Page 266 - Optical Communications Essentials
P. 266
Performance Impairments
256 Chapter Fifteen
10,000
Effective length (km) 1000 75-km
100
amplified systems
10
Nonamplified system
1
10 100 1000 10,000
Link length (km)
Figure 15.4. Effective length as a function of the actual link length
for a link without amplifiers and a link with optical amplifiers spaced
75km apart.
Input wavelengths Output wavelengths
Optical fiber
λ 1 λ 2 λ 3 λ 4 λ 1 λ 2 λ 3 λ 4
Figure 15.5. SRS generates scattered light at a
longer wavelength, thereby decreasing the power
in the pump wavelength signal.
15.5.2. Stimulated Raman scattering
Stimulated Raman scattering is an interaction between light waves and the
vibrational modes of silica molecules. If a photon with energy hν 1 is incident on
a molecule having a vibrational frequency ν m , the molecule can absorb some
energy from the photon. In this interaction the photon is scattered, thereby
attaining a lower frequency ν 2 and a corresponding lower energy hν 2 . The modi-
fied photon is called a Stokes photon. Because the optical signal wave that is
injected into a fiber is the source of the interacting photons, it is often called the
pump wave, since it supplies power for the newly generated wave.
This process generates scattered light at a wavelength longer than that of the
incident light. If another signal is present at this longer wavelength, the SRS
light will amplify it and the pump wavelength signal will decrease in power.
Figure 15.5 illustrates this effect. Consequently, SRS could limit the perform-
ance of a multichannel optical communication system by transferring energy
from short-wavelength channels to neighboring higher-wavelength channels.
This is a broadband effect that can occur in both directions. Powers in channels
separated by up to 16THz (125nm) can be coupled through the SRS effect,
thereby producing crosstalk between wavelength channels.
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