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Optical Free Space Communication

Introduction
With the improvements in semiconductors optical free space communication has become n attractive alternative to existing signal conducts as fibre or wireless (RF). It offers high data rates and is much less expensive than underground fibre. But there are a few restrictions, which may mean it remains an alternative. Many engineers are familiar with one of two successful existing technologies for data communication: (1) Guided wave (e.g. fibre optics) and (2) wireless free space communication (e.g. RF). Fibre optics offers extremely high bandwidths for high data rate, while RF communications offers the advantage of wireless connectivity and the ability to broadcast over a wide area. Optical free space communication fits into the 2nd group, but has some properties of the 1st. Many aspects are related to fibre optics with the difference, that the media is air and not fibers. Therefore are the required devices similar to those used with fibers.

The advantages of such a system are the following:
_ No need for digging up the roads
_ No radio frequency licenses are required
_ Bandwidth is equal or superior to fibre systems and much better than RF
_ Doesn't cause interference with existing electromagnetic equipment because interference on narrow band point-to-point connections is rather unlikely.

But there are as well some limitations:
_ Absorption (caused primarily by the water vapor and carbon dioxide)
_ Scattering (depends of the used wavelength and the number and size of scattering elements in the air e.g. fog)
_ Shimmer (due to a combination of factors, including atmospheric turbulence, air density, light refraction, cloud cover, and wind which cause a similar disturbance when a laser beam is transmitted through the atmosphere)
_ Moving building (although we are not aware of the movement, buildings often sway from side to side even settle into the ground causing offset to the laser beam and making the receiver laser receive less power)

Applications

Free space applications
Due to its properties optical free space communications is suited very well for providing high bandwidth to point-to-point connections. The attenuation characteristics of free space make inter-satellite connections a promising application for optical free space communication. Systems of this kind are already being deployed as a communications media in a number of key applications around the world.

Terrestrial applications
For terrestrial applications emerged a solution to the .last mile problem.. This consists in the fact that needs for bandwidth cannot be satisfied because the next fibre backbone is too far away. Laser links between the backbone and the user can provide quickly a high bandwidth much more cost effective than with other technologies. Due to the atmospheric effects on the propagation of light only short distance links can be established assuring a comparable bit error rate to fibre networks. With optical free space communication devices it's as well possible to replace lower speed wired lines between computer networks with fibre-like delay-free high bandwidth. It turned out as well that optical free space communication can be used to interconnect LAN's in campus or industrial environments in a reasonable way or to deploy temporary internet connection while the fibre installation can be done. Another application is to use laser links to bypass a broken data connection.

Military applications
There are not many information about military applications available. However there have been successful experiments to track moving objects with a laser beam as well for communication purposes using so called spatial light modulators.

 

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