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Black Box Networks PICO systems
Pico Manual v5.1
V5.1 10
2 Optical free space transmission
The principle used in free space laser transmission is very similar to the one is used for
fiber optic transmission. The difference is while fiber optic devices use electronics and
optics optimised for optical fiber, free space communication equipment deploys
electronics and optics optimised for transmission through the air. Also you can observe
the similarity in the transmission properties. No galvanic contact, no ground loops, no
need for surge protection, noise immunity, high bandwidth.
What makes it unique – and difficult to design – is that it does not require any
transmission medium like fiber or copper, but has to cope with the dynamically changing
parameters. For instance while the attenuation of an optical fiber is constant, the
attenuation of the atmosphere between the laser units can change dramatically
(depending on the weather conditions).
The Black Box heads are usually placed on top of buildings, where clear line of sight is
guaranteed and the beam cannot be interrupted.
In the head the incoming signal is amplified, encoded, then drives the laser diode. The
transmitter optics assures the proper beam shape. The receiver optics perceives and directs
the transmitted signal to the photodiode. The diode converts it back into electrical, then 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, transmitters 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.