Abstract
An analytic model for interchannel four-wave mixing (FWM)
is developed for return to zero differential phase shift keying (RZ-DPSK)
single span transmission including interchannel walk-off for degenerate FWM
(D-FWM) and nondegenerate FWM (ND-FWM). The model is verified by simulations
and reasonable agreement is obtained for a 10-Gb/s single span link. Results
are presented for different channel spacing, launch pulse duty-cycle, and
launch power. The model is strictly valid for systems with low fiber dispersion
as it ignores dispersion related pump pulse distortion. It is found that the
walk-off effect makes a significant contribution to the detected unfiltered
FWM noise and this contribution is mostly concentrated at high frequencies
determined by the channel spacing alone. The contribution of the walk-off
and its frequency content is understood in terms of intracollision and intercollision
interference. In the typical situation when a sub-bit-rate electrical low-pass
filter is used, the walk-off effect is effectively removed for all types of
FWM. The analytic model is validated by comparing to split-step-Fourier (SSF)
simulations. Extension to the general case of multiple FWM lines is considered.
It is found that the FWM noise standard deviation is 2 times smaller for RZ-DPSK
as compared to that for return to zero on-off keying (RZ-OOK) at the same
average launch power for a system with 25-GHz channel spacing.
© 2008 IEEE
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