344 CHAPTER 8. MULTICHANNEL SYSTEMS
and can be accomplished in a switching time of less than 10μs. Acousto-optic tunable
filters are also suitable for wavelength routing and optical cross-connect applications
in dense WDM systems.
Another category of tunable optical filters operates on the principle of amplification
of a selected channel. Any amplifier with a gain bandwidth smaller than the channel
spacing can be used as an optical filter. Tuning is realized by changing the wavelength
at which the gain peak occurs. Stimulated Brillouin scattering (SBS), occurring nat-
urally in silica fibers [59], can be used for selective amplification of one channel, but
the gain bandwidth is quite small (<100 MHz). The SBS phenomenon involves in-
teraction between the optical and acoustic waves and is governed by a phase-matching
condition similar to that found for acousto-optic filters. As discussed in Section 2.6,
SBS occurs only in the backward direction and results in a frequency shift of about
10 GHz in the 1.55-μm region.
To use the SBS amplification as a tunable optical filter, a continuous-wave (CW)
pump beam is launched at the receiver end of the optical fiber in a direction opposite to
that of the multichannel signal, and the pump wavelength is tuned to select the channel.
The pump beam transfers a part of its energy to a channel down-shifted from the pump
frequency by exactly the Brillouin shift. A tunable pump laser is a prerequisite for
this scheme. The bit rate of each channel is even then limited to 100 MHz or so. In a
1989 experiment in which a 128-channel WDM network was simulated by using two
8 ×8 star couplers [60], a 150-Mb/s channel could be selected with a channel spacing
as small as 1.5 GHz.
Semiconductor optical amplifiers (SOAs) can also be used for channel selection
provided that a DFB structure is used to narrow the gain bandwidth [61]. A built-in
grating can easily provide a filter bandwidth below 1 nm. Tuning is achieved using a
phase-control sectionin combination with a shift of Bragg wavelength through elec-
trorefraction. In fact, such amplifiers are nothing but multisection semiconductor lasers
(see Section 3.4.3) with antireflection coatings. In one experimental demonstration,
two channels operating at 1 Gb/s and separated by 0.23 nm could be separated by se-
lective amplification (>10 dB) of one channel [62]. Four-wave mixing in an SOA
can also be used to form a tunable filter whose center wavelength is determined by the
pump laser [63].
8.2.2 Multiplexers and Demultiplexers
Multiplexers and demultiplexers are the essential components of a WDM system. Sim-
ilar to the case of optical filters, demultiplexers require a wavelength-selective mecha-
nism and can be classified into two broad categories.Diffraction-based demultiplexers
use an angularly dispersive element, such as a diffraction grating, which disperses in-
cident light spatially into various wavelength components.Interference-based demul-
tiplexersmake use of devices such as optical filters and directional couplers. In both
cases, the same device can be used as a multiplexer or a demultiplexer, depending on
the direction of propagation, because of the inherent reciprocity of optical waves in
dielectric media.
Grating-based demultiplexers use the phenomenon of Bragg diffraction from an
optical grating [64]–[67]. Figure 8.9 shows the design of two such demultiplexers. The