9.6. HIGH-SPEED SOLITON SYSTEMS 445
Several other techniques can be used for controlling the timing jitter. One approach
consists of inserting a fast saturable absorber periodically along the fiber link. Such
a device absorbs low-intensity light such as ASE and dispersive waves but leaves the
solitons intact by becoming transparent at high intensities. To be effective, it should
respond at a time scale shorter than the soliton width. It is difficult to find an absorber
that can respond at such short time scales. A nonlinear optical-loop mirror (see Section
8.4) can act as a fast saturable absorber and reduces the timing jitter of solitons while
also stabilizing their amplitude [138]. Re-timing of a soliton train can also be accom-
plished by taking advantage of cross-phase modulation [139]. The technique overlaps
the soliton data stream and another pulse train composed of only 1 bits (an optical
clock) inside a fiber where cross-phase modulation (XPM) induces a nonlinear phase
shift on each soliton in the signal bit stream. Such a phase modulation translates into
a net frequency shift only when the soliton does not lie in the middle of the bit slot.
Similar to the case of synchronous phase modulation, the direction of the frequency
shift is such that the soliton is confined to the center of the bit slot. Other nonlinear ef-
fects such as stimulated Raman scattering [140] and four-wave mixing (FWM) can also
be exploited for controlling the soliton parameters [141]. The technique of distributed
amplification also helps in reducing the timing jitter. As an example, if solitons are
amplified using distributed Raman amplification, timing jitter can be reduced by about
a factor of 2 [125].
9.6 High-Speed Soliton Systems
As seen in Section 8.4, the optical time-division multiplexing (OTDM) technique can
be used to increase the single-channel bit rate to beyond 10 Gb/s. As early as 1993,
the bit rate of a soliton-based system was extended to 80 Gb/s with this method [147].
The major limitation of such systems stems from nonlinear interaction between two
neighboring solitons. This problem is often solved by using a variant of polarization
multiplexing in which neighboring bit slots carry orthogonally polarized pulses. The
80-Gb/s signal could be transmitted over 80 km with this technique. The same tech-
nique was later used to extend the bit rate to 160 Gb/s [148]. At such high bit rates, the
bit slot is so small that the soliton width is typically reduced to below 5 ps, and sev-
eral higher-order nonlinear and dispersive effects should be considered. This section is
devoted to such issues.
9.6.1 System Design Issues
Both fiber losses and dispersion need to be managed properly in soliton systems de-
signed to operate at high bit rates. The main design issues are related to the choice of a
dispersion map and the relationship between the map periodLmapand amplifier spac-
ingLA. As discussed in Section 9.4.2, the parameterTmapgiven in Eq. (9.4.11) sets
the scale for the shortest pulse that can be propagated in a periodic fashion. Thus, the
important design parameters are the local GVD and the length of various fiber sections
used to form the dispersion map.