Michele R. Suite

Atmospheric turbulence effects measured along horizontal-path optical retro-reflector links.

The scintillation measured over close-to-ground retro-reflector links can be substantially enhanced due to the correlations experienced by both the direct and reflected echo beams. Experiments were carried out at China Lake, California, over a variety of ranges. The emphasis in this paper is on presenting the data from the 1.1 km retro-reflecting link that was operated for four consecutive days. The dependence of the measured irradiance flux variance on the solar fluence and on the temperature gradient above the ground is presented. The data are consistent with scintillation minima near sunrise and sunset, rising rapidly during the day and saturating at irradiance flux variances of ~10. Measured irradiance probability distributions of the retro-reflected beam are compared with standard probability density functions. The ratio of the irradiance flux variances on the retro-reflected to the direct, single-pass case is investigated with two data sets, one from a monostatic system and the other using an off-axis receiver system.

Passive optical monitor for atmospheric turbulence and windspeed

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.

Fast steering mirror implementation for reduction of focal-spot wander in a long-distance free-space optical communication link

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).

Modulating Retro-Reflector Lasercom Systems for Small Unmanned Vehicles

Lasercom, also known as free space optical (FSO) communication, has enjoyed a renewal of interest driven by increasing data rate requirements and the crowding of the RF spectrum, affecting both commercial and military sectors. Military communications must also deal with intentional or unintentional jamming, as well as frequency allocation restrictions, neither of which affects lasercom. The U.S. Naval Research Laboratory (NRL) has been conducting research on lasercom since 1998 with an emphasis on tactical applications. NRLs lasercom research has covered propagation studies in the maritime domain, component development, and systems demonstrations. NRL has developed both conventional lasercom systems and retro-reflecting systems for small platforms. This paper reviews some of the retro-reflecting work, discusses applications of lasercom in the areas of explosive ordnance disposal (EOD) unmanned ground vehicles (UGVs), and unmanned aerial vehicles (UAVs), and describes future directions.

Analysis of long-term measurements of laser propagation over the Chesapeake Bay

Parameters characterizing the atmospheric turbulence in a 16 km maritime optical link were measured for the months of January through June of 2007 on a continuous basis, as conditions allowed. Both the scintillation index sigmaI(2) and the atmospheric structure constant Cn(2) are found to have a strong dependence on the air-minus-water temperature difference. There is no obvious diurnal variation of Cn(2) or of sigmaI(2) akin to the reduction in turbulence level seen in terrestrial links in the hour before sunrise and the hour after sunset. Results from the analysis of these data highlight a need for new approaches to modeling beam propagation in a maritime environment.

Spatial intensity correlation and aperture averaging measurements in a 20-mile retroreflected lasercom link

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.

Atmospheric Turbulence Studies of a 16 km Maritime Path

The Naval Research Lab (NRL) is currently operating a lasercom test facility (LCTF) across the Chesaepeake Bay between NRLs 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 spotlights 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.

Interference effects and aperture averaging in retroreflected light

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.

Free-space optical data link to a small robot using modulating retroreflectors.

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.

Power spectra of a free space optical link in a maritime environment

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.