306 CHAPTER 7. DISPERSION MANAGEMENT
Figure 7.15: Recirculating fiber loop used to demonstrate transmission of a 10-Gb/s signal over
10,000 km of standard fiber using a DCF periodically. Components used include laser diode
(LD), electroabsorption (EA) modulator, optical switch (SW), fiber amplifier (EDFA), single-
mode fiber (SMF), and DCF. (After Ref. [135];©c2000 IEEE; reprinted with permission.)
spread over several bit slots before the GVD was compensated. This is not the case
when nonlinear effects cannot be neglected. The nonlinear interaction among optical
pulses of the same channel (intrachannel effects), and among pulses of neighboring
channels in a WDM system (interchannel effects), degrade the signal quality to the
extent that the GVD compensation at the receiver alone fails to work for long-haul
systems.
A simple solution is provided by the technique ofperiodicdispersion management.
The underlying idea is quite simple and consists of mixing fibers with positive and
negative GVDs in a periodic fashion such that the total dispersion over each period is
close to zero. The simplest scheme uses just two fibers of opposite dispersions and
lengths with the average dispersion
D ̄=(D 1 L 1 +D 2 L 2 )/Lm, (7.8.1)whereDjis the dispersion of the fiber section of lengthLj(j= 1 , 2 )andLm=L 1 +L 2
is the period of dispersion map, also referred to as the map period. IfD ̄is nearly zero,
dispersion is compensated over each map period. The lengthLmis a free design param-
eter that can be chosen to meet the system-performance requirements. In practice, it is
common to chooseLmto be equal to the amplifier spacingLAas this choice simplifies
the system design. TypicallyLm=LA≈80 km for terrestrial lightwave systems but is
reduced to about 50 km for submarine systems.
Because of cost considerations, most laboratory experiments use a fiber loop in
which the optical signal is forced to recirculate many times to simulate a long-haul