8.2. WDM COMPONENTS 355
Figure 8.18: Scanning electron micrograph of an 8×8 MEMS optical switch based on free-
rotating micromirrors. (After Ref. [112];©c2000 IEEE; reprinted with permission.)
ing an OXC. For this reason, a micro-electro-mechanical system (MEMS) is used
for switching [109]. Figure 8.18 shows an example of a 8×8 MEMS optical switch
containing a two-dimensional array of free-rotating micromirrors [112]. Optical path
lengths are far from being uniform in such a two-dimensional (2-D) geometry. This fea-
ture limits the switch size although multiple 2-D switches can be combined to increase
the effective size. The three-dimensional (3-D) configuration in which the input and
output fibers are located normal to the switching-fabric plane solves the size problem
to a large extent. The switch size can be as large as 4096×4096 in the 3-D configu-
ration. MEMS-based switches were becoming available commercially in 2002 and are
likely to find applications in WDM networks. They are relatively slow to reconfigure
(switching time>10 ms) but that is not a major limitation in practice.
A MZ interferometer similar to that shown in Fig. 8.8(b) can also act as a 2× 2
optical switch because the input signal can be directed toward different output ports
by changing the delay in one of the arms by a small amount. The planar lightwave
circuit technology uses the thermo-optic effect to change the refractive index of silica
by heating. The temperature-induced change in the optical path length provides op-
tical switching. As early as 1996, such optical switches were used to form a 8× 16
OXC [103]. By 1998, such an OXC was packaged using switch boards of the standard
(33×33 cm^2 ) dimensions [107]. The extinction ratio can be increased by using two
MZ interferometers in series, each with its own thermo-optic phase shifter, since the
second unit blocks any light leaked through the first one [110]. Polymers are some-
times used in place of silica because of their large thermo-optic coefficient (more than
10 times larger compared with that of silica) for making OXCs [111]. Their use re-
duces both the fabrication cost and power consumption. The switching time is∼1ms
for all thermo-optic devices.
A directional coupler also acts as a 2×2 optical switch because it can direct an
input signal toward different output ports in a controlled fashion. In LiNbO 3 -based di-
rectional couplers, the refractive index can be changed by applying an external voltage
through the electro-optic effect known as electrorefraction. The LiNbO 3 technology
was used by 1988 to fabricate an 8×8 OXC [108]. Switching time of such cross-