They
allow the creation of a narrow beam of light to be transmitted. They also allow for the
collection of optical signals at the receiving end. The optics technology used in FSO systems
are the same ones found in other optical systems such as telescopes and cameras.
Therefore, the technology is very mature and well proven.
Transmit Optics The transmit optics in an FSO system consist of optical components
such as lenses and/or mirrors. The transmit optics serve the purpose of collecting
the light from the transmit source such as the LED, laser, or fiber-optic cable and then
transmitting it in the form of a narrow beam of light. Such beams are characterized
by two parameters: beamwidth and divergence. A simple form of transmit optics is illustrated
in Figure 9.2.
Transmit Beam Width The beamwidth is the measure of the diameter of the transmit
beam as it launches out of the system. The desirability for a larger beamwidth is in its
ability to transmit more optical power while meeting the safety requirements mandated
by government agencies. FSO system safety, as regulated by government agencies,
Optical Wireless Mesh Networks 239
depends on the optical power per unit cross-sectional area of the beam.
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