Black Box Networks PICO systems
Pico Manual v5.1
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.
V5.1 | 10 |
