378 CHAPTER 8. MULTICHANNEL SYSTEMS
Figure 8.27: Demultiplexing schemes for OTDM signals based on (a) cascaded LiNbO 3 modu-
lators, (b) XPM in a nonlinear optical-loop mirror, and (c) FWM in a nonlinear medium.
multiplexing of a 6.3-Gb/s channel from a 100-Gb/s OTDM signal was demonstrated
in 1993. By 1998, the NOLM was used to demultiplex a 640-Gb/s OTDM signal [221].
The third scheme for demultiplexing in Fig. 8.27 makes use of FWM in a nonlinear
medium and works in a way similar to the wavelength-conversion scheme discussed
in Section 8.2.5. The OTDM signal is launched together with the clock signal (at a
different wavelength) into a nonlinear medium. The clock signal plays the role of the
pump for the FWM process. In time slots in which a clock pulse overlaps with the 1
bit of the channel that needs to be demultiplexed, FWM produces a pulse at the new
wavelength. As a result, the pulse train at this new wavelength is an exact replica of
the channel that needs to be demultiplexed. An optical filter is used to separate the
demultiplexed channel from the OTDM and clock signals. A polarization-preserving
fiber is often used as the nonlinear medium for FWM because of the ultrafast nature of
its nonlinearity and its ability to preserve the state of polarization despite environmen-
tal fluctuations. As early as 1996, error-free demultiplexing of 10-Gb/s channels from