316 CHAPTER 7. DISPERSION MANAGEMENT
Figure 7.19: Pulse shapes after a 2.6-ps input pulse propagated over 300 km of dispersion-
shifted fiber (β 2 =0). Left and right traces compare the improvement realized by compensating
the third-order dispersion. (After Ref. [169];©c1996 IEE; reprinted with permission.)
This requirement is nearly the same as that obtained earlier in Eq. (7.9.2) for DCFs used
in WDM systems becauseβ 3 is related to the dispersion slopeSthrough Eq. (2.3.13).
For a single-channel system, the signal bandwidth is small enough even at bit rates
of 500 Gb/s that it is sufficient to satisfy Eq. (7.9.5) over a 4-nm bandwidth. This re-
quirement is easily met by an optical filter or a chirped fiber grating [51]. Consider
the case of optical filters first. Planar lightwave circuits based on multiple MZ inter-
ferometric filters (see Section 7.5) have proved quite successful because of the pro-
grammable nature of such filters. In one experiment [169], such a filter was designed
to have a dispersion slope of− 15 .8 ps/nm^2 over a 170-GHz bandwidth. It was used
to compensate third-order dispersion over 300 km of a dispersion-shifted fiber with
β 3 ≈ 0 .05 ps/(km-nm^2 ) at the operating wavelength. Figure 7.19 compares the pulse
shapes at the fiber output observed with and withoutβ 3 compensation when a 2.6-ps
pulse was transmitted over 300 km of such a fiber. The equalizer eliminates the long
oscillatory tail and reduces the width of the main peak from 4.6 to 3.8 ps. The increase
in the pulse width from its input value of 2.6 ps is attributed to polarization-mode dis-
persion (PMD), a topic covered later.
Chirped fiber gratings are often preferred in practice because of their all-fiber na-
ture. Long fiber gratings (∼1 m) were developed by 1997 for this purpose [170]. In
1998, a nonlinearly chirped fiber grating was capable of compensating the third-order
dispersion over 6 nm for distances as long as 60 km [171]. Cascading of several
chirped gratings can provide a dispersion compensator that has arbitrary dispersion
characteristics and is capable for compensating dispersion to all higher orders [172].
An arrayed-waveguide grating [173] or a sampled fiber grating [174] can also com-
pensate for second- and third-order dispersion simultaneously. Although a sampled
fiber grating chirped nonlinearly can provide tunable dispersion for several channels
simultaneously [177], its bandwidth is still limited. An arrayed-waveguide grating in
combination with a spatial phase filter can provide dispersion-slope compensation over
a bandwidth as large as 8 THz and should be suitable for 40-Gb/s multichannel sys-
tems [178]. The feasibility of transmitting of a 100-Gb/s signal over 10,000 km has
also been investigated using midway optical phase conjugation in combination with
third-order dispersion compensation [179].
Several single-channel experiments have explored the possibility of transmitting a
single channel at bit rates of more than 200 Gb/s [181]–[183]. Assuming that a 2-ps bit