8.4. TIME-DIVISION MULTIPLEXING 377
of lasers are commonly used for this purpose [219]. In one approach, gain switching
or mode locking of a semiconductor laser provides 10–20 ps pulses at a high repetition
rate, which can be compressed using a variety of techniques [40]. In another approach,
a fiber laser is harmonically mode locked using an intracavity LiNbO 3 modulator [40].
Such lasers can provide pulse widths∼1 ps at a repetition rate of up to 40 GHz. More
details on short-pulse transmitters are given in Section 9.2.4.
8.4.2 Channel Demultiplexing
Demultiplexing of individual channels from an OTDM signal requires electro-optic
or all-optical techniques. Several schemes have been developed, each having its own
merits and drawbacks [216]–[220]. Figure 8.27 shows three schemes discussed in this
section. All demultiplexing techniques require aclock signal—a periodic pulse train
at the single-channel bit rate. The clock signal is in the electric form for electro-optic
demultiplexing but consists of an optical pulse train for all-optical demultiplexing.
The electro-optic technique uses several MZ-type LiNbO 3 modulators in series.
Each modulator halves the bit rate by rejecting alternate bits in the incoming signal.
Thus, an 8-channel OTDM system requires three modulators, driven by the same elec-
trical clock signal (see Fig. 8.27), but with different voltages equal to 4V 0 ,2V 0 , andV 0 ,
whereV 0 is the voltage required forπphase shift in one arm of the MZ interferome-
ter. Different channels can be selected by changing the phase of the clock signal. The
main advantage of this technique is that it uses commercially available components.
However, it has several disadvantages, the most important being that it is limited by
the speed of modulators. The electro-optic technique also requires a large number of
expensive components, some of which need high drive voltage.
Several all-optical techniques make use of anonlinear optical-loop mirror(NOLM)
constructed using a fiber loop whose ends are connected to the two output ports of
a 3-dB fiber coupler as shown in Fig. 8.27(b). Such a device is also referred to as
the Sagnac interferometer. The NOLM is called a mirror because it reflects its input
entirely when the counterpropagating waves experience the same phase shift over one
round trip. However, if the symmetry is broken by introducing a relative phase shift
ofπbetween them, the signal is fully transmitted by the NOLM. The demultiplexing
operation of an NOLM is based on the XPM [59], the same nonlinear phenomenon that
can lead to crosstalk in WDM systems.
Demultiplexing of an OTDM signal by an NOLM can be understood as follows.
The clock signal consisting of a train of optical pulses at the single-channel bit rate
is injected into the loop such that it propagates only in the clockwise direction. The
OTDM signal enters the NOLM after being equally split into counterpropagating direc-
tions by the 3-dB coupler. The clock signal introduces a phase shift through XPM for
pulses belonging to a specific channel within the OTDM signal. In the simplest case,
optical fiber itself introduces XPM. The power of the optical signal and the loop length
are made large enough to introduce a relative phase shift ofπ. As a result, a single
channel is demultiplexed by the NOLM. In this sense, a NOLM is the TDM counter-
part of the WDM add–drop multiplexers discussed in Section 8.2.3. All channels can
be demultiplexed simultaneously by using several NOLMs in parallel [220]. Fiber non-
linearity is fast enough that such a device can respond at femtosecond time scales. De-