Data Encryption on Laser Beams: Pros and Cons

For over a century, telecommunication systems have relied on radio-frequency (RF) systems to transmit and receive information.  However, because of the inherently long wavelengths of radio frequencies, transmission requires large and expensive operating equipment (e.g., terrestrial antennas) in addition to inherent limitations on available bandwidth and scattered power concentration.

An intriguing solution to this problem, and one which has quickly gained interest over the past decade, is to encrypt data on the much shorter wavelengths of laser beams, using laser communication systems in place of RF systems.  Laser communication systems, or high-data-rate free-space optical (FSO) laser communication systems, have become more popular given the obvious benefits: the ability to use smaller and lighter equipment components and antennas; narrow-beam power concentration; and increased modulation bandwidth [1].

There are associated drawbacks, however, and one of the most serious is a result of the atmosphere itself.  While the longer wavelengths of RF systems can power through atmospheric turbulence–including temperature fluctuations, fog, rain, and wind–with a fair amount of ease, the more sensitive, short wavelengths of laser beams are prone to everything from power loss to beam wandering under such conditions, including scattering and loss of information [2]. Specifically, scintillation is the term used to describe the temporal and spatial fluctuations of a laser beam adversely affected by unstable atmospheric conditions [3].  In addition, the mechanics of pointing and directing a beam are more challenging than releasing wide-ranging radio waves from a tower, and the technology required for laser optics is not as well developed yet as that of RF communications.

Fortunately, recent developments in beam wander theory may provide a solution for this problem in the coming years [4].  In 2013, for example, scientists at Los Alamos National Laboratory and the Institute of Physics of the National Academy of Sciences in Kiev released their findings that the wandering effect could be decreased, at least theoretically, after studying the photon kinetics of both weak and strong turbulence through analytical and numerical calculations. These researchers claimed to obtain the variance of beam centroid deflections caused by scattering from turbulence.  Of significant interest, they proposed the creation of an artificial distortion of the initial coherence of the radiation in order to decrease the wandering effect.  Their paper, Beam Wandering in the Atmosphere: The Effect of Partial Coherence [5], can be found at:

http://arxiv.org/pdf/0709.2588.pdf

References:

[1] L.C. Andrews, “Free-Space Laser Propagation: Atmospheric Effects,” Photonics Society, IEEE (2005). Available:  http://photonicssociety.org/newsletters/oct05/free_space.html

[2] L. C. Andrews and R. L. Phillips, “Laser Beam Propagation through Random Media,” 2nd ed. (SPIE Press, 2005).

[3] L. C. Andrews, R. L. Phillips, and C. Y. Hopen, “Laser Beam Scintillation with Applications” (SPIE Press, 2001).

[4] L. C. Andrews, et al., “Beam wander effects on the scintillation index of a focused beam,” SPIE 5793 (2005), to appear.

[5] G.P. Berman, A.A. Chumak, and V.N. Gorshkov”Beam Wandering in the Atmosphere: The Effect of Partial Coherence.” Los Alamos National Laboratory, Theoretical Division, Los Alamos, NM 87545, Institute of Physics of the National Academy of Sciences Pr. Nauki 46, Kiev-28, MSP 03028 Ukraine (February 13, 2013) LAUR-07-5341.

 

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