Abstract:
Optical wavelength division multiplexing (WDM) system using low dispersion fibers and
erbium-doped fiber amplifiers (EDF A) is getting tremendous popularity for
communication networks due to its high information capacity, high-channel-count and
low system costs. WDM transmission systems effectively exploit the huge potential
bandwidth of an optical fiber. More than thousand channels can be multiplexed with high
bit rate if the fiber low dispersion region around 1.55 11m is fully utilized where
attenuation loss is minimum and EDFA provides gain. However, a number of nonlinear
effects arise due to low dispersion and high transmitting power, which severely degrade
the system performance. Among these nonlinear processes, four-wave mixing (FWM) is
the most serious effect that limits the allowable input power and system capacity by
generating new waves, which cause performance degradation of long-haul multi-channel
lightwave transmission systems.
In this thesis the performance of wavelength shift keying (WSK) technique in reducing
the effect of FWM in an optical WDM system is analyzed. WSK-WDM system is a
modification of the conventional WDM system that uses symmetric wavelength
assignment and balanced detection to mitigate the FWM crosstalk. Performance criteria
like probability of error, power penalty, allowable input power and bit rate-distance
product are estimated. The performance is evaluated for different types of fibers, different
number of channels, different channel spacings and different length of fibers. It is
observed that the conventional on-off WDM system suffers severe performance
degradation due to FWM interference particularly when fiber length, number of channels
or transmitting power is high. WSK- WDM is a simple scheme to reduce this effect and
i
excels conventional WDM at all signal power levels and allows higher allowable input
power and larger bit rate-distance product for a given bit error rate. The performance of
WSK-WDM is compared to that of conventional on-off WDM system and also to
unequal channel spacing and repeated unequal channel spacing schemes. New schemes
are proposed, which are the combinations of WSK technique and different dispersion management techniques. The combined systems offer much enhanced performance like
lower error probability, much lower power penalty and higher allowable input power per
channel and greater bit rate-distance product to achieve a given bit error rate compared to
those of WSK-WDM. The results obtained from this work will be useful for designing
very high capacity long-haul optical communication systems.
