| 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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