Page 708 - Introduction to Information Optics

692 12. Networking with Optics

fiber patch panel. FXCs are less complex than a wavelength-selective or
wavelength-interchanging cross-connect. In parts of the network where
protection against fiber cuts is the main concern, FXCs could be a viable
solution. They may also make the best use of current proven optical
technologies. While FXCs can provide simple provisioning and restora-
tion capabilities, they may not offer the flexibility required to promote
new end-to-end wavelength generating services.
* Wavelength selective cross-connect (WSXC) can switch a subset of the
wavelength channels from an input fiber to an output fiber. They require
demultiplexing of an incoming DWDM signal, and remultiplexing them
into its individual constituent wavelengths. This type of cross-connect
offers much more flexibility than an FXC, allowing the provisioning of
wavelength services, which in turn can support video distribution, distance
learning, or a host of other services. A WSXC also offers better flexibility
for service restoration; wavelength channels can be protected individually
using a mesh, ring, or hybrid protection scheme.
• Wavelength interchanging cross-connect (WIXC), which is a WSXC with
the added capacity to translate or change the frequency (or wavelength)
of the channel from one frequency to another. This feature reduces the
probability of not being able to route a wavelength from an input fiber to
an output fiber because of wavelength contention. WIXC offer the greatest
flexibility for the restoration and provisioning of services.

Optical switch is the fundamental building block for OXCs with optical
core. The key issues for switches are low insertion loss, low cross talk, relatively
fast switch time, reliable manufacturing at low cost. Optical switches can be
categorized into those based on free-space waves and those based on guided
waves. Broadly speaking, free-space optical switches exhibit lower loss than
their guided-wave counterparts. Moreover, switches for which the principle of
operation relies on the electro-optic effect have faster switching times than their
electromechanical counterparts.
The following technologies are being employed for optical switching appli-
cations:
• MEMS (microelectromechanical systems). These are arrays of tiny mir-
rors originally developed for the very large video screens seen at sports
events and pop music concerts. It is at the moment the hottest technology
lor optical switching, attenuation, and wavelength tuning.
* Liquid crystals. Borrowed from laptop-screen technology, electric volt-
age alters the properties of liquid crystals so that light passing through
them is polarized in different ways. Passive optical devices then steer
each wavelength of light one way or the other, depending on its polariz-
ation.