Finally, making FoV smaller poses the same challenges as reducing beam divergence.
It requires precision components, precision manufacturing, and complex alignment.
Active Tracking As discussed in the preceding sections, it is often desirable to use FSO
systems with narrow divergence and FoV. However, even a small scale mispointing of
such a narrow beam can easily disrupt the FSO link established by the beam.
There are several reasons for such involuntary mispointing. FSO equipment is generally
installed in open environments such as buildings and on poles that are likely
to exhibit small movements. For example, buildings are subject to daily sway due to
thermal expansion and contractions and poles exhibit oscillations under heavy winds.
In other cases, FSO systems often get installed too close to sources of vibration such as
large air conditioners causing the FSO systems to resonate along with the vibrating
equipment. All of these involuntary movements can cause beam mispointing.
There are two common ways to compensate for mispointing due to involuntary movement
for FSO links: (1) passively by means of a relatively large beam divergence and
FoV and (2) actively by means of tracking.
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