Page 136 - Optical Switching And Networking Handbook

06_200023_CH05/Batesx 1/17/01 8:19 AM Page 121

Wave-Division and Dense-Wave-Division Multiplexing 121

DWDM uses a composite optical signal inside, carrying multiple
data streams, each sent on a different optical wavelength (different
color of light). This has been around for years, but earlier versions
were not as flexible as today’s technologies. Earlier versions just
used two different wavelengths and carried very wide band capacity
on the same fiber. Recent developments finally have evolved to a
point where it is practical and affordable to use multiple wave-
lengths and pack these different waves parallel in the same fiber.
The signals now operate in the 192- to 200-terahertz (THz) frequency
range. However, a new problem has reared its head. With signals as
precise and dense as those employed with the cables now in use, we
needed a way to provide accurate signal separation,that is, filtration,
at the receiver. This had to be
Easy to implement

Maintenance-free
Affordable
Early receivers were not very precise, so it was difficult to sepa-
rate the signals back to the originating input. Moreover, cross-talk
became a problem in handling the multiple wavelengths.
Another problem is the ability to regenerate the signals across the
fibers. We needed something that would satisfy the fiber demands
and yet enable the amplification of all wavelengths. The more we
had to extract the signal to amplify it, the more the risk there was of
failure. Moreover, this was expensive. The advent and implementa-
tion of erbium-doped fiber amplifiers (EDFAs) enabled the commer-
cial development and implementation of DWDM systems by
providing a way to amplify all the light waves at the same time. The
way in which this is done is that when the fibers are created (pulled),
erbium ions (a natural element and amplifier of light) are embedded
in the core of the glass; this process is called doping. These dopants
are fiber additives designed to produce the solution needed. Instead
of using multiple electric regenerators, the providers now can use
EDFAs to optically boost the signal, providing savings on the cost
and maintenance of the electrical regenerators.