5.3. LONG-HAUL SYSTEMS 199
The third generation of lightwave systems became available commercially in 1991.
They operate near 1.55μm at bit rates in excess of 2 Gb/s, typically at 2.488 Gb/s,
corresponding to the OC-48 level of the synchronized optical network (SONET) [or the
STS–16 level of the synchronous digital hierarchy (SDH)] specifications. The switch
to the 1.55-μm wavelength helps to increase the loss-limited transmission distance to
more than 100 km because of fiber losses of less than 0.25 dB/km in this wavelength
region. However, the repeater spacing was limited to below 100 km because of the
high GVD of standard telecommunication fibers. In fact, the deployment of third-
generation lightwave systems was possible only after the development of distributed
feedback (DFB) semiconductor lasers, which reduce the impact of fiber dispersion by
reducing the source spectral width to below 100 MHz (see Section 2.4).
The fourth generation of lightwave systems appeared around 1996. Such systems
operate in the 1.55-μm region at a bit rate as high as 40 Gb/s by using dispersion-
shifted fibers in combination with optical amplifiers. However, more than 50 million
kilometers of the standard telecommunication fiber is already installed in the world-
wide telephone network. Economic reasons dictate that the fourth generation of light-
wave systems make use of this existing base. Two approaches are being used to solve
the dispersion problem. First, several dispersion-management schemes (discussed in
Chapter 7) make it possible to extend the bit rate to 10 Gb/s while maintaining an am-
plifier spacing of up to 100 km. Second, several 10-Gb/s signals can be transmitted
simultaneously by using the WDM technique discussed in Chapter 8. Moreover, if
the WDM technique is combined with dispersion management, the total transmission
distance can approach several thousand kilometers provided that fiber losses are com-
pensated periodically by using optical amplifiers. Such WDM lightwave systems were
deployed commercially worldwide beginning in 1996 and allowed a system capacity
of 1.6 Tb/s by 2000 for the 160-channel commercial WDM systems.
The fifth generation of lightwave systems was just beginning to emerge in 2001.
The bit rate of each channel in this generation of WDM systems is 40 Gb/s (correspond-
ing to the STM-256 or OC-768 level). Several new techniques developed in recent
years make it possible to transmit a 40-Gb/s optical signal over long distances. New
fibers known as reverse-dispersion fibers have been developed with a negative GVD
slope. Their use in combination with tunable dispersion-compensating techniques can
compensate the GVD for all channels simultaneously. The PMD compensators help to
reduce the PMD-induced degradation of the signal. The use of Raman amplification
helps to reduce the noise and improves the signal-to-noise ratio (SNR) at the receiver.
The use of a forward-error-correction technique helps to increase the transmission dis-
tance by reducing the required SNR. The number of WDM channels can be increased
by using the L and S bands located on the long- and short-wavelength sides of the
conventional C band occupying the 1530–1570-nm spectral region. In one 3-Tb/s ex-
periment, 77 channels, each operating at 42.7-Gb/s, were transmitted over 1200 km
by using the C and L bands simultaneously [48]. In another experiment, the system
capacity was extended to 10.2 Tb/s by transmitting 256 channels over 100 km at 42.7
Gb/s per channel using only the C and L bands, resulting in a spectral efficiency of
1.28 (b/s)/Hz [49]. The bit rate was 42.7 Gb/s in both of these experiments because
of the overhead associated with the forward-error-correction technique. The highest
capacity achieved in 2001 was 11 Tb/s and was realized by transmitting 273 channels