Therefore, the amount of transmitted
power needed to achieve comparable distances is significantly higher in the case
of FSO than in the case of fiber optics.
Based on the transmitter and receiver techniques, FSO systems can be divided into
two broad categories: active systems and passive systems.
Active System Active FSO systems consist of active electro-optic components to
transmit and receive data. Electro-optical devices such as light-emitting diodes (LEDs)
or laser diodes are used to generated modulated signals to be transmitted. Electrooptical
devices such as PN diodes or avalanche photodiodes (APDs) are used to receive
and demodulate the received optical signals.
LEDs used in FSO devices are close cousins of LEDs used widely as electronic displays
and are even closer to LEDs used in remote controls and IrDA devices. In a
typical FSO system, LEDs are modulated at a much higher rate than LEDs in IrDA.
Besides being less expensive than laser diodes, LED also has the benefit of being a
source of incoherent light. The incoherent light makes FSO systems based on LEDs
safer (for eyes) than those based on laser diodes. Additionally, it also makes such systems
less susceptible to effects of atmospheric scintillation, a topic to be discussed in
more detail later.
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