The U. S. Naval Research Laboratory (NRL) is developing a small size, weight and power (SWaP) free space lasercomm terminal for small unmanned airborne platforms. The terminal is based on a small gimbal developed by CloudCap Technology. A receiver with a large field of view and with sensitivity sufficient to meet the program range goals is required for this terminal. An InGaAs Avalanche Photodiode (APD) with internal structures engineered to reduce excess noise and k(eff) in high gain applications was selected as the detector. The detector is a 350 micron diameter impact ionization engineered ((IE)-E-2) APD developed by Optogration, Inc. Results of development and characterization of the receiver will be presented.

Introduction: As reliance on small unmanned aerial systems (UASs) expands, and sensors requiring higher-bandwidth downlinks are developed, the need for secure high-data-rate communications increases. Laser communication (lasercom), also known as free-space optical (FSO) communication, is inherently low-probability-of-interception and detection (LPI/ LPD). Lasercom has been previously demonstrated on manned aircraft; however, those systems require a high-precision pointing system and correspondingly high-accuracy navigation information hardware, both of which are prohibitively heavy for small UASs. The work described here opens up the use of lasercom to small UASs, which could not otherwise support lasercom terminals. NRL developed an alternative to conventional lasercom terminals and applied it to small UASs. The enabling technology in these new terminals is the modulating retro-reflector (MRR). NRL has actively developed MRRs since 1998, primarily for terrestrial and shipboard applications. An MRR architecture shifts most of the power, weight, and pointing requirements to one end of the link, allowing the other end to be extremely small, low-power, and require only rough pointing. Airborne Lasercom Transceiver Components: NRL's MRR transmitters and photoreceivers require only coarse pointing (±15°). An optical amplitude modulator is mounted in the path of a cornercube retro-reflector. The ground station's laser beam is retro-reflected back with data impressed on the beam. MRR transmitters and photoreceivers are installed in modified, low-cost, lightweight (65 g) camera gimbals to allow hemispherical coverage. Dakota Wingpod Lasercom System: The MRR transceiver gimbals, stabilized camera, and electronics are installed in a wingpod. Hardware providing GPS position, inertial sensing, and heading are included. An onboard processor maintains pointing to the laser ground station. An RF modem in the tail of one pod adds the capability to make hardware configuration changes while in flight for testing purposes. The final weights of the pods are 3.6 kg and 3.1 kg, including gimbaled lasercom transmitter and receiver, stabilized camera, video compressor/modem, navigation data sources, antennas, pod structure, and mounting hardware. The combined power draws and weights of the components required for communication are 6 W and 1 kg, and can readily be further reduced. The pods require only power and an optional GPS antenna feed from the Dakota. Figure 1 shows the Dakota with wingpods. Lasercom Ground Station: The lasercom ground station is based on the Dual Mode Optical Interrogator (DMOI) developed by NovaSol and NRL under the Office of Naval Research DMOI program. NRL extended the capabilities of the DMOI to allow it to track aircraft. …

Free-space optical communication can allow high-bandwidth data links that are hard to detect, intercept, or jam. This makes them attractive for many applications. However, these links also require very accurate pointing, and their availability is affected by weather. These challenges have limited the deployment of free-space optical systems. The U.S. Naval Research Laboratory has, for the last 15 years, engaged in research into atmospheric propagation and photonic components with a goal of characterizing and overcoming these limitations. In addition several demonstrations of free-space optical links in real-world Navy applications have been conducted. This paper reviews this work and the principles guiding it.

We report on the free space optical transmission of FM audio/video signals using a 6.3mm diameter InGaAs modulating retro-reflector.

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.

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.

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.

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.

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.

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 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 atmospheric turbulence conditions is critical for predicting the performance of a free-space optical laser communication (FSO lasercomm) link. A C-n(2) monitor based on angle-of-arrival (AOA) fluctuations has been built for characterization of atmospheric conditions at the NRL FSO Lasercomin 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.

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 similar to500 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 at 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).

