16-08-2012, 11:45 AM
OPTICAL FIBER COMMUNICATION: FROM TRANSMISSION TO NETWORKING
[attachment=31613]
INTRODUCTION
Optical fiber communication is now firmly established
as the preferred means of communication
for signals over a few hundred megabits per second
over distances more than a few hundred
meters. Compared to transmission over electrical
cables, optical fiber offers an almost perfect
transmission medium: low loss over a very high
bandwidth, low levels of undesirable transmission
impairments, immunity to electromagnetic
interference, and long life-spans. Measurements
of fiber plants over 20 years have indicated little
degradation. To paraphrase from [l], we can
think of optical fiber and wireless communications
as quite complementary. Optical fiber
doesn’t go everywhere, but where it does go, it
provides a huge amount of available bandwidth
(well over tens of terabits per second over a single
fiber). Wireless, on the other hand, does go
almost everywhere, but provides a highly bandwidth-
constrained transmission channel, susceptible
to a variety of impairments.
EARLY DAYS:M ULTIMODFEIB ER
Figure 1 captures the evolution of optical fiber
transmission systems. Early experiments in the
mid 1960s by Kao and Hockham [2] demonstrated
that information encoded in light signals
could be transmitted over a glass fiber waveguide.
These early experiments proved that optical
transmission. over fiber was feasible.
However, it was not until the development of
processes to fabricate low-loss optical fiber in
the early 1970s by researchers at Corning [3] and
Bell Labs [4] that optical fiber transmission systems
really took off. This silica-based optical
fiber has three low-loss windows in the 0.8, 1.3,
and 1.55 pm infrared wavelength bands. The
lowest loss is around 0.25 dB/km in the 1.55 pm
band, and about 0.5 dB/km in the 1.3 pm band,
as shown in Fig. 2. These fibers enabled transmission
of light signals over distances of several
tens of kilometers before they needed to be
regenerated. A regenerator converts the light signal
into an electrical signal and retransmits a
fresh copy of the data as a new light signal.
OPTICAL AMPLIFIERS AND
WAVELENGTDHI VISIONM ULTIPLEXING
The next major milestone in the evolution of
optical fiber transmission systems was the development
of erbium-doped fiber amplifiers (EDFAs)
in the late 1980s and early 1990s. The EDFA
basically consists of a length of optical fiber, typically
a few meters to tens of meters, doped with
the rare earth element erbium.
The erbium atoms in the fiber are pumped
from their ground state to an excited state at a
higher energy level using a pump source. An
incoming signal photon triggers these atoms to
come down to their ground state. In the process,
each atom emits a photon. Thus incoming signal
photons trigger the emission of additional photons,
resulting in optical amplification.
FROM OPTICAL TRANSMISSIONTO
OPTICAL NETWORKING
The history of optical communication has been
mostly about transmission and how to provide
higher bandwidths while simultaneously reducing
the cost per bit transmitted. The future is likely
to be about optical networking. Transmission will
continue to play a key role, but the new game is
to reduce the cost per connecfed bit transmitted,
The implication of this statement is that the
optical layer will move from providing simple
transmission pipes to a managed optical network.
This allows service providers to deliver a range
of new services using the optical network. In
order to be successful at accomplishing this
objective, service providers and equipment manufacturers
will need to figure out how to get the
best nctwork cfficiencies by combining the optical
layer with higher layers such as SONET (for
TDM selviccs) and IP (for statistical multiplexed
selvices).
[attachment=31613]
INTRODUCTION
Optical fiber communication is now firmly established
as the preferred means of communication
for signals over a few hundred megabits per second
over distances more than a few hundred
meters. Compared to transmission over electrical
cables, optical fiber offers an almost perfect
transmission medium: low loss over a very high
bandwidth, low levels of undesirable transmission
impairments, immunity to electromagnetic
interference, and long life-spans. Measurements
of fiber plants over 20 years have indicated little
degradation. To paraphrase from [l], we can
think of optical fiber and wireless communications
as quite complementary. Optical fiber
doesn’t go everywhere, but where it does go, it
provides a huge amount of available bandwidth
(well over tens of terabits per second over a single
fiber). Wireless, on the other hand, does go
almost everywhere, but provides a highly bandwidth-
constrained transmission channel, susceptible
to a variety of impairments.
EARLY DAYS:M ULTIMODFEIB ER
Figure 1 captures the evolution of optical fiber
transmission systems. Early experiments in the
mid 1960s by Kao and Hockham [2] demonstrated
that information encoded in light signals
could be transmitted over a glass fiber waveguide.
These early experiments proved that optical
transmission. over fiber was feasible.
However, it was not until the development of
processes to fabricate low-loss optical fiber in
the early 1970s by researchers at Corning [3] and
Bell Labs [4] that optical fiber transmission systems
really took off. This silica-based optical
fiber has three low-loss windows in the 0.8, 1.3,
and 1.55 pm infrared wavelength bands. The
lowest loss is around 0.25 dB/km in the 1.55 pm
band, and about 0.5 dB/km in the 1.3 pm band,
as shown in Fig. 2. These fibers enabled transmission
of light signals over distances of several
tens of kilometers before they needed to be
regenerated. A regenerator converts the light signal
into an electrical signal and retransmits a
fresh copy of the data as a new light signal.
OPTICAL AMPLIFIERS AND
WAVELENGTDHI VISIONM ULTIPLEXING
The next major milestone in the evolution of
optical fiber transmission systems was the development
of erbium-doped fiber amplifiers (EDFAs)
in the late 1980s and early 1990s. The EDFA
basically consists of a length of optical fiber, typically
a few meters to tens of meters, doped with
the rare earth element erbium.
The erbium atoms in the fiber are pumped
from their ground state to an excited state at a
higher energy level using a pump source. An
incoming signal photon triggers these atoms to
come down to their ground state. In the process,
each atom emits a photon. Thus incoming signal
photons trigger the emission of additional photons,
resulting in optical amplification.
FROM OPTICAL TRANSMISSIONTO
OPTICAL NETWORKING
The history of optical communication has been
mostly about transmission and how to provide
higher bandwidths while simultaneously reducing
the cost per bit transmitted. The future is likely
to be about optical networking. Transmission will
continue to play a key role, but the new game is
to reduce the cost per connecfed bit transmitted,
The implication of this statement is that the
optical layer will move from providing simple
transmission pipes to a managed optical network.
This allows service providers to deliver a range
of new services using the optical network. In
order to be successful at accomplishing this
objective, service providers and equipment manufacturers
will need to figure out how to get the
best nctwork cfficiencies by combining the optical
layer with higher layers such as SONET (for
TDM selviccs) and IP (for statistical multiplexed
selvices).