Steven Michael

Architecture overview and data summary of a 5.4 km free-space laser communication experiment

MIT Lincoln Laboratory designed and built two free-space laser communications terminals, and successfully demonstrated error-free communication between two ground sites separated by 5.4 km in September, 2008. The primary goal of this work was to emulate a low elevation angle air-to-ground link capable of supporting standard OTU1 (2.667 Gb/s) data formatting with standard client interfaces. Mitigation of turbulence-induced scintillation effects was accomplished through the use of multiple small-aperture receivers and novel encoding and interleaver hardware. Data from both the field and laboratory experiments were used to assess link performance as a function of system parameters such as transmitted power, degree of spatial diversity, and interleaver span, with and without forward error correction. This work was sponsored by the Department of Defense, RRCO DDR&E, under Air Force Contract FA8721-05-C-0002. Opinions, interpretations, conclusions and recommendations are those of the authors and are not necessarily endorsed by the United States Government.

Air-to-ground lasercom system demonstration

This article presents an overview of the air to ground lasercom demonstration performed under the Free-Space Optical Communications Airborne Link (FOCAL) program. Techniques used to mitigate fading, demonstrated in 2008 and reported earlier, are reviewed as are the basic equipment approach, also reported earlier. We overview the new results for tracking, fiber coupling, channel measurements and communications performance for the air-to-ground link. This work was sponsored by the Department of Defense, RRCO DDR&E, under Air Force Contract FA8721-05-C-0002. Opinions, interpretations, conclusions and recommendations are those of the authors and are not necessarily endorsed by the United States Government.

Long-haul atmospheric laser communication systems

Optical communications provides an attractive means of achieving wideband data transfer over long distances. We review perceived challenges and enabling technology developments that promise to facilitate a new era of free-space laser communications.

A process for free-space laser communications system design

We present a design methodology for free-space laser communications systems. The first phase includes a characterization through numerical simulations of the channel to evaluate the range of extinction and scintillation. The second phase is the selection of fade mitigation schemes, which would incorporate pointing, acquisition, tracking, and communication system parameters specifically tailored to the channel. Ideally, the process would include sufficient flexibility to adapt to a wide range of channel conditions. We provide an example of the successful application of this design approach to a recent set of field experiments. This work was sponsored by the Department of Defense, RRCO DDR&E, under Air Force Contract FA8721-05-C-0002. Opinions, interpretations, conclusions and recommendations are those of the authors and are not necessarily endorsed by the United States Government.

Comparison of Scintillation Measurements from a 5 km Communication Link to Standard Statistical Models

As part of a free-space optical communications experiment over a 5km horizontal path, an extensive database of tilt-stabilized receiver data was collected for Cn2 n conditions ranging from benign to very strong. This paper focuses on the scintillation measurements made during those tests. Ensemble probability distributions are compiled from these results, and are subsequently compared with standard channel models such as the log-normal and gammagamma distributions. Statistical representations of temporal behavior are also developed from this database. Accurate statistical models of atmospheric channel effects have proved to be invaluable in the development of high-performance free-space transceivers.

Emulation of dynamic wavefront disturbances using a deformable mirror

Boundary-layer turbulence resulting from uneven airflow around window interfaces can impact airborne laser communications (lasercom). In the focal plane, these distortions can produce fast jitter and beam break-up, posing challenges for tracking and communications. We demonstrate an experimental emulator that reproduces aircraft aero-optical distortions using a deformable mirror. This boundary-layer emulator resides in a hardware testbed that experimentally mimics air-to-space lasercom links in a controlled, laboratory environment. The boundary-layer emulator operates in the 1.55-mum band and accurately recreates aero-optical distortions at a rate of 2 kilo-frames per second.

Experimental comparison of tracking algorithms in the presence of aircraft boundary-layer distortions for emulated free-space laser communication links.

We report experiments comparing different focal plane array (FPA) tracking algorithms for emulated laser communication links between an aircraft and spacecraft. The links include look-angle-dependent phase disturbances caused by boundary-layer turbulence replicated by using a deformable mirror. Impairments from platform jitter, atmospheric scintillation, and propagation delay are also included. We study a hyperhemispherical dome geometry that provides a large field of regard but generates boundary-layer turbulence. Results from experiments comparing peak and centroid FPA tracking algorithms in various environments show that power delivered to the optical fiber varies with algorithm and look angle. An improvement in steady-state fiber-coupled power of up to 1.0 dB can be achieved through appropriate choice of algorithm. In a real system, this advantage could be realized by implementing a tracking processor that dynamically changes its tracking algorithm depending on look angle and other parameters correlated to boundary-layer turbulence.

Comparisons of Cn2 Measurements and Power-in-Fiber Data from Two Long-Path Free-Space Optical Communication Experiments

Over a two-year period beginning in early 2008, MIT Lincoln Laboratory conducted two free-space optical communication experiments designed to test the ability of spatial beam diversity, symbol encoding, and interleaving to reduce the effects of turbulence-induced scintillation. The first of these exercises demonstrated a 2.7 Gb/s link over a ground-level 5.4 km horizontal path. Signal detection was accomplished through the use of four spatially-separated 12 mm apertures that coupled the received light into pre-amplified single-mode fiber detectors. Similar equipment was used in a second experiment performed in the fall of 2009, which demonstrated an error-free air-to-ground link at propagation ranges up to 60 km. In both of these tests power levels at all fiber outputs were sampled at 1 msec intervals, which enabled a high-rate characterization of the received signal fluctuations. The database developed from these experiments encompasses a wide range of propagation geometries and turbulence conditions. This information has subsequently been analyzed in an attempt to correlate estimates of the turbulence profile with measurements of the scintillation index, characteristic fading time constant, scintillation patch size, and the shape parameters of the statistical distributions of the received signals. Significant findings include observations of rapid changes in the scintillation index driven by solar flux variations, consistent similarities in the values of the alpha and beta shape parameters of the gamma-gamma distribution function, and strong evidence of channel reciprocity. This work was sponsored by the Department of Defense, RRCO DDR&E, under Air Force Contract FA8721-05-C-0002. Opinions, interpretations, conclusions and recommendations are those of the authors and are not necessarily endorsed by the United States Government.

Analysis of rapid Cn2 fluctuations observed during a 5-km communication link experiment

The path-integrated turbulence strength is usually thought of as a parameter that varies slowly with time. In a recent free-space communications experiment the Cn2 n value over a 5-km horizontal path was monitored almost continuously for a period of nearly a month. In addition to well defined and repeatable diurnal fluctuations, strong short-term fluctuations were observed in which the turbulence strength changed by an order of magnitude within a period of minutes. These rapid changes were independently measured by a commercial scintillometer and the high-rate output from the communications transceiver. The characteristics and probable causes for these dynamic atmospheric events and their impact on the design of free-space communication systems are discussed in this article.

Observations of Power-in-Fiber Statistics in Two Recent Free-Space Communication Link Experiments

Lincoln Laboratory-conducted two free-space optical-ommunication experiments designed to test the ability of beam-diversity, symbol-encoding, and interleaving to reduce the effects of turbulence-induced-scintillation. This article presents a small sample of the power-in-fiber-data obtained from those experiments.