For a typical FSO system, it means transmitted
optical power of about 100 mW to 1 W, a much more realistic number. This example
illustrates that FSO links can be immune from all weather conditions when they are
deployed over short distances.
Another aspect of weather to consider is the probability of occurrence of a particular
weather event. For example, in cities such as Phoenix, the probability of
fog events denser than a thin fog may be negligible. In such environments, a link
with 12 dB of margin at 250 m can also be immune from all likely weather conditions.
Therefore, the viability of long-distance FSO links depends on local weather
patterns.
Scintillation Scintillation is a phenomenon experienced quite often in our daily life.
It is an atmospheric effect commonly observed as the twinkling of stars or distant
light sources. It is caused by variations of air density through the different parts of
the atmosphere constantly changing over time. The variation is caused by the turbulent
mixing of warmer and cooler air and is more pronounced during hotter days than
cooler days.
The primary cause of scintillation in FSO is due to the constant changing of the course
of a beam.
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