University of Dayton, School of Engineering, 300 College Park College Park Center, LOCI, Dayton ... random breaks in communication data traffic known as atmospheric signal ... "prevention" of long-term data loses (rather than data recovery ).
OSA / ASSP/LACSEA/LS&C 2010 a404_1.pdf LSMA1.pdf
Adaptive Optics for Free Space Laser Communications Mikhail Vorontsov1, Thomas Weyrauch1, Gary Carhart2 and Leonid Beresnev2 University of Dayton, School of Engineering, 300 College Park College Park Center, LOCI, Dayton, OH, 45469-2951 US Army Research Laboratory, CISD, 2800 Powder Mill Rd. Adelphi, MD, 20783
Abstract: We discuss adaptive optics (AO) role in free-space laser communications with focus on two major challenges: the high cost of AO deployment, and high intensity scintillation levels that are typical for most communication scenarios.
Although the term "free-space optical wave propagation" is typically associated with propagation in vacuum, in most applications of so-called "free-space optical (FSO) communication" technology, wave propagation occurs in an optically inhomogeneous media such as turbulent atmosphere. Atmospheric turbulence may severe impact performance of FSO communication systems resulting in communication link deteriation shown as an increase of number of errors in received signal. The bit error rate (BER) - the major characteristic of communication system performance - depends on both short-term errors resulted from an electronic circuit related noise, and the turbulence induced long-term (up to tens of milliseconds) random breaks in communication data traffic known as atmospheric signal fading. The short-term data losses can be (at least partially) "recovered" using various data coding techniques developed for "wire" and fiber-based communication systems. Contrary, the atmospheric turbulence-induced deep signal fadings represent an unique and significantly more challenging problem, which cann't be solved using conventional data coding techniques without sacrificing communication system efficiency (data throughput rate). Major thrust for adaptive optics (AO) technique in atmospheric laser communication is active "prevention" of long-term data loses (rather than data recovery ). This active information loses "prevention" is implemented by "on-the-fly" controlling communication optical antenna (telescope) aberrations using active/adaptive optical elements (wavefront correctors) incorporated into receiver or transmitter optical system. These, introduced into optical antenna, "controllable aberrations" aim cancellation (mitigation) of atmospheric turbulence-induced phase aberrations and thus communication signal fading. Atmospheric turbulence mitigation with adaptive optics technique will houpfuly result in communication system performance compared with performance of truly "free-space" communication systems. In this sense the term free-space laser communication being applied to atmospheric systems represents an advanced (optimistic) view of future development of atmospheric laser communication technology. Direct "replication" of the adaptive optics techniques developed for astronomical observations to free-space laser communications faces two major challenges: the high cost associated with adaptive system deployment, and the extremely high intensity scintillation level that is typical for near-horizontal or lowlatitude laser beam propagation paths. On the positive side, the laser communication scenario is different from AO astronomical or military applications in that communication occurs between two (or more) friendly parties. This allows the active exchange of information regarding the instantaneous state of the communication channel. The use of this information could greatly simplify adaptive optics beam control, and minimize system cost. Results of numerical simulations for different wavefront control system configurations, as well as some preliminary experimental results, are discussed.
