical measurements of the effects of scintillation are collected.
The Naval Research Laboratory has established a lasercom test bed across the Chesapeake Bay. The test bed uses a bi-static transmitter/receiver arrangement on the western shore of the Chesapeake Bay and various configurations of 5 cm retro-reflectors on the eastern shore to produce a 32 km retro-reflected lasercom test range. Experiments measuring the laser’s transverse spatial profile after propagation over the test range have been performed. These experiments use an InGaAs CCD to image the pupil plane of the 40 cm receiver telescope and a frame grabber to store contiguous images for analysis. Analysis of these image sequences allows measurement of transverse spatial correlations across the received beam after 32 km retro-reflected propagation of the beam. Various configurations and numbers of retro-reflectors were studied to investigate the impact of number and arrangement of retro-reflectors on the received beam’s spatial profile and spatial correlations. Additionally, since the CCD output is stored as a contiguous stream of images, analysis of these images’ intensity variance in time allows measurement of aperture averaging effects as a function of number of retro-reflectors and their geometry. Results from these experiments are presented.
Measurement of the atmospheric index of refraction structure constant (Cn^2) is critical for predicting the performance of a free-space optical laser communication (FSO lasercomm) link. A Cn^2 monitor based on angle-of-arrival (AOA) fluctuations has been built for characterization of atmospheric conditions at the NRL FSO Lasercomm Test Facility across the Chesapeake Bay. The monitor used existing lights in various locations as point sources for determining AOA fluctuations. Real time analysis of the AOA fluctuations was performed to determine the power spectrum of the fluctuations every few seconds. This additional power spectrum information allows much greater understanding of atmospheric conditions including estimation of average wind speed based on frequency shifts in the power spectrum distribution. The performance of the monitor was tested over short paths by comparison to a commercial scintillometer. In addition, the monitor was used at other sites to determine atmospheric conditions at a variety of locations. Results of these experiments are presented.
KEYWORDS: Mirrors, Telescopes, Free space optical communications, Analog electronics, Sensors, Receivers, Data acquisition, Space telescopes, Free space optics, Analytical research
One of the causes of power loss in a free-space optical communication link is beam motion or received spot wander. The power spectrum of the spot motion indicates that most of the frequency content is less than ~500 Hz. A fast steering mirror (FSM) controlled by a position-sensing detector (PSD) has the potential to correct for a significant portion of the focal spot position fluctuations and thus the power loss. A FSM controlled with a Germanium PSD was installed on the receiver of the NRL Chesapeake Bay free-space lasercomm test facility. Results are presented from the initial tests performed using this system to measure and correct for wander of an optical beam propagated across the bay (20 mile round-trip).
The Naval Center for Space Technology at the Naval Research Laboratory has been operating a long-range, maritime, free-space optical communications facility located between Chesapeake Beach, MD and Tilghman, Island, MD. The two sections of the facility are separated by 16.2 km of the Chesapeake Bay. The facility permits one-way communications with the transmitter and receiver at opposite ends as well as double pass communications using a retroreflector array on Tilghman Island and the transmitter and receiver located together at Chesapeake Beach. Over the past year, a ball lens has been incorporated to couple the returned free-space light into an optical fiber. This ball lens makes the coupling much less sensitive to angle. With the lens, averaged coupled power into the receive fiber increased from 50 mW to 130 mW. Link statistics including fade rate and bit error rate are included for a typical summer afternoon for the double pass configuration.
A 1550 nm eye-safe, free-space optical communications link has been demonstrated at rates up to 5 Mbps over a distance of two kilometers in the Chesapeake Bay, using quantum-well-based modulating retro-reflectors. In addition to waveform studies, video and audio transmission has also been carried out, as well as bit error rate measurements. Tests have been conducted under various atmospheric conditions over a time period of eight months.
An adaptive thresholding method is presented for optimum detection for optical receivers with large multiplicative noise. The technique uses low frequency sampling of the detected current that enables calculation of the bit means and variances and estimation of the optimum detection threshold. The regime in which this holds is when the sampling frequency is lower than the bit rate but higher than atmospheric turbulence frequency content. Simulations are done with data obtained from the NRL Chesapeake Bay Lasercomm Testbed. The results of simulations comparing BER performance versus sample rate and parameter estimation error will be presented. If the system parameters are characterized in advance with reasonable accuracy, the BER obtained will typically be an order of magnitude improvement over the equal variance threshold (depending on the signal to noise ratio).
It has been shown that for optical communication receivers with large, signal-dependant noise components (multiplicative noise), the optimum detection threshold can be derived from a Bayes' Likelihood Ratio Test (LRT); however, the mean and variance of the bit levels must be known to obtain the order of magnitude bit-error-rate (BER) improvement over the typical matched filter type detector which assumes equal variances of the bit levels. In free-space communication systems, atmospheric conditions can cause variations in optical transmission and subsequently in the bit level means and variances. The bit level means and variances must be tracked and estimated and the detection threshold updated at a rate greater than the frequency of atmospheric changes, or the BER performance may actually be worse than that of the equal-variance threshold. Adaptive thresholding methods have been proposed and developed which track the bit means and variances and update the detection threshold to maintain near optimum performance. In this paper, simulated data based on actual optical receiver component characteristics and measured average received power data containing atmospheric turbulence induced fluctuations are used to test the tracking and BER performance of adaptive thresholding algorithms. The results of simulations comparing performance of three adaptive methods, maximum likelihood estimation/prediction, Kalman filter predictor/smoother, and a Least-Mean-Square (LMS) adaptive predictor, will be presented.
Two methods of measuring atmospheric turbulence as it affects free space optical communications are presented. Each method yields a value of the structure constant of refractive index fluctuations, Cn2. A scintillometer is used as the basis or 'truth' for measurements taken by the first method by fitting data from the other instrument to the simultaneous scintillometer data. The first method utilizes a device conceived at New Mexico Tech using microphones to measure a pressure differential. This device was altered to provide both pressure and temperature measurements at two points separated by a specific distance. A thermocouple was added beside each microphone to provide temperature data, and the data collection method was altered. The device currently measures two pressure and temperature gradients. Also, the Naval Research Laboratory conducted the first efforts to quantify and calibrate the data collected by the device. Second, measurements are made from the angle of arrival of light from a laser transmitted across 16.4 km of the Chesapeake Bay. The variance of the angle of arrival over time is obtained from the variance of the centroid location of the focused light on a position-sensing detector. The same measurement is made over the same path using a halogen spotlight, a CCD camera, and a video tracker. The angle of arrival variance is used to calculate Cn2. The microphone/thermocouple apparatus took measurements over land alongside the scintillometer. Results from both methods are provided.
The Naval Research Laboratory (NRL) has established a free-space laser communication link across 16.2 km of the Chesapeake Bay between the Chesapeake Bay Detachment of NRL and Tilghman Island. The transmitter consists of a modulated 1550 nm oscillator amplified to 2 watts in an erbium doped fiber amplifier developed at NRL. The beam is fiber coupled to a 4 inch collimating lens on a remotely controllable gimbal mount. The beam is transmitted to a retro-reflector array at Tilghman Island and back to the receiver at CBD (32.4 km round trip). The receiver consists of a 16" Meade telescope either directly or fiber coupled to a variety of fast photo-detectors. Experiments have been conducted to study the stability and quality of the link. These include: bit-error rate measurements, probability density functions, power spectrum densities, and angle of arrival measurements of the received signal. Results of these experiments are presented.
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