Fig. 11. Normalized power crosstalk levels versus the percentage of dispersion
compensation in a lO-Gb/s, six-span system (100 km/span) with the fiber
dispersion of 2.9 ps/nm/km. The 8.5 dBm launched pump optical power at
each fiber span: (1) dispersion compensation after each span, (2) dispersion
compensation after every two spans, (3) dispersion compensation after every
three spans, and (4) one lumped dispersion compensation in front of the
receiver.
or the second span but not for both of them. The residual
XPM crosstalk level is higher in this case than that with
compensation in each span. Similarly, trace (3) in Fig. 11
was obtained with a dispersion compensator placed after every
three spans and trace (4) is with only one lumped compensator
placed in front of the receiver.
Obviously, when the number of dispersion compensators is
reduced, the level of residual XPM crosstalk is higher and
the optimum value of dispersion compensation is closer to
50% of the total system dispersion. Therefore, in systems
where XPM-induced crosstalk is a significant impairment, per
span dispersion compensation is recommended. However, this
will increase the number of dispersion compensators and thus
increase the cost.
V. CONCLUSION
We have investigated the spectral characteristics of XPM in
multispan IMDD optical systems, both experimentally and theoretically.
Interference between XPM-induced crosstalk components
created in different amplified fiber spans has a strong
impact on the overall frequency response of XPM crosstalk in
the system. Reasonably good agreement between theory and
experiment has been obtained.
In uncompensated optical systems, a decrease in fiber
dispersion will increase XPM-induced phase modulation
efficiency, while an increase in fiber dispersion will increase
phase-to-intensity noise conversion efficiency. Dispersion
compensation was shown to be an effective way to reduce
XPM-induced crosstalk in IMDD systems. Different schemes
of dispersion compensation in multispan optical fiber systems
were evaluated and per span dispersion compensation was
found to be the most effective way to minimize the effect of
XPM crosstalk.
The crosstalk level between high and low bit rate channels
was found to be similar to that between two low bit rate