14 CHAPTER 1. INTRODUCTION
Figure 1.9: Digital bit stream 010110...coded by using (a) return-to-zero (RZ) and (b)
nonreturn-to-zero (NRZ) formats.
The use of the RZ format in the optical domain began to attract attention around
1999 after it was found that its use may help the design of high-capacity lightwave sys-
tems [64]–[66]. An example of the RZ format is provided by the dispersion-managed
soliton systems where a chirped pulse propagates inside the fiber link in a periodic
fashion, and the average dispersion is used to counteract the buildup of the nonlin-
ear effects [67]. In an interesting variant of the RZ format, known as the chirped RZ
(or CRZ) format, optical pulses in each bit slot are chirped before they are launched
into the fiber link but the system is operated in a quasi-linear regime [68]. In other
schemes, modulation formats well known in the field of microwave communications
are applied to the optical domain. Such formats are known as carrier-suppressed RZ
(CSRZ), single-sideband, or vestigial-sideband formats [59]. Such RZ formats benefit
from a reduced bandwidth compared to the standard RZ format.
An important issue is related to the choice of the physical variable that is modulated
to encode the data on the optical carrier. The optical carrier wave before modulation is
of the form
E(t)=ˆeAcos(ω 0 t+φ), (1.2.5)
whereEis the electric field vector,eˆis the polarization unit vector,Ais the amplitude,
ω 0 is the carrier frequency, andφis the phase. The spatial dependence ofEis sup-
pressed for simplicity of notation. One may choose to modulate the amplitudeA, the
frequencyω 0 , or the phaseφ. In the case of analog modulation, the three modulation
choices are known as amplitude modulation (AM), frequency modulation (FM), and
phase modulation (PM). The same modulation techniques can be applied in the digital
case and are called amplitude-shift keying (ASK), frequency-shift keying (FSK), and