Page 728 - Introduction to Information Optics

712 12. Networking with Optics

efficiency modulation, dispersion slope management, and dynamic gain
equalization are the key technologies under study to combat these dif-
ficulties. Another effort to realize ultra-long-haul transmission is soliton
transmission.
Opaque OXC — optical cross-connect using O/E/O and electrical switch
core has been deployed in the field since late 2000. Transparent core OXC uses
optical switch core and some O/E/O, and totally transparent OXC uses optical
switch and no O/E/O. OXCs based on optical switch core are still striving to
make their way to real-world applications. Electrical switch fabrics offer clear
advantages. They allow use of proven, off-the-shelf chip technology, and they
provide channel monitoring and management capabilities. On the other hand,
they bring clear disadvantages as well. Electrical switch fabric requires O/E/O
that uses expensive optoelectronic components such as lasers and modulators,
and limits the port speed and ultimately scalability of OXC. In contrast,
all-optical switches use MEMs and other transparent optical components to
redirect lightwaves without regard to the speed of the signal. Thus, they have
the benefit of port-speed independence, so that carriers can upgrade their
transport equipment without having to replace their switches. However,
all-optical switches offer little channel monitoring and line rate bandwidth
grooming, making service provisioning and network management quite chal-
lenging. Therefore, opaque OXC will be around for quite a while, perhaps
forever at the edge of the network. Transparent core OXC is a future-ready
interim solution. Totally transparent OXC will dominate the deep core of the
network when optical switch as well as optical monitoring technologies
mature.
While wavelengths hold the key to commercial optical networking, they are
generated by wavelength-stabilized DFB lasers. Wavelength-tunable semicon-
ductor lasers will provide networking flexibility and cost reduction in the
following areas:

Spares. Today, for every discrete wavelength deployed in an 8- or 80-
channel DWDM system, 8 or 80 spare transponders must be in inventory
in the event of transmitter failure. Tunable semiconductor lasers have
been proposed as a means to solve this problem.
Hot backup. An idle channel containing a tunable laser is used for
redundancy to immediately replace any failed transponder.
Replacement. Using tunable lasers to fully replace the fixed-wavelength
lasers in DWDM networks solves inventory management and field-
deployment complexity.
Reconfigurable and/or dynamic OADM. In optical networks using
OADM, tunable lasers enable network operators to dynamically allocate