380 CHAPTER 8. MULTICHANNEL SYSTEMS
(a) (b)Figure 8.28: (a) Mach–Zehnder TOAD demultiplexer with two SOAs placed asymmetrically.
Insets show the device structure. (b) Gain variations inside two SOAs and the resulting switching
window. (After Ref. [225];©c2001 IEEE; reprinted with permission.)
is commonly used for this purpose. Schemes based on a NOLM or an injection-locked
laser can also be used [219]. Self-pulsing semiconductor lasers as well as mode-locked
fiber lasers have been used for injection locking.
8.4.3 System Performance
The transmission distanceLof OTDM signals is limited in practice by fiber disper-
sion because of the use of short optical pulses (∼1 ps) dictated by relatively high bit
rates. In fact, an OTDM signal carryingNchannels at the bit rateBis equivalent to
transmitting a single channel at the composite bit rate ofNB, and the bit rate–distance
productNBLis restricted by the dispersion limits found in Sections 2.4.3. As an ex-
ample, it is evident from Fig. 2.13 that a 200-Gb/s system is limited toL<50 km even
when the system is designed to operate exactly at the zero-dispersion wavelength of
the fiber. Thus, OTDM systems require not only dispersion-shifted fibers but also the
use of dispersion-management techniques capable of reducing the impact of both the
second- and third-order dispersive effects. Even then, PMD becomes a limiting factor
for long fiber lengths and its compensation is often necessary. The intrachannel nonlin-
ear effects also limit the performance of OTDM systems; the use of soliton-like pulses
is often necessary for OTDM systems [217].
In spite of the difficulties inherent in propagation of single-carrier OTDM systems
operating at bit rates exceeding 100 Gb/s, many laboratory experiments have realized
high-speed transmission using the OTDM technique [219]. In a 1996 experiment, a
100-Gb/s OTDM signal consisting of 16 channels at 6.3 Gb/s was transmitted over
560 km by using optical amplifiers (80-km spacing) together with dispersion manage-
ment. The laser source in this experiment was a mode-locked fiber laser producing
3.5-ps pulses at a repetition rate of 6.3 GHz (the bit rate of each multiplexed channel).
A multiplexing scheme similar to that shown in Fig. 7.26 was used to generate the 100-
Gb/s OTDM signal. The total bit rate was later extended to 400 Gb/s (forty 10-Gb/s
channels) by using a supercontinuum pulse source producing 1-ps pulses [226]. Such
short pulses are needed since the bit slot is only 2.5-ps wide at 400 Gb/s. It was neces-