Page 267 - Optical Communications Essentials

Performance Impairments

Performance Impairments 257

Interaction up to 125 nm
Peak Stokes shift (90 nm)
Gain peak
Relative amplitude gain curve Input signal
Raman

@ 1610 nm

Input signal SRS-generated signal
@ 1530 nm @ 1610 nm
Figure 15.6. As a result of SRS, the signal at 1530nm acts as
a pump for the signal at 1610nm.

In general, if the optical power per channel is not excessively high (e.g., less
than 1mW each), then the effects of SRS do not contribute significantly to the
eye closure penalty as a function of transmission distance. Furthermore, since
the generated wavelength is around 90nm away from the signal wavelength,
crosstalk from SRS is not a significant problem unless the system operates over
a wide spectral range. As an example, consider a WDM system that operates over
both the C-band and the L-band (from 1530 to 1620nm). As shown in Fig. 15.6 for
two specific wavelengths in this range, as a result of SRS the signal at 1530nm
acts as a pump for the signal at 1610nm.

15.5.3. Stimulated Brillouin scattering
Stimulated Brillouin scattering arises when light waves scatter from acoustic
waves. The resultant scattered wave propagates principally in the backward
direction in single-mode fibers. This backscattered light experiences gain from
the forward-propagating signals, which leads to depletion of the signal power.
The frequency of the scattered light experiences a Doppler shift given by

2nV s
ν B (15.7)
λ
where n is the index of refraction and V s is the velocity of sound in the material.
In silica this interaction occurs over a very narrow Brillouin linewidth of
∆ν B 20MHz at 1550nm. For V s 5760m/s in fused silica, the frequency of
the backward-propagating light at 1550nm is downshifted by 11GHz (0.09nm)
from the original signal. This shows that the SBS effect is confined within a sin-
gle wavelength channel in a WDM system. Thus, the effects of SBS accumulate
individually for each channel, and consequently they occur at the same power level
in each channel as occurs in a single-channel system.

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