5.5. COMPUTER-AIDED DESIGN 217
Figure 5.15: Steps involved in computer modeling of fiber-optic communication systems.reflectivities of the two interfaces [128]. Figure 4.19 can be used to estimate the power
penalty. It shows that power penalty can become infinite and lead to BER floors when
reffexceeds 0.2. Such BER floors have been observed experimentally [128]. They can
be avoided only by eliminating or reducing parasitic reflections along the entire fiber
link. It is therefore necessary to employ connectors and splices that reduce reflections
through the use of index matching or other techniques.
5.5 Computer-Aided Design
The design of a fiber-optic communication system involves optimization of a large
number of parameters associated with transmitters, optical fibers, in-line amplifiers,
and receivers. The design aspects discussed in Section 5.2 are too simple to provide
the optimized values for all system parameters. The power and the rise-time budgets
are only useful for obtaining a conservative estimate of the transmission distance (or
repeater spacing) and the bit rate. The system margin in Eq. (5.2.4) is used as a ve-
hicle to include various sources of power penalties discussed in Section 5.4. Such a
simple approach fails for modern high-capacity systems designed to operate over long
distances using optical amplifiers.
An alternative approach uses computer simulations and provides a much more real-
istic modeling of fiber-optic communication systems [141]–[156]. The computer-aided
design techniques are capable of optimizing the whole system and can provide the op-
timum values of various system parameters such that the design objectives are met at
a minimum cost. Figure 5.15 illustrates the various steps involved in the simulation
process. The approach consists of generating an optical bit pattern at the transmitter,
transmitting it through the fiber link, detecting it at the receiver, and then analyzing it
through the tools such as the eye diagram and theQfactor.