cont. from previous post, Corning article:
"Fibre Optic Cable: Unsung Hero"
All-Optical Destiny
For years, the commercial use of optical fibres for voice and data
transmission was merely a dream. The problem was one of light loss
over distance. In 1970, three Corning, Inc. researchers, Robert
Maurer, Donald Keck and Peter Schultz successfully solved the
problem. They created the first optical waveguides -- glass fibres
made from fused silica -- which maintained the strength of laser light
signals over significant distances.
In the early 1980s, standard single-mode optical fibre became
available for commercial use. This fibre was optimised for use in the
1310 nm operating window. This window had minimal chromatic
dispersion, but not a minimal attenuation. In the mid-1980s,
dispersion-shifted optical fibres were developed which shifted the
minimum chromatic dispersion to the 1550 nm operating window
where attenuation is lower. End users could then take advantage of
both minimal chromatic dispersion and attenuation. With the
development of the erbium-doped fibre amplifier (EDFA) in the early
1990s, end users began to move toward multiple wavelengths on a
single optical fibre. Unfortunately, multiple wavelengths can lead to
unwanted interactions in the presence of very low chromatic
dispersion. To combat this, non-zero dispersion-shifted optical fibres
were developed. These fibres continued to be optimised for use in the
1550 nm operating window; however, they had a small amount of
chromatic dispersion to mitigate such non-linear effects as four wave
mixing in dense wavelength division multiplexing (DWDM) systems.
The development of dispersion-shifted and non-zero
dispersion-shifted optical fibres has thus enabled higher data rates
over longer distances. The need for regeneration electronics has been
decreased, lowering system cost and improving reliability.
With the advent of DWDM systems, customers are able to place
more data on a single fibre without incurring the cost of installing new
cable.
At the same time, the increase in demand for bandwidth has brought
two reactions from the optical fibre industry: increased data rates, and
increased fibre penetration. The move to non-zero dispersion-shifted
fibres has allowed for huge amounts of data to be transmitted over a
single fibre. Increases in both the speed of the data rates and the
number of wavelengths in use have allowed single fibres to carry data
in the range of 1 Tbps with 2 Tbps on the horizon.
Additionally, the amount of fibre demand has increased dramatically.
Where cables containing 100 and 200 fibres used to be common,
backbone lines containing fibre counts of 400 or 800 optical fibres are
now beginning to be used. In addition to the higher fibre count
backbones, fibre is being used closer to the end user. Ten years ago
fibre was primarily used in the backbone networks, but today
subscriber loops are commonly using optical fibre. As telephone and
cable TV companies rebuild their existing systems, fibre is being used
to replace copper even in the neighbourhood. Eventually, fibre will be
used to the home itself in an effort to satisfy the increasing demand for
bandwidth.
Broadband Band-Aids
Coaxial cable systems were installed about 30 years ago as
tree-and-branch systems with one-way amplifiers. That makes
upstream signals difficult. Upgrading them for interactivity is proving
very costly. Further, cable modem specifications are just now
extending to integrated voice capabilities. As a shared medium, cable
modem speeds decrease as more users go online.
xDSL requires a long foot copper drop from the curb to the house
plus the copper from the terminations to the DLC and to the central
office. These factors are conspiring to make it more difficult for
operators to guarantee the kind of quality of service (QoS)
expectations that are increasing as quickly as the demand for
bandwidth itself. Moreover, shared bandwidth cable and xDSL
engender security risks that fibre does not. With FTTH
(fibre-to-the-home) for example, the subscriber is connected to the
splitter, which is often miles from the termination, via a two-fibre
(upstream/downstream) drop.
Nevertheless, as fibre moves closer to the end user and displaces
copper, there are some adaptations that need to be achieved. Cable
designs will need to evolve from being solely designed for backbone
applications with higher fibre counts to being small size, low fibre
count solutions needed in FTTH applications. Additionally, media
conversion will be a big challenge. Today in the home or office, there
are very few appliances capable of receiving an optical signal. The
home television or telephone uses electrical signals. Where such a
media conversion will take place is still in dispute. Should the fibre
stop at the curb? the house? or the TV? Electronics manufacturers will
have to provide solutions for changing the high-speed optical signal
into a format useable by the average consumer. This may result in the
creation of set-top boxes for the television, or a whole new breed of
telephones for the home. The issue of powering such electronics is
also unresolved. Should such electronics be powered off the electric
grid, or should they be powered independently by the service
provider?
For the horizontal run, from the telecoms or wiring closet to the work
area, copper is still by far the most widely used transmission medium.
Although the cost of installed, horizontal fibre optic cabling has come
down, optical network interface cards for desktops and hubs are still
expensive compared to copper, but so was the desktop calculator at
one time.
Since the 1980s, the impact of fibre optics and the information
superhighway with photonics, subsequently led to the improvement of
devices that split, modify, amplify and manage photons in high-speed
optical systems. Through photonic manipulation enabling cabling that
interconnects hub locations, fibre optic cabling remains the most
practical approach. While it may be tempting to utilise copper twisted
pairs to support connections between hubs that are located in close
proximity to each other, fibre optics present an approach that can be
used consistently to allow for maximum flexibility as the network
grows. Furthermore, the use of fibre to the individual network nodes
is often discussed in the context of providing sufficient network
bandwidth to the user to support applications such as the transfer of
multimedia files and desktop videoconferencing.
Multiplying Capacity
Single transmission fibres have been considerably fortified via the
development and implementation of multiplexing devices such as
erbium-doped optical amplifiers (EDFAs) and dense wavelength
division multiplexers (DWDMs). These act to solve bandwidth
problems on the installed base so that carriers can readily
accommodate high bandwidth desktop applications and services.
