298 CHAPTER 7. DISPERSION MANAGEMENT
Wavelength (nm)Normalized Reflectivity (dB)Time Delay (ps)Figure 7.12: Measured reflectivity and time delay for a linearly chirped fiber grating with a
bandwidth of 0.12 nm. (After Ref. [73];©c1996 IEEE; reprinted with permission.)
over whichdφ/dωvaries linearly. The slope of the group delay (about 5000 ps/nm)
is a measure of the dispersion-compensation capability of the grating. Such a grating
can recover the 10-Gb/s signal by compensating the GVD acquired over 400 km of the
standard fiber. The chirped grating should be apodized in such a way that the coupling
coefficient peaks in the middle but vanishes at the grating ends. The apodization is
essential to remove the ripples that occur for gratings with a constantκ.
It is clear from Eq. (7.6.6) thatDgof a chirped grating is ultimately limited by
the bandwidth∆λover which GVD compensation is required, which in turn is deter-
mined by the bit rateB. Further increase in the transmission distance at a given bit rate
is possible only if the signal bandwidth is reduced or a prechirp technique is used at
the transmitter. In a 1996 system trial [72], prechirping of the 10-Gb/s optical signal
was combined with the two chirped fiber gratings, cascaded in series, to increase the
transmission distance to 537 km. The bandwidth-reduction technique can also be com-
bined with the grating. As discussed in Section 7.3, a duobinary coding scheme can
reduce the bandwidth by up to 50%. In a 1996 experiment, the transmission distance
of a 10-Gb/s signal was extended to 700 km by using a 10-cm-long chirped grating in
combination with a phase-alternating duobinary scheme [73]. The grating bandwidth
was reduced to 0.073 nm, too narrow for the 10-Gb/s signal but wide enough for the
reduced-bandwidth duobinary signal.
The main limitation of chirped fiber gratings is that they work as a reflection filter.
A 3-dB fiber coupler is sometimes used to separate the reflected signal from the incident
one. However, its use imposes a 6-dB loss that adds to other insertion losses. An
optical circulator can reduce insertion losses to below 2 dB and is often used in practice.
Several other techniques can be used. Two or more fiber gratings can be combined to
form a transmission filter, which provides dispersion compensation with relatively low
insertion losses [74]. A single grating can be converted into a transmission filter by
introducing a phase shift in the middle of the grating [75]. A Moir ́e grating, formed
by superimposing two chirped gratings formed on the same piece of fiber, also has a