Page 67 - Optical Switching And Networking Handbook

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52 Chapter 2

Polarity Modulation or Polarity-Inversion-Shift Keying
(PISK) Lasers produce linearly polarized light.A modulation tech-
nique might work by changing the light’s polarity. Unfortunately,
current fiber changes the polarization of light during transit. There-
fore, this may result in a conflict, producing a reversal of the pulse
being transmitted. This is not available commercially yet.

Directional Transmission Fiber typically is a one-way trans-
mission system. Two fibers are used for two-way transmission. The
size of a single fiber and the number of fibers that can be packed into
a cable make this attractive and the predominant mode of operation.
However, a single fiber can carry light in both directions simultane-
ously. Devices doing this today use different wavelengths in differ-
ent directions.
Many new techniques emerge as fast as the existing ones are
installed. The market today is ripe for innovation in the use of fiber-
based networks and capitalizes on the huge amount of bandwidth
available.
Current fiberoptic systems produce a single bit per pulse. In the
future, however, researchers probably will achieve 2, 3, or even 5 bits
per pulse. This, of course, will expand the bandwidth of the fiber by
orders of magnitude. With a single bit per pulse, the theoretical
bandwidth (throughput in bits per second) is approximately 3 10 13
bps (30 Tbps). Even if no improvements are made in the current
technology, we can get a 10,000-fold increase using a single fiber over
what we get today.
This is where the action is taking the industry. The industry is
looking toward the following goals:

To find a way to increase the modulation techniques to get 4, 5,
or 6 bits per hertz

To achieve a higher number of wavelengths on a single fiber
To optically switch the light instead of having to convert it back
to an electrical signal and then into photonic again
To increase the distances between repeaters so that fiber is less
expensive to deploy on long-haul circuits