426 CHAPTER 9. SOLITON SYSTEMS
Figure 9.13: Setup used for soliton transmission in a 1990 experiment. Two EDFAs after the
LiNbO 3 modulator boost pulse peak power to the level of fundamental solitons. (After Ref. [70];
©c1990 IEEE; reprinted with permission.)
be divided into two categories, depending on whether a linear fiber link or a recircu-
lating fiber loop is used for the experiment. The experiments using fiber link are more
realistic as they mimic the actual field conditions. Several 1990 experiments demon-
strated soliton transmission over fiber lengths∼100 km at bit rates of up to 5 Gb/s
[70]–[72]. Figure 9.13 shows one such experimental setup in which a gain-switched
laser is used for generating input pulses. The pulse train is filtered to reduce the fre-
quency chirp and passed through a LiNbO 3 modulator to impose the RZ format on
it. The resulting coded bit stream of solitons is transmitted through several fiber sec-
tions, and losses of each section are compensated by using an EDFA. The amplifier
spacing is chosen to satisfy the criterionLALDand is typically in the range 25–40
km. In a 1991 experiment, solitons were transmitted over 1000 km at 10 Gb/s [73].
The 45-ps-wide solitons permitted an amplifier spacing of 50 km in the average-soliton
regime.
Since 1991, most soliton transmission experiments have used a recirculating fiber-
loop configuration because of cost considerations. Figure 9.14 shows such an exper-
imental setup schematically. A bit stream of solitons is launched into the loop and
forced to circulate many times using optical switches. The quality of the signal is
monitored after each round trip to ensure that the solitons maintain their width during
transmission. In a 1991 experiment, 2.5-Gb/s solitons were transmitted over 12,000 km
by using a 75-km fiber loop containing three EDFAs, spaced apart by 25 km [74]. In
this experiment, the bit rate–distance product ofBL=30 (Tb/s)-km was limited mainly
by the timing jitter induced by EDFAs. The use of amplifiers degrades the signal-to-
noise ratio (SNR) and shifts the position of solitons in a random fashion. These issues
are discussed in Section 9.5.
Because of the problems associated with the lumped amplifiers, several schemes
were studied for reducing the timing jitter and improving the performance of soliton
systems. Even the technique of Raman amplification was revived in 1999 and has