Abstract:
In recent years, traffic on the internet has increased rapidly because of the increase in the
number of users. Use of VOIP, video conference, multimedia, online data transfer and
various software applications has also increased the data rate. As a result, the backbone
network should have high speed and capacity which can be provided by the optical fiber. The
advantages of using fiber-optic communication over wireless communication are that it is
free from Electro-Magnetic Interference (EMI), Radio Frequency Interference (RMI) and
highly secured etc. On-off keying (OOK) modulation has been used in optical transmission
system for a long time because of its simplicity and low cost. Currently most commercial
systems use OOK format at 10 Gbps data rate. If the bit rate is increased greater than 40 Gbps
then the dispersion, nonlinear effect, intra-channel effects will increase much. As a result
OOK modulation is not suitable in high-speed optical communication systems. On the other
hand, phase shift keying (PSK) modulation techniques may provide higher spectral efficiency
and better performance than OOK since PSK signals have constant amplitude and compact
spectrum. Moreover PSK signals are robust to dispersion and nonlinearity. PSK receiver with
coherent detection is costly and complex. However, direct detection receiver can be used in
differential multilevel PSK modulation schemes which are simpler and comparatively low
cost. That is why PSK modulation schemes are becoming popular for next generation optical
communication system.
In this thesis, the performance of DBPSK, DQPSK and 8-DPSK modulation schemes is
evaluated in high-speed long-haul fiber-optic communication system. The performance
parameter is quality factor Q, which is observed by varying bitrates, input power, distance
and residual dispersion. The Split-Step Fourier method is used as numerical technique for
solving Nonlinear Schrodinger Equation which governs the propagation of light in optical
fiber. In this simulation dispersion-managed transmission line is used in order to achieve
perfect dispersion compensation for each of the three modulation schemes for different fibers
and bit rates. However, practically zero dispersion is not possible, even if one channel can be
made approximately perfectly compensated, other channels could find residual dispersion due
to dispersion slope. The performance parameter Q is measured from eye pattern which is
attained from numerical simulations. Therefore, optimum PSK format has been checked
which is suitable for high-speed communication. The result obtained from this work will be
useful for designing high-speed long-haul fiber-optic communication system.
