Learning About Options in Fiber - Cables Plus USA

Channel Separation
Channel separation describes how well a coupler
can distinguish wavelengths. In most couplers, the
wavelengths must be widely separated, such as
820 nm and 1300 nm. Such a device will not distinguish
between 1290 nm and 1310 nm signals.
WDM allows the potential information-carrying
capacity of an optical fiber to be increased significantly.
The bandwidth-length product used to specify the
information-carrying capacity of a fiber applies only
to a single channel—in other words, to a signal
imposed on a single optical carrier.
SECTION 2—FIBER-OPTIC BASICS
When used in a ring network, however, failure of a
single terminal will shut down the entire network.
The fiber-optic bypass switch overcomes this
problem. Two settings on this switch permit the
light signal to be transmitted to the terminal
receiver or to bypass the terminal and continue on
the ring to the next terminal. A directional coupler
after the switch must also be used in conjunction
with the switch.
The switch uses a relay arrangement to move the
fiber between positions. A switch can be constructed
so that it automatically switches to the
bypass position if the power is removed, either
from turning off the terminal intentionally or from
unexpected disruption. The result is a certain
degree of “fail-safe” operation.
OPTICAL SWITCH
It is sometimes desirable to couple light from one
fiber to one of two fibers, but not to both. A
passive coupler (described earlier) does not allow
such a choice. The division of light is always the
same. An optical switch, however, does allow
such a choice. It is analogous to an electrical
switch, since it permits one of two circuit paths to
be chosen, depending on the switch setting.
The drawback to these switches is the difficulty of
manufacturing low loss switches. Maintaining
alignment on moving parts and over repeated
switchings compounds the already difficult task of
holding the tight tolerances imposed by the need
for precise alignment in fiber optics.
For this reason and many others, great care
should be exercised when selecting the manufacturer
of the fiber-optic system for your application.
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