LaserBit Giga Series 4.3 page
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2.2 Optical Free-space Transmission
The principle used in free space laser transmission is very similar to the one is used
for fibre optic transmission. The difference is while fibre optic devices use electronics
and optics optimized for transmission to the air. Also one can observe to the
similarity in the transmission properties. No galvanic contact, no ground-loops, no
need for surge protection, noise immunity, long distances, high bandwidth.
What makes it unique – and difficult to design – is that it does not require any
transmission medium like fibre or copper, but it has to cope with the dynamically
changing parameters. For instance while the attenuation of an optical fibre is
constant, the attenuation of the atmosphere between the laser units can change
dramatically (depending on the weather conditions).
The laser-heads are usually placed on top of building, where the clean line of site is
guaranteed and the beam cannot be interrupted.
In the head the incoming signal is amplified, encoded, and then drives the laser-
diode. The transmitter optics assures the proper beam shape and controls the beam
divergence. The receive optics perceives and directs the transmitter signal to the
photo diode. The diode converts it back into electrical, than it is decoded, amplified
and converted.
There are several things that can influence the quality of transmission. We can
classify those factors into three main groups.
System conditions - transmitting power, transmitter’s wavelength, beam divergence,
receiver optics diameter, receiver sensitivity, parameters of optical system and
casing. These parameters determine the system’s characteristic at a certain
distance and are controlled by system design and factory set up.
Weather conditions - molecular absorption, particle scattering and turbulence. These
elements have great effect on the operational conditions of the system. We do not
have very much influence on them; proper product selection can eliminate the
undesirable effects.
Environmental conditions - building movements, direct sunlight, refractive surfaces.
These are also key factors related to the installation sites and can be controlled by
appropriate site survey and system installation
