5.3. LONG-HAUL SYSTEMS 201
optimized to combat such effects. Even then, amplifier spacing is typically limited to
50 km, and the use of an error-correction scheme is essential to ensure a bit-error rate
of< 2 × 10 −^11.
A second category of undersea lightwave systems requires repeaterless transmis-
sion over several hundred kilometers [52]. Such systems are used for interisland com-
munication or for looping a shoreline such that the signal is regenerated on the shore
periodically after a few hundred kilometers of undersea transmission. The dispersive
and nonlinear effects are of less concern for such systems than for transoceanic light-
wave systems, but fiber losses become a major issue. The reason is easily appreciated
by noting that the cable loss exceeds 100 dB over a distance of 500 km even under the
best operating conditions. In the 1990s several laboratory experiments demonstrated
repeaterless transmission at 2.5 Gb/s over more than 500 km by using two in-line am-
plifiers that were pumped remotely from the transmitter and receiver ends with high-
power pump lasers. Another amplifier at the transmitter boosted the launched power to
close to 100 mW.
Such high input powers exceed the threshold level for stimulated Brillouin scatter-
ing (SBS), a nonlinear phenomenon discussed in Section 2.6. The suppression of SBS
is often realized by modulating the phase of the optical carrier such that the carrier
linewidth is broadened to 200 MHz or more from its initial value of<10 MHz [54].
Directly modulated DFB lasers can also be used for this purpose. In a 1996 experi-
ment. a 2.5-Gb/s signal was transmitted over 465 km by direct modulation of a DFB
laser [55]. Chirping of the modulated signal broadened the spectrum enough that an
external phase modulator was not required provided that the launched power was kept
below 100 mW. The bit rate of repeaterless undersea systems can be increased to
10 Gb/s by employing the same techniques used at 2.5 Gb/s. In a 1996 experiment [56],
the 10-Gb/s signal was transmitted over 442 km by using two remotely pumped in-line
amplifiers. Two external modulators were used, one for SBS suppression and another
for signal generation. In a 1998 experiment, a 40-Gb/s signal was transmitted over
240 km using the RZ format and an alternating polarization format [57]. These results
indicate that undersea lightwave systems looping a shoreline can operate at 10 Gb/s or
more with only shore-based electronics [58].
The use of the WDM technique in combination with optical amplifiers, dispersion
management, and error correction has revolutionized the design of submarine fiber-
optic systems. In 1998, a submarine cable known as Atlantic-Crossing 1 (AC–1) with
a capacity of 80 Gb/s was deployed using the WDM technology. An identically de-
signed system (Pacific-Crossing 1 or PC–1) crossed the Pacific Ocean. The use of
dense WDM, in combination with multiple fiber pairs per cable, resulted in systems
with much larger capacities. By 2001, several systems with a capacity of>1 Tb/s be-
came operational across the Atlantic Ocean (see Table 5.3). These systems employ a
ring configuration and cross the Atlantic Ocean twice to ensure fault tolerance. The
“360Atlantic” submarine system can operate at speeds up to 1.92 Tb/s and spans a
total distance of 11,700 km. Another system, known as FLAG Atlantic-1, is capable
of carrying traffic at speeds up to 4.8 Tb/s as it employs six fiber pairs. A global net-
work, spanning 250,000 km and capable of operating at 3.2 Tb/s using 80 channels (at
10 Gb/s) over 4 fibers, was under development in 2001 [53]. Such a submarine network
can transmit nearly 40 million voice channels simultaneously, a capacity that should be