In fact, a
system with four separate transmit beams can achieve a better scintillation immunity.
The downside of a system with either wider transmit beams or multiple transmit
beams is the size, weight, and cost of the system. The optical components required to
create large beamwidths are not only bigger and heavier but can be significantly more
Figure 9.2 A typical transmit optic and transmit beam profile
The width of the beam as
it comes out of the
transmitter receiver
defines the beamwidth of
the system.
The angle of the
transmit beam defines
the divergence of the
transmitter.
Transmit beam
FSO
system
FSO
system
Signal source
(LED, laser diode)
Lens
Transmit beam
Transmit beam
creates a large
footprint at the
receiving end.
Transmit Optics and Beamwidth
Beam Divergence
240 Chapter 9
costly than smaller optical components. Some of the benefits of the large transmit beam
may not be of any significance for the particular application being considered. For example,
for short-range links, the effect of scintillation is insignificant.
Transmit Beam Divergence Transmit beam divergence measures the degree of beam
spreading as it propagates away from the transmitter.
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