356 CHAPTER 8. MULTICHANNEL SYSTEMS
(a) (b)Figure 8.19: Examples of optical switches based on (a) Y-junction semiconductor waveguides
and (b) SOAs with splitters. (After Ref. [105];©c1996 IEEE; reprinted with permission.)
connects can be quite small (<1 ns) as it is only limited by the speed with which
electrical voltage can be changed. An OXC based on LiNbO 3 switches was used for
the MONET project [21].
Semiconductor waveguides can also be used for making optical switches in the
form of direction couplers, MZ interferometers, or Y junctions [105]. The InGaAsP/InP
technology is most commonly used for such switches. Figure 8.19(a) shows a 4× 4
switch based on the Y junctions; electrorefraction is used to switch the signal between
the two arms of a Y junction. Since InGaAsP waveguides can provide amplification,
SOAs can be used for compensating insertion losses. SOAs themselves can be used for
making OXCs. The basic idea is shown schematically in Fig. 8.19(b) where SOAs act
as a gate switch. Each input is divided intoNbranches using waveguide splitters, and
each branch passes through an SOA, which either blocks light through absorption or
transmits it while amplifying the signal simultaneously. Such OXCs have the advan-
tage that all components can be integrated using the InGaAsP/InP technology while
providing low insertion losses, or even a net gain, because of the use of SOAs. They
can operate at high bit rates; operation at a bit rate of 2.5 Gb/s was demonstrated in
1996 within an installed fiber network [106].
Many other technologies can be used for making OXCs [115]. Examples include
liquid crystals, bubbles, and electroholography. Liquid crystals in combination with
polarizers either absorb or reflect the incident light depending on the electric voltage,
and thus act as an optical switch. Although the liquid-crystal technology is well devel-
oped and is used routinely for computer-display applications, it has several disadvan-
tages for making OXCs. It is relatively slow, is difficult to integrate with other optical
components, and requires fixed input polarization. The last problem can be solved by
splitting the input signal into orthogonally polarized components and switching each
one separately, but only at the expense of increased complexity.
The bubble technology makes use of the phenomenon oftotal internal reflectionfor
optical switching. A two-dimensional array of optical waveguides is formed in such a
way that they intersect inside liquid-filled channels. When an air bubble is introduced
at the intersection by vaporizing the liquid, light is reflected (i.e., switched) into another
waveguide because of total internal reflection. This approach is appealing because of