7.9. HIGH-CAPACITY SYSTEMS 311
7.9.1 Broadband Dispersion Compensation
As discussed in Chapter 8, a WDM signal typically occupies a bandwidth of 30 nm or
more, although it is bunched in spectral packets of bandwidth∼ 0 .1 nm (depending on
the bit rate of individual channels). For 10-Gb/s channels, the third-order dispersion
does not play an important role as relatively wide (>10 ps) optical pulses are used
for individual channels. However, because of the wavelength dependence ofβ 2 ,orthe
dispersion parameterD, the accumulated dispersion will be different for each channel.
Any dispersion-management scheme should compensate the GVD of all channels si-
multaneously to be effective in practice. Several different methods have been used for
dispersion compensation in WDM systems. One can use either a single broadband fiber
grating or multiple fiber gratings with their stop bands tuned to individual channels. Al-
ternatively, one can take advantage of the periodic nature of the WDM spectrum, and
use an optical filter with periodic transmission peaks. A common approach consists of
extending the DCF approach to WDM systems by designing the DCF appropriately.
Consider first the case of fiber gratings [51]. A chirped fiber grating can have a
stop band as wide as10 nm if it is made long enough. Such a grating can be used in a
WDM system if the number of channels is small enough (typically<10) that the total
signal bandwidth fits inside its stop band. In a 1999 experiment, a 6-nm-bandwidth
chirped grating was used for a four-channel WDM system, each channel operating at
40 Gb/s [139]. When the WDM-signal bandwidth is much larger than that, one can use
several cascaded chirped gratings in series such that each grating reflects one channel
and compensates its dispersion [140]–[144]. The advantage of this technique is that
the gratings can be tailored to match the GVD of each channel. Figure 7.16 shows
the cascaded-grating scheme schematically for a four-channel WDM system [143].
Every 80 km, a set of four gratings compensates the GVD for all channels while two
optical amplifiers take care of all losses. The gratings opened the “closed eye” almost
completely in this experiment. By 2000, this approach was applied to a 32-channel
WDM system with 18-nm bandwidth [144]. Six chirped gratings, each with 6-nm-
wide stop band, were cascaded to compensate GVD for all channels simultaneously.
The multiple-gratings approach becomes cumbersome when the number of chan-
nels is so large that the signal bandwidth exceeds 30 nm. A FP filter has multiple
transmission peaks, spaced apart periodically by the free spectral range of the filter.
Such a filter can compensate the GVD of all channels if (i) all channels are spaced
apart equally and (ii) the free spectral range of the filter is matched to the channel spac-
ing. It is difficult to design FP filters with a large amount of dispersion. A new kind of
fiber grating, referred to as thesampledfiber grating, has been developed to solve this
problem [145]–[147]. Such a grating has multiple stop bands and is relatively easy to
fabricate. Rather than making a single long grating, multiple short-length gratings are
written with uniform spacing among them. (Each short section is a sample, hence the
name “sampled” grating.) The wavelength spacing among multiple reflectivity peaks
is determined by the sample period and is controllable during the fabrication process.
Moreover, if each sample is chirped, the dispersion characteristics of each reflectivity
peak are governed by the amount of chirp introduced. Such a grating was first used in
1995 to demonstrate simultaneous compensation of fiber dispersion over 240 km for
two 10-Gb/s channels [145]. A 1999 experiment used a sampled grating for a four-