5 miles from a transmitter, the transmitted beam
needs to have a divergence of about 0.034 degrees. Such a system requires much more
precise components and manufacturing processes than a system with 1 degree of
divergence. Though technically feasible, the cost of such high precision systems may not
make them economically viable in all applications.
Systems with a narrower beam divergence also pose a significant challenge to the
task of aligning FSO links and maintaining alignment during their operation. For example,
for the system with 0.035 degrees divergence, a deflection of the transmit beam
by as little as 0.035 degrees can mispoint the transmit signal away from the receiver.
As explained later, such mispointing is quite common, but mechanisms to maintain
alignment within such small angles, though technically feasible, can be very costly.
Receive Optics The receiver optics serve purposes exactly complementary to those
of the transmit optics. The receiver optics collect the light signal and focus it onto
the detector (or into the fiber-optic cable in the case of a passive system). They are
made out of combination of one or more lenses and/or mirrors.
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