Divergence is the property, measured
in degrees or radian, that identifies the spreading factor of the transmit beam.
The smaller the divergence, the less spread out the beam is.
In any wireless communication system, such spreading of the signal is one of the
greatest sources of signal loss. To illustrate the point, consider Figure 9.2 where two
FSO systems are located a half mile from each other. Let??™s assume the transmit beam
divergence is about 1 degree, a typical value for an FSO system. By the time the signal
arrives at the location of the receiver, the transmit beam would have spread enough
to create a beam 46 ft in radius. Unless a receiver with a diameter of 46 ft is used to
collect all the light, an impractical proposition, any practically sized receiver would not
be able to collect most of the signal. In fact, in the case of this example, a typical FSO
system with a 6-in diameter receiver would be able to collect only about 1/10000th of
the total power arriving at the receiving end.
Significantly reducing divergence requires higher precision optics and a higher precision
manufacturing process. For example, to recover 1/10th the transmitted signal
by a 6-in receiver at a distance of 0.
Pages:
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591