Page 723 - Introduction to Information Optics

12.4. Applications and Future Development of Optical Networks 707

Data in

Fiber

Fig. 12.23. Implementation of FEC in an optical communication system.

FEC. In other words, a system powered by FEC will achieve a longer
transmission distance with the same BER.
The extra bits introduced by FEC will increase the bit rate. Practically, a
7% increase in bit rate will comfortably decrease system OSNR requirements
by 5 dB or more, which in turn, easily doubles the transmission distance.

12.4.1.2.2. Raman Amplification
In high-capacity DWDM transmission systems, every channel has to keep
enough OSNR in order to achieve required BER. Consequently, high-power
amplifiers are used to boost the launch power into every fiber span. There are
two direct impairments associated with this method, however:

• High-power amplifiers produce more ASE than low-power amplifiers.
ASE is the major noise source degrading transmission system perfor-
mance.
• High launch power causes more severe fiber nonlinearity impairment.

Mixed together with EDFA, Raman amplification becomes a key technology
to enable ULH transmission.
Raman scattering was discussed in the previous section. Raman amplifica-
tion actually takes advantage of Raman scattering. It occurs when high-energy
pump photons scatter off the optical phonons of an amplification materials
lattice matrix and add energy coherently to the signal photons. When applied
to optical networks, pump light is launched directly into the transmission fiber
and the transmission fiber serves as the gain medium to the optical channels.
Hence, it is also called distributed Raman amplification. The advantages of
using distributed Raman amplification are as follows:

• Distributed Raman amplification offers an effective noise figure which is
much smaller than EDFA, resulting in higher system OSNR.