William J. Scharpf

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).

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.

Latest results from the 32 km maritime lasercom link at the Naval Research Laboratory, Chesapeake Bay Lasercom Test Facility

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.

Overview of NRL's maritime laser communication test facility

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.

Characterization of the marine atmosphere for free-space optical communication

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.

Packet testing in free-space optical communication links over water

NRLs 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.

Progress in high-speed communication at the NRL Chesapeake Bay lasercomm testbed

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.

Low-frequency sampling adaptive thresholding for free-space optical communication receivers with multiplicative noise

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).