Page 270 - Optical Communications Essentials

Performance Impairments

260 Chapter Fifteen

ν 1 ν 2
Sideband Sideband
2ν 1 - ν 2 2ν 2 - ν 1

Frequency
Figure 15.8. Two optical waves at frequencies ν 1 and
ν 2 mix to generate two third-order sidebands.

are located near the zero-dispersion point, three optical frequencies (ν i , ν j , ν k )
will mix to produce a fourth intermodulation product ν ijk given by
with i, j
k (15.9)
ν ijk ν i ν j ν k
When this new frequency falls in the transmission window of the original fre-
quencies, it can cause severe crosstalk.
Figure 15.8 shows a simple example for two waves at frequencies ν 1 and ν 2 .
As these waves copropagate along a fiber, they mix and generate sidebands at
2ν 1 ν 2 and 2ν 2 ν 1 . Similarly, three copropagating waves will create nine new
optical sideband waves at frequencies given by Eq. (15.9). These sidebands will
travel along with the original waves and will grow at the expense of signal
strength depletion. In general, for N wavelengths launched into a fiber, the
number of generated mixing products M is

N 2
M (N 1) (15.10)
2
If the channels are equally spaced, a number of the new waves will have the
same frequencies as the injected signals. Thus, the resultant crosstalk interfer-
ence plus the depletion of the original signal waves can severely degrade multi-
channel system performance unless steps are taken to diminish it.
The efficiency of four-wave mixing depends on fiber dispersion and the chan-
nel spacings. Since the dispersion varies with wavelength, the signal waves and
the generated waves have different group velocities. This destroys the phase
matching of the interacting waves and lowers the efficiency at which power is
transferred to newly generated frequencies. The higher the group velocity mis-
matches and the wider the channel spacings, the lower the four-wave mixing. The
G.655 and G.655b fibers were developed specifically to address the FWM issues.

15.6. Summary
Signal dispersion factors and nonlinear effects in fibers limit the information
carrying capacity in optical links. For high-performance single-mode fibers,

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