284 CHAPTER 7. DISPERSION MANAGEMENT
Figure 7.2: Dispersion compensation using FSK coding: (a) Optical frequency and power of the
transmitted signal; (b) frequency and power of the received signal and the electrically decoded
data. (After Ref. [13];©c1994 IEEE; reprinted with permission.)
between 1 and 0 bits while leaving the power unchanged (see Chapter 10). During
propagation inside the fiber, the two wavelengths travel at slightly different speeds.
The time delay between the 1 and 0 bits is determined by the wavelength shift∆λand
is given by∆T=DL∆λ, as shown in Eq. (2.3.4). The wavelength shift∆λis chosen
such that∆T= 1 /B. Figure 7.2 shows schematically how the one-bit delay produces
a three-level optical signal at the receiver. In essence, because of fiber dispersion, the
FSK signal is converted into a signal whose amplitude is modulated. The signal can be
decoded at the receiver by using an electrical integrator in combination with a decision
circuit [13].
Several transmission experiments have shown the usefulness of the dispersion-
supported transmission scheme [13]–[15]. All of these experiments were concerned
with increasing the transmission distance of a 1.55-μm lightwave system operating at
10 Gb/s or more over the standard fibers. In 1994, transmission of a 10-Gb/s signal
over 253 km of standard fiber was realized [13]. By 1998, in a 40-Gb/s field trial,
the signal was transmitted over 86 km of standard fiber [15]. These values should be
compared with the prediction of Eq. (7.1.2). Clearly, the transmission distance can be
improved by a large factor by using the FSK technique when the system is properly
designed [17].
Another approach for increasing the transmission distance consists of transmitting
an optical signal whose bandwidth at a given bit rate is smaller compared with that of
the standard on–off coding technique. One scheme makes use of theduobinary coding,
which can reduce the signal bandwidth by 50% [18]. In the simplest duobinary scheme,
the two successive bits in the digital bit stream are summed, forming a three-level
duobinary code at half the bit rate. Since the GVD-induced degradation depends on the
signal bandwidth, the transmission distance should improve for a reduced-bandwidth
signal. This is indeed found to be the case experimentally [19]–[24].
In a 1994 experiment designed to compare the binary and duobinary schemes, a