8.4. TIME-DIVISION MULTIPLEXING 379
a 500-Gb/s OTDM signal was demonstrated by using clock pulses of about 1 ps dura-
tion [222]. This scheme can also amplify the demultiplexed channel (by up to 40 dB)
through parametric amplification inside the same fiber [223].
The main limitation of a fiber-based demultiplexer stems from the weak fiber non-
linearity. Typically, fiber length should be 5 km or more for the device to function at
practical power levels of the clock signal. This problem can be solved in two ways.
In one approach, the required fiber length is reduced by up to a factor of 10 by using
special fibers designed such that the nonlinear parameterγis enhanced because of a
reduced spot size of the fiber mode [223]. Alternatively, a different nonlinear medium
can be used in place of the optical fiber. The nonlinear medium of choice in practice is
the SOA. Both the XPM and FWM schemes have been shown to work using SOAs. In
the case of a NOLM, an SOA is inserted within the fiber loop. The XPM-induced phase
shift occurs because of changes in the refractive index induced by the clock pulses as
they saturate the SOA gain (similar to the wavelength-conversion scheme discussed
earlier). As the phase shift occurs selectively only for the data bits belonging to a spe-
cific channel, that channel is demultiplexed. The refractive-index change induced by
the SOA is large enough that a relative phase shift ofπcan be induced at moderate
power levels by an SOA of<1-mm length.
The main limitation of an SOA results from its relatively slow temporal response
governed by the carrier lifetime (∼1 ns). By injecting a CW signal with the clock
signal (at a different wavelength), the carrier lifetime can be reduced to below 100 ps.
Such demultiplexers can work at 10 Gb/s. Even faster response can be realized by
using a gating scheme. For example, by placing an SOA asymmetrically within the
NOLM such that the counterpropagating signals enter the SOA at different times, the
device can be made to respond at a time scale∼1 ps. Such a device is referred to
as the terahertz optical asymmetrical demultiplexer (TOAD). Its operation at bit rates
as high as 250 Gb/s was demonstrated by 1994 [224]. A MZ interferometer with two
SOAs in its two branches (see Fig. 8.20) can also demultiplex an OTDM signal at
high speeds and can be fabricated in the form an integrated compact chip using the
InGaAsP/InP technology [125]. The silica-on-silicon technology has also been used
to make a compact MZ demultiplexer in a symmetric configuration that was capable
of demultiplexing a 168-Gb/s signal [217]. If the SOAs are placed in an asymmetric
fashion, the device operates similar to a TOAD device. Figure 8.28(a) shows such a
MZ device fabricated with the InGaAsP/InP technology [225]. The offset between the
two SOAs plays a critical role in this device and is typically<1 mm.
The operating principle behind the MZ-TOAD device can be understood from
Fig. 8.28. The clock signal (control) enters from port 3 of the MZ interferometer and
is split into two branches. It enters the SOA1 first, saturates its gain, and opens the MZ
switch through XPM-induced phase shift. A few picoseconds later, the SOA2 is satu-
rated by the clock signal. The resulting phase shift closes the MZ switch. The duration
of the switching window can be precisely controlled by the relative location of the two
SOAs as shown in Fig. 8.28(b). Such a device is not limited by the carrier lifetime and
can operate at high bit rates when designed properly.
Demultiplexing of an OTDM signal requires the recovery of a clock signal from the
OTDM signal itself. An all-optical scheme is needed because of the high bit rates asso-
ciated with OTDM signals. An optical phase-locked loop based on the FWM process