218 CHAPTER 5. LIGHTWAVE SYSTEMS
Each step in the block diagram shown in Fig. 5.15 can be carried out numerically
by using the material given in Chapters 2–4. The input to the optical transmitter is a
pseudorandom sequence of electrical pulses which represent 1 and 0 bits. The lengthN
of the pseudorandom bit sequence determines the computing time and should be chosen
judiciously. Typically,N= 2 M, whereMis in the range 6–10. The optical bit stream
can be obtained by solving the rate equations that govern the modulation response
of semiconductor lasers (see Section 3.5). The equations governing the modulation
response should be used if an external modulator is used. Chirping is automatically
included in both cases. Deformation of the optical bit stream during its transmission
through the optical fiber is calculated by solving the NLS equation (5.3.1). The noise
added by optical amplifiers should be included at the location of each amplifier.
The optical signal is converted into the electrical domain at the receiver. The shot
and thermal noise is adding through a fluctuating term with Gaussian statistics. The
electrical bit stream is shaped by passing it through a filter whose bandwidth is also
a design parameter. An eye diagram is constructed using the filtered bit stream. The
effect of varying system parameters can be studied by monitoring the eye degradation
or by calculating theQparameter given in Eq. (4.5.11). Such an approach can be
used to obtain the power penalty associated with various mechanisms discussed in
Section 5.4. It can also be used to investigate trade-offs that would optimize the overall
system performance. An example is shown in Fig. 5.12, where the dependence of
the calculated system penalty on the frequency chirp and extinction ratio is found.
Numerical simulations reveal the existence of an optimum extinction ratio for which
the system penalty is minimum.
Computer-aided design has another important role to play. A long-haul lightwave
system may contain many repeaters, both optical and electrical. Transmitters, receivers,
and amplifiers used at repeaters, although chosen to satisfy nominal specifications, are
never identical. Similarly, fiber cables are constructed by splicing many different pieces
(typical length 4–8 km) which have slightly different loss and dispersion characteris-
tics. The net result is that many system parameters vary around their nominal values.
For example, the dispersion parameterD, responsible not only for pulse broadening
but also for other sources of power penalty, can vary significantly in different sections
of the fiber link because of variations in the zero-dispersion wavelength and the trans-
mitter wavelength. A statistical approach is often used to estimate the effect of such
inherent variations on the performance of a realistic lightwave system [146]–[150]. The
idea behind such an approach is that it is extremely unlikely that all system parameters
would take their worst-case values at the same time. Thus, repeater spacing can be
increased well above its worst-case value if the system is designed to operate reliably
at the specific bit rate with a high probability (say 99.9%).
The importance of computer-aided design for fiber-optic communication systems
became apparent during the 1990s when the dispersive and nonlinear effects in optical
fibers became of paramount concern with increasing bit rates and transmission dis-
tances. All modern lightwave systems are designed using numerical simulations, and
several software packages are available commercially. Appendix E provides details on
the simulation package available on the CD-ROM included with this book (Courtesy
OptiWave Corporation). The reader is encouraged to use it for a better understanding
of the material covered in this book.