Multiplexing technology allows many data channels at different
wavelengths to be bundled together, transmitted and subsequently
unbundled at the end of the route. DWDM is capable of multiplexing
at least 160 channels and is limited by the channel spacing, fibre
dispersion and the availability of fibre amplifiers throughout the optical
fibre spectrum. There is as yet is no perceptible limit to the number of
channels that can be multiplexed; these figures are constantly changing
on the R&D level. The most limiting factor right now for the high bit
rate transmission is dispersion. Fibre non-linear effects and chromatic
dispersion can be performance limiting factors because of the high
output of optical amplifiers and the simultaneous transmission of
multiple wavelengths. For installed conventional single-mode fibre, the
high dispersion at the 1550 nm region limits the transmission distance
at high bit rates. New fibre designs, such as non-zero
dispersion-shifted fibre and non-zero dispersion shifted fibre having a
large effected area, which are capable of controlling chromatic
dispersion, reducing fibre nonlinear effects, and operating at high bit
rates.
Corning believes that the drive to enhance the portion of the installed
base that consists of standard unshifted single-mode fibre -- optimised
for operations at 1310 nm -- has led to the large scale replacement of
regenerators with EDFAs, which operate at 1550 nm. OC-48 WDM
technology is well suited to this type of fibre and gives the carrier the
flexibility to incrementally add wavelengths when needed.
Regardless of cable design, it is the optical fibre that determines
whether a DWDM system is possible. Today, the non-zero
dispersion-shifted fibres are best suited for DWDM systems requiring
high data rates over long distances. However, if the data rate required
is somewhat lower, and distances are shorter, the standard
single-mode fibre is still an acceptable fibre for supporting DWDM. In
the outside plant environment, stranded loose tube cable and central
tube cables are best suited for dealing with the environmental and
mechanical rigors. Depending on the application, the customer may
find a lower fibre count cable with individual fibres to be the best
solution, or perhaps a high fibre count ribbon cable is better suited.
Older Fibre and DWDM
Some problems may exist in very old vintage optical fibres (circa early
1980s) when it comes to using DWDM systems. Additionally,
systems using dispersion-shifted optical fibre may also have trouble
with DWDM. These dispersion-shifted systems, however, are taking
on new life through the use of wavelengths above or below the 1550
nm operating window. When operated beyond the 1550 nm window,
dispersion-shifted fibres begin to operate similarly to non-zero
dispersion-shifted fibres and have a new application with respect to
DWDM systems.
Dispersion compensation modules can be added to a system to
correct for the higher chromatic dispersion typically seen in
dispersion-unshifted fibres at the 1550 nm operating window. Limiting
the data rate may allow older fibre types to be used with newer
systems as well. Using dispersion-shifted fibre above the 1550 nm
operating window can make that fibre useful for DWDM. A number
of solutions are available from electronics and photonics
manufacturers depending on the nature of the existing plant and the
new application desired.
One of the greatest challenges facing optical fibre is misunderstanding.
Many customers, familiar with copper products and installation, do
not completely understand the issues involved with optical fibre.
Optical fibres are housed inside hollow, cylindrical tubes, called buffer
tubes. The inside diameter of the buffer tubes is much larger than the
outside diameter of the optical fibre. This difference in diameter allows
free movement of the optical fibre within the buffer tube so that the
optical fibres are decoupled from the rest of the cable. A jelly like
substance fills the tube around the optical fibres, preventing moisture
from entering and allowing the optical fibre to ?float? within the buffer
tube. It also eliminates the possibility of water freezing in the vicinity of
the optical fibre, which could lead to an increase in attenuation or fibre
breakage. With an average tensile breaking strength of 600,000
pounds per square inch, fibre exceeds the strength requirements of all
of today?s communications applications (Figure 1).
In addition, glass is an extremely stable material, as shown by rigorous
environmental testing. Similarly, data gathered in the field over several
years has helped the industry establish fibre standards that suggest a
very long service life. Optical fibre has indeed been shown to have
virtually unlimited information-carrying capacity and exhibits little
susceptibility to severe weather or interference from outside
electromagnetic forces. Fibre?s immunity to adverse conditions such
as moisture ingress, corrosion and fatigue make it possible to project
its useful life out to 20 years or more.
Let There be Light
The increase in demand for bandwidth has brought two reactions from
the optical fibre industry: increased data rates and increased fibre
penetration. The move to non-zero dispersion-shifted fibres has
allowed for huge amounts of data to be transmitted over a single fibre.
Increases in both the speed of the data rates and the number of
wavelengths in use have allowed single fibres to carry data in the
range of 1 Tbps with 2 Tbps now looking possible.
Additionally, the amount of fibre demand has increased dramatically.
Where cables containing 100 and 200 fibres used to be common,
backbone lines containing fibre counts of 400 or 800 optical fibres are
now beginning to be used. In addition to the higher fibre count
backbones, fibre is being used closer to the end user. Ten years ago
fibre was primarily used in the backbone networks, but today
subscriber loops are commonly using optical fibre. As telephone and
cable TV companies rebuild their existing systems, fibre is being used
to replace copper even in the neighbourhood. Eventually, fibre will be
used to the home itself in an effort to satisfy the increasing demand for
bandwidth, thereby claiming its rightful legacy as a hero of the
broadband revolution.
Malcom Barnett is senior vice president, Sales and Marketing
Europe, Corning Cable Systems. |