We present a CW LIDAR utilizing telecommunications components and operating at eye-safe wavelength of 1550 nm. Our direct detection LIDAR relies on a pseudo-noise phase coded RF sub-carrier to perform range determination. We demonstrated a range resolution of 15 m at a distance of 16 km in fog using retro-reflectors.

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.

The plasma density distribution within a capillary discharge determines its performance as a waveguide for high intensity laser pulses. This paper describes direct, spatially resolved density measurements of a discharge capillary plasma channel, obtained using transverse interferometry. A transparent glass discharge capillary with square cross section was designed and constructed to provide a clear side view for interferometric imaging. Localized density profiles were reconstructed both inside and outside the capillary. Density channels suitable for high-intensity laser guiding were directly measured, revealing significant axial density gradients inside the capillary near the ends, as well as possible curvature of the channel.

The implementation of an innovative technique for measuring the propagation of intense laser pulses through plasma channels is described. At high laser intensities, temporally resolved stimulated Raman backscattering can be used to diagnose both the electron density and the laser intensity inside the plasma channel, observations which are not possible using other techniques. This diagnostic is demonstrated in experiments using an open-ended capillary in which a plasma channel was created. The plasma channel was generated using either an electrical discharge or laser ablation by a second laser pulse.

To achieve multi-GeV electron energies in the laser wakefield accelerator (LWFA), it is necessary to propagate an intense laser pulse long distances in a plasma without disruption. One of the purposes of this paper is to evaluate the stability properties of intense laser pulses propagating extended distances (many tens of Rayleigh ranges) in plasma channels. A three-dimensional envelope equation for the laser field is derived that includes nonparaxial effects such as group velocity dispersion, as well as wakefield and relativistic nonlinearities. It is shown that in the broad beam, short pulse limit the nonlinear terms in the wave equation that lead to Raman and modulation instabilities cancel. This cancellation can result in pulse propagation over extended distances, limited only by dispersion. Since relativistic focusing is not effective for short pulses, the plasma channel provides the guiding necessary for long distance propagation. Long pulses (greater than several plasma wavelengths), on the other hand, experience substantial modification due to Raman and modulation instabilities. For both short and long pulses the seed for instability growth is inherently determined by the pulse shape and not by background noise. These results would indicate that the self-modulated LWFA is not the optimal configuration for achieving high energies. The standard LWFA, although having smaller accelerating fields, can provide acceleration for longer distances. It is shown that by increasing the plasma density as a function of distance, the phase velocity of the accelerating field behind the laser pulse can be made equal to the speed of light. Thus electron dephasing in the accelerating wakefield can be avoided and energy gain increased by spatially tapering the plasma channel. Depending on the tapering gradient, this luminous wakefield phase velocity is obtained several plasma wavelengths behind the laser pulse. Simulations of laser pulses propagating in a tapered plasma channel are presented. Experimental techniques for generating a tapered density in a capillary discharge are described and an example of a GeV channel guided standard LWFA is presented.

Most laser wakefield accelerator (LWFA) experiments to date have operated in the self-modulated (SM) regime and have been self-guided. A channel-guided LWFA operating in the standard or resonant regime is expected to offer the possibility of high electron energy gain and high accelerating gradients without the instabilities and poor electron beam quality associated with the SM regime. Plasma channels such as those produced by a capillary discharge have demonstrated guiding of intense laser pulses over distances of several centimeters. Optimizing the performance in a resonant LWFA constrains the on-axis plasma density in the channel to a relatively narrow range. A scaling model is presented that quantifies resonant LFWA performance in terms of the maximum accelerating gradient, dephasing length, and dephasing-limited energy gain. These performance quantities are expressed in terms of laser and channel experimental parameters, clearly illustrating some of the tradeoffs in the choice of parameters. The predicted energy gain in this model is generally lower than that indicated by simpler scaling models. Simulations agree well with the scaling model in both low and high plasma density regimes. Simulations of a channel-guided, self-modulated LWFA are also presented. Compared with the resonant LWFA regime, the requirements on laser and channel parameters in the SM regime are easier to achieve, and a channel-guided SM-LWFA is likely to be less unstable than a self-guided SM-LWFA.