Page 710 - Introduction to Information Optics

694 12. Networking with Optics

redirect light. But in this approach, each mirror has multiple possible posi-
tions— at least N positions. It is called 2N architect because two arrays of N
mirrors are used to connect N input to N output fibers. The 3D approach can
scale to thousands of ports with low loss and high uniformity since the distance
of light propagation does not increase as the port count grows, whereas in 2D
switch, port count increase results in squared increase in light travel distance
and increase in the pitch of the micromirrors and the diameter of the light
beam, placing tight constraints on collimator performance and mirror align-
ment tolerance. Such a trade-off can rapidly become unmanageable, leading to
very large silicon devices and low yields. Thirty-two ports are currently
considered a top-end size for a single-chip solution in 2D MEMS switch. The
catch for 3D switch is that a sophisticated analog-driving scheme is needed to
ensure that the mirrors are in the correct position at all times. Although
MEMS technology can produce 2N 3D mirror arrays with impressive stability
and repeatability by using a simple open-loop driving scheme, closing the loop
with active feedback control is fundamental to achieving the long-term stability
required in carrier-class deployment of all optical OXCs. Using a closed-loop
control scheme implies that monitoring the beam positions must be implemen-
ted in conjunction with computation resources for the active feedback loop and
very linear high-voltage drivers.

12.2.7. OPTICAL MONITORING

Optical monitoring provides the base for optical networking. It is crucial
not only to offer large capacity for various traffics but also to manage all the
traffic streams. Network management (NM) functions include performance
monitoring (PM), alarm, provision, and protection/restoration. NM in electri-
cal networks is usually done by monitoring some predefined overhead (OH)
bytes or bits in digital signals. For example, SONET/SDH defines path, line,
and section overhead bytes. SONET equipment reads these OH bytes for
network management. In principle, an optical network should perform the
same functions at the optical layer. Unfortunately, optical monitoring technol-
ogy is still in its infancy stage, way behind optical transmission technology.
The earliest (and still widely used) optical monitoring system uses an
asymmetric optical coupler, called a tap coupler, to take a small portion of the
traffic-bearing optical signal. After O-to-E conversion, this tapped signal
provides the optical power level of the mainstream optical signal to NM, based
on which LOS (loss of signal) alarms and protection switching are determined.
In some DWDM systems, optical channel or wavelength information is
encoded using a low-frequency dither tone to modulate the transmitter laser.
This low-frequency content is extracted at the optical monitoring point, so that