Page 729 - Introduction to Information Optics

12.4. Applications and Future Development of Optical Networks 71 3

which channels are dropped from the network and which channels are
added.
* Optical cross-connect and wavelength routing. Tunable laser transmitters
are a way to achieve additional flexibility, reliability, and functionality in
an optical network. Tuning the laser from one wavelength to another
wavelength can change the path that is taken by an optical signal, A
tunable filter at the receiving node is also required.

In the future, nanosecond-speed tunable lasers may make it possible to
switch packets in the optical domain. Today, there are four basic technology
candidates for tunable lasers:

1. DFB lasers. Limited tuning (5 to 15 nm) is achieved by tuning the
temperature of the laser chip.
2. Distributed Bragg Reflector (DBR) lasers. The gratings of DBR lasers are
built in the one of the passive sections of the laser cavity, instead of inside
the active section as is the case for DFBs. The biggest benefit is that
injection current to the grating can result in a much bigger change in the
refractive index of the cavity, since carriers in the passive section are not
clamped as they are in the active section; this in turn results in wider
wavelength tuning through injection current [33].
3. External-cavity diode lasers, which consist of a Fabry-Perot laser diode
and an external high-reflectivity diffraction grating. Tuning is achieved
through mechanically tilting the grating.
4. Vertical-cavity surface-emitting lasers (VCSELs). VCSELs are compound
semiconductor microlaser diodes that emit light vertically perpendicular
to their p-n junctions from the surface of a fabricated wafer [34].

Tuning is achieved by moving the top reflection mirror of the laser cavity
implemented with MEMS. All of these four tunable laser technologies have
their advantages and disadvantages.
Tunable optical filters, which pass any desired wavelength via software-
based electrical control, are the next step toward current fixed-wavelength
mux/demux used in DWDM networks. Tunable lasers and tunable filters
together, they enable the dynamic routing of all wavelengths in an optical
network; that is, any wavelength can be routed to any destination via software
provisioning.
Optical monitoring must extract all information for transmission, such as
power, spectrum, noise, dispersion, and nonlinearity, and for networking, such
as destination, routs, priority, and content type, at the optical layer so that the
optical network has an OAM&P (operation, administration, maintenance, and
provisioning) intelligence close to that of the electrical network. This is a very
important area for optical networking with a great deal left to be deployed.